selecting a model for a lithography process, the model including an optical model and a resist model, the optical model having initial values of optical model parameters and the resist model having initial values of resist model parameters;

measuring an aerial image using an image sensor array to produce a measured sensor image, the aerial image being produced by a lithography tool from a mask created using circuit design data;

simulating the lithography process using the optical model and the circuit design data to produce a simulated aerial image;

determining differences between the simulated aerial image and the measured sensor image; and

modifying the values of the optical model parameters based on the differences between the simulated aerial image and the measured sensor image to identify optimal values for the optical model parameters, wherein a simulated aerial image produced using the optimal values for the optical model parameters is an optimal simulated aerial image that is a best match for the measured sensor image.