Optical system for surface topography measurement

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, 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.

Claim 10

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.

Claim 11

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.