Reference interferometer with variable wavelength

The interferometer has an interferometer splitter and a reference beam path. A folded measurement beam path (Z) runs between two planar mirrors (8,9). At least one distance holding element (10) is provided to keep the planar mirrors a fixed distance apart. A light beam on the beam path is coupled into the space between the planar mirrors. This is reflected between the mirrors several times and is finally sent out of this space again. The two planar mirrors are arranged relative to each other such that the reflection points (13) of the light beam to the reflecting surfaces of the two mirrors lie on a parabolic curve. An inlet aperture (11) is provided in the first (8) of the two mirrors for coupling the light beam into the space. An outlet aperture (12) is provided in the other mirror for coupling the light beam out of the space. A retro-reflector is arranged after the second planar mirror (9) such that the light beam incident on it is fed back to or near to the inlet aperture on the same or nearly the same path.