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:
  1. Mathematical definition: M = d P / d S. If the radiant powerP is constant over the surface area considered, M = P / S.
  2. 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 wavelengthλ.
  3. Same as spherical radiant exitance. Formerly called radiant emittance.
Source:
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 409