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'''Luminous efficacy''' is a measure of how well a light source produces visible light. It is the ratio of [[luminous flux]] to [[power (physics)|power]]. Depending on context, the power can be either the [[radiant flux]] of the source's output, or it can be the total power (electric power, chemical energy, or others) consumed by the source.<ref>{{cite book |author=Allen Stimson |title=Photometry and Radiometry for Engineers |publisher=Wiley and Son |location=New York |year=1974}}</ref><ref>{{cite book |title=Optical Radiation Measurements, Vol 1 |publisher=Academic Press |location=New York |author=Franc Grum, Richard Becherer |year=1979}}</ref><ref>{{cite book |title=Radiometry and the Detection of Optical Radiation |publisher=Wiley and Son |location=New York |author=Robert Boyd |year=1983}}</ref> 
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Which sense of the term is intended must usually be inferred from the context, and is sometimes unclear. The former sense is sometimes called '''luminous efficacy of radiation''', and the latter '''luminous efficacy of a source'''.  
 
The luminous efficacy of a source is a measure of the efficiency with which the source provides visible light from  electricity.<ref>{{cite book  |title=Photovoltaic systems engineering |edition=2 |author=Roger A. Messenger; Jerry Ventre |publisher=CRC Press |year=2004 |isbn=978-0-8493-1793-4 |page=123}}</ref>
The luminous efficacy of radiation describes how well a given quantity of electromagnetic radiation from a source produces visible light: the ratio of [[luminous flux]] to [[radiant flux]].<ref>{{cite book |title=Color imaging: fundamentals and applications |author=Erik Reinhard, Erum Arif Khan, Ahmet Oğuz Akyüz, Garrett Johnson |publisher=A K Peters, Ltd |year=2008 |isbn=978-1-56881-344-8 |page=338}}</ref>
Not all wavelengths of light are equally visible, or equally effective at stimulating human vision, due to the [[spectral sensitivity]] of the [[human eye]]; radiation in the [[infrared]] and [[ultraviolet]] parts of the spectrum is useless for illumination. The overall luminous efficacy of a source is the product of how well it converts energy to electromagnetic radiation, and how well the emitted radiation is detected by the human eye.  
 
==Efficacy and efficiency==<!--Luminous coefficient redirects here-->
 
In some [[Units of measurement|systems of units]], luminous flux has the same units as radiant flux. The luminous efficacy of radiation is then [[dimensionless]]. In this case, it is often instead called the '''luminous efficiency''', and may be expressed as a percentage. A common choice is to choose units such that the maximum possible efficacy, 683&nbsp;lm/W, corresponds to an efficiency of 100%. The distinction between ''efficacy'' and ''efficiency'' is not always carefully maintained in published sources, so it is not uncommon to see "efficiencies" expressed in lumens per watt, or "efficacies" expressed as a percentage.
 
The '''luminous coefficient''' is luminous efficiency expressed as a value between zero and one, with one corresponding to an efficacy of 683&nbsp;lm/W.
 
==Luminous efficacy of radiation==
===Explanation===
 
[[File:CIE 1931 Luminosity.png|right|thumb|The [[Luminosity function|response of a typical human eye to light]], as standardized by the [[International Commission on Illumination|CIE]] in 1924. The horizontal axis is wavelength in nm]]
 
[[Wavelength]]s of light outside of the [[visible spectrum]] are not useful for illumination because they cannot be seen by the [[human eye]]. Furthermore, the eye responds more to some wavelengths of light than others, even within the visible spectrum. This response of the eye is represented by the [[luminosity function]]. This is a standardized function which represents the response of a "typical" eye under bright conditions ([[photopic vision]]). One can also define a similar curve for dim conditions ([[scotopic vision]]). When neither is specified, photopic conditions are generally assumed.
 
Luminous efficacy of radiation measures the fraction of electromagnetic power which is useful for lighting. It is obtained by dividing the [[luminous flux]] by the [[radiant flux]]. Light with wavelengths outside the [[visible spectrum]] reduces luminous efficacy, because it contributes to the radiant flux while the luminous flux of such light is zero. Wavelengths near the peak of the eye's response contribute more strongly than those near the edges.
 
In [[SI]], luminous efficacy has units of [[Lumen (unit)|lumen]]s per [[watt]] (lm/W). Photopic luminous efficacy of radiation has a maximum possible value of 683&nbsp;lm/W, for the case of monochromatic light at a wavelength of 555&nbsp;nm (green). Scotopic luminous efficacy of radiation reaches a maximum of 1700&nbsp;lm/W for narrowband light of wavelength 507&nbsp;nm.
 
===Mathematical definition===
 
The dimensionless luminous efficiency measures the [[integral|integrated]] fraction of the [[radiant power]] that contributes to its luminous properties as evaluated by means of the standard [[luminosity function]].<ref>{{cite book|title=Van Nostrand's Scientific Encyclopedia, 3rd Edition| publisher=D. Van Nostrand Company, Inc.| location=Princeton, New Jersey, Toronto, London, New York|date=January 1958}}</ref> The luminous coefficient is
 
:<math>\frac{ \int^\infty_0 y_\lambda J_\lambda d\lambda } { \int^\infty_0 J_\lambda d\lambda },</math>
 
where
:''y''<sub>&lambda;</sub> is the standard [[luminosity function]],
:''J''<sub>&lambda;</sub> is the [[spectral power distribution]] of the radiant intensity.
 
The luminous coefficient is unity for a narrow band of wavelengths at 555 [[nanometre]]s.
 
Note that <math>\int^\infty_0 y_\lambda J_\lambda d\lambda</math> is an [[inner product]] between <math>y_\lambda</math> and <math>J_\lambda</math> and that <math>\int^\infty_0 J_\lambda d\lambda</math> is the [[norm (mathematics)|one-norm]] of <math>J_\lambda</math>.
 
===Examples===
 
[[File:Wiens law vis limits.svg|right|thumb|300px|[[Spectral radiance]] of a [[black body]]. Energy outside the [[Visible spectrum|visible wavelength]] range (~380–750 nm, shown by grey dotted lines) reduces the luminous efficiency.]]
 
[[File:Blackbody efficacy 1000-16000K.svg|right|thumb|500px]]
 
{| class="wikitable"
|-
!Type<br>&nbsp;||Luminous efficacy of radiation<br>(lm/W)||Luminous efficiency<ref group="note" name="max">Defined such that the maximum value possible is 100%.</ref><br>&nbsp;
|-
|Typical tungsten light bulb at 2800&nbsp;K
|15<ref name="ideal-white">{{cite web |title=Maximum Efficiency of White Light |url=http://physics.ucsd.edu/~tmurphy/papers/lumens-per-watt.pdf |accessdate=2011-07-31}}</ref>
|2%
|-
|Class M star ([[Antares]], [[Betelgeuse]]), [[Color temperature|3000 K]]
|30
|4%
|-
|ideal [[black-body]] radiator at 4000&nbsp;K
|54.7<ref name="blackbody">[[:De:Bild:Blackbodyvisiblerp.png|Black body visible spectrum]]</ref>
|8%
|-
|Class G star ([[Sun]], [[Capella (star)|Capella]]), 5800 K
|93<ref name="ideal-white"/>
|13.6%
|-
|ideal black-body radiator at 7000&nbsp;K
|95<ref name="blackbody"/>
|14%
|-
|ideal 5800&nbsp;K black-body, truncated to 400–700&nbsp;nm (ideal "white" source) <!--most efficient source you can do that mimics solar spectrum only within range of visual sensitivity-->
|251<ref name="ideal-white"/><ref group="note" name="ideal_white">Integral of truncated [[Planck function]] times photopic [[luminosity function]] times 683&nbsp;W/sr, according to the definition of the [[candela]]. [http://jap.aip.org/resource/1/japiau/v111/i10/p104909_s1]</ref>
|37%
|-
|ideal monochromatic 555&nbsp;nm source
|683<ref name="luminosity">{{cite book | author=Wyszecki, Günter and Stiles, W.S. | title=Color Science – Concepts and Methods, Quantitative Data and Formulae |edition=2nd | publisher=Wiley-Interscience | year=2000 | isbn =0-471-39918-3 }}</ref>
|100%
|}
 
==Lighting efficiency== <!--Many terms redirect to this section-->
 
Artificial light sources are usually evaluated in terms of luminous efficacy of a source, also sometimes called ''overall luminous efficacy''. This is the ratio between the total luminous flux emitted by a device and the total amount of input power (electrical, etc.) it consumes. It is also sometimes referred to as the '''wall-plug luminous efficacy''' or simply '''wall-plug efficacy'''. The overall luminous efficacy is a measure of the efficiency of the device with the output adjusted to account for the spectral response curve (the “luminosity function”). When expressed in dimensionless form (for example, as a fraction of the maximum possible luminous efficacy), this value may be called '''overall luminous efficiency''', '''wall-plug luminous efficiency''', or simply the '''lighting efficiency'''.
 
The main difference between the luminous efficacy of radiation and the luminous efficacy of a source is that the latter accounts for input energy that is lost as [[heat]] or otherwise exits the source as something other than electromagnetic radiation. Luminous efficacy of radiation is a property of the radiation emitted by a source. Luminous efficacy of a source is a property of the source as a whole.
 
===Examples===
The following table lists luminous efficacy of a source and efficiency for various light sources. Note that all lamps requiring [[Electrical ballast|electrical/electronic ballast]] are unless noted (see also voltage) listed without [[Electrical efficiency|losses]] for that, reducing total efficiency.
{| class="wikitable"
|-
!Category<br>&nbsp;||Type<br>&nbsp;||Overall<br>luminous efficacy (lm/W)||Overall<br>luminous efficiency<ref group="note" name="max" />
|-
| rowspan="2" style="text-align:center;"|Combustion
|[[candle]]
|0.3<ref group="note">1 [[candela]]*4&pi;&nbsp;[[steradian]]s/40 W</ref>
|0.04%
|-
|[[gas mantle]]
| 1–2<ref>{{cite journal | title=Recent Developments in Gas Street Lighting | journal=The American City |volume=22 |issue=5 |publisher=Civic Press |location=New York | page=490 | first=F. V. |last=Westermaier | year=1920 | url=http://books.google.com/?id=rWxLAAAAMAAJ&dq=mantle%20lamp&pg=PA490#v=onepage&q=mantle%20lamp}}</ref>
|0.15–0.3%
|-
| rowspan="6" style="text-align:center;"|[[incandescent light bulb|Incandescent]]
|100–200 W tungsten incandescent (230&nbsp;V)
|13.8<ref>{{cite web |url=http://www.bulbs.ch/index.php?cPath=49_41_55_61_94 |title=Bulbs: Gluehbirne.ch: Philips Standard Lamps (German) |publisher=Bulbs.ch |date= |accessdate=2013-05-17}}</ref>–15.2<ref name="philc">[http://www.lighting.philips.com/de_de/tools_downloads/pricelist_lamps/downloads/preisliste_dede_20081023.pdf Philips Product Catalog] (German)</ref>
|2–2.2%
|-
|100–200–500 W tungsten glass halogen (230&nbsp;V)
|16.7<ref>{{cite web |url=http://www.osram.de/_global/pdf/osram_de/tools_services/downloads/allgemeinbeleuchtung/halogenlampen/haloluxhalopar.pdf |title=Osram halogen |work=osram.de |language=German |accessdate=2008-01-28 |format=PDF |archiveurl=http://web.archive.org/web/20071107054500/http://www.osram.de/_global/pdf/osram_de/tools_services/downloads/allgemeinbeleuchtung/halogenlampen/haloluxhalopar.pdf |archivedate=November 7, 2007}}</ref>–17.6<ref name="philc"/>–19.8<ref name="philc"/>
|2.4–2.6–2.9%
|-
|5–40–100 W tungsten incandescent (120&nbsp;V)
|5–12.6<ref name="incandescent">{{cite web |title=The Nature of Light |last=Keefe |first=T.J. |year=2007 |url=http://www.ccri.edu/physics/keefe/light.htm |accessdate=2007-11-05 |archiveurl=http://archive.is/me55 |archivedate=2012-07-24}}</ref>–17.5<ref name="incandescent"/>
|0.7–1.8–2.6%
|-
|2.6 W tungsten glass halogen (5.2&nbsp;V)
|19.2<ref>{{cite web |url=http://www.bulbtronics.com/Search-The-Warehouse/ProductDetail.aspx?sid=0000747&pid=OS6406330&AspxAutoDetectCookieSupport=1 |title=Osram 6406330 Miniwatt-Halogen 5.2V |publisher=bulbtronics.com |accessdate=2013-04-16}}</ref>
|2.8%
|-
|tungsten quartz halogen (12–24&nbsp;V)
|24
|3.5%
|-
|photographic and projection lamps
|35<ref name="bulbguide">{{cite web | author=Klipstein, Donald L.| year=1996 | title=The Great Internet Light Bulb Book, Part I | url=http://freespace.virgin.net/tom.baldwin/bulbguide.html | accessdate=2006-04-16}}</ref>
|5.1%
|-
| rowspan="8" style="text-align:center;"|[[Light-emitting diode]]
|white LED (raw, without power supply)
|4.5–150 <!-- Do not add values for experimental LEDs, unless the number is supported by a reference to a reliable, *independent* source. Manufacturer press releases are not independent sources! See: [[Talk:Luminous efficacy/Archive_1#new record for LED efficiency]] --><ref>{{cite web |title=White LED Offers Broad Temp Range And Color Yield |url=http://electronicdesign.com/lighting/white-led-offers-broad-temp-range-and-color-yield |publisher=Electronicdesign |date=2001-04-02 |accessdate=2013-05-16}}</ref><ref>{{cite web |url=http://www.nichia.co.jp/specification/en/product/led/NSPWR70CS-K1-E.pdf |title=Nichia NSPWR70CSS-K1 specifications |publisher=Nichia Corp. |format=PDF |accessdate=2013-05-16}}</ref><ref name="Klipstein-LED">{{cite web |author=Klipstein, Donald L. |title=The Brightest and Most Efficient LEDs and where to get them |work=Don Klipstein's Web Site |url=http://members.misty.com/don/led.html#ln |accessdate=2008-01-15}}</ref><ref>{{cite web |url=http://www.cree.com/~/media/Files/Cree/LED%20Components%20and%20Modules/XLamp/Data%20and%20Binning/XLampXPG.pdf |title=Cree XLamp XP-G LEDs Data Sheet}}</ref>
|{{Rnd|{{#expr:4.5/6.83002}}|2}}–{{Rnd|{{#expr:150/6.83002}}|0}}%
|-
|4.1 W LED [[Edison screw|screw base]] lamp (120&nbsp;V)
|{{Rnd|{{#expr:240/4.1}}|1}}–{{Rnd|{{#expr:340/4.1}}|1}}<ref name="Toshiba-LED">{{cite web |url=http://en.item.rakuten.com/alllight/lelaw8l_toshiba/ |title=Toshiba E-CORE LED Lamp |publisher=item.rakuten.com |date= |accessdate=2013-05-17}}</ref>
|{{Rnd|{{#expr:240/4.1/6.83002}}|1}}–{{Rnd|{{#expr:340/4.1/6.83002}}|0}}%
|-
|5.4 W LED screw base lamp (100&nbsp;V 50/60&nbsp;Hz)
|{{Rnd|{{#expr:550/5.4}}|1}}<ref name="Toshiba-LED LDA5N-E17">{{cite web |title=Toshiba E-CORE LED Lamp LDA5N-E17 |url=http://www.tlt.co.jp/tlt/new/lamp/hp_led/minikry_lda5.htm |archiveurl=http://web.archive.org/web/20110719165551/http://www.tlt.co.jp/tlt/new/lamp/hp_led/minikry_lda5.htm |archivedate=2011-07-19}}</ref>
|{{Rnd|{{#expr:550/5.4/6.83002}}|1}}%
|-
|6.9 W LED screw base lamp (120&nbsp;V)
|{{Rnd|{{#expr:380/6.9}}|1}}–{{Rnd|{{#expr:565/6.9}}|1}}<ref name="Toshiba-LED"/>
|{{Rnd|{{#expr:380/6.9/6.83002}}|1}}–{{Rnd|{{#expr:565/6.9/6.83002}}|0}}%
|-
|7 W LED [[Parabolic aluminized reflector light|PAR20]] (120&nbsp;V)
|{{Rnd|{{#expr:200/7}}|1}}<ref>{{cite web |url=http://www.amazon.com/gp/product/B001NP8N9G/ref=oss_T15_product |title=GE 73716 7-Watt Energy Smart PAR20 LED Light Bulb |publisher=Amazon.com |date=2008-12-12 |accessdate=2013-05-17}}</ref>
|{{Rnd|{{#expr:200/7/6.83002}}|1}}%
|-
|7 W LED [[Parabolic aluminized reflector light|PAR30]] (110-230&nbsp;V)
|{{Rnd|{{#expr:420/7}}|0}}<ref>{{cite web |url=http://www.bax-shop.nl/par-30-lampen/lite-gear-led-par-30-7w-lamp-ww-edison-45g/product-details.html |title=Lite Gear LED PAR 30 7W Light Bulb |publisher=Bax-shop.nl |date=2010-07-01 |accessdate=2013-05-17}}</ref>
|{{Rnd|{{#expr:420/7/6.83002}}|1}}%
|-
|8.7 W LED screw base lamp (120&nbsp;V)
|{{Rnd|{{#expr:600/8.7}}|0}}–{{Rnd|{{#expr:810/8.7}}|1}}<ref name="Toshiba-LED"/><ref>[http://ledsreview.com/news/367/ Toshiba to release 93 lm/W LED bulb] Ledrevie</ref>
|{{Rnd|{{#expr:600/8.7/6.83002}}|1}}–{{Rnd|{{#expr:810/8.7/6.83002}}|1}}%
|-
|Theoretical limit (white LED with phosphorescence color mixing)
|{{Rnd|{{#expr:260}}|0}}–{{Rnd|{{#expr:300}}|0}}<ref name="physorg.com">[http://www.physorg.com/news202453100.html White LEDs with super-high luminous efficacy] physorg.com</ref>
|{{Rnd|{{#expr:260/6.83002}}|1}}–{{Rnd|{{#expr:300/6.83002}}|1}}%
|-
| rowspan="4" style="text-align:center;"|[[Arc lamp]]
|[[xenon arc lamp]]
|30–50<ref name="xenon">{{cite web | title=Technical Information on Lamps | work=Optical Building Blocks | url=http://www.pti-nj.com/products/High-Speed-Spectrofluorometer/TechNotes/TechnicalInformationLamps.pdf | format=PDF|accessdate=2010-05-01}} Note that the figure of 150&nbsp;lm/W given for xenon lamps appears to be a typo. The page contains other useful information.</ref><ref>{{cite book|title=OSRAM Sylvania Lamp and Ballast Catalog|year=2007}}</ref>
|4.4–7.3%
|-
|[[mercury (element)|mercury]]-[[xenon]] arc lamp
|50–55<ref name="xenon"/>
|7.3–8%
|-
|[[Ultra-high-performance lamp|UHP]] – ultra-high-pressure [[mercury-vapor lamp|mercury-vapor]] arc lamp: initial, free mounted
|58–78<ref>[http://www.koti.mbnet.fi/jahonen/Electronics/Stuff/UHP_Lamp.pdf REVIEW ARTICLE: UHP lamp systems for projection applications] Journal of Physics D: Applied Physics</ref>
|8.5-11.4%
|-
|UHP – ultra-high-pressure mercury-vapor arc lamp: rated, with reflector for [[Digital Light Processing|projectors]]
|30–50<ref>[http://www.beamerlampen.biz/EASYLAMP_OSRAM_VIP_Projector_Lamp.pdf OSRAM P-VIP PROJECTOR LAMPS] Osram</ref>
|4.4–7.3%
|-
| rowspan="5" style="text-align:center;"|[[fluorescent lamp|Fluorescent]]
|[[Vacuum|very low]] [[pressure]] [[Mercury-vapor lamp|mercury-vapor]] [[gas-discharge lamp]] with [[fluorescence]] as T12 tube with magnetic ballast
| 60<ref name=FEMP>{{cite paper|url=http://www1.eere.energy.gov/femp/procurement/eep_fluortube_lamp.html |title=How to buy an energy-efficient fluorescent tube lamp |author =Federal Energy Management Program |publisher=U.S. Department of Energy |date=December 2000}}</ref>
| 9%
|-
|9–32 W [[Compact fluorescent lamp|compact fluorescent]] (with ballast)
|46–75<ref name="philc"/><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>
|8–11.45%<ref name="CF_efficiency">{{cite web | title=Global bulbs | url=http://www.1000bulbs.com/32-Watt-Compact-Fluorescents/37889/ | publisher= 1000Bulbs.com accessdate=2010-2-20}}|</ref>
|-
|T8 tube with electronic ballast
| 80–100<ref name=FEMP/>
| 12–15%
|-
|PL-S 11 W U-tube, excluding ballast loss
| 82<ref name="U-tubes">{{cite web | author=Phillips | title=Phillips Master | url=http://skinflint.co.uk/a416644.html | accessdate=2010-12-21}}</ref>
| 12%
|-
|T5 tube
| 70–104.2<ref name="energyrating">{{cite web | author=Department of the Environment, Water, Heritage and the Arts, Australia | title=Energy Labelling—Lamps | url=http://www.energyrating.gov.au/appsearch/download.asp | accessdate=2008-08-14}}</ref><ref name="Plusrite">{{cite web | url=http://www.bulbamerica.com/osram-24w-t5-miniature-bi-pin-compact-fluorescent-light-bulb-1.html | publisher=Bulbamerica.com| accessdate=2010-02-20|title=BulbAmerica.com}}</ref>
| 10–15.63%
|-
| rowspan="5" style="text-align:center;"|[[Gas-discharge lamp|Gas discharge]]
|1400 W [[sulfur lamp]]
|100<ref>{{cite news |url=http://www.iaeel.org/IAEEL/iaeel/newsl/1996/ett1996/LiTech_b_1_96.html |title=1000-watt sulfur lamp now ready |work=IAEEL newsletter |year=1996 |issue=1 |publisher=IAEEL |archiveurl=http://web.archive.org/web/20030818061414/195.178.164.205/IAEEL/iaeel/newsl/1996/ett1996/LiTech_b_1_96.html |archivedate=2003-08-18}}</ref><!--Prototype, not production unit.-->
|15%
|-
|[[metal halide lamp]]
|65–115<ref>{{cite web |url=http://www.venturelighting.com/TechCenter/Metal-Halide-TechIntro.html |title=The Metal Halide Advantage |year=2007 |work=Venture Lighting |accessdate=2008-08-10}}</ref>
|9.5–17%
|-
|[[Sodium vapor lamp#High pressure / HPS / SON|high pressure sodium lamp]]
|85–150<ref name="philc"/>
|12–22%
|-
|[[Sodium vapor lamp#Low pressure / LPS / SOX|low pressure sodium lamp]]
|100–200<ref name="philc"/><ref name="sodium">{{cite web |title=LED or Neon? A scientific comparison |url=http://www.signweb.com/index.php/channel/12/id/138/}}</ref><ref name="lightning">{{cite web |url=http://webexhibits.org/causesofcolor/4.html |title=Why is lightning coloured? (gas excitations) |publisher=webexhibits.org}}</ref>
|15–29%
|-
|[[Plasma display panel]]
|2-10<ref>{{cite web |url=ftp://ftp.panasonic.com/pub/Panasonic/consumer_electronics/whitepapers/Future_Looks_Bright_for_Plasma_TVs.pdf |title=Future Looks Bright for Plasma TVs |publisher=Panasonic |year=2007 |accessdate=2013-02-10}}</ref>
|0.3–1.5%
|-
| style="text-align:center;"|[[Electron stimulated luminescence|Cathodoluminescence]]
|[[electron stimulated luminescence]]
|30 {{cn|date=December 2012}}
|5%
|-
| rowspan="2" style="text-align:center;"|Ideal sources
|Truncated 5800&nbsp;K blackbody<ref group="note" name="ideal_white"/>
|251<ref name="ideal-white"/>
|37%
|-
|Green light at 555&nbsp;nm (maximum possible luminous efficacy)
|683.002<ref name="luminosity"/>
|100%
|}
 
Sources that depend on thermal emission from a solid filament, such as [[incandescent light bulb]]s, tend to have low overall efficacy because, as explained by Donald L. Klipstein, “An ideal thermal radiator produces visible light most efficiently at temperatures around 6300&nbsp;°C (6600&nbsp;K or 11,500&nbsp;°F). Even at this high temperature, a lot of the radiation is either infrared or ultraviolet, and the theoretical luminous [efficacy] is 95 lumens per watt. No substance is solid and usable as a light bulb filament at temperatures anywhere close to this. The surface of the sun is not quite that hot.”<ref name="bulbguide"/> At temperatures where the [[tungsten]] filament of an ordinary light bulb remains solid (below 3683 kelvins), most of its emission is in the [[infrared]].<ref name="bulbguide"/>
 
==SI photometry units==
 
{{SI_light_units}}
 
==See also==
*[[Photometry (optics)|Photometry]]
*[[Light pollution]]
*[[Wall-plug efficiency]] – a related principle, but slightly different
*[[Coefficient of utilization]]
 
==Notes==
<references group="note" />
 
==References==
{{Reflist|30em}}
 
==External links==
*[[Hyperphysics]] has these [http://hyperphysics.phy-astr.gsu.edu/hbase/vision/bright.html#c2 graphs of efficacy] that do not quite comply with the standard definition
*[http://www.cus.net/electricity/subcats/eleclighting.html Energy Efficient Light Bulbs]
*[http://www.otherpower.com/otherpower_lighting.html Other Power]
 
[[Category:Photometry]]
[[Category:Physical quantities]]
[[Category:Lighting]]
[[Category:Energy economics]]
 
[[ru:Световая отдача]]

Latest revision as of 21:10, 2 January 2015

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