The volumetric photon flux density (qP) was determined by means of ferrioxalate actinometry according to Hatchard and Parker. 25, 50 or 75 mL of a freshly prepared ferrioxalate solution (6 µM in 0.05 M H2SO4 (98% Merck)) was added to the reactor and irradiated either with a varying amount of WLE or the external UV LED. At defined intervals, 25 µL samples of the irradiated ferrioxalate solution were taken and mixed with 175 µL of a solution consisting of 7.5 mL H2SO4 (0.05 M), 2 mL 0.1% 1,10-phenantroline (Carl Roth) solution, 5 mL sodium acetate solution (1 M (Carl Roth)) and 3 mL deionized water. The absorbance of the resulting ferroin complex was measured at the maximum absorption of 510 nm using a UV-Vis microplate reader (PowerWave HT, BioTek). The respective iron(II)concentration has been determined using iron(II)sulfate as an authentic standard.