Liquid crystal display

Reflective twisted nematic liquid crystal display.

Vertical filter film to polarize the light as it enters.
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.
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.

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 or reflector. It is prized by engineers because it uses very small amounts of electric power, and is therefore suitable for use in battery-powered electronic devices.

Each pixel (picture element) consists of a column of liquid crystal molecules suspended between two transparent electrodes, and two polarizing filters, the axes of polarity of which are perpendicular to each other. Without the liquid crystals between them, light passing through one would be blocked by the other. The liquid crystal twists the polarization of light entering one filter to allow it to pass through the other.

The molecules of the liquid crystal have electric charges on them. By applying small electrical charges to transparent electrodes over each pixel or subpixel, the molecules are twisted by electrostatic forces. This changes the twist of the light passing through the molecules, and allows varying degrees of light to pass (or not pass) through the polarizing filters.

Before applying an electrical charge, the liquid crystal molecules are in a relaxed state. Charges on the molecules cause these molecules to align themselves in a helical structure, or twist (the "crystal"). In some LCDs, the electrode may have a chemical surface that seeds the crystal, so it crystallizes at the needed angle. 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.

When an electrical charge is applied to the electrodes, the molecules of the liquid crystal 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.

Many LCDs are driven to darkness by an alternating current, which disrupts the twisting effect, and become light or transparent when no current is applied.

To save cost in the electronics, LCD displays are often multiplexed. In a multiplexed display, electrodes on one side of the display are grouped and wired together, and each group gets its own voltage source. On the other side, the electrodes are also grouped, with each group getting a voltage sink. The groups are designed so each pixel has a unique, unshared combination of source and sink. The electronics, or the software driving the electronics then turns on sinks in sequence, and drives sources for the pixels of each sink.

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 location of the light source. A transmissive LCD is illuminated from the back by a backlight and viewed from the opposite side (front). This type of LCD is used in applications requiring high luminance levels such as computer displays, personal digital assistants, and mobile phones. The illumination device used to illuminate the LCD in such a product usually consumes much more power than the LCD itself.

Reflective LCDs, often found in digital watches and calculators, are illuminated by external light reflected by a (sometimes) diffusing reflector behind the display. This type of LCD has higher contrast than the transmissive type since light must pass through the liquid crystal layer twice and thus is attenuated 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. 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

Image:lcd_defects.gif

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 pixels 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 LCDs are in competition with electronic paper.

External links

How LCDs Work (http://electronics.howstuffworks.com/lcd.htm)
LCD Monitor (http://www.lcdmonitor.org/) - Covers basics, frequently asked questions, and product database.
Liquid Crystal Institute (Kent State University) (http://www.lci.kent.edu)