The lens-free conoscope that we have developed for lecture demonstrations1 is very simple and effective, but the crystals that are required are expensive and difficult to find. This limits the wide use of the method we described in Ref. 1.
In this note, we describe an attempt to replace the crystals used in Ref. 1 with transparent cellophane sheets that are used, for example, to wrap flower bouquets. The double refraction in cellophane is caused by mechanical strain in the sheets during production. Since the optical axis of the material is located within the plane of the sheet, the interferential picture obtained in a conoscopic measurement has the form of a double system of color curves, similar to hyperbolas. The image shown in Fig. 1 is obtained with eight sheets of cellophane, placed between crossed polarizers. The optical axes of all sheets are directed in the same direction...