In 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.
In SI derived units these are measured in lux (lx) or lumens per square metre (cd·sr·m−2). In the CGS system, the unit of illuminance is the phot, which is equal to 10,000 lux. The foot-candle is a non-metric unit of illuminance that is used in photography.
Illuminance was formerly often called brightness, but this leads to confusion with other uses of the word. "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, while at the bright end, it is possible to read large text at 108 lux, or about 1,000 times that of direct sunlight, although this can be very uncomfortable and cause long-lasting afterimages.
See also 
- Luminous exitance Drdrbill.com
- One phot = 929.030400001 foot-candles, according to http://www.unitconversion.org/unit_converter/illumination.html
- Knowledgedoor, LLC (2005) Library of Units and Constants: Illuminance Quantity
- Illuminance Converter
- A Kodak guide to Estimating Luminance and Illuminance using a camera's exposure meter. Also available in PDF form.
|Luminous energy||Qv [nb 2]||lumen second||lm⋅s||T⋅J [nb 3]||units are sometimes called talbots|
|Luminous flux||Φv [nb 2]||lumen (= cd⋅sr)||lm||J [nb 3]||also called luminous power|
|Luminous intensity||Iv||candela (= lm/sr)||cd||J [nb 3]||an SI base unit, luminous flux per unit solid angle|
|Luminance||Lv||candela per square metre||cd/m2||L−2⋅J||units are sometimes called nits|
|Illuminance||Ev||lux (= lm/m2)||lx||L−2⋅J||used for light incident on a surface|
|Luminous emittance||Mv||lux (= lm/m2)||lx||L−2⋅J||used for light emitted from a surface|
|Luminous exposure||Hv||lux second||lx⋅s||L−2⋅T⋅J|
|Luminous energy density||ωv||lumen second per metre3||lm⋅s⋅m−3||L−3⋅T⋅J|
|Luminous efficacy||η [nb 2]||lumen per watt||lm/W||M−1⋅L−2⋅T3⋅J||ratio of luminous flux to radiant flux|
|Luminous efficiency||V||1||also called luminous coefficient|
|See also: SI · Photometry · Radiometry|
- Standards organizations recommend that photometric quantities be denoted with a suffix "v" (for "visual") to avoid confusion with radiometric or photon quantities.
- Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ or K for luminous efficacy.
- "J" here is the symbol for the dimension of luminous intensity, not the symbol for the unit joules.