photon exitance, M p

https://dx.doi.org/10.1351/goldbook.P04631
Photon flux, number basis, q p, emitted by an element of the surface containing the source point under consideration divided by the area S of that element. SI unit is s −1 m −2.
Notes:
  1. Mathematical definition: d q p / d S. If q p is constant over the surface area considered, M p = q p / S.
  2. Formerly called photon emittance.
  3. This quantity can be expressed on a chemical amount basis by dividing M p by the Avogadro constant. In this case the symbol is M n , p, the name 'photon exitance, amount basis', SI unit is mol s −1 m −2; common unit is einstein s −1 m −2.
Source:
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 394