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 first 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 first wavefront intersects the test optic at the apex of the aspheric surface and at one or more radial positions where the first wavefront and the aspheric surface intersect at points of common tangency in circular zones around said scanning axis;
interfering the first wavefront with a second wavefront 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;
directing 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 first 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
, for determining information corresponding to the coordinates axial distance z and radial distance 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.
24 Citations
20 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 first 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 first wavefront intersects the test optic at the apex of the aspheric surface and at one or more radial positions where the first wavefront and the aspheric surface intersect at points of common tangency in circular zones around said scanning axis; interfering the first wavefront with a second wavefront 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; directing 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 first 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
, for determining information corresponding to the coordinates axial distance z and radial distance h of the aspheric surface at the common points of tangency. - View Dependent Claims (3)
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2. An interferometric scanning method for measuring test optics having aspheric surfaces including those with large departures from spherical, the method comprising the steps of:
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generating a first wavefront upstream of a known origin along a scanning axis; generating a second wavefront upstream of the known origin; reflecting the first wavefront off an aspherical surface of a test optic that is movable along the scanning axis relative to the known origin; interfering the first wavefront with the second wavefront to generate interferograms containing phase information about differences in optical path lengths between locations on the test optic away from an apex of the aspherical surface relative to the location of the apex; directing the interferograms onto a detector to provide an electronic signal carrying the phase information; evaluating from the interferograms, as a function of an axial distance, ν
, along the scanning axis, an angle α
between the scanning axis and a surface normal of the aspherical surface at each of the locations of the test optic having a common tangency with the first wavefront; anddetermining, based on α
, information corresponding to the coordinates axial distance z and radial distance h of the aspheric surface at each of the locations. - View Dependent Claims (4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14)
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15. Interferometric scanning apparatus for measuring test optics having aspheric surfaces including those with large departures from spherical, the apparatus having a scanning axis and comprising:
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a radiation source and optical arrangement for generating first and second wavefronts upstream of a known origin along the scanning axis; a precision manipulator for aligning a test optic on the scanning axis and selectively moving the test optic an axial distance, ν
, with respect to the origin along the scanning axis so that the first wavefront has a radius of curvature that varies as a function of the axial distance ν
, as the first wavefront reflects from an aspherical surface of the test optic;a distance measuring device for measuring the axial distance ν
;a two-dimensional detector; optics for directing the interferograms onto the detector to provide an electronic signal carrying phase information about differences in optical path lengths between locations of the aspherical surface away from an apex of the aspherical surface relative to the location of the apex; and a programmable device for; evaluating from the interferograms, as a function of an axial distance, ν
, along the scanning axis, an angle α
between the scanning axis and a surface normal of the aspherical surface at each of the locations having a common tangency with the first wavefront; anddetermining, based on α
, information corresponding to the coordinates axial distance z and radial distance h of the aspheric surface at each of the locations. - View Dependent Claims (16, 17, 18, 19, 20)
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Specification