absorbed (spectral) photon flux density

Number of photons of a particular wavelength per time interval (spectral photon flux, number basis, $q p , λ$ , or spectral photon flux, amount basis, $q n , p , λ$ ) absorbed by a system per volume, $V$ . On number basis, SI unit is $s −1 m −4$ common unit is $s −1 cm −3 nm −1$ On amount basis, SI unit is $mol s −1 m −4$ common unit is $einstein s −1 cm −3 nm −1$ .

Notes:

Mathematical expression: $q p , λ 0 1 − 10 − A λ V$ on number basis, $q n , p , λ 0 1 − 10 − A λ V$ on amount basis, where $A λ$ is the absorbance at wavelength $λ$ and superscript $0$ (zero) indicates incident photons.
Absorbed spectral photon flux density (number basis or amount basis) should be used in the denominator when calculating a differential quantum yield and using in the numerator the rate of change of the number concentration, $d C d t$ or the rate of change of the amount concentration, $d c d t$ , respectively.

Source:

PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 297