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{{Short description|Luminous flux incident on a surface per area}}
[[Image:Lux meter.jpg|thumb|right|A [[lux meter]] for measuring illuminances in work environments]]▼
{{Distinguish|Luminance}}
In [[photometry (optics)|photometry]], '''illuminance''' is the total [[luminous flux]] incident on a surface, per unit [[area]]. 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. 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>▼
{{Infobox physical quantity
| name = Illuminance
| unit = [[lux]]
| otherunits = [[phot]], [[foot-candle]]
| symbols = {{math|''E''<sub>v</sub>}}
| baseunits = [[candela|cd]]·[[steradian|sr]]·[[meter|m]]<sup>−2</sup>
| dimension = <math>\mathsf{L}^{-2} \mathsf{J}</math>
}}
[[File:Illuminance Diagram.tif|alt=Illuminance diagram with units and terminology.|thumb|372x372px|Illuminance diagram with units and terminology]]
▲In [[photometry (optics)|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 derived units|SI units]] illuminance is measured in [[lux]] (lx), or equivalently in [[lumen (unit)|lumens]] per [[square metre]] ([[Lumen (unit)|lm]]·[[meter|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">
[[File:3 illuminance.PNG|thumb|Illuminance]]
InInternational [[SIElectrotechnical derivedCommission units]](IEC): these''International areElectrotechnical measured inVocabulary.'' [[lux]]https://www.electropedia.org/iev/iev.nsf/display?openform&ievref=845-21-081 (lx)ref. or845-21-081, [[lumenluminous (unit)|lumens]] per [[square metre]] ([[candela|cd]]·[[steradian|sr]]·[[meter|m]exitance]<sup>−2</supref>). 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&nbsp;μlx), while at the bright end, it is possible to read large text at 10<sup>8</sup> lux (100&nbsp;Mlx), or about 1000 times that of direct [[sunlight]], although this can be very uncomfortable and cause long-lasting [[afterimage]]s.{{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-candle]]s !! [[Lux]]
|-
| Sunlight || 10,000 <ref>{{cite web
| Full daylight || 1,000 <ref>{{cite web |archive-url=https://web.archive.org/web/20150112024752/http://academy.autodesk.com:80/library/building-science/measuring-light-levels
|archive-url=https://web.archive.org/web/20220403223446/https://www.engineeringtoolbox.com/light-level-rooms-d_708.html
|url=http://academy.autodesk.com/library/building-science/measuring-light-levels
|url=https://www.engineeringtoolbox.com/light-level-rooms-d_708.html
|title=Measuring Light Levels
|title=Illuminance - Recommended Light Level
|accessdate=Nov 16, 2017
|archiveaccess-date=JanuaryJuly 127, 20152022
|archive-date=April 3, 2022
|publisher=Autodesk Design Academy
|publisher=The Engineering ToolBox
}}</ref> || 10,000▼
|url-status=live
▲}}</ref> || 10100,000
|-
| OvercastShade on a sunny day || 100{{0}}1,000 || 1{{0}}10,000
|-
| Very darkOvercast day || 10{{0|00}}100 || 100{{0|00}}1,000
|-
| TwilightVery dark day || 1{{0|000}}10 || 10{{0|000}}100
|-
| Deep twilightTwilight || {{0.|0000}}1 || 1{{0|0000}}10
|-
| FullDeep moontwilight || {{0|0000}}0.011 || {{0.|00000}}1
|-
| QuarterFull moon || {{0|0000}}0.00101 || {{0|00000}}0.011
|-
| StarlightQuarter moon || {{0|0000}}0.0001001 || {{0|00000}}0.00101
|-
| 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 magnitude]]s in the visible band.<ref>{{cite web |url=http://stjarnhimlen.se/comp/radfaq.html#7 |title=Radiometry and photmetryphotometry 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 |accessdateaccess-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_m_\mathrm{v})/2.5},</math>,
where ''E''<sub>v</sub> is the illuminance in lux, and ''Mm''<sub>v</sub> is the apparent magnitude. The reverse conversion is
:<math display="block">M_m_\mathrm{v} = -14.18 - 2.5 \log(E_\mathrm{v}).</math>.
 
==Relation to luminance==
[[File:photometry_radiometry_units.svg|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==
Line 49 ⟶ 78:
 
==References==
{{reflistReflist}}
 
== 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}}
 
{{SI light units}}
{{Authority control}}
 
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