Validation and Universalization of Daylight Glare Probability Index

Pierson, Clotilde [UCL] Bodart, Magali [UCL]

(eng) In the present climatic and energetic context, the design of buildings taking advantage of natural daylight is of crucial importance. However, some annoyances due to poor designs could cancel out all the benefits linked to daylight penetration in buildings. One of these annoyances is glare. Glare may likely drive the occupants to the use of electrical lighting, which is easier to control. It is thus of major importance, when designing or renovating a building, to be able to evaluate the glare risks linked to daylight penetration in the building. The Daylight Glare Probability is at the moment the only daylight glare index developed under real daylighting conditions. The DGP evaluates the probability that a person is disturbed by daylight glare. Whereas most of the glare index formulas are based on the contrast between the luminance of the glare source and the luminance of the background, the DGP is mainly influenced by the vertical illuminance at eye level. During Wienold’s experiment, the DGP showed promising results for glare assessment, making it the most widely used index to date. However, recent studies indicate that the DGP could misevaluate glare perception in some situations such as under direct sun-facing conditions, under low luminance levels, or in open plan spaces. In order to clarify these issues and to validate the DGP for a wider range of situations, this research focuses on three main objectives. The first one is the validation of the DGP as is. This implies to assess the proper relevance of the DGP when it is used in a real office environment, with similar characteristics to Wienold’s adjacent cells. The second objective of the research project is the extension of the DGP scope. In addition to the validation of the index in an individual south-oriented office, there is a need to assess the proper functioning of the DGP when used under other conditions, e.g. with various shading devices, different daylighting levels, multiple space arrangements, etc. Finally, the third objective of this research project is the evaluation of the adaptive nature of glare perception according to usual climatic conditions. Indeed, the literature shows that humans perceive glare at varying intensity depending on different parameters, such as age, seasons, time of day, current task, or even information from the outside view. Moreover, Cauwerts suggested that a cultural adaptability might exist regarding tolerance to glare: in her experiments, same scenes were perceived as less glaring by the French sample of participants than by the Belgian one.