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A Liquid Crystal display (commonly abbreviated '''LCD''') is a thin, flat Display Device made up of any number of color or Monochrome Pixel s 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 typically consists of a layer of Molecule s aligned between two transparent electrodes, and two Polarizing Filter s, the axes of transmission of which are (in most of the cases) perpendicular to each other. With no Liquid Crystal between the polarizing filters, Light passing through the first filter would be blocked by the second (crossed) polarizer. The surface 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 e.g. a cloth. The direction of the liquid crystal alignment is then 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 (still the most common liquid crystal device), the surface alignment directions at the two electrodes are perpendicular to each other, 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 incident 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 in the center of the layer are almost completely untwisted and the polarization of the incident light is not rotated as it passes through the liquid crystal layer. This light will then be mainly polarized perpendicular to the second filter, and thus be 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 thus constituting different levels of gray. The optical effect of a twisted nematic device in the voltage-on state is far less dependent on variations in the device thickness than that in the voltage-off state. Because of this, these devices are usually operated between crossed polarizers such that they appear bright with no voltage (the eye is much more sensitive to variations in the dark state than the bright state). These devices can also be operated between parallel polarizers, in which case the bright and dark states are reversed. The voltage-off dark state in this configuration appears blotchy, however, because of small thickness variations across the device. Both the liquid crystal material and the alignment layer material contain Ionic Compound s. 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. SPECIFICATIONS Important factors to consider when evaluating an LCD monitor:
BRIEF HISTORY
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).
A detailed description of the origins and the complex history of liquid crystal displays from the perspective of an insider during the early days has been published by Joseph A. Castellano in "Liquid Gold, The Story of Liquid Crystal Displays and the Creation of an Industry" LIQUID GOLD, The Story of Liquid Crystal Displays and the Creation of an Industry, 2005 World Scientific Publishing Co. Pte. Ltd., ISBN 981-238-956-3. The same history seen from a different perspective has been described and published by Hiroshi Kawamoto (''The History of Liquid-Crystal Displays'', Proc. IEEE, Vol. 90, No. 4, April 2002Hiroshi Kawamoto: ''The History of Liquid-Crystal Displays'', Proc. IEEE, Vol. 90, No. 4, April 2002), This paper is publicly available at the IEEE History Center . COLOR DISPLAYS In color LCDs each individual Pixel is divided into three cells, or subpixels, which are coloured 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 CRTs 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 ADDRESSED LCDS LCD, with two lines of 16 characters.]] LCDs with a small number of segments, such as those used in Digital Watch es and Pocket Calculator s, have individual electrical contacts 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), and (CSTN) colour-STN (a technology where colour is added by using a internal colour-filter). 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 ''passive-matrix addressed'' 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 Time s and poor Contrast are typical of passive-matrix addressed LCDs. High- Resolution color displays such as modern LCD Computer Monitors and Televisions use an Active Matrix structure. A matrix of Thin-film Transistor s (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 addressed displays look "brighter" and "sharper" than passive-matrix addressed displays of the same size, and generally have quicker response times, producing much better images. ACTIVE MATRIX TECHNOLOGIES Main article: 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 single transistor needed for a standard thin-film transistor (TFT) display. This results in blocking more transmission area, thus requiring a brighter backlight, which will consume more power, making this type of display less desirable for notebook computers. Vertical alignment (VA) Vertical alignment displays are a form of LC displays in which the liquid crystal material naturally exists in a horizontal state removing the need for extra transistors (as in IPS). When no voltage is applied the liquid crystal cell, it remains perpendicular to the substrate creating a black display. When voltage is applied, the liquid crystal cells shift to a horizontal position, parallel to the substrate, allowing light to pass through and create a white display. VA liquid crystal displays provide some of the same advantages as IPS panels, particularly an improved viewing angle and improved black level. QUALITY CONTROL Some LCD panels have defective Transistor s, causing permanently lit or unlit pixels which are commonly referred to as Stuck Pixel s or Dead Pixel s 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 greatly. At one point, Samsung held a zero-tolerance policy for LCD monitors sold in Korea.1 Currently, though, Samsung adheres to the more restrictive ISO 13406-2 standard.2 Other companies have been known to tolerate as many as 11 dead pixels in their policies.3 Dead pixel policies are often hotly debated 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 300 mm SVGA LCD has 8 defects and a 150 mm 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 .EBU – TECH 3320, ''"User requirements for Video Monitors in Television Production"'', EBU/UER, May 2007, p. 11. 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 color ZBD devices. A French company, Nemoptic , has developed another zero-power, Paper -like LCD technology which has been mass-produced since July 2003. This technology is intended for use in applications such as Electronic Shelf Labels, E-books, E-documents, E-newspapers, E-dictionaries, Industrial sensors, Ultra-Mobile PCs, etc. Zero-power LCDs are a category of 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 is slow refresh rate, especially with low temperatures. In 2004 researchers at the University of Oxford also demonstrated two new types of Zero Power bistable LCDs based on Zenithal bistable techniques.5 DRAWBACKS LCD technology still has a few drawbacks in comparison to some other display technologies:
SEE ALSO LCD technologies
Other display technologies
Display applications
Manufacturers
REFERENCES EXTERNAL LINKS - TUTORIALS
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