Patent ID: 7791731

Claim:
A partial coherence interferometer of a type that employs a measuring beam having a given spectral bandwidth comprising a test arm for conveying a test beam portion of the measuring beam along a pathway encountering a test surface, a reference arm for conveying a reference beam portion of the measuring beam along a pathway encountering a reference surface, the test arm including a focusing optic for focusing the test beam onto the test surface and for collecting a reflected portion of the test beam from the test surface, a spectrally sensitive detector system coupled to the test and reference arms that monitors interference as a function of wavelength throughout a range of the measuring beam spectral bandwidth as a measure of unsigned optical path length differences between the test and reference arms, a focus detector system calibrated with the interferometer for distinguishing optical path length differences between the test and reference arms that are significantly greater than zero from optical path length differences between the test and reference arms that are significantly less than zero for resolving ambiguities associated with the measure of unsigned optical path length differences between the test and reference arms, the focus detector system including a focus arm that conveys light reflected from the test surface through the focusing optic to a focus detector, and the focus detector system being calibrated with the interferometer by positioning the focus of the light passing through the focusing optic at a predetermined offset from a null condition at which the optical path length of the test arm equals the optical path length of the reference arm.