The potassium ferrioxalate actinometer (Equation ), a well-established standard, was employed to characterize the setups using ultraviolet UV-A light-emitting diode (LED) light sources. The actinometer conversion was measured at various flow rates to generate kinetic curves under each set of conditions. Nitrogen was introduced as an inert gas to examine the effect of gas phase addition on photon absorption, while gas-phase effects from carbon dioxide formation were neglected (Figure ). By utilizing photon flux data obtained via ray-tracing and radiometry, the effective optical path length was determined by fitting the kinetic curves to an appropriate reactor model.