The Influence of Spectral Composition on Discomfort Glare for Large Size Sources

We have previously demonstrated that brightness perception in full field is influenced by scotopic luminance, even at light levels in the photopic range. We asked here if glare discomfort is distinguishable when comparing a scotopically enhanced source with a scotopically deficient source at the same photopic luminance. Similarly to our previous study on discomfort glare, we used both objective and subjective techniques to assess glare response.

Discomfort glare responses were assessed for 12 subjects who viewed two broad-band glare sources (illurninants) of size 1.22 x 0.91 m (4 x 3 ft) with maximum photopic luminances of 3,700 cd/m². The two sources were approximately matched for photopic luminance but due to their spectra they had markedly different scotopic luminances. Both sources were presented separately at three different photopic luminance levels. These six glare conditions were each presented five times, for four second periods, in a randomized sequence. Electromyographic (EMG) responses from the facial orbicularis oculi muscles were subjected to Fourier analysis and integration of the power spectrum provided a measure of EMG activity. Our objective index of the response to glare was the ratio between EMG samples taken before and during the presentation of the source. For the subjective method, discomfort severity was indicated by subjects marking a Visual Analog Scale (VAS) punctuated with four descriptors — perceptible, annoying, disturbing and intolerable.

Both objective and subjective indices systematically increased with increasing luminance. However, the objective measure showed a significantly higher value for the scotopically deficient source at all luminances, while the subjective response was higher only at the greater luminances.

We conclude that discomfort glare is related to both the photopic luminance of the source, and its spectral composition with the absence of long wave length energy in the spectrum associated with lower levels of discomfort.