1. A method for determining a process model, the method comprising: receiving a first optical model which models a first optical system of a photolithography process; using 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; using 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; receiving process data which is obtained by subjecting a test layout to the photolithography process; and determining a process model using the first optical model, the second optical model, the third optical model, the test layout, and the process data.