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 liquidcrystal 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 passivematrix 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)