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Liquid crystal display
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A liquid crystal display (LCD) is an electronically-modulated optical device shaped into a thin, flat panel made up of any number of color or monochrome pixels filled with liquid crystals and arrayed in front of a light source (backlight) or reflector. It is often utilized in battery-powered electronic devices because it uses very small amounts of electric power.
A comprehensive classification of the various types and electro-optical modes of LCDs is provided in the article LCD classification.
pixel of an LCD typically consists of a layer of molecules aligned between two transparent electrodes, and two polarizing filters, the axes of transmission of which are (in most of the cases) perpendicular to each other.
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Encyclopedia
A liquid crystal display (LCD) is an electronically-modulated optical device shaped into a thin, flat panel made up of any number of color or monochrome pixels filled with liquid crystals and arrayed in front of a light source (backlight) or reflector. It is often utilized in battery-powered electronic devices because it uses very small amounts of electric power.
A comprehensive classification of the various types and electro-optical modes of LCDs is provided in the article LCD classification.
Overview
Each pixel of an LCD typically consists of a layer of molecules aligned between two transparent electrodes, and two polarizing filters, the axes of transmission of which are (in most of the cases) perpendicular to each other. With no actual 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, for example, a cloth. The direction of the liquid crystal alignment is then defined by the direction of rubbing. Electrodes
are made of a transparent conductor called Indium Tin Oxide (ITO).
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. This reduces the rotation of the polarization of the incident light, and the device appears grey. 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 variations of thickness across the device.
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 either by applying 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 are needed in a display, it is not technically possible 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:
- Resolution: The horizontal and vertical size expressed in pixels (e.g., 1024x768). Unlike monochrome CRT monitors, LCD monitors have a native-supported resolution for best display effect.
- Dot pitch: The distance between the centers of two adjacent pixels. The smaller the dot pitch size, the less granularity is present, resulting in a sharper image. Dot pitch may be the same both vertically and horizontally, or different (less common).
- Viewable size: The size of an LCD panel measured on the diagonal (more specifically known as active display area).
- Response time: The minimum time necessary to change a pixel's color or brightness. Response time is also divided into rise and fall time. For LCD Monitors, this is measured in btb (black to black) or gtg (gray to gray). These different types of measurements make comparison difficult. A response time of <16ms is sufficient for video-gaming, and the difference between response times once they're below 10ms begin becoming imperceptible due to limitations of the human eye
- Refresh rate: The number of times per second in which the monitor draws the data it is being given. Since activated LCD pixels do not flash on/off between frames, LCD monitors exhibit no refresh-induced flicker, no matter how low the refresh rate. Many high-end LCD televisions now have a 120 Hz (current and former NTSC countries) or 200 Hz (PAL/SECAM countries) refresh rate. The rate of 120 was chosen as the least common multiple of 24 fps (cinema) and 30 fps (NTSC TV), and allows for less distortion when movies are viewed due to the elimination of telecine (3:2 pulldown). For PAL/SECAM at 25 fps, 200 Hz is used as a compromise of one-third the least common multiple of 600 (24 x 25). This is most effective from a 24p-source video output (available on Blu-ray DVD).
- Matrix type: Active TFT or Passive.
- Viewing angle: (coll., more specifically known as viewing direction).
- Color 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: The ratio of the intensity of the brightest bright to the darkest dark.
- Aspect ratio: The ratio of the width to the height (for example, 4:3, 5:4, 16:9 or 16:10).
- Input ports (e.g., DVI, VGA, LVDS, DisplayPort, or even S-Video and HDMI).
Displays that have built-in TV Tuners or decoders may feature
Brief history
- 1888: Friedrich Reinitzer (1858-1927) discovers the liquid crystalline nature of cholesterol extracted from carrots (that is, two melting points and generation of colors) and published his findings at a meeting of the Vienna Chemical Society on May 3, 1888 (F. Reinitzer: Beiträge zur Kenntniss des Cholesterins, Monatshefte für Chemie (Wien) 9, 421-441 (1888)).
- 1904: Otto Lehmann publishes his work "Flüssige Kristalle" (Liquid Crystals).
- 1911: Charles Mauguin first experiments of liquids crystals confined between plates in thin layers.
- 1922: George Friedel describes the structure and properties of liquid crystals and classified them in 3 types (nematics, smectics and cholesterics).
- 1962: The first major English language publication on the subject "Molecular Structure and Properties of Liquid Crystals", by Dr. George W. Gray.
- 1962: Richard Williams of RCA found that liquid crystals had some interesting electro-optic characteristics and he realized an electro-optical effect by generating stripe-patterns in a thin layer of liquid crystal material by the application of a voltage. This effect is based on an electro-hydrodynamic instability forming what is now called “Williams domains” inside the liquid crystal.
- 1964: In the fall of 1964 George H. Heilmeier, then working in the RCA laboratories on the effect discovered by Williams realized the switching of colors by field-induced realignment of dichroic dyes in a homeotropically oriented liquid crystal. Practical problems with this new electro-optical effect made Heilmeier to continue work on scattering effects in liquid crystals and finally the realization of the first operational liquid crystal display based on what he called the dynamic scattering mode (DSM). Application of a voltage to a DSM display switches the initially clear transparent liquid crystal layer into a milky turbid state. DSM displays could be operated in transmissive and in reflective mode but they required a considerable current to flow for their operation. George H. Heilmeier was inducted in the National Inventors Hall of Fame and credited with the invention of LCD.
- 1960s: 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).
- 1970: On December 4, 1970, the twisted nematic field effect in liquid crystals was filed for patent by Hoffmann-LaRoche in Switzerland, () with Wolfgang Helfrich and Martin Schadt (then working for the Central Research Laboratories) listed as inventors. Hoffmann-La Roche then licensed the invention to the Swiss manufacturer Brown, Boveri & Cie who produced displays for wrist watches during the 1970s and also to Japanese electronics industry which soon produced the first digital quartz wrist watches with TN-LCDs and numerous other products. James Fergason at the Westinghouse Research Laboratories in Pittsburgh while working with Sardari Arora and Alfred Saupe at Kent State University Liquid Crystal Institute filed an identical patent in the USA on April 22, 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 to improvements of lower operating voltages and lower power consumption.
- 1972: The first active-matrix liquid crystal display panel was produced in the United States by T. Peter Brody.
- 2007: In the 4Q of 2007 for the first time LCD surpassed CRT in worldwide sales.
- 2008: LCD TVs are the mainstream with 50% market share of the 200 million TVs forecast to ship globally in 2008 according to Display Bank.
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". Another report on the origins and history of LCD from a different perspective has been published by Hiroshi Kawamoto, available at the IEEE History Center.
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. CRT monitors employ a similar 'subpixel' structures via phosphors, although the 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 the 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.
To reduce smudging in a moving picture when pixels do not respond quickly enough to color changes, so-called pixel overdrive may be used.
Passive-matrix and active-matrix addressed LCDs
LCDs with a small number of segments, such as those used in digital watches and pocket calculators, 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—the latter of which addresses a color-shifting problem with the former—and color-STN (CSTN)—wherein color is added by using an internal 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 times 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 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 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
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 comprehensive 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.
Advanced Fringe Field Switching (AFFS)
Advanced Fringe Field Switching is a similar technology to IPS or S-IPS offering superior performance and color gamut besides high luminosity. AFFS is developed by Boe Hydis Displays, Korea.
AFFS-applied notebook applications minimize color distortion while maintaining its superior wide viewing angle for a professional display. Color shift and deviation caused by light leakage is corrected by optimizing the white gamut which also enhances white/grey reproduction.
In premium IBM ThinkPad series notebooks, Boe Hydis AFFS displays are used to provide higher resolutions up to 1600x1200 in a relatively small 15 inch display setting. IBM also advertised these high end panels under their FlexViewTM label. AFFS panels are mostly classified under the VIEWIZTM name by Boe Hydis resembling premium performance.
As of 2008, Hitachi acquired AFFS license to manufacture high end panels in their product line. Boe Hydis suspended their production of high quality displays however the company still advertises the benefits of the superior technology.
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.
Blue Phase mode
Blue phase LCDs do not require an LC top layer. Blue phase LCDs are relatively new to the market,and very expensive because of the low volume of production. They provide a higher refresh rate than normal LCDs, but normal LCDs are still cheaper to make and actually provide better colors and a sharper image.
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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 (ICs), 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. Currently, though, Samsung adheres to the less restrictive ISO 13406-2 standard. Other companies have been known to tolerate as many as 11 dead pixels in their policies. 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. 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 the example to the right, 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", which is an extra screening process which can then determine "A" and "B" grade panels. Many manufacturers 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. It is most visible in dark or black areas of displayed scenes. Defects in various LCD panel components can cause mura effect.
Zero-power (bistable) 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 demonstrated two new types of zero-power bistable LCDs based on Zenithal bistable techniques.
Several bistable technologies, like the 360° BTN and the bistable cholesteric, depend mainly on the bulk properties of the liquid crystal (LC) and use standard strong anchoring, with alignment films and LC mixtures similar to the traditional monostable materials. Other bistable technologies (i.e. Binem Technology) are based mainly on the surface properties and need specific weak anchoring materials.
See Ferro Liquid Display for more information about ferro fluid based bistable displays.
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" and is susceptible in general to multiple kinds of HDTV blur. Many LCDs are incapable of displaying very low resolution screen modes (such as 320x200) due to these scaling limitations.
- Smaller color gamut compared to CRT and Plasma displays.
- Some types of LCD displays have a more limited color resolution than advertised, and must use spatial and/or temporal dithering to increase the apparent color depth. This can cause a shimmering effect with some types of displays which can be distracting for some users.
- 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 or even a bluish / purple tint with TN-film based displays. However, as of December 2007, the very best LCDs can approach the contrast ratios of plasma displays in terms of delivering a deep black.
- 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 quickly on an LCD, multiple cursors can sometimes be seen.
- LCDs appear to exhibit motion blur as the human eye follows moving objects, where CRT screens do not. This is because an individual LCD pixel is constantly visible for the entire duration of a frame (typically 16.7ms), whereas a CRT pixel is lit for only a fraction of a microsecond once per frame as the electron beam scans past it. The means that even on a hypothetical LCD panel with a response time of zero, a panning image will appear to have motion blur whereas a panning image on a CRT monitor will not. This is caused by the movement of our eyes during the time the frame is visible.
- Some LCD TVs have significant input lag due to slow video processing. 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 other LCD panels with negligible amounts of input lag. Some LCD TVs have a "game mode" (the term used by Sony) that reduces both the amount of video processing and the amount of input lag.
- LCD panels using TN tend to have a limited viewing angle relative to CRT and plasma displays. This reduces the number of people able to conveniently view the same image – laptop screens are a prime example. Usually when looking below the screen, it gets much darker; looking from above makes it look lighter. Many displays based on thin film transistor variants such as IPS, MVA, or PVA, have much improved viewing angles; typically the color only gets a little brighter when viewing at extreme angles.
- 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.
- Dead pixels can occur when the screen is damaged or pressure is put upon the screen; few manufacturers replace screens with dead pixels under warranty.
- Horizontal and/or vertical banding is a problem in some LCD screens. This flaw occurs as part of the manufacturing process, and cannot be repaired (short of total replacement of the screen). Banding can vary substantially even among LCD screens of the same make and model. The degree is determined by the manufacturer's quality control procedures.
- The cold cathode fluorescent lamps typically used for back-lights in LCD screens contain mercury, a toxic substance, though LED-backlit LCD screens are mercury-free.
See also
Related technology
Other display technologies
Display applications
Manufacturers
External links - Tutorials
- ,LCD Interfacing with microcontroller
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- from Case Western Reserve University
- from the Liquid Crystals Group, University of Colorado
- from the Liquid Crystal Technology Group, Oxford University
- , good tutorial
- Liquid Crystals Interactive Online (not updated since 1999)
- Kent State University
- a journal by Taylor&Francis
- a journal by Taylor&Francis
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- from Chalmers University of Technology, Sweden
General information
- , TFT LCD guide for dummies.
- , an interactive demonstration from AUO (LCD manufacturer).
- – Video by the Vega Science Trust.
- – Presentation and extracts from the book Crystals that Flow: Classic papers from the history of liquid crystals by its co-author Timothy J. Sluckin
- , Presentation Technology
- , Diamond Electronics
- , Techmind offers a free test screen to get a better LCD picture quality than the LCDs "auto-tune" function.
- About.com: PC Support
- LCD Monitor Reviews, Specs, Articles and News
- - LCD Monitor and LCD-TV Reviews, Articles and News
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