The cultivation of photosynthetic microorganisms in photobioreactors mainly occurs under limitation by radiant light energy transfer at a rate controlled by the total radiant light energy entering the reactor. Consequently, the mean incident radiant light flux onto the reactor is a key technical quantity in both kinetic and energetic studies of such processes. In this paper, a general revised method for determining the incident radiant light flux is proposed using a chemical actinometer, Reinecke’s salt. New easy procedures are given for preparing the reactive salt and measuring the photoreaction rate. Additionally, a mathematical model makes it possible to apply the technique to different reactor geometries and lighting systems with no limiting operating conditions for the incident flux. The method gives accurate results and constitutes a simple, general and reliable method for evaluating the maximum performance of photobioreactors. It can also be a tool for developing monodimensional models for simulation.