In photometry, this is used as a measure of the irradiance, as perceived by the spectrally unequally responding human eye, of light that hits or passes through a surface.

It is analogous to the radiometric unit watt per square metre, but with the power at each wavelength weighted according to the luminosity function, a model of human visual brightness perception, standardized by the CIE and ISO.

Achieving an illuminance of 500 lx might be possible in a home kitchen with a single fluorescent light fixture with an output of 12000 lumens.

Thus, lighting a larger area to the same illuminance (lux) requires a greater luminous flux (lumen).

[17] The unobscured Sun provides an illumination of up to 100 kilolux (klx) on the Earth's surface, the exact value depending on time of year and atmospheric conditions.

The peak of the luminosity function is at 555 nm (green); the eye's image-forming visual system is more sensitive to light of this wavelength than any other.

For a light source with mixed wavelengths, the number of lumens per watt can be calculated by means of the luminosity function.

This means that white (or whitish) light sources produce far fewer lumens per watt than the theoretical maximum of 683.002 lm/W.

The ratio between the actual number of lumens per watt and the theoretical maximum is expressed as a percentage known as the luminous efficiency.

Still cameras do not use such a specification, since longer exposure times can generally be used to make pictures at very low illuminance levels, as opposed to the case in video cameras, where a maximal exposure time is generally set by the frame rate.

Since one foot-candle is the illuminance cast on a surface by a one-candela source one foot away, a lux could be thought of as a "metre-candle", although this term is discouraged because it does not conform to SI standards for unit names.