Evaluation methods and development of a new glare prediction model for daylight environments with the use of CCD cameras

Daylighting and the impact of daylighting strategies on the visual environment continue to be a vital issue for building occupants due to visual comfort and user acceptance of luminous indoor environments. Some of the critical factors affecting the level of visual comfort and quality in daylit office spaces include glare, window luminances, and luminance ratios within the field of view. One of the goals of this study was to provide new insight into the impact of luminance distributions on glare. The luminance distribution within the field of view was recorded using CCD camera-based luminance mapping technology. The technology provides a great potential for improved understandings of the relation between measured lighting conditions and user response. With the development of the RADIANCE based evaluation tool "evalglare", it became possible to analyse glare according to a number of daylight glare prediction models as well as contrast ratios in various daylit situations (workplace, VDU). User assessments at two locations (Copenhagen, Freiburg) with more than 70 subjects under various daylighting conditions were performed in order to assess existing glare models and to provide a reliable database for the development of a new glare prediction model. The comparison of the results of the user assessments with existing models clearly shows the great potential for improving glare prediction models. For the window luminance a squared correlation factor of only 0.12 and for the daylight glare index (DGI) of 0.56 were found. Due to the low predictive power of existing glare prediction models a new index, daylight glare probability (DGP), was developed and is presented in this paper. DGP is a function of the vertical eye illuminance as well as on the glare source luminance, its solid angle and its position index. The DGP showed a very strong correlation (squared correlation factor of 0.94) with the user's response regarding glare perception.