1. A method of generating a model for simulating an imaging performance of an optical imaging system having an illumination stage that illuminates a mask, and a pupil that projects the illumination from the mask to a substrate stage, said method comprising the steps of: defining said optical imaging system and a process to be utilized by said optical imaging system; identifying one or more existing model associated with said optical imaging system and said process; defining a functional representation of said pupil; modifying said functional representation of said pupil by calibrating with said one or more existing model to generate a calibrated pupil kernel, wherein said calibrated pupil kernel represents a model of the pupil performance; defining a model equation including said calibrated pupil kernel, wherein said model equation represents said optical imaging system and said process; and generating said simulation model using said model equation to simulate said imaging performance of said optical imaging system.