${\displaystyle M_{\text{e}}=\frac{\text{d}{\Phi }_{\text{e}}}{\text{d}A}}$

where Φ_e is radiant flux and A is the area from which the radiant flux leaves

Note 1 to entry: For Planckian radiation, M_e = σT⁴ where σ is the Stefan-Boltzmann constant and T is thermodynamic temperature.

Note 2 to entry: The corresponding photometric quantity is "luminous exitance". The corresponding photon quantity is "photon exitance".

Note 3 to entry: The radiant exitance is expressed in watt per square metre (W·m⁻²).

Note 4 to entry: This entry was numbered 845-01-47 in IEC 60050-845:1987.