Temperature dependence liquid crystal colour

In order to produce black-and-white as well as full-colour STN-LCDs, the monochrome interference colours must first be eliminated. This was achieved initially by using two STN-LCDs in a combined double-layer (DSTN) LCD configuration. This involves the use of another non-addressed, passive STN cell in addition to the active display STN-LCD. However, the non-addressed cell has an opposite sense of twist of the nematic director in the cell to that of the addressed STN-LCD. The second STN-LCD, which is identical to the first, but not addressed at all, acts as a retardation compensation layer. The use of an identical second STN-LCD in combination with the active STN-LCD has the advantage that both displays exhibit exactly the same temperature dependence of the birefringence with the same dispersion, assuming that both cells are filled with the same liquid crystal mixture. The second STN-LCD is not addressed and, therefore, there is no increase in power consumption. However, the use of two identical STN-LCDs instead of only one clearly increases the cost and weight of the final product significantly. 

Fig. 9. Schematic dependence of the wavelength of selectively reflected light on temperature for a typical side chain polymer liquid crystal. Tsch denotes the cholesteric to smectic A phase transition and above this temperature the reflected colour changes markedly on heating. Rapid cooling of the material to below Tg stores the reflectance colour obtained during the heating cycle.

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