LAUSR

We derive an estimator for the step size in phase-shifting interferometry. Using a minimum of five samples, it avoids the occasional indeterminate results that afflict the traditional Carré step-size estimate. The estimator can be understood as a generalization of the modified-covariance frequency estimator for a real-valued sinusoid with an unknown mean. We describe its use in the NRC Gauge Block Interferometer for monitoring the motion of a phase-shifting mirror mount.