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

unit : m²

NOTE 1 – Since M = πL, and in the particular case where all points on S₁ are seen from all points on S₂

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

where l is the distance between the elements of areas dA₁ and dA₂ on the surfaces S₁ and S₂, 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

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

where dΩ₁ (or dΩ₂) is the solid angle which the area dA₂ (or dA₁) subtends from the centre of dA₁ (or dA₂).

NOTE 3 – The radiance or luminance of the beam delimited by the boundaries of dA₁ and dA₂ is

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