photon exitance, \(M_{\textrm{p}}\)

Also contains definition of: specific photon emission
https://doi.org/10.1351/goldbook.P04631
Photon flux, number basis, \(q_{\textrm{p}}\), emitted by an element of the surface containing the source point under consideration divided by the area \(S\) of that element. SI unit is \(\textrm{s}^{-1}\textrm{ m}^{-2}\).
Notes:
  1. Mathematical definition: \(\textrm{d}q_{p}/\textrm{d}S\). If \(q_{\textrm{p}}\) is constant over the surface area considered, \(M_{\textrm{p}} = q_{\textrm{p}}/S\).
  2. Formerly called photon emittance.
  3. This quantity can be expressed on a @C01019@ basis by dividing \(M_{\textrm{p}}\) by the @A00543@. In this case the symbol is \(M_{\textrm{n,p}}\), the name 'photon @E02264@, amount basis', SI unit is \(\textrm{mol s}^{-1}\textrm{ m}^{-2}\); common unit is \(\textrm{einstein s}^{-1}\textrm{ m}^{-2}\).
Source:
PAC, 2007, 79, 293. (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 394 [Terms] [Paper]