An embodiment provides systems and techniques for determining a process model. During operation, the system may receive a first optical model which models a first optical system of a photolithography process. Next, the system may use the first optical model to determine a second optical model that models a second latent image that is formed by the first optical system at a second distance. The system may also use the first optical model to determine a third optical model that models a third latent image that is formed by the first optical system at a third distance. Next, the system may receive process data which is obtained by subjecting a test layout to the photolithography process. The system may then determine a process model using the first optical model, the second optical model, the third optical model, the test layout, and the process data.