Accurate prediction of the light distribution field is important for designing large-scale photoreactors. So far, many incident light models have been developed that can be used for an accurate assessment of the light distribution field in annular photoreactors. Sometimes coated reflectors are used in the reactors to recycle photons, which are otherwise wasted; however, in these situations, the performance of the above models has not been tested. In this study, a new light distribution model based on a line source with diffuse emission model has been developed that considers the reflector as a second light source in addition to the original light installed along the centerline of the annular photoreactor. The performance of the new model is satisfactory in comparison with the experimental data obtained using chemical actinometry in a gas-phase photoreactor.