The outcome of our ray-tracing simulations and experimental endeavors aims to shed light on irradiation profiles and photon absorption, offering predictive capabilities for absorption properties across diverse photochemical systems. Additionally, by integrating the unique geometry of different reactor designs, photon fluxes and effective path lengths can be ascertained through data fitting from a single kinetic experimental curve. Our methodology underwent rigorous validation using a non-intensified batch reactor, and its application extended to determining photon flux and effective optical path length in both a microflow and a photo rotor-stator spinning disk reactor. Intriguingly, the effective optical path length derived did not mirror the results obtained by using the frequently applied geometric average path length, underscoring the pivotal roles played by factors like diffuse emission, reflection, and refraction. Indeed, resorting to geometric average path lengths when deducing photon flux could yield results that defy physical possibility, given the constraints of the light sources and radiometric data at hand. This revelation underscores the paramount importance of accurately obtaining effective optical path lengths and attests to the robustness of our proposed workflow.