Accuracy analysis of the infrared materials refractive index measurements by the minimum deviation method

The problems of metrological assurance for measuring the refractive index of solid optical materials (silicon, germanium, etc.) used in the infrared range of the spectrum (infrared materials) are considered. The refractive index of optical materials in the infrared wavelength range must be known with high accuracy when developing optics for thermal imagers, night vision devices, etc. Currently, the tasks of developing domestic instruments for measuring the refractive index in the infrared region of the spectrum and analyzing measurement errors are relevant. The scheme of the developed infrared refractometer based on a goniometer is presented. Using this refractometer, the method of the minimum deviation for refractive index of infrared materials measurements in automatic mode is implemented. The error of measuring the refractive index of infrared materials by the method of minimum deviation using a developed infrared refractometer based on a goniometer is analyzed, considering the infl uence of measurement errors of the prism angle, beam deviation angle, radiation wavelength and temperature. The infl uence of diffraction on the measurement error of the deviation angles of infrared radiation is considered. The total error of refractive index measurements in the ranges 3–5 and 8–12 microns for germanium and silicon was estimated, which was 4.1·10^–5 and 2.5·10^–5, respectively. The results obtained are important for developing requirements for the components of the State Primary Standard of the Refractive Index Unit GET 138-2021 when improving it in order to expand the wavelength range to the middle and far infrared ranges.

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