Disability and discomfort are two commonly experienced forms of glare associated with conventional lighting,
1 experienced when exposed to a visual scene of extremely non-uniform illumination. Discomfort glare is the glare that causes discomfort without necessarily impairing the vision of objects; disability glare is that which impairs the vision of objects without necessarily causing discomfort.
2 Although disability glare, caused by scattered light forming a luminous veil over the retina, is reasonably well understood, there is insufficient understanding of the cause of discomfort glare.
1
In numerous studies,
3–12 discomfort glare was measured using psychophysical procedures such as category rating, and from these data metrics have been developed to predict the degree of discomfort glare, including visual comfort probability,
13 discomfort glare rating, and unified glare rating.
14 Quantitative subjective measurements such as the category rating scales used to evaluate glare are prone to many forms of bias.
15 Involuntary physiological responses such as pupil dilation provide an alternative, objective evaluation of the discomfort glare and may help to understand the cause.
In two studies, pupil diameter was investigated alongside ratings of discomfort. Hopkinson
16 concluded there was no relationship between pupil diameter and the degree of discomfort from glare. In that experiment using a constant background luminance, the glare luminance was slowly increased to the point at which it was considered just perceptible, at which point pupil size was determined using flash photography. Following 5 minutes of adaptation, the glare luminance was increased to find the points at which glare was considered just acceptable, just uncomfortable, just intolerable, and definitely intolerable. Hopkinson
16 reported that pupil diameter varied “hardly at all,” despite the variation in discomfort from just perceptible to definitely intolerable. However, this may be due to stimulus range bias
15 or to errors in the method of pupil size measurement rather than lack of relationship between discomfort glare and pupil size. Using a more precise methodology, Stringham et al.
17 examined discomfort glare under a limited range of conditions (two background luminances and one glare luminance), with discomfort evaluated using a 10-point scale (ranging from not noticeable glare to unbearable glare) and monitored pupil diameter using infra-red photography. They found that pupil diameter reduced as glare become more unbearable (correlation coefficient for bivariate analysis [
r] = −0.429,
P = 0.037): the smaller the pupil size, the higher the discomfort rating, which is somewhat paradoxical as less light is reaching the retina. To extend this background, we needed further confirmation of the change in pupil size with discomfort.
In previous studies, the parameter recorded was absolute pupil size.
16,17 However, absolute pupil size depends also on the luminous condition to which the eye is adapted.
16 In other words, pupil size may be jointly affected by background illumination condition and glare source. Investigating how the pupil constricts when it is exposed to glare in comparison to when it is adapted to background illumination may characterize discomfort glare more accurately.
A reaction to glare causing discomfort is a flinch response in the muscles surrounding the eye (i.e., squinting), and the intensity of the electrical activity in these muscles, the electromyography (EMG), can be measured easily.
18 Murray et al.
18 compared EMG and subjective responses (using a 10-point scale similar to that by Stringham et al.
17) for glare presenting an illuminance at the eye of between 20 and 6000 lux and concluded that the correlation between these measurements was significant (using coefficient of determination for bivariate analysis [
R2] of ≥0.659,
P < 0.001). Berman et al.
19 also used EMG as an objective measure but recorded the subjective response by using a horizontal line with key words (e.g., perceptible and intolerable) to identify points along the line, and concluded that there was significant between-subject variation in both measurements. Stringham et al.
20 investigated photophobia, which their definition suggests is a case of extreme (intolerable) discomfort glare, and found significant correlation (
r = 0.98,
P < 0.001) between subjective ratings of photophobia and EMG-derived thresholds from their two participants.
Electromyography records the electrical activity produced by skeletal muscles, which in glare studies are the extra-ocular muscles,
18 and has uses including diagnosis of categories of disease to aid with the diagnosis of nerve injury and with other problems of the muscles or nerves.
21 Electro-oculography (EOG) is an alternative measure of biopotential, measuring the corneoretinal standing potential that exists between the front and back of the eye to record eye movements. If the eye moves from its center position toward one of two electrodes (placed either above and below or to the left and right of the eye), this electrode sees the positive side of the retina, and the opposite electrode sees the negative side of the retina, resulting in a potential difference between the electrodes, which gives a measure of the eye's position. A main application of EOG is recording eye movement,
21 and it has been used in previous studies to investigate the relationship between eyestrain and visual fatigue.
22–25 Thus, in the present study, we investigated the use of EOG as an alternative to EMG in measuring the involuntary vertical movement of the eyeball in response to discomfort glare and nystagmus.
21
This article presents the results of experiments in which pupil size and eye movement responses to discomfort glare were measured under various lighting conditions and based on more human participants than in previous studies. This extends previous work by using EOG rather than EMG to record eye muscle movement and by considering pupil size following on-set of glare relative to its size when adapted to the background lighting rather than absolute size.