The intensity of light per unit of area on a surface that is exposed to light: Illuminance is the amount of radiance, evaluated according to its capacity to produce visual stimulation and which reaches a section of a surface area during a period of time.

Illuminance is measured in lux (the SI unit of illumination), equal to one lumen per square and a "lumen" is the unit of luminous flux in the International System (SI) equal to the amount of light given out through a solid angle by a source of one "candela" intensity radiating equally in all directions.

A "candela", which is the SI unit of luminous intensity and symbolized as cd, is equal to the strength of illuminance in a given direction of a source that emits monochromatic (one color) radiation of frequency.