Patent ID: 8031327

Claim:
An illumination system having an optical axis, the illumination system comprising: a first wedge plate having a first wedge direction extending perpendicularly to the optical axis in a direction of a maximum thickness of the first wedge plate; and a second wedge plate having a second wedge direction extending perpendicularly to the optical axis in a direction of a maximum thickness of the second wedge plate; wherein: the first and second wedge plates are arranged rotatably about the optical axis; the first wedge plate comprises a second birefringent crystal material having a first optical crystal axis; the second wedge plate comprises a first birefringent crystal material having a second optical crystal axis; the first and second wedge plates are rotatable independently of each other about the optical axis; when the first and second wedge directions are parallel to each other, the first and second optical crystal axes are oriented at an angle from 42° to 48° relative to each other; and the illumination system is configured to be used in a microlithographic projection exposure apparatus.