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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real (2007)

Chapter:Appendix C Intermediate-Term Feature Descriptions

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Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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C
Intermediate-Term Feature Descriptions

This appendix has in-depth descriptions of the innovative banknote features that could be implemented in a time frame of fewer than 7 years and that are discussed in Chapter 4 of this report. Each feature description includes subheadings dealing with various aspects of the feature:

  • Description—An explanation of the physical principle(s) on which the feature is based. Also, the feature application as visible, machine-readable, applicable to the visually impaired, forensic applicability, and so on, is described. Furthermore, the benefits and limitations of the feature are presented; graphics may be included to depict the feature and its operation.

  • Feature Motivation—A summary of the reasons why the feature is highly rated by the committee and reference to its uniqueness.

  • Materials and Manufacturing Technology Options—A summary of the materials and manufacturing process that could be used to produce the feature, as well as initial thoughts on how the feature could be integrated into a Federal Reserve note.

  • Simulation Strategies—A discussion of potential ways in which a counterfeiter could simulate or duplicate the feature and the expected degree of difficulty in attempting to do so.

  • Key Development Risks and Issues—A discussion of the durability challenges, feature aesthetics, anticipated social acceptability, and description of the key technical challenges that must be addressed during the first phase of the development process to demonstrate the feasibility of the feature idea,

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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that is, to demonstrate feature capabilities and determine the usefulness of the feature in counterfeit deterrence. (The development phases are defined in Chapter 6.)

  • Phase I Development Plan—A characterization of the current maturation level of the feature technology, key milestones to be achieved during the first development phase, and known current and planned related developments external to the Bureau of Engraving and Printing (BEP).

  • Estimate of Production Cost—An initial assessment of additional BEP operational steps that would be required at the BEP to produce a banknote with the feature, incremental cost (higher, lower, the same) relative to the cost of the current security thread, and an indication of whether additional BEP capital equipment would be required for production.

  • References and Further Reading—Selected references related to the feature and its associated components. Such references could include, for example, papers and conference proceedings for background on any work done relating to this feature. These lists are not exhaustive but are intended to provide a snapshot of current work related to the feature concept.

The features described in this appendix are as follows:

  • Color Image Saturation

  • Fiber-Infused Substrate

  • Fresnel Lens for Microprinting Self-Authentication

  • Grazing-Incidence Optical Patterns

  • High-Complexity Spatial Patterns

  • Hybrid Diffractive Optically Variable Devices

  • Metameric Ink Patterns

  • Microperforated Substrate

  • Nanocrystal Pigments

  • Nanoprint

  • Refractive Microoptic Arrays

  • See-Through Registration Feature

  • Subwavelength Optical Devices

  • Tactile Variant Substrate

  • Thermoresponsive Optically Variable Devices

  • Window

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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COLOR IMAGE SATURATION

Description

By watermarking an image, its authenticity can be assessed. In most cases, watermarking is done in one or more of the various data channels of the image. The luminance, or brightness, channel of the image has been used, as well as the frequency space of the image. Since the eye is very sensitive to luminance variations, using the luminance channel results in any image degradation or manipulation being very obvious, and watermarks—even authentic ones—are often noticeable to a human observer. The frequency channel requires considerable processing if it is used, and it can also contain image-degrading artifacts.

The technique on which this proposed currency feature—color image saturation— is based uses the saturation channel of color images to embed watermarking or other secure data. Color images are usually captured via red (R), green (G), and blue (B) data channels. Saturation is data derived from the RGB channels using various computational techniques already known in the imaging industry and does not require the development of any new technology. In watermarking via the saturation channel, no visible artifacts would generally be realized, and hence the image can be watermarked without impacting its quality in any noticeable way. The human visual system is much less sensitive to saturation channel variations (essentially color intensity) than to pure luminance variations as previously described. With the BEP capability to create, process, and print such a watermarked image, the counterfeiter would not know how the watermarking was done and, as a consequence, would be at a considerable disadvantage in attempting to create a passable note utilizing this feature.

The paper cited in the “Further Reading” section below outlines the techniques used in performing this type of watermarking. One key benefit of this approach is that it should be very robust, and it is noteworthy that only an instrument can determine the authenticity of the image so marked, since unassisted visual inspection of the note would not be adequate to authenticate it. The complexity of the authentication hardware and software is not expected to be so costly or complex that it would incur prohibitive hardware or software implementation costs.

Implementing a color-saturation feature requires that the image being used is in more than one color rather than being pure monochrome. Depending on the color model chosen for the image or on how the data are created and stored, there should be a hue, value, and chroma channel—for example, the chroma channel might be used for the watermarking. Hue, for example, is the color of the image, such as red, green, blue, cyan, magenta, and so on. Value is the brightness of the color and can be similar to luminance. The chroma channel is the intensity of the color, such as its “redness,” for example. The eye is about 10 times less sensitive to

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×

chroma variations than to luminance variations. Color images in other encodings can be converted to have a saturation channel as required; it is just a matter of image preparation. The choice of whether the image is full color, pseudo-color, or just multicolor is optional and can be made at the time of note design. However, the image chosen would require selection based on its color characteristics and suitability for saturation channel watermarking use.

Feature Motivation

This feature would deter counterfeiting owing to the need to make an acceptable image with watermarking in the saturation channel. Also, it is expected that a watermarking scheme that would permit copying detection could be implemented. Thus, if a counterfeiter copied real currency and attempted to place the image on a counterfeit, the copied image would be detectable via the appropriate analysis mechanism. While this complete capability has yet to be verified, it would, if successful, be a very robust feature indeed. This feature is quite unique in that it uses the saturation channel of a color image to encode data; since this channel is not generally observable, a secure method of authenticating the note is provided. Furthermore, since the image is usually watermarked as a multibit-per-pixel image and then rendered as a binary image for printing via a halftoning or other binarization scheme, the would-be counterfeiter would not have access to the original image and would have great difficulty in determining from the binary image on the authentic currency the pixel values of the original image.

It is expected that this feature would deter the opportunist and the petty criminal counterfeiter and that many professional criminal counterfeiters would be highly challenged in attempting to duplicate or simulate the feature. Furthermore, the would-be counterfeiter would have to reverse-engineer the authentication hardware and software. This multitiered robustness of challenging image modification and detection methodology replication would be highly frustrating and time-consuming. Perhaps one of the strongest values of this technique is for forensic detection. The value for other users would depend on whether the technology to detect and authenticate the watermark would be shared with commercial banks or retail outlets.

Materials and Manufacturing Technology Options

A color-saturation watermark feature would be printed on the note similar to the other Federal Reserve note (FRN) features. No special processes or ink would be required. The feature’s strength lies in the data encoded in the image. Since most notes do not have a full-color ink set such as cyan, magenta, yellow, and black, some effort would be necessary to develop a production process and an

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×

image that employs the inks that the Bureau of Engraving and Printing uses or is planning to use. These requirements are not restrictions but do represent design and manufacturing choices to be made.

Simulation Strategies

Duplication of this feature would require the capability of at least a criminal of professional level. The opportunist or petty criminal would be easily prohibited from making this feature work by copying. Additionally, the data encoded in the saturation watermark would be unknown to the counterfeiter, since the data would be verified by a scanning and analysis mechanism only. The design of this mechanism would not generally give away the data that it processed in order to determine authenticity.

A key characteristic of most images is that they exist in a continuous-tone or multibit-per-pixel data format. However, the printing process, such as intaglio, is a binary process in that there is either ink or no ink deposited on the substrate. There can be substantial proprietary technology in turning the continuous-tone image into the proper binary image that is capable of being rendered from a device such as an intaglio or offset printer. Again, the would-be counterfeiter would have no knowledge of the original image’s continuous-tone data and hence could not readily determine either the binarization process used by the BEP or the original image data. Without this information, the would-be counterfeiter would have no idea what the original data looked like and hence could not readily determine what an acceptable forgery would look like to an analysis instrument. This kind of feature would, therefore, provide the note with substantial security advantages that are not accessible by the criminal from the currency itself. A visually similar appearance would be no guarantee that a counterfeit note would pass an authenticity examination via the scanner and processor at the point of use.

Key Development Risks and Issues
Durability

The durability of this feature should be high, since it is contained in a printed image and the image should be quite robust, as any printed feature would be. Therefore, durability is not an issue.

Aesthetics

The look and feel of the currency should not be negatively impacted by the use of this feature. The images would have to be in color or pseudo-color, and color is already present on U.S. FRNs.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Social Acceptability

There should be no issues regarding social acceptability, since the feature is just an image. However, it is conceivable that the scanning and processing of the note might cause some concern about maintaining the anonymity of currency. This feature would only facilitate the authentication of the note, however, and would not be used as a tracking feature.

Key Technical Challenges

The first key technical challenge would be to make sure that the embedding of the required data in the saturation channel did not noticeably deteriorate the image being watermarked.

The second key challenge would be to make sure that the watermark data were detectably altered if the image was copied in an unauthorized fashion. In this way, any attempt to copy and reproduce the image would cause detectable errors that would flag the currency as counterfeit.

Lastly, a scanning and processing mechanism would need to be designed that properly analyzed the note and did so at an acceptable speed coupled with tolerable cost and complexity.

Phase I Development Plan
Maturity of the Technology

This technology is modestly mature. Any required scanners and data-processing schemes are already known and tested. The only remaining issue is how well the embedded data degrade upon copying so that forgeries are easily detectable. The state of this knowledge is unknown.

Current and Planned Related Developments

No related developments in the public domain are known to the committee except as described in the reference work cited in “Further Reading,” below.

Key Milestones

The key milestones required are as follows:

  • Select an image or images suitable for use on currency.

  • Watermark the images, scan them or copy them, and process the data.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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  • Investigate how well the saturation watermark passes through the currency engraving and generation process.

  • In approximately 1 year following the achievement of the above results, currency could be in production, depending on resources and priorities.

Development Schedule

The committee estimates that the development of Phase I of this feature could be completed well within 2 years.

Estimate of Production Cost
Compatibility with Current BEP Equipment and Processes

The production cost should be very minimal owing to the fact that watermarking is only a printed feature. The cost impact of printing an image in more than one ink needs to be assessed, but this is likely already known.

Incremental Production Cost

The cost of this feature should be very minimal, since it is just another printed feature on the currency. For the required color image, color inks and a more complex printing process are involved, but the additional cost impact should be low to very low in the volumes of currency produced.

Required Capital Equipment

There is little in expected capital cost incurred with this feature. The need to process the watermark and scan the image would require some capital equipment, but it should be a relatively small amount. The software processing required should be capable of being developed on systems already in-house.

Further Reading

Huang, P.S., and C.-S. Chiang. 2005. Novel and robust saturation watermarking in wavelet domains for color images. Optical Engineering 44(11): 117002.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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FIBER-INFUSED SUBSTRATE

Description

This proposed feature involves fiber-infused paper—that is, small-diameter optical fiber segments placed in the currency substrate. These fiber segments could be glass, acrylic, or other materials. Even metallic fibers could be placed in the substrate to radiate signals when illuminated by radio-frequency (RF) signals, for example. Optical-fiber segments, when illuminated by laser light or narrow-spectrum illumination, create a signature pattern that would be easily recognizable. This feature is envisioned as an upgrade to the current fiber content of the substrate of U.S. FRNs. To employ this feature, optical fibers, or more preferably fiber segments, are placed in the substrate. As the substrate is manufactured, these fiber segments are mixed in before the paper is dried. When the finished substrate is illuminated with light, especially laser light, the fibers light up as the incident light emanates from the ends of the fibers.

The first deterrent example would be for a user to notice the speckles of light from the substrate when it is illuminated. The mere speckles of the substrate with its embedded fibers would be somewhat complex for counterfeiters to reproduce, since the counterfeiters would have to create their own substrate. This elevates the complexity of their counterfeiting task considerably. The limitation of this approach is that anything that causes the substrate to produce visible speckles might be misconstrued as authentic. One key element of this feature is that the substrate is no longer passive when illuminated by optical or other electromagnetic radiation. The way that the substrate responds can be highly controlled.

Figure C-1 illustrates the fibers embedded in the substrate. The references in “Further Reading,” below, give additional illustration of the concept and use of this technique.

FIGURE C-1 Fiber-infused substrate.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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A more robust feature would be to authenticate a note by a scan or digital photograph of the substrate and compare it with the known speckle pattern from a registered original or from data encoded on the note—this kind of longer-term feature is discussed in Appendix D.

Feature Motivation

The fiber-infused substrate feature has a good rating in the committee’s analysis owing to the difficulty of implementing the cause of the feature—that is, the fibers in the substrate—and the utility of visual inspection. Furthermore, this feature would not be reproducible using electronic printing and scanning techniques and hence would frustrate a large number of would-be counterfeiters. The feature idea is also compelling owing to its requiring both the design and manufacture of the currency substrate. A counterfeiter would be challenged not only to provide a good paper substitute for authentic currency but also to build the special fibers into the substrate. This process is most likely well beyond the capabilities of all but the most dedicated and resourced operations.

This feature is quite unique, although similar techniques were used for missile verification in the Strategic Arms Limitation Treaty—SALT 1—of 1993 when fiber-embedded placards that could not be duplicated were placed on missiles. Furthermore, the costs associated with this technique would be quite low, since the cost of the materials is low, and it is their being embedded randomly that gives the technique value.

Materials and Manufacturing Technology Options

The manufacturing requirements for this feature would involve paper manufacturing and integrating the fiber fragments into the paper or other substrate material. Since the BEP’s paper supplier produces the authentic substrate, it would be tasked with implementing this feature. It is not expected that this would be a difficult operation, although some tooling and process changes would no doubt be required. Once the substrate had been produced, further note production would proceed as usual.

Simulation Strategies

Simulation of this feature by would-be counterfeiters would not be easy. Furthermore, only the professional criminal or state-sponsored counterfeiter might be able to do a decent job of embedding fibers in the substrate and doing it well enough to make the operation a profitable one. Since the BEP could also control

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×

the fiber materials in the substrate, the counterfeiter would be faced with the difficult task of creating the fibers as well as making the substrate, eliminating the vast majority of criminals from attempting to do this.

Key Development Risks and Issues
Durability

The durability of this feature is unknown, but it would be dependent on the lengths of fiber embedded in the currency. If the currency was folded, the fibers could break if they were too long. Thus, the fibers would have to be short. No degradation of the fibers themselves is expected, and the only deterioration would be from breakage of the fibers if they were too long.

Aesthetics

There should be no aesthetic issues with the fiber-infused substrate feature. Unless illuminated, the note would look and feel identical to one without the feature. Even when illuminated, the feature should not detract from the note’s appearance. Furthermore, the speckles that would be generated by illumination would be a comforting feature to the receiver of the note. Thus, the feature is aesthetically neutral and conforms to the look and feel of current notes, as far as is known.

Social Acceptability

There should be no issues of social acceptability surrounding this feature.

Key Technical Challenges

The key technical challenge would be the incorporation of the fiber-infusing process into the substrate production process. A key technical challenge of this feature would be the development and use of instrumentation for the analysis of the fibers. Such instrumentation could range from the simple, such as a solid-state laser diode in a penlight configuration, to the more complex, such as a small scanner that reads the currency and produces a result that could be read by a user or that gives a “go” or “no-go” signal. Such an instrument should be simple, reliable, and cost-effective, which may require a development effort, depending on what requirements are placed on the instrument itself.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Phase I Development Plan
Maturity of the Technology

The maturity level of this technology is relatively low for currency-related efforts. The science behind its use and verification is known and highly reliable, but this technique has not been implemented in high-volume, low-cost applications such as that envisioned here.

Current and Planned Related Developments

There are currently no known programs that use this feature. The papers cited below in “Further Reading” describing its use are the only ones known to relate to this effort. There may be related proprietary efforts in companies, but this is not known at present. A key issue regarding this feature is one of feature-assessment methods such as instrumentation. There may be levels of authentication methods that are desired and that are improved over time. With the increasing miniaturization of instrumentation and sensors via technologies such as microelectromechanical systems (MEMS), the state of the art is advancing rapidly and should only enhance the usability and value of this feature.

Key Milestones

The expected key milestones for Phase I would be as follows:

  • Place fiber fragments in paper substrates to determine the applicability of the technique and any operational or manufacturing difficulties that might arise.

  • Assess authentication techniques for a phased development of passive and active instrumentation methods over time.

Development Schedule

It is expected that achieving Phase I development would take between 2 and 3 years.

Estimate of Production Cost
Compatibility with Current BEP Equipment and Processes

Other than obtaining the substrate from the supplier, the currency-manufacturing operation should remain unchanged. The conventional intaglio printing currently used would not be impacted.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Incremental Production Cost

The capital cost of this feature on a per note basis should be quite small—likely less than $0.02—since only the cost of the fibers used would impact the substrate cost. The visual check method would have no additional cost.

Required Capital Equipment

Although there should be no capital costs for the BEP, there might be costs for the substrate manufacturer. Equipment that would prepare, add, and mix any fiber materials with the pulp would be required. The fiber material could be provided in bulk from a supplier. Post-processing of the fiber materials, such as doping and so on, could occur at the substrate manufacturer for security purposes if required. The fibers could be glass, plastic, micro, or nano materials, with custom design of the properties as required. It is unclear what the capital equipment costs would be until an acceptable fiber material design is realized. The development of the fibers would likely be coordinated with authentication technologies so that the maximum benefit from the investment is realized.

Further Reading

Chen, Y., M.K. Mihcak, and D. Kirovski. 2005. Certifying authenticity via fiber-infused paper. ACM SIGecom Exchanges 5(3): 29-37.

DeJean, G., and D. Kirovski. 2006. Certifying authenticity using RF waves. Presented at IST Mobile Summit.

National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design, Washington, D.C.: National Academy Press, pp. 74-75 and 117-120.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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FRESNEL LENS FOR MICROPRINTING SELF-AUTHENTICATION

Description

Microprinting is currently impossible to replicate using ink-jet and laser printers, but its effectiveness in deterring counterfeits is hampered by the difficulty for cash handlers—including the public, cashiers, and tellers—of verifying the authenticity of an FRN by checking for the presence and quality of microprinting. A simple loupe is probably the best way to easily distinguish counterfeit from genuine notes because of very marked differences in microprinting and fine-line details between the two. A thin lens embedded within the banknote itself can provide a simple means of self-authentication for microprinting and fine-line detail. This feature would be most useful for the general public as protection against opportunist and petty criminals. The benefit of this feature would be to make it easy to see poor-quality printing.

This feature works by cutting a hole in the paper substrate and bonding a transparent Fresnel lens over the hole. The lens is fabricated from plastic and has a curved sawtooth profile, providing the refractive index variations of a lens without the bulk (see Figure C-2). The banknote has to be bent, not folded, to position the lens about an inch above the microprinting to be viewed. Note that microprinting does not have to be text; the clock face on the $100 note is a good example of an image that can be verified with a lens. The time on the clock can be easily read using a lens on a genuine note, but ink-jet counterfeits cannot accurately reproduce the hands or the numerals on the clock. Public education would be required to alert people about what to look for and where.

FIGURE C-2 The Fresnel lens shown here resembles a planoconvex lens that is cut into narrow rings and flattened. If the steps are narrow, the surface of each step is generally made conical and not spherical. The convex surface is reduced to concentric ridges. Fresnel lenses are flat rather than thick in the center and can be stamped out in a mold.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Feature Motivation

This Fresnel lens feature is intended for unassisted use by the general public. Cashiers and bank tellers could also use the feature, but most likely as a second line of defense because of the time required to bend the note. This feature could also assist visually impaired people in denominating their currency by using differently shaped or sized lenses for each denomination, and it could help deter “note washing” by using progressively smaller-area lenses on higher denominations. Opportunist and petty criminals should be deterred because they lack access to printing technology with sufficient spatial resolution to reproduce microprinting.

There are two risks associated with this feature: (1) home printers will be able to achieve sufficient spatial resolution to reproduce microprinting and fine-line details accurately, and (2) robust embedding of a plastic lens into the paper substrate may be difficult. The lenses themselves are robust and will work effectively even with substantial scratching and mild dirt.

This feature is expected to be cost-effective, since the cost of the mass-produced plastic lens and the cost to add a window are expected to be low.

This feature was highly rated in the committee’s systems evaluation because of the likelihood that it could be easily used by the general public without requiring an external device and because it is currently difficult for opportunist and petty criminals to reproduce microprinting and fine lines. It also has potential benefit for cashiers, tellers, and the visually impaired, and could prevent banknote washing for the reuse of $5 notes as counterfeit $20s or higher. It is not expected that this feature would be of benefit to machine readers.

This feature is unique among banknotes, although a patent exists for a similar feature applied to credit cards. Plastic Fresnel lenses are commonplace as inexpensive magnifiers and are easily available in hobby stores and drugstores, although they are too thick to be embedded within current banknote substrates.

Materials and Manufacturing Technology Options

Molded plastic Fresnel lenses are commercially available and can be currently used to inspect banknotes. Fresnel lenses are commonly injected molded from PMMA plastic and are typically ~½ mm thick. Microfabrication techniques have been used to make very thin lenses (10 microns thick) but not for use in the visible spectrum.

Fresnel lenses are in common use in a variety of industries but not in banknotes. They are commercially available at craft stores and drugstores for less than $1. Holes in paper substrates have been manufactured by De La Rue, Ltd., which has also bonded metal films across these holes. This company believes that plastic can be robustly applied across paper windows. Two new processes would be required for

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×

the BEP: cutting a hole and bonding the lens to the paper. This feature enhances the effectiveness of two existing features: microprinting and fine-line printing.

Simulation Strategies

This feature can be crudely simulated by buying a cheap Fresnel lens, cutting a hole in forged paper, and gluing the lens over the hole. However, the quality of this means of reproduction is likely to be low, since available lenses are relatively thick. Microprinting cannot now be easily replicated, so even if the lens could be simulated, the magnified image would still be poor, exposing the counterfeit. If microprinting becomes easily reproducible, this feature would lose much of its effectiveness as a deterrent.

Opportunist criminals cannot simply use a computer to reproduce this feature, and therefore it may be harder for them to rationalize their actions because of the extra effort required to manually add the lens. Opportunist and petty criminals are currently limited in their ability to print with high spatial resolution, so while it might be easy for them to embed a lens, by doing so they would be adding a means for the general public to see easily the low quality of their printed note.

Professional and state-sponsored criminals should have little trouble reproducing this feature.

Key Development Risks and Issues
Durability

The key durability issue is the attachment of the lens over a hole in the substrate. Commercial banknote vendors have recently demonstrated the feasibility of attaching metallic foils over holes in paper substrates. The adhesion process should be similar for a thin plastic sheet (lens), with similar performance in durability tests. This feature looks promising as a durable feature in the short term.

Aesthetics

This feature can enhance the existing banknote by enabling users to look closely at the fine detail of the notes, but adding a plastic window may be considered unattractive by some.

Social Acceptability

No problems of social acceptability are anticipated with this feature.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Key Technical Challenges

The key technical challenge is the ability to manufacture a sufficiently thin Fresnel lens (on the order of 40 microns thick) and to attach it reliably across a hole in the banknote.

Phase I Development Plan
Maturity of the Technology

Commercially available Fresnel lenses are about 0.5 mm thick and so are unsuitable for application within banknotes. Microfabricated, thin lenses have been demonstrated in the laboratory. The committee knows of no plastic films that have yet been adhered to a paper substrate. The manufacturing readiness level is currently low.

Current and Planned Related Developments

Researchers at the University of California at Los Angeles (UCLA) have manufactured Fresnel lenses in silicon for use in the infrared using microfabrication techniques (Lin et al., 1994). Researchers at the University of Maryland have manufactured Fresnel lenses for use with x-rays using deep reactive ion etching (Morgan et al., 2004). These are not suitable for use as visual lenses as required by this feature, but they represent the current state of microfabrication. If the lenses can be manufactured from embossed plastic, microfabrication techniques may not be necessary.

Key Milestones
  • Demonstrate fabrication of a Fresnel lens with appropriate dimensions. Reasonable targets are thickness equal to or less than 40 microns, aperture equal to or greater than 10 mm, and focal length about 25 mm, operating in the visible spectrum.

  • Demonstrate an approach for integrating the lens over a hole in the substrate such that it passes all durability tests.

  • Define a lens-fabrication process that has the potential to be affordably scaled up.

Development Schedule

The committee believes that Phase I of the development of this feature could be completed within 2 to 3 years.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Estimate of Production Cost
Compatibility with Current BEP Equipment and Processes

The committee expects that this feature would be provided by the manufacturer of the paper substrate, therefore minimally affecting BEP operations. The substrate would be delivered to the BEP with the lens already attached, and the BEP would continue to add microprinting using existing processes.

Incremental Production Cost

Microfabrication techniques should be able to provide this feature at a low incremental cost. This capability would be dependent on successful research, as outlined above.

Required Capital Equipment

The substrate provider would likely subcontract the manufacture of the lens, requiring no special equipment for the BEP or the bureau’s paper suppliers. The paper supplier would need to develop a process to produce the hole.

References and Further Reading

Finkelstein, A., D.A. Dixon, and R.H. Boede. 1995. Credit Card with a Fresnel Magnifying Lens Formed in a Section of the Transparent. U.S. Patent 5,434,405. July 19, 1995.

Kingslake, R. 1992. Optics in Photography. Bellingham, Wash.: SPIE Optical Engineering Press. [Fresnel lens explanation on p. 53.]

Lin, L.Y., S.S. Lee, K.S.J. Pister, and M.C. Wu. 1994. Three-dimensional micro-Fresnel lenses fabricated by micromachining technique. Electronics Letters 30(5): 448-449.

Morgan, B., C.M. Waits, J. Krizmanic, and R. Ghodssi. 2004. Development of a deep silicon phase Fresnel lens using gray-scale lithography and deep reactive ion etching. Journal of Microelectromechanical Systems 13(1): 113-120.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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GRAZING-INCIDENCE OPTICAL PATTERNS

Description

The evaluation of many optical features on currency employs the reflection of light. Most often, the light illuminating the feature impinges on the currency at near-normal incidence—that is, vertical to the plane of the substrate. The concept behind a grazing-incidence optical pattern feature is that it is intended to use light that impinges on the substrate at very high angles of incidence—that is, at about 85 to 90 from the normal—and to exploit substrate-surface irregularities, either incidental or intentional, and the patterns that they generate from the reflected light. Substrates have fairly unique characteristics at the microscopic scale and, when illuminated with either spectrally narrow or coherent light, such as from a laser, particular substrate characteristics could enable differentiation between substrates. An example of a substrate feature that could be probed this way would be an impress watermark for which the pressure from the intaglio process or other mechanical impression on the substrate used to make the watermark produced a surface relief on the substrate.

Figure C-3 illustrates the grazing-incidence model. The illumination of the substrate could be either coherent or incoherent, and the scattered light as seen by the observer could be seen as a pattern or, in the case of a watermark, as a relief image. Since substrates have unique properties that are under the control of the BEP, the scattered pattern from a banknote could be customized in a unique and secret way that would deter duplication by the counterfeiter. For instance, the intaglio process could impress highly complex depressions into the currency substrate. Additionally, the substrate surface microscopic profile could be prepressed into the substrate material so as to minimize any negative impact on the FRN production process.

FIGURE C-3 Concept behind the grazing-incidence optical pattern feature.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Public use of this feature would require a gadget, and use by a cashier or bank teller would also be assisted.

Feature Motivation

The most compelling aspect of this proposed feature is that most counterfeiters do not produce their own substrates and therefore would be unable to begin to duplicate or simulate this feature. The currency paper supplier could properly design patterns and materials into their substrate that would make its reproduction or simulation highly complicated for all but the most determined and well-resourced counterfeiters. Different customization for different denominations would also deter the use of one FRN substrate for the counterfeiting of higher-denomination notes. This type of feature would be a deterrent for the opportunist and petty criminal counterfeiter in particular and somewhat of a deterrent for the professional criminal counterfeiter.

Another compelling aspect is that this feature is quite unique in its use of the z-axis or vertical dimension of the substrate to produce the salient effect of the feature, not ink absorption or other x, y schemas to produce the pattern or other feature characteristic.

Materials and Manufacturing Technology Options

A grazing-incidence feature is simple in that it is just an additional designed characteristic of the substrate and not produced in the printing process.

Simulation Strategies

The dedicated, very motivated, and patient counterfeiter could attempt some form of simulation of this feature, but it would require an expensive pressure plate that was engraved with a pattern that simulated the authentic feature. It is expected that only the professional criminal or state-sponsored counterfeiter would have any hope of creating even a poor facsimile of the substrate properties necessary to produce the right pattern. But it is clear that anyone wanting to re-create this feature would be greatly challenged.

Key Development Risks and Issues
Durability

This feature should have excellent durability if properly implemented. If the substrate was impressed with micro patterns over its surface, occasional crushing or folding of the currency would not damage the entire surface—making the note

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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quite durable for general use. Only the crushing of the substrate surface, which would be hard to do completely, would abrogate the use of this feature.

Aesthetics

This feature should be aesthetically neutral, since it would not be generally observable without shining some form of illumination on it at a grazing-incidence angle on the note. Thus, normal viewing of the note would not reveal any observable pattern in general. The look and feel of the note would likely be unchanged, since the pattern or its roughness would not be noticeable to the casual user.

Social Acceptability

There should be no issues with this feature relative to privacy or environmental concerns. Properly designed, it might have characteristics that would enable visually challenged persons, since features for their benefit could also be placed on the note as part of the impression process.

Key Technical Challenges

The key technical challenges are the development of appropriate patterns and patterns that are compatible with tolerable analysis procedures. Once the patterns have been properly designed and analyzed, the production of such patterns in the currency substrate should not be difficult.

Phase I Development Plan
Maturity of the Technology

The current technology is somewhat in its operational infancy. No known implementations of this type of feature have been found in the literature.

Current and Planned Related Developments

No known development programs exist. As discussed above, most features are x, y features—that is, features such as printed patterns in the plane of the substrate. A grazing-incidence feature uses the z or vertical axis of the substrate. The committee is unaware of currency development or production programs that employ such features, other than watermarks, which use depressions in the surface of the substrate to cause images to appear when viewed vertically or nearly vertically. This feature is both illuminated and viewed at high angles of incidence.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Key Milestones

There are some key milestones to consider in the development and use of this feature:

  • Design test impressions for implementation and evaluation.

  • Generate impressions in substrate material using equipment that would be used for production of the substrate or note.

  • Develop evaluation systems for analysis of the feature.

  • Design production feature setup and testing.

Development Schedule

The first of the milestones listed above should require about a 1-year effort to design and evaluate the proper feature configuration; thus, the committee estimates that Phase I of the development of a grazing-incidence feature could be completed within 2 years, since some efforts could overlap in the development and testing process.

Estimate of Production Cost
Compatibility with Current BEP Equipment and Processes

The production cost for this feature should be quite low, since only the impression of the substrate before, during, or after printing is required. Producing this impression could be an additional step, but it is not much of a complication since the impression process is already known and operational via the intaglio printing already used.

Incremental Production Cost

The incremental production cost should be very low. Only the amortization of the wear and tear on the impression equipment incurs cost, and it is expected that this cost is already well known, since the intaglio process is well understood.

Required Capital Equipment

A small amount of optical bench equipment would be required to experiment with the use and evaluation of this feature.

Further Reading

No additional reading is suggested.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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HIGH-COMPLEXITY SPATIAL PATTERNS

Description

One basic high-value attribute of current currency production is the use of intaglio presses for the printing operation. The intaglio process uses engraved or otherwise produced “masters” with which to impress the currency substrate and print the pattern. As discussed in Chapter 3 of this report, with the advent of electronic printing, scanning, and image processing, many of the high-quality features of new currency can be reasonably scanned, processed, and reproduced on ink-jet, laser-printer, and other electronic output devices. Such electronic printing, scanning, and image processing are only going to improve in the future, although the current high-quality capabilities available result in less motivation for improving these capabilities significantly in the foreseeable future. Desktop and professional publishing markets are demanding but do not require significantly higher quality than is already available. Therefore, development resources will not press current technology beyond what commercial markets require, and hence a natural performance limit has been set by market needs.

However, there is one key difference between currency production and the electronic imaging tools used by would-be counterfeiters. That difference is the analog versus digital production of the final result. Intaglio is an analog technique. One can engrave or create on the master virtually any pattern at any location on the master within the limits of the creator’s art. This process is not a digital process. The electronic printer, however, is a digital process and as such has specific addressability limits. For example, a 2,500 pixel per inch printer can only lay down a pixel every 0.0004 inch. A pixel cannot be laid down 0.00027 inch from the last one because of the digitized nature of the device. Therefore, it is intended with this feature to place patterns at locations and in arrays of patterns that particularly frustrate digital printers of all kinds.

Examples of two possible patterns are shown in Figure C-4, which illustrates patterns often used for testing purposes in facsimile systems and photographic systems. While it is not necessarily suggested that these particular patterns be used, they are shown here as examples of patterns in which the gradually decreasing spacing, whether radial or lineal, will at some point cause the addressability limits of the digital printer to place two lines together without a space or two spaces without a line, and so on. With proper design and adequate intaglio/substrate printing quality, the analog system should be able to frustrate the digital system consistently.

One should notice that this feature is visible and does not require instrumentation, although a magnifier might be of some help depending on the quality of the user’s visual capabilities. These features are optical in nature, and the usefulness of this high-complexity spatial pattern will depend on superior intaglio performance, line-width control, and minimum permissible line width and/or line spacing of

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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FIGURE C-4 Examples of possible high-complexity features. Upper: Starburst pattern. Lower: Sayce target.

the original currency production equipment. The basic intent with this feature is to use patterns whose full spatial bandwidth cannot be reproduced completely by digital systems and hence will show a visible defect on such systems, regardless of the image processing used. In the past, wavy patterns, chevrons, and so on could

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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often be simulated well enough not to show errors without close examination. The type of feature discussed here is intended to create patterns in the intaglio master that cannot be reproduced in full fidelity by digital systems available on the market today or in the near future.

Feature Motivation

The high-complexity spatial patterns feature is both unique and nonunique in the following ways. The pattern is spatial and hence similar to those already included on present currency. However, it differs from current features because rather than having a fixed shape or fixed spatial frequency, this feature is composed of a multiplicity of spatial frequencies and line spacings that digital printers would be unable to reproduce all at the same time.

Currently, if counterfeiters fake a currency pattern, they might “tune” the pattern to work on their printers. Even though the resulting fake pattern does not have the same exact frequency, the ordinary user could not discern the differences without instrumentation. The inherent characteristics of this new feature prevent counterfeiters from accurately reproducing the pattern, since they cannot reproduce all the frequencies in the pattern without degrading the appearance of the pattern. Thus, the spatial band pass of the intaglio printing system exceeds that of the electronic printer owing to the latter’s digitized pixel positioning requirements. The digital system is challenged by the analog nature of the original currency pattern. Clearly, a digital system could be specifically designed to have sufficient spatial bandwidth to reproduce the highly complex feature, but the current and future performance of intaglio printing and the foreseen improvements of commercial digital printer performance are such that this feature will remain a deterrent into the foreseeable future.

Should the counterfeiter decide to hand-engrave the patterns used or simulate them, he or she might succeed, with enough patience, but the effort would be detectable via pattern matching by investigators. There is the risk that digital imaging equipment will advance to the point that this feature would not have value, but if the intaglio printing process is of sufficiently high quality, as it appears to be, this is unlikely.

This feature is compelling because of several salient characteristics. First, the addition of the feature to currency uses the present intaglio printing process and hence requires little change to the present production methodology. Second, by its very nature, the pattern is designed to challenge digital scanners and printers attempting to reproduce the pattern by means other than the original intaglio technique. Third, the quality of the pattern is assessed by means of human visual capabilities and hence no equipment—just the human observer—is needed. An

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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optical magnifier could be used for assistance, but this is a fairly simple and easily obtained tool should the user wish to have one.

The characteristics listed above make this feature a considerably attractive approach. The BEP production process would not need to be changed, and the increasing quality goals being driven by the BEP only enhance the utility of this feature. It is also a feature that does not “wear out,” so to speak, and would last the life of the currency. As discussed earlier, this feature deters counterfeiting by being patterned in such a way as to frustrate digital reproduction methods. Any digital reproduction system has an inherent spatial bandwidth. When this bandwidth is exceeded, either the system cannot reproduce the signal or the signal is degraded in visually obvious ways. By creating special patterns in the analog masters for the intaglio process, it is intended to stress digital reproduction systems, which are the preferred scheme of most counterfeiters.

Materials and Manufacturing Technology Options

This feature would be an additional printed pattern on currency and hence its production would integrate well into the current manufacturing process. Few if any changes would be required to the current manufacturing process for banknotes to employ this deterrent method. Perhaps most advantageous is that as printing quality increases, the value of this feature would increase, since its complexity can be upgraded as the manufacturing quality of the currency is upgraded. Capabilities such as line-width control, line-space control, edge raggedness of printed lines, and so on all enhance the usefulness of this feature as they are improved.

Simulation Strategies

Simulating this feature would be done by using the best digital scanning and printing equipment available. However, it would be expected that the counterfeiter would be constantly frustrated by the scanning and printing system’s inability to replicate the pattern in its entirety. The pattern could be designed to maximize the difficulties faced by those wishing to copy this pattern other than by re-creating it. Every imaging system has quality capabilities and noise characteristics. Building a pattern that capitalizes on the weaknesses of digital-scanning and especially digital-printing systems is key to the success of this approach. It is expected that all classes of counterfeiters would find replication of this feature difficult. Only those willing to re-create the pattern and print it using intaglio printing would have any chance at success. Therefore, state-sponsored counterfeiters might attempt to re-create the pattern, but it is expected that no one else would.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Key Development Risks and Issues
Durability

Durability should not be an issue with this feature, since it would be printed on the same currency using the same techniques now used.

Aesthetics

This feature should not degrade the aesthetics of the banknote, since the pattern can be used to print any desired image and does not need to look like a test target such as those illustrated in Figure C-4. Thus, this feature should be aesthetically neutral, would conform to the current look and feel of FRNs, and could even enhance the aesthetics of the banknotes.

Social Acceptability

There should be no issues regarding social acceptability with the use of this feature.

Key Technical Challenges

The key technical challenge with this feature is the differential quality of the intaglio spatial bandwidth and that of current and expected digital printers. If the two processes were to come to parity, this feature would have less value than if the printing process of the authentic currency continued to exceed that of digital printers. The higher the differential quality between intaglio and a counterfeiter’s digital printer, the more compelling the use of this feature. Line-width control, line spacing, edge raggedness, and so on are quality metrics that would play an important role in the use of this feature. Both the jetting of ink in ink-jet printers and the fusing of toner in laser electrophotographic printers cause edge raggedness and line-width variations, weaknesses that could be exploited in the design and use of this feature.

Phase I Development Plan
Maturity of the Technology

The technology readiness of this approach is high because of the maturity of the intaglio process and its current use in FRN production. Patterns would need to be identified and tested with both the currency production equipment and the

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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relevant electronic printers to determine the ability of the feature to perform as expected. It should be possible to perform these tests quickly and economically.

Current and Planned Related Developments

There are no known development programs for evaluating this feature, but since the process is fairly straightforward, testing its utility should be straightforward. Appropriate pattern generation could be conducted by optical test target producers already operating in the imaging industry.

Key Milestones

There are three key milestones in evaluating this feature’s usefulness:

  • Develop the images and evaluate the targets containing the desired spatial patterns.

  • Render the desired target in a form that can be printed with the intaglio process on substrates of interest.

  • Attempt to reproduce the pattern using current high-quality digital copying and/or reproduction methods.

Development Schedule

The Phase I development and evaluation of this feature should easily be carried out within a 2-year time frame. Using current high-quality digital printers and scanners would result in low development costs in the feasibility phase. The evaluation would be partly conducted using already available production equipment, so the transition from prototype to production should be fairly swift since only the creation of the intaglio masters would be required for production use.

Estimate of Production Cost
Compatibility with Current BEP Equipment and Processes

The effect on current BEP processes would be minimal, since the current process would generate the feature required. Only the costs associated with creating or engraving the original pattern would be additional. The usefulness of this feature would improve with the increased quality of output from the BEP process.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Incremental Production Cost

The cost of this feature should be very low, since it is just another printed pattern. No special inks or other equipment would be required if this feature works as envisioned.

Required Capital Equipment

For a thorough evaluation of this feature’s usefulness, access to high-quality digital reproduction equipment would be required. This feature could be in one or more colors and hence color digital reproduction equipment should be available. Using the latest in reproduction equipment would allow an assessment of the robustness of this feature; it may be that this equipment is already available in-house at the BEP. If so, no special purchases would be required. Certainly, microscopes and so on would be needed to assess the feature’s quality, but it is assumed that such equipment is already available to those requiring it in the BEP.

Further Reading

Johnson, J.L. 1986. Principles of Non Impact Printing. Irvine, Calif.: Palatino Press.

Smith, W. 1990. Modern Optical Engineering. New York: McGraw-Hill.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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HYBRID DIFFRACTIVE OPTICALLY VARIABLE DEVICES

Description

In the most general sense, hybrid diffractive optically variable devices (h-DOVDs) appear different depending on viewing angle, and at least one part of the device operates on the basis of diffractive effects. These elements consist of diffractive optically variable devices (DOVDs)—holograms, kinegrams, exelgrams, pixelgrams, or similar structures—that also incorporate patterned reflective layers, moiré patterns, color images, nonuniform coatings, interference filters, or other features to create optical effects that are more elaborate and difficult to simulate or duplicate than those achievable with conventional DOVDs. Figure C-5 shows two examples of h-DOVDs. In sophisticated embodiments, the devices include encrypted information or hidden features that can be observed with specialized equipment, light sources or optics (for example, systems capable of retrieving phase information from a reflected or transmitted image).

The DOVD part of the device uses relief structures and/or spatial variations in the index of refraction or absorption of a material to produce diffraction patterns that create images whose appearance depends on viewing angle. The DOVD can operate in transmission or reflection modes or in both modes simultaneously. Reflection-mode surface-relief DOVDs are often coated with thin layers of metal to increase their brightness. Protective coatings are used to prevent a gradual wearing away of the relief during circulation of the note.

The other components of the h-DOVD can include printed patterns of colored

FIGURE C-5 Representative hybrid diffractive optically variable devices. Left: Thin aluminum patterns on a hologram. Right: Thin aluminum patterns and Fabry-Pérot interference filters on holograms. SOURCE: R.W. Phillips and A. Argoitia. 2005. Using roll coaters to produce anti-counterfeiting devices. Vacuum and Coating Technology 6(10):46-54.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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inks, thin-film interference filters, or patterns of reflective metal formed on top of reflection-mode DOVDs or on the top or bottom of transmission-mode DOVDs. Advanced integration might involve combining color-shifting technologies such as optically variable interference-based pigments and Zero Order Devices with the DOVD.

Feature Motivation

An h-DOVD can provide both visible and hidden features to deter counterfeiting. The striking optical appearance of the device provides a distinctive visual indicator of the authenticity of a currency note. Also, the phase and other information in the images produced by the devices can be encoded and retrieved using specialized equipment that is very difficult for even sophisticated replicas or simulating structures to reproduce. All of the functionality is integrated into the currency note in a single feature, the h-DOVD, so clutter and feature proliferation are minimized.

The deterrence occurs at several levels:

  • Visual inspection of a counterfeit note by the recipient can prevent its passing.

  • The striking appearance and the possibility of hidden encoded information can deter attempts to generate counterfeit notes.

  • The feature provides forensic information that can aid law enforcement against sophisticated counterfeiters who might be able to simulate some of the overt visual features of the devices.

Conventional DOVDs are used in more than 200 currency denominations from 78 issuing authorities worldwide, and they are widely employed in software packaging, audio and video media, tickets, and other products in which security is a concern. The implementation of more sophisticated versions that include polarization-dependent effects, encrypted images, three-dimensional volume optical effects, and other features appears to be straightforward but less well developed for commercial applications.

The nondiffractive components of h-DOVDs are also well established (for example, interference structures are used in optically variable inks, patterned reflective layers on holograms have been demonstrated, and so on), although they are typically not implemented directly with DOVDs. The combination of these features into a single device with visible and hidden functionality is less well explored. The attractiveness of this combination is that it increases the functionality and visibility of the feature while avoiding the clutter associated with separate features placed on different parts of the currency note.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Materials and Manufacturing Technology Options

The DOVDs are either created through optical exposures of photosensitive materials or through physical embossing of relief structures in thermoplastics or thermosets. Most implementations are of the surface-relief type, owing to the ease of low-cost manufacturing. Silver halide films, dichromated gelatin films, and certain classes of photopolymers are typically used for index or absorption DOVDs. In these cases, the DOVDs are written directly using an optical recording process.

For surface-relief-type DOVDs, optically or lithographically produced embossing tools can be used with many different thermoplastics or thermosets (for example, polyvinyl chloride, polyester, and so on). The metal coatings for reflective DOVDs are usually deposited by physical vapor deposition. The additional features to create h-DOVDs could consist of conventional inks (for color images, moiré patterns, and so on), vapor-deposited thin-film stacks (for interference filters), or patterns of metal deposited by physical vapor deposition (using, for example, lift-off processes with flexographically printed patterns of volatile oils). These features could be added immediately after or before the fabrication of the DOVDs. The DOVDs can be very low in cost, especially for the surface-relief type.

The additional features needed to create h-DOVDs can also be low cost, since most rely on well-established processes. The h-DOVDs can exist in the form of patches (50 percent of holograms used for currency worldwide are this type), stripes (40 percent), and threads (10 percent). These elements, like DOVDs, can be applied to the currency substrates prior to printing.

Simulation Strategies

In their conventional form, DOVDs can be simulated, at a crude level, by the use of off-the-shelf holograms (obtained from packaging materials, tickets, art supply stores, and other sources and items) that are integrated with a counterfeit by simple cutting and pasting by primitive and opportunist counterfeiters. Although not readily amenable to large-scale production, this procedure can provide, in some cases, an effective simulation strategy. The more sophisticated classes of counterfeiters can use readily available hot stamping presses and laminating equipment for the attachment of the simulated shiny strips. The hybrid nature of h-DOVDs offers an opportunity to achieve designs that provide a more unique and striking visual appearance than is possible with conventional DOVDs, thereby degrading the effectiveness of crude simulations. Also, the integration of an h-DOVD into the note substrate, for example as a woven strip, might add even more challenge to the counterfeiter.

Actual copies of the DOVD component of an h-DOVD feature can be made in several ways. For example, surface-relief-type DOVDs, which represent the

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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lowest-cost and most widely used form of DOVDs, can be copied relatively easily by separating them from their protective coatings and then using them as tools to create copies by embossing other substrates. This method cannot be used with DOVDs that involve modulations in the index or absorption of a material. In these cases, copies can be made by using the diffracted light from a DOVD to write a new DOVD in a photosensitive material. The h-DOVD’s design includes nondiffracting structures that serve to frustrate such duplication techniques and to render the simple simulation strategy ineffective.

Other countermeasures include the use of encrypted information, hidden features, polarization-dependent effects, imposed distortions, phase encoding, and other (mostly covert) features. Such attributes (both overt and covert) would make h-DOVDs difficult or impossible to reproduce accurately for all but state-sponsored organizations.

Key Development Risks and Issues
Durability

The durability of an h-DOVD is expected to be comparable with that of a DOVD or an optically variable image (OVI). Common degradation modes include wearing away of the protective coating and surface relief structures, for the case of relief-based DOVDs, and debonding from the currency substrate. Index-modulation-based diffractive elements avoid the former degradation pathway. Additional work is needed to explore issues related to durability.

Aesthetics

The emerging widespread use of DOVDs in various currencies around the world and their implementation in product packaging provide some evidence of the good aesthetic value for h-DOVDs. Suitable implementation can conform to the look and feel of U.S. currency while still providing an attention-getting feature with striking appearance.

Social Acceptability

No social concerns have been raised about the use of DOVDs in packaging, currency, or other applications. The materials used to produce these features pose no environmental concern. The same conclusions apply to h-DOVDs.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Key Technical Challenges

Technical challenges include developing optimized designs that have suitable lifetime and acceptable manufacturing cost.

Phase 1 Development Plan
Maturity of the Technology

The separate components of h-DOVDs have already been demonstrated for currency applications. Research and design are needed to define an optimized way to integrate these elements into a single currency feature that provides the desired level of overt and covert protection against counterfeiting and with acceptable manufacturing costs and aesthetics.

Current and Planned Related Developments

The production of h-DOVDs can exploit existing manufacturing capacities for DOVDs, OVIs, and other features now used in currency applications. Optimized designs for h-DOVDs are needed. Study of the primary degradation modes and lifetime of the elements is required to establish durability.

Key Milestones

There are two key milestones:

  • Establish the designs and layouts and implement them in the currency substrates.

  • Conduct durability tests to study the effects of wear and tear on these devices.

Development Schedule

If acceptable durability is demonstrated, this technology can complete Phase I within 2 to 3 years.

Estimate of Production Costs
Compatibility with Current BEP Equipment and Processes

The h-DOVD feature could be incorporated into the paper by the substrate manufacturer. Alternatively, some of the nondiffractive components of the feature

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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could conceivably be overprinted by the BEP using existing processes (for example, printing for an OVI feature). The effects on the BEP operations could, therefore, be minimal, especially in the former scenario.

Overprinting with intaglio ink, for example, might require ink and/or DOVD development to ensure good adhesion to both the plastic and the cloth fibers simultaneously.

Incremental Production Cost

The cost of an h-DOVD is expected to be incrementally more than that of a conventional DOVD.

Required Capital Equipment

The capital equipment would be the same as that used for DOVDs and systems, like those for the OVI inks, that can provide the additional functionality. For patterned metallization, for example, continuous reel-to-reel systems that use flexographic printers, metal sputtering chambers, and a lift-off process could be used.

Further Reading

Chesak, C.E. 1995. Holographic counterfeit protection. Optics Communications 115: 429-436.

Colburn, W.S. 1997. Review of materials for holographic optics. Journal of Imaging Science and Technology 41(5): 443-456.

Javidi, B., and T. Nomura. 2000. Securing information by use of digital holography. Optics Letters 25(1): 28-30.

Lancaster, I.M., and A. Mitchell. 2004. The growth of optically variable features on banknotes, in Optical Security and Counterfeit Deterrence Techniques V, R.L. van Renesse (ed.), Proceedings of SPIE-IS&T Electronic Imaging, Vol. 5310, pp. 34-45.

Phillips, R.W., and A. Argoitia. 2005. Using roll coaters to produce anti-counterfeiting devices, Vacuum and Coating Technology 6(10): 46-54.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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METAMERIC INK PATTERNS

Description

Patterns printed with metameric ink appear different under different illumination sources. In other words, a pattern’s appearance can change depending on the color of light shining on it. In the commercial printing industry such effects are problematic and to be avoided, since having two nearby colors appear the same under one light source (for example, a fluorescent lamp) and different under another source (for example, a conventional tungsten lamp) can be quite frustrating. The currency feature proposed here would employ metameric inks to enhance any difference in appearance when the feature is illuminated with, for example, a simple light-emitting diode (LED) penlight or tungsten penlight to get patterns to show or disappear. A metameric ink feature would not always require special instrumentation for the average user, since the patterns could be designed to allow for evaluation in the home or a place of business using fluorescent or tungsten lights, both of which are commonly found.

The property on which a metameric ink feature would depend is the inks’ spectral reflectances which, when combined with different illuminants, produce different colors. An extreme example of metamerism, but perhaps most illustrative of the effect, can be seen when a maroon or red car is parked in a lot that has sodium vapor lighting. Sodium vapor is used for its efficiency, since its spectral output is mainly in the yellow range, and in such illumination maroons and reds often look gray.

Figure C-6 illustrates a metameric ink pair along with a “standard” color that these inks attempt to reproduce. As described in more detail below, electronic color printers can reproduce a color that results from a metameric ink under a particular illuminant, but such printers cannot reproduce the metameric effect because the same primary colorants are used in these printers for all imaging purposes. The need to provide special inks or toners to simulate the real metameric inks would be a significant deterrent to most counterfeiters.

Feature Motivation

The idea of using inks that display metameric effects in visible light is compelling, because most counterfeiters use digital printing systems such as color ink-jet or color laser printers to generate their illicit output. However, a metameric feature is highly problematic for the counterfeiter using equipment that often uses only four printing dyes—cyan (red absorber), magenta (green absorber), yellow (blue absorber), and black. In printing a picture or other pattern and attempting to reproduce a color that is perceived as maroon, the printer combines two and

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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FIGURE C-6 Spectral reflectance of a metameric ink pair and a standard color ink. Notice that there are spectral differences in the 400 nanometer (nm) to 500 nm region and the 550 mm to 650 nm region. Depending on the spectral content of the illuminant—that is, its color—these inks can generate different perceived colors. In some spectral regions the lines touch or nearly touch, and in that illumination the dyes appear to be the same color. But if the inks are illuminated with 600 nm (orange) light, for example, they appear to be different colors. SOURCE: Viggiano (2004). Reprinted with permission of the Society for Imaging Science and Technology, sole copyright owners of Proceedings of the Second European Conference on Colour in Graphics, Imaging, and Vision.

perhaps three of the colored inks in some proportion to achieve the desired color. The printer might be able to produce different-looking maroon colors, but since the printing “primaries,” that is, the four inks used by the printer, are the same as any other maroon, no metamerism would result. The cyan dye, for instance, is the same for all images. Unless users created their own special inks or toners in the case of electrophotographic laser printers, reproducing metameric inks would be very troublesome.

The major benefits of visible metameric features are that they use inks which already function within the BEP manufacturing process, and they could be specially designed with novel materials and chemistry to act as a significant deterrent to those who might try to duplicate the color without being able to duplicate the

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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metameric effects. While the metameric patterns would also be useful to machine readers that could detect detailed data such as their spectra, the main advantage here is that ordinary users with little additional equipment, using either environmental illumination or inexpensive penlights, could verify the authenticity of a banknote, even in low-light conditions. The only limitation of this feature is that it would require two light sources so that the metameric effects could be seen by human observers with normal vision—either using a penlight kind of device against ambient lighting or using two common light sources in the home or workplace. It is also possible that patterns could be designed so that an observer with deficient color vision could generally detect the effects. The major color-vision deficiency in humans is red-green color blindness. Green would not need to play a role in most metameric designs.

Materials and Manufacturing Technology Options

This feature is fairly unique in that one would expect to use the latest in materials technology to create the inks for printing the metameric currency pattern(s). The use of nanocrystals and other novel chemistry for making the inks would be difficult to reproduce because a wide range of different materials would be required. While metameric inks have been used for certificates and some currency, the approach suggested here goes beyond standard metameric inks.

A compelling characteristic of this proposed feature is that including it in the manufacture of an FRN is merely a matter of printing the desired pattern with special inks using existing printing presses at the BEP. The use of special inks may or may not require an additional printing step. However, the inks would integrate well into the intaglio printing process currently used and would not require a significant change in tooling or major production processes. Laying down ink on the currency substrate is already a well-controlled process, and whether the ink is metameric or not would not alter the basic process steps in producing the currency.

Simulation Strategies

This feature would deter most counterfeiters because access to the exact inks would be problematic and simulation of this feature would require the very inks used in the currency itself. The professional criminal and state-sponsored counterfeiter would likely have access to resources for duplicating or simulating this feature; however, the other classes of counterfeiters, using digital imaging tools such as scanners, printers, and their associated software, could not print patterns that would simulate the unique characteristics of a custom-designed metameric pattern. Furthermore, trace elements in the inks or their novel nanostructure could

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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be exploited as a forensic feature and would greatly complicate the reproduction or simulation process for most counterfeiters.

Key Development Risks and Issues
Durability

The durability of this feature should not be an issue—it is just ink on the currency substrate and would only have to tolerate ultraviolet and other types of exposure similar to what currency inks must already be able to endure. The feature would have an expected durability that does not vary much from today’s ink durability.

Aesthetics

The aesthetics of the note should not be altered by the use of this feature, since the ink could be used in portraits on the currency, in chevron patterns, and so on. In fact, this feature should be aesthetically neutral and would conform to the look and feel of U.S. currency today. It is even possible that this feature might enhance the note’s appearance.

Social Acceptability

This feature should be very innocuous to most users. It would be designed intentionally to look like just another printed pattern. As far as is known, there would be no privacy or environmental hazards. The actual ink chemistry might not be a foodstuff but would be designed not to emit or give off harmful effluents. Any metameric inks used would have to conform to the same safety standards that apply to current currency inks. It is not expected that this would be a debilitating issue for this feature or its proper use.

Key Technical Challenges

The key technical issues to be addressed with respect to this feature would be the design of inks that stress many if not most electronic printers to the greatest extent possible. Also, the custom-designed inks should display metameric effects under simple and available light sources, such as a portable penlight, an LED penlight, commonplace indoor lighting, and so on. Little ink would be needed per note, so the ink cost would not be expected to be an issue even if exotic materials were used in the ink chemistry. Such materials might cause the ink to fluoresce.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Phase I Development Plan
Maturity of the Technology

Little needs to be developed for this feature beyond the special metameric inks to be used—a development process that could take a few months, as the general knowledge about inks is currently at a high level. Placing special requirements on the inks could require more development time, since any such requirements are unknown at this time and they cannot be readily assessed until specified.

Current and Planned Related Developments

The committee is unaware of any related programs at the present time.

Key Milestones

There are three milestones to pass in order to properly implement this feature:

  • Develop metameric inks with proper cost and durability as well as adequate metamerism in the expected user environments.

  • Develop and/or identify simple light sources, such as penlights with filters or LED penlights that could be distributed and used at point-of-sale locations, with which to quickly assess the authenticity of the currency.

  • Evaluate the ink’s applicability to the BEP printing equipment. This effort would involve chemical interactions with the intaglio plates as well as any other chemical effects on the printing equipment.

Development Schedule

Phase I of the development of this feature could be completed within 6 to 9 months and the full development of a metameric feature is estimated to take up to 18 months, depending on equipment availability and the scale-up requirements for the ink producer. The costs for this effort are unknown but are not expected to be very high, since considerable expertise in ink chemistry is available.

Estimate of Production Costs
Compatibility with Current BEP Equipment and Processes

The compatibility of this feature with current BEP processes is expected to be very high. The efficiency of the intaglio process and the simplicity of the ink

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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printing requirements (basically, having the substrate wet and the ink’s having a higher affinity for the substrate than for the printing plate) are the basic needs for the BEP printing operation. This feature would be very adaptable to the current BEP operations.

Incremental Production Cost

It is expected that the cost would be very low because a low volume of ink would likely be used per note. There would be no unusual tools that would wear out and need to be amortized over the life of the note’s production.

Required Capital Equipment

As long as the BEP is not generating the inks used in this feature, little capital cost would be required. The ink vendor might require some capital expenditures for producing the ink in sufficient quantities, but whether this would be the vendor’s cost or BEP’s shared cost is unknown. The development of special penlights would be placed on the penlight vendor if that identification method was chosen. There should be no BEP expense incurred in this aspect of the feature.

Further Reading

For a description of ink features used in currency production, see <http://www.currencyproducts.com/what_to_look_for/ink_features.html>. Accessed February 2007.

For a discussion of the basics of color separation by Dr. Jan Pekarovic, University of Michigan, see <www.wmich.edu/ppse/pekarovicova/071099.html>. Accessed February 2007.

Viggiano, J.A.S. 2004. Metrics for evaluating spectral matches: A quantitative comparison. Proceedings of CGIV-2004: The Second European Conference on Colour Graphics, Imaging, and Vision. Springfield, Va.: Society for Imaging Science and Technology. See <www.acolyte-color.com/papers/CGIV_2004.pdf>. Accessed February 2007.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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MICROPERFORATED SUBSTRATE

Description

Transmitted light features provide security by preventing simple scanning as an image-acquisition method. Features that are not printed deliver an additional challenge to the opportunist and petty counterfeiter. Microperforation, in which very small holes are removed from the paper substrate by a laser, is one such feature.

Microperforation is a commercial technology, currently used in the manufacture of currency and the production of other secure documents. A carbon dioxide laser creates a pattern of tiny holes in the paper substrate. Microperforation is a descendant from mechanical perforation, long used to cancel checks and securities, but the use of a laser permits much smaller, more closely spaced holes. The holes are small enough that they do not significantly impact the strength of the paper, and since paper is removed rather than its being punctured, the holes do not affect the feel of the paper. They are essentially invisible in reflected light.

In currency, microperforation is typically used to form a pattern that is unique to each denomination. The pattern can be observed by means of transmitted light, either ambient light (holding the note up to a light) or, more dramatically, a light source (penlight, light table).

Because the holes are small, the viewing angle is also small. However, if the pattern is extensive enough, its existence can be verified with little effort. The details of the pattern can then be confirmed if necessary.

Microperforation is a visual feature, but it could potentially be read by a machine, either optically or conductively. It is primarily a feature for the general public but could be useful for the cash handler and teller.

Microperforation is used in a few international currencies, including the Swiss franc and some former Soviet Union currencies. Typically, the pattern of holes indicates the denomination, but a pictorial image is certainly feasible. Although not unique as a feature since it is used in other countries, microperforation is considered effective.

Feature Motivation

Microperforation rates highly for its effectiveness at deterring the opportunist and petty criminal. It is straightforward to implement in the BEP process using commercially available equipment, and costs are presumed to be reasonable. It offers advantages to both human and machine cash handlers, although as a new feature, it would require some education of human users and handlers.

While there are some simulation options with this feature, microperforation would require an additional production step in the counterfeiting process and is not amenable to simulation by printing.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Materials and Manufacturing Technology Options

Microperforation is already used in other nations’ currencies, and the carbon dioxide laser is commercially available. Implementing this feature would require an additional manufacturing step at the BEP, but this step would be separate from the current process flow. Experts would need to decide at what point to add the microperforations to prevent the holes from becoming occluded during subsequent steps. Presumably, the pattern selected would reflect the denomination (perhaps even overlaid on one of the printed denominating numbers) or an image that would enhance the note design. Microperforation could be added as a design element to another transmitted light feature such as the watermark.

Simulation Strategies

Microperforation can be simulated inexpensively using mechanical punctures (a needle), but this leaves a texture that is not present in the real note and is easily detected. Microperforation can, of course, be duplicated using lasers, but this is an expensive capital undertaking, currently out of the opportunist or petty counterfeiter’s range. It has been suggested that a printed image, say of small black dots, might be used to simulate a “dirty” note with holes occluded. This is not an optimal counterfeiting strategy, because, whether occluded or open, the holes in authentic notes would not be visible in reflected light. A visible pattern of dots would be a red flag to the cash handler. Further, dirt that occludes the entire pattern of holes is not typical, again alerting the cash handler.

Key Development Risks and Issues
Durability

Microperforation passes durability tests in other currency. The minimal amount of material removed in creating the holes should not impact paper strength. It is possible that holes could be occluded in dirty bills, and the pattern could become hard to see in a crumpled bill if the viewing angle changed locally owing to wrinkles. Both effects could reduce feature effectiveness as the bill became worn. Most features, however, lose some effectiveness with wear.

Aesthetics

Because the feature is not seen in reflected light, it poses little aesthetic challenge. Typical implementations use an aesthetically neutral design, such as the denomination; however, a more aesthetically pleasing design, similar to the watermark, is an option.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Social Acceptability

There are no issues of loss of privacy or environmental hazards related to the implementation of this feature.

Key Technical Challenges

The key technical challenge for microperforation is that of adding the holes at an appropriate point in the process in order to prevent hole occlusion by subsequent manufacturing steps and to permit correct placement of the feature.

Phase I Development Plan
Maturity of the Technology

It appears that this feature is technically mature, as it is in use in other currencies.

Current and Planned Related Developments

Development programs for this feature probably exist in other countries that use microperforation. Because this feature is quite simple, the development of a similar program in the United States would require relatively little investment to implement.

Key Milestones

The milestones for implementing this feature are the following:

  • Design a test microperforation feature that is both aesthetically pleasing and easy to authenticate.

  • Identify where to insert the microperforation step in the manufacturing process; find appropriate space and infrastructure for doing so.

  • Acquire microperforation equipment sufficient for the production level required.

  • Test and debug the microperforation system.

  • Implement microperforation in the manufacturing process.

Development Schedule

Since this feature would rely on commercial technology already used in currency manufacture, it is likely that it could be implemented within 1 to 2 years.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Estimate of Production Cost
Compatibility with Current BEP Equipment and Processes

Microperforation technology is completely compatible with the current BEP operation, although it requires a new manufacturing step.

Incremental Production Cost

It is presumed that the incremental production cost would be low, as this feature is used in other currencies. Laser paper removal is also used in other low-cost products, such as greeting cards and cigarette papers.

Required Capital Equipment

Microperforation requires automated laser microperforation equipment sufficient for the production level required, as well as supporting infrastructure.

Further Reading

For a discussion of the application of laser microperforation in Russian rubles by Yelena Kiseleva on the Web site of the Water Mark magazine, see <http://www.watermark.ru/magazine/bum7.htm>. Accessed February 2007.

For a description of some commercial laser perforation systems, see <http://www.microlasertech.com/>. Accessed February 2007.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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NANOCRYSTAL PIGMENTS

Description

Dispersions of semiconductor or metallic nanocrystals have the potential to provide unusual classes of inks with unique color and spectral characteristics, in both reflection and transmission modes, to form images on currency or, as embedded in a plastic matrix, to create colored features in a security strip. By synthesizing nanocrystals for specific ink formulations, complex color characteristics (measured with visible, ultraviolet, or infrared light, and in reflection, transmission, and scattering modes) can be achieved with an overall degree of control of all of these characteristics that would be difficult to duplicate using other approaches. Figure C-7 shows the kinds of colors that can be achieved using nanocrystals of CdSe, silver, and gold. An image or security strip formed with such inks could be designed to have a unique appearance as an overt feature, with unique combinations of reflecting, scattering, absorbing, and transmitting properties that would be impossible to reproduce using ordinary pigments. A key value would be the machine-readable and forensic functionality provided by the detailed spectral fingerprint, as measured in reflection, transmission, and scattering modes simultaneously, especially when implemented with features in wavelength regions (for example, the infrared) that are difficult to address with conventional organic pigments.

Metal and semiconductor nanocrystals provide optical properties (that is, absorption, scattering, and fluorescence) that depend not only on the constituent materials but on the shapes and sizes of the crystals. Bulk synthetic methods can grow such materials with well-controlled sizes and shapes, thereby providing the capability to achieve tunable optical properties throughout the visible, ultraviolet, and near infrared. Examples include gold or silver metallic nanocrystals, and semiconductor nanocrystals of InP, InAs, ZnS, ZnSe, ZnTe, CdS, CdSe, CdSe, or CdTe. These materials can be loaded into support matrices and printed as conventional inks, with the nanocrystals providing precisely defined optical properties. Nanocrystals of different sizes, shapes, or materials could be mixed to produce complex but well-defined reproducible, absorption, scattering, or fluorescent signatures. Although organic dyes might be able to simulate certain of these spectral properties, it is unlikely that they could effectively reproduce the combined wavelength-dependent scattering, absorbing, transmitting, and reflecting characteristics of suitably designed nanocrystal inks.

The feature on a banknote would consist of printed images or patterns or security strips that incorporate the nanocrystal inks. Such features would be visible to the unaided eye and would be machine-readable—the latter gathering data on the unique spectral properties of the inks. These materials might also be well suited for use in metameric inks, of the type described previously in this appendix (see the section “Metameric Ink Patterns”).

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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FIGURE C-7 Top: Photograph of ink-jet-printed squares of inks that consist of different-sized CdTe nanocrystals in polymer matrices. Middle: Optical micrograph of silver nanoparticles of different sizes and shapes. Bottom: Gold nanocrystals with different sizes and shapes and spectral extinction. SOURCES: Top: Tekin et al. (2006). Middle: Reprinted, with permission, from McFarland and Van Duyne (2003), ©2003 American Chemical Society. Bottom: Reproduced from Hu et al. (2006) by permission of the Royal Society of Chemistry.

Feature Motivation

As described above, dispersions of semiconductor or metallic nanocrystals provide unique color and spectral characteristics in an ink format that could be used to form images on currency with complex color characteristics (measured with visible,

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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ultraviolet, or infrared light, and in reflection, transmission, or scattering modes) or as additives to produce color in a security strip. High levels of complexity in the appearance and spectral properties can be achieved using this approach, with a degree of control that would be difficult to duplicate using conventional dyes or with diffraction or interference-based structures. An image formed with such inks could be designed to have a unique appearance as an overt feature—such as a metameric feature as described in detail in the section “Metameric Ink Patterns,” above.

A primary value of a feature printed with nanocrystal inks would be the machine-readable and forensic functionality provided by the detailed spectral fingerprint, as measured in the ultraviolet, visible, and infrared wavelength regions with a spectrometer or other optical device. These attributes could deter the passing of a counterfeit as well as attempts to generate a counterfeit.

The fact that metallic nanocrystals are already well developed and cost-effective also motivates the exploration of these materials for anticounterfeiting use, although their incorporation in inks for currency would require some development. Metallic nanocrystals, primarily gold, have been used for centuries as materials to provide coloration in items of various sorts and are especially striking because of the different colors that appear on transmission and reflection of light. Semiconductor nanocrystals are newer types of materials that need additional development for this application and have important issues such as toxicity of the materials. These crystals and the controlled means to synthesize them were discovered about 20 years ago. They are currently used for biological tagging and imaging, and some applications in security devices have been explored. To the committee’s knowledge, nanocrystal inks formed from these materials have not been used in currency and therefore could be unique to U.S. currency if developed in a proprietary way.

In essence, the committee judged this feature highly because it exploits a new class of material to achieve wide-ranging optical properties in printed images or security strips. The extreme level of tunability provides the ability to define complex optical signatures that could be evaluated using suitable measurement apparatus. The high level of chemical, thermal, and physical stability in these materials also represents an advantage. Simulations of certain properties or aspects of this feature might be possible with conventional dyes and pigments, but it is unlikely that the full spectral response of the reflected, the scattered, and, possibly, the transmitted light could be reproduced without access to the specialized nanocrystal ink formulations.

Materials and Manufacturing Technology Options

Compared with organic dyes for application in currency features, semiconductor and metallic nanocrystal inks offer significant advantages in terms of brightness/contrast and stability and other attributes. For example, they do not bleach

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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like organic dyes. In fact, the hues in centuries-old vases and goblets in which gold nanocrystals were used to generate red and green colors remain brightly visible today. The implementation in currency could involve the formulation of printable inks that consist of matrices loaded to some level with the nanocrystals. Alternatively, preformed security strips loaded with these materials, in patterned or unpatterned formats, could be integrated with the currency substrates. Certain types of metallic nanoparticles are available at low cost. The potential toxicity of these materials, particularly the compound semiconductors, must be investigated.

Simulation Strategies

Images formed using nanocrystal inks could be simulated with conventional inks, printers, and scanners to produce a counterfeit that looks, by the unaided eye and with ambient illumination, similar to an actual note. Inspection by fluorescence and/or with spectrometry equipment or in systems that evaluate the transmitted, reflected, or scattered light could, however, easily detect such simulations. Duplication would be very difficult, owing to the challenge of accurately matching the optical signatures achieved with the nanocrystal inks.

Key Development Risks and Issues
Durability

The durability of the nanocrystals (of suitable materials) is known to be good—much better than that of organic-based dyes. The durability of images formed using inks of these nanocrystals has not been explored, to the committee’s knowledge. The committee expects that the durability, using properly supported matrices, could be as high as that of conventional inks that are currently used in currency applications.

Aesthetics

The nanocrystal ink feature would be aesthetically neutral.

Social Acceptability

There are serious environmental toxicity concerns associated with many of the semiconductor nanocrystals. Some of these issues are currently being addressed, as the range of applications for this class of material expands to include things such as organic LEDs for displays. Additional work on this topic is required. Funded by

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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the National Science Foundation (NSF), the Nanoscale Science and Engineering Center at Rice University is carrying out investigations of toxicity issues with these and related classes of nanomaterials; these activities could be useful to the BEP in assessing any toxicity risk of these inks.

Key Technical Challenges

The key technical challenge is to demonstrate ink formulations that exploit the unique optical properties of the nanocrystals while at the same time enabling low-cost printing and high durability in nontoxic forms.

Phase I Development Plan
Maturity of the Technology

Gold nanocrystals have been used for centuries in various decorative and other items and therefore are a fairly mature technology, although these types of inks have not, as far as the committee is aware, been incorporated into currency. Semiconductor nanocrystals are less developed (although the synthetic procedures for forming them are nearly 20 years old) and would require more development for implementation into inks for currency applications. Toxicity issues must be investigated.

Current and Planned Related Developments

There are many industrial and academic research programs on metallic and semiconductor nanocrystals. These programs are funded by several federal agencies, including NSF, the National Institutes of Health, and the Defense Advanced Research Projects Agency (DARPA).

Key Milestones

The key milestones associated with the development of these inks are the following:

  • Prepare the laboratory-scale formulation of an ink or a security strip using nanocrystals that is compatible with BEP systems.

  • Provide the proof of concept of unique color and spectral characteristics.

  • Overcome the toxicity issues related to certain classes of metallic and semiconductor crystals.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Development Schedule

Completing Phase I of the development of the use of metallic nanocrystal inks for currency use could be achieved within 2 to 3 years. Similar levels of development for the semiconductor inks might require twice as long.

Estimate of Production Cost
Compatibility with Current BEP Equipment and Processes

The nanocrystal ink formulations have the potential to be designed for compatibility with existing BEP printing systems.

Incremental Production Cost

The cost is estimated to be low to medium—approximately similar to that of an OVI ink.

Required Capital Equipment

Suitably designed inks could be used with existing BEP equipment.

References and Further Reading

See articles in Synthesis and Plasmonic Properties of Nanostructures. 2005. Y. Xia and N.J. Halas (eds.). MRS Bulletin 30(5).

Haes, A.J., C.L. Haynes, A.D. McFarland, G.C. Schatz, R.P. Van Duyne, and S. Zou. 2005. Plasmonic materials for surface-enhanced sensing and spectroscopy. MRS Bulletin 30(5): 368-375.

Hu, M., J. Chen, Z.-Y. Li, L. Au, G.V. Hartland, X. Li, M. Marqueze, and Y. Xia. 2006. Gold nanostructures: Engineering their plasmonic properties for biomedical applications. Chemical Society Reviews 35: 1084-1094.

McFarland, A.D., and R.P. Van Duyne. 2003. Single silver nano particles as real time optical sensors with zeptomole sensitivity. Nano Letters 3: 1057-1062.

Murray, C.B., C.R. Kagan, and M.G. Bawendi. 2000. Synthesis and characterization of monodisperse nanocrystals and close-packed nanocrystal assemblies. Annual Review of Materials Science 30: 545-610.

Tekin, E., P.J. Smith, S. Hoeppener, A.M.J. van den Berg, A.S. Sush, A.L. Rogach, J. Feldman, and U.S. Schubert. 2006. Inkjet printing of luminescent CdTe nanocrystal-polymer composites. Advanced Functional Materials 17(1): 23-28.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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NANOPRINT

Description

The concept of nanoprint as a currency feature involves printed text, images, or regular arrays of patterns with critical dimensions in the micron and submicron range, and perhaps as small as tens of nanometers, formed on smooth surfaces of components of the currency, such as a windowed security strip. This feature concept represents an extension of microprint, in which the extremely high level of resolution prevents duplication by even the most sophisticated existing (or envisioned) commercial printing equipment. The technical feasibility of nanoprint derives from the discovery, through recent research in nanotechnology, of printing-like processes that offer resolution down to tens of nanometers. These approaches can be used to form nanoprint features in wide-ranging classes of materials, thereby providing an opportunity for an additional level of security. Initial implementations of these methods for nonsecurity applications indicate a potential for low-cost, high-volume operation. The nanoprint can be viewed directly using high-magnification electron microscopes—for example for covert or forensic operations. Suitable designs can produce visible collective effects for overt operations.

Feature Motivation

Nanoprint has value as a currency feature because it provides resolution and patterning characteristics that lie well outside the capabilities of the most sophisticated commercial printer systems. Owing to the small dimensions of the patterns, nanoprint can be viewed directly only by means of specialized equipment such as scanning optical microscopes. These nanoprint features can be observed indirectly, however, by exploiting visible and/or machine-readable collective effects such as diffractive or moiré effects. As an example of this mode, an array of lines with micron widths viewed through a transparent element with a similar pattern can generate moiré patterns that have spatial frequencies much lower than those of the nanoprint features themselves. By using transparent elements on the currency, such moiré effects can be generated by self-referencing with a folded piece of currency or by referencing one piece of currency to another. Such implementations would eliminate the need for a separate reading device.

In addition to feature resolution, the techniques for forming nanoprint, as described below, are compatible with a broad diversity of inks, ranging from organic molecular materials, to hybrid organic/inorganics, to nanoparticles, to biomaterials, to polymers. This materials flexibility can be exploited for additional, machine-readable forensic functionality. In particular, by combining high-resolution features with unique ink compositions, nanoprint has the potential to provide an extremely

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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FIGURE C-8 Examples of nanoprint. Patterns of (A) fluorescently tagged proteins formed by rubber stamping, (B) polymer structures formed by imprint lithography, and (C) organic molecular monolayers formed by dip-pen writing. SOURCES: (A) Reprinted with permission from Bernard et al. (1998), ©1998 American Chemical Society. (B) Reprinted from Resnick et al. (2005), ©2005, with permission from Elsevier. (C) Mirkin (2001).

high level of security. Figure C-8 provides some examples of printing at the length scales contemplated for nanoprint, with several different classes of ink materials.

Nanoprint rated highly with the committee because it provides an extremely high level of security in a feature format that has the potential to be manufactured at low cost. The material composition of the inks and the geometries of the printed patterns can be evaluated using specialized equipment (for example, scanning electron microscopes), thereby providing valuable forensic and/or machine-readable functionality. The previously mentioned collective effects provide a strategy for the use of a nanoprint feature by the general public or by high-speed machine readers without the need for specialized microscopes.

Materials and Manufacturing Technology Options

Recent research in nanotechnology has produced several methods that can “print” structures and patterns with dimensions in the deep submicron and nano-

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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meter regime and, as described above, have the potential to create nanoprint features for currency applications. There are three different approaches that are well developed for research applications and are now in various stages of commercial development for application outside of security printing:

  • Dip-pen lithography, in which the stylus of an atomic force microscope is used as a pen to write patterns of various materials, with lateral resolution down to tens of nanometers; this method is being developed for writing security markers on pharmaceuticals.1

  • Soft lithography, in which an elastomeric stamp prints patterns of materials, with resolution in the 1 nm to 10 nm regime; this method is being commercially explored for applications in photonics and electronics.2

  • Imprint lithography, in which an embossing element produces features of relief in a thermally softened polymer or a photocurable liquid prepolymer; this method is being commercialized for applications in microelectronics and optics.3

The manufacturing costs associated with these methods can be low, since the materials can be selected to be low in cost and the processes can be scaled for high-volume production, as evidenced by the development activities in microelectronics, photonics, pharmaceuticals, and so on. These manufacturing techniques are most effective when applied to smooth substrates. As a result, nanoprint would be most easily implemented in security strips or other structures that are separately processed and subsequently integrated into the paper.

Simulation Strategies

Nanoprint, of course, cannot be viewed directly with the unaided eye. The collective effects of nanoprint features (for example, diffraction, moiré effects) might be simulated in various ways, similar to those that are used to simulate DOVDs. The moiré effects that are possible with nanoprint are expected to be difficult to simulate accurately using structures with larger dimensions. Also, these simulation strategies would be ineffective in defeating the forensic or machine-readable functionality associated with the features themselves or the material inks.

Duplication of nanoprint would be very difficult. Although the fabrication

1

NanoInk, Inc., is an emerging growth technology company specializing in nanometer-scale manufacturing and application development for the life science and semiconductor industries. See <http://www.nanoink.net>. Accessed February 2007.

2

For example, by IBM, Inc., by Royal Philips Electronics, and by DuPont, Inc.

3

For example, by NanoOpto Corporation, by Nanonex, Inc., and by Molecular Imprint, Inc.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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techniques for nanoprint have low-cost operational characteristics, they are sufficiently sophisticated that they only exist in research laboratories and certain specialized development systems. Only state-sponsored organizations could gain access to or reproduce these systems. Even in this case, the inks themselves—for example, uniquely designed biological molecules—provide an additional level of security that could make duplication difficult even for such organizations.

Key Development Risks and Issues
Durability

The durability of the nanoprint feature is determined by the selection of the inks and could be expected to be good, especially when protective layers are used on top of the print. The committee envisions that the most natural embodiment will involve nanoprint formed on a plastic substrate that is subsequently integrated with the currency.

Aesthetics

Nanoprint, since it is not directly observable by the eye, would be aesthetically neutral. Nanoprint features that also provide collective effects (for example, moiré patterns) to generate visible features could improve the FRN’s aesthetics and appeal.

Social Acceptability

There are no known concerns regarding social acceptability with respect to nanoprint.

Key Technical Challenges

The key technical challenge is to produce the nanoprint feature at acceptable cost.

Phase I Development Plan
Maturity of the Technology

The procedures based on imprint lithography currently appear to be the most well developed, with soft lithography as a close second. The approach of employing pen writing is being explored for use in security systems for pharmaceuticals.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Current and Planned Related Developments

There are many efforts in the area of nanomanufacturing at industrial and academic research laboratories. Some of these are funded by organizations such as NSF and DARPA. NSF Centers for Nanomanufacturing exist at the University of Illinois at Urbana-Champaign, UCLA/University of California, Berkeley, and Northeastern University.

Key Milestones

The key milestones for Phase I are the following:

  • Develop a preliminary nanoprint feature that is visible without a device.

  • Explore technical approaches to producing nanoprint features at acceptable cost on a controlled plastic substrate.

  • Explore methods to integrate plastic substrate into the paper substrate with high durability.

Development Schedule

The committee estimates that Phase I of the development of nanoprint could be completed in about 2 to 3 years, with additional development time for design layouts that exploit collective effects. The key assumption is that the basic manufacturing procedures can be scaled to achieve cost-effective operation for currency applications.

Estimate of Production Cost
Compatibility with Current BEP Equipment and Processes

Nanoprint would be incorporated into the paper of the currency through security strips, plastic inserts, or related strategies. In this scenario, the nanoprint would not affect manufacturing operations at the BEP.

Incremental Production Cost

There is a potential for low cost, given the successful development of manufacturing approaches that use scaled versions of existing methods.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Required Capital Equipment

A tool for performing the printing would be needed. The systems for integrating the feature into the paper would be similar to those used in diffractive optical elements or security strips.

References and Further Reading

See articles in Materials Today. 2005. February: 50-56.

See MRS Bulletin. 2001. 26(7).

Bernard, A., E. Delamarche, H. Schmid, B. Michel, H.R. Bosshard, and H. Biebuyck. 1998. Printing patterns of proteins. Langmuir 14(9): 2225-2229.

Chou, S.Y., P.R. Krauss, W. Zhang, L.J. Guo, and L. Zhuang. 1997. Sub-10 nm imprint lithography and applications. Journal of Vacuum Science and Technology B 15(6): 2897-2904.

Michel, B., A. Bernard, A. Bietsch, E. Delamarche, M. Geissler, D. Juncker, H. Kind, J.P. Renault, H. Rothuizen, H. Schmid, P. Schmidt-Winkel, R. Stutz, and H. Wolf. 2001. Printing meets lithography: Soft approaches to high-resolution printing. IBM Journal of Research and Development 45(5): 697-719.

Mirkin, C.A. 2001. Dip-pen nanolithography: Automated fabrication of custom multicomponent, sub-100-nanometer surface architectures. MRS Bulletin 26(7): 535-538.

Piner, R.D., J. Zhu, F. Xu, S.H. Hong, and C.A. Mirkin. 1999. Dip-pen nanolithography. Science 283(5402): 661-663.

Resnick, D., S.V. Sreenivasan, and C.G. Willson. 2005. Step and flash imprint lithography. Materials Today February: 34-42.

Salaita, K., S.W. Lee, X.F. Wang, L. Huang, T.M. Dellinger, C. Liu, and C.A. Mirkin. 2005. Sub-100 nm, centimeter-scale, parallel dip-pen nanolithography. Small 1(10): 940-945.

Xia, Y., J.A. Rogers, K.E. Paul, and G.M. Whitesides. 1999. Unconventional methods for fabricating and patterning nanostructures. Chemical Reviews 99(7): 1823-1848.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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REFRACTIVE MICROOPTIC ARRAYS

Description

The foundational principle of features based on the incorporation of microoptics into an FRN is that of deterring counterfeiting by presenting an easy-to-observe-and-remember visual effect, applicable to all classes of currency users, that cannot be simulated in printing alone. This feature class employs arrays of microoptic elements, lenslets, to create obvious interactive optical effects. The user typically evaluates this type of feature by moving or tilting the substrate to generate a change in the observed image (Rossi et al., 2005). The feature requires the integration of an additional material into a currency paper substrate (Bartz, 2002).

Numerous image effects are possible using microlens arrays.4 A pair of images recorded with stereo (binocular) separation creates a three-dimensional visual perception (Iizuka, 2006; Johnson and Jacobsen, 2005). Using a sequence of images can give the appearance of action and or morphing. Completely different scenes are displayed by slightly changing the viewing direction, enabling unexpected shifts in position including reverse zooming, flipping, inverting, and distinct color or background changes. The three-dimensional perception can be combined with these tilting-induced changes. Replacing the cylindrical lenses with spherical lenses creates additional effects (Steenblik et al., 2006). In this case, tilting in either direction can cause different changes in scenes.

The detection time, with minimal or no prior training, should be less than a few seconds, and detection should be sufficiently obvious that the user is confident in the authentication. In addition to the visual aspect, the feature will have a plastic versus currency-paper tactile response and an audio response created by being scratched with a fingernail (“buzzing bee”) (De Heij, 2006). Varying the location and the size of this area on the substrate of the different denominations might assist the blind in verification and reduce paper washing. The absence of surface texture should trigger further investigation of a possible counterfeit by money handlers. No human-assisted device is necessary for evaluation.

Microoptic arrays have a wide range of applications, including advertisement publishing, optical systems in photocopiers, laser printers, facsimile machines, visual displays, and high-technology fields of photonics, optical metrology, and telecommunications. With the development of low-cost plastic-production methods, advertisement publishing has become the largest-volume application. The

4

Additional information is available from Human Eyes® Technology, <http://www.humaneyes.com/3d-technology/invention/>; plastic sheet supplier Spartech Plastics, <http://www.spartech.com/plastics/lenticular_sheet.html>; Epigem Limited, <http://www.epigem.co.uk/products-microlens.htm>; and SUSS MicroOptics, <http://www.suss-microoptics.com/index.html>. Accessed February 2007.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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eye-catching property of these devices has led to applications on vending machine displays, magazine and CD inserts along with front covers, plastic packaging materials, greeting cards, business cards, and more. The advertisement devices use standard printing technologies directly on the back side of stiff, self-supporting plastic lenticular array stock. Printing at a resolution of only 300 dots per inch is needed for these thick-lens arrays. Desktop image analysis and publishing software is also becoming available for creating individual displays using ink-jet printed images and lens array sheets.5 The thickness of these devices is typically 0.5 mm, which is at least 5 times thicker than the device that would have to be integrated into an FRN. Therefore, the major challenge for currency application is to achieve microoptical arrays with dynamic visual effect in a robust, at least “laundry-tolerant,” plastic film integrated into the 100-μm-thick paper currency.

Feature Motivation

The microoptic array is a visual feature that goes beyond the two-dimensional printed optical image by making it interactive using three-dimensional perception and animation. A feature using microoptics can be easy to remember and use. It could also have a triggering component that is tactile and tactile/audio responsive, along with an eye-catching color design. For these reasons the committee rates these features as having high detection efficiency by the general public with no assistance and as being useful in authentication by cashiers and tellers. The tactile response of this feature might be designed to help the blind authenticate an FRN without device assistance. To implement such a feature in the thickness of an FRN requires special equipment to make microoptical three-dimensional structures and micron-resolution printing. This type of equipment is typically only available in microelectronics or similar high-technology fabrication facilities. Therefore, this type of feature will be technology blocking for all but the state-sponsored class of counterfeiter. Current state-of-the-art security document manufacturers introduced the first of this type of feature.6 These early features have many of the properties envisioned and are compatible with high-volume production methods that will likely lead to only a moderate cost increase over the holographic stripe and embedded security strip.

5

For additional information, see HP Global Solutions Catalog, <http://h30156.www3.hp.com/solutionview.cfm?SOLUTIONID=1523>, and software producer Imagiam High Image Techs, SL, <http://www.imagiam.com/content/english/lenticular_versions.htm>. Accessed February 2007.

6

For example, the Crane and Company Motion™ curency feature uses lens arrays that give a response when tilting in both horizontal and vertical directions. The images translate 90 degrees with respect to the tilting direction. The feature is monochrome. It is based on woven thread integration with the currency. The paper and thread thickness is very smooth and uniform and thus distinct from simulations that will involve appliqués and lamination of multiple sheets. The spherical lenses in a hexagonal array weaken the fingernail scratching, “buzzing bee,” audible response.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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The weaknesses seen result from the use of very simple monochromatic images with significant haziness and marginal tactile contrast.

Materials and Manufacturing Technology Options

Figure C-9 shows a cross section and an oblique transverse view of an array of cylindrical lenslets, which are typical of lenticular arrays (De Heij, 2006). Each lens focuses light from specific directions to a corresponding spot on the back surface. The thickness of this device, t, for an ideal system, is typically 20 percent shorter than the focal length, f, for this system, f = nR / (n − 1), where R is the radius of curvature of the refractive surface and n is the material’s refractive index. Using a typical refractive index for plastic material, the focal length is about three times the radius of curvature. To obtain full areal coverage, the lenslet’s pitch has to be less than 2R. The spacing of the interlaced scenes, s, for a viewing angle change of θ is approximately s = (tR)θ. Thus, the scene height will be a fraction of the lens thickness; for a modest separation angle, θ, of 6 degrees (0.1 radian), a scene height is one-tenth the element thickness. For currency applications, the array must be less than 100 μm thick to fit physically within the thickness of an FRN. Since both focal length and the scene spacing scale with the thickness of the refractive lens, arrays will need focal lengths of 40-20 μm and the scene will need printing line widths of 4-2 μm.

The microoptic currency device must also have sufficient flexibility to pass a minimum of robustness tests such as the “laundry”-type test or similar methods proposed for examination of the wear of woven strips (De Heij, 2006). Optically

FIGURE C-9 Schematic cross section of a lenticular array showing thickness, t, pitch, and scene spacing. The blue scene is viewed straight on, while the green scene is viewed at an angle. Scenes are printed in stripes on the back side parallel to the cylindrical lenses.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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transparent plastic substrates exist in currency applications in more than 23 countries. Therefore, the material robustness of a plastic material of this thickness is not likely a problem. Lenses with pitches of 50 μm are already commercially available, so embossing methods exist that can create features with nearly sufficient size; but to also achieve the comparable short focal lengths will require improvements in replication accuracy.

Although a 40-20 μm focal-length lens is just beyond the current capability of advertising-based lenticular arrays, lenslets arrays of this size are routinely manufactured for photonics device integration, fiber-optic coupling, and microelectronics (Borelli, 1999). These are made using processing methods that are slower and costly. Advances, driven primarily by consumer demand for very wide screen televisions and large-area displays, are driving down the cost of semiconductor processing lithography on large areas and prompting the development of new methods for the creation of three-dimensional structures (Chabinyc et al., 2005; Michel et al., 2002). The embossing of plastic films, which are currently used to make holographic strips at high speed and low cost, has sufficient feature line-width capability for the small pitch lenses; however, improvements in three-dimensional replication accuracy, the shape accuracy, will be needed to produce high-quality very short focal length refractive lens arrays. Eventually arrays of optical elements might be printed using advanced forms of flexography and intaglio. These printing methods already have ultraviolet and electron-beam curing7 that would assist in transferring methods from semiconductor lithography.

The required scene-printing resolution is beyond the capability of all current ink-based color printing technologies. This resolution can be achievable using semiconductor processing lithography and related methods. A special flexographic printing process coupled with a vacuum roll coating (Phillips and Argoitia, 2005), which is similar to the security film printing methods used in currency, has the potential to achieve low-cost, high-speed production rates. Unfortunately, this method is currently only monochrome. Monochrome scenes significantly reduce the eye-catching nature of this class of feature. Even with advances in resolution, color processes using printing pigments and dyes will eventually fail because of insufficient optical absorption depth. Advances in very strong absorbers are needed. These might include microstructures such as interference filters and gratings, along with increased scene-printing resolution.

The microoptical lens arrays can be made extremely thin by the use of diffractive lenses; these are closely related to transmission holograms. Typical low-cost diffractive lens elements are made using a relief structure comprising features less than 2 μm in width and depth. Due to the binary nature of the relief and the very

7

Electron-beam (e-beam) curing involves the curing of composite materials using a beam of highly energetic electrons at controlled doses.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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flat profile, these structures are not as challenging for plastic embossing as those needed for high-quality refractive curved surfaces. Patterning volume changes in a material refractive index is another method of creating diffractive optical elements. Diffractive elements can be added to refractive lenses to reduce aberrations or used separately (Burger, 2002; Herzig, 1997). A very thin lens also would enable lens combinations that provide additional optical effects, such as a change with distance between the observer and the element and many advanced features similar to those in holographic security features. Diffractive lenses typically have lower efficiency, which leads to less-distinct and dimmer images. Further information on diffractive elements is presented in the section above, “Hybrid Diffractive Optically Variable Devices.”

In addition to the refractive and diffractive microoptic lenses described above, microstructures with size and shape on the order of the wavelength of light, 0.5 μm, can create numerous additional optical effects. Examples of these effects that do not have printed two-dimensional equivalents include spectral dispersion (splitting of white light into a rainbow of colors) using gratings; angle-dependent high reflectivity or opaqueness using total internal reflection; advanced optically variable color shifts using interference filters; image offsets using Fresnel prisms or other wave-guiding structures; glistening (like a cut diamond) by creating a microstructure in a very high index material; and opalescence, with artificial photonic crystals.

The micron printed optical material needs to be integrated with the paper substrate. Most current currency with holographic plastic strips uses hot pressed stamping to attach plastic strips on the surface, as shown in Figure C-10(A). These are very thin plastic films. The thickness of refractive microoptical elements makes this method impractical. A newer approach, shown in Figure C-10(B), weaves the strip into the paper substrate, creating regions in which the plastic strip is buried and other regions in which it is exposed to the surface. In the woven-thread approach, a 20 μm to 40 μm thick plastic strip will compromise paper robustness, especially near the edges of the plastic windows. The device, which only needs to be thick in the windowed areas, suggests an approach similar to that shown in Figure C-10(C) to improve robustness.

The intaglio printing creates 5 μm to 10 μm thick ink that can be formulated to adhere to plastic and paper. Using this ink to print across the boundary might reduce delaminating. Intaglio printing over the microoptic element could also be used to fill in selected areas of the microstructures, which will nullify the optical effect in the inked regions. Besides this additive three-dimensional structure, the high-pressure intaglio printing combined with deformable plastics could create additional patterned optical functions. These novel enhancements to the microoptical base feature would benefit from higher-resolution intaglio printing that might be-

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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FIGURE C-10 Schematics of methods of integrating plastic material (blue) into a paper substrate (green). (A) A thin plastic film attached to the surface. (B) A woven security strip used for thicker films. Regions of the film are buried within the paper, and others regions are exposed to the surface. (C) A variable-thickness windowed strip in which the device regions are thicker, while the other regions are buried in the paper. The thin regions could be created by laminating the optical devices onto a plastic security strip such as that already used in U.S. currency. This approach also has the advantage of easily variable shaped windows.

come possible with changes in curing methods8 combined with higher-resolution plates made using laser engraving (Deinhammer et al., 2006).

The shape of the variable-thickness area is easily controlled, as exemplified in Figure C-10(c), and its contrasting surface texture, owing to the material change in the windowed area, creates a tactile feature that is easily made note-specific. This type of feature might be effective for the blind. It also acts as an enhanced trigger for all currency users and can be used discretely if needed.

The microoptical array also has an audio signature when scratched with a sufficiently sharp fingernail, pen, or pencil. This feature is similar to the scratch feature on the euro (De Heij, 2006). The audio effect is stronger for cylindrical lens arrays than for spherical. A fingernail is sufficient for even 40 μm pitched arrays.

Simulation Strategies

Printing methods are not expected to be able to simulate the optically variable display, the three-dimensional perception, the audio signature, and the tactile feel

8

Sun Chemical WetFlex™ echnology with Energy Sciences, Inc., <http://www.ebeam.com/>, and <http://www.specialchem4coatings.com/news-trends/displaynews.aspx?id=5368>. Accessed February 2007.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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of this feature. The latter two characteristics of the feature could be simulated by add-on artwork, but since these would not be integrated into the substrate, they should be obvious if tested.

It is expected that although the state-sponsored counterfeiter would find the optically variable feature difficult to produce, without the cost pressure of high production rates this feature would not likely deter production by this class of counterfeiter. The state-sponsored class of counterfeiter also has the ability to integrate the optical device with the substrate.

The counterfeits of lower quality, typical of those made by the professional criminal down to the opportunist, would not match the tactile response accurately and would completely miss the optical effects. For example, a hot stamped holographic strip might yield an optically variable feature but would lack the physical integration of the optical material with the paper. Also, the holographic simulation would not have the audio signature. Printed image simulations would be static. The lower class of simulation might use an advertising-type microoptic array, which would be so thick that it would look and feel as if it was glued onto the substrate.

Simulating the machine authentication at the level of near-infrared signature and simple optical signature might be possible for the professional criminal counterfeiter class. Either simulation would require proprietary knowledge of the tests methods designed into the machine or very extensive reverse engineering or experimentation.

Key Development Risks and Issues
Durability

The plastic used and the adhesion of this material to the paper have been worked on during the creation of the embedded security strip. A thin plastic strip has been demonstrated; however, this device needs to be as thick as practicable to enhance the working of the device. Likely failure will be on the paper edges of the interface with the plastic.

To counter the bleaching of intaglio ink on an FRN, a plastic that has chemical behavior similar to that of the cured ink would be advantageous. In addition, if the ink from the intaglio printing was used as an adhesive bond between the plastic and the paper, once that ink was removed the likely resulting separation of the feature from the substrate would render the substrate useless for the counterfeiting of higher denominations.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Aesthetics

The feature has the capability of engaging the user’s attention without making the image complex and intricate. This type of feature has the potential to enhance the aesthetic appeal of U.S. currency.

Social Acceptability

This feature is a human-interfaced device and should not present any concerns over loss of privacy. The materials used are already in current U.S. currency. However, switching to laser-engraved intaglio plates should reduce the use and cost of disposing of water-soluble nickel salts. If it can be made compatible with FRN substrate materials, switching to e-beam cured inks might enable a reduction in the emission of volatile organic compounds. Ultraviolet acrylic ink would likely add literally more headache at the BEP.

Key Technical Challenges

An advanced feature in this class should be close to that used by advertisers for catching and holding the consumer’s attention. The visual effect should have an eye-catching three-dimensional perception effect, and the need to tilt the note to see the feature’s effects should promote interaction with the user. The use of color shifting would help make the feature harder to simulate by requiring even higher-resolution printing. The fact that two different materials are integrated into the note should be used to enhance the tactile contrast. The use of structures that enhance the nail-scratching buzzing bee sound and tactile response might help the blind.

Commercially available plastic microoptic arrays that yield bright sets of images are typically four times thicker than an FRN substrate.9 Several technical challenges exist in reducing the thickness without significantly increasing cost or compromising functionality. Similar challenges exist in advertisements and security features for advance packaging, consumer product labeling, a potential desktop-publishing three-dimensional photography product, and fabrication of large-area displays (Levinson, 2005). Arrays with the basic functionality needed for currency application are easily made in sophisticated microfabrication facilities with slow and expensive processes (Borrelli, 1999). Printing of the image on the back side of

9

Additional information is available from Human Eyes® Technology, <http://www.humaneyes.com/3d-technology/invention/>; plastic sheet supplier Spartech Plastics, <http://www.spartech.com/plastics/lenticular_sheet.html>; Epigem Limited, <http://www.epigem.co.uk/products-microlens.htm>; and SUSS MicroOptics, <http://www.suss-microoptics.com/index.html>. Accessed February 2007.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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these arrays would require line widths of only a few microns. This is not currently possible with high-speed commercial printing processes. Expensive lithographic methods similar to those used in microelectronics fabrication achieve this resolution, however with limited color selection (Phillips and Argoitia, 2005). Additional optical effects besides dye and pigment-based coloring might be needed to achieve a full-color display in currency.

These are incremental improvements beyond what is becoming available from currency substrate suppliers. The key challenges will be durability and cost. The durability issues will likely limit the use of this type of feature to high-denomination notes. Improvements in the integration of paper and plastic films will be important. Challenges include efforts to do the following:

  • Add a color three-dimensional perception, which will require printing at micron line widths or integrating advanced microstructure optical devices.

  • Achieve robust integration of the microoptical device with the paper substrate, enabling varying size, shape, and positioning of the windows.

  • Increase the fingernail scratching audio and tactile effects.

Commercially available lenticular plastic elements can be obtained as single sheets and continuous web rolls produced by extrusion, embossing, molding, and casting. These are available in a variety of optical-quality plastic materials with lens pitches from 1.7 mm to 40 μm. A design challenge for these devices is to minimize aberrations, ghost images, and out-of-focus scattered light. A thinner, more-flexible device would enable additional applications in packaging advertisement and might create a three-dimensional computer printing photography market creating unaided three-dimensional displays made on the home office computer printer. Individual consumers could record images using digital cameras with just the addition of a special lens. The images recorded could then be printed on special lenticular stock, viewed with animation effects on ordinary computer screens, sent in e-mails, and possibly viewed statically on specially modified computer displays. No special glasses would be needed for viewing just a “special paper” for ink-jet or thermal transfer printers. This is not much different from current practice.

The typical array with 16 images resolved in a ±30 degree tilt using a 525 μm thick lens array will need interlaced printed scenes of 50 μm. This is in the range of current printing resolutions, including flexographic printing on thin plastics and desktop-publishing printers. Thinner devices with corresponding shorter focal lengths need proportionally narrower scene lines, which will challenge existing high-speed and desktop-publishing printer resolution. Paper alignment in the printer will need improvement in order to print reliably directly on these thin

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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arrays. Current lens array stock for individual consumers is a few dollars, but advertisement material cost is approaching that of high-quality photo paper.

The advertisement elements are typically freestanding, made with stiff materials. Flexibility will be the consequence of thinner elements, and it would be further improved by using different materials. The thinner elements with the shorter focal lengths will require higher replication accuracy to maintain the shape of the lens surface.

Phase I Development Plan
Maturity of the Technology

The first introduction of an example of this feature-class product has occurred. Prototypes are available for testing, and medium-scale manufacturing has started. Future improvements are under development. The small-scale manufacturing will yield real-life product experience.

Current and Planned Related Developments

Knowledge gained from the first product introduction should be used to refine and improve the design for the FRN.

Key Milestones
  • Define and show the initial feasibility of printing optical elements at low cost. This effort would include demonstrating a high-resolution stamping process for optical elements of this size or embossed films. Determine if sufficient resolution exists to print simultaneously on the reverse side while also attaining the required registry, and explore a viable approach for printing the elements on a high-speed drum printing flexographic/offset press.

  • Explore approaches to adding color three-dimensional perception to the optical effect. This would include a pigment and dye approach, or, alternately, an interference filter stack; high printing resolution; and the integration of advanced dye or pigment with high-resolution offset printing.

  • Develop robust integration of a microoptic device with the paper substrate, including laminated plastic film optical elements to achieve the required durability.

  • Explore creating varying window sizes, shapes, and positioning.

  • Investigate increasing the tactile effect.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Development Schedule

The committee believes that Phase I of the development of this feature could be completed within 2 to 3 years.

Estimate of Production Cost
Compatibility with Current BEP Equipment and Processes

This feature would have to be outsourced to the substrate manufacturer. Minimal changes in BEP operation would be needed. The BEP would play an important role in refining the robustness requirements and product improvements. New testing and quality-control measures would be needed.

Incremental Production Cost

From the BEP perspective, this material should have a mature product cost 1.5 to 2 times that of holographic strips and significantly higher than that of the current security strip. The holographic strips in high-volume production cost $7/m2 to $9/m2 in 2004, which amounts to $6 to $7 per 1,000 FRNs. Therefore, this $9 to $12 per 1,000 note feature is classified as having a high incremental cost.

The feature’s development program and the resulting high-volume cost schedule should follow a trail similar to that of the development of the euro holographic strip, which dropped in cost from $12 per 1,000 notes to $6 to $7 per 1,000 notes in 5 years. For a production rate of 1 billion notes per year, the cost amounts to an estimated $30 million for development. The capital equipment cost for a rate of 1 billion notes per year with an 8-year fixed depreciation and assuming a 100 percent margin above incremental cost suggests that a capital investment of between $20 million and $30 million would be required. Very high resolution vacuum roll coating demetallization equipment costs about $20 million installed. The embossing and plastic laminating equipment are likely less expensive but still approach $10 million installed each. Therefore, an initial cost for this type of feature will be in the range of $12 to $16 per 1,000 FRNs, with an outlook of $9 per 1,000 notes.

Printing equipment is likely about the same capital investment but with production speeds 10 times higher. Replacing the optical element formation and or the micron line process with a printing process would have significant impact on cost reduction.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Required Capital Equipment

The BEP would be required to acquire additional testing and quality-control equipment, and if advanced intaglio was required, major capital for press replacements would be needed. If printed optical elements are needed, the film will likely be printed after integration into the substrate, and this would require the new printing process equipment.

References and Further Reading

Bartz, W.J. 2002. Durability of optically variable devices on bank notes. Proc. SPIE, Optical Security and Counterfeit Deterrence Techniques IV, R.L. van Renesse (ed.), Vol. 4677, pp. 81-88.

Borrelli, N.F. 1999. Microoptics Technology: Fabrication and Applications of Lens Arrays and Devices. New York: Marcel Dekker.

Burger, R.J. 2002. Lenslet Array Systems and Methods. U.S. Patent 6381072, April 30, 2002.

Chabinyc, M.L., W.S. Wong, A.C. Arias, et al. 2005. Printing methods and materials for large-area electronic devices. Proc. IEEE 93(8): 1491-1499.

De Heij, H.A.M. 2006. Public feedback for better banknote design. Proc. SPIE, Optical Security and Counterfeit Deterrence Techniques VI, R.L. van Renesse (ed.), Vol. 6075, pp. 1-40.

Deinhammer, H., D. Schwarzbach, R. Kefeder, and P. Fajmann. 2006. The implications of direct laser engraved intaglio plates on banknote security. Proc. SPIE, Optical Security and Counterfeit Deterrence Techniques VI, R.L. van Renesse (ed.), Vol. 6075.

Herzig, H.P. 1997. Microoptics Elements, Systems and Applications. Philadelphia: Taylor and Francis, Ltd.

Iizuka, K. 2006. Welcome to the wonderful world of 3D. Optics and Photonics News 17: 43-51.

Jang, S.-J., S.-C. Kim, J.-S. Koo, J.-I. Park, and E.-S. Kim. 2004. 100-inch 3D real-image rear-projection display system based on Fresnel lens. Proc. SPIE, Integrated Optical Devices, Nanostructures, and Displays, Keith L. Lewis (ed.), Vol. 5618, pp. 204-211.

Johnson, R.B., and G.A. Jacobsen. 2005. Advances in lenticular lens arrays for visual display. Proc. SPIE, Current Developments in Lens Design and Optical Engineering VI, P.Z. Mouroulis, W.J. Smith, R.B. Johnson (eds.), Vol. 5874, pp. 59-69.

Lancaster, I.M., and A. Mitchell. 2004. The growth of optically variable features on banknotes. Proc. SPIE-IS&T Electronic Imaging, Optical Security and Counterfeit Deterrence Techniques V, R.L. van Renesse (ed.), Vol. 5310, pp. 34-45.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Levinson, H.J. 2005. Principles of Lithography. 2nd ed. SPIE Press Monograph Vol. PM146. Washington, D.C.: SPIE—The International Society for Optical Engineering.

Michel, B., A. Bernard, A. Bietsch, E. Delamarche, M. Geissler, D. Juncker, H. Kind, J.P. Renault, H. Rothuizen, H. Schmid, P. Schmidt-Winkel, R. Stuts, and H. Wolf. 2002. Printing meets lithography: Soft approaches to high-resolution patterning. CHIMIA 56(10): 527-542.

Phillips, R., and A. Argoitia. 2005. Using vacuum roll coaters to produce anticounterfeiting devices. Vacuum Technology and Coating 6(10): 46-53.

Rossi, L.M., J. Saarinen, and M. Gale. 2005. MICRO-OPTICS: Micro Optics technology fulfills its promise. Laser Focus World. October; <http://lfw.pennnet.com/articles/article_display.cfm?article_id=238630>. Accessed March 2007.

Steenblik, R., M.J. Hunt, and M.E. Knotts. Nanoventions, Inc. 2006. Microoptics for Article IdentificationCrane, U.S. Patent 7006294, February 28, 2006; <http://www.nanoventions.com/nano4/index.html>. Accessed March 2007.

Watson, B. 1999. Science makes a better lighthouse lens. Smithsonian 30(5): 30. Available at <http://www.smithsonianmag.com/issues/1999/august/object_aug99.php>. Accessed March 2007.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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SEE-THROUGH REGISTRATION FEATURE

Description

For a see-through registration feature, one side of a substrate aligns with an associated feature on the reverse side—that is, when the FRN is held up to a light source, the combination of the two features on the two sides of the note gives rise to a new “complete” feature.

With a non-opaque substrate held up to a light source, features on both sides of the currency can become connected in a significant and obvious manner, becoming a new feature. Types of registration can include but are not limited to the following: (1) the continuation of a line shape; (2) two (individually nonrecognizable) parts of an image or design that together render a complete image; and (3) lines or shapes that line up exactly with identical features on both sides of the FRN. These features can also be combined to create a transmitted color different from either of the printed-image colors. The acuity of human vision has an impressive and rapid ability to discern between good and bad registration—about 10 seconds of arc, meaning ~0.03 mm from 60 cm.

In order for this feature to be most effective, the images involved must be large enough to attract the user’s attention, and the design must be clear to make the feature obvious. A small see-through feature would be ineffective. In addition, the design of this feature must take full advantage of the simultaneous front- and back-side offset printing capability in spot colors at the BEP so as to challenge the improving ink-jet printers’ registration, color simulation, and resolution.

See-through registration is a visual feature in transmitted light, and although it could be read by a machine, its contribution is to create an overt visual feature primarily for the public and secondarily for the cash handler and teller.

A see-through registration feature can be found on many international currencies. Typically, the large numbers designating the denomination are split in half, with part of the number on one side and the remaining portion on the other. When held to the light, the entire number becomes visible. Because the public is very focused on the denomination of any particular note, there is a special focus on these numbers. Although see-through registration is not unique as a feature, as it is used in other countries, there is still room enough for innovative designs that it should remain effective against the primitive class of counterfeiter. The innovative designs could include more-complex patterns and the inclusion of subtractive color creations.

One possible registration feature would be to print a set of fine lines on both sides of the note that are precisely on top of each other. On a genuine note, the gaps between the lines would be clearly visible when the feature was held up to a light source. But on a counterfeit, in most instances, the variation in printer toler-

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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ances would result in the lines not being on top of each other, effectively closing the transmitted image gap, making the image look dark or solid.

Feature Motivation

With see-through registration offering an impressive combination of high saliency, easy implementability at the BEP, and the ability to deter the petty criminal, this feature is highly rated. In addition, this feature would have an immediate and positive impact on the public. Education of the public regarding this feature would be required, but a relatively simple explanation would suffice.

A carefully designed see-through registration feature could be an excellent and inexpensive way to deter the lower-level criminals from producing counterfeit notes. This feature can only be successful with the very precise alignment of the printing on the front and back sides of the FRN in spot colors. At present, there are no inexpensive two-sided printers available on the market, and there appears to be little motivation for creating this type of printer in the future beyond the environmental pressure to reduce paper usage by means of two-sided printing. The feature would have to be designed so that it was sophisticated enough to overcome the “good enough” registration possible on inexpensive ink-jet printers that counterfeiters have already used, according to data provided to the committee by the Secret Service.

The fine-line-feature approach might satisfy the basic requirements of being hard to replicate by the opportunist counterfeiter and easy to observe. However, of direct challenge to this feature class is the remarkably accurate print registration on the paper available with common, low-speed ink-jet printers.10 Print registration is called for in a variety of special printing tasks that still rely on the generation of layers of artwork or graphics. Special printer drivers are being marketed for even dot-level control of the printing process with registration on paper of special material. Currently, the BEP has production processes in place that allow for the necessary degree of precise registration in spot colors, and therefore little to no cost would be incurred to implement this new feature.

Materials and Manufacturing Technology Options

The BEP already has the capability to control the precise location of the printed image on the substrate and, to the committee’s knowledge, it appears that the BEP can obtain a level of precision that is necessary for creating the see-through registration feature.

10

Some printers’ photo ink-jet printer drivers add dithering algorithms to the printing on a page. It is not clear why this is done, but alternative drivers and work-arounds can be easily found.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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There is a narrow design window for this feature type that will make it difficult to simulate using ink-jet printing. The design needs to take into account the pixelated and digitally addressable aspect of the ink-jet printer. The BEP equipment can avoid the image-processing methods of conventional printing and use or develop special security inks that are out of the normal color gamut to create a see-through feature that might fill this gap.

All ink-jet and commercial digital printing uses pixelated images with multiple dots partially overlapping to create the desired color. This process results in color lines that are wider than, or a subpixel offset compared with, a single ink-dot line. In addition, using special dyes in the offset ink can exacerbate this aspect of the simulated ink-jet-printed image. The unique security ink colors could also be used to BEP advantage in the same feature, to give a combined unique transmitted image color that is neither of the printed colors and will appear only with very high resolution single-pixilated straight-line printing. Again, the additive aspect of the spectrum will be washed out to some extent in wider, less-distinct-line ink-jet printing. An additional aspect is to combine see-through registration with a substrate feature such as the cream color of the FRN currency or a special dye added in the security thread.

Simulation Strategies

As most commercially available printers do not print two sides of a substrate simultaneously, precise registration requires some additional effort. Possibly only state-sponsored counterfeiters have access to the types of printers that could duplicate the registration precision required. Primitive and opportunist classes of counterfeiter would not be able to simulate this feature well enough to fool the general public. The petty criminal would have to obtain special inks and use specialized software drivers to ensure registration of the front and back sides of the note in separate printing. Also, using the multidimensional aspects such as incorporating a property of the substrate with this feature would force lamination or other measures to be used, similar to the simulation of other transmitted optical features by professional criminals.

Key Development Risks and Issues
Durability

With the degradation of the note over its lifetime, the transparency of the FRN may be obscured by the accumulation of dirt, which would limit the effectiveness

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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of the see-through registration feature. However, it could be argued that the effectiveness of most features would be reduced by the accumulation of dirt.

Aesthetics

Using see-through registration as a feature would entail using a pattern large enough for observers to actually determine whether the registration was correct (authentic notes) or not aligned (counterfeit notes). Using a set of parallel lines or having only half of a number visible on one side (as is done in other international currencies) may appear too modern in design and potentially not appropriate for U.S. notes. However, there are many other ways to incorporate a registration feature by using other patterns that would be in keeping with the aesthetics of the note.

Social Acceptability

There are no issues of loss of privacy or environmental hazards related to the implementation of this feature.

Key Technical Challenges

The key technical challenge for the see-through registration feature would be the development of a design that took advantage of the nonpixelated, nondigitally addressable image-creation capability of the BEP using its offset spot-color printing, with the possible inclusion of a substrate color. The design would have to include precision in alignment or registration of the front and back sides of the note that is beyond the capability of desktop systems now and for the next 5 to 10 years. Although the machines at the BEP are capable of this level of precision, they are not used in this fashion at present. The stretching of the substrate in the presses could complicate registration. Also, the opacity of the paper may have to be adjusted to make this feature more visible in normal transmitted light.

Phase I Development Plan
Maturity of the Technology

Aspects of this feature are technically mature as they are in use in other currencies. However, to establish feature designs as described above would require standard manufacturing quality-control testing at the BEP. Some additional equipment would be needed for measuring the performance of the transmission aspect

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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of this feature, and the BEP would have to test its machines for precise registration capability and determine the optimum level of paper opacity.

Current and Planned Related Developments

This feature design has some aspects in common with the high-complexity spatial patterns feature previously discussed. Development programs for this feature probably exist in other countries that use see-through registration. Because this feature is quite simple, the development of a similar program in the United States would require modest investment to implement.

Key Milestones

The milestones for implementing this feature are the following:

  • Design a test see-through registration feature that is both pleasing to the eye and easy to authenticate and technologically challenging to the inkjet printer, assuming continued improvement in resolution and printing registration.

  • Test existing BEP printing equipment for precise registration and stability of colors.

  • Modify the paper opacity specification as required.

Development Schedule

This feature is relatively simple. It is expected that it could be ready for production within 3 years.

Estimate of Production Cost
Compatibility with Current BEP Equipment and Processes

Adding the see-through registration feature would have minimal effect on manufacturing operations at the BEP. Only the level of precision at the BEP would need to be increased. It is possible that the scrap rate might increase.

Incremental Production Cost

The incremental production cost for this feature is estimated to be very low.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Required Capital Equipment

No major capital equipment would be necessary to produce this feature.

Further Reading

Alpern, M. 1978. The Eyes and Vision Handbook of Optics, W.G. Driscoll and W. Vaughan (eds.). NewYork: McGraw-Hill, Ch. 12.

Boucher, P.E. 1955. Fundamentals of Photography. New York: Van Nostrand.

Dowling, J., L. Stryer, and T. Wiesel. 1999. Vision: From photon to perception, pp. 1-3 in National Academy of Sciences Colloquium on Vision. Available at <http://www.nap.edu/catalog/9965.html>. Accessed February 2007.

Garcia, D.D. 2007. C What U C: A Visual Acuity Simulator. Available at <http://http.cs.berkeley.edu/optical/SPIE/SPIE98_CWhatUC.pdf>. Accessed February 2007.

Tassi, P., N. Pellerin, M. Moessinger, A. Hoeft, and A. Muzet. 2000. Visual resolution in humans fluctuates over the 24 hour period. Chronobiology International 17: 187-195.

Westheimer, G. 1981. Visual Hyperacuity: Progress in Sensory Physiology. Berlin: Springer-Verlag.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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SUBWAVELENGTH OPTICAL DEVICES

Description

Conventional inks and printers offer powerful capabilities for accurately duplicating visible monochrome or color images, as evaluated by the unaided eye. Duplicating images that modulate the state of polarization or phase, or that have complex spectroscopic signatures, however, is much more difficult. Subwavelength optical elements (SOEs) provide visible images that achieve colors, polarization contrast, and/or other optical effects through the use of structures, such as relief features or narrow conducting wires that have dimensions substantially smaller than the wavelength of light. These optical characteristics, and especially those that provide polarization-dependent effects (for example, polarization-dependent transmission or reflection, or polarization transformation), offer a high level of control over the appearance of an image as viewed by the unaided eye under suitable lighting conditions. The subwavelength structures of SOEs can also provide forensic or machine-readable functionality.

The concept of an SOE feature is based on the wide range of optical characteristics that can be obtained from a single material by structuring it on length scales shorter than the wavelength of light—that is, substantially less than a few hundred nanometers. For example, parallel arrays of thick, subwavelength metal lines on a transparent substrate are transparent to light polarized along the lengths of the lines but do not transmit light with the orthogonal polarization. Similar structures in dielectric materials can rotate the polarization of the transmitted light, reflect light in narrow wavelength ranges, or serve as antireflection surfaces. Spatially variable structures of this type, used alone or in combination with conventional inks, can yield images with complex optical effects. For example, radially oriented subwavelength metal lines can produce polarizing elements for which the high-transmission direction has circular symmetry. This type of element would be difficult to construct using conventional polarizing optics. The chromatic effects, especially for the combined transmission, reflection and scattering effects, available in SOEs would be similarly difficult to simulate using conventional inks.

This feature is intended for use by the general public but is well suited also for machine readers. Of the various optical effects that are achievable with SOEs, the polarizing and polarization transforming operations appear to have particularly unique potential for the general public, provided that the public is educated on the conditions needed to view these effects—such as through polarizing sunglasses or with light from a backlit liquid-crystal display. The complex spectroscopic properties (for example, narrowband reflection) could also be useful in this sense, but in a manner that is most useful when combined with the assistance of another optical element or gadget—such as a narrowband filter—for viewing or in metameric

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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patterns. These types of features, as well as the polarization effects, could easily be detected, at high speed, with a machine reader. The very small sizes of the structures in SOEs and the ability to incorporate nearly arbitrary geometries provide opportunities for functionality that could be useful for forensics.

Feature Motivation

The committee found the concept of the SOE feature to be useful because this feature has both overt functionality that can be used by the general public and by machine readers, and functionality for forensics. As a result, an SOE feature has potential value for protection against all classes of counterfeiter. Also, the manufacturing baseline and durability established by existing diffractive optically variable device (DOVD) currency features suggests technical feasibility for SOEs.

The main risk with this feature is that certain of its effects (for example, polarization response) are apparent only to an educated public. The manufacturing costs for fabrication at the required resolution must be established, although there is some promise for using scaled versions of the embossing techniques currently used for low-cost DOVDs such as holograms. The level of robustness of an SOE will be comparable to that of a DOVD.

SOEs are used in a variety of optical systems, and they are commercially available, currently for niche applications, from several vendors. The use of these items for applications in the visible or ultraviolet is described in the scientific and technical literature.

The committee is unaware of SOEs being used in currency or security applications. The structures in SOEs are similar to, in some cases, but smaller than those found in diffractive optical devices. The design of a particular type of SOE feature (that is, the colors, polarization behavior, and so on) would influence its uniqueness.

Materials and Manufacturing Technology Options

SOEs use established materials, structured into geometries that have subwavelength scales. Some of the manufacturing processes used to fabricate them rely on procedures borrowed from the microelectronics and display hologram industries. Newer techniques based on embossing procedures similar to those used in the production of blue semiconductor lasers for use in high-definition digital videodisc players are also suitable, as demonstrated in the recent scientific literature (Wang et al., 2006). The SOEs are similar in some ways to DOVDs, except that the sizes of the structures in SOEs are substantially smaller (2 to 10 times) than those in DOVDs. The SOEs can be integrated into currency in ways similar to those for DOVDs.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Simulation Strategies

Some of the color effects in SOEs could be simulated, for viewing with the unaided eye, using conventional inks. Simple polarization-dependent behaviors might be simulated by cutting and pasting conventional polarizers and waveplates onto a counterfeit banknote. These simulation methods are not, however, readily scalable for larger-scale counterfeit production. In addition, neither conventional inks nor cutting and pasting can reproduce many of the characteristics of SOEs—such as the circular polarizer configuration illustrated in Figure C-11. As a result, simulating SOEs with conventional inks would yield poor-quality simulations that could be identified by an educated public—that is, by a public that understands the viewing conditions needed to observe the features.

Strategies to duplicate SOEs that incorporate only surface relief structures would be similar to those used for surface-relief-type reflective or transmissive DOVDs. In this approach, the counterfeiter uses the optical element itself as an embossing tool to create duplicate relief structures with similar geometries, followed, in some cases, by coating with metal films by physical vapor deposition. The level of difficulty in using these methods with an SOE would be higher than the level of difficulty for a DOVD because the feature sizes are considerably smaller (for example, by 2 to 10 times). The effectiveness of the embossing approach could be reduced by including features in the SOE, such as patterns of isolated subwavelength metal lines, patterns of color, or other hybrid features (see the section above entitled “Hybrid Diffractive Optically Variable Devices”) that could not be reproduced directly by embossing. The fabrication of such features requires sophisticated setups that are available only to professional criminal or state-sponsored counterfeiters.

FIGURE C-11 Scanning electron micrograph (left) of a subwavelength optical device, which, in this case, consists of an array of metal lines. This structure forms a polarizer, whose implementation as a circular polarizing element is illustrated on the right. SOURCE: Schnabel et al. (1999).

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Key Development Risks and Issues
Durability

The committee estimates that the durability of an SOE feature would be comparable with that of a diffractive optical device such as holograms or kinegrams. The degradation modes, for instance, would be similar.

Aesthetics

The specific design of an SOE feature determines its aesthetics. In some embodiments, an SOE feature would be aesthetically similar to a diffractive optical device. In others in which, for example, the SOE simply provides a polarization contrast, the SOE feature would alter only by a small amount the current look and feel of U.S. currency. In any case, the SOE would require integration with the note through a bonded plastic element similar to that used for a diffractive optical device.

Social Acceptability

The committee does not foresee any social acceptability issues with regard to this proposed feature.

Key Technical Challenges

The key technical question is whether the embossing-based fabrication approaches for SOEs, which are similar to those used for diffractive optical devices, but require higher resolution, can be scaled up for low-cost manufacturing. These methods are described in the scientific and technical literature, and they are in use by several companies for niche applications, primarily in the infrared or near-infrared.

Phase I Development Plan
Maturity of the Technology

SOEs have been described in multiple scientific and technical publications, and they form the basis of a set of niche commercial products, primarily for applications in the infrared and near-infrared. The basic materials and many aspects of the SOEs are similar to those of diffractive optical devices.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Current and Planned Related Developments

There are a number of research efforts at industrial and academic laboratories focused on the development of high-resolution embossing approaches for fabricating sub-100 nm structures for microelectronics and photonics. Many of these programs have been and are currently supported by agencies such as NSF and DARPA. For example, the NSF Center on Nanomanufacturing based at the University of California at Los Angeles and the University of California at Berkeley includes development efforts on related lithographic procedures based on embossing. These and similar methods represent the most promising path to the low-cost manufacture of SOEs.

Key Milestones

The key milestones for Phase I development are the following:

  • Develop an approach to achieve high-volume production of SOEs for operation in the visible range.

  • Develop suitable designs that balance aesthetics, security, and cost of manufacturing.

  • Develop scanners and associated processing software to detect phase-encoded and other covert responses of SOEs.

Development Schedule

The committee estimates that the Phase I development for an SOE feature could be achieved with 2 to 4 years of development work. The key assumption is that the basic manufacturing approaches and materials used for SOEs that operate in the infrared and near-infrared will be suitable for SOEs in the visible.

Estimate of Production Cost
Compatibility with Current BEP Equipment and Processes

The SOEs could be integrated into a security strip or other component in a manner similar to that used for diffractive optical devices that are found in other currencies. The SOE would be integrated into the paper itself, thereby minimizing the effects on BEP manufacturing operations.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Incremental Production Cost

The cost to incorporate an SOE is estimated to be similar to that for a diffractive optical device.

Required Capital Equipment

New capital equipment would be required. It would be similar to that used for diffractive optical devices but with enhanced resolution capabilities.

References and Further Reading

Kikuta, H., H. Toyota, and W.J. Yu. 2003. Optical elements with subwavelength structured surfaces. Optical Review 10(2): 63-73.

Pelletier, V., K. Asakawa, M. Wu, D.H. Adamson, R.A. Register, and P.M. Chaikin. 2006. Aluminum nanowire polarizing grids: Fabrication and analysis. Applied Physics Letters 88: 211114.

Schnabel, B., E.-B. Kley, and F. Wyrowski. 1999. Study on polarizing visible light by subwavelength-period metal-stripe gratings. Optical Engineering 38(2): 220-226.

Wang, J.J., X. Deng, X. Liu, A. Nikolov, P. Sciortino, F. Liu, and L. Chen. 2006. Ultraviolet wave plates based on monolithic integration of two fully filled and planarized nanograting layers. Optics Letters 31(12): 1893-1895.

<http://www.nanoopto.com/>. Accessed March 2007.

<http://www.nanonex.com/>. Accessed March 2007.

<http://www.molecularimprints.com/>. Accessed March 2007.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×

TACTILE VARIANT SUBSTRATE

Description

The principal idea behind the proposed tactile variant feature on an FRN involves creating changes in the feel across the face of the bill that can be detected by the touch of a hand. The idea is that each denomination would have its own unique tactile signature.

To date, the only tactile feature on the FRN is the substrate itself. The special combination of cotton and linen combine to make a substrate that is not only unique, but very difficult to simulate. In general, any feature will be most salient if there is little “noise” in the “background.” In this case, “noise” refers to other tactile features and “background” refers to the entire note. With the absence of other tactile features, the saliency of a new tactile variance feature would be even more pronounced. Introducing a tactile feature would involve roughening a specific area on the note in such a way that would make it “readable”—that is, easily detectable and discernible from the tactilely different areas on other denominations.

One example of a tactile variant feature could be vertical strips (in the same direction as the existing plastic ribbon) that are rougher than the base substrate. These strips could be of different widths, and they could vary in number, similar to a bar code (see Figure C-12). The challenge would be to choose a tactile variant pattern that would clearly indicate a certain denomination—for example, one thin strip for the $100 note, two thin strips for the $50, three thin strips for the $20, one medium strip for the $10, two medium strips for the $5, and one thick strip for the $1. The visually impaired would also be able to use the different denominations without confusion and with confidence. A teller could also use the feel of the currency to determine the authenticity of a note and potentially could use illumination from the side to inspect the tactile variant feature visually.

With the U.S. banknotes having a unique substrate of cotton and linen, creating small areas that are rougher to the touch would make the substrate even more unique and difficult to simulate. Also, with careful design of different tactile variant areas for each of the denominations, counterfeiters would find it difficult to reuse a bleached note of a smaller denomination to simulate a note of a larger denomination.

This feature is primarily a human-perceptible, not a machine-readable, feature. However, given the three-dimensional quality of a rougher substrate, it is conceivable that this feature would have visibility with illumination from the side of the note. A tactile variant note would also provide a unique opportunity for the blind and the visually impaired to recognize the denomination. A tactile variant feature would not require additional devices for its authentication or denomination; to the contrary, it would require a simple tactile test.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×

FIGURE C-12 Schematics of two denominations with different bar-code-like strips for the tactile identification of each.

Creating this feature would require the substrate manufacturer to add another step to the process. As paper-roughening machines already exist, it would appear that the cost to include this new feature would be minimal.

Feature Motivation

U.S. banknotes arguably have one of the most unique substrates of all the international currencies. Its feel is not only easily recognizable to the touch, but it is also extremely difficult to simulate. This special substrate has been and continues to be one of the most important features of the FRN. A tactile variant feature on the FRN is an excellent way to enhance the uniqueness of the cotton and linen substrate, while also creating a first-of-its-kind tactile feature. This feature would be particularly important because for the first time it would allow the visually impaired and the blind to differentiate between the various denominations.

A tactile variant feature allows the substrate of each denomination to be unique. As the quality of inexpensive printers continues to rise, the substrate is

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×

again becoming an important and critical feature. Not only would counterfeiters be less inclined to attempt to pass a fake note produced on poor-quality paper, but a tactile variant feature would make simulation even more difficult. Also, as indicates above, counterfeiters have been known to bleach FRNs in order to use the substrate to make counterfeits of higher denominations, and this new feature would reduce such counterfeiting considerably. This feature would probably be well received by the public, which is already very aware of the feel of the note.

Materials and Manufacturing Technology Options

To create a tactile variant feature, a machine process would have to be created at the manufacturing facility of the substrate supplier. The machine would have an abrasive quality that would precisely roughen up the surface in specific areas. The BEP would receive the substrate with this feature already added. The feature could, for instance, be added to the side of the portrait, in an area where other features would not be adversely affected.

Simulation Strategies

Simulation of this feature is possible in a crude fashion—that is, using a substrate-roughening machine. However, because the careful design of a tactile feature would not allow a bleached lower denomination to be used to simulate a high-denomination note, simulation would become very difficult. Thus, this feature would be effective in deterring the primitive, opportunist, and petty criminal, and to some extent the professional criminal.

Key Development Risks and Issues
Durability

The tactile qualities of the substrate could potentially change over the course of the note’s lifetime, with the rough areas (feature) becoming softer over time. However, the main characteristic of this feature is that the rougher areas remain distinct and different from the normal tactile qualities of the substrate. It is the committee’s determination that this difference in roughness would remain over the note’s lifetime, although durability experiments would be needed to confirm this.

Aesthetics

Using a tactile variant feature would not degrade the feel of the substrate; in fact, it might even bring more attention to the tactile quality of the FRN. Placement

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×

of the feature could potentially degrade or distort the look of the portrait; thus, it perhaps would be prudent to place this feature to the right of the portrait.

Social Acceptability

There are no obvious hazards or public concerns created by introducing this feature. Although a simple bar-code design is suggested, this design would be very rudimentary (only a few strips) and therefore could not contain the amount of information that a normal bar code could. Thus, there would not be any loss of privacy. There are no environmental hazards. The inclusion of a feature that the visually impaired and blind population could use to denominate and authenticate FRNs would be of great social benefit.

Key Technical Challenges

The technical challenges for the tactile variant feature involve establishing the process by which the substrate becomes rough and understanding the manner in which the ink will respond to a substrate with varying levels of roughness. Adding roughened areas to FRNs needs to be done in a such a way that whole sheets of bills can still be printed. The use of a bar-code-like design has its benefits, as one could apply roughened strips straight across the entire sheet. This could be the simplest format, thereby using a roughening machine directly on the roll of paper as it exits the papermaking machine—that is, before it is cut into sheets. Using roughened shapes that repeat themselves along an imaginary vertical line is another approach to implementing this feature. High-pressure printing on a substrate with varying roughness will probably not cause any adverse effects to the precision and clarity of the note. Experiments will be needed to confirm this.

Phase I Development Plan
Maturity of the Technology

Experiments for durability and ink clarity need to be designed and performed, a design for this feature needs to be carefully chosen, and a machine to create the unique rough areas on the FRN needs to be created or modified from an existing one.

Current and Planned Related Developments

There are no known development programs related to this feature, but the feature is quite simple, and it is expected that relatively little investment would be needed to implement it.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×
Key Milestones

The milestones for the Phase I development of this feature are the following:

  • Conduct a laboratory demonstration of a substrate-roughening method that can produce roughened areas of different shapes.

  • Develop test features incorporating tactile variance.

  • Carry out experiments to estimate the durability of a roughened substrate.

  • Conduct initial experiments to determine how intaglio images would be affected by a substrate with different degrees of roughness.

Development Schedule and Cost Estimate

It is expected that the Phase I, and indeed perhaps the total development, of this feature could be completed within 2 to 3 years.

Estimate of Production Cost
Compatibility with Current BEP Equipment and Processes

The roughening machine to create this feature would be used by and located at the supplier of the substrate. Thus, the effect on BEP operations would be minimal.

Incremental Production Cost

The cost for the roughening machine should be very modest, and it is expected there would be a low incremental cost.

Required Capital Equipment

The technology to create a roughening machine already exists. In fact, these machines themselves already exist. They would, however, need to be modified so that they are capable of creating the unique feature shapes on the FRN.

Further Reading

No additional reading is suggested.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×

THERMORESPONSIVE OPTICALLY VARIABLE DEVICES

Description

A thermoresponsive optically variable device (TOVD) provides an appearance that changes reversibly with temperature—for example, blue when cool and red when hot—over a temperature range near room temperature. Heating this type of responsive device (or cooling it, depending on its temperature) by touching it with a finger, for example, changes its appearance. In certain embodiments, such as those that involve liquid crystals, the colors can also change with viewing angle, and the properties of the reflected or transmitted light can depend on polarization. Such devices would be difficult or impossible to reproduce with conventional scanner or printer technologies. They provide additional functionality—that is, responsiveness—and are challenging to simulate or duplicate, compared with conventional OVI features or DOVDs. As such, they provide enhanced security benefits. Their highly visible, reversible, and optically variable appearance and the polarized nature of the reflected or transmitted light provide a level of banknote feature functionality that is valuable for the general public.

One type of TOVD can be formed with thermotropic chiral liquid-crystal (LC) materials. The nematic phase of a chiral LC is known as the cholesteric phase, and is observed in LCs with chiral nature or in achiral LCs that have chiral additives. This phase consists of a helical arrangement of LC molecules, with a well-defined pitch. Circularly polarized light that strikes a layer of cholesteric LC is reflected when its wavelength is comparable to the distance associated with a full turn of the helix (that is, the pitch of the cholesteric phase). This effect causes the LC to appear brightly colored. The color depends on the viewing angle owing to the geometry of the Bragg effects that generate the reflected light. The reflected light also can be circularly polarized, providing additional benefits for security. The color varies with temperature owing to the temperature dependence of the helical pitch of LC molecules in the cholesteric phase. The chemical structure of the LC molecules determines the pitch and its dependence on temperature.

Figure C-13 illustrates the thermal effects in a typical system. A TOVD currency feature could consist of a uniform patch of this type of material, a printed image formed on top of such a patch, or an image formed directly with the thermoresponsive material. Separate fabrication of the feature followed by integration with the paper represents a path to insertion into currency. TOVDs based on liquid crystals are used in thermometers, mood rings, car paints, and battery testers. They are also being explored for use in low-cost systems, such as “re-printable” paper. These devices can be used in security applications, but the committee is unaware of any widespread use for such purposes. The main challenges for currency applications are durability and cost.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×

FIGURE C-13 Images showing the thermal response of a cholesteric liquid crystal. At room temperature, the device is black (upper left in the left frame). Heating by a hand causes changes in the observed color (right frame). SOURCE: Adapted from the University of Wisconsin-Madison IPSE Liquid Crystal Activity Guide <http://mrsec.wisc.edu/Edetc/supplies/ActivityGuides/LC_Activity_Guide_Expo.pdf>, Materials Research Science and Engineering Center on Nanostructured Interfaces, University of Wisconsin-Madison.

Another route to TOVDs uses thermoresponsive inks known as leucodyes. Although these materials are much better explored for security applications than are liquid-crystal-based devices, they have the disadvantage that they do not provide unique viewing-angle and polarization-dependent properties. They are, however, more fully developed for low-cost implementations and are used widely in packaging applications. Durability represents the main challenge for currency applications.

Feature Motivation

The committee considered the concept of a TOVD feature to be valuable owing to its easily identifiable, unique, and highly visible responsive functionality, suitable for use by the general public even with little education provided on the nature of the feature. It is also usable by high-speed machine readers. A TOVD feature would provide, however, limited forensic functionality.

As described above, heating or cooling a TOVD feature by touching it with a finger, for example, changes its appearance. Such a characteristic would be difficult to reproduce with conventional scanner or printer technologies. Implementations with cholesteric liquid crystals offer the widest diversity of visual indicators, including viewing-angle-dependent appearance and polarization-selective operation. Leucodyes, however, are more fully developed for low-cost applications. In both cases, robustness for currency applications must be demonstrated.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×
Materials and Manufacturing Technology Options

A TOVD requires a thermoresponsive material (for example, thermotropic chiral LCs or leucodyes), a suitable substrate, and a top layer to seal the system. The manufacturing processes for cholesteric and leucodye devices are well established. Specially designed materials, especially in the case of the LCs, could provide unique colors and temperature responses. Such a device could be formed into an element, such as a security strip, and then integrated with the paper prior to printing. The ability of such a structure to withstand the direct pressure of the intaglio process is unknown. It might be possible, however, to adjust the position to avoid direct contact during printing or to integrate the TOVD after printing.

The designs would need to take into account a wide range of operating temperatures—for use, for example, in regions from Alaska to Arizona. However, the functioning of the devices—that is, their responsiveness—only requires changes in temperature due to contact with a finger or other device. As a result, the ambient temperature does not limit the device’s operation except in the special case that the temperature of the note is the same as body temperature, as in the case of inspection of the feature with a finger. To expand the complexity of the functioning of a TOVD and to address the variable ambient temperatures, it would be possible to construct a feature that consisted of a composite array of different features, each of which responds in different temperature ranges.

Simulation Strategies

For liquid-crystal-based devices, simulation might be possible at a crude level, by cutting and pasting off-the-shelf devices obtained from battery testers, decorative items, thermometers, clothing, and so on. TOVD features that involve printed patterns, or their integration with other printed images, would make this sort of simulation strategy difficult. Also, the colors and temperature responses of TOVDs used in currency could be custom-designed to differ in identifiable ways from commercially available devices. Nevertheless, it is reasonable to expect that crude simulations could be generated by petty criminals and that acceptable simulations could be produced by professional criminals. Duplication of TOVDs, while more difficult than simulation, is within the range of capabilities of a state-sponsored organization. Devices based on leucodyes might be easier to simulate, owing to the wider availability of the inks as well as the feasibility of directly printing patterns of leucodyes without concern for careful control of thickness or molecular alignment, which are necessary in the case of liquid crystals. A path to simulation of a liquid-crystal device could use, in fact, combinations of leucodyes and conventional pigments to simulate the color changes. The viewing angle and polarization-dependent behavior would, however, be difficult to capture using this approach.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×
Key Development Risks and Issues
Durability

The durability of TOVDs is sufficient for use in a range of existing applications, as noted above, but their durability, at the level required for currency, is unproven. The degradation modes would be a function mainly of the device packaging, particularly in the case of the liquid-crystal systems, since the materials themselves are known to be very stable—liquid-crystal display applications provide an example. The leucodyes can be degraded by prolonged exposure to ultraviolet light. Suitable packages (for example, ultraviolet-absorbing encapsulation layers) would need to be developed to avoid these sorts of limitations.

Aesthetics

A responsive feature, such as a TOVD, could, if properly incorporated, enhance the aesthetics and appeal of U.S. currency.

Social Acceptability

There are no known environmental hazards or public concerns with respect to TOVDs.

Key Technical Challenges

A key challenge would be the development of low-cost, high-volume manufacturing approaches for TOVD production. The extremely high levels of reliability and durability demanded by currency applications—including in this instance the range of ambient temperatures in which a thermal device would have to operate effectively—represent the main difficulty. Tests must be performed to assess the durability of existing devices for use in currency. The outcome of such tests can provide guidance on the development of suitable packaging systems. Methods to reduce the cost of the liquid-crystal-based devices, in particular, appear necessary.

Phase I Development Plan
Maturity of the Technology

The existing TOVD devices in the applications noted above suggest that the technology is mature for applications similar to but with less stringent operational demands than those in currency. These devices have not been demonstrated to

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×

achieve the necessary durability and reliability for currency applications, to the committee’s knowledge.

Current and Planned Related Developments

There are many development efforts in liquid crystals generally, and in cholesteric liquid crystals in particular, to support existing product applications and to develop new systems, such as bistable reflective displays and “re-printable” paper, that use these materials. Additional efforts are required, however, to address the technical needs of the currency application, and in particular the cost.

Key Milestones

The key milestones to the development of a TOVD feature are the following:

  • Conduct initial durability tests of several different TOVD features.

  • Develop a reasonable approach to achieving cost-effective integration of TOVD features in the currency substrate.

Development Schedule

The committee estimates the time for completion of Phase I of development for a TOVD feature to be within 2 to 3 years. Successful systems-level tests would require, primarily, adequate packaging of the feature and cost-effective manufacturing approaches. The key assumption is that durability of a TOVD can be achieved by suitable packaging and integration approaches.

Estimate of Production Cost
Compatibility with Current BEP Equipment and Processes

The cost of a liquid-crystal-based TOVD is expected to be relatively high compared with that of other nonresponsive complex features such as diffractive optical devices. Integration of a liquid-crystal TOVD would occur through a strip bonded to or woven into the paper. TOVDs that use microencapsulated leucodyes might be printed directly. The layout of the printed parts of the currency might need to be designed to avoid high-pressure contact with the TOVD associated with the printing. Alternatively, development efforts could be directed to yield TOVDs that are compatible with these pressures.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×
Incremental Production Cost

The cost of a liquid-crystal TOVD would likely be in the medium to high range. Other approaches, such as those based on leucodyes, would be lower in cost.

Required Capital Equipment

Capital equipment used for existing TOVDs could be implemented directly, or in some variants, for currency applications. Liquid-crystal TOVDs would most easily be integrated through a security strip or on a plastic substrate that is integrated with the paper note, similar to a diffractive optical device. Leucodyes could be printed directly, although studies would be needed to determine whether existing BEP printers could be used for this purpose.

Further Reading

For more information on color-changing inks, see <http://www.screenweb.com/inks/cont/brighten981119.html>. Accessed February 2007.

Bahadur, B. 1998. Liquid Crystal Applications and Uses, Vols. 1-3. Singapore: World Scientific.

Broan, L., and C.L. Saluja. 1978. The use of cholesteric liquid crystals for surface temperature visualization of film cooling processes. Journal of Physics E: Scientific Instrumentation 11: 1068-1072.

Ireland, P.T., and T.V. Jones. 2000. Liquid crystal measurements of heat transfer and surface shear stress. Measurement Science and Technology 11: 969-986.

Parker, R. 1988. Flexible Resistive Heat Battery Tester and Holder. U.S. Patent 4726661.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×

WINDOW

Description

The principle of the window feature is the inclusion of a denomination-specific window, possibly integrated into the substrate of U.S. banknotes. The window could be shaped differently on different notes, or different notes could simply have windows of different sizes—in particular it is suggested that the lower-denomination notes have larger windows, thereby deterring their use for counterfeiting larger-denomination notes that would have smaller windows. Alternatively, higher-denomination notes could have no window.

Feature Motivation

The motivation for the window feature is its effectiveness in deterring the use of lower-denomination notes as sources of currency substrate for the counterfeiting of larger-denomination notes by the inclusion of a denomination-specific window or hole in the note. This feature is intended for unassisted use by the general public—that is, a large-denomination note would have a window, or the wrong window, if it was a counterfeit manufactured by the “washing” of a lower-denomination note.

In particular, therefore, the feature is proposed as a specific deterrent for the opportunist and petty criminal classes of counterfeiter. Also, the committee believes that the careful design of the window could add to this feature’s value as a denominating aid for the blind.

The major risk with this feature is that robust embedding of a plastic window into the paper substrate might be difficult. This feature is expected to be cost-effective once the manufacturing challenge is solved, since the cost of the mass-produced plastic windows is expected to be less than that of the security thread. While clear plastic windows are in use in banknotes with plastic substrates—such as those in Australia and Mexico—the committee is not aware of integrated plastic windows in paper notes.

Materials and Manufacturing Technology Options

Substrate-integration technology similar to that needed to implement windows is also needed for the Fresnel lens, hybrid diffractive optically variable device, microoptic array, and subwavelength optical element features discussed earlier in this appendix. In order to produce this feature so that it is challenging to the counterfeiter, the window would have to be integrated into the substrate. This would require a change in the substrate manufacturing process, perhaps as a variation of the process used to integrate the current security strip.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×
Simulation Strategies

The windows could be simulated and duplicated by professional criminal and state-sponsored counterfeiters in counterfeiter-produced substrates. However, simulation by a petty or opportunist counterfeiter would be challenging, as it would require the holes in the lower-denomination notes to be “filled in” in order to allow for the use of those notes by bleaching for instance, as a substrate for higher-denomination notes.

Key Development Risks and Issues
Durability

The positioning, size, and shape of the window would have to be investigated to minimize durability issues. The committee believes that given the remarkable strength of U.S. currency paper, the durability of the feature and the note has a high probability of being satisfactory.

Aesthetics

The feature will change the aesthetics of the U.S. FRN, but properly designed, the window should not affect the overall look and feel of the notes.

Social Acceptability

There should be no social acceptability issues involved with the implementation of the window feature.

Key Technical Challenges

The key challenge is the development of the window design and production process that are respectively durable and inexpensive.

Phase I Development Plan
Maturity of the Technology

The window feature is a low-technology feature that could be implemented in the short term once the durability and production cost issues were resolved.

Current and Planned Related Developments

The committee is unaware of any research specifically targeting this type of feature. The committee knows of no plastic films that have yet been integrated into

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×

a paper substrate, although there does not appear any compelling reason to think that this cannot be done.

Key Milestones

The key milestones for the Phase I development of this feature are the following:

  • Demonstrate that a plastic window can be effectively integrated into the substrate.

  • Resolve durability issues.

Development Schedule

The committee believes that this feature could be fully developed within 2 years.

Estimate of Production Costs
Compatibility with Current BEP Equipment and Processes

The window feature might require an additional processing step in the production at the BEP, depending on whether the holes are produced prior to printing (by the substrate manufacturer) or after printing (at the BEP).

Incremental Production Cost

The committee believes that this feature would not add significantly to the cost of FRN production.

Required Capital Equipment

The need for extra equipment at the BEP would depend on whether the holes were produced prior to printing (by the substrate manufacturer) or after printing (at the BEP).

Further Reading

No additional reading is suggested.

Suggested Citation:"Appendix C Intermediate-Term Feature Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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The rapid pace at which digital printing is advancing is posing a very serious challenge to the U.S. Department of the Treasury’s Bureau of Printing (BEP). The BEP needs to stay ahead of the evolving counterfeiting threats to U.S. currency. To help meet that challenge, A Path to the Next Generation of U.S. Banknotes provides an assessment of technologies and methods to produce designs that enhance the security of U.S. Federal Reserve notes (FRNs). This book presents the results of a systematic investigation of the trends in digital imaging and printing and how they enable emerging counterfeiting threats. It also provides the identification and analysis of new features of FRNs that could provide effective countermeasures to these threats and an overview of a requirements-driven development process that could be adapted to develop an advanced-generation currency.

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