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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 <math>4\pi</math> 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. |
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 <math>4\pi</math> 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. |
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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 [[ |
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. |
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==Projector output== |
==Projector output== |
Revision as of 03:08, 27 May 2008
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
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.[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.
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". [3]
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.[4]
SI photometry units
Quantity | Unit | Dimension [nb 1] |
Notes | ||
---|---|---|---|---|---|
Name | Symbol[nb 2] | Name | Symbol | ||
Luminous energy | Qv[nb 3] | lumen second | lm⋅s | T⋅J | The lumen second is sometimes called the talbot. |
Luminous flux, luminous power | Φv[nb 3] | lumen (= candela steradian) | lm (= cd⋅sr) | J | Luminous energy per unit time |
Luminous intensity | Iv | candela (= lumen per steradian) | cd (= lm/sr) | J | Luminous flux per unit solid angle |
Luminance | Lv | candela per square metre | cd/m2 (= lm/(sr⋅m2)) | L−2⋅J | Luminous flux per unit solid angle per unit projected source area. The candela per square metre is sometimes called the nit. |
Illuminance | Ev | lux (= lumen per square metre) | lx (= lm/m2) | L−2⋅J | Luminous flux incident on a surface |
Luminous exitance, luminous emittance | Mv | lumen per square metre | lm/m2 | L−2⋅J | Luminous flux emitted from a surface |
Luminous exposure | Hv | lux second | lx⋅s | L−2⋅T⋅J | Time-integrated illuminance |
Luminous energy density | ωv | lumen second per cubic metre | lm⋅s/m3 | L−3⋅T⋅J | |
Luminous efficacy (of radiation) | K | lumen per watt | lm/W | M−1⋅L−2⋅T3⋅J | Ratio of luminous flux to radiant flux |
Luminous efficacy (of a source) | η[nb 3] | lumen per watt | lm/W | M−1⋅L−2⋅T3⋅J | Ratio of luminous flux to power consumption |
Luminous efficiency, luminous coefficient | V | 1 | Luminous efficacy normalized by the maximum possible efficacy | ||
See also: |
- ^ The symbols in this column denote dimensions; "L", "T" and "J" are for length, time and luminous intensity respectively, not the symbols for the units litre, tesla and joule.
- ^ Standards organizations recommend that photometric quantities be denoted with a subscript "v" (for "visual") to avoid confusion with radiometric or photon quantities. For example: USA Standard Letter Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967
- ^ a b c Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ for luminous efficacy of a source.
See also
References
- ^ "ANSI lumen article". PC Magazine Encyclopedia. Retrieved 2006-12-20.
- ^ "Projector Guide". CPILive.net. 2004. Retrieved 2006-12-20.
{{cite web}}
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ignored (help) - ^ "ANSI method of light output measurement" (doc). 1993. Retrieved 2008-01-15.
- ^ "Sony G70 Brochure" (PDF).