 Absorbed photon flux

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,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.^[ref]

For example, $q_0=2\mu \mathrm{mol}{\mathrm{s}}^{-1}$ and $A=0.30$ giveA product-formation rate of $0.20\mu \mathrm{mol}{\mathrm{s}}^{-1}$ 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.^[ref]