1. A method for designing a lithographic process including simulating an image of a topographic mask under an illumination having an associated electric field, wherein a design of the topographic mask is provided, the mask design including a mask feature having two-dimensional polygon feature edges in a horizontal plane of the mask, wherein the feature edges have a finite thickness along a non-horizontal direction of the mask, the method comprising the steps of: determining a thin mask representation of the electric field on the mask plane corresponding to said mask feature; determining a first field difference between the electric field on the mask plane along a first edge of said mask feature and the electric field on the mask plane along the corresponding edge of a thin mask representation of said mask feature under illumination of a first component of the electric field and a second field difference between the electric field on the mask plane along said first edge of said mask feature and the electric field on the mask plane along said corresponding first edge of said thin mask representation of said mask feature under illumination of a second component of the electric field orthogonal to said first component of the electric field; forming an isotropic field perturbation (IsoField Perturbation IsoΔM) to the thin mask electric field on the mask plane corresponding to said first edge of said mask feature comprising a weighted coherent combination of said first and second field differences; providing an imaging model for simulating an image on a wafer plane, the imaging model comprising a representation of the electric fields on the mask plane responsive to illumination from an imaging system, wherein said representation of the electric fields on the mask plane comprises said thin mask representation combined with said Isofield Perturbation; and simulating an image on the wafer plane corresponding to said mask feature under the illumination using said imaging model.