Effect of Background Luminance on Discomfort Glare in Relation to the Glare Source Size

Discomfort glare from bright luminaires or windows is not a simple matter but consists of a complex relationship between a number of factors. The objective of this study was to investigate the effect of the background luminance on discomfort glare in relation to the size of the glare source. An experiment was conducted with observers. In the experiment, the change in the discomfort glare, depending on the change in the background luminance, was measured using different sizes of glare sources. Seven sizes of steradian of the glare source were selected and five levels of background luminance were chosen for the experiment. The results showed that the higher the background luminance, the smaller the observer's degree of discomfort glare. The exponent of background luminance changed clearly into different patterns in relation to the sizes of the glare source. A new mathematical formula has been developed by this study to determine the exponent of the background luminance for use in the range from 0.0003 to 5 sr of the glare source. The use of this formula would extend the application limit of the current unified glare rating formula from the current limit of 0.1 to 5 sr. Using the formula, the change in the background luminance would have no impact on the discomfort glare, in case when the glare source occupies the entire visual field.