A liquid crystal display (commonly abbreviated 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.

Overview
Each pixel of an LCD consists of a layer of molecules aligned between two transparent electrodes, and two polarizing filters, the axes of polarity of which are perpendicular to each other. With no liquid crystal between the polarizing filters, light passing through one filter would be blocked by the electrodes.

The surfaces of the electrodes that are in contact with the liquid crystal material are treated so as to align the liquid crystal molecules in a particular direction. This treatment typically consists of a thin polymer layer that is unidirectionally rubbed using a cloth (the direction of the liquid crystal alignment is defined by the direction of rubbing).

Before applying an electric field, the orientation of the liquid crystal molecules is determined by the alignment at the surfaces. In a twisted nematic device (the most common liquid crystal device), the surface alignment directions at the two electrodes are perpendicular, and so the molecules arrange themselves in a helical structure, or twist. Because the liquid crystal material is birefringent, light passing through one polarizing filter is rotated by the liquid crystal helix as it passes through the liquid crystal layer, 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 a voltage is applied across the electrodes, a torque acts to align the liquid crystal molecules parallel to the electric field, distorting the helical structure (this is resisted by elastic forces since the molecules are constrained at the surfaces). This reduces the rotation of the polarization of the incident light, and the device appears gray. If the applied voltage is large enough, the liquid crystal molecules are completely untwisted and the polarization of the incident light is not rotated at all as it passes through the liquid crystal layer. This light will then be polarized perpendicular to the second filter, and thus be completely blocked and the pixel will appear black. By controlling the voltage applied across the liquid crystal layer in each pixel, light can be allowed to pass through in varying amounts, correspondingly illuminating the pixel.

With a twisted nematic liquid crystal device it is usual to operate the device between crossed polarizers, such that it appears bright with no applied voltage. With this setup, the dark voltage-on state is uniform. The device can be operated between parallel polarizers, in which case the bright and dark states are reversed (in this configuration, the dark state appears blotchy).

Both the liquid crystal material and the alignment layer material contain ionic compounds. If an electric field of one particular polarity is applied for a long period of time, this ionic material is attracted to the surfaces and degrades the device performance. This is avoided by applying either an alternating current, or by reversing the polarity of the electric field as the device is addressed (the response of the liquid crystal layer is identical, regardless of the polarity of the applied field).

When a large number of pixels is required in a display, it is not feasible to drive each directly since then each pixel would require independent electrodes. Instead, the display is multiplexed. In a multiplexed display, electrodes on one side of the display are grouped and wired together (typically in columns), and each group gets its own voltage source. On the other side, the electrodes are also grouped (typically 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.

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 or off. Vertical ridges etched on the surface are smooth.
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. (In a backlit LCD, this layer is replaced with a light source.)

Specifications of LCD
Important factors to consider when evaluating an LCD monitor include

• resolution: unlike CRT monitors, LCD monitors have a native-supported resolution for best display effect.
• dot pitch: the granularity of LCD pixels. The smaller the dot pitch size, the less granularity is present. Hence, a clearer presentation.
• viewable size: The diagonal length of a LCD panel (more specifically known as active display area)
• response time (sync rate)
• matrix type (passive or active)
• viewing angle (coll., more specifically known as viewing direction
• c olor support: How many types of colors are supported (coll., more specifically known as color gamut)
• brightness: The amount of light emitted from the display (coll., more specifically known as luminance).
• contrast ratio
• aspect ratio: 4 by 3, 16 by 9, 16 by 10, etc.
• input ports (e.g. DVI, VGA, or even S-Video ).

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".
• 1962: 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 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 Hoffmann-LaRoche in Switzerland (Swiss patent No. 532 261) with Martin Schadt and Wolfgang Helfrich (then working for the Central Research Laboratories) listed as inventors. Hoffmann-La Roche then licensed the invention to the Japanese electronics industry which soon produced the first digital quartz wrist watches with TN-LCDs and numerous other products. 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 2007, the first Double-sided LCD panel[1] and the World's slimmest LCD panel[2] are produced by Samsung Electronics.

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 'subpixel' structures via the use of phosphors, although the analog electron beam employed in CRT's do not hit exact 'subpixels'.

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 super-twisted 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

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.

For a more comprehensible description refer to the section on the twisted nematic field effect.

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 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 less desirable 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 defective pixels because LCD panels are much larger than ICs. Manufacturers have different standards for determining a maximum acceptable number of defective pixels. The maximum acceptable number of defective pixels for LCD varies a lot (such as zero-tolerance policy[3] and 11-dead-pixel policy[citation needed] ) from one brand to another, often a hot debate between manufacturers and customers. To regulate the acceptability of defects and to protect the end user, ISO released the ISO 13406-2 standard[4]. However, not every LCD manufacturer conforms to the ISO standard and the ISO standard is quite often interpreted in different ways.

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.

LCD panels also have defects known as Mura, which look like a small-scale crack with very small changes in luminance or color

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, LCDs produce crisp images only in their "native resolution" and, sometimes, fractions of that native resolution. Attempting to run LCD panels at non-native resolutions usually results in the panel scaling the image, which introduces blurriness or "blockiness".
• Although LCDs typically have more vibrant images and better "real-world" contrast ratios (the ability to maintain contrast and variation of color in bright environments) than CRTs, they do have lower contrast ratios than CRTs in terms of how deep their blacks are. A contrast ratio is the difference between a completely on (white) and off (black) pixel, and LCDs can have "backlight bleed" where light (usually seen around corners of the screen) leaks out and turns black into gray.
• Many LCDs cannot "truly" display as many colors as their CRT and plasma counterparts, typically ones that have lower-end panel types (see List of LCD matrices) such as Twisted Nematic panels (TN).
• LCDs typically have longer response times than their plasma and CRT counterparts, especially older displays, creating visible ghosting when images rapidly change. For example, when moving the mouse too fast on an LCD, multiple cursors can sometimes be seen.
• Some LCDs have significant input lag. If the lag delay is large enough, such displays can be unsuitable for fast and time-precise mouse operations (CAD, FPS gaming) as compared to CRT displays or smaller LCD panels with negligible amounts of input lag.
• LCD panels tend to have a limited viewing angle relative to CRT and plasma displays. This can reduce the number of people able to conveniently view the same image - laptop screens are one example.
• Some LCD monitors can cause migraines and eyestrain problems due to flicker from fluorescent backlights fed at 50 or 60 Hz.
• A small percentage of LCD screens suffer from image persistence, which is similar to screen burn on CRT and plasma displays.
• Many LCDs are incapable of displaying very low resolution screen modes (such as 320x200) due to scaling limitations.
• Consumer LCD monitors tend to be more fragile than their CRT counterparts. The screen may be especially vulnerable due to the lack of a thick glass shield as in CRT monitors.