Optical insturment having stage support

An optical instrument for gauging the velocity of a travelling surface with the aid of a gauging light beam emitted by a laser and a detector receiving stray reflections of said light beam water a reflection on said surface, comprising a housing having a base plate,

• at least one optical assembly enclosed by said housing and mounted on a stage plate, said stage plate being supported on said base plate by supporting means
• which fixes (1) said stage plate at one first point inhibiting translation movement and allowing at least some rotational movement of said stage plate at said one point, (2) said stage plate at one second point inhibiting transactional movement except for movement along an axis through both of said first and second points and allowing rotational movement of said stage late at said second point, and (3) said stage plate at one third point inhibiting transactional movement and allowing rotational movement of said stage plate at said third point,
• thus preventing thermal, torsional or bending deformation of said stage plate irrespectively of a mechanical or thermal attack onto said housing.