A position measuring apparatus includes a radiation source mounted on an isolated
reference frame and a two-dimensional radiation detector mounted adjacent the radiation
source. The object whose position is to be detected has a retro-reflector mounted
on it so as to reflect light emitted from the radiation source along a return path
that is parallel to but displaced from the incident light path. The amount of displacement
is dependent on the position of the object and is measured by the two-dimensional
detector. Three such apparatus can be combined in a system to measure the position
of the object in all six degrees of freedom.
