PERFORMANCE: Human Factors
Energy & Cost
Codes & Standards
Too much daylight can produce excessive glare, which is particularly undesirable in computer and other work environments. In the context of windows, glare causes difficulty to see due to bright light—typically occurring when there is a large difference between the area lit by entering sunlight and the area that an occupant is trying to see. As the eye attempts to even out the contrast between the task and the surrounding surfaces, the muscles of the eye have to work harder and more frequently. Tired eyes and increased levels of stress result. Glare within the range that the eye can handle is called discomfort glare; glare preventing us from doing a task is called disability glare. In addition to these two glare categories, there is direct and indirect glare. Direct glare is caused when a person views the source of illumination. Indirect glare results from light being reflected off surfaces.
Like the strain associated with glare, the eye has a difficult time adapting to high brightness ratios between differently illuminated tasks within one's field of view. Because brightness is a function of reflectance and illumination, the brightness ratio is controllable through good design. Anatomically, the eye is more sensitive in brightness ratios at the center of the field of vision, but brightness ratios in the periphery nevertheless invoke a reflex to center the eye on the brightness difference. Thus, changes in brightness ratios due to daylighting or artificial lighting need to be kept low over large areas of an occupant's field of view in a space. By keeping the reflectance of wall surfaces within appropriate levels, excessive brightness ratios can be minimized. Direct sun also must be controllable. The Illuminating Engineering Society (IES) recommends that small patches of sunlight be controlled to less than 79 candelas per square foot.