Twisted nematic displays rotate the director of the liquid crystal by
, but super-twisted nematic displays employ up to a
rotation. This extra rotation gives the crystal a much
steeper voltage-brightness response curve and also widens the angle at
which the display can be viewed before losing much contrast. With the
sharper response, it is possible to achieve higher contrast with the
same voltage selection ratio. Therefore, the degree to which
multiplexing is possible is greatly increased. The largest common
super-twist displays have up to 500 rows.
Figure 4: Response curve of a super-twisted nematic LCD compared
with the response of an ordinary twisted nematic LCD. The
steeper response exhibited by the super-twist display makes a
wider range of brightness possible.
In dual-scan super-twist displays, two essentially separate displays are placed adjacent to each other and scanned simultaneously by separate electronics. This cuts the number of rows in each part by half. The reduced multiplexing in each half means higher selection ratios and consequently, even better contrast. In a double-cell super-twist display, there are two cells on top of each other with helical directions opposite each other. Since the circular birefringence is wavelength dependent, this provides a way to undo the wavelength disturbance created when passing through only one layer, thereby providing slightly improved contrast and a true black and white display.