Scanning interferometric methods and apparatus for measuring aspheric surfaces and wavefronts
First Claim
1. An interferometric scanning method for measuring test optics having aspheric surfaces including those with large departures from spherical, said method comprising the steps of:
- generating a reference wavefront from a known origin along a scanning axis;
aligning a test optic on the scanning axis and selectively moving it along said scanning axis relative to said known origin so that said reference wavefront intersects the test optic at the apex of the aspheric surface and at one or more radial positions where the reference wavefront and the aspheric surface intersect at points of common tangency in circular zones around said scanning axis to generate interferograms containing phase information about the differences in optical path length between the center of the test optic and the one or more radial positions;
imaging the interferograms onto a detector to provide an electronic signal carrying the phase information;
measuring the axial distance, ν
, by which the test optic is moved with respect to said origin;
determining the detector pixel height corresponding to where the reference wavefront and test surface slopes match for each scan position;
determining the angles, α
, of the actual normal to the surface of points at each zone as a function of the distance along the scanning axis; and
using the angles, α
, determining the coordinates z and h of the aspheric surface at the common points of tangency.
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Abstract
Interferometric scanning method(s) and apparatus for measuring test optics having aspherical surfaces including those with large departures from spherical. A reference wavefront is generated from a known origin along a scanning axis. A test optic is aligned on the scanning axis and selectively moved along it relative to the known origin so that the reference wavefront intersects the test optic at the apex of the aspherical surface and at one or more radial positions where the reference wavefront and the aspheric surface intersect at points of common tangency (“zones”) to generate interferograms containing phase information about the differences in optical path length between the center of the test optic and the one or more radial positions. The interferograms are imaged onto a detector to provide an electronic signal carrying the phase information. The axial distance, ν, by which the test optic is moved with respect to the origin is interferometrically measured, and the detector pixel height corresponding to where the reference wavefront and test surface slopes match for each scan position is determined. The angles, α, of the actual normal to the surface of points Q at each “zone” are determined against the scan or z-axis. Using the angles, α, the coordinates z and h of the aspheric surface are determined at common points of tangency and at their vicinity with αmin≦α≦αmax, where αmin and αmax correspond to detector pixels heights where the fringe density in the interferogram is still low. The results can be reported as a departure from the design or in absolute terms.
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Citations
25 Claims
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1. An interferometric scanning method for measuring test optics having aspheric surfaces including those with large departures from spherical, said method comprising the steps of:
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generating a reference wavefront from a known origin along a scanning axis; aligning a test optic on the scanning axis and selectively moving it along said scanning axis relative to said known origin so that said reference wavefront intersects the test optic at the apex of the aspheric surface and at one or more radial positions where the reference wavefront and the aspheric surface intersect at points of common tangency in circular zones around said scanning axis to generate interferograms containing phase information about the differences in optical path length between the center of the test optic and the one or more radial positions; imaging the interferograms onto a detector to provide an electronic signal carrying the phase information; measuring the axial distance, ν
, by which the test optic is moved with respect to said origin;determining the detector pixel height corresponding to where the reference wavefront and test surface slopes match for each scan position; determining the angles, α
, of the actual normal to the surface of points at each zone as a function of the distance along the scanning axis; andusing the angles, α
, determining the coordinates z and h of the aspheric surface at the common points of tangency. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15)
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16. Interferometric scanning apparatus for measuring test optics having aspheric surfaces including those with large departures from spherical, said apparatus having a scanning axis and comprising:
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a radiation source and optical arrangement for generating a reference wavefront from a known origin along said scanning axis; a precision manipulator for aligning a test optic on said scanning axis and selectively moving it along said scanning axis relative to said known origin so that said reference wavefront intersects the test optic at the apex of the aspheric surface and at one or more radial positions where the reference wavefront and the aspheric surface intersect at points of common tangency in circular zones around said scanning axis to generate interferograms containing phase information about the differences in optical path length between the center of the test optic and the one or more radial positions; a two-dimensional detector; optics for imaging the interferograms onto said detector to provide an electronic signal carrying the phase information; a distance measuring device for measuring the axial distance, ν
, by which the test optic is moved with respect to said origin; anda programmable device for; determining the detector pixel height corresponding to where the reference wavefront and test surface slopes match for each scan position; determining the angles, α
, of the actual normal to the surface of points at each zone as a function of the distance along the scanning axis; andusing the angles, α
, determining the coordinates z and h of the aspheric surface at the common points of tangency. - View Dependent Claims (17, 18, 19, 20, 21, 22, 23, 24, 25)
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Specification