The measured irradiance can then be combined with the side-view area of the dispersion to calculate the power incident on the dispersion (e.g. in mW). At this point is must be mentioned that the incident power is not equivalent to the photon flux that actually reaches the dispersion inside the photoreactor, for example due to scattering by the water mantle, glass walls etc. The photon flux that reaches the inside of the photoreactor can be measured using chemical actinometry . Importantly, there is a linear correlation between measured incident power and the photon flux inside the reactor . Therefore, for a specific photoreactor geometry, it is sufficient to perform a calibration between incident power and chemical actinometry once. Using this calibration, measured incident powers from photocatalytic tests can then be converted to photon flux within the reactor, which can be used e.g. for quantum efficiency calculations. Readers are directed to Schneidewind et al., 2021 for a detailed description of the conversion from incident power measurements to photon flux.