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 equivalently in lumens per square metre (cd·sr·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.

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 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



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. V-magnitudes can be converted to lux using the formula

E v = 10 ( − 14.18 − M v ) / 2.5 {\displaystyle 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 v = − 14.18 − 2.5 log ⁡ ( E v ) {\displaystyle M_{\mathrm {v} }=-14.18-2.5\log(E_{\mathrm {v} })} .


  • SI photometry quantities

    Quantity Unit Dimension Notes
    Name Symbol Name Symbol Symbol
    Luminous energy Qv lumen second lm⋅s TJ The lumen second is sometimes called the talbot.
    Luminous flux, luminous power Φv 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 lux lx L−2J Luminous flux emitted from a surface
    Luminous exposure Hv lux second lx⋅s L−2TJ Time-integrated illuminance
    Luminous energy density ωv lumen second per cubic metre lm⋅s/m3 L−3TJ  
    Luminous efficacy η lumen per watt lm/W M−1L−2T3J Ratio of luminous flux to radiant flux or power consumption, depending on context
    Luminous efficiency, luminous coefficient V     1 Luminous efficacy normalized by the maximum possible efficacy