${\displaystyle N_{\text{p}}=\frac{Q_{\text{e}}}{h\nu }}$

where Q_e is radiant energy, h is the Planck constant, and ν is the frequency of the corresponding electromagnetic wave

Note 1 to entry: Photon number can also be expressed by the time integral of the photon flux, Φ_p, over a given duration, Δt:

${\displaystyle N_{\text{p}}={\displaystyle {\int }_{\Delta t}{\Phi }_{\text{p}}}\text{d}t}$

Note 2 to entry: The photon number has unit one.

Note 3 to entry: This entry was numbered 845-01-29 in IEC 60050-845:1987.