A simple model for determining potential bottlenecks of a photoreactor setup focusing on the photon fluxes is presented. The application of the concept can reveal optimization potentials and gives insights into the sensitivity of the reactor setup to different optimization possibilities. The introduced model benefits from the concept of using only data already available from optimization studies of the process conditions. Applying the introduced concept to the characterization of a previously developed modular organic light-emitting diode reactor setup revealed great optimization potentials, especially with respect to the external photonic efficiency. Interestingly, the attempt to enhance the external photonic efficiency by increasing the projection area of the reactor did not provide any improvement. This is attributed to a significant influence of reflection and scattering within the setup.