partially coherent euv radiation suitable for projection lithography application_number US 37794703 A description the k u00f6hler illuminator is an important class of illumination systems that can be described as providing a fourier transform relationship between the source and the target illumination plane a simple manifestation of such a system is a single lens where the source is placed in the front focal plane and the target illumination plane in the back focal plane in this configuration the lens can be seen as a collimating optic with respect to the source each individual source point where a source point is defined as an elemental independent point radiator can then be viewed as generating a plane wave exiting the lens at some unique angle for a source point centered on the lens optical axis the plane wave will travel along the optical axis and for any other source point the exit angle will be determined by the angle defined by the source point offset from the optical axis and the distance to the lens the front focal length of the lens because there exists a fourier transform relationship between illumination angular bandwidth and spatial coherence the coherence area at the target illumination plane can be seen to be inversely proportional to the source size or extent in the front focal plane where source size.