An interferometer provides a large dynamic range for perpendicular
displacement measurements. In operation, a measurement reflector on an
object reflects a measurement beam to an overlying Porro prism, and a
reference reflector on the object returns a reference beam to the
interferometer. A second Porro prism in the interferometer can return the
reference beam for a second pass to the reference reflector, while the
measurement beam complete only one pass. Reductions in beam walk-off
result from retroreflections in the Porro prisms and the matching effects
that some object rotations have on measurement and reference beams.
Perpendicular motion of the object relative to the first Porro prism
causes a Doppler shift only in the measurement beam. Accordingly, a beat
frequency found when combining the measurement and reference beams can
indicate a residual Doppler shift associated with the motion in the
perpendicular direction.