View source for Luminous flux

{{short description|Perceived luminous power}}
{{Infobox physical quantity
| name = Luminous flux
| unit = [[Lumen (unit)|lumen]]
| otherunits =
| symbols = {{math|''Φ''<sub>[[Luminosity function|v]]</sub>}}
| baseunits = [[candela|cd]]⋅[[steradian|sr]]
| dimension = <math>\mathsf{J}</math>
}}
[[Image:Luminosity.png|thumb|right|400px|Photopic (black line) and scotopic <ref>{{Cite web|url=http://www.cvrl.org/database/text/lum/scvl.htm|title = Scotopic luminosity function}}</ref> (green line) luminosity functions.  The photopic includes the CIE 1931 standard (solid),<ref>{{Cite web|url=http://www.cvrl.org/database/text/cmfs/ciexyz31.htm|title=CIE 2-deg CMFS}}</ref> the Judd-Vos 1978 modified data (dashed),<ref>{{Cite web|url=http://www.cvrl.org/database/text/lum/vljv.htm|title = Judd-Vos modified Photopic luminosity function}}</ref> and the Sharpe, Stockman, Jagla & Jägle 2005 data (dotted).<ref>{{cite web|url=http://www.cvrl.org/database/text/lum/ssvl2.htm |title=Sharpe, Stockman, Jagla & Jägle (2005) 2-deg V*(l) luminous efficiency function |access-date=2007-05-10 |url-status=dead |archive-url=https://web.archive.org/web/20070927222337/http://www.cvrl.org/database/text/lum/ssvl2.htm |archive-date=2007-09-27 }}</ref> The horizontal axis is wavelength in nm.]]
[[Image:Luminance Chamber.jpg|thumb|right|upright|[[Integrating sphere]] used for measuring the luminous flux of a light source.]]
In [[photometry (optics)|photometry]], '''luminous flux''' or '''luminous power''' is the measure of the perceived power of [[light]]. It differs from [[radiant flux]], the measure of the total power of [[electromagnetic radiation]] (including [[infrared]], [[ultraviolet]], and visible light), in that luminous flux is adjusted to reflect the varying sensitivity of the [[human eye]] to different [[wavelength]]s of light.

==Units==
The [[SI]] unit of luminous flux is the [[lumen (unit)|lumen]] (lm). One lumen is defined as the luminous flux of light produced by a light source that emits one [[candela]] of [[luminous intensity]] over a solid angle of one [[steradian]].

<math display=block> 1\ \text{lm} = 1\ \text{cd} \times 1\ \text{sr}</math>

In other systems of units, luminous flux may have units of [[power (physics)|power]].

==Weighting==
The luminous flux accounts for the sensitivity of the eye by [[weighting]] the power at each wavelength with the [[luminosity function]], which represents the eye's response to different wavelengths. The luminous flux is a [[weighted sum]] of the power at all wavelengths in the visible band. Light outside the visible band does not contribute. The ratio of the total luminous flux to the radiant flux is called the [[luminous efficacy]]. This model of the [[human]] visual brightness perception, is standardized by the CIE and [[International Organization for Standardization|ISO]].<ref>{{cite book | url=https://www.iso.org/standard/83178.html | title=ISO/CIE 23539:2023 CIE TC 2-93 Photometry — The CIE system of physical photometry | publisher=ISO/CIE | date=2023 | language=en | doi=10.25039/IS0.CIE.23539.2023}}</ref>

==Context==
Luminous flux is often used as an objective measure of the useful light emitted by a [[Light#Light sources|light source]], and is typically reported on the packaging for [[Lamp (electrical component)|light bulbs]], although it is not always prominent. Consumers commonly compare the luminous flux of different light bulbs since it provides an estimate of the apparent amount of light the bulb will produce, and a lightbulb with a higher ratio of luminous flux to consumed power is more efficient.

Luminous flux is not used to compare [[brightness]], as this is a subjective perception which varies according to the distance from the light source and the angular spread of the light from the source.
{{clear}}
==Measurement== 

Luminous flux of artificial light sources is typically measured using an [[integrating sphere]], or a [[goniophotometer]] outfitted with a [[photometer]] or a spectroradiometer. <ref>{{cite book |last1=Schneider |first1=T. |last2=Young |first2=R. |last3=Bergen |first3=T. |last4=Dam-Hansen |first4=C |last5=Goodman |first5=T. |last6=Jordan |first6=W. |last7=Lee |first7=D.-H |last8=Okura |first8=T. |last9=Sperfeld |first9=P. |last10=Thorseth |first10=A |last11=Zong |first11=Y. |title=CIE 250:2022 Spectroradiometric Measurement of Optical Radiation Sources |date=2022 |publisher=CIE - International Commission on Illumination |location=Vienna |isbn=978-3-902842-23-7 |url=https://orbit.dtu.dk/en/publications/cie-2502022-spectroradiometric-measurement-of-optical-radiation-s}}</ref>
{{SI light units}}

=== Relationship to luminous intensity ===
[[File:photometry radiometry units.svg|thumb|upright=1.5|Comparison of photometric and radiometric quantities]]
Luminous flux (in lumens) is a measure of the total amount of light a lamp puts out. The luminous intensity (in candelas) is a measure of how bright the beam in a particular direction is. If a lamp has a 1 lumen bulb and the optics of the lamp are set up to focus the light evenly into a 1 [[steradian]] beam, then the beam would have a luminous intensity of 1 candela. If the optics were changed to concentrate the beam into 1/2 steradian then the source would have a luminous intensity of 2 candela. The resulting beam is narrower and brighter, however the luminous flux remains the same.

==Examples==
{|class="wikitable" style="width:500px;"
|+ '''Table of comparative luminous flux of several light sources'''<ref>{{Cite book
|title=Introduction to Architectural Science: The Basis of Sustainable Design
|last= Szokolay
|first=S. V. 
|edition=Second
|year=2008
|page=143
|publisher=Routledge
|url=https://books.google.com/books?id=jfISC1m3FqUC&pg=PA143
|isbn=9780750687041
}}</ref><ref>{{Cite book
|title=BeLight
|volume=3
|year=2010
|pages=10–12
|publisher=Trendforce
|url=https://books.google.com/books?id=Mgrd6ecnBnwC&pg=PA12 <!--|isbn=9772074075000-->
}}</ref><ref>{{Cite book
|title=Practical Handbook on Image Processing for Scientific and Technical Applications
|last=Jahne
|first=Bernd 
|edition=Second
|year=2004
|page=111
|publisher=CRC
|url=https://books.google.com/books?id=HcGBWtPF0PUC&pg=PA111
|isbn=9780849390302
}}</ref>
|-
! scope="col" style="text-align: left;"| Source
! scope="col" | Luminous flux (lumens)
|-
|align="left"|37&nbsp;mW "Superbright" white [[Light-emitting diode|LED]]||align="center"|0.20
|-
|align="left"|15&nbsp;mW green [[laser]] (532&nbsp;nm wavelength) ||align="center"|8.4
|-
|align="left"|1 W high-output white LED||align="center"|25–120
|-
|align="left"|[[Kerosene lamp|Kerosene lantern]]||align="center"|100
|-
|align="left"|40 W [[incandescent light bulb|incandescent lamp]] at 230 volts||align="center"|325
|-
|align="left"|7 W high-output white LED||align="center"|450
|-
|align="left"|6 W [[Chip on board|COB]] filament LED lamp||align="center"|600
|-
|align="left"|18 W [[fluorescent lamp]]||align="center"|1250
|-
|align="left"|100 W incandescent lamp||align="center"|1750
|-
|align="left"|40 W fluorescent lamp||align="center"|2800
|-
|align="left"|35 W xenon bulb||align="center"|2200–3200
|-
|align="left"|100 W fluorescent lamp||align="center"|8000
|-
|align="left"|127 W [[sodium-vapor lamp#Low-pressure sodium|low pressure sodium vapor lamp]]||align="center"|25,000
|-
|align="left"|400 W [[metal-halide lamp]]||align="center"|40,000
|-
| colspan="2" align="left" style="padding-left: 2em; font-size: 86%;"|''Values are given for newly manufactured sources.  The output from many sources decreases significantly over their lifetime.''
|}

==References==
{{reflist}}

[[Category:Physical quantities]]
[[Category:Photometry]]
[[Category:Temporal rates]]