Noise Reduction Techniques for Electronic Speckle Interferometry

Abstract

Video-based speckle interferometric methods such as electronic speckle pattern interferometry (ESPI) allow us to measure full-field surface deformation of a diffuse object. In this paper we show, in a first step, that the susceptibility of ESPI to noise can be substantially reduced [1] by synchronizing the optical interferometer and the object stressing system with the CCD image acquisition and processing system, and by performing what amounts to a repetitive sequence of rapid ESPI tests. In this manner, a stable fringe pattern can be obtained as long as the ambient noise is of sufficiently lower frequency than the video acquisition rate (typically 30 Hz). This scheme will be referred in this paper as reference-updating subtractive correlated ESPI to distinguish it from classical ESPI. We then take this issue further with a hybrid additive-subtractive decollated ESPI technique that will freeze out unwanted environmental noise of frequencies much higher than video rates [2], The basic principle of this proposed technique can be summed up as follows: (i) acquire speckle images containing information about the same two deformed object states in every frame of the image acquisition sequence, (ii) decorrelate the speckles between every frame, and (iii) compare every consecutive pair of speckle images to extract visible fringes corresponding to the two deformed states of the test object.