Cellophane of thickness to give 180° phase difference between O and E beams may be combined with polarizing sheets to make three types of windows. A design is cut in the Cellophane and it is mounted on the polarizer with effective optic axis 45° to the plane of vibration transmitted. The three types are (a) linear polarizer with half-wave Cellophane (b) linear/circular polarizer with half-wave Cellophane mounted on the side opposite the quarter-wave plate (c) linear/circular polarizer with half-wave Cellophane mounted on the same side as the quarter-wave plate.
Two windows of type (a) give eight different patterns depending on the arrangement. One window of type (a) permits detection of small degrees of polarization. With one window of type (b) and one of type (c) there are six possible patterns two of which do not change with rotation of one window with respect to the other. These effects result from the ability of a half-wave plate to rotate the plane of vibration through any angle or reverse the sense of circularly polarized light.
