An Objective Measure of Discomfort Glare
Although it is relatively easy to perceive and report the sensation of discomfort caused by the presence of an offending light source of high luminance, no one has yet found a reliable objective correlate correlate of discomfort glare.
In order to find an objectively measured correlate of discomfort glare, we have examined electrical activity associated with the two major facial muscles that surround the eye, viz. the orbicularis oculi and the corrugator supercilii. We have made electromyographic (EMG) recordings using small silver/silver chloride electrodes applied to the skin above the muscles and measured electrical potentials while lighting glare conditions have been changed. Intensities were varied over a range of glare luminance determined by a separate procedure according to subjective ratings. For this subjective method, subjects indicated the level of discomfort by marking a Visual Analogue Scale (VAS) punctuated with four descriptions of discomfort level: perceptible, annoying, disturbing, and intolerable. We have determined that the VAS is much more reliable with much less variability than the previously used border Between Comfort and Discomfort (BCD) method.
For 19 subjects, discomfort glare was assessed under three conditions: 2 deg. diameter glare source with low room illumination, 2 deg. glare source with medium room illumination, and I deg. glare source with medium room illumination. The glare source was a projector beam, 11 deg. to the right of a fixation target on a video monitor. Six different glare luminance levels were presented for 2 second periods. Each glare level was presented six times in a randomized order giving 36 presentations.
EMG responses were subjected to Fourier analysis and the power frequency spectrum was determined with appropriate digital filtering used to eliminate power line artifacts. Blinking causes an artifact whose power spectrum is markedly different and can be determined independently of the glare source. The integrated power spectrum of the EMG activity during exposure to the glare source was compared to the same integral prior to exposure to obtain a quantitative measure of glare induced activity. For each of the 19 subjects and a variety of glare conditions, the objective measure and the VAS have been plotted as a function of glare luminance. For individual subjects we found increasing objective measure and increasing subjective discomfort with increased glare luminance. We conclude that the EMG technique is a valid objective means of assessing discomfort glare.