A study of mechanisms for discomfort glare

The presence of a bright light source in the visual field, particularly when viewed against a dark background, can generate a form of discomfort, which is often described as ‘discomfort glare’. The mechanisms for discomfort glare remain poorly understood, even after 50 years of multidisciplinary research in this field. The aim of this investigation was to investigate a number of relevant parameters that can affect discomfort glare in order to gain insights into the corresponding mechanisms. We measured retinal illuminance levels for discomfort glare at threshold as a function of source size, eccentricity and surrounding background luminance. In addition, the pupil size was measured throughout and related to the measured thresholds for discomfort glare. A group of 50 subjects with normal visual acuity and no clinical signs of eye disease took part in the primary study that measured discomfort glare thresholds as a function of source size. A light ‘homogenizer’ was used to integrate the concentrated light output from a quad LED light source. Pulse frequency modulation was used to control the intensity of the source and continuous pupil size measurements made it possible to calculate retinal illuminance. Discomfort glare thresholds were estimated by measuring the retinal illuminance of the glare source at threshold using a staircase procedure. Discomfort glare thresholds were measured as a function of glare source area, eccentricity and background luminance. The amplitude of pupil constriction was also measured both below and above the discomfort glare threshold. A model of contrast vision with the filtering of a photoreceptor signal through centre-surround ganglion cells was developed to account for the small size dependence of discomfort glare thresholds that was observed experimentally. Another model for scattered light was applied to compute the corresponding pupil constriction amplitude caused by the integrated photoreceptor signals generated by the glare source both within and outside the stimulus area. The threshold for discomfort glare decreased gradually with glare source size and increased with background luminance and showed little dependence on glare source eccentricity. The effect of forward light scatter in the eye was also investigated and a model was developed to account for the continued increase in pupil response amplitude well above the discomfort glare threshold. The effect of glare source size on discomfort glare thresholds could be predicted by a model involving photoreceptor saturation and edge response. When the scattered light outside the stimulus area was also taken into account, the pupil constriction amplitude increased log-linearly with stimulus retinal illuminance both below and above discomfort glare thresholds. These findings suggest that discomfort glare depended largely on the localised retinal illuminance and could be accounted for by the saturation of photoreceptor signals in the retina. The results and the pupil modeling work also suggest that the pupil response to light flux increments continued well above the discomfort glare threshold, largely as a result of light scattered outside the area of the glare source.

Name dc:type.qualificationname

phd

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

City, University of London

Year dc:date.issued

2014

Author dc:creator

• Jia, Y.

• RE Ophthalmology