The influence of the size of the hot gas volume on the temperature decrease due to radiative losses above a burner with a flat porous inert surface has been studied using a 1D and a 2D axisymmetric model. Based on the results of simulations with these models and stimulated by a formulation based on the weighted sum of gray gases model, a simple expression accounting for the effect of the hot gas volume size in 1D simulations is proposed. This model was validated with temperature measurements above a few burners with different sizes and the agreement is satisfactory. It is shown that the optically thin limit is applicable only for very small burners. The uncertainty of the optically thin limit grows with the diameter d of the burner and is already 10% for d = 1 cm. On the other hand, a 1D model including self-adsorption is valid only for very large burners. For most applications, detailed computations of the radiative transfer should be applied. For fast computations it is advised that the above-mentioned simple model be used to account for emission and self-absorption. (C) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.