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Apparatus for the measurement of surface shape

  • US 5,260,761 A
  • Filed: 12/05/1991
  • Issued: 11/09/1993
  • Est. Priority Date: 12/07/1990
  • Status: Expired due to Fees
First Claim
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1. A method of measuring the shape of a remote surface relative to a fixed reference datum, comprising:

  • providing a coherent radiation beam,splitting the beam into a measuring beam and a reference beam,directing the measuring beam at a first measurement point on the remote surface,providing a fixed datum for the reference beam whereby a static optical path length traversed by the reference beam is nominally the same as a path length traversed by the measuring beam,modulating accurately and repetitively an instantaneous optical path length traversed by one of the reference beam and the measuring beam over a dynamic range,collecting a portion of the measuring beam reflected back from the remote surface,collecting a portion of the reference beam reflected back from the datum point,recombining the reflected portions of the measuring beam and the reference beam,detecting a variation of interferometric modulation between the measuring beam and the reference beam during a path length modulation cycle,determining from the detected variation of interferometric modulation a condition when the instantaneous path length of the reference beam is identical to the path length of the measuring beam at a first measurement point,determining a corresponding path difference between the first measurement point and the fixed reference datum,determining coordinates of the first measurement point transverse to a measuring beam axis,moving the measuring beam to a second and subsequent measurement points on the remote surface, andrepeating the above steps for the second and each subsequent measurement point to determine a path difference and transverse coordinates of the second and subsequent measurement points until a desired number of points have been measured on the remote surface.

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