Power Per Unit Area of Surface

The power per unit area on an illuminated surface, sometimes called areance, is distinguished from the similar quantity for the source. In radiometry the surface areance may be called irradiance and luminous areance may be called illuminance. This is the quantity of practical importance in judging whether an area is lighted well enough for reading or other activities. The illuminance is measured in lux, but the older unit footcandle is still encountered.

Lux

The lux is defined as a lumen per square meter and is a unit of illuminance. An equivalent term is luminous flux density. As such, it measures the amount of visible light striking a surface. The standard symbol for it is E_v. It must take into account the sensitivity of the eye for the wavelengths of light involved, but that is taken care of in establishing the number of lumens.

Most light meters measure this quantity, and it is the quantity most commonly used to state requirements for architectural lighting.

Nit

The nit is the unit of luminance and is defined as a candela per square meter. The unit could be written as cd/m² or lm/m²sr. It is most often used to characterize the "brightness" of flat emitting or reflecting surfaces. Palmer gives the following examples: laptop computer screens 100 to 250 nits, while those which are sunlight-readable must have more than 1000 nits. Typical CRT monitors are said to have luminances of 50-125 nits.

The standard symbol for the luminance in nits is L_v.