Overview
Each pixel of an LCD 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.

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.
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

                        
 
A general purpose alphanumeric LCD, with two lines of 16 characters.
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.

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, sometimes, 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 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. In brightly lit rooms contrast of LCD monitors can, however, exceed some CRT displays due to higher maximal brightness.
LCDs have longer response time than their plasma and CRT counterparts, older displays creating visible ghosting when images rapidly change; this drawback, however, is continually improving as the technology progresses and is hardly noticeable in current LCD Computer Displays and TVs with overdrive technology. 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 fed at 50 or 60 Hz. This does not happen with most modern displays which feed backlights with high-frequency current.
LCD screens occasionally suffer from image persistence, which is similar to screen burn on CRT and plasma 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.
Consumer LCD monitors are more fragile than their CRT counterparts, with the screen especially vulnerable. However, lighter weight makes falling less dangerous, and some displays may be protected with glass shields.
LCD monitors may have stuck pixels or dead pixels problems.

                                     LCD projector

An LCD projector is a device utilized for displaying video images or data. It is the modern equivalent to the slide projector and overhead projector used in the past.

 Overview
LCD (liquid crystal display) projectors usually contain three separate LCD panels, one each for the red, green, and blue components of the video signal. However single panel LCD projectors have been produced in the past. Light from a halogen lamp, which outputs an ideal color temperature and a broad spectrum of color is split by a prism into the three component colors. These lamps also have the ability to produce an extremly large amount of light within a small area, on average for current projectors of 2,000-4,000 ANSI Lumins. As light passes through the LCD panels, individual pixels can be opened to allow light to pass, or closed to block the light, as if each little pixel were fitted with a Venetian blind. This activity modulates the light and produces the image that is projected onto the screen by allowing many different shades from each color LCD panel.

With a lens that "projects" the image on any flat surface and does not require large "furniture" (like a big TV would), LCD projectors tend to be smaller and much more portable than older systems. The best image quality can be accomplished with a blank white or grey surface to project on, and for this reason dedicated projection screens are often used. However since white is more of a nuetral color it is best suited for people wanting "natural color tones". However it is also true that the darkest your darkest black will get is the equivilant of how dark your screen on which you're projecting on is. This is why some prefer to use grey screens which make the user percieve higher contrast levels due to the image being projected on a darker backgound. But the trade-off that is made with this "percieved higher contrast" levels is that the color tones will be off (like purple lips...ect), this can adjusted through the use of the color and hue settings of the projector but can never be completly and correctly adjusted.

 History
Early LCD systems were often intended to be used with existing overhead projectors, built as a large "plate" that was put on the projector in place of the transparencies. This provided the market with a stop-gap solution in the era when the computer was not yet the universal display medium so that there was a market for LCD projectors before their current main use became popular.

This technology is employed in some sizes of rear projection television consoles, as there are cost advantages when employed in mid size sets (40 to 50 inch diagonal). This is not expected to have much longevity in the "home theater" marketplace due to expected improvements cost/performance of competing technologies, particularly in direct-view LCD panels at the lower range of sizes and DLP projection in the larger sizes. Another advantage of using this LCD projection system in large television sets is to allow better image quality as opposed to a single 60 inch LCD panel. A common rule of thumb is that an LCD's image quality will decrease with its size. A workaround is to use a small lcd panel (or panels) and project them through a lens onto a rear projection screen to give a larger screensize (with a decreased contrast ratio) but without the quality loss.

In 2004 and 2005, LCD front projection has been enjoying a come-back because of the addition of the dynamic iris which has improved perceived contrast up to the levels of DLP.

The basic design of an LCD projector is frequently used by hobbyists who build their own DIY projection systems. The basic technique is to combine a high CRI HID lamp and ballast with a condenser and collector fresnel, an LCD removed from a common computer display and a triplet.

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                                                Consumer Electronics Technicians Group