Abstract: The invention relates to a standing wave interferometer for measuring optical path differences in which a standing wave is produced between a monochromatic light source and a reflector. The displacement of the bulges and nodal points of the standing wave due to displacement of the reflector is sampled by a sampling normal inserted into the standing wave and having two photodetecting layers at a mutual space of k.multidot..lambda./8, k being any desired odd integer. The sampling signals are fed into an electronic evaluation unit where these are processed. The measuring object, when having a well-reflecting surface can be the reflector itself, without the necessity of adjusting the same accurately to the incident light beam.