USE OF ELECTRONIC SPECKLE PATTERN INTERFEROMETERS FOR THE ANALYSIS OFCONVECTIVE STATES OF LIQUIDS IN WEIGHTLESSNESS

Interferometry has always been a powerful tool to diagnose the respons e of liquids when changes of status parameters (e.g., temperature or c oncentration) induce modifications in their optical properties. Interf erometric measurements are based on the ability to measure variations in the optical path length around a reference configuration. Investiga tions done so far on heat convection driven by capillary forces indica te that the observation of both the bulk phase and of the free surface is instrumental for understanding the physical mechanisms steering th e heat transfer phenomena in ''weightless liquids.'' When used in spac e applications, conventional interferometers suffer some fundamental d rawbacks because of the severe requirements in terms of the mechanical stability of the optical elements. Holographic interferometry removes the most stringent limitations of classical interferometry, but requi res precise positioning of the recording plate, with accuracy better t han half a wavelength. The superior feature of an electronic speckle p attern interferometer (ESPI) is that it enables real time correlation fringes to be recorded by a video camera and displayed on a television monitor, without recourse to any form of photographic processing or p late relocation. This comparative ease of operation enables the techni que of electronic speckle pattern interferometry to be extended to con siderably more complex problems of deformation analysis and measuremen t of refractive index modulation. Since it basically works as a time d ifferential interferometer, measurements can always be referred to a w ell known configuration and condition of the test sample, reducing or even eliminating the requirements on mechanical stability. We describe how a double-path ESPI is accommodated within the optical diagnostics of a microgravity payload, fluid physics facility (FluidPac), due for launch in 1999 on the Russian retrievable capsule Foton. The ESPI her e described enables one to observe and quantify the deformation of the free surface of a liquid subjected to a thermal gradient. Motions ind uced by the convective flows in the bulk phase can be monitored at the same time. The main features of the ESPI are presented together with design outlines and optical performances. (C) 1998 society of Photo-Op tical Instrumentation Engineers.