National Academies Press: OpenBook

New Facility for Calibrating Retroreflectometers (2005)

Chapter: Report Contents

Page 1
Suggested Citation:"Report Contents." National Academies of Sciences, Engineering, and Medicine. 2005. New Facility for Calibrating Retroreflectometers. Washington, DC: The National Academies Press. doi: 10.17226/23311.
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Page 1
Page 2
Suggested Citation:"Report Contents." National Academies of Sciences, Engineering, and Medicine. 2005. New Facility for Calibrating Retroreflectometers. Washington, DC: The National Academies Press. doi: 10.17226/23311.
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Page 2
Page 3
Suggested Citation:"Report Contents." National Academies of Sciences, Engineering, and Medicine. 2005. New Facility for Calibrating Retroreflectometers. Washington, DC: The National Academies Press. doi: 10.17226/23311.
×
Page 3
Page 4
Suggested Citation:"Report Contents." National Academies of Sciences, Engineering, and Medicine. 2005. New Facility for Calibrating Retroreflectometers. Washington, DC: The National Academies Press. doi: 10.17226/23311.
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Page 4

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reflector lamps. The exit port is imaged by an Abbe projection system similar to the system used in the strip lamp system. The sphere system has similar characteristics to the strip lamp system with more op- erating complications. The real advantage of the sphere projection system is that any light can be cou- pled into the sphere without changing any of the pro- jection optics. The high intensity discharge (HID) lamps that are available in cars have a very distinct spectral pattern. The retroreflectance of devices can be calculated if spectral coefficients of retroreflection are measured, but, to experimentally verify the re- sults, the sphere projection system is the better option. The goniometer of the reference retroreflec- tometer is mounted on a rail system. The illumina- tion distance is variable from 5 to 35 m and will have an absolute uncertainty of 0.005 m (k = 2). The largest retroreflective device the goniometer can ac- commodate is a device 95 cm in diameter. It has a clear view to allow almost any length of pavement marking. The sample mounting plate uses vacuum cups to hold the retroreflective devices against a pre- cision register. The mounting bracket has an ad- justable depth to accommodate different sample thicknesses. The detector package can also be mounted to the sample plate to measure the illumi- nance at the sample plane. In addition to the three automated rotation axes, the goniometer is able to translate along three axes using stepping motors pro- viding an accuracy of 0.25 mm along the illumina- tion axis and 0.05 mm perpendicular to the illumi- nation axis. These translations are for research purposes, such as studying uniformity of the source and the sample. The detector is supported by the observation angle positioner, which comprises a 2-m translation stage, a rotation stage and a 0.2-m translation stage. Each of these motions has an optical encoder to en- sure accuracy. The observer apertures will range from 3 to 20 arc minutes. With this detector system and source, the detection limit (signal-to-noise of 2:1) for the NIST reference retroreflectometer is 0.17 mcd/m2/lx. A calibration limit can be defined as the magnitude of the coefficient of retroreflected lumi- nance where the signal-to-noise ratio does not dom- inate the uncertainty budget (typically 1000:1, in this case 500:1); therefore, the calibration limit for the NIST reference retroreflectometer is 42 mcd/m2/lx. The reference retroreflectometer has been ana- lyzed and characterized for over forty different as- pects that are components in the overall uncertainty budget for the calibration of retroreflective material, including signage and pavement marking material. The typical calibration of a white encapsulated bead retroreflective sheeting material for coefficient of retroreflected luminous intensity is expected to have a relative expanded uncertainty of 1% (k = 2). The uncertainty will be somewhat higher, up to 2% (k = 2) for microprismatic and/or colored materials. Typ- ical white or yellow pavement marking material will have a relative expanded uncertainty of 2% (k = 2). CALIBRATION SERVICES AND TRACEABILITY TO NIST The mission of NIST is to develop and promote measurement, standards, and technology to enhance productivity, facilitate trade, and improve the qual- ity of life. To help meet the measurement and stan- dards needs of retroreflective materials manufactur- ers and users, NIST provides calibrations, test methods, proficiency evaluation materials, measure- ment quality assurance programs, and laboratory ac- creditation services that assist a customer in estab- lishing traceability of results of measurements or values of standards. Traceability requires the establishment of an un- broken chain of comparisons to stated reference ar- tifacts or materials. NIST assures the traceability of results of measurements or values of standards that NIST itself provides or calibrates. Other organiza- tions are responsible for establishing the traceability of their own results or values to those of NIST or other stated references. The role of this project was to develop a calibration program to provide the stated references. The specific calibration service procedures at NIST are expanded upon in a series of documents la- beled Special Publications. A Special Publication in the NIST 250 series is to be published in the spring of 2005 on the calibration of retroreflective material. The publication provides a general description and then describes the scale realization, appropriate arti- facts for calibration, equipment for calibration, calibration procedures, and uncertainty budget of the calibration. The cost of the calibration service changes with time depending on factors determined by the NIST management. All of these factors are based on the number of hours required to perform the calibration. 2

A time budget for the calibration of retroreflective samples has been prepared. With the basic automa- tion of the equipment and the complete uncertainty analysis, an estimate of the time required for cali- bration has been determined: Number of hours = [n * (m + 0.3) + 2] where n is the number of test samples and m is the number of angle combinations per sample. NIST also assists in helping laboratories establish traceability of their measurements through a variety of support mechanisms that include round robin com- parisons, workshops, the Measurement Assurance Program (MAP), and the National Voluntary Labora- tory Accreditation Program (NVLAP). The MAP and NVLAP programs are described in more detail in the final report for Project 05-16. FACILITY CONTACT C. Cameron Miller National Institute of Standards and Technology Optical Technology Division 100 Bureau Drive Gaithersburg, MD 20899 301/975-4713 c.miller@nist.gov REPORT AVAILABILITY The complete report for NCHRP Project 05-16 is available on TRB’s website as NCHRP Web-Only Document 72. ACKNOWLEDGMENTS This study was conducted under NCHRP Proj- ect 05-16, “National Calibration Facility for Retroreflective Traffic Control Materials.” It was guided by NCHRP Project Panel 05-16, chaired by Mr. John B. Arens, with members Mr. Arthur H. Breneman, Dr. Paul J. Carlson, Ms. Wendy Ealding, Mr. Mitch Gipson, Ms. Kelly Morse, Mr. Wallace C. Richardson, Mr. Gilbert Rushton, Mr. Carl An- dersen, and Mr. Frank N. Lisle. 3

Transportation Research Board 500 Fifth Street, NW Washington, DC 20001 These digests are issued in order to increase awareness of research results emanating from projects in the Cooperative Research Programs (CRP). Persons wanting to pursue the project subject matter in greater depth should contact the CRP Staff, Transportation Research Board of the National Academies, 500 Fifth Street, NW, Washington, DC 20001.

Transportation Research Board 500 Fifth Street, NW Washington, DC 20001 These digests are issued in order to increase awareness of research results emanating from projects in the Cooperative Research Programs (CRP). Persons wanting to pursue the project subject matter in greater depth should contact the CRP Staff, Transportation Research Board of the National Academies, 500 Fifth Street, NW, Washington, DC 20001. Non-profit Org. U.S. Postage PAID Washington, DC Permit No. 8970

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TRB’s National Cooperative Highway Research Program (NCHRP) Research Results Digest 297: New Facility for Calibrating Retroreflectometers announces the results of NCHRP Project 05-16 (National Calibration Facility for Retroreflective Traffic Control Materials), which developed dedicated reference instrumentation to provide national calibration standards for retroreflectivity, thereby improving the accuracy of measurements made by other instruments.

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