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The results to be presented are related to a series of investigations carried out at the Department of Hydrooptics of SOI during a number of years with the aim of development, design and practical employment of optical instruments for measuring the statistical characteristic of turbulent inhomogeneities in the sea water. The values of magnitude and characteristic scales of refractive index fluctuations caused by turbulent convection in the sea water with temperature and salinity stratification, as well as the presence of suspended hydrosol particles, stipulate the choice of devices based on photo-electrical registration of small-angle scattering of light beam propagating through the volume of analyzed medium. The developed modifications of phase-contrast methods (shadow methods) make it possible to detect the variations of light beam angle-of-arrival at the level of 0.03" - 0.05"; the instrument may be mounted on remotely controlled underwater carrier towed by research vessel with the velocity 4 - 12 knots at the desired depth in the range from 0 to 300 m. The interpretation of registered signals from photo-electrical output of the optical instrument is based on the connection between the signal correlation function or spectrum and the energetic spectrum of refractive index fluctuations. The corresponded relations are obtained with the help of the theory of light propagation in random media, namely, the closed equations for light field statistical moments in parabolic approximation. The results of theoretical analysis allows the optimization of optical scheme to meet the requirements of linearity of the signal relation to the analyzed fluctuations magnitude, high spatial resolution and low sensitivity threshold, as well as sufficient resistance to vibrations and changes in pressure and temperature. A measurement of optical fluctuations spectrum in turbulent flow behind a heated grating in hydrodynamic tube are shown to demonstrate the designed instruments abilities. Some results of in-situ experiments made in calm waters and in turbulized layer near thermocline are presented.
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