In the new recommendations for roadway lighting, the visibility of objects is used to characterize the quality. The visibility level (VL) is based on the perceptability of a small target of 10 minutes of arc that is considered the critical size the driver has to see to ensure traffic safety. VL is defined as the ratio of the existing contrast of that target to its threshold of perception. A metric has been developed that allows to calculate the threshold contrast for foveal observation, this means the target is fixated. Outside the fovea the target contrast has to increase for perception. As from the literature data for peripheral perception are known only in the horizontal, the threshold values for targets in the lower half of the visual field had to be experimentally determined. The results have been modeled to allow for the calculation of the elevation of the target contrast for pheripheral perception over that at the foveal location. This ratio is identical with the visibility level VL. For VL = constant follow curves that enclose an area in which a target can be seen. It appears that VL has to be in the order of 10 to provide fields of visibility large enough for safe driving conditions. (A)
Abstract