Patent ID: 8279404

Claim:
An optical system, comprising: a plurality of elements arranged to image radiation at a wavelength λ along a radiation path from an object field in an object surface to an image field in an image surface, wherein: the plurality of optical elements comprise mirror elements having a reflective surface comprising a reflective coating positioned along the radiation path; at least one of the plurality of mirror elements has a rotationally asymmetric reflective surface which deviates from a best-fit rotationally symmetric reflective surface by about λ or more at one or more locations; and the plurality of optical elements comprise an apodization correction element configured to correct a spatial intensity distribution present in an exit pupil of the optical system relative to a spatial intensity distribution in the exit pupil of the optical system that would be present in the absence of the apodization correcting element, wherein the apodization correction element is configured to increase symmetry of the spatial intensity distribution present in the exit pupil of the optical system relative to symmetry of spatial intensity distribution that would be present in the exit pupil of the optical system in the absence of the apodization correcting element.