= Absorbed photon flux
\b[Absorbed photon flux] is the rate at which a specified sample absorbs <photons>. Number-based flux counts <photons> per second; amount-based flux counts moles of <photons> per second. The absorbed rate differs from the incident rate and supplies the denominator of a rate-based <photochemical quantum yield>. For monochromatic light passing through a homogeneous absorbing sample,
$$
q_{\mathrm{abs}}=q_0\left(1-10^{-A(\lambda)}\right),
$$
where $q_0$ is incident photon flux and $A(\lambda)$ is the decadic <absorbance> at the excitation <wavelength>. Reflection and scattering must be absent or accounted for before transmission loss is interpreted entirely as <absorption>.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry>[Entry Q04991, differential quantum-yield denominator and monochromatic-excitation note.]{ref}
For example, $q_0=2\,\mathrm{\mu mol\,s^{-1}}$ and $A=0.30$ give
$$
q_{\mathrm{abs}}=2(1-10^{-0.30})\approx0.998\,\mathrm{\mu mol\,s^{-1}}.
$$
A product-formation rate of $0.20\,\mathrm{\mu mol\,s^{-1}}$ would then give <photochemical quantum yield> approximately $0.20/0.998=0.200$, whereas division by incident flux gives $0.10$. The two calculations answer different questions.
The distinction also matters during an experiment: if <absorbance> changes as a reactant is consumed, one initial flux value cannot represent the whole exposure. Absorbed photon amount is the time integral of the absorbed rate. If the numerator is a change of concentration per second, divide the total absorbed amount rate by the illuminated sample volume as well. A photon rate, a rate per unit area and a rate per unit volume have different dimensions; the word flux alone should not conceal that distinction.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry>[Q04991, note 1 on number/amount concentrations and absorbed spectral photon flux density.]{ref}
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