The composition, operating principle, and main metrological characteristics of GET 138-2021 State Primary Refractive Index Standard are presented. This standard provides uniform measurements of the spectral dependence and spatial distribution of the refractive index in solid and liquid substances within the extended wavelength range of 0.4–1.55 μm by means of using 12 replaceable light sources, a dynamic gonio-spectrometer equipped with a ring laser and an autocollimator-based null-indicator. Performance of highly accurate and reliable refractometric measurements constitutes an important scientific field that is crucial in improving the resolution of lenses and other optical instruments used in the optical industry, as well as in controlling quality and determining the composition of substances in the food, chemical, and pharmacological industries.