Lateral shearing properties of twisted nematic liquid-crystal cell and its potential application to differential interference contrast imaging

A liquid crystal (LC) cell behaves as an optically uniaxial crystal and lateral shear properties can be obtained under considerably low voltage, because an oblique optical axis distribution state appears in the middle voltage level between ON and OFF. In this work, a pair of twisted nematic (TN) LC cells is introduced to the normal polarization microscope system to implement differential interference contrast (DIC) imaging, which is a powerful observation tool for weak phase samples such as a bio cell. DIC imaging is usually obtained using a pair of Nomarski prisms, which are inserted at the back focal plane of the objective and condenser lenses to separate and combine the input image laterally. However, if we use LC cells, DIC images can be easily obtained by just sandwiching the test sample between a pair of LC cells. The lateral shear distance, which influences DIC sensitivity, becomes tunable with fast response speed by using LC cells, although the shear distance is fixed in the normal DIC system. Furthermore, unique lateral shearing properties of the TN cell help in achieving self-compensation of optical retardation, and we can then use the incoherent illumination of a normal microscope system for DIC observation as usual. Here, fundamental lateral shearing properties and the operational mode for DIC imaging are investigated using a pair of TN cells.