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Use of electronic speckle pattern interferometers for the analysis of convective states of liquids in weightlessness

Interferometry has always been a powerful tool to diagnose the response of liquids, when changes of status parameters induce modifications in their optical properties. Interferometric measurements are based on the ability to measure variations, around a reference configuration, in the optical path length or the refractive index. 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 the understanding of the physical mechanisms steering the heat transfer phenomena in 'weightless liquids'. When used in space application, conventional interferometers suffer of some fundamental drawbacks, because of the severe requirements in terms of 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 allows the technique of ESPI 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. This paper describes how double-path ESPI are accommodated within the optical diagnostics of a microgravity payload, fluid physics facility, due to launch in 1998 on the Russian retrievable capsule FOTON. The two- ESPI layout permits 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.