<title>Simultaneous measurements of vector components of displacement by ESPI and FFT techniques</title>

Abstract

To measure a number of components of displacement with electronic speckle pattern interferometry (ESPI), multiple interferograms are usually captured in succession for different sensitivity vectors. Simultaneous measurement of a number of orthogonal components of displacement using ESPI can be carried out by application of the Fourier transform method. An object is illuminated by several object beams, and the scattered light is combined with one reference beam to form a first speckle image. After providing different tilts to the object beams, a second speckle image is recorded. A third speckle image is then recorded with the object loaded. The difference of the first and second images contains a set of straight carrier fringes. The difference of the first and third images contains a corresponding separate set of modulated carrier fringes due to the loading. The Fourier transforms of these two difference images show multiple peaks which correspond to the carrier fringes of the different object beams. By appropriately masking the corresponding peaks in the two Fourier transforms, the different sets of carrier fringes for the loaded and unloaded object configurations, can be separated. The phase corresponding to the different orthogonal components of displacement can then be retrieved from the ratios of the real to imaginary parts of the two inverse Fourier transforms of each filtered peak. An example of the measurement is presented.