A model to calculate the amount of photons radiated to the reaction medium is important to choose the appropriate reactor type and light source in accordance with the objectives (e.g., space time yield maximization) the reaction is evaluated for. Photon flux received in the Vapourtec UV-150 photochemical reactor was studied using a potassium ferrioxalate in the ESI. Experimental procedure, measurement techniques, and equations are reported in detail in the ESI.