PERFORMANCE: Human Factors

Glare

USEFUL INFORMATION

ASHRAE Standard 55-2004 Thermal Environmental Conditions for Human Occupancy

ASHRAE Standard 62.1-2007 Ventilation for Acceptable Indoor Air Quality

Need to reference IES

Glare

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.

June
double clear double tint double clear with interior fixed Venetian blind double clear with exterior 2' overhang
December
double clear double tint double clear with interior fixed Venetian blind double clear with exterior 2' overhang

Radiance images of an office building in Chicago, south orientation, 40% window area, and no daylighting controls generated using COMFEN.

Tools such as the Facade Design Tool and COMFEN demonstrate the environmental and human factors impacts of various design scenarios—allowing for decisions to be made early in the design process.

This image illustrates the annual glare index for 4 scenarios in Chicago, south orientation, 40% window area, and 4 unshaded glazing types. Since this is an annual average on a south elevation, the "averaged" glare index is very low. If more specific glare information is needed, use COMFEN.

 

This image illustrates the glare illuminance levels per distance from the facade for the same 4 scenarios as in the previous illustration using the Facade Design Tool. COMFEN can also show Radiance images for glare with a diffused or clear sky.

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