The use of the Light-Beam-Induced-Current (LBIC) and Electron-Beam-Induced-Current (EBIC) analysis for characterizing polycrystalline materials can sometimes result in unreasonably high or unreasonably low recombination velocities at grain boundaries (GBs). In this work, we present a theoretical LBIC expression by taking into account the band bending and the reduced optical transmittance (T) at a grain boundary (GB). The recombination velocity Vr, the diffusion length L of the minority carriers, the potential barriers height (qVg) and the transmittance T of the GB are determined by fitting the theoretical LBIC current to the experimental one.