The visibility level necessary for the performance of a visual task is defined relative to the visibility reference function. This function, as at present proposed for use by the CIE, is to be considered illustrative only at background luminances below 1 cd/m2. It may therefore not be representative of actual threshold values for visual tasks performed in this luminance region. Driving at night on unlighted roads with dipped beam is an example of such a task. It is shown that experimental data for this case indicate lower threshold values than those given by the visibility reference function.
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References
1.
Implementation procedures for evaluating visual performance aspects of lighting, preliminary fourth draft, CIE Technical Committee 3. 1 (1975).
2.
A unified framework of methods for evaluating visual performance aspects of lighting, CIE publication 19 (1972).
