Effect of light source size on discomfort glare thresholds

Abstract Purpose Bright light sources, such as car headlamps or residential street lights, can cause visual discomfort. The origins of discomfort glare (DG) or the factors that contribute to it remain poorly understood, even after 50 years of multidisciplinary research [American Journal of Ophthalmology, 153(4):587‐593(2012)]. This study examined the effect of the light‐source intensity, size, surround luminance and pupil size on DG thresholds. Methods The pupil was measured in real time to calculate retinal illuminance, a more pertinent parameter in studies of DG. DG thresholds were estimated with a staircase procedure: the retinal illuminance was increased or decreased based on whether the participant indicated the absence or presence of DG, respectively. The subjects were required to view a source of light presented against a simulated residential street background. The size of the disc source was selected randomly under computer control. Five different source sizes were used to produce a 40 fold variation in stimulus area. The light source behind the aperture disc was designed to produce uniform flashes of light of varying intensity, while the surround light level remained unchanged. Results It was found that at the threshold for DG, the retinal illuminance is approximately constant and independent of source size. On the other hand, pupil plane illuminance increases as source size increases. Conclusion These findings suggest that DG depends mostly on localised retinal illuminance, rather than the total amount of light entering the eye. DG therefore appears to be largely related to saturation of photoreceptor signals. The results also suggest that higher levels of street illumination can be achieved by increasing the size of light source without causing any increase in DG.