To describe the thermal radiation behaviour of particle-laden flows a mathematical model, based on a physical approach, was developed which takes into account the effects of absorption, scattering and backscattering. In contrast to existing calculational methods the required optical coefficients are not obtained empirically or by measurement. By applying the Mie theory these parameters can be calculated in an explicit manner. As backscattering is taken into consideration, the model is suitable for applications even at very high particle loadings including the limiting case of stationary fluidized beds.