We present the theoretical foundations of the method of using multipass Fabry-Perot interferometers and an iodine light filter to get high-contrast spectra. Plane Fabry--Perot interferometers are considered with two, three, or five successive passes of the light through them, as well as one- and two-pass spherical Fabry--Perot interferometers, and also their tandem combinations. Examples are given of experimental studies of molecular light-scattering spectra in phase transitions in imperfect crystals and liquid crystals, pressed powders, opaque crystals, and also acoustic magnon spectra. The resultant contrast of the spectra was as large as 10$^{12}$ when multipass and tandem interferometers were used, and 10$^5$ when an iodine light filter was used.