liquid crystal display
LCD
(LCD) An electro-optical device used to display
digits, characters or images, commonly used in digital
watches, calculators, and portable computers.
The heart of the liquid crystal display is a piece of liquid
crystal material placed between a pair of transparent
electrodes. The liquid crystal changes the phase of the
light passing through it and this phase change can be
controlled by the voltage applied between the electrodes.
If such a unit is placed between a pair of plane polariser
plates then light can pass through it only if the correct
voltage is applied. Liquid crystal displays are formed by
integrating a number of such cells, or more usually, by using
a single liquid crystal plate and a pattern of electrodes.
The simplest kind of liquid crystal displays, those used in
digital watches and calculators, contain a common electrode
plane covering one side and a pattern of electrodes on the
other. These electrodes can be individually controlled to
produce the appropriate display. Computer displays, however,
require far too many pixels (typically between 50,000 and
several millions) to make this scheme, in particular its
wiring, feasible. The electrodes are therefore replaced by a
number of row electrodes on one side and column electrodes on
the other. By applying voltage to one row and several columns
the pixels at the intersections are set.
The pixels being set one row after the other, in passive
matrix displays the number of rows is limited by the ratio of
the setting and fading times. In the setup described above
(known as "twisted nematic") the number of rows is limited
to about 20. Using an alternative "supertwisted nematic"
setup VGA quality displays (480 rows) can be easily built.
As of 1995 most notebook computers used this technique.
Fading can be slowed by putting an active element, such as a
transistor, on the top of each pixel. This "remembers" the
setting of that pixel. These active matrix displays are of
much better quality (as good as CRTs) but are much more
expensive than the passive matrix displays.
LCDs are slimmer, lighter and consume less power than the
previous dominant display type, the cathode ray tube, hence
their importance for portable computers.
(1995-12-09)
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