Illuminance is the total amount of visible light illuminating (incident upon) a point on a surface from all directions above the surface. This “surface” can be a physical surface or an imaginary plane. Therefore illuminance is equivalent to irradiance weighted with the response curve of the human eye.

Standard unit for illuminance is Lux (lx) which is lumens per square meter (lm/m2).
In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, 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 both measured in lux (lx) or lumens per square metre (cd•sr•m^-2). In the CGS system, the unit of illuminance is the phot. One phot 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 10^8 lux, or about 1,000 times that of direct sunlight, although this can be very uncomfortable and cause long-lasting afterimages.

footcandle 1 fc = 10.764 lx.
dalx (in canadian
safety regulations) 1 dalx = 10.764 lx.
phot 1 ph = 10’000 lx

Typical illuminance values are:

1 lx full moon
10 lx street lighting
100-1’000 lx workspace lighting
10’000 lx surgery lighting
100’000 lx plain sunshine

 SI photometry units



SI unit



Luminous energy


lumen second


units are sometimes called talbots

Luminous flux


lumen (= cd·sr)


also called luminous power

Luminous intensity


candela (= lm/sr)


an SI base unit



candela per square metre


units are sometimes called “nits”



lux (= lm/m2)


Used for light incident on a surface

Luminous emittance


lux (= lm/m2)


Used for light emitted from a surface

Luminous efficacy

lumen per watt


ratio of luminous flux to radiant flux

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