Patent ID: 7046371

Claim:
A method comprising: imaging test light reflected from a test surface to interfere with reference light on a camera and form an interference pattern, the test light and reference light being derived from a spatially extended, common source, having a mean wavelength λ, and comprising chief rays and marginal rays that propagate along different paths, wherein the test light and reference light propagate along different paths in a main cavity comprising the test surface and also propagate along different paths in at least one remote cavity, and wherein a single pass through the main cavity introduces a non-zero optical path difference δ 0 between the chief rays of the test and reference light and a single pass through the remote cavities introduce an optical path length difference δ r between the chief rays of the test and reference light; and adjusting the optical path difference δ r to vary a total optical path difference between the chief rays at the camera, wherein the imaging comprises coupling the test light and reference light between the main cavity and the at least one remote cavity to cause a total optical path difference between the marginal rays of the test and reference light at the camera to be smaller than λ when the total optical path difference between the chief rays of the test and reference light at the camera is zero.