Apparatus wherein diffracted light does not return to the source

Abstract

A lattice interference-type displacement detection device including a scale having a penetration-type diffraction lattice formed on one side thereof, and an optical system positioned adjacent to the diffraction lattice and remote from the scale. The optical system includes a light source which emits a light beam along a light source optical path, a light beam divergence arrangement for branching the light beam into first and second divergence beams along first and second optical paths, respectively, and for causing the first and second divergence beams to be incident at a diffraction point on the diffraction lattice, a light beam mixing arrangement for generating first and second mixing light beams from a first group of diffraction light beams generated at the diffraction point, and an arrangement for converting the first and second mixing light beams into respective electrical signals. The scale also includes a light reflection surface remote from the optical system with respect to the diffraction lattice. The light reflection surface reflects the first group of diffraction light beams generated at the diffraction point to the light beam mixing arrangement along third and fourth optical paths, respectively. The optical system is arranged so that a second group of diffraction light beams generated at the diffraction point are not reflected back along the light source optical path to the light source. The light beam divergence arrangement and light beam mixing arrangement are constructed of a single prism.