A liquid crystal display (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 consists of a layer 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.

Before applying an electrical charge, the liquid crystal molecules are in a relaxed state. Charges on the molecules cause these molecules to align themselves with microscopic grooves on the electrodes. The grooves on the two electrodes are perpendicular, so the molecules arrange themselves in a helical structure, or twist (the "crystal"). Light passing through one filter is rotated as it passes through the liquid crystal, allowing it to pass through the second polarized filter. Half of the light is absorbed by the first polarizing filter, but otherwise the entire assembly is transparent.

When an electrical charge is applied to the electrodes, the molecules of the liquid crystal are pulled parallel to the electric field, thus reducing the rotation of the 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 through in varying amounts, correspondingly illuminating the pixel.

It is normal to align the polarizing filters so that pixels are transparent when relaxed and become opaque in the presence of an electric field, however the opposite is sometimes done for special effect.

The electric field necessary to align the liquid crystal molecules rapidly is also enough to pull them out of position, damaging the display. This is solved by using an alternating current to rapidly pull the molecules in alternate directions.

To save cost in the electronics, LCDs are often multiplexed. In a multiplexed display, electrodes on one side of the display are grouped and wired together (say, in columns), and each group gets its own voltage source. On the other side, the electrodes are also grouped (say, in rows), 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 resolution, viewable size, response time (sync rate), matrix type (passive or active), viewing angle, color support, brightness and contrast ratio, aspect ratio, and input ports (e.g. DVI or VGA).

Brief history

1904: Otto Lehmann publishes his work "Liquid Crystals"

1911: Charles Mauguin describes the structure and properties of liquid crystals.

1936: The Marconi Wireless Telegraph company patents the first practical application of the technology, "The Liquid Crystal Light valve".

1963: The first major English language publication on the subject "Molecular Structure and Properties of Liquid Crystals", by Dr. George W. Gray.

Pioneering work on liquid crystals was undertaken in the late 1960s by the UK's Royal Radar Establishment at Malvern. The team at RRE supported ongoing work by George Gray and his team at the University of Hull who ultimately discovered the cyanobiphenyl liquid crystals (which had all of the correct stability and temperature properties for application in LCDs).

The first operational LCD was based on the Dynamic Scattering Mode (DSM) and was introduced in 1968 by a group at RCA in the USA headed by George Heilmeier. Heilmeier founded Optel, which introduced a number of LCDs based on this technology.

In December 1970, the twisted nematic field effect in liquid crystals was filed for patent by M. Schadt and W. Helfrich, then working for the Central Research Laboratories of Hoffmann-LaRoche in Switzerland (Swiss patent No. 532 261). James Fergason at Kent State University filed an identical patent in the USA in February 1971. In 1971 the company of Fergason ILIXCO (now LXD Incorporated) produced the first LCDs based on the TN-effect, which soon superseded the poor-quality DSM types due improvements of lower operating voltages and lower power consumption.

In 1972, the first active-matrix liquid crystal display panel was produced in the United States by T. Peter Brody.(1)

In 2005, Mary Lou Jepsen developed a new type of LCD display for the One Laptop Per Child project to reduce power consumption and manufacture cost of the Children's Machine. This display uses a plastic diffraction grating and lenses on the rear of the LCD to illuminate the colored subpixels. This method absorbs very little light, allowing for a much brighter display with a lower powered backlight. Replacing the backlight with a white LED allows for largely reduced costs and increased durability as well as a wider color gamut.

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, televisions, 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 can produce darker 'blacks' than the transmissive type since light must pass through the liquid crystal layer twice and thus is attenuated twice. Because the reflected light is also attenuated twice in the translucent parts of the display image, however, contrast is usually poorer than in a transmissive display. 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 work as either transmissive or reflective LCDs, depending on the ambient light. They work reflectively when external light levels are high, and transmissively in darker environments via a low-power backlight.

Buyer's guide on (Transflective) displays

Color displays

In color LCDs each individual pixel is divided into three cells, or subpixels, which are colored red, green, and blue, respectively, by additional filters (pigment filters, dye filters and metal oxide 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.

The first operational LCD was based on the Dynamic Scattering Mode (DSM) and was introduced in 1968 by a group at RCA in the USA headed by George Heilmeier. Heilmeier founded Optel, which introduced a number of LCDs based on this technology.

In December 1970, the twisted nematic field effect in liquid crystals was filed for patent by M. Schadt and W. Helfrich, then working for the Central Research Laboratories of Hoffmann-LaRoche in Switzerland (Swiss patent No. 532 261). James Fergason at Kent State University filed an identical patent in the USA in February 1971. In 1971 the company of Fergason ILIXCO (now LXD Incorporated) produced the first LCDs based on the TN-effect, which soon superseded the poor-quality DSM types due improvements of lower operating voltages and lower power consumption.

In 1972, the first active-matrix liquid crystal display panel was produced in the United States by T. Peter Brody.(1) for displaying color. Color LCDs initially were used only for handheld video games, but thanks to improvements in quality and price they are now becoming the dominant form of computer display.

Color components may be arrayed in various pixel geometries, depending on the monitor's usage. If software knows which type of geometry is being used in a given LCD, this can be used to increase the apparent resolution of the monitor through subpixel rendering. This technique is especially useful for text anti-aliasing.

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 less feasible. Very slow response times and poor contrast are typical of passive-matrix LCDs.

High-resolution color displays such as modern LCD computer monitors and televisions use an active matrix structure. A matrix of thin-film transistors (TFTs) is added to the polarizing and color filters. Each pixel has its own dedicated transistor, allowing 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, producing much better images.

Active matrix technologies

Main article: TFT LCD, Active-matrix liquid crystal display

Twisted nematic (TN)

Twisted nematic displays contain liquid crystal elements which twist and untwist at varying degrees to allow light to pass through. When no voltage is applied to a TN liquid crystal cell, the light is polarized to pass through the cell. In proportion to the voltage applied, the LC cells twist up to 90 degrees changing the polarization and blocking the light's path. By properly adjusting the level of the voltage almost any grey level or transmission can be achieved.

In-plane switching (IPS)

In-plane switching is an LCD technology which aligns the liquid crystal cells in a horizontal direction. In this method, the electrical field is applied through each end of the crystal, but this requires the need for two transistors for each pixel instead of the one needed for a standard thin-film transistor (TFT) display. This results in blocking more transmission area requiring brighter backlights, which consume more power making this type of display undesirable for notebook computers.

Quality control

Some LCD panels have defective transistors, causing permanently lit or unlit pixels which are commonly referred to as stuck pixels or dead pixels respectively. 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.

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 SVGA LCD panel with 4 defective pixels is usually considered defective and customers can request an exchange for a new one. Some manufacturers, notably in South Korea where some of the largest LCD panel manufacturers, such as LG, are located, now have "zero defective pixel guarantee" and would replace a product even with one defective pixel. Even where such guarantees do not exist, the location of defective pixels is important. A display with only a few defective pixels may be unacceptable if the defective pixels are near each other. Manufacturers may also relax their replacement criteria when defective pixels are in the center of the viewing area.

Zero-power displays

The zenithal bistable device (ZBD), developed by QinetiQ (formerly DERA), can retain an image without power. The crystals may exist in one of two stable orientations (Black and "White") and power is only required to change the image. ZBD Displays is a spin-off company from QinetiQ who manufacture both grayscale and colour ZBD devices.

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.

Kent Displays, has also developed a "no power" display that uses Polymer Stabilized Cholesteric Liquid Crystals(ChLCD). The major drawback to the ChLCD display is slow refresh rate, especially with low temperatures.

Drawbacks

LCD technology still has a few drawbacks in comparison to some other display technologies:

• While CRTs are capable of displaying multiple video resolutions without introducing artifacts, LCD displays produce crisp images only in their "native resolution" and fractions of that native resolution. Attempting to run LCD display panels at non-native resolutions usually results in the panel scaling the image, which introduces blurriness or "blockiness".
• LCD displays generally have a lower contrast ratio than that on a plasma display or CRT. This is due to their "light valve" nature: some light always leaks out and turns black into gray.
• Older LCDs had longer response time than their plasma and CRT counterparts, creating ghosting when images rapidly change; this drawback, however, is continually improving as the technology progresses and is almost imperceptible in current LCD Computer Displays and TVs.^{citation needed]} Most newer LCDs have response times at approximately 8ms, with the exact response time varying according to the type of panel and manufacturer.
• LCD display panels have a limited viewing angle, thus reducing the number of people who can conveniently view the same image. As the viewer moves closer to the limit of the viewing angle, the colors and contrast appear to deteriorate. However, this negative has actually been capitalized upon in two ways. Some vendors offer screens with intentionally reduced viewing angle, to provide additional privacy, such as when someone is using a laptop in a public place. Such a set can also show two different images to one viewer, providing a three-dimensional effect.
• Some users of older (around pre-2000) LCD monitors complain of migraines and eyestrain problems due to flicker from fluorescent backlights.
• LCD screens occasionally suffer from image persistence, which is similar to screen burn on CRT displays. This is becoming less of a problem as technology advances, with newer LCD panels using various methods to reduce the problem. Sometimes the panel can be restored to normal by displaying an all-white pattern for extended periods of time.
• Some light guns do not work with this type of display since they do not have flexible lighting dynamics that CRTs have. However, the field emission display will be a potential replacement for LCD flat-panel displays since they emulate CRTs in some technological ways.
• Some panels are incapable of displaying low resolution screen modes (such as 320x200). However, this is due to the circuitry that drives the LCD rather than the LCD itself.
• LCD monitors may tend to be more fragile than their CRT counterparts.
• LCD monitors may have stuck pixels or dead pixels problems.

References

(1) Brody, T.P., "Birth of the Active Matrix," Information Display, Vol. 13, No. 10, 1997, pp. 28-32.

See also

LCD technologies

• Color LCD
• List of LCD matrices
• TFT LCD
• Active-matrix liquid crystal display (AMLCD)

Other Display Technologies

• Comparison of display technology
• Cathode ray tube (CRT)
• Vacuum fluorescent display (VFD)
• Digital Light Processing (DLP)
• Plasma display panel (PDP)
• Light-emitting diode (LED)
• Organic light-emitting diode (OLED)
• Surface-conduction electron-emitter display (SED)
• Field emission display (FED)
• Liquid crystal on silicon (LCOS)

Display Applications

• Television and digital television
• Liquid crystal display television (LCD TV)
• LCD projector
• Computer monitor

Manufacturers

• Sharp Corporation
• Corning Inc.
• LXD Incorporated
• International Display Works

External links

General Information

• Development of Liquid Crystal Displays - George Gray, Hull University Freeview video by the Vega Science Trust.
• History of Liquid Crystals, presentation and extracts from the book Crystals that Flow: Classic papers from the history of liquid crystals by its co-author Timothy J. Sluckin
• LCD Monitor.org LCD monitors basics, frequently asked questions, and product database
• Display Technology, by Geoff Walker in the September 2001 issue of Pen Computing
• How LCDs Work by Jeff Tyson for HowStuffWorks
• CRT vs. LCD Monitors Concise comparison matrix by Sam C. Chan for the network integrator Bravo Technology Center

Institutional

• Liquid Crystal Institute of the Kent State University

Manufacturers

• LXD Incorporated, harsh environements dispays manufacturing
• Displaze Ltd., Flat Panel reseller with specifications for many brands
• G-NET Inc., LCD Displays For Vehicle Applications From 7" to 12" Sizes.
• Hannstar Display Corp. LCD panel manufacturer for computers and televisions.

Usage of LCDs

• LCD (Liquid Crystal Display) IO, source code and examples for driving small LCD displays by techref.massmind.org, updated: 2006/4/10
• PIC Microcontroler LCD IO routines, source code and examples for driving LCD displays with the Microchip PIC embedded controllers, by techref.massmind.org, updated: 2006/3/5
• LCD Info Page, LCD Simulators for smaller character and graphics LCDs, by Dincer Aydin