IUPAC

radiant exitance, $M$

https://dx.doi.org/10.1351/goldbook.R05041

Radiant power, $P$ , emitted at all wavelengths by an element of surface containing the source point under consideration divided by the area of the source. SI unit is $W m −2$ .

Notes:

Mathematical definition: $M = d P / d S$ . If the radiant power $P$ is constant over the surface area considered, $M = P / S$ .
Equivalent to the integration of the radiant power leaving a source over the solid angle and over the whole wavelength range. Mathematical definition: $M = ∫ λ M λ ⁢ d λ$ , where $M λ$ is the spectral radiant exitance at wavelength $λ$ .
Same as spherical radiant exitance. Formerly called radiant emittance.