Patent ID: 7426039

Claim:
An optical instrument for measuring dimensional changes of a test object between two opposite side faces of the test object comprising a measuring loop having a plurality of optical components and a mounting for the test object, a beam routing system for routing a measuring beam from a beam source to the measuring loop and from the measuring loop to a data acquisition system; the optical components of the measuring loop being arranged (a) for circulating a first transverse portion of the measuring beam as an object beam that encounters both of the opposite side faces of the test object and the optical components between entering and exiting the measuring loop and (b) for circulating another transverse portion of the measuring beam as a loop beam that encounters the optical components without encountering the two opposite side faces of the test object between entering and exiting the measuring loop, and the data acquisition system providing for directly or indirectly comparing the object and loop beams to distinguish length variations between the two opposite side faces of the test object and path length variations of the measuring loop.