Effects of Target Positions on Purkinje Effect

The International Commission on Illumination (CIE) approved a compensated mesopic photometry system (MES2) to bridge between V (lambda) and V' (lambda). However, before the system is applied to real road lighting applications, several problems need to be solved. Among them, we investigated the effects of target positions on the Purkinje phenomenon and therefore on the current mesopic photometry system. We conducted two experiments by using a high-pressure sodium (HPS) lamp and three ceramic metal halide (CMH) lamps. The experimental results showed that there were small variations in the Purkinje effect depending on target positions. Such experimental results seemed to reflect retinal density distributions of cones and rods. Practically, however, mesopic luminances obtained from the CIE mesopic photometry system (MES2) were correlated to reaction times to targets presented at eccentricity angles between 5- and 15-degrees. The experimental results also suggested that a glare source appeared to exaggerate differences in reaction time for peripheral target detections between HPS and CMH illuminations, and therefore that the use of lamps with higher S/P ratios improved drivers' visual performance more than the mesopic photometry predicts.