Optical properties of a nematic twist cell

We show that many of the complicated static and dynamic optical transmission characteristics of a Schadt-Helfrich twist cell, including the “bounce” in transmission that is observed at negative oblique angles of incidence after switching it off, can be predicted quite well from a very simple dynamic model. The model ignores flow and has only a viscous resistance to angular reorientation, in addition to the Ossen-Frank elastic properties. The bounce observed in some cases with normally incident light cannot be predicted by the model. As expected, the model yields equilibrium configurations at any applied voltage that give the observed transmission very accurately. Dynamic observations in the upper part of the nematic temperature range, where the observed bounce at normal incidence is small, are well reproduced by the model. At lower temperatures the discrepancies are larger. When interpreted with the model, both dynamic and static measurements concur in indicating that the directors of the liquid crystal adjacent to the surfaces assumed a more or less permanent tilt of a few degrees in our cell. Alignment was obtained by rubbing the conducting electrodes in one direction with linen cloth.