Diffraction modeling of a diffracting structure employing at least two distinct differential equation solution methods. In an embodiment, a rigorous coupled wave (RCW) method and a coordinate transform (C) method are coupled with a same S-matrix algorithm to provide a model profile for a scatterometry measurement of a diffracting structure having unknown parameters. In an embodiment, a rigorous coupled wave (RCW) method and a coordinate transform (C) method generate a modeled angular spectrum of diffracted orders as a prediction for how a diffracting photolithographic mask images onto a substrate.