A computer-implemented method for simulating a scattered radiation field of a patterning device including one or more features, in a lithographic projection apparatus, the method including: determining a scattering function of the patterning device using one or more scattering functions of feature elements of the one or more features; wherein at least one of the one or more features is a three-dimensional feature, or the one or more scattering functions characterize scattering of incident radiation fields at a plurality of incident angles on the feature elements.