Illuminance is the luminous flux incident on a surface of unit area. It measures how much energy has fallen on a surface.

This quantity is useful for describing the level of illumination incident on a surface without making the measurement dependent on the size of the surface itself. The lux (lx) is the International System (SI) unit of illuminance. The American System (AS) unit for illuminance is the footcandle (fc), equivalent to 1 lumen per square foot. Illuminance is a function of the distance from the light source. To specify the illuminance of a light, you must enter a value in lx, and the distance at which that illuminance is measured.