The photon irradiance $E_{\text{p}}$, at wavelength $\lambda$ per unit wavelength interval. The SI unit is ${\text{s}}^{-1}{\text{m}}^{-3}$, but a commonly used unit is ${\text{s}}^{-1}{\text{m}}^{-2}{\text{nm}}^{-1}$. Alternatively, the term can be used with the amount of photons (mol or its equivalent einstein), the SI unit then being $\mathrm{mol}{\mathrm{s}}^{-1}{\mathrm{m}}^{-3}$ and the common unit $\mathrm{mol}{\mathrm{s}}^{-1}{\mathrm{m}}^{-2}{\mathrm{nm}}^{-1}$.