density of radiant 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 radiance 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 radiant intensity,
The equation in the definition can mathematically be interpreted as a derivative (i.e. a rate of change of radiant intensity with projected area) and could alternatively be rewritten in terms of the average radiant intensity, , as:
Hence, radiance 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 irradiance,
Note 3 to entry: An equivalent formula is , where
Note 4 to entry:
Radiant flux can be obtained by integrating radiance 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 radiant flux,
Note 7 to entry: The corresponding photometric quantity is "luminance". The corresponding quantity for photons is "photon radiance".
Note 8 to entry: The radiance is expressed in watt per square metre per steradian (W⋅m−2·sr−1).
Note 9 to entry: This entry was numbered 845-01-34 in IEC 60050-845:1987.
Note 10 to entry: This entry was numbered 17-1012 in CIE S 017:2011.