The issues of metrological support of the instruments for measuring a two-dimensional spatial distribution (profile) of a unit of refractive index for solids are considered. A reference system has been developed, which is included into the State Primary Refractive Index Standard (GET 138-2021). The composition, principle of operation, and main metrological characteristics of the reference system, based on a digital interferometer for measuring a two-dimensional spatial distribution (profile) of the refractive index for thin sections of the preforms of graded-index glass fibers, are presented. The reference system according to GET 138-2021 provides the uniformity of measurements for the instruments used to measure the refractive index profile—so-called optical analyzers of preforms of the graded-index and optical glass fibers. The results of experimental studies of GET 138-2021 in terms of reproducing a two-dimensional spatial distribution of the refractive index unit are presented.