Liquid crystal displays take advantage of a liquid crystal's ability to reflect and scatter light. This light reflecting ability is in part due to liquid crystal's tendency to form textures. The term texture describes the molecular orientations within a liquid crystal display cell. Cholesteric liquid crystals exhibit three alignments. These are the planar texture, focal conic texture, and homeotropic texture. Cholesteric crystals exhibit a helical molecular structure. The helical structure is formed by stacked long molecules that are progressively displaced through a small angle. When these liquid crystals are in the focal conic texture, the individual helical domains are in a random arrangement. This random arrangement weakly scatters light. The helical axis is more or less parallel to the supporting surfaces. In the homeotropic texture, the liquid crystal material adopts a completely undeformed director configuration. In this configuration, the director points perpendicular to the supporting surfaces. Finally, in the planar texture, the helical axis is aligned perpendicular to the supporting surfaces. As the liquid crystal material moves from one of these textures to another, its light propagating attributes change.
Cholesteric liquid crystals are used for reflective displays because they exhibit Bragg reflection in the planar texture. In the focal conic texture, cholesteric liquid crystal material scatters light. They are both stable at zero field. For a regular cholesteric liquid crystal with a positive dielectric anisotropy, the transition from the planar texture to the focal conic texture is direct and is achieved by applying a low voltage pulse. However, the transition from the focal conic texture to the planar texture is indirect. The material must be switched from the focal conic texture to a third state, a homeotropic texture, by a high voltage pulse, and then the material relaxes to the planar texture. The need to switch the material to the homeotropic texture is disadvantageous because the voltage required to switch the material to homeotropic texture is high, response time is increased, and it is difficult to make use of cumulative effect with the homeotropic texture. These disadvantages make it impractical to use known cholesteric liquid crystals in video rate displays.
It is known to provide a dual frequency cholesteric liquid crystal material responsive to high and low frequency voltages. However, it is only known to apply a single high or low frequency of varying duration to change the appearance of the material. This results in a slow and unacceptable addressing speed.
Thus, it is desirable to develop a drive scheme for switching directly from the focal conic texture to the planar texture without first switching to a homeotropic texture. It is also desirable to provide a cholesteric display that would be conducive to video rate applications.
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