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