The anisotropic extinction and absorption coefficients of a high porosity material with a semi-transparent solid phase are directly obtained from identification of the statistical cumulated distribution function of extinction distances in the material, linked to that of the chords, and the corresponding function in the equivalent semi transparent medium. The bidirectional phase function is then determined without any hypothesis. The model only requires the knowledge of the material morphology, given by a X-ray tomography and the phases local radiative properties. (c) 2006 Elsevier Ltd. All rights reserved.