A general methodology integrating the Fresnell, Snell, and Beer-Lambert laws for modeling the radiant distribution in a medium is presented. The model considers refraction/reflection through/from the body of the UV lamp and sleeve, as well as the reflections from other sources, such as the reactor body. The measured boundary conditions are applied to realistically simulate the fluence/irradiance rate around the radiant source, in particular, in the zone closest to the radiant source. Different low-pressure UV lamps were tested under different operating conditions using photodiodes and a radiometer. The experimentally measured irradiance rate was in a very good agreement with the simulation results of the presented model. (C) 2010 Elsevier Ltd. All rights reserved.