17-797

(mutual) exchange coefficient (between 2 surfaces S1 and S2, when the radiance or luminance of S1 (or S2) is the same at all points and for all directions) [g]

quotient of the radiant flux or luminous flux Φ2 (or Φ1) that surface S1 (or S2) sends to surface S2 (or S1), by the radiant exitance or luminous exitance M1 (or M2)

       

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, G is the geometric extent of the beam delimited by the boundaries of S1 and S2, and θ1 and θ2 are the angles between the surface normals of the 2 elementary areas dA1 and dA2 respectively and the line joining the 2 elementary areas.

NOTE 2 For 2 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 L of the beam delimited by the boundaries of dA1 and dA2 is

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