During the experiments, mass transfer effects were ruled out based on the absence of dark zones in the photoreactors, along with the use of high flow rates and/or rotation speeds. Plug flow behavior was assumed for the microcapillary reactor, while a continuously stirred tank reactor (CSTR) model in series was applied to the high-shear reactor. These models were assumed to be valid across all operational conditions. Any variation in photon absorption due to changes in the reactor model is captured in the effective optical path length, which serves as an effective descriptor for photon absorption. This path length was obtained using photon fluxes determined through a combination of radiometry and raytracing. The methodology employed allows for the quantification of the average fraction of transmitted photons within the photoreactor system. The applicability of this method extends beyond the use of UV-A light and the potassium ferrioxalate actinometer. In principle, any actinometer and non-absorbing gas can be used, provided that the irradiated reactor-medium interface is wetted, allowing for precise determination of the effective optical path length in most photoreactor systems. As demonstrated by the unexpected findings in this study, this characterization is crucial for accurately assessing performance and operational expenditures.