Interferometry is a trusted and widely used optical technique for measurements of the refractivity of objects, from which related quantities like temperature, or concentration can be determined. In particular, it is considered as most precise method for measuring diffusion in transparent fluids. A reason why this precise and powerful technique has not yet found application for measuring thermodiffusion obviously lies not in the accuracy of the method itself but rather in a set of some complementary factors. Consideration of the main factors affecting accuracy of thermodiffusion measurement by interferometry is one of topics of present work.

For the measurements of thermodiffusion by means of optical diagnostics, we use a classical Soret cell with transparent lateral walls clamped between two thermostabilized blocks. Although this system is generally stable with downward temperature gradient, some small lateral heat flux provokes residual convection which in turn disturbs the measurement. Ability of the method to locate and exclude from consideration the regions disturbed by convection is demonstrated for cells of different geometries. Cell design optimized with help of digital interferometry has been successfully used to measure Soret and diffusion coefficients in different binary mixtures.