Methodologies for Glare Analysis

Green Building

Methodologies for Glare Analysis

Any complete analysis of a daylit interior environment should include both the amount of light arriving to specified surfaces (such as work planes) as well as an assessment of the visual comfort of the space. Both of these factors are dynamic. Thus, they will change throughout any given time period (e.g., hour, day, week, month, season).

The former can be quantified in a rather straightforward sense because it’s just physics. There is a specific amount of lumens entering a space and the characteristics of the interior will determine how the daylight is distributed. This phenomenon is called illuminance and it can be specifically defined as the density of the luminous flux incident on any given surface. Illuminance is expressed in units of footcandles (or lux), however analytic data can be expressed in terms of:

Daylight Factor (DF): The ratio of internal light level to external light level.

Daylight Autonomy (DA): The fraction of the occupied times per year when the required minimum illuminance level at a specific point can be maintained by daylight alone.

Illuminance analysis of a gymnasium using LBNL’s Radiance software (with Autodesk Ecotect as a graphic user interface).
Illustration by Daniel Overbey.

Glare Defined

Aside from illuminance, the issue of visual comfort is more difficult to quantitatively evaluate because “comfort” is a subjective human sensation. Whether dealing with thermal comfort, acoustical comfort, or visual comfort, every person will perceive the quality of an environment differently.

Even LEED has shied away from specifying any sort of quantitative glare analysis to compliment its daylighting credit – which use to reference Daylight Factor, then accepted computationally derived illumination simulations, and will soon specify Daylight Autonomy analyses.

This concept of visual comfort is often referred to as glare. In a more academic sense, glare can be defined as the sensation produced by luminance within the visual field that is sufficiently greater than the luminance to which the eye is adapted, thus causing annoyance, discomfort, or loss in visual performance and/or visibility.

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