In this study, we present a comprehensive workflow to ascertain the photon flux and effective optical path length for intensified photoreactors. Recognizing that the effective optical path length in intricate photochemical reactors and setups is challenging to define with a single predetermined value, we proposed its determination through a synergistic approach involving radiometry, ray-tracing, and chemical actinometry. Instead of relying solely on manufacturer-provided optical output data, we conduct radiometric measurements to quantify light intensity across multiple distinct points, factoring in potential light source decay. This information serves as a calibration benchmark for virtual light source simulations. Ray-tracing emerges as an invaluable instrument, granting profound insights into light sources and photoreactors. Our modeling approach, though necessitating precise light source and reactor representations, proves adept at handling even reactors with complex geometries and nontrivial effective optical path lengths, such as those incorporating static mixers or featuring nontransparent backplates. Only a description of the reactor's outer surface and reactor-medium interface suffices for determining the received photon flux, thereby obviating intricate photon path assessments within ray-tracing software.