Liquid crystal display

LCD layers.svg|thumb|300px|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 or off. 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.

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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 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 to 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 faint or transparent when no current is applied.

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, 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 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 1969, the twisted nematic field effect in liquid crystals was discovered by James Fergason at Kent State University in the USA, and in 1971 his company ILIXCO (now LXD Incorporated) 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, 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, however because the reflected light is also attenuated twice in the translucent parts of the display image contrast is usually poorer than 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. 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 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 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, thus allowing people to view images much more easily.

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 most 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.

Resolution	Bright Dots	Dark dots	Total
2048×1536 (QXGA)	15	16	16
1600×1200 (UXGA)	11	16	16
1400×1050 (SXGA+)	11	13	16
1024×768 (XGA)	8	8	9
800×600 (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 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 Samsung, 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. However, consumers should be sure to check the manufacturer's policies before they buy.

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 usually produce only crisp images in their "native resolution" or even fractions of it.
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 making black grey.
LCDs have longer response time than their plasma and CRT counterparts, creating ghosting and mixing when images rapidly change; this drawback, however, is continually improving as the technology progresses and is impercetible in current LCD Computer Displays and TV's.
The viewing angle of a LCD is usually less than that of most other display technologies, thus reducing the number of people who can conveniently view the same image. 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. It also allows multiple TV outputs from the same LCD screen just by changing the angle from where the TV is seen. Such a set can also show two different images to one viewer, providing a three-dimensional effect.
Many users of older (around pre-2000) LCD monitors get migraines and severe eyestrain problems from the flicker nature of the fluorescent backlights. If you experience eyestrain issues with LCDs, consider these possibilities: you are using too small a resolution for reading text, there is glare from another light, screen brightness is set too low or high, there is a defective backlight, the LCD monitor is too close or too far away, or you need to change the refresh rate. If your operating system has antialiasing with subpixel accuracy (called "ClearType" on Windows XP), this generally helps improve font visibility, although its style is not to everyone's liking.
LCD screens occasionally suffer from image persistence, which is similar to screen burn on CRT displays. Unlike CRTs, however, LCDs can be restored to normal by displaying an all-white pattern for extended periods of time.
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.
Many models can't display some low resolution screenmodes (such as 320x240).

External links

General Information

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
Display Technology, by Geoff Walker in the september 2001 issue of Pen Computing
How LCDs Work by Jeff Tyson for HowStuffWorks
Learn About LCD TV and TFT LCD Displays from avdeals.com, Samaung Electronics information.
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.

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

Other LCD Information

LCD Monitor.org LCD monitors basics, frequently asked questions, and product database
LCD Buyer's Guide, Anandtech forum sticky
LCD vs CRT Consumer Advice by PC review.co.uk, 22-05-04
Guide: Contemporary LCD Monitor Parameters and Characteristics by Oleg Artamonov, X-bit labs, 10/26/2004
Backlight Bleeding: An LCD Problem
How to Mount an LCD provides advice on Audio visual mounting hardware

Liquid crystal displays

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