17-711

luminance (in a given direction, at a given point of a real or imaginary surface) [Lv; L]

quantity defined by the formula:

        

where

dΦv is the luminous flux transmitted by an elementary beam passing through the given point and propagating in the solid angle, dΩ, containing the given direction;

dA is the area of a section of that beam containing the given point;

Θ is the angle between the normal to that section and the direction of the beam

Unit: cd·m-2 = lm·m-2·sr-1

NOTE 1 The above equation does not represent a derivative (i.e. a rate of change of flux with solid angle or area) but rather the quotient of an element of flux by an element of solid angle and an element of area. In strict mathematical terms the definition could be written:

       

In practical measurements, A and Ω should be small enough that variations in Φv do
not affect the result. Otherwise, the ratio gives the average luminance and the exact measurement conditions must be specified.

NOTE 2 See NOTES 2 to 7 for “radiance”.

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