liquid crystal display
<hardware> (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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