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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.[1]

In SI derived units these are measured in lux (lx), or equivalently in lumens per square metre (lm·m−2). 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.[2]

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, while at the bright end, it is possible to read large text at 108 lux, or about 1000 times that of direct sunlight, although this can be very uncomfortable and cause long-lasting afterimages.

Common illuminance levels
A lux meter for measuring illuminances in work environments
Lighting condition Foot-candles Lux
Full daylight 1,000 [3] 10,000
Overcast day 100 1,000
Very dark day 10 100
Twilight 1 10
Deep twilight 0.1 1
Full moon 0.01 0.1
Quarter moon 0.001 0.01
Starlight 0.0001 0.001
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.[4] V-magnitudes can be converted to lux using the formula[5]

$$E_{\mathrm {v} }=10^{(-14.18-M_{\mathrm {v} })/2.5},$$

where Ev is the illuminance in lux, and Mv is the apparent magnitude. The reverse conversion is

$$M_{\mathrm {v} }=-14.18-2.5\log(E_{\mathrm {v} }).$$

Exposure value
Luminance

References

Luminous exitance Drdrbill.com
One phot = 929.030400001 foot-candles, according to http://www.unitconversion.org/unit_converter/illumination.html
"Measuring Light Levels". Autodesk Design Academy. Archived from the original on January 12, 2015. Retrieved Nov 16, 2017.
Schlyter, Paul. "Radiometry and photmetry in astronomy FAQ, section 7".

"Formulae for converting to and from astronomy-relevant units" (PDF). Retrieved Nov 23, 2013.

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 quantities
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−2J 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−2J Luminous flux incident on a surface
Luminous exitance, luminous emittance Mv lumen per square metre lm/m2 L−2J Luminous flux emitted from a surface
Luminous exposure Hv lux second lx⋅s L−2T J Time-integrated illuminance
Luminous energy density ωv lumen second per cubic metre lm⋅s/m3 L−3T J
Luminous efficacy (of radiation) K lumen per watt lm/W M−1L−2T3J Ratio of luminous flux to radiant flux
Luminous efficacy (of a source) η[nb 3] lumen per watt lm/W M−1L−2T3J Ratio of luminous flux to power consumption
Luminous efficiency, luminous coefficient V 1 Luminous efficacy normalized by the maximum possible efficacy

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
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.
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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