A radiation model has been developed to estimate Light energy absorbed in a photoreactor with multiple lamps, In this study a diffused line source emission model is adopted and a light path length distribution is used for computing the light absorption rate. The results of computation were confirmed to be adequate by chemical actinometric experiments. The optimum lamp location in a photoreactor with a lamps in symmetry is determined to attain the maximum average light intensity. As a rule of thumb, the optimum ratio of the distance between the lamp center and the reactor center to the reactor radius is 1/3 for n = 2, 2/5 for n = 3, and 1/2 for n = 4.