Patent ID: 8031326

Claim:
An optical system having an optical system axis, the optical system comprising: at least one element group comprising first, second and third birefringent elements each of which comprise optically uniaxial material and have an aspheric surface, wherein: the first birefringent element of the group has an optical crystal axis with a first orientation; the second birefringent element has an optical crystal axis with a second orientation rotated relative to the first orientation by an angle that does not correspond to a rotation around the optical system axis by an angle of 90° or an integer multiple thereof; the third birefringent element of the group has an optical crystal axis with a third orientation rotated relative to the second orientation by an angle that does not correspond to a rotation around the optical system axis by an angle of 90° or an integer multiple thereof; and the optical system is an illumination system of a microlithographic exposure system or a projection lens of a microlithographic exposure system, wherein each birefringent element has a variation of its thickness profile so that a minimum phase retardation of Δφmin≈25 mrad is obtained at a given operating wavelength of the optical system.