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Evaluating the appearance of retroreflectors involves the application of the basic principles of appearance measurement. It is typical of many appearance measurement problems in that it involves a source (or sources) of illumination, a viewer, and an object or surface of concern. The objective is a numerical description of the appearance properties of the material. For such descriptions, the measurement geometry must be established, the spectral conditions considered, and meaningful appearance related quantities agreed upon.
In the measurement technique described here, computers are used to control the instrumentation for geometric and spectral analysis while simultaneously calculating appearance related quantities, along an appropriate application parameter. A shoulder mounted retroreflective road sign is the example considered, and the application parameter is the distance from vehicle to sign. Local viewing and illuminating geometry are translated into a laboratory measurement coordinate system. These transformations are made at distances corresponding to the incremental motion of a vehicle along the roadway. From these application-oriented measurements, intrinsic photometric results are calculated and displayed in terms of quantities such as coefficient of retroreflection. Once the material characteristics are measured, along the application parameter, additional analysis using the actual illuminating source can be completed. In the case of the retroreflector, the luminance in the direction of the viewer is an important result. Evaluation based on practical viewing conditions provides a useful insight into appearance without the necessity of completely describing the spectraogoniophotometric properties of the retroreflector.
goniometers, retroreflectors, luminance, appearance, application parameter, coefficient of retroreflection, entrance angle, goniophotometry, observation angle, retroreflectance, retroreflection, rotation angle
Physics specialist, 3M Co., St. Paul, MN