Patent ID: 6151115
Filing Date: 2000-11-21
Classification: G01B,G03F

Abstract:
A method of focusing a phase-shifting point diffraction interferometer that defines an optical path that includes (i) a source of electromagnetic energy in the optical path, (ii) an optical element under test in the optical path, and (iii) an irradiance detector in the optical path downstream from the optical element under test, comprising the steps of:(a) positioning a mask in the image plane of the optical system under test positioned upstream of the irradiance detector wherein the mask defines a test-beam window and at least one reference pinhole and a focus-aid pinhole having a diameter of at least about 20% larger than that of the at least one reference pinhole;(b) directing a beam focused by the optical element under test through the test-beam window;(c) rough-focusing the beam onto the image plane;(d) positioning a beam divider in the optical path for dividing the electromagnetic radiation from the source into a test-beam and a reference-beam;(e) aligning the mask so that the test-beam passes through the test-beam window and the reference-beam passes through the focus-aid pinhole;(f) fine-focusing the beams on the image plane by detecting interference patterns with the irradiance detector and recovering the defocus term from the wavefront; and(g) maneuvering the mask so that the reference-beam passes through the reference-beam pinhole.