View source for Luminous efficacy

{{short description|Measure of how well a light source produces visible light}}
'''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]], measured in [[Lumen (unit)|lumens]] per [[watt]] in the [[International System of Units]] (SI). 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 |author1=Franc Grum |author2=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>
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''' or '''overall luminous efficacy'''.<ref>{{cite book |title=Photovoltaic systems engineering |edition=2 |author1=Roger A. Messenger |author2=Jerry Ventre |publisher=CRC Press |year=2004 |isbn=978-0-8493-1793-4 |page=123}}</ref><ref>{{cite book |title=Color imaging: fundamentals and applications |author1=Erik Reinhard |author2=Erum Arif Khan |author3=Ahmet Oğuz Akyüz |author4=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 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-->
Luminous efficacy can be normalized by the maximum possible luminous efficacy to a [[dimensionless]] quantity called '''luminous efficiency'''. 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.

==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.

Photopic luminous efficacy of radiation has a maximum possible value of {{nowrap|683 lm/W}}, for the case of monochromatic light at a wavelength of approximately {{nowrap|555 nm}} (green). Scotopic luminous efficacy of radiation reaches a maximum of {{nowrap|1700 lm/W}} for monochromatic light at a wavelength of {{nowrap|507 nm}}.

===Mathematical definition===
'''Luminous efficacy''', denoted ''K'', is defined as<ref name="CIE">{{cite web|url=http://eilv.cie.co.at/term/730|title=Luminous efficacy (of radiation)|publisher=[[International Commission on Illumination|CIE]]|accessdate=2016-06-07}}</ref>
: <math>K = \frac{\Phi_\mathrm{v}}{\Phi_\mathrm{e}} = \frac{\int_0^\infty K(\lambda) \Phi_{\mathrm{e},\lambda}\,\mathrm{d}\lambda}{\int_0^\infty \Phi_{\mathrm{e},\lambda}\,\mathrm{d}\lambda},</math>

where
* Φ<sub>v</sub> is the [[luminous flux]];
* Φ<sub>e</sub> is the [[radiant flux]];
* Φ<sub>e,λ</sub> is the [[radiant flux|spectral radiant flux]];
* {{nowrap|1=''K''(''λ'') = ''K''<sub>m</sub>''V''(''λ'')}} is the [[luminosity function|spectral luminous efficacy]].

===Examples===
====[[Photopic vision]]====
{| class="wikitable sortable"
|-
! Type
! Luminous efficacy <br/>of radiation (lm/W)
! Luminous <br/>efficiency{{efn|group=note|name=max|Defined such that the maximum possible luminous ''efficacy'' corresponds to a luminous ''efficiency'' of 100%.}}
|-
| Tungsten light bulb, typical, 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{{nbsp}}K]]
| 30
| 4%
|-
| [[Black-body]], 4000&nbsp;K, ideal
| 54.7<ref name="blackbody">[[:De:Bild:Blackbodyvisiblerp.png|Black body visible spectrum]]</ref>
| 8%
|-
| Class G star ([[Sun]], [[Capella (star)|Capella]]), 5800{{nbsp}}K
| 93<ref name="ideal-white" />
| 13.6%
|-
| Black-body, 7000&nbsp;K, ideal
| 95<ref name="blackbody" />
| 14%
|-
| Black-body, 5800&nbsp;K, truncated to 400–700&nbsp;nm (ideal "white" source){{efn|group=note|Most efficient source that mimics the solar spectrum within range of human visual sensitivity.}} 
| 251<ref name="ideal-white" />{{efn|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]].}}<ref name="max-eff-truncated-1" />
| 37%
|-
| Black-body, 5800&nbsp;K, truncated to ≥&nbsp;2% photopic sensitivity range{{efn|group=note|name=max-eff-truncated|Omits the part of the spectrum where the eye's sensitivity is very poor.}}
| 292<ref name="max-eff-truncated-1">
 {{cite journal|doi=10.1063/1.4721897
  | title=Maximum spectral luminous efficacy of white light|journal=Journal of Applied Physics
  | volume=111 | issue=10 | page=104909 | year=2012
  | last1=Murphy | first1=Thomas W.
  | arxiv = 1309.7039
  | bibcode = 2012JAP...111j4909M
 }}</ref>
| 43%
|-
| Black-body, 2800&nbsp;K, truncated to ≥&nbsp;2% photopic sensitivity range{{efn|group=note|name=max-eff-truncated}} 
| 299<ref name="max-eff-truncated-1" />
| 44%
|-
| Black-body, 2800&nbsp;K, truncated to ≥&nbsp;5% photopic sensitivity range{{efn|group=note|name=max-eff-truncated-1|Omits the part of the spectrum where the eye's sensitivity is low (≤&nbsp;5% of the peak).}}
| 343<ref name="max-eff-truncated-1" />
| 50%
|-
| Black-body, 5800&nbsp;K, truncated to ≥&nbsp;5% photopic sensitivity range{{efn|group=note|name=max-eff-truncated-1}}
| 348<ref name="max-eff-truncated-1" />
| 51%
|-
| Ideal monochromatic 540&nbsp;THz source
| 683<ref name=SI-statement>
{{cite web
 |url = https://www.bipm.org/utils/common/pdf/SI-statement.pdf
 |title = BIPM statement: Information for users about the proposed revision of the SI
 |access-date = 5 May 2018
 |archive-url = https://web.archive.org/web/20180121160000/https://www.bipm.org/utils/common/pdf/SI-statement.pdf
 |archive-date = 21 January 2018
 |url-status = live
 |df = dmy-all
 }}
</ref>
| 100%
|}

====[[Scotopic vision]]====
{| class="wikitable"
|-
! Type
! Luminous efficacy <br/>of radiation (lm/W)
! Luminous <br/>efficiency{{efn|group=note|name=max}}
|-
| Ideal monochromatic 507&nbsp;nm source
| 1699<ref name="Kohei2004">
 {{cite book
  | title   = Light Pollution Handbook
  | author1  = Kohei Narisada
  | author2  = Duco Schreuder
  | publisher = Springer
  | year   = 2004
  | isbn   = 1-4020-2665-X
 }}</ref> or 1700<ref name="Casimer1998">
 {{cite book
  | title   = Handbook of Applied Photometry
  | author  = Casimer DeCusatis
  | publisher = Springer
  | year   = 1998
  | isbn   = 1-56396-416-3
 }}</ref>
| 100%
|}

[[File:Blackbody efficacy 1000-16000K.svg|left|thumb|500px]]
[[File:Wiens law vis limits.svg|none|thumb|300px|[[Spectral radiance]] of a [[black body]]. Energy outside the [[Visible spectrum|visible wavelength]] range (~380–750{{nbsp}}nm, shown by grey dotted lines) reduces the luminous efficiency.]]

==Lighting efficiency== <!--Many terms redirect to this section-->
{{main|Wall-plug efficiency}}
Artificial light sources are usually evaluated in terms of luminous efficacy of the source, also sometimes called ''wall-plug 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. The luminous efficacy of the source 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 ''luminous efficiency of a source'', ''overall luminous efficiency'' or ''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 sortable"
|-
! Category
! Type
! data-sort-type="number" | Overall luminous <br />efficacy (lm/W)
! data-sort-type="number" | Overall luminous <br />efficiency{{efn|group=note|name=max}}
|-
| rowspan="2" style="text-align:center;" | Combustion
| [[Candle]]
| 0.3{{efn|group=note|1{{nbsp}}[[candela]] × 4π{{nbsp}}[[steradian]]s/40{{nbsp}}W}}
| 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=https://books.google.com/books?id=rWxLAAAAMAAJ&dq=mantle%20lamp&pg=PA490#v=onepage&q=mantle%20lamp}}</ref>
| 0.15–0.3%
|-
| rowspan="2" style="text-align:center;" | [[Incandescent light bulb|Incandescent]]
| 15, 40, 100{{nbsp}}W tungsten incandescent (230&nbsp;V)
| 8.0, 10.4, 13.8<ref>{{cite web |url=http://www.lighting.philips.com/main/prof/lamps/incandescent-lamps/standard-t-a-e-shape/classictone-standard/920119143329_EU/product |title=Philips Classictone Standard 15 W clear}}</ref><ref>{{cite web |url=http://www.lighting.philips.com/main/prof/lamps/incandescent-lamps/standard-t-a-e-shape/classictone-standard/920053843329_EU/product |title=Philips Classictone Standard 40 W clear}}</ref><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><ref name="philc">[http://www.lighting.philips.com/de_de/tools_downloads/pricelist_lamps/downloads/preisliste_dede_20081023.pdf Philips Product Catalog] {{dead link|date=January 2018|bot=medic}}{{cbignore|bot=medic}} (German)</ref>
| 1.2, 1.5, 2.0%
|-
| 5, 40, 100{{nbsp}}W tungsten incandescent (120&nbsp;V)
| 5, 12.6, 17.5<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=2016-04-15 |archiveurl=https://web.archive.org/web/20120118001547/http://www.ccri.edu/physics/keefe/light.htm |archivedate=2012-01-18}}</ref>
| 0.7, 1.8, 2.6%
|-
| rowspan="5" style="text-align:center;"|[[halogen lamp|Halogen incandescent]]
| 100, 200, 500{{nbsp}}W tungsten halogen (230&nbsp;V)
| 16.7, 17.6, 19.8<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=https://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><ref name="philc" />
| 2.4, 2.6, 2.9%
|-
| 2.6{{nbsp}}W tungsten 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 |archive-url=https://web.archive.org/web/20160213071457/http://www.bulbtronics.com/Search-The-Warehouse/ProductDetail.aspx?sid=0000747&pid=OS6406330&AspxAutoDetectCookieSupport=1 |url-status=dead |archive-date=2016-02-13 |title=Osram 6406330 Miniwatt-Halogen 5.2V |publisher=bulbtronics.com |accessdate=2013-04-16 }}</ref>
| 2.8%
|-
| Halogen-IR (120&nbsp;V)
| 17.7–24.5<ref>{{cite web |url=http://www.gelighting.com/LightingWeb/na/images/71886_HIR_Plus_Halogen_PAR38_SellSheet_tcm201-20752.pdf |title=GE Lighting HIR Plus Halogen PAR38s |publisher=ge.com |accessdate=2017-11-01}}</ref>
| 2.6–3.5%
|-
| 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 |archive-url=https://web.archive.org/web/20010909055127/http://freespace.virgin.net/tom.baldwin/bulbguide.html |url-status=dead |archive-date=2001-09-09 |accessdate=2006-04-16 }}</ref>
| 5.1%
|-
| rowspan="4" style="text-align:center;" | [[Light-emitting diode]]
| LED [[Edison screw|screw base]] lamp (120&nbsp;V)
| Up to {{Rnd|{{#expr:550/5.4}}|0}}<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><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=https://web.archive.org/web/20110719165551/http://www.tlt.co.jp/tlt/new/lamp/hp_led/minikry_lda5.htm |archivedate=2011-07-19}}</ref><ref>[http://ledsreview.com/news/367/ Toshiba to release 93 lm/W LED bulb] Ledrevie</ref>
| Up to {{Rnd|{{#expr:550/5.4/6.83002}}|1}}%
|-
| 11{{nbsp}}W LED screw base lamp (230{{nbsp}}V)
| 138<ref name=":0">{{Cite web|url=http://www.p4c.philips.com/cgi-bin/cpindex.pl?ctn=8718696742396&hlt=Link_ProductInformation&mid=Link_ProductInformation&scy=FR&slg=AEN|title=8718696742396 Philips LED Bulb 8718696742396 11W (100W) E27 Warm white Non-dimmable - Philips Support|last=Care|first=Philips Consumer Electronics Global Consumer|website=www.p4c.philips.com|access-date=2017-09-07}}</ref>
| 20.3%
|-
| 21.5{{nbsp}}W LED retrofit for T8 fluorescent tube (230{{nbsp}}V)
| 172<ref>{{Cite web|url=http://www.lighting.philips.com/main/prof/led-lamps-and-tubes/led-tubes/master-ledtube-em-mains-t8/929001377002_EU/product|title=MAS LEDtube 1500mm UE 21.5W 840 T8|access-date=2018-01-10}}</ref>
| 25%
|-
| Theoretical limit for a 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="5" style="text-align:center;" | [[Arc lamp]]
|[[Carbon arc lamp]]
| 2–7<ref name="carbon">{{cite web | title=Arc Lamps | publisher=Edison Tech Center | url=http://www.edisontechcenter.org/ArcLamps.html
|accessdate=2015-08-20}}</ref>
| 0.29–1.0%
|-
| [[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|Ultra-high-pressure]] (UHP) [[mercury-vapor lamp|mercury-vapor]] arc lamp, free mounted
| 58–78<ref>[http://www.koti.mbnet.fi/jahonen/Electronics/Stuff/UHP_Lamp.pdf REVIEW ARTICLE: UHP lamp systems for projection applications]{{Dead link|date=November 2016 |bot=InternetArchiveBot |fix-attempted=yes }} Journal of Physics D: Applied Physics</ref>
| 8.5–11.4%
|-
| Ultra-high-pressure (UHP) mercury-vapor arc lamp, 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="6" style="text-align:center;" | [[Fluorescent lamp|Fluorescent]]
| 32{{nbsp}}W T12 tube with magnetic ballast
| 60<ref name=FEMP>{{cite journal|url=http://www1.eere.energy.gov/femp/procurement/eep_fluortube_lamp.html |archive-url=https://web.archive.org/web/20070702014038/http://www1.eere.energy.gov/femp/procurement/eep_fluortube_lamp.html |url-status=dead |archive-date=2007-07-02 |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{{nbsp}}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 |url-status=dead |archiveurl=https://web.archive.org/web/20081013115302/http://www.energyfederation.org/consumer/default.php/cPath/25_44_3006 |archivedate=October 13, 2008 }}</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 |url-status=dead |archiveurl=https://web.archive.org/web/20081014010312/http://www.energyfederation.org/consumer/default.php/cPath/25_44_784 |archivedate=October 14, 2008 }}</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=2010-02-20}}|</ref>
|-
| T8 tube with electronic ballast
| 80–100<ref name=FEMP />
| 12–15%
|-
| PL-S 11{{nbsp}}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 |url-status=dead |archiveurl=https://web.archive.org/web/20080723003909/http://www.energyrating.gov.au//appsearch/download.asp |archivedate=July 23, 2008 }}</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 |url-status=dead |archiveurl=https://web.archive.org/web/20121201003233/http://www.bulbamerica.com/osram-24w-t5-miniature-bi-pin-compact-fluorescent-light-bulb-1.html |archivedate=December 1, 2012 }}</ref>
| 10–15.63%
|-
| 70–150{{nbsp}}W inductively-coupled electrodeless lighting system
| 71–84<ref name="ICETRON®">{{cite web | author=SYLVANIA | title=Sylvania Icetron Quicktronic Design Guide | url=http://www.acuitybrands.com/products/-/media/files/acuity/products/lighting/featured%20technology/induction/icetron.pdf | accessdate=2015-06-10}}</ref>
| 10–12%
|-
| rowspan="5" style="text-align:center;" | [[Gas-discharge lamp|Gas discharge]]
| 1400{{nbsp}}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=https://web.archive.org/web/20030818061414/http://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/content/led-or-neon?page=0%2C1}}</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{{citation needed|date=December 2012}}
| 5%
|-
| rowspan="2" style="text-align:center;" | Ideal sources
| Truncated 5800&nbsp;K black-body{{efn|group=note|name=ideal_white}}
| 251<ref name="ideal-white" />
| 37%
|-
| Green light at 540&nbsp;THz (maximum possible luminous efficacy by definition)
| 683<ref name=SI-statement/>
| 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 kelvin), 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]]
* [[Coefficient of utilization]]
* [[List of light sources]]

==Notes==
{{noteslist|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]

{{Artificial light sources}}

[[Category:Photometry]]
[[Category:Physical quantities]]
[[Category:Lighting]]
[[Category:Energy economics]]