Interferometer with a virtual reference surface

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 the 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,
• wherein the second path includes a third optical element positioned and arranged to direct reference light from the first optical element toward the second optical element;
• and
• wherein the first and second paths between the first and second optical elements, in combination with the third optical element positioned in the second path, define a geometry corresponding substantially to a triangle.

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,
• wherein said first beam-splitting optic, said second beam-splitting optic and said third optic, together define a geometry corresponding substantially to a triangle;
• 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.