If the absorbance is high (high optical density of dispersion and/or long photoreactor pathlength), all the supplied light will be absorbed. At the point of complete light absorption, the observed activity (again in μmol(product) h -1 ) will reach a plateau. Further increasing the catalyst concentration will not increase the activity, instead the mass normalized activity (e.g. in μmol(product) g(catalyst) -1 h -1 ) will go down. This can be explained by the fact that in the regime of high absorbance, the light beam will not penetrate through the entire dispersion. Hence, a part of the dispersed photocatalyst particles will not be irradiated at all, thus not contributing to photocatalytic activity, which in turn leads to a decrease in mass normalized activity.