${\displaystyle f_{21}=\frac{{\Phi }_2}{A_2{M}_1}=\frac{g}{A_2}}$

or

${\displaystyle f_{12}=\frac{{\Phi }_1}{A_1{M}_2}=\frac{g}{A_1}}$

with

$A_2{f}_{21}=g=A_1{f}_{12}$

where Φ₂ (or Φ₁) is the radiant flux or luminous flux received over surface S₂ (or S₁) from surface S₁ (or S₂), A₂ (or A₁) is the area of surface S₂ (or S₁) and g is the mutual exchange coefficient

Note 1 to entry: The form factor has unit one.

Note 2 to entry: This entry was numbered 845-09-73 in IEC 60050-845:1987.