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Interferometry has always been a powerful tool to diagnose the response of liquids when changes of status parameters (e.g., temperature or concentration) induce modifications in their optical properties. interferometric measurements are based on the ability to measure variations in the optical path length around a reference configuration. investigations done so far on heat convection driven by capillary forces indicate that the observation of both the bulk phase and of the free surface is instrumental for understanding the physical mechanisms steering the heat transfer phenomena in "weightless liquids." When used in space applications, conventional interferometers suffer some fundamental drawbacks 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 requires precise positioning of the recording plate, with accuracy better than half a wavelength. The superior feature of an electronic speckle pattern 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 plate relocation. This comparative ease of operation enables the technique of electronic speckle pattern interferometry to be extended to considerably more complex problems of deformation analysis and measurement of refractive index modulation. Since it basically works as a time differential interferometer, measurements can always be referred to a well 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 here described enables one to observe and quantify the deformation of the free surface of a liquid subjected to a thermal gradient. Motions induced 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.
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