Illuminance

Common symbols

| Ev
SI unit| lux

Other units

| phot, foot-candle
In SI base units| cd·sr·m−2
Dimension| L−2J

File:Illuminance Diagram.tif In photometry, illuminance is the total luminous flux incident on a surface, per unit area.[1] 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.[2] Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance.[3][4]

In SI units illuminance is measured in lux (lx), or equivalently in lumens per square metre (lm·m−2).[2] Luminous exitance is measured in lm·m−2 only, not lux.[4] In the CGS system, the unit of illuminance is the phot, which is equal to 10000 lux. The foot-candle is a non-metric unit of illuminance that is used in photography.[5]

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 5×10−5 lux (50 μlx), while at the bright end, it is possible to read large text at 108 lux (100 Mlx), or about 1000 times that of direct sunlight, although this can be very uncomfortable and cause long-lasting afterimages.

## Contents

* 1 Common illuminance levels
* 2 Astronomy
* 3 Relation to luminous intensity
* 4 Relation to luminance
* 5 See also
* 6 References
* 7 External links

## Common illuminance levels

A lux meter for measuring illuminances in work environments Lighting condition | Foot-candles | Lux

|
|

Sunlight | 10,000 [6] | 100,000
Shade on a sunny day | 01,000 | 010,000
Overcast day | 00100 | 001,000
Very dark day | 00010 | 000100
Twilight | 00001 | 000010
Deep twilight | 00000.1 | 000001
Full moon | 00000.01 | 000000.1
Quarter moon | 00000.001 | 000000.01
Starlight | 00000.0001 | 000000.001
Overcast night | 00000.00001 | 000000.0001

## 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.[7] V-magnitudes can be converted to lux using the formula[8] Ev=10(−14.18−mv)/2.5, where Ev is the illuminance in lux, and mv is the apparent magnitude. The reverse conversion is mv=−14.18−2.5log⁡(Ev).

## 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[9][10] the cosine law with the inverse-square law: Ev=Ivcos⁡(θ)D2 where

* Iv is the luminous intensity of the source
* θ is the angle of incidence, and
* D is the distance between the source and the surface.

## Relation to luminance

Comparison of photometric and radiometric quantities

The luminance of a reflecting surface is related to the illuminance it receives: ∫ΩΣLvdΩΣcos⁡θΣ=Mv=EvR where the integral covers all the directions of emission ΩΣ, and

* Mv is the surface's luminous exitance
* Ev is the received illuminance, and
* 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 Lv=EvRπ

## See also

* Irradiance
* Exposure value
* Luminance

## References

1. ↑ "Illuminance, 17-21-060". CIE S 017:2020 ILV: International Lighting Vocabulary, 2nd edition.. CIE - International Commission on Illumination. 2020. https://cie.co.at/eilvterm/17-21-060. Retrieved 20 April 2023.
2. ↑ 2.0 2.1 International Electrotechnical Commission (IEC): International Electrotechnical Vocabulary. ref. 845-21-060, illuminance
3. ↑ Luminous exitance Drdrbill.com
4. ↑ 4.0 4.1 International Electrotechnical Commission (IEC): International Electrotechnical Vocabulary. ref. 845-21-081, luminous exitance
5. ↑ One phot = 929.030400001 foot-candles, according to http://www.unitconversion.org/unit_converter/illumination.html
6. ↑ "Illuminance - Recommended Light Level". The Engineering ToolBox. https://www.engineeringtoolbox.com/light-level-rooms-d_708.html.
7. ↑ Schlyter, Paul. "Radiometry and photometry in astronomy FAQ, section 7". http://stjarnhimlen.se/comp/radfaq.html#7.
8. ↑ "Formulae for converting to and from astronomy-relevant units". http://members.ziggo.nl/jhm.vangastel/Astronomy/Formules.pdf.
9. ↑ IES (2022). "ANSI/IES LS-1-22, Lighting Science: Nomenclature And Definitions For Illuminating Engineering". ANSI/IES LS-1-22, Lighting Science: Nomenclature And Definitions For Illuminating Engineering. New York: Illuminating Engineering Society. §9.1.2, cosine law. https://ies.org/standards/definitions/. Retrieved 15 Dec 2025.
10. ↑ CIE (2020). "e-ILV, online version of CIE S 017:2020, International Lighting Vocabulary". e-ILV, online version of CIE S 017:2020, International Lighting Vocabulary (2 ed.). Vienna: International Commission on Illumination. §17-25-104, photometric distance law. https://cie.co.at/e-ilv. Retrieved 15 Dec 2025.

## External links

* Illuminance Converter
* Knowledgedoor, LLC (2005) Library of Units and Constants: Illuminance Quantity
* Kodak's guide to Estimating Luminance and Illuminance using a camera's exposure meter. Also available in PDF form.

SI photometry quantitiesv * d * e

Quantity  | Unit  | Dimension | Notes

|
|
|

Name | Symbol[nb 1] | Name | Symbol | Symbol[nb 2]
Luminous energy | Qv[nb 3] | lumen second | lm⋅s  | T⋅J | The lumen second is sometimes called the talbot.
Luminous flux, luminous power  | Φv[nb 3] | lumen (= candela steradians)  | lm (= cd⋅sr)  | J | Luminous energy per unit time
Luminous intensity | Iv | candela (= lumen per steradian)  | cd (= lm/sr)  | J | Luminous flux per unit solid angle
Luminance | Lv | candela per square metre | cd/m2 | L−2⋅J | Luminous flux per unit solid angle per unit projected source area. The candela per square metre is sometimes called the nit.
Illuminance | Ev | lux (= lumen per square metre)  | lx (= lm/m2)  | L−2⋅J | Luminous flux incident on a surface
Luminous exitance, luminous emittance  | Mv | lux  | lx  | L−2⋅J | Luminous flux emitted from a surface
Luminous exposure | Hv | lux second | lx⋅s  | L−2⋅T⋅J | Time-integrated illuminance
Luminous energy density  | ωv | lumen second per cubic metre  | lm⋅s/m3 | L−3⋅T⋅J |
Luminous efficacy (of radiation)  | K | lumen per watt | lm/W | M−1⋅L−2⋅T3⋅J | Ratio of luminous flux to radiant flux
Luminous efficacy (of a source)  | η[nb 3] | lumen per watt | lm/W | M−1⋅L−2⋅T3⋅J | Ratio of luminous flux to power consumption
Luminous efficiency, luminous coefficient  | V |  |  | 1 | Luminous efficacy normalized by the maximum possible efficacy
See also: SI * Photometry * Radiometry

1. ↑ Standards organizations recommend that photometric quantities be denoted with a subscript "v" (for "visual") to avoid confusion with radiometric or photon quantities. For example: USA Standard Letter Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967
2. ↑ The symbols in this column denote dimensions; "L", "T" and "J" are for length, time and luminous intensity respectively, not the symbols for the units litre, tesla and joule.
3. ↑ 3.0 3.1 3.2 Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ for luminous efficacy of a source.

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