The experimental results of testing the operation features of a SF-56 spectrophotometer are presented. Methods for processing the results of measuring the intensity of radiation sources using Maxima and Gnuplot software packages are developed to increase the accuracy of measurements. The use of the Gaussian function in approximation of the emission curve of low-pressure gas lamps for a nominal slit width of 0.3 nm increases the accuracy of determination of the wavelength of the maximum of emission lines by 2.5 times compared to the passport data. However, the effective width of the slit defined as the half-width of the reference line 486 nm (or 656.1 nm) always exceeds the passport values by 15 - 20%, and for a nominal slit of 1 nm by a factor of 1.5. The wavelength of the maximum of the SF-56 reference line (during the measurement time up to 6000 sec) linearly depends on time. The dependence of the reference line intensity on the heating time turned out to be nonlinear and is well approximated by the formula for an oscillator with a damping factor equal to the natural frequency. An analytical expression for the spectrum of the tungsten emission coefficient in the wavelength range of 300 - 1100 nm was derived. The Planck curve for the tungsten emissivity describes the spectrum of the signal of a standard incandescent lamp only in the wavelength range of 300 - 420 nm. The spectral function of the instrument is also determined. It is noted that SF-56 spectrophotometers can be used to measure the spectra of radiation sources placed in the lighting unit. The presented data can be used both to improve this type of devices, and to improve the accuracy and reliability of measurement results.