Black-and-White STN-LCDs

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. 

Simpler, cheaper and more practical approaches adopted subsequently to compensate for the interference colours of STN-LCDs involve the use of a passive optical element, such as selective polarisers and colour filters, see Chapter 2. A black-on-white appearance is produced most efficiently by using optical retardation layers with a high birefringence and an opposite twist sense to the STN-LCD. However, this also reduces the brightness of the display due to additional light absorption. 

---