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A frequency-scanning interferometer is modified to include a diffuse reference surface. An illuminating system produces an expanding measuring beam, portions of which reflect from a test object surface and the diffuse reference surface on converging paths to an imaging system. Interference patterns

A frequency-scanning interferometer is modified to include a diffuse reference surface. An illuminating system produces an expanding measuring beam, portions of which reflect from a test object surface and the diffuse reference surface on converging paths to an imaging system. Interference patterns between overlapping images of the object and reference surfaces are generated at a plurality of frequencies for measuring the object surface with respect to the reference surface.

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We claim: 1. An interferometer for comparing an object surface to a reference surface for measuring departures of the object surface from a nominal geometric surface form of the reference surface comprising: an illuminating system that illuminates the object surface with an object beam and the refe

We claim: 1. An interferometer for comparing an object surface to a reference surface for measuring departures of the object surface from a nominal geometric surface form of the reference surface comprising: an illuminating system that illuminates the object surface with an object beam and the reference surface with a reference beam along separate optical pathways such that the object beam does not pass through the reference surface on route to the object surface; an imaging system that produces overlapping images of the object surface and the reference surface with the object and reference beams containing interferometric data based on path length differences between the object and reference beams reflected from a plurality of corresponding points on the object and reference surfaces; a frequency regulator that varies a frequency of the object and reference beams at which the interferometric data is produced over a range of different beam frequencies; a data acquisition system that gathers the interferometric data from the plurality of corresponding points within the overlapping images of the object and reference surfaces as the interferometric data varies through conditions of constructive and destructive interference as a result of the variation in the beam frequency; a processor that converts the variations in the interterometric data as a function of the variation in beam frequency into measures of the path length differences between the object and reference beams from the plurality corresponding points on the object and reference surfaces and converts the path length differences between the object and reference beams from the plurality of corresponding points on the object and reference surfaces into measures of departures of the object surface from the nominal geometric surface form of the reference surface; and the reference surface being a substantially diffuse surface that scatters rays of the reflected reference beam through a plurality of different angles that contribute to the imaging of the reference surface by the imaging system. 2. The interferometer of claim 1 in which the reference surface has a nominal geometric surface form, and measured with respect to normals to the reference surface's nominal geometric surface form, rays of the reference beam reflect from the reference surface at angles that depart from angles at which the reference beam rays are incident upon the reference surface. 3. The interferometer of claim 2 in which the interferometric data contains information relating to differences between the object surface and the reference surface gathered as a result of reflections from both surfaces, and further comprising a data acquisition system that acquires the interferometric data at a plurality of different frequencies of the object and reference beams. 4. The interferometer of claim 3 in which the interferometric data for local areas within the overlapping images of the object and reference surfaces cycles through conditions of constructive and destructive interference with progressive variation in the beam frequencies, and the processor converts cyclical variations in the interferometric data as a function of frequency into path length differences between the object and reference beams and converts the path length differences between the object and reference beams into measures of departures of the object surface from the nominal geometric surface form of the reference surface. 5. The interferometer of claim 2 in which the reference surface is a nominally planar surface, and the interferometric data contains information relating to differences between the object surface and the nominally planar form of the reference surface. 6. The interferometer of claim 5 in which the reference beam propagates toward the reference surface along an optical axis, and more of the rays the reference beam converge toward the optical axis immediately after reflection from the reference surface than immediately before reflection from the reference surface. 7. An interferometer for comparing an object surface to a reference surface comprising: an illuminating system that illuminates the object surface with an object beam and the reference surface with a reference beam; an imaging system that produces overlapping images of the object surface and the reference surface with the object and reference beams for gathering interferometric data based on path length differences between the object and reference beams; a frequency regulator that regulates a frequency of the object and reference beams at which the interferometric data is gathered over a range of different beam frequencies; a processor that converts variations in the interferometric data as a function of beam frequency into measures of the path length differences between the object and reference beams; the reference surface being substantially non-specular so that the reference beam is altered upon reflection to enhance the imaging of the reference surface; the reference surface having a nominal geometric surface form, and measured with respect to normals to the reference surface's nominal geometric surface form, rays of the reference beam reflect from the reference surface at angles that depart from angles at which the reference beam rays are incident upon the reference surface; and the non-specular reference surface reducing anisotropic reflection of the reference beam. 8. The interferometer of claim 2 in which the non-specular reference surface includes beam-shaping structures that produce an at least partially ordered alteration of the reference beam. 9. The interferometer of claim 1 in which the illuminating system produces a non-collimated reference beam that expands upon approach to the reference surface. 10. The interferometer of claim 9 in which representative portion of the reference beam reflects from the reference surface on a converging path toward the imaging system. 11. The interferometer of claim 1 in which the illuminating system produces non-collimated object and reference beams that expand upon approach to the object and reference surfaces, and representative portions of both the object beam and the reference beam reflect from the object and reference surfaces on converging paths toward the imaging system. 12. The interferometer of claim 11 in which the imaging system has a imaging aperture area, and the object surface imaged by the imaging system has a transverse area that is larger than the imaging aperture area. 13. The interferometer of claim 12 in which the illuminating system has an illuminating aperture area, and the transverse area of the object surface is larger than the illuminating aperture area. 14. The interferometer of claim 13 in which a traverse area of the reference surface that is imaged by the imaging system is larger in area than both the imaging aperture area and the Illuminating aperture area. 15. The interferometer of claim 1 in which the overlapping images of the object surface and the reference surface produce a series of interference patterns over the range of different frequencies, and the path length variations introduced by the reference surface into the reference beam at least partially obscure the identification of interference fringes in the interference patterns. 16. The interferometer of claim 15 in which the path length variations introduced by the reference surface into the reference beam are of an order of magnitude or more smaller than the path length variations introduced by surface height variations of the object surface. 17. The interferometer of claim 1 further comprising a beam filter positioned between the illuminating and imaging systems for relatively adjusting beam strength between the object and reference beams. 18. The interferometer of claim 17 in which the beam filter provides for more evenly distributing beam intensity throughout a beam aperture. 19. In frequency-scanning interferometer of a type that produces a series of interference patterns between object and reference beams over the range of different frequencies, the object beam being arranged for encountering an object surface, the reference beam being arranged for encountering a reference surface, the object beam encountering the object surface without encountering the reference surface, the series of interference patterns being subject to differences between the object and reference surfaces, interferometric data being gathered from a plurality of points in the interference patterns corresponding to overlapping points on the object and reference surfaces, and the interferometric data from each of the points in the interference patterns cycling through conditions of constructive and destructive interference as a result of progressive variations in the beam frequency, wherein the improvement comprises: the reference surface being a diffuse surface that scatters rays of the reference beam in a substantially random manner for contributing to the formation of individual points in the interference patterns from a plurality of different directions. 20. In frequency-scanning interferometer of a type that produces a series of interference patterns between object and reference beams over the range of different frequencies, the object beam being arranged for encountering an object surface, the reference beam being arranged for encountering a reference surface, and the series of interference patterns being subject to differences between the object and reference surfaces, wherein the improvement comprises: the reference surface being a diffuse surface for altering the reference beam, the diffuse surface producing a largely random alteration of the reference beam, and the diffuse surface being a non-specular reflective surface that reduces anisotropic reflection of the reference beam. 21. The improvement of claim 19 in which the reference beam contains a plurality of rays, and the rays of the reference beam propagate in an increased number of directions after encountering the reference surface. 22. The improvement of claim 19 in which the reference beam contains a plurality of rays and propagates toward the reference surface along an optical axis, and an increasing percentage of the rays of the reference beam converge toward the optical axis after encountering the diffuse reference surface. 23. The improvement of claim 19 in which the diffuse reference surface introduces path length variations between the object and reference beams that at least partially obscure the identification of interference fringes in the interference patterns. 24. In frequency-scanning interferometer of a type that produces a series of interference patterns between object and reference beams over the range of different frequencies, the object beam being arranged for encountering an object surface, the reference beam being arranged for encountering a reference surface, and the series of interference patterns being subject to differences between the object and reference surfaces, wherein the improvement comprises: the reference surface being a diffuse surface for altering the reference beam, the diffuse reference surface introducing path length variations between the object and reference beams that at least partially obscure the identification of interference fringes in the interference patterns, and surface height variations of the object surface introducing path length variations between the object and reference beams of an order of magnitude larger than the path length variations introduced by the diffuse reference surface. 25. The improvement of claim 19 in which the diffuse reference surface has a nominal geometric form, and the series of interference patterns record differences between the object surface and the nominal geometric form of the diffuse reference surface. 26. The improvement of claim 25 in which the nominal geometric form of the diffuse reference surface is a plane. 27. The improvement of claim 26 in which the reference beam propagates toward the nominally planar reference surface as an expanding beam and a representative portion of the reference beam propagates away from the nominally planar reference surface as a converging beam. 28. The improvement of claim 19 in which the diffuse reference surface is a non-specular reflective surface. 29. The improvement of claim 28 in which the reference surface has a nominal geometric surface form, and measured with respect to normals to the reference surfaces' nominal geometric surface form, rays of the reference beam reflect from the reference surface at angles that depart from angles at which the reference beam rays are incident upon the reference surface. 30. The improvement of claim 29 in which the reference beam propagates toward the reference surface along an optical axis, and more of the rays the reference beam converge toward the optical axis immediately after reflection from the reference surface than immediately before reflection from the reference surface. 31. The improvement of claim 30 in which the reference surface has a nominally planar form. 32. The improvement of claim 19 further comprising a beam filter for more evenly distributing beam intensity throughout a beam aperture. 33. A method of interferometrically measuring surface features of an object surface of a test object as measures of departures of the object surface from a nominal geometric surface form of a reference surface comprising steps of: illuminating the object surface with an object beam and the reference surface with a reference beam along separate optical pathways such that the object beam does not pass through the reference surface on route to the object surface; non-specularly reflecting the reference beam from the reference surface so that directional components of the reference beam are altered upon reflection through a plurality of different angles; producing overlapping images of the object surface and the reference surface with the object and reference beams for gathering interferometric data based on path length differences between the object and reference beams reflected from a plurality of corresponding points on the object and reference surfaces; the reference surface being a diffuse surface that is imaged through the plurality of different angles at which the reference beam is reflected from the reference surface; varying a frequency of the object and reference beams at which interferometric data is gathered over a range of different frequencies; converting variations in the interferometric data as a function of the variation in frequency into measures of the path length differences between the object and reference beams from the plurality corresponding points on the object and reference surfaces; and converting the path length differences between the object and reference beams from the plurality of corresponding points on the object and reference surfaces into measures of departures of the object surface from the nominal geometric surface form of the reference surface. 34. The method of claim 33 in which the step of illuminating includes expanding the reference beam along a path of propagation toward the reference surface, and the step of non-specularly reflecting includes contracting a portion of the reference beam along a path of propagation away from the reference surface. 35. The method of claim 33 in which the step of non-specularly reflecting includes producing a largely random alteration of the reference beam. 36. The method of claim 35 in which the step of non-specularly reflecting provides for scattering the reference beam throughout a range of different directions. 37. A method of interferometrically measuring surface features of test objects comprising steps of: illuminating an object surface with an object beam and a reference surface with a reference beam; non-specularly reflecting the reference beam from the reference surface so that directional components of the reference beam are altered upon reflection; producing overlapping images of the object surface and the reference surface with the object and reference beams for gathering interferometric data based on path length differences between the object and reference beams; regulating a frequency of the object and reference beams at which interferometric data is gathered over a range of different frequencies; converting variations in the interferometric data as a function of frequency into measures of the path length differences between the object and reference beams; the step of non-specularly reflecting including producing a largely random alteration of the reference beam; and the step of non-specularly reflecting reducing anisotropic reflection of the reference beam. 38. The method of claim 33 in which the step of non-specularly reflecting includes producing an at least partially ordered alteration of the reference beam. 39. The method of claim 33 in which the step of illuminating includes producing non-collimated object and reference beams that expand upon approach to the object and reference surfaces, and at least portions of both the object beam and the reference beam non-specularly reflect from the object an reference surfaces on converting paths. 40. The method of claim 33 in which the step of producing overlapping images includes producing a series of interference patterns over the range of discrete frequencies, and the step of non-specularly reflecting includes introducing path length variations between the object and reference beams that at least partially obscure the identification of interference fringes in the interference patterns. 41. The method of claim 40 in which the path length variations introduced by the reference surface are of an order of magnitude or more less than path length variation introduced by the object surface. 42. The method of claim 33 including an additional step of non-specularly reflecting the object beam from the object surface so that directional components of the object beam are altered upon reflection. 43. The method of claim 33 in which the step of illuminating includes illuminating the object and reference surfaces at field-dependent angles of incidence. 44. The method of claim 43 including a further step of applying a field-dependent scale factor in the processing of the interferometric data to compensate for illuminating the object and reference surfaces at field-dependent angles of incidence.