Patent ID: 7715017

Claim:
An apparatus for measuring the spacing between a first body and a second body comprising: at least one source of light; source optics producing at least two light beams from the at least one source, including a measurement light beam and a reference light beam; at least one beam splitter that separates the measurement light beam into a first measurement portion and a second measurement portion, and that separates the reference light beam into a first reference portion and a second reference portion; a measurement optical path that directs the first measurement portion to shine upon the first and second bodies, and that directs the second measurement portion to shine upon a third body; a reference optical path that directs the first reference portion to shine upon the first body but not the second body, and that directs the second reference portion to shine upon the third body; a first light detector disposed to receive light that is reflected from the first, second, and third bodies via the measurement optical path, and that produces a first output signal; a second light detector disposed to receive light that is reflected from the first and third bodies via the reference optical path, and that produces a second output signal; a movable mount that can hold the second body in the measurement optical path during a first period, and that can hold the second body away from the measurement optical path during a second period; a phase difference comparator coupled to the first and second light detectors, capable of comparing a phase difference between the first and second output signals during the first period with a phase difference between the first and second output signals during the second period, and an actuator that moves the third body and thereby creates a frequency shift in the second measurement portion and a substantially identical frequency shift in the second reference portion.