Performance evaluation of fringe thinning algorithms for interferometric tomography

Abstract

The present study is concerned with the sensitivity of reconstruction algorithms to fringe thinning operations, in the context of interferometric tomography. Interferograms obtained from a Mach–Zehnder interferometer are considered as the fringe system. The physical problem considered is fluid convection in a rectangular enclosure that is square in plan, heated from below and cooled from the top. The fringes are line-of-sight integrals of the refractive index and hence the temperature field. The fringe patterns are digitized and stored as strings of numbers using a CCD camera along with an image processing system. Three methods of fringe thinning algorithms are considered. The first algorithm is programmable and is based on the search of minimum intensity within the dark bands of the fringe system. The other two algorithms are, respectively, semi-automatic and manual search procedures for location of the midpoints of the dark bands. The thinned fringes contain information about the projection of the temperature field in the direction of the light beam. These data have been used to reconstruct the three-dimensional temperature field in the fluid layer using principles of tomography. The fringe thinning algorithms have been evaluated in terms of effort required, and differences in the reconstructed temperature field as well as the wall heat transfer rate. The automatic search procedure developed in the present work is found to be best suited in terms of these criteria.