{{Short description|Luminous flux incident on a surface per area}} {{Distinguish|Luminance}} {{Infobox physical quantity | name = Illuminance | unit = lux | otherunits = phot, foot-candle | symbols = {{math|''E''<sub>v</sub>}} | baseunits = cd·sr·m<sup>−2</sup> | dimension = <math>\mathsf{L}^{-2} \mathsf{J}</math> }} alt=Illuminance diagram with units and terminology.|thumb|372x372px|Illuminance diagram with units and terminology In photometry, '''illuminance''' is the total luminous flux incident on a surface, per unit area.<ref>{{cite encyclopedia | title=Illuminance, 17-21-060 | encyclopedia=CIE S 017:2020 ILV: International Lighting Vocabulary, 2nd edition. | publisher=CIE - International Commission on Illumination | accessdate=20 April 2023 | year=2020 | url=https://cie.co.at/eilvterm/17-21-060}}</ref> It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception.<ref name="IEC_845-21-060">International Electrotechnical Commission (IEC): ''International Electrotechnical Vocabulary.'' [https://www.electropedia.org/iev/iev.nsf/display?openform&ievref=845-21-060 ref. 845-21-060, illuminance]</ref> Similarly, '''luminous emittance''' is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as '''luminous exitance'''.<ref>[http://www.drdrbill.com/downloads/optics/photometry/Exitance.pdf Luminous exitance] ''Drdrbill.com''</ref><ref name="IEC_845-21-081" />
In SI units illuminance is measured in lux (lx), or equivalently in lumens per square metre (lm·m<sup>−2</sup>).<ref name="IEC_845-21-060" /> Luminous exitance is measured in lm·m<sup>−2</sup> only, not lux.<ref name="IEC_845-21-081"> International Electrotechnical Commission (IEC): ''International Electrotechnical Vocabulary.'' [https://www.electropedia.org/iev/iev.nsf/display?openform&ievref=845-21-081 ref. 845-21-081, luminous exitance]</ref> In the CGS system, the unit of illuminance is the phot, which is equal to {{gaps|10|000|u=lux}}. The foot-candle is a non-metric unit of illuminance that is used in photography.<ref>One phot = {{gaps|929.030|400|001|u=foot-candles}}, according to http://www.unitconversion.org/unit_converter/illumination.html</ref>
Illuminance was formerly often called brightness, but this leads to confusion with other uses of the word, such as to mean luminance. "Brightness" should never be used for quantitative description, but only for nonquantitative references to physiological sensations and perceptions of light.
The human eye is capable of seeing somewhat more than a 2 trillion-fold range. The presence of white objects is somewhat discernible under starlight, at {{val|5|e=-5|u=lux}} (50 μlx), while at the bright end, it is possible to read large text at 10<sup>8</sup> lux (100 Mlx), or about 1000 times that of direct sunlight, although this can be very uncomfortable and cause long-lasting afterimages.{{Citation needed|date=July 2008}}
==Common illuminance levels== [[Image:Lux meter.jpg|thumb|right|A lux meter for measuring illuminances in work environments]] {| class="wikitable sortable" |- ! Lighting condition !! Foot-candles !! Lux |- | Sunlight || 10,000 <ref>{{cite web |archive-url=https://web.archive.org/web/20220403223446/https://www.engineeringtoolbox.com/light-level-rooms-d_708.html |url=https://www.engineeringtoolbox.com/light-level-rooms-d_708.html |title=Illuminance - Recommended Light Level |access-date=July 7, 2022 |archive-date=April 3, 2022 |publisher=The Engineering ToolBox |url-status=live }}</ref> || 100,000 |- | Shade on a sunny day || {{0}}1,000 || {{0}}10,000 |- | Overcast day || {{0|00}}100 || {{0|00}}1,000 |- | Very dark day || {{0|000}}10 || {{0|000}}100 |- | Twilight || {{0|0000}}1 || {{0|0000}}10 |- | Deep twilight || {{0|0000}}0.1 || {{0|00000}}1 |- | Full moon || {{0|0000}}0.01 || {{0|00000}}0.1 |- | Quarter moon || {{0|0000}}0.001 || {{0|00000}}0.01 |- | Starlight || {{0|0000}}0.0001 || {{0|00000}}0.001 |- | Overcast night|| {{0|0000}}0.00001 || {{0|00000}}0.0001 |}
==Astronomy== In astronomy, the illuminance stars cast on the Earth's atmosphere is used as a measure of their brightness. The usual units are apparent magnitudes in the visible band.<ref>{{cite web |url=http://stjarnhimlen.se/comp/radfaq.html#7 |title=Radiometry and photometry in astronomy FAQ, section 7 |first=Paul |last=Schlyter}}</ref> V-magnitudes can be converted to lux using the formula<ref>{{cite web |url=http://members.ziggo.nl/jhm.vangastel/Astronomy/Formules.pdf |title=Formulae for converting to and from astronomy-relevant units |access-date=Nov 23, 2013 |archive-date=December 2, 2013 |archive-url=https://web.archive.org/web/20131202231237/http://members.ziggo.nl/jhm.vangastel/Astronomy/Formules.pdf |url-status=dead }}</ref> <math display="block">E_\mathrm{v} = 10^{(-14.18-m_\mathrm{v})/2.5},</math> where ''E''<sub>v</sub> is the illuminance in lux, and ''m''<sub>v</sub> is the apparent magnitude. The reverse conversion is <math display="block">m_\mathrm{v} = -14.18 - 2.5 \log(E_\mathrm{v}).</math>
==Relation to luminous intensity== When the light source is sufficiently far away to be treated as a point source, the illuminance on a surface is related to the luminous intensity of light it receives by combining<ref name="LS-1">{{cite encyclopedia |author=IES |date=2022 |title=ANSI/IES LS-1-22, Lighting Science: Nomenclature And Definitions For Illuminating Engineering |url=https://ies.org/standards/definitions/ |at=§9.1.2, cosine law |location=New York |publisher=Illuminating Engineering Society |access-date=15 Dec 2025}}</ref><ref name="e-ILV">{{cite encyclopedia |author=CIE |title=e-ILV, online version of CIE S 017:2020, International Lighting Vocabulary |edition=2 |url=https://cie.co.at/e-ilv |at=§17-25-104, photometric distance law |location=Vienna |publisher=International Commission on Illumination |year=2020 |access-date=15 Dec 2025}}</ref> the cosine law with the inverse-square law: <math display="block">E_\mathrm{v} = \frac{I_\mathrm{v} \cos(\theta)}{D^2}</math> where * {{var|I}}<sub>v</sub> is the luminous intensity of the source * {{var|θ}} is the angle of incidence, and * {{var|D}} is the distance between the source and the surface.
==Relation to luminance== thumb|upright=1.5|Comparison of photometric and radiometric quantities The luminance of a reflecting surface is related to the illuminance it receives: <math display="block">\int_{\Omega_\Sigma} L_\mathrm{v} \mathrm{d}\Omega_\Sigma \cos \theta_\Sigma = M_\mathrm{v} = E_\mathrm{v} R</math> where the integral covers all the directions of emission {{math|Ω<sub>Σ</sub>}}, and * {{var|M}}<sub>v</sub> is the surface's luminous exitance * {{var|E}}<sub>v</sub> is the received illuminance, and * {{var|R}} is the reflectance.
In the case of a perfectly diffuse reflector (also called a Lambertian reflector), the luminance is isotropic, per Lambert's cosine law. Then the relationship is simply <math display="block">L_\mathrm{v} = \frac{E_\mathrm{v} R}{\pi}</math>
==See also== *Irradiance *Exposure value *Luminance
==References== {{Reflist}}
== External links== * [http://www.convertthis.com/converters/illuminance.aspx Illuminance Converter] {{Webarchive|url=https://web.archive.org/web/20100210000727/http://www.convertthis.com/converters/illuminance.aspx |date=2010-02-10 }} * Knowledgedoor, LLC (2005) [http://www.knowledgedoor.com/1/Library_of_Units_and_Constants/Quantity_Index/illuminance.htm Library of Units and Constants: Illuminance Quantity] * Kodak's guide to [http://www.kodak.com/cluster/global/en/consumer/products/techInfo/am105/am105kic.shtml Estimating Luminance and Illuminance] using a camera's exposure meter. Also available in [https://web.archive.org/web/20070709163424/http://www.kodak.com/cluster/global/en/consumer/products/techInfo/am105/am105kic.pdf PDF form].
{{SI light units}} {{Authority control}}
<!--Categories--> Category:Physical quantities Category:Photometry