A differential polarization interferometer comprising: a tunable laser source of coherent radiation generating an input beam; a first polarizer which is configured to convert said input beam into a polarized beam with adjustable intensities of orthogonally polarized components; a broadband non-polarizing beam splitter forming two optically connected arms, first one with a transmitted beam; wherein in the first arm formed with said transmitted beam there are sequentially arranged: a birefringent element configured to split the polarized input beam into two separate, parallel, orthogonally polarized beams, namely a reference beam and an object beam, and mounted on a base with fine rotation; and a transparent substrate, on the part of which a film under test is deposited, mounted on a base with adjustable shift and rotation; and a chopper wheel with a system of holes that is configured to open sequentially for some time object and reference beams separately, then simultaneously, providing interference of said beams; and a broadband plane mirror configured to reflect said incident beams in the opposite direction and to provide double passing of the beams through the transparent substrate and the birefringent element which is configured to convert said two beams into orthogonally polarized components of a single output beam falling on the non-polarizing beam splitter; wherein in the second arm of the interferometer formed with said reflected beam there are sequentially arranged: a controllable phase retarder, configured to introduce phase shift only into one polarization component of said beam; and a second polarizer that is configured to equalize and mix the polarization components of the input beam and to transform said input beam into a beam, which is the result of interference of polarization components of the input beam; and a photodetector configured to receive an input beam and to transform an intensity of said beam into an output electric signal, which can be amplified and analyzed by various electronic devices. a tunable laser source of coherent radiation generating an input beam; a first polarizer which is configured to convert said input beam into a polarized beam with adjustable intensities of orthogonally polarized components; a broadband non-polarizing beam splitter forming two optically connected arms, first one with a transmitted beam; wherein in the first arm formed with said transmitted beam there are sequentially arranged: a birefringent element configured to split the polarized input beam into two separate, parallel, orthogonally polarized beams, namely a reference beam and an object beam, and mounted on a base with fine rotation; and a transparent substrate, on the part of which a film under test is deposited, mounted on a base with adjustable shift and rotation; and a chopper wheel with a system of holes that is configured to open sequentially for some time object and reference beams separately, then simultaneously, providing interference of said beams; and a broadband plane mirror configured to reflect said incident beams in the opposite direction and to provide double passing of the beams through the transparent substrate and the birefringent element which is configured to convert said two beams into orthogonally polarized components of a single output beam falling on the non-polarizing beam splitter; a birefringent element configured to split the polarized input beam into two separate, parallel, orthogonally polarized beams, namely a reference beam and an object beam, and mounted on a base with fine rotation; and a transparent substrate, on the part of which a film under test is deposited, mounted on a base with adjustable shift and rotation; and a chopper wheel with a system of holes that is configured to open sequentially for some time object and reference beams separately, then simultaneously, providing interference of said beams; and a broadband plane mirror configured to reflect said incident beams in the opposite direction and to provide double passing of the beams through the transparent substrate and the birefringent element which is configured to convert said two beams into orthogonally polarized components of a single output beam falling on the non-polarizing beam splitter; wherein in the second arm of the interferometer formed with said reflected beam there are sequentially arranged: a controllable phase retarder, configured to introduce phase shift only into one polarization component of said beam; and a second polarizer that is configured to equalize and mix the polarization components of the input beam and to transform said input beam into a beam, which is the result of interference of polarization components of the input beam; and a photodetector configured to receive an input beam and to transform an intensity of said beam into an output electric signal, which can be amplified and analyzed by various electronic devices. a controllable phase retarder, configured to introduce phase shift only into one polarization component of said beam; and a second polarizer that is configured to equalize and mix the polarization components of the input beam and to transform said input beam into a beam, which is the result of interference of polarization components of the input beam; and a photodetector configured to receive an input beam and to transform an intensity of said beam into an output electric signal, which can be amplified and analyzed by various electronic devices.