Lumen

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. The lumen is defined in relation to the candela by
1 lm = 1 cd•sr = 1 lx•m2
That is, a light source that uniformly radiates one candela in all directions radiates a total of 4π lumens. If the source were partially covered by an ideal absorbing hemisphere, that system would radiate half as much luminous flux—only 2π lumens. The luminous intensity would still be one candela in those directions that are not obscured.


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 1500–1700 lumens, while a standard European 230 V model emits 1200–1400 lm. A 100 watt high-pressure sodium vapor lamp emits around 15,000 lumens. The number of lumens produced per watt of power consumed is the wall-plug luminous efficacy of the source.

Projector output
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. 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. This allows projectors to be more easily compared on the basis of their brightness specifications.
The method for measuring ANSI lumens is defined in the IT7.215 document which was created in 1992. First the projector is set up to display an image in a room at a temperature of 25 degrees celsius. The brightness and contrast of the projector are adjusted so that on a full white field, it is possible to distinguish between a 5% screen area block of 95% percent peak white, and two identically sized 100% and 90% peak white boxes at the center of the white field. The light output is then measured on a full white field at nine specific locations around the screen and averaged. This average is then multiplied by the screen area to give the brightness of the projector in "ANSI lumens".

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Peak lumens
Peak lumens is a measure of light output normally used with CRT video projectors. The testing uses a test pattern with typically at either 10 and 20 percent of the image area as white at the center of the screen, the rest as black. The light output is measured just in this center area. Limitations with CRT video projectors result in them producing greater brightness when just a fraction of the image content is at peak brightness. For example the Sony VPH-G70Q CRT video projector produces 1200 "peak" lumens but just 200 ANSI lumens.


SI photometry units
SI photometry units
v • d • e
Quantity Symbol SI unit Abbr. Notes
Luminous energy
Qv lumen second
lm•s units are sometimes called talbots

Luminous flux
F lumen (= cd•sr)
lm also called luminous power
Luminous intensity
Iv candela (= lm/sr)
cd an SI base unit

Luminance
Lv candela per square metre
cd/m2 units are sometimes called nits

Illuminance
Ev lux (= lm/m2)
lx Used for light incident on a surface
Luminous emittance
Mv lux (= lm/m2) lx Used for light emitted from a surface
Luminous efficacy
lumen per watt
lm/W ratio of luminous flux to radiant flux

SI • Photometry

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