A method is provided for determining an OPC model comprising: recording an aerial image by use of a mask inspection microscope, wherein the aerial image comprises at least one segment of a mask; simulating a plurality of aerial images which comprise at least the segment, proceeding from a mask design and from predefined parameters of an optical model which is part of the OPC model, wherein the parameters differ for each of the simulated aerial images of the plurality of aerial images; determining differences between the measured aerial image and the simulated aerial images; determining those parameters for which the differences between simulated aerial image and measured aerial image are the least.In addition, a mask inspection microscope for carrying out the method is provided.