High thermal stability plane mirror interferometer

An interferometer having high thermal stability, comprising:

• light source means for producing a coherent light beam;
• a light detector;
• beam splitter means for separating the coherent light beam into a measurement beam and a reference beam;
• reflecting means, having substantially no optical path length, mounted on a movable measurement plane;
• and
• optical means having optical elements in thermal contact and in thermal equilibrium with the beam splitter, for cooperating with the beam splitter means to direct the reference beam along a reference path to said detector, and to direct the measurement beam along a measurement path to said detector, said measurement path including the round trip distance to the reflecting means on the movable measurement plane;
• and
• wherein the reference path passes through different optical elements than the measurement path, but the reference path and the measurement path have substantially the same optical path length through said optical means and pass through optical elements of substantially the same refractive index, so that the thermally induced changes in the optical path length of the reference path and the optical path length of the measurement path through the beam splitter and the optical means compensate one another.