The theoretical background for a type of common path interferometer, the so-called axial twist interferometer, is discussed. Two devices capable of producing axially twisted bundles of rays are illustrated. Applications of the system are suggested with particular reference to interference microscopy. A simpler, related device, the planar inversion interferometer, is also described, and the characteristics and applicability of the two instruments are compared.
Ronald Alpiar, "Axial Twist and Planar Inversion Interferometers," Appl. Opt. 7, 1461-1465 (1968)