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