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{{for|other uses of the word|Lumen}}
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 [[wavelength]]s of light. The lumen is defined in relation to the [[candela]] by
: 1 lm = 1 [[candela|cd]]·[[steradian|sr]] = 1 [[lux|lx]]·[[metre|m]]<sup>2</sup>
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 <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. The number of candelas or lumens from a source also depends on its spectrum, via the nominal response of the human eye as represented in the [[luminosity function]].
A standard 120 V, 100 [[watt]] [[incandescent light bulb]] emits 1500–1700 lumens, while a standard 230 V model emits 1200–1400 lm.<ref>{{cite web |url=http://www.bulbs.ch/index.php?cPath=49_41_55_61 |title=Standardlampen |language=German |accessdate=2008-06-01}}</ref> A 23 watt [[compact fluorescent lamp]] emits about 1500 lm.<ref>{{cite web |url=http://www.osram.dk/osram_dk/Professionelle/Brochurer_&_kataloger/Energispareprer_og_kompaktrr/e21_DULUX_EL_teknisk_guide.pdf |title=Osram Dulux EL electronic energy-saving lamps |format=pdf |work=Osram.dk |accessdate=Jan. 04, 2009}}</ref><ref name="cf">{{cite web | title=Low Mercury CFLs | url=http://www.energyfederation.org/consumer/default.php/cPath/25_44_3006 | publisher=Energy Federation Incorporated | accessdate=2008-12-23}}</ref><ref>{{cite web | title=Conventional CFLs | url=http://www.energyfederation.org/consumer/default.php/cPath/25_44_784 | publisher=Energy Federation Incorporated | accessdate=2008-12-23}}</ref> The number of lumens produced per watt of power consumed is the [[wall-plug luminous efficacy]] of the source.
=== Differences between lumens and lux ===
The difference between the units lumen and [[lux]] is that the lux takes into account the area over which the luminous flux is spread. A flux of 1000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1000 lux. The same 1000 lumens, spread out over ten square metres, produces a dimmer illuminance of only 100 lux.
Achieving an illuminance of 500 lux might be possible in a home kitchen with a single [[fluorescent light]] fixture with an output of 12000 lumens. To light a factory floor with dozens of times the area of the kitchen would require dozens of such fixtures. Thus, lighting a larger area to the same level of lux requires a greater number of lumens.
==Projector output==
===ANSI lumens===
The light output of [[projector]]s (including [[video projector]]s) 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.<ref>{{cite web|url=http://www.pcmag.com/encyclopedia_term/0,2542,t=ANSI+lumen&i=37802,00.asp|title= ANSI lumen article|work= PC Magazine Encyclopedia|accessdate=2006-12-20}}</ref> For marketing purposes, the luminous flux of projectors that have been tested according to this procedure may be quoted in "ANSI lumens", to distinguish them from those tested by other methods. ANSI lumen measurements are in general more accurate than the other measurement techniques used in the projector industry.<ref>{{cite web|url=http://www.cpilive.net/news_ver2_old/guides_2004/projector_guide_issue_04/office.htm|title=Projector Guide| work=CPILive.net| month=February| year=2004| accessdate=2006-12-20}}</ref> This allows projectors to be more easily compared on the basis of their brightness specifications.<ref>{{cite web|url=http://www.boxlight.com/Boxlight-Projector-Resources/Boxlight-Projector-Guides/Lumen-Guide|title= Lumen Guide|work= Boxlight Corporation}}</ref>
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".<ref>{{cite web |url=http://www.hometheater1.com/tech/TB93-2.doc |title=ANSI method of light output measurement |year=1993 |format=doc |accessdate=2008-01-15}}</ref>
===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.<ref>{{cite web|url=http://www.curtpalme.com/docs/SonyG70_Brochure.pdf|title=Sony G70 Brochure}}</ref>
==SI photometry units==
{{SI light units}}
==See also==
*[[Brightness]]
*[[Lux]]
==References==
{{reflist}}
[[Category:Photometry]]
[[Category:Units of luminous flux]]
[[Category:SI derived units]]
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