Lux (lx) is the internationally standardized physical unit of measurement for the illuminance of a light source. The name is derived from the Latin word for light.

The unit lux is calculated from the luminous flux incident on a given surface, i.e. the number of lumens per m². Thus, the illuminance of one lux corresponds to the uniform illumination of a 1 m² surface with a luminous flux of one lumen (1 lux is 1 lumen/m²). Alternatively, 1 lux can be defined as the illuminance at 1 meter from a point light source of luminous intensity 1 candela (1 cd).

Thus, the lux number depends on the distance between the light source and the surface: The greater the distance, the lower the number of lux.

Unlike lumen and candela, which are transmit quantities, lux is a receive quantity.