Luminous efficacy

{{Short description|How efficiently a source produces visible light}} {{Infobox physical quantity | name = Luminous efficacy | image = | caption = | unit = lm⋅W{{superscript|−1}} | symbols = ''K'' | baseunits = cd⋅sr⋅s{{superscript|3}}⋅kg{{superscript|−1}}⋅m{{superscript|−2}} | dimension = <math>\mathsf J \mathsf{T}^{3} \mathsf{M}^{-1} \mathsf{L}^{-2}</math> | extensive = | conserved = | derivations = }}

'''Luminous efficacy''' is a measure of how efficiently a light source produces visible light. It is the ratio of luminous flux to power, measured in 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|bibcode=1974wi...book.....S }}</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''',<ref name="IEC_845-21-090">International Electrotechnical Commission (IEC): ''International Electrotechnical Vocabulary'', [http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=845-21-090 ref. 845-21-090, Luminous efficacy of radiation (for a specified photometric condition)]</ref> and the latter '''luminous efficacy of a light source'''<ref name="IEC_845-21-089">International Electrotechnical Commission (IEC): ''International Electrotechnical Vocabulary'', [http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=845-21-089 ref. 845-21-089, Luminous efficacy (of a light source)]</ref> or '''overall luminous efficacy'''.<ref>{{cite book |title=Photovoltaic systems engineering |url=https://archive.org/details/photovoltaicsyst00mess_644 |url-access=limited |edition=2 |author1=Roger A. Messenger |author2=Jerry Ventre |publisher=CRC Press |year=2004 |isbn=978-0-8493-1793-4 |page=[https://archive.org/details/photovoltaicsyst00mess_644/page/n145 123]}}</ref><ref>{{cite book |title=Color imaging: fundamentals and applications |url=https://archive.org/details/colorimagingfund00rein |url-access=limited |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=[https://archive.org/details/colorimagingfund00rein/page/n353 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 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==

By definition, light outside the visible spectrum cannot be seen by the standard human vision system, and therefore does not contribute to, and indeed can subtract from, luminous efficacy.

===Explanation=== [[File:CIE 1931 Luminosity.png|right|thumb|The typical response of human vision to light under daytime or bright conditions, as standardized by the CIE in 1924. The horizontal axis is wavelength in nanometers.<ref>{{Cite report |url=https://www.iso.org/standard/41641.html |title=ISO 23539:2005 Photometry — The CIE system of physical photometry |last=ISO |date=2005 |access-date=2022-01-05}}</ref>]]

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.<ref name="IEC_845-21-090" /> Light wavelengths outside the visible spectrum reduce luminous efficacy, because they contribute 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.

Wavelengths of light outside of the visible spectrum are not useful for general illumination{{efn|group=note|There are special cases of illumination involving wavelengths of light that are outside the human visible range. One example is Ultraviolet light which is not itself visible, but can excite some pigments to fluoresce, where the pigments re-emit the light into the visible range. Such special cases are not a contributing part of luminous efficacy calculations.}}. Furthermore, human vision responds more to some wavelengths of light than others. This response of the eye is represented by the luminous efficiency function. This is a standardized function representing photopic vision, which models the response of the eye's cone cells, that are active under typical daylight conditions. A separate curve can be defined for dark/night conditions, modeling the response of rod cells ''without'' cones, known as scotopic vision. (Mesopic vision describes the transition zone in dim conditions, between photopic and scotopic, where both cones and rods are active.)

Photopic luminous efficacy of radiation has a maximum possible value of {{nowrap|683.002 lm/W}}, for the case of monochromatic light at a wavelength of {{nowrap|555 nm}} {{Colorsample|#78ff00|border=#55b900}}.{{efn|group=note|Standard vision typically perceives {{nowrap|555 nm}} as a hue of yellowish-green {{Colorsample|#78ff00|border=#55b900}}, which can be emulated on an sRGB display with CSS color value <code>rgb(120,255,0)</code> or hex <code>#78ff00</code>.}} Scotopic luminous efficacy of radiation reaches a maximum of {{nowrap|1700 lm/W}} for monochromatic light at a wavelength of {{nowrap|507 nm}}.{{efn|group=note|Under standard photopic vision {{nowrap|507 nm}} is perceived as a blue-green hue similar to viridian {{Colorsample|#00a880|border=#005a43}}, however scotopic rod-only vision does not create a color sensation in the standard human vision system.}}

===Mathematical definition=== '''Luminous efficacy (of radiation)''', denoted ''K'', is defined as<ref name="IEC_845-21-090" /> : <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 * Φv is the luminous flux; * Φe is the radiant flux; * Φe,λ is the spectral radiant flux; * {{nowrap|1=''K''(''λ'') = ''K''m''V''(''λ'')}} is the spectral luminous efficacy.

===Examples=== ====Photopic vision==== {| class="wikitable sortable" |- ! Type ! Luminous efficacy of radiation (lm/W) ! Luminous 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 K | 15<ref name="ideal-white" /> | 2% |- | Class M star (Antares, Betelgeuse), 3300{{nbsp}}K | 30 | 4% |- | Black body, 4000 K, ideal | 54.7{{efn|group=note|name="blackbody"|Black body visible spectrum}} | 8% |- | Class G star (Sun, Capella), 5800{{nbsp}}K | 93<ref name="ideal-white" /> | 13.6% |- | Black-body, 7000 K, ideal | 95{{efn|group=note|name="blackbody"}} | 14% |- | Black-body, 5800 K, truncated to 400–700 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 683.002 lm/W.}} | 37% |- | Black-body, 5800 K, truncated to ≥ 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="ideal-white" /> | 43% |- | Black-body, 2800 K, truncated to ≥ 2% photopic sensitivity range{{efn|group=note|name=max-eff-truncated}} | 299<ref name="ideal-white" /> | 44% |- | Black-body, 2800 K, truncated to ≥ 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 (≤ 5% of the peak).}} | 343<ref name="ideal-white" /> | 50% |- | Black-body, 5800 K, truncated to ≥ 5% photopic sensitivity range{{efn|group=note|name=max-eff-truncated-1}} | 348<ref name="ideal-white" /> | 51% |- | Monochromatic source at {{val|540|u=THz}} | 683 (exact)<ref name=BIPM-source/> | 99.9997% |- | Ideal monochromatic source: {{val|555|u=nm}} (in air) | 683.002<ref name="BIPM-source">{{cite web |title=Principles Governing Photometry 2nd edition |url=https://www.bipm.org/documents/20126/52718503/G1-2019.pdf/692275b2-049d-5222-35de-c975dd1850fc |publisher=Bureau International des Poids et Mesures |access-date=30 December 2025 |archive-url=https://web.archive.org/web/20220703191421/https://www.bipm.org/documents/20126/52718503/G1-2019.pdf/692275b2-049d-5222-35de-c975dd1850fc |archive-date=3 July 2022 |page=10 |url-status=live}}</ref> | 100% |}

==== Scotopic vision ==== {| class="wikitable" !Type !Luminous efficacy of radiation (lm/W) !Luminous efficiency{{efn|group=note|name=max}} |- |Ideal monochromatic 507 nm source |1699<ref name="Kohei20042">{{cite book|author1=Kohei Narisada|title=Light Pollution Handbook|author2=Duco Schreuder|publisher=Springer|year=2004|isbn=1-4020-2665-X}}</ref> or 1700<ref name="Casimer19982">{{cite book|author=Casimer DeCusatis|title=Handbook of Applied Photometry|publisher=Springer|year=1998|isbn=1-56396-416-3}}</ref> |100% |} left|thumb|500x500px [[File:Wiens_law_vis_limits.svg|none|thumb|300x300px|Spectral radiance of a black body. Wavelengths outside the visible light band (~380–750{{nbsp}}nm, bounded within grey dotted lines) have very low luminous efficiency.]]

== Lighting efficiency ==  {{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/electronic ballast are unless noted (see also voltage) listed without losses for that, reducing total efficiency.

{| class="wikitable sortable" |- ! Category ! Type ! data-sort-type="number" | Overall luminous efficacy (lm/W) ! data-sort-type="number" | Overall luminous efficiency{{efn|group=note|name=max}} |- | style="text-align:center;" | Combustion | 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&q=mantle+lamp&pg=PA490}}</ref> | 0.15–0.3% |- | rowspan="2" style="text-align:center;" | Incandescent | 15, 40, 100{{nbsp}}W tungsten incandescent (230 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 |access-date=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=October 2021|bot=medic}}{{cbignore|bot=medic}} (German)</ref> | 1.2, 1.5, 2.0% |- | 5, 40, 100{{nbsp}}W tungsten incandescent (120 V) | 5.0, 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 |access-date=2016-04-15 |archive-url=https://web.archive.org/web/20120118001547/http://www.ccri.edu/physics/keefe/light.htm |archive-date=2012-01-18}}</ref> | 0.7, 1.8, 2.6% |- | rowspan="5" style="text-align:center;"|Halogen incandescent | 100, 200, 500{{nbsp}}W tungsten halogen (230 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=de |access-date=2008-01-28 |archive-url=https://web.archive.org/web/20071107054500/http://www.osram.de/_global/pdf/osram_de/tools_services/downloads/allgemeinbeleuchtung/halogenlampen/haloluxhalopar.pdf |archive-date=November 7, 2007}}</ref><ref name="philc" /> | 2.4, 2.6, 2.9% |- | 2.6{{nbsp}}W tungsten halogen (5.2 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 |access-date=2013-04-16 }}</ref> | 2.8% |- | Halogen-IR (120 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 |access-date=2017-11-01}}</ref> | 2.6–3.5% |- | Tungsten quartz halogen (12–24 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 |access-date=2006-04-16 }}</ref> | 5.1% |- | rowspan="5" style="text-align:center;" | Light-emitting diode | LED screw base lamp (120 V) | {{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 |access-date=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 |archive-url=https://web.archive.org/web/20110719165551/http://www.tlt.co.jp/tlt/new/lamp/hp_led/minikry_lda5.htm |archive-date=2011-07-19}}</ref><ref>[http://ledsreview.com/news/367/ Toshiba to release 93 lm/W LED bulb] Ledrevie</ref> | {{Rnd|{{#expr:550/5.4/6.83002}}|1}}% |- | 5–16{{nbsp}}W LED screw base lamp (230 V) | 75–217<ref>{{Cite web |title=EGLO 110326 technical datasheet |url=https://tools.eglo.com/tds/uk-EN/110326 |access-date=2024-09-13 |website=EGLO |format=PDF}}</ref><ref>{{Cite web |title=LED Bulb Filament A60 / E27 / 5 W (75 W) / 1 060 lm / neutral white EN {{!}} EMOS |url=https://en.b2b.emos.cz/led-bulb-filament-a60-e27-5-w-75-w-1-060-lm-neutral-white-2 |access-date=2024-05-09 |website=en.b2b.emos.cz}}</ref><ref name=":0">{{Cite web|url=https://www.lighting.philips.com/main/prof/led-lamps-and-tubes/led-bulbs#pfpath=0-LED_GR|title=Philips - LED bulbs|access-date=2020-03-14}}</ref><ref>{{Cite web|title=LED CLA 60W A60 E27 4000K CL EELA SRT4 {{!}} null|url=https://www.lighting.philips.co.uk/consumer/p/led-bulb/8719514343801/specifications|access-date=2021-09-26|website=www.lighting.philips.co.uk|language=en-gb}}</ref> | 11–32% |- | 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_LED_efficacy">{{cite web |last1=Zyga |first1=Lisa |title=White LEDs with super-high luminous efficacy could satisfy all general lighting needs |url=https://phys.org/news/2010-08-white-super-high-luminous-efficacy.html |publisher=Phys.org |access-date=17 November 2021 |language=en |date=2010-08-31}}</ref> | {{Rnd|{{#expr:260/6.83002}}|1}}–{{Rnd|{{#expr:300/6.83002}}|1}}% |- |Red LED 660nm | |83%<ref>{{Cite web |title=[Exclusive Interview] Focusing on Energy Efficiency and Spectral Innovation, Cree LED Addresses a New Cycle in Horticultural Lighting with High Reliability and IP-Free Strategy |url=https://www.ledinside.com/interview/2026/1/interview_cree_led_horticultural_lighting |access-date=2026-01-07 |website=www.ledinside.com |language=english}}</ref> |- | 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 |access-date=2015-08-20}}</ref> | 0.29–1.0% |- | Xenon arc lamp | 30–90<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 |access-date=2010-05-01}} Note that the figure of 150 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><ref>{{Cite web|title=XENARC ORIGINAL D1S {{!}} OSRAM Automotive|url=https://www.osram.com/ecat/XENARC%20ORIGINAL-Xenon%20headlight%20lamps-Truck%20lighting-Automotive/com/en/GPS01_1057100/ZMP_83508/|access-date=2021-09-30|website=www.osram.com}}</ref> | 4.4–13.5% |- | Mercury-xenon arc lamp | 50–55<ref name="xenon" /> | 7.3–8% |- | Ultra-high-pressure (UHP) 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 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 | 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 (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 |access-date=2008-12-23 |url-status=dead |archive-url=https://web.archive.org/web/20081013115302/http://www.energyfederation.org/consumer/default.php/cPath/25_44_3006 |archive-date=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 |access-date=2008-12-23 |url-status=dead |archive-url=https://web.archive.org/web/20081014010312/http://www.energyfederation.org/consumer/default.php/cPath/25_44_784 |archive-date=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 |access-date=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 | access-date=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 |access-date=2008-08-14 |url-status=dead |archive-url=https://web.archive.org/web/20080723003909/http://www.energyrating.gov.au//appsearch/download.asp |archive-date=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 |access-date=2010-02-20 |title=BulbAmerica.com |url-status=dead |archive-url=https://web.archive.org/web/20121201003233/http://www.bulbamerica.com/osram-24w-t5-miniature-bi-pin-compact-fluorescent-light-bulb-1.html |archive-date=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 | access-date=2015-06-10}}</ref> | 10–12% |- | rowspan="5" style="text-align:center;" | 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 |archive-url=https://web.archive.org/web/20030818061414/http://195.178.164.205/IAEEL/iaeel/newsl/1996/ett1996/LiTech_b_1_96.html |archive-date=2003-08-18}}</ref> | 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 |access-date=2008-08-10 |archive-url=https://web.archive.org/web/20120215041921/http://www.venturelighting.com/techcenter/metal-halide-techintro.html |archive-date=2012-02-15 |url-status=dead }}</ref> | 9.5–17% |- | High-pressure sodium lamp | 85–150<ref name="philc" /> | 12–22% |- | 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><ref>{{cite book |doi=10.1109/PLASMA.1997.605090 |chapter=The low-pressure sodium lamp |title=IEEE Conference Record - Abstracts. 1997 IEEE International Conference on Plasma Science |date=1997 |last1=Hooker |first1=J.D. |page=289 |isbn=0-7803-3990-8 |s2cid=102792535 }}</ref> | 15–29% |- | Plasma display panel | 2–10<ref>{{cite web |url=http://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 |access-date=2013-02-10}}</ref> | 0.3–1.5% |- | style="text-align:center;" | Cathodoluminescence | Electron-stimulated luminescence | 30–110<ref>{{cite web |url=https://www.osa-opn.org/home/newsroom/2019/july/tv-tube_technology_builds_an_efficient_light_bulb/ |title=TV-Tube Technology Builds an Efficient Light Bulb |publisher=OSA |year=2019 |access-date=2020-09-12}}</ref><ref>{{cite journal |url=https://avs.scitation.org/doi/abs/10.1116/1.5070108 |title=Prototype of cathodoluminescent lamp for general lighting using carbon fiber field emission cathode |publisher=AVS |year=2019 |doi=10.1116/1.5070108 |access-date=2020-09-12|last1=Sheshin |first1=Evgenii P. |last2=Kolodyazhnyj |first2=Artem Yu. |last3=Chadaev |first3=Nikolai N. |last4=Getman |first4=Alexandr O. |last5=Danilkin |first5=Mikhail I. |last6=Ozol |first6=Dmitry I. |journal=Journal of Vacuum Science & Technology B |volume=37 |issue=3 |page=031213 |bibcode=2019JVSTB..37c1213S |s2cid=155496503 |url-access=subscription }}</ref> | 15% |- | rowspan="2" style="text-align:center;" | Ideal sources | Truncated 5800 K black-body{{efn|group=note|name=ideal_white}} | 251<ref name="ideal-white" /> | 37% |- | Green light at {{val|555|u=nm}} (maximum possible luminous efficacy by definition) | 683.002<ref name=BIPM-source/><ref name="Choudhury">{{cite book | last = Choudhury | first = Asim Kumar Roy | author-link = | date = 2014 | title = Principles of Colour and Appearance Measurement: Object appearance, colour perception and instrumental measurement | volume = 1 | chapter = Characteristics of light sources: luminous efficacy of lamps | url = | location = | publisher = Woodhead Publishing | page = 41 | isbn = 978-0-85709-229-8 | doi = 10.1533/9780857099242.1 | quote = If the lamp emits all radiation at 555 nm (where Vλ = 1), the luminous efficacy will be of about 680 lm W−1, the theoretical maximum value. The lamp efficacy will be 26 and 73 lm W−1, when the whole light is emitted at 450 and 650 nm respectively. The luminous coefficient is luminous efficiency expressed as a value between zero and one, with one corresponding to an efficacy of 683 lm W−1. }}</ref> | 100%<ref name="Choudhury" /><ref>{{Cite web |title=Figure 5. Wall plug efficiency (WPE), luminous efficacy and eye... |url=https://www.researchgate.net/figure/Wall-plug-efficiency-WPE-luminous-efficacy-and-eye-response-curve-full-line-curve-of_fig4_338355749 |archive-url=http://web.archive.org/web/20250806073252/https://www.researchgate.net/figure/Wall-plug-efficiency-WPE-luminous-efficacy-and-eye-response-curve-full-line-curve-of_fig4_338355749 |archive-date=2025-08-06 |access-date=2026-01-07 |website=ResearchGate |language=en}}</ref> |}

Sources that depend on thermal emission from a solid filament, such as incandescent light bulbs, 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 °C (6600 K or 11,500 °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 * Light pollution * Wall-plug efficiency * Coefficient of utilization * List of light sources * SI defining constants, including ''K''cd (used in the definition of candela)

==Notes== {{noteslist|group=note}}

==References== {{reflist|30em|refs= <ref name="ideal-white"> {{cite journal|doi=10.1063/1.4721897 | url=https://tmurphy.physics.ucsd.edu/papers/JAP_111_104909.pdf | title=Maximum spectral luminous efficacy of white light|journal=Journal of Applied Physics | volume=111 | issue=10 | year=2012 | last1=Murphy | first1=Thomas W. | author-link=Tom Murphy (physicist) | pages=104909–104909–6| article-number=104909 | arxiv = 1309.7039 | bibcode = 2012JAP...111j4909M | s2cid=6543030}}  </ref> }}

==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