Optical system for surface topography measurement
First Claim
1. A non-contacting method for measuring deviations between a reference wavefront representing a reference surface and a test wavefront representing a test surface comprising,establishing a white light interference pattern on a detector between two wavefronts, one wavefront from at least one point on a reference surface of known topography and another wavefront from a corresponding point on a test surface of unknown topography,repeatedly translating one of the wavefronts from said surfaces by known incremental distances relative to the detector from a starting position and establishing other white light interferences patterns, one for each translation, while establishing zero optical path differences between wavefronts from locations on the reference surface and corresponding locations on the test surface of unknown topography,reading light intensity levels at a number of sample points of the interference pattern for each translation,calculating a statistical function indicative of the contrast maximum for each of the sample points of the interference pattern over the range of wavefront translation,recording the positions of calculated maximum fringe contrast and the corresponding incremental distance of surface translation relative to the starting position for each sample point, whereby the points of maximum fringe contrast at a given step represent points, the locus of which defines contours for which there is zero optical path difference between the test surface and the reference surface.
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Abstract
An interferometric system for characterizing the surface of a test object, such as an aspheric surface. A white light interferogram is produced wherein the principal fringe indicates zero optical path difference between a test surface and a reference surface. Wavefronts from either of the test or reference surfaces are translated by incremental amounts. A multi-point detector array is used to make multi-point contrast readings. Points of contrast maxima for each detector point are computed by a statistical determination. A centroid function is preferred.
By recording the points of maximum contrast and the incremental wavefront translation, two dimensional plots showing zero optical path differences for the two surfaces are obtained, thereby comparing the test and reference surfaces.
39 Citations
3 Claims
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1. A non-contacting method for measuring deviations between a reference wavefront representing a reference surface and a test wavefront representing a test surface comprising,
establishing a white light interference pattern on a detector between two wavefronts, one wavefront from at least one point on a reference surface of known topography and another wavefront from a corresponding point on a test surface of unknown topography, repeatedly translating one of the wavefronts from said surfaces by known incremental distances relative to the detector from a starting position and establishing other white light interferences patterns, one for each translation, while establishing zero optical path differences between wavefronts from locations on the reference surface and corresponding locations on the test surface of unknown topography, reading light intensity levels at a number of sample points of the interference pattern for each translation, calculating a statistical function indicative of the contrast maximum for each of the sample points of the interference pattern over the range of wavefront translation, recording the positions of calculated maximum fringe contrast and the corresponding incremental distance of surface translation relative to the starting position for each sample point, whereby the points of maximum fringe contrast at a given step represent points, the locus of which defines contours for which there is zero optical path difference between the test surface and the reference surface.
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2. A non-contacting method for measuring deviations between a reference surface and a test surface comprising,
(a) establishing a white light interference pattern on a detector array for zero optical path difference between two wavefronts, one wavefront from at least one point on a reference surface of known topography and another wavefront from a corresponding point on a test surface of unknown topography; -
(b) repeatedly translating one of the wavefronts from said surfaces by known incremental distances relative to the detector from a starting position and establishing other white light interference patterns, one for each translation, while establishing zero optical path differences between wavefronts from locations on the reference surface and corresponding locations on the test surface of unknown topography; (c) repeatedly sampling detector points of the detector array; (d) storing in a first register intensity values, xo, at detector points in the starting position; (e) storing in a second register f(s) where f(s) equals ε
|x-xo |, with x being the intensity at the detector points other than at the starting position;(f) storing in a third register Sf(s) where S is the incremental slice number; (g) computing the centroid function ##EQU2## for all points on all slices where x is the computed intensity maximum for each point and storing said values of x in a fourth register; (h) storing in a fifth register, for corresponding positions in said fourth register, a step number representing an incremental distance at which the contrast maxima values, x in the fourth register occurred, whereby maximum contrast values indicate equal phase contours for establishing zero optical path differences between said test and reference surfaces.
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3. An apparatus for measuring deviations between a reference surface and a test surface comprising,
a white light interferometer having a mean wavelength of white light for producing an interference pattern from interfering wavefronts from a test surface and a reference surface; -
means for varying the optical path length difference for interfering wavefronts between said test and reference surfaces by known incremental distances; a detector positioned for sensing the intensity of said interfering wavefronts, said detector having a plurality of detector elements therein, means for scanning said plurality of detector elements at each incremental distance to obtain from each cell signals representative of intensity, a first register having cells for storing intensity values, xo, for each detector element in the detector starting position, a second register having cells for storing f(s) where f(s) equals ε
|x-xo |, with x being the intensity at the detector points other than at the starting position,a third register having cells for storing Sf(s) where S is the incremental slice number, means for computing the centroid function ##EQU3## for all points on all slices were x is the computed intensity maximum for each point, a fourth register having cells for storing the computed intensity maxima, x, and a fifth register having cells for storing the step number representing an incremental distance at which the contrast maxima values, x, in the fourth register occurred, whereby maximum contrast values indicate equal phase contours for establishing zero optical path differences between the test and reference surfaces.
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Specification