Three light source models, i.e., radial, specular, and diffuse line source models, are compared for light intensity profile, overall rate of reaction, and radial scale-up ratio of annular photochemical reactors, with the assumptions of constant absorption coefficient and constant quantum efficiency. The necessary condition for the radial light model to hold is that, when the ratio of inner radius to height of the reactor m, is 0.1, that of outer radius to inner radius ρ should be less than 3, and that when m is larger ρ should be close to 1. Published reaction data are analyzed according to this condition.