Counterfeit Deterrent Features for the Next-Generation Currency Design (1993)

There are many technologies that are currently used in the non-impact printing industry. These will be briefly discussed in terms of their principle of operation, performance characteristics, strengths, limitations, and technical trends.

Electrophotography, the leading non-impact technology, is widely used in copiers and printers, both monochrome and color. Devices using this technology, along with input scanners, are common in the modern workplace (Schaffert, 1980). They are used to copy, print, and reproduce almost any printed document.

The differences between copiers and printers are merging quite rapidly since both employ the same basic electrophotographic process to reproduce either printed or electronic data. A copier has been an analog device (copiers that convert the image into digital format are beginning to appear) that uses light to create a latent image of an entire original document on a photosensitive plate. The latent image is developed with toner using one of several electrophotographic developer techniques and is then transferred and fixed to paper. A printer, on the other hand, is not a stand alone device; it must be connected to a computer system or network. It relays the image information, in digital form, to the photosensitive member by use of a controllable light source, such as a laser, light-emitting diodes, light valves, or some other method of addressing many points using light. (The digital image information may have been generated by a computer scanner, by use of a software program, or by both.) The latent image is developed using the same choices of developer technology as for a copier.

Various methods can be used to develop the latent image. The objective of the development system is the delivery of electrostatically charged toner particles that partially neutralize the charged image to produce the printed image that is transferred to the paper. The types of development systems include dual component, monocomponent, non-magnetic monocomponent, and liquid (Hayes, 1991). These terms describe the composition of the development system, which vary in complexity and size (Jaffe and Burland, 1988).

In order to be able to produce the higher print quality that is required today, the size and uniformity of the toner must be optimized. In 1989, toner size was in the 10—15 µm range and was produced by a milling process that resulted in a wide, non-uniform distribution of both shape and size. As true resolution increases from today's standard 300 dots per inch (dpi) to 600 dpi, 1 toner size must become smaller and more uniform. Today's toner can be in the 9–12µm size, or smaller, for dry powders and from 1–2µm for liquid toner. New methods of producing toner can provide a very uniform spherical shape, allowing much crisper printed edges. In order to achieve the best color reproduction, both the average size and size distribution of toner particles must be controlled. The technological aspects of toner production are in place to support higher-quality levels of color electrophotography.

When copying or printing in full color, sequential passes must be made to lay down the subtractive primary colors of cyan, magenta, yellow, and black. This need for four separate development systems resulted in a physically large print engine in early models. Smaller-sized developers, such as nonmagnetic monocomponent or liquid ones, have led to process simplifications, smaller units, and cheaper, and more durable machines.

Many companies have patented or licensed basic print engine architectures that will lead to a new breed of smaller, cheaper, color copiers/printers. There are many configurations that can accomplish the sequential printing of the three primary colors plus black. One approach exposes and develops each color separately on the photoconductor. After the first color is toned, it is transferred to an intermediate member so the photoconductor can be cleaned and imaged with the next color, which is transferred on top of the first, and so on until all (four-color separation planes) have been completed. The finished image is then transferred to paper for a final fusing step. This method requires that the relationship between planes be maintained so the resulting image will be well registered. Since the intermediate member can be controlled both in position and material, this can be accomplished with great accuracy in both dry and liquid systems.

Another method uses the paper itself as the intermediate member. This method has more difficulty with registration, since the dimensional stability of most grades of paper is not very good or repeatable. Also, paper is abrasive and tends to wear the photoconductor excessively.

A simple method of developing the four planes would require that the photoconductor hold all four layers until the final transfer. But this is difficult to do in practice, because most development systems use direct contact with the photoconductor to develop the latent image, and contamination will occur on subsequent passes, because residual toner particles will be left from the previous passes. However, if development does not require direct contact with the photoconductor, the contamination problem can be controlled. This is possible with monocomponent and nonmagnetic monocomponent systems (Konica Corporation, 1992). This configuration is also being considered using liquid toner. These developments can lead to even smaller and simpler full-color copiers/printers.

Ink-jet technology has great potential for color printing (Jaffe et al., 1978, 1981). This potential was unrealized for some time due to the complexity of this technology and the unreliability of the nozzle system. As ink-jet technology progressed, the complexity of print heads has decreased and the reliability increased. Today there are many ink jet products on the market. The technology most widely used now is thermal ink-jet2.

Ink-paper interactions are the key to controlling the quality of the print (Jaffe et al., 1979). The need for special paper to ensure dependable print quality has been an issue for ink-jet printers. Much of the work conducted in recent years has been aimed at overcoming this limitation. New ink formulations greatly improve the ink-paper interaction (and paper manufacturers are designing special papers for ink-jet printing) helping pave the way for ink jet printing to become a low-cost, highly capable technology for everyday color printing. The most important ink formulation developments are described below.

Water-based pigmented inks, in which the colorant is a normal printing ink pigment, have been patented (E.I. DuPont de Nemours and Company, 1998)3. They have stabilized emulsions that use water as a carrier fluid that can be readily jetted. The water is absorbed into the paper or evaporates, leaving the coloring agent on the paper surface.

A paper-insensitive approach uses an ink that is normally solid at room temperature but is jetted at elevated temperatures where it is liquid (Titerington and Jaeger, 1992). Another method uses solvent-based inks in a print head that incorporates piezoelectrically grooved channels; it can use an ink that is similar to regular printing.

Technology advancements looming on the horizon include the elimination of nozzles. Acoustic ink jet and electrostatic-pull ink jet accomplish this in different ways. Another advance has been the development of a cartridge containing both the ink and thermal drivers produced by photolithographic methods; these drives are so inexpensive that the cartridge is disposable.

In summary, ink-jet technology inherently is a less complex color system than technologies requiring sequential imaging of the same spot. Breakthroughs in ink technology have resulted in color ink-jet printers becoming available now for less than $1,000. Further improvements in cost and print quality are expected in the future.

Thermal transfer printing is one of the technologies that is currently popular for color printing. Two ways are used to transfer color from a ribbon to paper. One implementation uses a colored wax that melts at a low temperature on a mylar base, and which transfers to

paper when heated with a thermal print head. It requires a very smooth paper that can make intimate contact with the ribbon for good transfer properties. The other uses a dye diffusion process (sometimes erroneously called dye sublimation) that melts the ribbon base. Diffusion across the ribbon/paper boundary requires a special layer to trap the dye. The paper needed for this process looks very much like the stock on which photographs are printed. Because diffusion is difficult to localize, the transferred areas spread laterally, and sharp lines and text are difficult to reproduce. This limits the applications that can be covered using this technology.

Recently, work has been done to develop ribbons that transfer ink to plain paper. One approach is to change the rheology of the ink to be more flexible in transfer and able to conform to the morphology of plain paper (Abe and Kitamura, 1991). The other approach uses a coating that is applied to the paper before the color is transferred. This coating is on a ribbon, like the colored wax, and is applied with the same print head to the paper in the areas to be printed by the regular wax transfer ribbon. The printed areas are calculated by the printer. In this manner, each paper appears the same to the ribbon that follows. This method is already used in a commercially available printer.

Magnetic printed has not progressed in the same manner as some of the more popular nonimpact printing technologies. The color printing capabilities have been projected, but it is difficult to produce pure colors from a toner that requires large amounts of dark magnetic material. Research on color-masked magnetic material has shown that it is theoretically feasible but not probable to make a commercially successful product in the foreseeable future.

The area of electrostatic printing and presses is one that has taken a mature technology and applied it to new applications. Large-format color printing has served the engineering design market, but newer markets such as the graphic arts, outdoor advertising, and lithography for short runs have recently emerged (Hard Copy Observer III, 1993). Even though this technology is suited for color, most of the implementations have been in large-format machines (i.e., large-size and high-cost ones). These machines, which cost between $40,000 and $100,000, are probably not available for the casual user. They require maintenance and trained operators. Future progress could bring the technology to smaller and cheaper models, but with all the other technologies available for the lower end market, the committee does not think this is probable in the foreseeable future.

This field is advancing very rapidly with new products appearing monthly. As demand for digital input increases, more and more ways to get data into computer systems will be implemented. Scanners and digital cameras are becoming more popular as their prices fall. High-quality scanners for color are already available for under $2,000, and, as generally occurs with electronic equipment, the prices will continue to decline, and function will increase (Imaging Magazine, 1992)4. It is evident that if a printed document can be seen in reflected light, it can usually be digitized.

Once an image is digitized, the manipulation of its pixels can begin. The lack of uniform standards and the difficulty of sending color information across various devices lack of uniform standards, are being addressed by the industry. The advent of many different input and output color devices has emphasized the need for software to manipulate the large amount of digital data dictated by color images. This field is rapidly growing, as evidenced by a conference covering only the data-manipulation segment of color transfer has been planned 5.

There are software programs that match colors, manipulate every pixel, and prepare color output for printing. Appropriate manipulation of the image can significantly improve the printed image. For example, each section can be optimally separated. Fine lines can be sharpened independently from the balancing of halftone areas. After the manipulation is complete, storage on one of the many available storage media allows the image to be available for reuse.

Digital technology has been leading advances that allow easier and cheaper access to color printing in the conventional offset printing industry, just as it has done in office and home printing. The digital nature of this technology allows data from many input sources, such as scanners and computers, to be used. Some equipment manufacturers are now employing digital presses to receive digital data that can directly generate a plate for printing6. This capability

has dramatically decreased the time and cost of short-run color printing to the extent that it is economic to print in color in runs as small as 20 to 1,000 copies.

Recently, advanced color presses have been announced by several European-based companies. These products have the advantage of flexibility, and no plates or films as intermediates7. Although these machines cost more than $200,000 each, the development of the market for short-run color will insure faster and more competitive machines for the future.

The past has relied on standard photographic methods to produce input separations for printing work. But current technology has moved so rapidly that digital camera technology is now good enough for prepress work. The advantages of digital photography are many. Its versatility and extendibility will allow its importance to expand in the future. For example, the possible dynamic range is even wider than for conventional photography. The rapid turnaround that is inherent in this digital process gives a new perspective for the designing and publishing business. The charged coupled device (CCD) camera technology, which uses the same technology as flatbed scanner, is relatively mature, and new products are being introduced rapidly. This coupled with Photo Compact Disk (CD) technology, allows images to become part of any computer-based system8. It is expected to become a widespread source of available images for printing.

Abe, T., and S. Kitamura. 1991. Relation between dynamic characteristics of thermo-fusible ink and print quality in thermal transfer printing. Journal of Imaging Technology17:119–122.

Color Business Report. 1993. New products for Xeikon and Indigo re-define high speed color electrophotography . Color Business Report3(7):1.

E.I. DuPont de Nemours and Company. 1998. Aqueous Pigmented Inks for Ink Jet Printers. European Patent Application 0, 518, 225.

Etchells, R. D.1993. Digital photography gets serious. Color Publishing3(4):19–24.

Hard Copy Observer III. 1993. QHS beats rivals to punch with first office color laser. C. Le Cohpte, Ed., No. 6 (June) p. 1.

Hard Copy Observer. Indigo and Xeikon roll out fist high speed digital color presses. C. Le Conpte, Ed., No. 7, P.10.

Hayes, D. A.1991. The evolution of color xerographic development systems. Journal of Imaging Technology17:252–258.

Hewlett Packard Company. 1991. Method for Enhancing the Uniformity and Consistency of Dot Formation Produced by Color Ink Jet Printing. U.S. Patent 4,999,646. March 12, 1991.

Imaging Magazine. 1992. High speed scanner roundup and color scanner roundup. M. Neilson, Ed., Volume 9, pp. 39-630.

Jaffe, A. B., and D. Burland. 1988. Electrophotographic printing. Pp. 221-260 in. Output Hardcopy Devices. Boston, Mass.: Academic Press.

Jaffe, A. B., W. Crooks, and T. Niewegha. 1979. Materials parameter affecting the quality of color printing. Second International Conference on Business Graphics, Washington, D.C. Springfield, Va.: SPSE,

Jaffe, A. B., E. W. Luttman, and W. Crooks. 1981. High quality color printing with continuous “ink jet.” in The First International Congress on Non-Impact Printing Technologies, Venice, Italy. Springfield, Va., SPSE.

Konica Corporation. 1992. Color Image Forming Apparatus. U.S. Patent 5,162,821. November 10, 1992.

Schaffert, R.1980. Electrophotography. New York, New York Focal Press.

Stoy, J.1993. Photo CD: A practical guide. Color Publishing3(4):10.

Titerington, D., and C. W. Jaeger. 1992. Design parameters for a phase change ink jet ink. Pg. 298in Proceedings, Society for Imaging Science and Technology, Eighth International Conference on Advances in Non-Impact Printing. Springfield, Va,: IS&T.

U.S. Patent 5,115,277. Electrostatically Assisted Transfer Roller and Method for Directly Transferring Liquid Toner to a Print Medium. Hewlett-Packard Co. Camis Thomas (US). May 19, 1992.

U.S. Patent 4,879,568. Droplet Deposition Apparatus. AM International. Bartky W. Scott(US); Michaelis A. John(US); Paton Anthony D.(GB). November 7, 1989.