The application of advanced oxidation processes using hydroxyl radicals in the degradation of organic contaminants from various waste streams has become increasingly popular over the last 2 decades. The relationship between the local volumetric rate of energy absorption in a photoreactor and the rate of photodegradation of organics is essential in scaling up photoreactors to treat large volumes of air and water. Thus, it is very important that the light intensity in a photoreactor is evaluated correctly. In this research, local light intensities as a function of both radius and longitudinal axis in a test photoreactor for the treatment of air were measured using chemical actinometry and compared with the simulated data using three radiation models. A good agreement exists between the experimental data and the model predictions, and no model exhibits a clear advantage over others. The line source with diffused emission model shows the best overall performance among the three models with regard to computational time, simplicity, and accuracy.