Abstract
The state of adaptation of the observer's eyes determines the required luminance in the entrance zone of a tunnel for safe entry. Based on the physiology of the visual system, it can be shown that the essential foveal adaptation is composed of the luminance in a 2° field of fixation and the equivalent straylight luminance L Seq caused by the environmental field luminance: L A = L 20 + L Seq . This adaptation level can be followed by the eye due to neuronal or network adaptation after a change of luminance within 2 log units. A method is suggested to obtain L Seq using a special graph paper with zones in which the luminance produces equal amounts of straylight in the center. This graph superimposed on that of a tunnel scene allows L Seq to be read off easily. An experiment shows that the L 20 concept put forward by the CIE necessarily falls short and cannot serve to determine the adaptation luminance. A comparative study of 12 tunnel scenes reveals poor correlation between L 20 and L Seq.
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