Encoder head with birefringent elements for forming imperfect retroreflection and exposure system utilizing the same

An encoder head configured to detect a relative positional relationship of a workpiece with respect to a reference diffraction grating, the encoder head comprising:

• a first optical system configured to deliver a first beam of light with a first polarization state to a measurement position on the diffraction grating, wherein the diffraction grating is configured to form a second beam of light by diffracting the first beam of light;
• a plurality of birefringent members spatially separated from one another, wherein a birefringent member from said plurality is made from an optically-birefringent material and disposed in an optical path of the second beam of light;
• a reflector having a bottom surface and disposed
• to form a third beam of light by reflecting the second beam of light, that has passed through the birefringent member, back to the birefringent member to be transmitted through the birefringent member and to impinge at the measurement position on the diffraction grating, wherein the second beam of light has a second polarization state;
• and
• a wave plate disposed between the reflector and two birefringent members from said plurality, said wave plate being in contact with the bottom surface of the reflector at a location between said two birefringent members to intercept the third beam upon propagation of said third beam of light from the measurement position on the diffraction grating towards the reflector.