National Calibration Facility for Retroreflective Traffic Control Materials (2005)

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CHAPTER 1 INTRODUCTION AND OVERVIEW Presented in this Final Report for Project 05-16 “National Calibration Facility for Retroreflective Traffic Control Materials” are the details required to construct and validate the reference retroreflectometer. Chapter 1 contains the problem statement along with the research approach for solving this problem. The requirements for the reference retroreflectometer are presented based on the data collected from national and international standards and meeting with various people knowledgeable in the field of retroreflective measurements. The conclusion of Chapter 1 presents a general overview of the Center for High Accuracy Retroreflection Measurements. Chapters 2, 3 and 4 discuss the source, the goniometer and the detectors. Each section describes the operation and capabilities of the component along with the contributions to the final uncertainty budget. Chapter 5 gives an in depth description of how the reference retroreflectometer is aligned and absolutely calibrated for (α, β1, β2, ε) parameters. Additional components of uncertainty not addressed in chapters 2, 3, and 4 are discussed. Chapter 6 provides the overall uncertainty budgets for the calibration of retroreflective material. Uncertainty analysis for additional angular parameters that are important in other representations: Intrinsic, Application and Road Marking (ωs, γ, a, b, e, d) are discussed in the Appendix B. Chapter 7 describes the calibration procedure, which includes a statement of traceability to NIST. The role of accreditation through the National Voluntary Laboratory Accreditation Program (NVLAP) or the Measurement Assurance Program (MAP) is discussed. PROBLEM STATEMENT AND RESEARCH APPROACH Retroreflective traffic control devices are widely used for nighttime visibility and safety. Congress has directed the U.S. Department of Transportation to establish “a standard for a minimum level of retroreflectivity that must be maintained for pavement markings and signs which apply to all roads open to public travel.” Establishing a national standard for minimum levels of retroreflectivity will require accurate methods to measure retroreflectivity. Instruments are commercially available for measuring the retroreflectivity of signs and markings, and 5

documented standards establish procedures for such measurements. However, there can be significant variability among instruments measuring the same object, and the standards do not ensure accuracy of the instruments. There are currently no traceable methods in the United States to determine the accuracy of measurements, because national calibration standards for retroreflectivity do not exist. The primary mission of the National Institute of Standards and Technology (NIST) is to provide such national calibration standards in a variety of areas important to government or industry. Within NIST, the Optical Technology Division maintains standards and provides calibrations for measurements involving optical radiation. The measurement of retroreflectivity is essentially a measurement of the reflectance of materials under specified geometrical and spectral conditions. It therefore involves the areas of photometry and spectrophotometry within the Optical Technology Division. While the distinction between these two areas can overlap, in general photometry maintains the national scales for luminous intensity, illuminance and luminance, while spectrophotometry maintains the national scales for spectral reflectance and transmittance. NIST was previously involved in retroreflectivity with a reference instrument and a Measurement Assurance Program. However, retirement of essential personnel and lack of modernization of the reference instrument, particularly automation, ended this program. The range used by the reference instrument and some of its components were used in the new reference instrument. A workshop held in 1997 stressed the need for NIST to again become involved in retroreflectivity, and so NIST personnel began to participate in national and international organizations dealing with retroreflectivity, namely the American Society of Testing and Materials (ASTM) and the International Commission on Illumination (CIE). The objective of this project was two-fold. First, to develop a dedicated reference instrument for measuring retroreflective materials, and second, to develop a calibration program that provides traceability to the relevant national scales maintained by NIST. The expertise of the personnel in the specific area of retroreflectivity and in the broader areas of photometry and spectrophotometry, the existing facilities, and the mission of NIST, has assured that the objective was attained. The research detailed in this report was directed toward developing a reference instrument that provides the basis for a national calibration facility for retroreflectivity measurements. This research was divided into two phases. Phase I consisted of those tasks 6

necessary to design the reference instrument, while the instrument was built and characterized in Phase II. The specific tasks in Phase I was as follows. Task 1. Literature review. This review included all of the existing national and international calibration and measurement methods of, and specifications for, retroreflective traffic control materials, with particular attention to the material, geometric, and spectral requirement capabilities. The review included visits to facilities to assess current retroreflectivity measurements, such as the 3M Company in Minnesota and the Turner Fairbanks Highway Research Center in Virginia. Facilities at the British Standards Institute (BSI) in England and the Federal Institute for Materials Research and Testing (BAM) in Germany have been visited. The list of standards is presented in Appendix A. Task 2. Requirements. These requirements describe all of the parameters necessary to calibrate retroreflective traffic control materials based upon the literature review conducted in Task 1. These parameters include the materials and sizes of retroreflecting materials, the geometry of the entrance and observation angles, apertures, and the spectral conditions. Also, included in this task are the requirements necessary to measure and characterize fluorescent materials. Specifically, a bi-spectral measurement is not included in this research plan. Task 2 is included in the Chapter 1 Summary of Instrument Requirements. Task 3. Preliminary design. A preliminary design for the reference instrument was produced, based upon the requirements identified in Task 2. This design included all of the components needed for the source, goniometer, detector, and data acquisition and control systems, as well as a plan for characterizing the operation of the instrument to ensure that it meets the requirements. The preliminary design was presented at the Annual Meeting of the Council for Optical Radiation Measurements, meeting on May 6th – 8th, 2002, specifically at OP4 “Retroreflection,” and at the 16th Biennial Symposium on Visibility and Simulation hosted by the Transportation Research Board and the University of Iowa on June 2nd - 4th, 2002. The preliminary design was described in the Interim Report to NCHRP and published as NISTIR 6940, “National Calibration Facility for Retroreflective Traffic Control Materials – Phase I.” Task 4. Uncertainty analysis. Based upon the design in Task 3, the uncertainties in the components of the reference retroreflectometer were calculated. The overall uncertainty budget 7

for calibration of typical retroreflective traffic control materials was finalized in this report and is presented in Chapter 7. Task 5. Traceability to NIST. Possible mechanisms for providing traceability of calibrated standards to NIST were investigated. There is not now, nor will there be, the resources for calibrating many samples in a rapid, inexpensive manner. Therefore, several mechanisms were developed to provide traceability to NIST that is agreeable both to NIST and to the customers. They include accreditation of secondary laboratories through NVLAP, measurement assurance program sets, and guidelines for customer standards submitted for the calibration service. The possibility of generating standard reference materials is discussed in Chapter 7. Task 6. Interim report. An interim report was prepared documenting Tasks 1 to 5 and including a detailed work plan for Phase II. This report was submitted to NCHRP and published as NISTIR 6940, “National Calibration Facility for Retroreflective Traffic Control Materials – Phase I.” Task 7. Meet with the project panel. This meeting occurred July 18th, 2002 at the National Academy of Sciences in Washington DC. The construction and validation of the reference instrument occurred in Phase II. In general terms, the goals for the instrument were divided into two categories. First, the instrument satisfies all the current requirements detailed in relevant documentary standards for materials, geometrical, and spectral conditions. Second, the instrument can accommodate additional geometrical and spectral capabilities to increase its utility in the future as standards change and develop. The specific tasks in Phase II were as follows. Task 8. Final design. A final design of the instrument was prepared based upon the results from Phase I. Task 9. Construct instrument. The components were acquired and assembled to produce a reference instrument for measuring the retroreflectivity of traffic control materials based upon the design in Task 8. Task 10. Validate instrument. The performance of the instrument was fully characterized to ensure that it meets the requirements and uncertainties identified in Tasks 2 and 4. Task 11. Demonstrate. On October 27th, 2003 a demonstration was conducted at NIST for the Project Panel to show the capabilities and operation of the instrument. 8

Task 12. Calibration program. The details of the calibration service described in Task 5 are finalized and presented in Chapter 7. The calibration service will be available starting the fall of 2004. Task 13. Final report. A final report was prepared that documents the entire research effort in Tasks 1 to 12, in conformance with guidelines set forth by the NCHRP. SUMMARY OF INSTRUMENT REQUIREMENTS No one document specifies the necessary requirements for a reference retroreflectometer. Appendix A is a list of the national and international retroreflection standards collected. The requirements presented here are the minimum based on these retroreflection standards, related support standards, and interviews with knowledgeable sources. The source of the reference retroreflectometer should be a projector type capable of uniformly overfilling the specimen. The uniformity of illuminance should be within 5% of the mean value obtained normal to the source. For most applications the relative spectral power distribution of the source should be equal to CIE standard illuminant A (2856 K) with an uncertainty of 20 K. The source should also be capable of providing other important relative spectral power distributions such as the new HID lamps. The source illuminance should not vary more than 1 % over the course of the measurement and should emit unpolarized light. Included in the source design is the source aperture. The recommended aperture sizes are 3, 6, 10, and 20 arc minutes for signage and 20 x 10 arc minutes for road marking materials. The uniformity of the source at the aperture has to be sufficiently uniform so as not to add positional uncertainty to the centroid of the aperture. The goniometer of the reference retroreflectometer should be capable of movement in three axes, entrance angle component β1, entrance angle component β2 and rotation angle, ε. Figure 1 shows the CIE system for specifying and measuring retroreflectors. The uncertainty in setting β1 and β2 should be less than 0.1° and the resolution should be better than 0.02°. The uncertainty in setting ε should be less than 0.2° and the resolution should be better than 0.04°. The goniometer should be able to accommodate sign specimens 0.3 meters square and pavement markings panels 10 to 15 cm wide and 60 to 120 cm long. The goniometer has to be able to substitute the detection system and the retroreflector specimen easily. Also associated with the goniometer is the illumination distance, which is the distance between the center of the 9

goniometer and the source aperture. The illumination distance needs to be variable from 7.5 to 30 m and should have an uncertainty less than 0.01 m. The detection system is composed of the observation angle positioner and photometer head. The observation angle positioner is designed to support and separate the photometer head from the light source. The observation distance should be maintained equal to the illumination distance with an uncertainty less than 0.01 m. The observation angle, α, should be set with an uncertainty less than 0.002°. The recommended aperture sizes for the photometer head aperture are 3, 6, 10, and 20 arc minutes. The responsivity and range of the photometer head should be sufficient that readings of the light source and the test retroreflector have a resolution of at least 1 part in 50. The linearity of the detection system over the range of the measurement should be within 1%. Correction factors may be used to correct non-linearities. The relative spectral responsivity of the photometer head should match the CIE V(λ)-function with an f1’ tolerance of at most 3%. Spectral mismatch corrections may be applied to the V(λ)-function. The readings of the photometer head from a constant source should not vary more than 1%. The specifications for nighttime color measurements or retroreflected color are not well specified. The spectroradiometer requirements are that it has a very good linear response and its wavelength scale must be calibrated. INSTRUMENT OVERVIEW The capabilities of the Center for High Accuracy Retroreflection Measurements at NIST are briefly described in this section and in more detail in the following chapters. Figure 2 is a conceptual drawing that shows the position of the devices in the tunnel. The source and detection system are on a 1.52 by 3.65 m optic table and the goniometer is on a rail system. The source and detector systems are behind a shield to reduce scattered light. Also, to reduce scattered light, a 3 square meter light trap is positioned at the end of the rail system and baffles mount on the rail system and slide into appropriate positions or can be removed. The source is composed of a 100 W strip lamp that is imaged by an Abbe projector. The uncertainty of the source luminance varies less than ± 0.056 % (k=2) over the course of a day. Experimentally, once the lamp has stabilized, the intensity fluctuation is less than ± 0.025 % (A- type uncertainty). The correlated color temperature produced by the system is 2856 K ± 10 K (k=2). The uniformity of the illuminance at the source aperture was determined to be within ± 3 10

% of the mean value. The uniformity of the illuminance at the retroreflector aperture surface was determined to be within ± 1.8 % of the mean value. The overall expanded uncertainty to the measurement of RL due to the source system is 0.33 % (k=2). A second system consists of a 5 cm diameter sphere made from Zenithpolymer* pumped by the light from four 410 W 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 provided similar characteristics to the strip lamp system with more operating complications. The real advantage of the sphere projection system is that any light can be coupled into the sphere without changing any of the projection 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 results, the sphere projection system is the best option. The goniometer of the reference retroreflectometer is mounted on a rail system. The illumination distance is variable from 5 to 35 m and has an absolute uncertainty of 0.005 m (k=2). The pitch and yaw axes have an absolute uncertainty of 0.02° (k=2) and both axes have a range of ± 95°. The rotation axis, ε, has an absolute uncertainty of 0.36° (k=2). The largest retroreflective device the goniometer can accommodate is a device 95 cm in diameter, and 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 precision register. The mounting bracket has an adjustable depth to accommodate different sample thicknesses. The detector package can also be mounted to the sample plate, to measure the illuminance at the sample plane. In addition to the three automated rotation axes, the goniometer is able to translate along three axes using stepping motors providing an accuracy of 0.25 mm along the illumination axis and 0.05 mm perpendicular to the illumination 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 is comprised of a 2 m translation stage, a rotation stage and a 0.2 m translation stage. Each of these motions has an optical encoder to ensure accuracy. The absolute uncertainty of the entrance angle, α, is 0.0004° * Certain commercial equipment, instruments, or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose. 11

(k=2). The observation distance is maintained equal to the illumination distance to an absolute uncertainty of 0.005 m. The observer apertures range from 3 to 20 arc minutes. An f1’= 1.6% was determined for our complete optical detection system. 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 luminance where the signal-to-noise does not dominated 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. 12