A coupled pair of simple partial differential equations extends
the Beer–Lambert law in photochemical applications. A function –
namely the left-hand side of Eq. (25) involving both the absorbance
and the transmittance – is shown to vary linearly with time during
a photochemical experiment. The quantum yield is calculable from
the slope of such a graph. A simulation has verified this result. The
method is valid for any degree of mixing