1. A method of improving an illumination pupil shape for an illumination source used in a lithographic process, the lithographic process being configured to image a design layout, the method comprising the steps of: identifying a target pattern included in the design layout; identifying at least one optimization point in said target pattern, wherein the optimization point comprises a spatial point within the design layout based on which the pupil shape of the illumination source is to be improved; identifying at least one design for manufacturing metric per optimization point; selecting a set of illumination source points based on the identified at least one design for manufacturing metric; and determining the improved illumination pupil shape based on the selected set of illumination source points.