The Online Encyclopedia and Dictionary






Liquid crystal display

LCD redirects here. For other meanings of LCD, see LCD (disambiguation).
Reflective twisted nematic liquid crystal display.Vertical filter film to the light as it enters. Glass substrate with electrodes. The shapes of these electrodes will determine the dark shapes that will appear when the LCD is turned on. Vertical ridges are etched on the surface so the liquid crystals are in line with the polarized light. Twisted nematic liquid crystals. Glass substrate with common electrode film (ITO) with horizontal ridges to line up with the horizontal filter. Horizontal filter film to block/allow through light. Reflective surface to send light back to viewer.
Reflective twisted nematic liquid crystal display.
  1. Vertical filter film to polarize the light as it enters.
  2. Glass substrate with ITO electrodes. The shapes of these electrodes will determine the dark shapes that will appear when the LCD is turned on. Vertical ridges are etched on the surface so the liquid crystals are in line with the polarized light.
  3. Twisted nematic liquid crystals.
  4. Glass substrate with common electrode film (ITO) with horizontal ridges to line up with the horizontal filter.
  5. Horizontal filter film to block/allow through light.
  6. Reflective surface to send light back to viewer.

A Liquid Crystal Display, or LCD, is a thin, flat display device made up of any number of color or monochrome pixels arrayed in front of a light source. Each pixel consists of a liquid crystal molecule suspended between two transparent polarizing filters, the axes of polarity of which are perpendicular to each other. In the absence of anything in between them, light passing through one would be blocked by the other. The twisted liquid crystal changes the polarization of light entering one filter to allow it to pass through the other. By applying measured electrical charges to each pixel or subpixel, the twist in the liquid crystal molecule can be controlled, allowing varying degrees of light to pass (or not pass) through.

Prior to the application of an electrical charge, the thin liquid crystal molecules are in a relaxed state. Perpendicular ridges in the opposing filters cause these molecules to align themselves in a helical structure, or twist. Light passing through one filter is rotated as it passes through the liquid crystal, allowing it to pass through the second polarized filter. A small amount of light is absorbed by the polarizing filters, but otherwise the entire assembly is transparent.

If and when an electrical charge is applied, the liquid crystals align themselves parallel to the electric field, thus limiting the rotation of entering light. If the liquid crystals are completely untwisted, light passing through them will be polarized perpendicular to the second filter, and thus be completely blocked. The pixel will appear unlit. By controlling the twist of the liquid crystals in each pixel, light can be allowed to pass though in varying amounts, correspondingly illuminating the pixel.

Important factors to consider when evaluating an LCD monitor include viewable size, response time (sync rate), matrix type (passive or active), viewing angle, color support, brightness and contrast ratio, resolution and aspect ratio, and input ports (e.g. DVI or VGA).


Brief History

The first operational LCD was based on the Dynamic Scattering Mode (DSM) and was introduced in 1968 by a group at RCA headed by George Heilmeier . Heilmeier founded Optel, which introduced a number of LCDs based on this technology. In 1969, the twisted nematic field effect in liquid crystals was discovered by James Fergason at Kent State University, and in 1971 his company (ILIXCO) produced the first LCDs based on it, which soon superseded the poor-quality DSM types.

Transmissive and Reflective Displays

LCDs can be either transmissive or reflective, depending on the light source. A transmissive LCD is illuminated from the back by a backlight and viewed from the opposite side. Uncharged pixels are fully lit. This type of LCD is used in applications requiring high brightness levels such as computer displays or some personal digital assistants. The lamp used to illuminate the LCD in such a product usually consumes more power than the LCD itself.

Reflective LCDs, often found in digital watches and calculators, are lit by external light reflected by a diffusing reflector behind the display. This type of LCD has lower contrast than the transmissive type since light must pass through the display twice. The absence of a lamp significantly reduces power consumption, allowing for longer battery life in battery-powered devices; small reflective LCDs consume so little power that they can rely on a photovoltaic cell, as often found in pocket calculators.

Transflective LCDs can work as either transmissive or reflective LCDs. They generally work reflectively when external light levels are high, and transmissively in darker environments via a low-power backlight.

Color Displays

In color LCDs each pixel is divided into three cells, or subpixels, which are colored red, green, and blue, respectively, by additional filters. Each subpixel can be controlled independently to yield thousands or millions of possible colors for each pixel. Older CRT monitors employ a similar method for displaying color. Color components may be arrayed in various pixel geometries, depending on the monitor's usage.

Passive-Matrix and Active-Matrix

LCDs with a small number of segments, such as those used in digital watches and pocket calculators, have a single electrical contact for each segment. An external dedicated circuit supplies an electric charge to control each segment. This display structure is unwieldy for more than a few display elements.

Small monochrome displays such as those found in personal organizers, or older laptop screens have a passive-matrix structure employing supertwist nematic (STN) or double-layer STN (DSTN) technology (DSTN corrects a color-shifting problem with STN). Each row or column of the display has a single electrical circuit. The pixels are addressed one at a time by row and column addresses. This type of display is called a passive matrix because the pixel must retain its state between refreshes without the benefit of a steady electrical charge. As the number of pixels (and, correspondingly, columns and rows) increases, this type of display becomes increasingly less feasible. Very slow response times and poor contrast are typical of passive-matrix LCDs.

For high-resolution color displays such as modern LCD computer monitors and televisions, an active-matrix structure is used. A matrix of thin-film transistors (TFTs) is added to the polarizing and color filters. Each pixel has its own dedicated transistor, which allows each column line to access one pixel. When a row line is activated, all of the column lines are connected to a row of pixels and the correct voltage is driven onto all of the column lines. The row line is then deactivated and the next row line is activated. All of the row lines are activated in sequence during a refresh operation. Active-matrix displays are much brighter and sharper than passive-matrix displays of the same size, and generally have quicker response times.

Quality control

Some LCD panels have defective transistors, causing permanently lit or unlit pixels which are always on or off. Unlike integrated circuits, LCD panels with a few defective pixels are usually still usable. It is also economically prohibitive to discard a panel with just a few bad pixels because LCD panels are much larger than ICs. Manufacturers have different standards for determining a maximum acceptable number of defective pixels. The following table presents the maximum acceptable number of defective pixels for IBM's ThinkPad laptop line.

Resolution Bright Dots Dark dots Total
QXGA 15 16 16
UXGA 11 16 16
SXGA+ 11 13 16
XGA 8 8 9
SVGA 5 5 9

LCD panels are more likely to have defects than most ICs due to their larger size. In this example, a 12" SVGA LCD has 8 defects and a 6" wafer has only 3 defects. However, 134 of the 137 dies on the wafer will be acceptable, whereas rejection of the LCD panel would be a 0% yield. (The standard is much higher now due to fierce competition between manufacturers and improved quality control. An LCD panel with 4 defective pixels is usually considered defective and customers can request an exchange for a new one.) The location of a defective pixel is also important. Often manufacturers relax their requirements when defective panels are in the center of the viewing area.

Some manufacturers offer a zero dead pixel policy.

Zero-power displays

The zenithal bistable device, developed in 2000 by ZBD Displays Limited, can retain an image without power, but this technology is not yet mass-manufactured.

A French company, Nemoptic, has developed another zero-power, paper-like LCD technology which has been mass-produced in Taiwan since July 2003. This technology is intended for use in low-power mobile applications such as e-books and wearable computers. Zero-power LCD displays are in competition with electronic paper.

See also

External links

The contents of this article are licensed from under the GNU Free Documentation License. How to see transparent copy