Discomfort Glare Issues discussed in Sun City
Jennifer Veitch, Director, Division 3 (Interior Environment and Lighting Design)
Limitations to the predominant models for discomfort glare prediction for electric lighting, the Unified Glare Rating and the Visual Comfort Probability, are well known. There is no single widely-accepted model as yet for discomfort experienced from daylight. Overhead glare from small, bright light sources is a newly-identified problem that has the potential for becoming more frequent with the use of LED systems. We have no metric to predict discomfort from such sources. Against this background, Division 3 held a workshop at the CIE Session in Sun City to animate a discussion of these issues among the world-wide lighting community there assembled. Five expert panellists presented their perspectives on the issues and there was lively audience participation. The full workshop report is available at the CIE Website and will appear in Volume 2 of the Proceedings, but here is a summary of the discussion:
1. Glare criteria and levels used in previous studies have typically mixed issues of comfort and acceptability. These should be clearly separated as the experience of discomfort is independent of whether or not someone accepts the discomfort experienced. One might, for example, accept a certain level of discomfort from daylight when there is a mitigating factor, such as a great view out across a wonderful landscape or the ocean. It might thus be useful to describe situations under which glare might be tolerated. This could also help in the design process.
2. When lighting technology (especially light sources and the respective luminaire) changes, the impact of that change on discomfort glare needs to be carefully studied. LEDs clearly pose a challenge, as their small size and high luminance present conditions very different from those studied while current metrics were developed. These mostly relied on larger areas of lower luminous intensity.
3. To study glare phenomena in real-world situations, better measurement protocols are needed so that the measurements from various studies can be compared and carefully assessed. Currently, different researchers or assessors appear to measure different things. Further development of measurement equipment seems also an important consideration.
4. Of special importance might be the transient adaptation, i.e. the changes in adaptation luminance a person experiences when moving around in a space or changing view directions. Current discomfort prediction models assume a fixed position and viewing direction to which an observer is adapted.
5. There is a great need to get a better handle on the fundamental mechanisms underlying the experience of discomfort. This should include work on physiological pathways between eye and brain. The current discomfort glare models may be too simple. Once more complex models can be understood, there might be a chance to simplify them again on a different basis, leading to better prediction models.
Further discussion of these issues is expected among members of Division 3, leading to the possible development of new technical committees following the completion of ongoing work in TCs 3-39 (Discomfort Glare from Daylight in Buildings) and 3-50 (Lighting Quality Measures for Interior Lighting with LED Lighting Systems), whose work also will benefit from the open discussion in Sun City.