A scanning interferometer comprising: - a light source generating a beam of light, - a beamsplitter-compensator pair splitting said beam of light into two partial beams, one of which is directed to one arm of the interferometer and one of which is directed to the other arm of the interferometer, - two stationary reflectors (12, 13), respectively, in the arms (14, 15) of the interferometer reflecting the partial beams of light back to the beamsplitter-compensator pair, - a scanning unit placed in one arm of the interferometer where one partial beam of light of the arm from the beamsplitter-compensator pair propagates through the scanning unit to the stationary reflector and back, - a stationary compensating prism placed in the other arm of the interferometer where the other partial beam of light of the arm from the beamsplitter-compensator pair propagates through said stationary compensating prism to the other stationary reflector and back, and - a detector to receive said pair of partial beams of light, characterised in that the scanning unit comprises a movable helicoidal prism having a shallow angle between its input and output surfaces, wherein one surface of the movable helicoidal prism is flat and the other surface is helicoidal, and the movable helicoidal prism is capable of rotating around its axis which is parallel to the optical axis of the arm of the interferometer, wherein the rotation of the scanning movable helicoidal prism refractively increases or decreases the optical path in one interferometer arm when scanning of the interferogram takes place, and the stationary compensating prism is a helicoidal prism having a shallow angle between its input and output surfaces, wherein one surface of the stationary compensating prism is flat and the other surface is helicoidal. - a light source generating a beam of light, - a beamsplitter-compensator pair splitting said beam of light into two partial beams, one of which is directed to one arm of the interferometer and one of which is directed to the other arm of the interferometer, - two stationary reflectors (12, 13), respectively, in the arms (14, 15) of the interferometer reflecting the partial beams of light back to the beamsplitter-compensator pair, - a scanning unit placed in one arm of the interferometer where one partial beam of light of the arm from the beamsplitter-compensator pair propagates through the scanning unit to the stationary reflector and back, - a stationary compensating prism placed in the other arm of the interferometer where the other partial beam of light of the arm from the beamsplitter-compensator pair propagates through said stationary compensating prism to the other stationary reflector and back, and - a detector to receive said pair of partial beams of light, characterised in that the scanning unit comprises a movable helicoidal prism having a shallow angle between its input and output surfaces, wherein one surface of the movable helicoidal prism is flat and the other surface is helicoidal, and the movable helicoidal prism is capable of rotating around its axis which is parallel to the optical axis of the arm of the interferometer, wherein the rotation of the scanning movable helicoidal prism refractively increases or decreases the optical path in one interferometer arm when scanning of the interferogram takes place, and the stationary compensating prism is a helicoidal prism having a shallow angle between its input and output surfaces, wherein one surface of the stationary compensating prism is flat and the other surface is helicoidal.