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

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