illuminance

Illuminance is a photometric quantity that expresses how much luminous flux is incident on a surface per unit area. It is measured in lux (lx), where one lux equals one lumen per square meter (lm/m^2). Illuminance describes the perceived brightness of a surface from the viewpoint of the human eye under a given light source, assuming standard observer conditions. It depends on the light source’s luminous intensity distribution, the distance from the source to the surface, the angle at which light strikes the surface, and the surface’s reflectance.

Illuminance is typically measured with a lux meter or photometer placed on or near the surface. For a point light source with luminous intensity I (candelas) at distance r, and with the surface normal to the line of sight (angle of incidence zero), the approximate relation is E = I / r^2. More generally, illuminance can be expressed as E = Φv / A for a uniformly illuminated area, where Φv is the luminous flux and A is the area. Illuminance is one of several photometric quantities; it is distinct from luminance, which measures the brightness emitted or reflected by a surface, and from irradiance, which is the radiant energy flux per unit area in watts.

Applications of illuminance include architectural and interior lighting design, workplace safety and ergonomics, photography, and horticulture, where adequate light levels are specified to meet tasks and standards. Typical indoor values vary by task, commonly ranging from about 100 to 500 lx in many work environments, while outdoor daylight can exceed several thousand to over 10,000 lx depending on sky conditions.