The practical implication is that for exploratory testing of new photocatalysts, it is often useful to first work in the regime of high optical density/high absorbance. This ensures that all supplied light is absorbed and the catalytic activity, in units of μmol(product) h -1 , is maximized. After a promising photocatalyst has been identified, its mass normalized activity (μmol(product) g(catalyst) -1 h -1 ) can be optimized by lowering the catalyst concentration, thus reducing the optical density of the dispersion to the optimal point. This optimal point is defined as the photocatalysts concentration where complete light absorption occurs while using the lowest amount of photocatalyst possible.