A Three Autocollimation Method for Measuring the Refractive Index of Optical Glass

International Journal of Applied Glass Science. 2026. Vol. 17. No. 2. Article e70022.

Yurin A.

This article presents a three autocollimation method (TAM) for measuring the refractive index (RI) of optical glass. The popular minimum deviation method (MDM) for RI measurement is accurate, but it is very laborious, since the procedure of determining the minimum possible angle requires many iterations. TAM allows the calculation of RI based on the measurement of the angles of incidence of the beam corresponding to the autocollimation from the mirror located behind the investigated prism, parallel to the entrance face. Measurements must be carried out for all three faces, but it is not necessary to measure the prism angles, since the RI is calculated by solving a system of equations relating the values of unknown parameters. A comparative analysis showed that the accuracy of the proposed method is about 20% higher than the accuracy of classical MDM under the same conditions. A limitation of TAM is the phenomenon of total internal reflection, which limits the measurement range. To implement TAM, a goniometric system with a photoelectric encoder and a set of three reference prisms made of optical glass is used. The experimental absolute error does not exceed ±1·10−5, which proves the prospect of using the method for high-precision RI measurements. TAM can also be used to study the dispersion characteristics of triangular prisms made of optically transparent materials. TAM increases the accuracy and simplifies the RI measurement process compared to MDM.

Keywords: dispersion characteristics, refractometry, goniometer, refractive index measurements, autocollimator

Publication based on the results of: Methods and tools for the development and implementation of digital twins for electronic engineering (2025)