Patent ID: 7907262

Claim:
Apparatus for indicating the departure of a shape of an object from a specified shape, the apparatus comprising: radiation means for directing an incident beam of radiation onto the object, inspecting means for inspecting a final beam, said object located optically between said radiation means and said inspecting means at least one wavefront shaping means, optically disposed between the radiation means and the inspecting means for shaping the final beam to have a substantially planar wavefront when said object has said specified shape, and said final beam comprises a beam which has been both transmitted by or reflected from said object and shaped by said wavefront shaping means, said at least one wavefront shaping means is arranged to compensate for non-planarity introduced by said object having said specified shape, and said inspecting means is arranged to determine any departure of the wavefront of the final beam from planarity, wherein said inspecting means comprises: beamsplitting means for splitting the final beam into two or more diffracted beams and for directing said two or more beams to laterally displaced locations; and detector means for detecting radiation intensity of said two or more beams on the detector means.