(mutual) exchange coefficient (between 2 surfaces S_{1} and S_{2}, when the radiance or luminance of S_{1} (or S_{2}) is the same at all points and for all directions) [*g*]

quotient of the radiant flux or luminous flux *Φ*_{2} (or *Φ*_{1}) that surface S_{1} (or S_{2}) sends to surface S_{2} (or S_{1}), by the radiant exitance or luminous exitance *M*_{1} (or *M*_{2})

Unit: m^{2}

NOTE 1 Since *M* = π*L*, and in the particular case where all points on S_{1} are seen from all points on S_{2}

where *l* is the distance between the elements of areas d*A*_{1} and d*A*_{2} on the surfaces S_{1} and S_{2}, *G* is the geometric extent of the beam delimited by the boundaries of S_{1} and S_{2}, and *θ*_{1} and *θ*_{2} are the angles between the surface normals of the 2 elementary areas d*A*_{1} and d*A*_{2} respectively and the line joining the 2 elementary areas.

NOTE 2 For 2 elementary areas d*A*_{1} and d*A*_{2}

where d*Ω*_{1} (or d*Ω*_{2}) is the solid angle which the area d*A*_{2} (or d*A*_{1}) subtends from the centre of d*A*_{1} (or d*A*_{2}).

NOTE 3 The radiance or luminance *L* of the beam delimited by the boundaries of d*A*_{1} and d*A*_{2} is

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