Both diffusion theory and similarity relations for light reflectance by semi-infinite turbid media have been analyzed by comparing their computational results with Monte Carlo simulation results. Since a large number of photon packets are traced, the variance of the Monte Carlo simulation results is small enough to reveal the detailed defects of diffusion theories and similarity relations. We have demonstrated that both diffusion theory and similarity relations provide very accurate results when the photon sources are isotropic and one transport mean free path below the turbid medium surface or deeper. This analysis has led to a hybrid model of Monte Carlo simulation and diffusion theory, which combines the accuracy advantage of Monte Carlo simulation and the speed advantage of diffusion theory. The similarity relations are used for the transition from the Monte Carlo simulation to the diffusion theory.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.