Abstract: An interferometer system for measuring the wavelength of laser light or and other optical radiation comprises an interferometer with a radiation input for a reference beam of a given wavelength, a sample radiation input for a sample beam of unknown wavelength, and a beam splitter for splitting up an incoming beam into two part-beams, and a system for changing the optical lengths of two part-beam optical paths as used by the reference beam and, parallel to it, the beam whose wavelength is to be measured. There is furthermore a detector for converting output beams from the interferometer into representative electrical signals and a signal processing circuit coupled to the detector means.Furthermore in the part-beam paths there is an optical part in the form of a parallelepipedic transparent body able to be rotated for the changing of the lengths of the paths of the beams in relation to each other, and a reflector system for reflecting back the part-beams, coming from the turning part, back into themselves.