luminance

_{v};

density of luminous intensity with respect to projected area in a specified direction at a specified point on a real or imaginary surface

where _{v} is luminous intensity,

Note 1 to entry: In a practical sense, the definition of luminance can be thought of as dividing a real or imaginary surface into an infinite number of infinitesimally small surfaces which can be considered as point sources, each of which has a specific luminous intensity, _{v}, in the specified direction. The luminance of the surface is then the integral of these luminance elements over the whole surface.

The equation in the definition can mathematically be interpreted as a derivative (i.e. a rate of change of luminous intensity with projected area) and could alternatively be rewritten in terms of the average luminous intensity, , as: _{.}

Hence, luminance is often considered as a quotient of averaged quantities; the area,

Note 2 to entry: For a surface being irradiated, an equivalent formula in terms of illuminance, _{v}, and solid angle,

Note 3 to entry: An equivalent formula is , where _{v} is luminous flux and

Note 4 to entry:

Luminous flux can be obtained by integrating luminance over projected area,

Note 5 to entry: Since the optical extent, expressed by ^{2}, where _{v}⋅^{−2} is also invariant along the path of the beam if the losses by absorption, reflection and diffusion are taken as 0. That quantity is called "basic luminance".

Note 6 to entry: The equation in the definition can also be described as a function of luminous flux, _{v}. In this case, it is mathematically interpreted as a second partial derivative of the luminous flux at a specified point (

where

Note 7 to entry: The corresponding radiometric quantity is "radiance". The corresponding quantity for photons is "photon radiance".

Note 8 to entry: The luminance is expressed in candela per square metre (cd·m^{−2} = lm·m^{−2}·sr^{−1}).

Note 9 to entry: This entry was numbered 845-01-35 in IEC 60050-845:1987.

Note 10 to entry: This entry was numbered 17-711 in CIE S 017:2011.