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Photometric measurement of retroreflection: a new calibration procedure

The problems involved in photometry of retroreflectors are considered: for example, low light levels, heterochromatic photometry, and critical effect of geometric parameters. In order to simulate practical use conditions the visibility of the retroreflector depends on the illumination received in normal driving situations at various distances and incidence angles from the headlamps of the following car and on the luminous itensity retroreflected in the direction of observation. The test apparatus consists of precise realisation of geometric parameters, adequate photometric components and practical calibration procedure with the highest accuracy. A new calibration technique, used at IEN, is here suggested. This requires one uncalibrated reference light source and two uncalibrated photo-detectors instead of calibrated light sources or calibrated photo-detectors as conventional procedures need.