Design of a Double‐Illumination espi system for the measurement of very slow motions

Abstract

Necessity appears in some contexts to measure velocity of objects moving at very low speed. In this work a double illumination speckle interferometer was used in order to determine the velocity of a plate moved by a PZT stage at velocity from 7 to 21 nm/s. When the time history for each pixel intensity is recorded oscillations in the signal appear due to phase variation connected to the movement of the object. By measuring the frequency of the oscillations of the autocorrelation function of the signal intensity and by knowing the geometrical and physical parameters of the set-up it is possible to calculate the velocity of the objects. Results show that the set up adopted in this work is able to measure very slow motion down to 7 nm/s with small percentage error and rather quickly.