Evaluation of discomfort glare through glare scales: what are we really measuring?

Pierson, Clotilde;Wienold, Jan;Altomonte, Sergio;Bodart, Magali
(2018) CIE Expert Workshop on Research Methods for Human Factors in Lighting — Location: Copenhagen, Denmark (13.August.2018)

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To collect subjective evaluations of discomfort glare, subjects are usually asked to rate a visual scene on a glare scale. Several glare scales have been defined in the literature, and many replications or derivatives of these scales are currently used. One of the most adopted glare scales is the 4-point Osterhaus’ scale – imperceptible/noticeable/disturbing/intolerable –, although scales that used modified versions of the 5 labels from the 9-point De Boer’s scale – unbearable/disturbing/just admissible/satisfactory/unnoticeable – are also common. Unfortunately, it is frequent practice by some experimenters to either mix labels from different scales or develop their own glare scale, hence making between-studies comparisons very difficult. There is a large consensus between researchers that most glare scales are unipolar, going from a “no glare” to a “very glary” end. These scales are usually treated as such in statistical analyses and for their correlation with discomfort glare indices. But, how do the subjects interpret these scales? Up to now, there has been only one study investigating subjects’ interpretation of a modified De Boer’s scale. The results showed that, besides the confusion brought by the terminology, only 15% of the subjects could reconfigure the labels of the scale in the right sequence. In an effort to clarify the use of glare scales, the data from two field studies have been analyzed.
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Pierson, C., Wienold, J., Altomonte, S., & Bodart, M. (2018). Evaluation of discomfort glare through glare scales: what are we really measuring? Proceedings of CIE Expert Workshop on Research Methods for Human Factors in Lighting. Published. CIE Expert Workshop on Research Methods for Human Factors in Lighting, Copenhagen, Denmark. https://hdl.handle.net/2078.5/228233