photon flux, q p,Φ p

https://dx.doi.org/10.1351/goldbook.P04636
Number of photons (quanta of radiation, N p) per time interval. SI unit is s −1.
Notes:
  1. Mathematical definition: q p = d N p / d t. If the number of photons is constant over the time interval, q p = N p / t.
  2. This quantity can be used on a chemical amount basis by dividing the photon flux, number basis, q p, by the Avogadro constant, the symbol then being q n , p, the name 'photon flux, amount basis', SI unit is mol s −1; common unit is einstein s −1.
  3. Although the symbol recommended by CEI is Φ p, the symbol q p is preferred since Φ is reserved for quantum yield.
Source:
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 395