The invention relates to a method and a device for generating a reference image in the characterization of a mask for microlithography, wherein the mask comprises a plurality of structures and wherein the reference image is generated by simulation of the imaging of said mask, said imaging being effected by a given optical system, both using a rigorous simulation and using a Kirchhoff simulation, wherein the method comprises the following steps: assigning each structure of said plurality of structures either to a first category or to a second category, calculating a plurality of first partial spectra for structures of the first category with implementation of rigorous simulations, calculating a second partial spectrum for structures of the second category with implementation of a Kirchhoff simulation, generating a hybrid spectrum on the basis of the first partial spectra and the second partial spectrum, and generating the reference image with implementation of an optical forward propagation of said hybrid spectrum in the optical system.