1. A method for determining a process model, the method comprising: receiving a first optical model which models a first image that is formed by a first optical system of a photolithography process at the first optical system's best focus distance; receiving a second optical model which models a second latent image that is formed by a second optical system at the first optical system's best focus distance; receiving a third optical model which models a third latent image that is formed by a third optical system at the first optical system's best focus distance; receiving process data which is obtained by subjecting a test layout to the photolithography process; determining an uncalibrated process model using the first optical model, the second optical model, and the third optical model; and fitting, by using a computer, the uncalibrated process model using the process data.