A new view of conoscopic illumination of optically active crystals

Conoscopic illumination is used in a microscope equipped with a rotating polarizer-circular analyser system and monochromatic light, and the resulting interference pattern is separated into its constituent parts. The technique allows identification of the sign of the optical indicatrix and birefringence to be determined without the need to insert extra wave plates. When the technique is applied to an optically active uniaxial crystal, the so-called Airy's spiral is revealed with high sensitivity, enabling the optical rotation of the crystal to be determined unambiguously. Theoretically computed images show a close correspondence with the observed images.