liquid crystal display
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.