Abstract <p>The effect of the imperfections of the polarizer, analyzer, and photomultiplier tube (PMT) on the results of measuring the spectral transmission dependences of catangasite (Ca₃TaGa₃Si₂O₁₄) crystals cut perpendicular to the optical axis has been theoretically and experimentally investigated. There is a difference between the spectra obtained for the <i>p</i> and <i>s</i> polarizations of incident light; jumps are also observed on the curves at λ&#xa0;= 1050 nm. This is due to the imperfection of the PMT and the optical activity of the crystal. The&#xa0;PMT parameters are estimated in dependence of the light wavelength proceeding from experimental data. The influence of the imperfection of the PMT and polarizers on the results of calculating the rotation of the plane of polarization ρ is studied. It is shown that transmission spectra must be measured at angles between the polarizer and analyzer of ±45° in order to to calculate exactly the ρ value. The measurement errors are determined by the set of optical elements in a particular device.</p>