Optical system for surface topography measurement

• US 4,340,306 A
• Filed: 02/04/1980
• Issued: 07/20/1982
• Est. Priority Date: 02/04/1980
• Status: Expired due to Term

• Current Assignee: Miles, Inc. (Bayer AG)
• Original Assignee: N Balasubramanian
• Inventors: Balasubramanian, N.
• Primary Examiner(s): Corbin, John K.
• Assistant Examiner(s): ARNOLD, BRUCE
• Application Number:
  US06/117,855
• Time in Patent Office: 897 Days
• Field of Search: 356/360, 356/354
• US Class Current: 356/513
• CPC Class Codes: G01B 11/2441 using interferometry
• CPC Class Codes: G01B 11/303 using photoelectric detection means

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 and after each translation the position of the principal fringe is noted by observing the loci of points of maximum contrast. 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.

Claims

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,

determining positions of maximum fringe contrast of the white light interference pattern thereby establishing zero optical path differences between said wavefronts,

repeatedly translating one of the wavefronts from said surfaces by known incremental distances relative to the detector and establishing other white light interference patterns while establishing zero optical path differences between wavefronts from points on the reference surface and corresponding points on the test surface of unknown topography,

recording the positions in each interference pattern of maximum fringe contrast and the corresponding step of incremental distance of surface translation relative to the starting point for each interference pattern, 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.

2. The method of claim 1 wherein the determining maximum fringe contrast is carried out by modulating the phase of one of the wavefronts by a predetermined amount, measuring maximum and minimum wavefront intensities and calculating contrast from maximum and minimum intensities.

3. The method of claim 2 further defined by calculating maximum fringe contrast at a plurality of elements in the detector array and recording the highest contrast value of each element for all of the white light interference patterns.

4. The method of claim 1 wherein the extent of translation for said one wavefront is of the order of a tenth of the mean wavelength of said white light.

5. A non-contacting method for measuring deivations 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 signal surface of known topography and another wavefront from a corresponding point on a test surface of unknown topography;

(b) modulating the phase of one of said wavefronts by less than plus and minus one-half of the mean wavelength;

(c) repeatedly sampling the detector array and storing in a first register maximum intensities for sample positions in the interference pattern during modulation and in a second register corresponding minimum intensities;

(d) after several sampling cycles, storing in a third register contrast values for sample positions in the interference pattern computed using corresponding maximum and minimum intensities in the first and second registers;

(e) repeatedly translating the wavefronts from one of said surfaces by known incremental distances relative to the detector and establishing other white light interference patterns, in other spatial positions;

(f) repeating steps (a), (b), (c) and (d) for each spatial position white light interference pattern across the test surface;

(g) replacing stored contrast values in said third register with any higher contrast value computed in step (f), while simultaneously discarding lower contrast values,

(h) storing in a fourth register, for corresponding positions in said third register, a step number representing an incremental distance at which the contrast value in the third register was stored, whereby maximum contrast values indicate equal phase contours for establishing zero optical path differences between said test surface and said reference surface.

6. The method of claim 5 wherein said modulating step is carried out by vibrating one of said surfaces back and forth.

7. The method of claim 5 wherein the extent of translation for the wavefront of one of said surfaces is of the order of a tenth of the mean wavelength of said white light.

8. 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 surface and reference surface by known incremental distances;

means for modulating the optical path length difference for interfering wavefronts between said test surface and reference surface continuously by up to plus and minus one half of the mean wavelength of said interferometer,

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 maximum and minimum intensity,

a first register having cells for storing maximum intensities for each detector element,

a second register having cells for storing minimum intensities for each detector element,

a third register connected to said first and second register and having cells for storing contrast values derived from maximum and minimum intensities in corresponding cells of said first and second registers;

a fourth register connected to said third register and having cells for storing step numbers, each number representing an incremental distance at which a contrast value in a corresponding cell in the third register was stored;

control means connected to said third and fourth registers for discarding contrast values lower than the stored value in each cell and replacing said stored value with any higher value detected during steps of incremental distances, while simultaneously recording in said fourth register the step number at which maximum contrast is measured, whereby maximum contrast locations indicate contours of equal phase and zero path differences between said test surface and said reference surface.

9. The apparatus of claim 8 wherein the reference surface is a planar surface and the test surface is a continuous, near planar surface.

10. The apparatus of claim 8 wherein the reference surface is a planar surface and the test surface is a discontinuous, near planar surface.

11. The apparatus of claim 8 wherein the reference surface is a convex spherical surface and the test surface is a convex aspherical surface.

12. The apparatus of claim 8 wherein the reference surface is a concave spherical surface and the test surface is a concave aspherical surface.

13. The apparatus of claim 8 wherein the reference surface is a planar surface and the test surface is an aspherical mirror surface.

14. The apparatus of claim 8 wherein said interferometer is a Twyman-Green interferometer.

15. The apparatus of claim 8 wherein said interferometer is a scatter plate interferometer.