Photon number flux density
= Photon number flux density
{title2=$\Phi_N=I/(hf)$}
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 $\mathrm{m^{-2}s^{-1}}$. For a mixed spectrum, integrate the spectral <light intensity> divided by the corresponding <photon> <energy>.