Patent ID: 8107084

Claim:
An apparatus comprising: an interference microscope comprising a test objective positioned relative to a test object to receive primary test light reflected from a surface of the test object, the interference microscope being configured to generate an interference pattern by combining the primary test light with primary reference light reflected from a reference object, the interference microscope further comprising a first optic and a second optic, the first optic being positioned relative to the test object to direct at least a part of a monitor test beam to the surface of the test object at a first incident angle and the second optic being positioned relative to the reference object to direct at least a part of a monitor reference beam to the reference object at a second incident angle, and recombine the first part of the test and reference monitor beams after they reflect from the test and reference objects to interfere with one another and form a monitor pattern, wherein the first and second optics are arranged so that the first and second angles cause the monitor pattern to have spatial interference fringes that vary in response to changes in a relative position between the test and reference objects, and the first optic is not in the path of the primary test light.