mutual exchange coefficient, <of radiance or luminance between surfaces>

exchange coefficient, <of radiance or luminance between surfaces>

g

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

Note 1 to entry: 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 two elementary areas dA₁ and dA₂ respectively and the line joining the two elementary areas.

Note 2 to entry: 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 to entry: The radiance or luminance L of the beam delimited by the boundaries of dA₁ and dA₂ is
.

Note 4 to entry: The mutual exchange coefficient is expressed in square metre (m²).

Note 5 to entry: This entry was numbered 845-09-71 in IEC 60050-845:1987.

Note 6 to entry: This entry was numbered 17-797 in CIE S 017:2011.