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.