PERFORMANCE EVALUATION OF FRINGE THINNING ALGORITHMS FOR INTERFEROMETRIC TOMOGRAPHY

The present study is concerned with the sensitivity of reconstruction algorithms to fringe thinning operations, in the context of interferom etric tomography. Interferograms obtained from a Mach-Zehnder interfer ometer are considered as the fringe system. The physical problem consi dered 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 held. The fringe patterns are digitized and stored as strings of numbe rs using a CCD camera along with an image processing system. Three met hods of fringe thinning algorithms are considered. The first algorithm is programmable and is based on the search of minimum intensity withi n the dark bands of the fringe system. The other two algorithms are, r espectively, semi-automatic and manual search procedures for location of the midpoints of the dark bands. The thinned fringes contain inform ation about the projection of the temperature held in the direction of the light beam. These data have been used to reconstruct the three-di mensional temperature field in the fluid layer using principles of tom ography. The fringe thinning algorithms have been evaluated in terms o f effort required, and differences in the reconstructed temperature fi eld as well as the wall heat transfer rate. The automatic search proce dure developed in the present work is found to be best suited in terms of these criteria. (C) 1998 Elsevier Science Ltd All rights reserved.