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

unit: s^{- 1}

NOTE – For a beam of radiation whose spectral distribution is ${\displaystyle \frac{\text{d}{\Phi }_{\text{e}}\left(\lambda \right)}{\text{d}\lambda }}$ or ${\displaystyle \frac{\text{d}{\Phi }_{\text{e}}\left(v\right)}{\text{d}v}}$ the photon flux Φ_p is ${\displaystyle {\Phi }_{\text{p}}={\displaystyle {\int }_0^{\infty }\frac{\text{d}{\Phi }_{\text{e}}(\lambda )}{\text{d}\lambda }}\cdot \frac{\lambda }{h{c}_0}\text{d}\lambda ={\displaystyle {\int }_0^{\infty }\frac{\text{d}{\Phi }_{\text{e}}(v)}{\text{d}v}}\cdot \frac{1}{hv}\text{d}v}$

h, Planck's constant= (6,626 0755 ± 0,000 0040) × 10⁻³⁴ J • s
c₀, speed of light in vacuum = 299 792 458 m • s⁻¹.