Optical system

摘要

An optical system comprising an optical element arranged on an optical axis in the path of a radiation beam. The optical element ( 2; 116; 202 ) comprises a birefringent material and has a non-planar face ( 4 ) through which the radiation beam passes. The optical system comprises a polarisation control system for controlling polarisation of the radiation beam such that the radiation beam has a polarisation which is non-uniform across a cross section ( 21; 24 ) taken perpendicular to the optical axis, the non-uniform polarisation having a distribution corresponding with a shape of the said non-planar face.