Patent ID: 7446951

Claim:
A system having an optical axis, the system being configured so that during use the system directs radiation along an optical path from an object plane to an image plane, the system comprising: a first optical element which comprises a cubic crystal material having a crystallographic (111) cut, a second optical element which comprises a cubic crystal material having a crystallographic (111) cut, the second optical element being rotated about the optical axis of the system relative to the first optical element; a third optical element which comprises a cubic crystal material having a crystallographic (100) cut and; and a fourth optical element which comprises a cubic crystal material having a crystallographic (100) cut, the fourth optical element being rotated about the optical axis of the system relative to the third optical element, wherein: for each set of two adjacent optical elements of the first, second, third and fourth optical elements, a convex surface of one of the optical elements faces a concave surface of the other optical element; and the first, second, third and fourth optical elements are between the object plane and the image plane along the optical path.