Mechanisms for discomfort glare in central vision.

PURPOSE The presence of a bright light source in the visual field can generate visual discomfort. Based on empirical observations we can predict to a reasonable degree of accuracy how uncomfortable a given lighting installation is likely to be; yet very little is known about the mechanism or physiological underpinnings that lead to visual discomfort. This study attempts to elucidate some of the underlying mechanisms by controlling the amount of light reaching the retina and by varying photometric properties of the glare source. METHODS The participants were required to view a source of light presented against a simulated residential street background in the form of uniform flashes of light of varying intensity. Discomfort-glare thresholds were estimated using a staircase procedure; the dependent variable was retinal illuminance. The size of the glare source and the luminance of the surrounding background were varied systematically. RESULTS Across glare-source sizes or background luminances the discomfort-glare threshold varied less in terms of retinal illuminance than it did in terms of pupil-plane illuminance or light flux. A two-stage model based on saturation of photoreceptors followed by summation of an edge response signal that defines the edges of the glare-source accurately predicted the data. CONCLUSIONS Discomfort glare in central vision is more closely associated with the spatial properties of the glare source, such as contrast-defined edges, than the overall amount of light entering the eye. The results suggest that discomfort glare in lighting installations could be reduced while maintaining adequate illuminance levels by an appropriate choice of illuminant source size.