Patent ID: 6111646
Filing Date: 2000-08-29
Classification: G01B,G01M,G03F

Abstract:
A method of measuring the accuracy of a phase-shifting point diffraction interferometer defining an optical path that includes (i) a source of electromagnetic energy in the optical path, (ii) a beam divider in the optical path for dividing electromagnetic radiation from the source into a reference beam and a test beam, (iii) an optical element under test in the optical path, and (iv) a fixed reference coordinate system in the optical path downstream from the source of electromagnetic energy, wherein the method comprises the steps of(a) positioning a null test mask in the image plane of the optical system under test positioned upstream of the fixed reference coordinate system wherein the mask has a region that is at least semi-transparent and at least one pair of pinholes and a corresponding alignment window, wherein each pair of pinholes comprises a first pinhole and a second pinhole that are spaced apart at a predetermined distance, whereby a reference beam and a test beam are directed to said region of the mask the two beams create first and second image points on said region;(b) aligning the image points and pinholes with respect to rotation orientation such that the image point orientation is substantially the same as that of the pinholes;(c) aligning the image points and the pinholes with respect to separation such that the images points are separated by a distance that is substantially the same as that between the pinholes; and(d) measuring aberrations of the phase-shifting point diffraction interferometer.