Supertwisted Nematic LCDs

The biggest problem with early multiplexed LCDs was the reduction in contrast ratio with number of addressed lines. This problem was essentially eliminated with the invention of the supertwisted nematic (STN) LCD in the early 1980s. It was found that if the twist angle was increased to 270 degrees, the slope of the brightness-voltage curve approached infinity; under this condition, a large number of lines could be multiplexed. This higher twist angle was achieved by adding higher concentrations of cholesteric liquid crystal to the nematic mix and by increasing the tilt angle atthe glass surface.

The first successful STN LCDs used a birefringence mode to create a “yellow mode” and a “blue mode.” Although the result was not optimum for general display use, it was possible to demonstrate 200:1 multiplexing with greater than 5:1 contrast ratio. For the first time, LCDs could be seriously considered for use in portable computers.

The next advance was the development of compensated STN LCDs to produce true black-and-white images. Using either a second STN LCD with opposite twist or a retardation film, several manufacturers were able to produce black- and-white LCDs with high contrast and multiplexibility. Today, the film- compensated STN (FSTN) is preferred because of its thin profile and low weight compared to the double STN (DSTN) type. FSTN LCDs with multiplexing ratios as high as 480:1 have been demonstrated in both black and white and full color. Full color is achieved in the same manner as in active matrix LCDs; that is, RGB filters are patterned on one of the glass plates to control the color of the light transmitted through the LCD.





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