Number of photons of a particular wavelength per time interval (spectral photon flux, number basis, $q_{\text{p},\lambda }$, or spectral photon flux, amount basis, $q_{n,\text{p},\lambda }$) absorbed by a system per volume, $V$. On number basis, SI unit is ${\text{s}}^{-1}{\text{m}}^{-4}$, and the common unit is ${\text{s}}^{-1}{\text{cm}}^{-3}{\text{nm}}^{-1}$. On amount basis, SI unit is $\mathrm{mol}{\mathrm{s}}^{-1}{\mathrm{m}}^{-4}$, and a common unit is $\text{einstein}{\text{s}}^{-1}{\text{cm}}^{-3}{\text{nm}}^{-1}$.