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For a sinusoidal plane electromagnetic wave in vacuum, write the maximum electric field as $E_0$. The magnetic field maximum is $B_0=E_0/c$, so the cycle-averaged Poynting vector magnitude, or light intensity, isEach photon carries energy $hf$, where $h$ is the Planck constant. Therefore the photon number flux density, namely the number per unit normal area per unit time, isMultiplying by an illuminated normal area gives the photon arrival rate. Each absorbed photon transfers momentum $hf/c$ at normal incidence, giving the radiation pressure on a perfectly absorbing surfaceThese are time-averaged quantities. At oblique angle of incidence $\theta$, the normal radiation pressure is $I{\cos }^2\theta /c$; the simple displayed expression assumes normal incidence.

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.