Photon flux, number basis,
,
__emitted__
by an element of the surface containing the source point under consideration divided
by the area
of that element. SI unit is
.

Notes:

- Mathematical definition: . If is constant over the surface area considered, .
- Formerly called photon emittance.
- This quantity can be expressed on a chemical amount basis by dividing by the Avogadro constant. In this case the symbol is , the name 'photon exitance, amount basis', SI unit is ; common unit is .

Source:

PAC, 2007, *79*, 293*
(Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006))
* on page 394