An optical model of a porous silicon layer used as a light diffuser in thin-film solar cells is proposed in the framework of Twersky＇s multiple scattering theory of electromagnetic waves in random media. A clear distinction is made between the coherent and diffuse components of the field. The optical properties of the coherent field are described in terms of a complex refractive index calculated using Foldy-Twersky＇s integral equation, while the. diffuse component is estimated by means of the Boltzmann-like radiative transfer equation. A brief description of the experimental verification of the model is then given. The model gives very good results for a low-porosity porous silicon layer sandwiched between the two crystalline silicon layers, but will hopefully give a good qualitative picture in more general situations. (C) 2000 Elsevier Science B.V. All rights reserved.