For a bistable nematic liquid crystal display, state transitions (ON-to-OFF or OFF-to-ON) occur under the influence of appropriately applied dynamic electric fields. See, for example, G. Boyd et al., "Liquid-Crystal Orientational Bistability and Nematic Storage Effects," Appl. Phys. Lett. 36, pp. 556-558 (1980) and J. Cheng et al., "Threshold and Switching Characteristics of a Bistable Nematic Liquid-Crystal Storage Display," Appl. Phys. Lett. 37, pp. 1072-1074 (1981). In particular, vertical electric fields cause orientational director transitions to a predominantly vertical alignment configuration, an ON state, for example, for the liquid crystal molecules. Similarly, horizontal electric fields cause orientational director transitions to a predominantly horizontal alignment configuration, an OFF state, for example, which is topologically distinct from the vertical alignment configuration. See U.S. patent application, Ser. No. 98,976 filed on Nov. 30, 1979, U.S. Pat. No. 4,333,708.
In this bistable nematic liquid crystal medium, the horizontal and vertical electric fields are produced with an array of interdigital electrodes as a matrix addressing arrangement for activating and deactivating individual display cells. Although this type of arrangement can provide low to moderate addressing speeds for a moderate electric field strength, it is incapable, from a practical viewpoint, of providing high speed operation. In addition, interdigital electrodes require both a large number of connections per display cell and complex control circuits to activate particular electrodes for switching.
Interdigital electrodes generate nonhomogeneous electric fields and exhibit two distinct switching thresholds for liquid crystal displays, namely, a longitudinal threshold and a transverse threshold. These thresholds represent minimum electric field strengths necessary for detaching disclinations in the liquid crystal medium from boundary discontinuities in surface alignment or topography. Existence of the two separate thresholds and the creation of nonhomogeneous electric fields substantially impair the effectiveness of interdigital electrodes for high speed matrix addressing purposes in liquid crystal displays.
Homogeneous or uniform vertical electric fields can be generated by an array of orthogonally disposed, continuous uniform strip electrodes as the matrix addressing arrangement. This type of electrode arrangement facilitates transitions from the horizontal state to the vertical state, that is, OFF to ON state transitions. Furthermore, it exhibits a single sharp switching threshold for horizontal to vertical state transitions which is of sufficient magnitude and definition to assure reliable, high speed switching. However, the array of strip electrodes is unsuited for generating the fields necessary for vertical to horizontal state transitions, that is, ON to OFF state transitions.