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 hig h luminance, no one has yet found a reliable objective correlate of di scomfort 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: the orbicularis o culi and the corrugator supercilii. We have made electromyographic (EM G) recordings using small silver-silver chloride electrodes applied to the skin above the muscles and measured electrical potentials while c hanging lighting glare conditions. Intensities varied over a range of glare luminance determined by a separate procedure according to subjec tive ratings. For this subjective method, subjects indicated the level of discomfort by marking a visual analog scale (VAS) punctuated with four descriptions of discomfort level: perceptible, annoying, disturbi ng, and intolerable. We have determined that the VAS is much more reli able with much less variability than the previously used border betwee n comfort and discomfort (BCD) method. For 20 subjects, discomfort gla re was assessed under three conditions: 2-degree-diameter glare source with low room illumination, 2-degree glare source with medium room il lumination, and 1-degree glare source with medium room illumination. T he glare source was a projector beam, 11 degrees to the right of a fix ation target on a video monitor. Six different glare luminance levels were presented for 2-s periods. Each glare level was presented six tim es in a randomized order giving 36 presentations. EMG responses were s ubjected to Fourier analysis and the power frequency spectrum was dete rmined with appropriate digital filtering used to eliminate powerline artifacts. Blinking causes an artifact whose power spectrum is markedl y different and can be determined independently of the glare source. T he integrated power spectrum of the EMG activity during exposure to th e glare source was compared to the same integral prior to exposure to obtain a quantitative measure of glare-induced activity. For each of t he 19 subjects and a variety of glare conditions, the objective measur e and the VAS have been plotted as a function of glare luminance. For individual subjects we found increasing objective measure and increasi ng subjective discomfort with increased glare luminance. We conclude t hat the EMG technique is a valid, objective means of assessing discomf ort glare.