VERIFICATION OF THE SUB-NANOMETRIC CAPABILITY OF AN NPL DIFFERENTIAL PLANE MIRROR INTERFEROMETER WITH A CAPACITANCE PROBE

A differential plane mirror interferometer developed at the NPL has be en built into a system designed to calibrate and certify displacement transducers. Two electrical signals generated from the optical outputs of the interferometer and a voltage output of the transducer to be ca librated are read by a computer and mathematically analysed in order t o calculate the proportionality and linearity of the displacement tran sducer. In order to realize the highest measurement accuracy from the interferometer the interferogram signals are analysed and corrected by an optimizing routine in order to interpolate the fractioning of the optical fringes accurately. Without computer optimization of the signa ls a standard deviation of measurement of a few nanometres is typical, whereas after optimization this is reduced to a fraction of a nanomet re. The interferometer has been used to calibrate a high-precision cap acitance probe and the results of these measurements are shown and dis cussed.