Stepwise interferometric method of measuring the refractive index of liquid samples

An iterative interferometric technique for accurately determining the refractive index (RI) of liquid samples is presented. The liquid is measured in an extremely stable stepped cell that is inserted into one arm of a Michelson-type interferometer. The uncertainty of the RI measurement is repeatedly improved by taking successive measurements of the interferometric fringe shifts on adjacent steps in the cell. It is shown that in practice the temperature nonuniformity in the liquid limits the ultimate uncertainty of the RI measurement. The RI resolution of the apparatus described is designed to be 4.3 x 10E-6, and the final RI uncertainty is 1.2 x 10E-5 (2sigma) for a liquid with a RI temperature coefficient of 4 x 10E-4.