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1. WO2012067940 - INTERFEROMETER WITH A VIRTUAL REFERENCE SURFACE

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[ EN ]

WHAT IS CLAIMED IS:

1. An interferometer, comprising:

a first beam splitter positioned to derive test light and reference light from input light and to direct the test light along a first path to contact a test object and direct the reference light along a second path different from the first path;

a second beam splitter positioned in the first and second paths and arranged to combine the reference light with test light after the test light contacts the test object; a detector positioned to receive the combined test and reference light; and an objective arranged to image the test object and a virtual reference surface onto the detector,

wherein the virtual reference surface corresponds to virtual surface optically conjugate to the detector between the first and second beam splitters.

2. The interferometer of claim 1, wherein an optical path length of the first path from the first beam splitter to the second beam splitter via the test object is substantially equal to an optical path length of the second path from the first beam splitter to the second beam splitter.

3. The interferometer of claim 1, further comprising an optical element positioned in the second path and arranged to direct reference light from the first beam splitter toward the second beam splitter.

4. The interferometer of claim 3, wherein an optical path length for the test light between the first and second beam splitters is substantially equal to an optical path length for the reference light between the first beam splitter and the optical element positioned in the second path.

5. The interferometer of claim 3, wherein the first and second paths between the first and second beam splitters, in combination with the optical element positioned in the second path, define a geometry corresponding substantially to an isosceles triangle.

6. The interferometer of claim 1 , wherein the test light corresponds to input light transmitted by the first beam splitter and the reference light corresponds to input light reflected by the first beam splitter.

7. The interferometer of claim 1, further comprising an alignment channel corresponding to a path of light combined at the first beam splitter, where the combined light propagates from the second beam splitter to the first beam splitter along the first or second paths.

8. The interferometer of claim 7, further comprising a second detector positioned at the alignment channel to detect the combined light.

9. The interferometer of claim 1 , further comprising a first compensating optical element positioned in the second path between the first and second beam splitters, wherein the first compensating optical element reduces an optical path length difference within an optical material between the test and reference light.

10. The interferometer of claim 9, wherein the first compensating optical element is arranged to reduce a lateral displacement between the test and measurement light at the detector.

11. The interferometer of claim 9, wherein the first compensating optical element is a plane parallel plate tilted with respect to the second path.

12. The interferometer of claim 9, further comprising one or more additional compensating optical elements positioned in the first and/or second paths, wherein the one or more additional compensating optical elements reduce the optical path length difference within optical material between the test and reference light and/or to reduce a lateral displacement between the test and measurement light at the detector.

13. The interferometer of claim 1, further comprising an illuminator configured to provide the input light during operation of the interferometer.

14. The interferometer of claim 13, wherein the illuminator comprises a light source and one or more optical elements arranged to receive light from the light source and direct the light toward the first beam splitter.

15. The interferometer of claim 14, wherein the one or more optical elements are arranged so that the test light is telecentric at the test object.

16. The interferometer of claim 14, wherein the light source is configured to provide light having a spectral bandwidth greater than 10 nm.

17. The interferometer of claim 14, wherein the light source comprises a light emitting diode (LED).

18. The interferometer of claim 1, wherein the first and second beam splitters comprise plane parallel optical elements.

19. The interferometer of claim 1, wherein the interferometer is arranged to image a surface of the test object within a field onto the detector, the field having a dimension of 10 mm or more.

20. The interferometer of claim 1, further comprising an electronic processor in communication with the detector, wherein during operation the electronic processor receives signals comprising interferometric information about an optical path length difference between the test and reference light at the detector and determines information about the test object based on the signals.

21. An interferometry method for determining information about a test object, the method comprising:

deriving test light and reference light from input light at a first optical element; directing the test light along a first path to contact a test object and directing the reference beam along a second path different from the first path;

combining, at a second optical element different from first optical element, the reference light with test light after the test light contacts the test object;

imaging the test object and a virtual reference surface onto a detector,

wherein the virtual reference surface corresponds to a virtual surface in the second path optically conjugate to the image at the detector in the second path.

22. An interferometer for measuring a characteristic of a test surface, said interferometer comprising:

(a) optics defining an interferometric cavity, said optics including:

a first beam-splitting optic positioned to separate an input beam into a test beam and a reference beam,

a second beam-splitting optic positioned to transmit the test beam to the test surface, receive the test beam back from the test surface, and thereafter recombine the test beam with the reference beam, and

a third optic positioned to direct the reference beam from the first optic to the second optic,

wherein the interferometric cavity defines a virtual reference surface positioned along a path for the reference beam between the second and third optics; and

(b) an imaging channel positioned to receive the recombined test and reference beams, said imaging channel including an imaging detector, and at least one imaging element configured to image the test surface and the virtual reference surface onto the detector.