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.

[3][4] In SI units illuminance is measured in lux (lx), or equivalently in lumens per square metre (lm·m−2).

[4] In the CGS system, the unit of illuminance is the phot, which is equal to 10000 lux.

[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 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.

[citation needed] In astronomy, the illuminance stars cast on the Earth's atmosphere is used as a measure of their brightness.

The luminance of a reflecting surface is related to the illuminance it receives:

where the integral covers all the directions of emission ΩΣ, and In the case of a perfectly diffuse reflector (also called a Lambertian reflector), the luminance is isotropic, per Lambert's cosine law.