${\displaystyle g=\frac{{\Phi }_2}{M_1}=\frac{{\Phi }_1}{M_2}}$

Note 1 to entry: Since M = πL, and in the particular case where all points on S₁ are seen from all points on S₂

${\displaystyle g=\frac{1}{\pi }{\displaystyle \underset{A_2}{\int }{\displaystyle \underset{A_1}{\int }\frac{\cos {\theta }_1\cos {\theta }_2}{l^2}\text{d}{A}_1\text{d}{A}_2=\frac{1}{\pi }G}}}$

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

${\displaystyle \text{d}g=\frac{1}{\pi }\text{d}{A}_1\text{d}{\Omega }_1\cos {\theta }_1=\frac{1}{\pi }\text{d}{A}_2\text{d}{\Omega }_2\cos {\theta }_2}$

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

${\displaystyle L=\frac{1}{\pi }\frac{\text{d}\Phi }{\text{d}g}}$.

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.