Patent ID: 7755833

Claim:
A correcting device to compensate for perturbations of a polarization distribution over a cross section of a light beam in an optical system, comprising a correcting member which, in a given arrangement, comprises: a first birefringent correcting element having two substantially parallel and substantially planar surfaces, including a first surface and a further first surface, and a substantially constant first thickness in a direction perpendicular to the surfaces of the first element, a second birefringent correcting element having two substantially parallel and substantially planar surfaces, including a second surface and a further second surface, and a substantially constant second thickness in a direction perpendicular to the surfaces of the second element, wherein at least one of the first surface, the further first surface, the second surface, and the further second surface is reprocessed so as to create local thickness variations Δd by which the perturbations of the polarization distribution are compensated at least approximately, and wherein the arrangement, the first and the second thicknesses and birefringence properties of the first and the second correcting elements are selected so that birefringent effects within the correcting member cancel each other out at least approximately, when the local thickness variations Δd are neglected.