The external irradiation with a single source (or at least singular irradiation area on the reactor), features a varying irradiance at said irradiation spot if the light intensity is varied. For example, based on a circular irradiation spot with 3 cm diameter, the irradiance increases from 30.1 mW cm -2 at 13 µM s -1 photon flux density up to 429.0 mW cm -2 at the highest employed light intensity for external irradiation (185 µM s -1 ). This value is more than 4 times as high as for the internal irradiation and can therefore explain the accompanied efficiency losses, whereas at low light intensity both the observed reaction rate and estimated irradiances of internal and external irradiation are comparable with similar irradiances.