A new technique for measuring reflectivity differences between two samples is reported. An electrically driven tuning fork is used to place the two samples alternately in the beam of a reflectometer, and a lock-in amplifier is used for detection of the difference signal. Advantages of this system include short deadtime, vacuum operation, and extremely high Q. The sensitivity is such that changes in reflectivity of the order of 0.001% can be observed.
W. R. Scott, L. Muldawer, and M. A. Graber, "Sensitive Technique for Measuring Differences in Reflectivity," Appl. Opt. 13, 1956-1958 (1974)