A proprietary field emission scanning electron microscopy (FESEM) image analysis method is proposed to support the optical modelling of spectroscopic ellipsometry data evaluation of complex porous silicon multilayer stacks (PSM). Sample structure was a 27-layer Fabry-Perot type PSM stack, which consists of nominally lambda(0)/4 (optical thickness) layers of high and low refractive indices (low and high porosity). Two types of optical models were adopted (derived from FESEM imaging): one considering step-like, abrupt interfaces and another one taking gradual transitions between the layers of high and low index of refraction into account. The results suggest that there is no sense to refine the optical models beyond any limit, when the uncertainties given by the tolerances involved in the interface determination of this random PSM structure do not justify the efforts any more. (C) 2004 Elsevier B.V. All rights reserved.