LCD

LCD Abbrev. for liquid-crystal display. A flat-panel display that is incorporated in most portable computers, where it can produce a monochrome or color image. LCDs are also used in other digital instruments. Early LCDs suffered from poor contrast between light and dark combined with narrow viewing angles. Several different forms of construction now offer improved viewing characteristics.

LCD technology is based on liquid crystals. These are common organic compounds that, between specific temperature limits, change their crystal structure to allow them to flow like a liquid. Supertwisted nematic displays use rod-shaped (nematic) crystals. The crystals are organized between two transparent polarized layers with 90° between the directions of polarization. The crystals form a spiral between the two layers so that light can be rotated and passed through the material unchanged. When an electric field is applied, the orientation of the crystals is disturbed thus stopping the light passing. Controlling the electric field applied to each pixel results in an image. It is possible to switch modes up to 120 hertz. In consequence, by shuttering white light through colored dye filters it is possible to turn a monochrome display into a color one. The same shuttering system can be used to generate stereo images, though this is not widely available.

Supertwisted nematic displays may be passive-matrix LCDs, containing no active (switching) electronic components. Nowadays much higher performance, especially for color displays, is obtained from active-matrix LCDs. In this construction, a thin-film transistor is added to each pixel to ensure an adequate and constant drive is maintained between refresh cycles. This gives a more uniform display and wider viewing angle, though in this respect it has some way to go to match the CRT.

LCDs are expensive when compared to CRT displays of similar resolution. They also have a much lower power consumption than CRT displays but since they do not emit light they must rely on external ambient illumination or be provided with back lighting.