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.

All of these terms are descriptions of solar radiation that is received indirectly on the surface of the earth, rather than directly from solar rays: "Diffuse radiation, etc. are all caused by the scattering that is a result of clouds, fog, haze, dust, or other barriers in the atmosphere."