Validity Range of Luminance and Luminous Intensity for a Right Characterization of Rear Lamps under Mesopic Vision Conditions

International automotive lighting standards like ECE or SAE are based on luminous intensity as a benchmark for brightness limitation. First field studies on taillights at the Technical University Darmstadt have shown that human brightness perception is probably independent of luminous intensity up to a distance of 100 metres between taillight and subject. In order to achieve a better brightness design of taillights in road traffic, there has to be developed a model of brightness perception as a function of distance between the signal lamp and the subject. For instance, this also would offer the possibility to develop a new generation of adaptive rear lamps. As a first step, it had to be clarified which photometric unit is relevant for the human brightness perception. For this purpose two test methods, a psychometric and a direct procedure, were used. In the psychometric tests, we used the manufacturing and the staircase method to get the optimal adjusted brightness level of a homogenous lamp as a function of distance. The distance steps were between 2 and 50 meters, simulated by using apertures in front of the signal lamp. This corresponds to a visual angle between 1,43° and 0,05°. As the signal’s shape we used a square and a circle to also determine the dependency of form on brightness perception. In the direct procedure the signal luminance was fixed but the signal’s size was changed in several steps so that the subjects had to decide if they see a difference in brightness. As a result of both methods we got a validity range of luminance and luminous intensity as well as an influence of the signal’s form on perception for a better characterization of rear lamp brightness