Abstract

Subject of study. We study a modification of the Littrow–Abbe method for refractive index measurement. Aim of study. The study aims to enhance the capability of the precision measurement of the refractive index of transparent solid and liquid optical materials by modifying the Littrow–Abbe method. The modified method is applicable in optically transparent objects in the form of trihedral prisms made of the material under study or filled with the substance under study. Method. An autocollimation method (Littrow–Abbe method) is often used to measure the refractive index of trihedral prisms. In this case, the directions of the incident beam and the beam reflected from the output face are matched. A modification of this method enabling the use of prisms with large apex angles is proposed. Main results. The results of the measurement of the refractive index of two trihedral prisms made of glasses of various types, i.e., N-BK7 and SF-1, using the proposed method and a goniometric system are presented. The measurement error did not exceed 2.5×10⁻⁴ when compared with the nominal value for these prisms at the wavelength of the radiation source of the goniometric system autocollimator, proving the prospect of using this method for high-precision measurements of the refractive index. Practical significance. The refractive index measurement method proposed in this article is applicable for trihedral prisms made of optically transparent materials with different apex angles in cases where the Littrow–Abbe method cannot be applied owing to the total internal reflection. The method can also be used for optically transparent liquid substances poured in a hollow trihedral prism.

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