Photometric measurements for visibility level computations

Abstract

Visibility level is a standard quality index in road lighting design. Three photometric values are needed to compute the visibility level of a target: its luminance, the luminance of its near background and the adaptation luminance. We discuss the consequences of how these parameters are set on the ability to predict the performance of a driver at target detection. An experiment was designed using a closed-road circuit. Sixteen targets were presented twice to 34 subjects. Six visibility level assessment methods were compared. The background luminance was set as the luminance at the bottom of the target, as the mean luminance around the target, or as the luminance associated with the maximum target contrast. The adaptation luminance was set either to the background luminance or to the mean luminance. The best non-linear fit between computed visibility level and target detection performance was found using the maximum contrast over the four sides of the square target, and setting the adaptation luminance to a unique estimated road luminance instead of the local background luminance. Even so, the variability of the visibility level data suggests great caution when using it as an index of road visibility.