${\displaystyle {\Phi }_{\text{p}}=\frac{\text{d}{N}_{\text{p}}}{\text{d}t}}$

where N_p is photon number, transmitted or received, and t is time

Note 1 to entry: Photon flux, Φ_p, is related to radiant flux, Φ_e, of monochromatic radiation by

${\displaystyle {\Phi }_{\text{p}}=\frac{{\Phi }_{\text{e}}}{h\nu }}$

where h is the Planck constant, and ν is the frequency of the corresponding electromagnetic wave.

Note 2 to entry: For a beam of radiation whose spectral distribution is ${\displaystyle \frac{\text{d}{\Phi }_{\text{e}}\left(\lambda \right)}{\text{d}\lambda }}$ or ${\displaystyle \frac{\text{d}{\Phi }_{\text{e}}\left(\nu \right)}{\text{d}\nu }}$, the photon flux can be expressed by ${\displaystyle {\Phi }_{\text{p}}={\displaystyle {\int }_0^{\infty }\frac{\text{d}{\Phi }_{\text{e}}(\lambda )}{\text{d}\lambda }}\cdot \frac{\lambda }{h{c}_0}\text{d}\lambda ={\displaystyle {\int }_0^{\infty }\frac{\text{d}{\Phi }_{\text{e}}(\nu )}{\text{d}\nu }}\cdot \frac{1}{h\nu }\text{d}\nu }$, where h is Planck's constant and c₀ is the speed of light in vacuum.

Note 3 to entry: The corresponding radiometric quantity is "radiant flux". The corresponding photometric quantity is "luminous flux".

Note 4 to entry: The photon flux is expressed in second to the power minus one (s⁻¹).

Note 5 to entry: This entry was numbered 845-01-26 in IEC 60050-845:1987.