density of luminous intensity with respect to projected area in a specified direction at a specified point on a real or imaginary surface
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,
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,
Note 3 to entry: An equivalent formula is , where
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
Note 6 to entry: The equation in the definition can also be described as a function of luminous flux,
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.