Lumen (unit)

The lumen (symbol: lm) is the SI unit of luminous flux, a measure of the perceived power of light. Luminous flux differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light.

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Definition

1 lm = 1 cd·sr = 1 lx·m²

Explanation

If a light source emits one candela of luminous intensity uniformly across a solid angle of one steradian, its total luminous flux emitted into that angle is one lumen. Alternatively, an isotropic one-candela light source emits a total luminous flux of exactly 4π lumens. The lumen can be thought of casually as a measure of the total "amount" of visible light in some defined beam or angle, or emitted from some source.

A standard North American 100 watt incandescent light bulb emits approximately 1700 lumens, while a 100 watt Sodium vapor lamp emits around 15000 lumens, about 8 times as much. See Luminous efficacy for the specific efficiency of various types of electric light sources.

ANSI lumens

The light output of projectors (including video projectors) is typically measured in lumens. A standardized procedure for testing projectors has been established by the American National Standards Institute, which involves averaging together several measurements taken at different positions.^[1] For marketing purposes, the luminous flux of projectors that have been tested according to this procedure may be quoted in "ANSI lumens", to distinguish devices that have been so tested from those tested by other methods. ANSI lumen measurements are in general more accurate than the other measurement techniques used in the projector industry.^[2] This allows projectors to be more easily compared on the basis of their brightness specifications.

SI photometry units

See also

• Brightness
• Lux

References