Everything about Lumen Unit totally explained
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.
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
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 15,000 lumens, about 9 times as much. See
Luminous efficacy for the specific efficiency of various types of electric light sources.
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's 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".
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
Further Information
Get more info on 'Lumen Unit'.
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