Encoder head designs

An encoder apparatus, which measures a relative movement with respect to a grating, the apparatus comprising:

• a light source situated to emit a measurement beam and first and second reference beams;
• an optical system including a beam splitter situated to reflect or transmit the measurement beam from the light source and situated to transmit or reflect the first and second reference beams, to direct the measurement beam from the beam splitter to a diffraction grating, to direct first and second diffracted beams which are caused by diffraction of the measurement beam irradiated on the diffraction grating, received from the diffraction grating, from first and second positions on the beam splitter and along first and second propagation directions respectively, and to direct the first reference beam from the first position along the first propagation direction and the second reference beam from the second position along the second propagation direction;
• and
• a detector situated to detect an interference of the first diffracted beam and the first reference beam, and an interference of the second diffracted beam and the second reference beam.