Modeling of lithographic processes for use in the design of photomasks for the manufacture of semiconductor integrated circuits, and particularly to the modeling of the complex effects due to interaction of the illuminating light with the mask topography, is provided. An isofield perturbation to a thin mask representation of the mask is provided by determining, for the components of the illumination, differences between the electric field on a feature edge having finite thickness and on the corresponding feature edge of a thin mask representation. An isofield perturbation is obtained from a weighted coherent combination of the differences for each illumination polarization. The electric field of a mask having topographic edges is represented by combining a thin mask representation with the isofield perturbation applied to each edge of the mask.