The present invention provides a method for calibrating a computational model of a lithography process by calculating a demerit function using an intensity measurement at a location of a wafer; and calibrating the lithography model or a mask making model by determining values of parameters of the computational model using the calculated demerit function. The method may also use a second demerit function that is defined by the sum of squares of differences between a simulated and measured critical dimensions of a feature on the wafer.