A new rapid method for determining the absorption spectrum of microalgae culture is proposed. The method based on the recording the emission spectrum of the light source before and after passing through the culture and using SpectralWorkbench free software. A home-made spectrometer in the form of a black model made of ABS plastic on a 3D printer ANET A6 with a diffraction grating with a resolving power of d = 0.74 μm was produced. The software layout of the spectrometer was made in the online editor Tinkercad. The optical spectrum of the light source obtained from the spectrometer was converted into a graphical or tabular dependence of the relative power of radiation on the wavelength using SpectralWorkbench. Test run of the spectrometer includes the recordings of the radiation spectrum of the fluorescent lamp Philips TL-D 18 / W 54 using various video cameras. It is shown that the quality of the radiation spectrum determination is assessed by the resolving power of the camera used. To determine the transmission spectrum of the microalgae culture, a ratio of the light intensity passing through the photobioreactor to the incident wavelength was calculated. The optical density was determined on the basis of the Bouguer-Lambert-Beer law. The obtained absorption spectrum of the S. platensis culture is characterized by the presence of maxima of all photosynthetic pigments. The drawn comparison between the results and absorption spectra recorded on spectrophotometers without the integrating sphere SF-2000 and Unico-4802 has shown the advantage of the proposed approach.