Photon number flux density (${\Phi }_N=I/(hf)$)

The photon number flux density is the number of photons crossing unit normal area per unit time. A monochromatic beam of light intensity $I$ and frequency $f$ has ${\Phi }_N=I/(hf)$ because each photon carries energy $hf$. Its units are ${\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$. For a mixed spectrum, integrate the spectral light intensity divided by the corresponding photon energy.