Noise Reduction in Amplitude-Fluctuation Electronic Speckle-Pattern Interferometry

Abstract

Amplitude-Fluctuation Electronic Speckle Pattern Interferometry is used in a variety of vibration analyses. The technique utilizes the fact that when the vibration frequency of the object is significantly higher than the frame rate of the imaging device, the interference term can be approximated by the lowest (0th) order of the first kind Bessel function. Since the 0th order Bessel function takes the maximum value of unity when the vibration amplitude is null, the amplitude of a given vibration can be estimated from the reduction of the interference term relative to the case when the object is still. In reality, however, various environmental noises, such as temperature fluctuation of air in the interferometric paths and floor motion transferred through the optical table, cause low frequency fluctuations of the interference term, and thereby compromise the integrity of data. In this paper, we discuss typical environmental noise on standard optical interferometer settings, and propose to reduce the effect of the noise on the signal by introducing a carrier fringe system and analyzing the fringe pattern in the spatial frequency domain. The effectiveness of the proposed method is assessed for our recent Michelson interferometer experiment in which vibration of thin-film specimen is characterized.