quotient of the radiant or luminous flux that surface S1 (or S2) sends to surface S2 (or S1),by the radiant or luminous exitance of surface S1 (or S2)
unit : m2
NOTE 1 – Since M = πL, and in the particular case where all points on S1 are seen from all points on S2
where l is the distance between the elements of areas dA1 and dA2 on the surfaces S1 and S2, and G is the geometric extent of the beam delimited by the boundaries of S1 and S2.
NOTE 2 – For two elementary areas dA1 and dA2
where dΩ1 (or dΩ2) is the solid angle which the area dA2 (or dA1) subtends from the centre of dA1 (or dA2).
NOTE 3 – The radiance or luminance of the beam delimited by the boundaries of dA1 and dA2 is
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