In the past, a number of different glare indices have been developed. All of them were basically aimed for artificial lighting and considered only small sized glare sources. Only one of them, the Daylight Glare Index (DGI) has been adapted to large glare sources and daylight conditions. Velds (2000) and Gall et al. (2000) found in their tests little or no correlation between the glare formula and the user assessments. This has been proven

by our own studies in 2003, when 27 subjects in test offices at the Fraunhofer ISE were tested under different lighting conditions. As can be seen in the following graph, there exists almost no correlation between DGI and user reaction.

The main reason for the big discrepancy is that, for the initial study, less than 10 people were used to develop the DGI formula. Another problem was that the studies were originally carried out under artificial and not real daylight conditions, nor under real office conditions.

Chauvel et al. (1982) argued that the weak correlation between the DGI and the observed glare from windows is compounded by other visual and aesthetic factors such as the quality of the view, the appearance of the window as well as the visual and aesthetic interior qualities of the room.

There was a greater tolerance of mild degrees of glare from the sky seen through the window than from a comparable artificial lighting situation with the same value of glare index, but that this tolerance did not extend to severe degrees of glare (Boubekri and Boyer, 1992; Chauvel et al., 1982). Chauvel et al. (1982) also observed that the discomfort glare resulting from the direct view through windows has been found to vary greatly from observer to observer and also to vary with factors associated with the appearance of the window, the view outside and the surroundings.

Iwata et al. (1990/91) showed that the subjects judged the light to be less uncomfortable even after only 30 seconds, suggesting that the most serious glare problems occur during the transition i. e. the time immediately after exposure to the glare source. Also, Osterhaus (1996) observed that the research subjects (32) in his experiment commented on becoming more sensitive to glare as the experiment progressed (2-2.5 hours) and that this impression was confirmed by experimental data. Osterhaus and Bailey (1992) also pointed out that no data is currently available on perceived comfort or discomfort and the relations between comfort and task performance under conditions in which the glare source borders or surrounds a work task. All existing discomfort glare indices were developed by assessments of subjects directly viewing the glare source rather than focusing on a work task.

Osterhaus (1996) also suggested carrying out glare experiments with subjects exposed to the daylighting situation for at least the eight hours of a regular workday. Decreasing work performance would be expected due to fatigue and distraction induced by glare discomfort. Sivak and Flannagan (1991) found that task difficulty affected discomfort glare. In their study, smaller gapsizes in a gap-detection task resulted in more discomfort glare responses concerning a simultaneous presented light source. They concluded that the assessment of discomfort glare requires the inclusion of the relevant visual task the observer is involved in during the presentation of the glare stimulus.