National Calibration Facility for Retroreflective Traffic Control Materials (2005)

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SUMMARY Retroreflective traffic control devices are widely used for nighttime visibility and safety. Congress has directed the United States 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. Before this project there were no traceable methods in the United States to determine the accuracy of measurements, because national calibration standards for retroreflectivity did 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. 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 Center for High Accuracy Retroreflection Measurements at NIST is composed of three components, the source, the goniometer, and the detector, and how these components are absolutely aligned. Each component is briefly described. The source is composed of a 100 W strip lamp that is imaged by an Abbe projector. The source luminance varies less than ± 0.056 % (k=2) over a day due to the current setting. Experimentally, once the lamp has stabilized, the intensity fluctuation is less than ± 0.025 %. 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 % 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 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 1

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 will have an absolute uncertainty of 0.005 m (k=2). The pitch and yaw axes have an absolute expanded uncertainty of 0.02° (k=2) and both axes have a range of ± 95°. The rotation axis, ε, has an absolute expanded 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 expanded uncertainty of the entrance angle, α, is 0.0002° (k=2). The observation distance is maintained equal to the illumination distance to an absolute expanded uncertainty of 0.005 m (k=2). The observer apertures will 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. The reference retroreflectometer has been analyzed and characterized for over forty different aspects 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 % 2

(k=2). The uncertainty will be somewhat higher, up to 2 % (k=2) for microprismatic and/or colored materials. Typical white or yellow pavement marking material will have a relative expanded uncertainty of 2 % (k=2). Traceability requires the establishment of an unbroken chain of comparisons to stated reference artifacts or materials. NIST assures the traceability of results of measurements or values of standards that NIST itself provides or calibrates. Other organizations are responsible for establishing the traceability of their own results or values to those of NIST. The role of this project was to develop a calibration program to provide the stated references. NIST also assists in helping laboratories establish traceability of their measurements through a variety of support mechanisms, which include round robin comparisons, workshops, the Measurement Assurance Program, and the National Voluntary Laboratory Accreditation Program. 3

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