A Monte Carlo ray tracing procedure is proposed to simulate thermal optical processes in heterogeneous materials. It operates within a detailed 3D image of the material, and it can therefore be used to investigate the relationship between the microstructure, the constituent optical properties, and the macroscopic radiative behavior. The program is applied to porous silica glass. A sample was first characterized by 3D x-ray tomography; then, its normal spectral emittance was calculated and compared with the experimental spectrum measured independently by high-temperature infrared emittance spectroscopy. We conclude with a discussion of the light-scattering mechanisms occurring in the sample.