Adaptation luminance when approaching a tunnel in daytime

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 LSeq caused by the environmental field luminance: LA = L20 + LSeq . 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 LSeq 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 LSeq to be read off easily. An experiment shows that the L20 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 L20 and LSeq.