Number of photons of a particular wavelength per time interval (spectral photon flux, number basis,
,
or spectral photon flux, amount basis,
)
absorbed by a system per volume,
.
On number basis, SI unit is
common unit is
On amount basis, SI unit is
common unit is
.

Notes:

- Mathematical expression: on number basis, on amount basis, where is the absorbance at wavelength and superscript (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, or the rate of change of the amount concentration, , respectively.

Source:

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