title: Discomfort glare, light scatter, and scene structure
Creators: name: Perry, Michael John
Publisher: name: The Open University
Year: 1995

description: Since the start of the Industrial Revolution there has been a general improvement in working conditions. As part of this process, light in the work place was recognised as an important environmental factor. In the early years of the 20th century it was also recognised that in providing adequate lighting for a particular working environment, there was a need to avoid the potential negative effects of too much, or inappropriately distributed, light. One of the negative effects of light in the work place was glare. <br><br> Holladay (Holladay, (1926)) attributed the negative effects of glare to impairment of vision caused by light scatter. Stiles (Stiles, (1929)) refuted Holladay's case by arguing that only a small proportion of the reduction in task visibility could be attributed to light scatter effects (where task visibility is a measure of how far above the visual threshold a task's contrast is). Stiles distinguished disability glare, a light scatter effect, from discomfort glare which was glare that could not be attributed to light scatter. The distinction made by Stiles resulted in the separate development of discomfort and disability glare models. Very few, if any, studies since Stiles have re-evaluated the potential association between subjectively rated discomfort glare, and physically based disability glare. <br><br> In the study reported here, subjects were asked to set the appearance of a 2° glare source so that it appeared at the Borderline between Comfort and Discomfort, or BCD (Guth, (1963)). Each subject's visual threshold for a 4 cycle per degree spatial grating was measured under BCD and control conditions, and a comparison made to assess if light scatter effects from the glare source influenced threshold contrast, C<sub>th</sub>. <br><br> The results of the study indicate that C<sub>th</sub>, can be lower in the presence of the glare source set to BCD. This anomaly may be explained by improvement in image quality caused by the glare source driving the pupil to a smaller diameter. <br><br> More significantly, there was found to be a strong correlation between subjective BCD settings and age, and also between BCD settings and control condition C<sub>th</sub>. Both of these results suggest an influence of light scatter on BCD settings of discomfort glare. This conclusion was further supported by the fitting to the data of the independently reported stray light function of Ijspeert et al (Ijspeert et al, (1990)). Thus the results strongly suggest a correlation between subjective BCD settings of a glare source and light scatter function. A conclusion that substantially weakens Stiles' argument that discomfort glare is not dependent on light scatter effects. Using the results of the study, a new threshold type model for assessing discomfort glare is proposed, which explicitly includes age as a parameter. <br><br> However, much variance remains to be explained in the glare data. Therefore, a second theme investigated in the dissertation is the possible association between scene visual structure and visual discomfort. The results of this study indicate that there is a small but significant difference in the image structure of natural and man made environments. This difference may contribute to visual discomfort, but will require further investigation.