Patent ID: 7355719

Claim:
A system for measuring an object, the system comprising: a measurement reflector mounted on the object and oriented to redirect a measurement beam traveling in an X direction to traveling in a Z direction; a reference reflector mounted on the object and oriented to redirect a reference beam traveling in the X direction back in an opposite direction; an optical system that directs the measurement beam in the X direction for a single pass to the measurement reflector and directs the reference beam in the X direction for a first pass to the reference reflector and then for a second pass to the reference reflector; an overlying reflector that overlies the object and is positioned to direct the measurement beam back to the measurement reflector which then redirects the measurement beam back to the optical system; and a detector that measures a difference in the measurement beam and the reference beam after the reference beam has completed the second pass and the measurement beam has complete only the single pass, wherein the difference indicates a relative displacement of the object in the Z direction.