Glossary: Terms and Definitions Used on this Site

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Light Reflectance Terms

Illuminance Levels
Illuminance is the quantity of light falling onto a surface. It is measured in footcandles or lux (metric measure). One footcandle is approximately 10 lux.

Luminance – Luminance is the quantity of light reflecting (or emitting) from a surface in the direction of the viewer. It is measured in foot-lamberts or candela/m2 (metric measure). Luminance is a measure of brightness.

Brightness Ratio – Brightness ratio is a measure of the quality of a lighting installation. It is an important measure of indirect lighting. A good indirect lighting system has a ceiling brightness ratio of less than 10 to 1.

Light Loss Factor (LLF) Used to calculate illuminance loss after a given period of time and under given conditions, such as, expected lamp light depreciation, expected lamp burn out, room surface dirt accumulation and ballast performance factors.

Light Reflectance (LR)
Light reflectance is a fundamental property of a material. It is a measure of a material’s ability to reflect visible light. The measure of light reflectance is that fraction of the specified incident light striking a surface that is reflected by the surface. (Defined in ASTM E 1477).

Nonspecular Surface Describes a surface which is diffuse, meaning it reflects light in equal amounts in all directions. This is important because a nonspecular ceiling’s brightness will be the same regardless of viewing position. Typically, nonspecular (diffuse) surfaces are less glossy and produce less offensive glare. 




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