A
liquid crystal display (
LCD) is a
flat panel displayFlat panel displays encompass a growing number of electronic visual display technologies. They are far lighter and thinner than traditional television sets and video displays that use cathode ray tubes , and are usually less than thick...
,
electronic visual displayAn electronic visual display is display technology which incorporates flat panel displays, performs as a video display, output device for presentation of images transmitted electronically, for visual reception, without producing a permanent record....
, or video display that uses the light modulating properties of liquid crystals (LCs). LCs do not emit light directly.
They are used in a wide range of applications, including computer monitors,
televisionTelevision is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
, instrument panels,
aircraft cockpit displaysFlight instruments are the instruments in the cockpit of an aircraft that provide the pilot with information about the flight situation of that aircraft, such as height, speed and altitude...
,
signageSignage is any kind of visual graphics created to display information to a particular audience. This is typically manifested in the form of wayfinding information in places such as streets or inside/outside of buildings.-History:...
, etc. They are common in consumer devices such as video players, gaming devices,
clockA clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...
s, watches,
calculatorAn electronic calculator is a small, portable, usually inexpensive electronic device used to perform the basic operations of arithmetic. Modern calculators are more portable than most computers, though most PDAs are comparable in size to handheld calculators.The first solid-state electronic...
s, and
telephoneThe telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
s. LCDs have replaced
cathode ray tubeThe cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
(CRT) displays in most applications.
They are available in a wider range of screen sizes than CRT and
plasma displayA plasma display panel is a type of flat panel display common to large TV displays or larger. They are called "plasma" displays because the technology utilizes small cells containing electrically charged ionized gases, or what are in essence chambers more commonly known as fluorescent...
s, and since they do not use phosphors, they cannot suffer image burn-in.
LCDs are more energy efficient and offer safer disposal than CRTs. Its low electrical power consumption enables it to be used in
batteryAn electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
-powered
electronicElectronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
equipment. It is an
electronically modulated optical deviceElectro-optic modulator is an optical device in which a signal-controlled element displaying electro-optic effect is used to modulate a beam of light. The modulation may be imposed on the phase, frequency, amplitude, or polarization of the modulated beam...
made up of any number of segments filled with
liquid crystalLiquid crystals are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be...
s and arrayed in front of a light source (
backlightA backlight is a form of illumination used in liquid crystal displays . As LCDs do not produce light themselves , they need illumination to produce a visible image...
) or
reflectorIn photography and cinematography, a reflector is an improvised or specialised reflective surface used to redirect light towards a given subject or scene.- Types :...
to produce images in color or
monochromeMonochrome describes paintings, drawings, design, or photographs in one color or shades of one color. A monochromatic object or image has colors in shades of limited colors or hues. Images using only shades of grey are called grayscale or black-and-white...
. The most flexible ones use an array of small
pixelIn digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
s. The earliest discovery leading to the development of LCD technology, the discovery of liquid crystals, dates from 1888.
By 2008, worldwide sales of televisions with LCD screens had surpassed the sale of CRT units.
Overview
Each
pixelIn digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
of an LCD typically consists of a layer of
moleculeA molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s aligned between two transparent
electrodeAn electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s, and two
polarizingA polarizer is an optical filter that passes light of a specific polarization and blocks waves of other polarizations. It can convert a beam of light of undefined or mixed polarization into a beam with well-defined polarization. The common types of polarizers are linear polarizers and circular...
filterOptical filters are devices which selectively transmit light of different wavelengths, usually implemented as plane glass or plastic devices in the optical path which are either dyed in the mass or have interference coatings....
s, the axes of transmission of which are (in most of the cases) perpendicular to each other. With no actual
liquid crystalLiquid crystals are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be...
between the polarizing filters,
lightLight or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
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
polymerA polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
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 OxideIndium tin oxide is a solid solution of indium oxide and tin oxide , typically 90% In2O3, 10% SnO2 by weight. It is transparent and colorless in thin layers while in bulk form it is yellowish to grey...
(ITO).
The Liquid Crystal Display is intrinsically a “passive” device, it is a simple light valve. The managing and control of the data to be displayed is performed by one or more circuits commonly denoted as
LCD drivers.
Before applying an
electric fieldIn physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
, the orientation of the liquid crystal molecules is determined by the alignment at the surfaces of electrodes. 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
helicalA helix is a type of smooth space curve, i.e. a curve in three-dimensional space. It has the property that the tangent line at any point makes a constant angle with a fixed line called the axis. Examples of helixes are coil springs and the handrails of spiral staircases. A "filled-in" helix – for...
structure, or twist. This reduces the rotation of the polarization of the incident light, and the device appears
greyGrey or gray is an achromatic or neutral color.Complementary colors are defined to mix to grey, either additively or subtractively, and many color models place complements opposite each other in a color wheel. To produce grey in RGB displays, the R, G, and B primary light sources are combined in...
. 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
perpendicularIn geometry, two lines or planes are considered perpendicular to each other if they form congruent adjacent angles . The term may be used as a noun or adjective...
to the second filter, and thus be blocked and the
pixelIn digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
will appear
blackBlack is the color of objects that do not emit or reflect light in any part of the visible spectrum; they absorb all such frequencies of light...
. 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 compoundIn chemistry, an ionic compound is a chemical compound in which ions are held together in a lattice structure by ionic bonds. Usually, the positively charged portion consists of metal cations and the negatively charged portion is an anion or polyatomic ion. Ions in ionic compounds are held together...
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 either by applying an
alternating currentIn alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
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).
Displays for a small number of individual digits and/or fixed symbols (as in digital watches, pocket calculators etc.) can be implemented with independent electrodes for each segment. In contrast full
alphanumericAlphanumeric is a combination of alphabetic and numeric characters, and is used to describe the collection of Latin letters and Arabic digits or a text constructed from this collection. There are either 36 or 62 alphanumeric characters. The alphanumeric character set consists of the numbers 0 to...
and/or variable graphics displays are usually implemented with pixels arranged as a matrix consisting of electrically connected rows on one side of the LC layer and columns on the other side, which makes it possible to address each pixel at the intersections. The general method of matrix addressing consists of sequentially addressing one side of the matrix, for example by selecting the rows one-by-one and applying the picture information on the other side at the columns row-by-row.
For details on the various matrix addressing schemes see Passive-matrix and active-matrix addressed LCDs.
Brief history
- 1888: Friedrich Reinitzer
Friedrich Richard Reinitzer was an Austrian botanist and chemist. In late 1880s, experimenting with cholesteryl benzoate, he discovered properties of liquid crystals ....
(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
Otto Lehmann was a German physicist and "father" of liquid crystal technology.-Life:...
publishes his work "Flüssige Kristalle" (Liquid Crystals).
- 1911: Charles Mauguin first experiments of liquids crystals confined between plates in thin layers.
- 1922: Georges Friedel
Georges Friedel was a French mineralogist and crystallographer.- Life :Georges was the son of the famous chemist Charles Friedel...
describes the structure and properties of liquid crystals and classified them in 3 types (nematics, smectics and cholesterics).
- 1927: Vsevolod Frederiks
Vsevolod Konstantinovich Frederiks was a Russian/Soviet physicist. His primary contribution to physics was in the field of liquid crystals...
devises the electrically switched light valve, called the Fréedericksz transitionThe Fréedericksz transition is a phase transition in liquid crystals produced when a sufficiently strong electric or magnetic field is applied to a liquid crystal in an undistorted state. Below a certain field threshold the director remains undistorted. As the field value is gradually increased...
, the essential effect of all LCD technology.
- 1936: The Marconi Wireless Telegraph company
The Marconi Company Ltd. was founded by Guglielmo Marconi in 1897 as The Wireless Telegraph & Signal 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.
- 1962: Richard Williams of RCA
RCA Corporation, founded as the Radio Corporation of America, was an American electronics company in existence from 1919 to 1986. The RCA trademark is currently owned by the French conglomerate Technicolor SA through RCA Trademark Management S.A., a company owned by Technicolor...
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: George H. Heilmeier
George Harry Heilmeier is an American engineer and businessman, who was a pioneering contributor to liquid crystal displays.-Biography:...
, then working in the RCA laboratories on the effect discovered by Williams achieved 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 continue to work on scattering effects in liquid crystals and finally the achievement of the first operational liquid crystal display based on what he called the dynamic scattering modeGeorge Heilmeier proposed the dynamic scattering effect which causes a strong scattering of light when the electric field applied to a special liquid crystal mixture exceeds a threshold value.A DSM cell requires the following ingredients:...
(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. HeilmeierGeorge Harry Heilmeier is an American engineer and businessman, who was a pioneering contributor to liquid crystal displays.-Biography:...
was inducted in the National Inventors Hall of Fame and credited with the invention of LCD. Heilmeier's work is an IEEE Milestone.
- 1960s: Pioneering work on liquid crystals was undertaken in the late 1960s by the UK
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
's Royal Radar EstablishmentThe name Royal Radar Establishment was given to the existing Radar Research Establishment following a visit by Queen Elizabeth II in 1957. Both names were abbreviated to RRE. The establishment had been formed, under its first name, in 1953 by merging the Telecommunications Research Establishment ...
at MalvernMalvern is a town and civil parish in Worcestershire, England, governed by Malvern Town Council. As of the 2001 census it has a population of 28,749, and includes the historical settlement and commercial centre of Great Malvern on the steep eastern flank of the Malvern Hills, and the former...
, England. The team at RRE supported ongoing work by George Gray and his team at the University of HullThe University of Hull, known informally as Hull University, is an English university, founded in 1927, located in Hull, a city in the East Riding of Yorkshire...
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
The twisted nematic effect was a main technology breakthrough that made liquid crystal displays practical. Unlike earlier displays, TN-cells did not require a current to flow for operation and used low operating voltages suitable for use with batteries...
in liquid crystals was filed for patent by Hoffmann-LaRoche in SwitzerlandSwitzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
, (Swiss patent No. 532 261) with Wolfgang Helfrich and Martin Schadt-Biography:In 1970 the physicists Martin Schadt and Wolfgang Helfrich invented the twisted nematic field effect in the Central Research Laboratories of F. Hoffmann-La Roche Ltd, in Basel, Switzerland...
(then working for the Central Research Laboratories) listed as inventors. Hoffmann-La Roche then licensed the invention to the Swiss manufacturer Brown, Boveri & CieBrown, Boveri & Cie was a Swiss group of electrical engineering companies.It was founded in Baden, Switzerland, in 1891 by Charles Eugene Lancelot Brown and Walter Boveri who worked at the Maschinenfabrik Oerlikon. In 1970 BBC took over the Maschinenfabrik Oerlikon...
who produced displays for wristwatches 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 FergasonJames L. Fergason was an inventor of an improved Liquid Crystal Display, or LCD.Fergason graduated from Carrollton High School in Carrollton, Missouri...
, while working with Sardari Arora and Alfred Saupe-Biography:Alfred Saupe was a German Physicist born in Badenweiler, who laid groundbreaking work in the area of liquid crystal studies....
at Kent State UniversityKent State University is a public research university located in Kent, Ohio, United States. The university has eight campuses around the northeast Ohio region with the main campus in Kent being the largest...
Liquid Crystal InstituteThe Glenn H. Brown Liquid Crystal Institute at Kent State University is a center of study for liquid crystal technology and education, blending basic and applied research on liquid crystals...
, filed an identical patent in the United States on April 22, 1971. In 1971 the company of Fergason ILIXCO (now LXD IncorporatedLXD, Incorporated is one of the first Liquid Crystal Display manufacturers in the world. The company has factories in the United States and China, including its corporate headquarters in Cleveland, Ohio.-History:...
) 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 Westinghouse
Westinghouse Electric was an American manufacturing company. It was founded in 1886 as Westinghouse Electric Company and later renamed Westinghouse Electric Corporation by George Westinghouse. The company purchased CBS in 1995 and became CBS Corporation in 1997...
, in Pittsburgh, PennsylvaniaPittsburgh is the second-largest city in the US Commonwealth of Pennsylvania and the county seat of Allegheny County. Regionally, it anchors the largest urban area of Appalachia and the Ohio River Valley, and nationally, it is the 22nd-largest urban area in the United States...
.
- 1983: Researchers at Brown, Boveri & Cie
Brown, Boveri & Cie was a Swiss group of electrical engineering companies.It was founded in Baden, Switzerland, in 1891 by Charles Eugene Lancelot Brown and Walter Boveri who worked at the Maschinenfabrik Oerlikon. In 1970 BBC took over the Maschinenfabrik Oerlikon...
(BBC), SwitzerlandSwitzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
, invent the super-twisted nematic (STN) structure for passive-matrix addressed LCDs. H. Amstutz et al. were listed as inventors in the corresponding patent applications filed in Switzerland on July 7, 1983, and October 28, 1983. Patents were granted in Switzerland CH 665491, Europe EP 0131216, and many more countries. Scientific details are published in the article referred to
- 1990: Under different titles inventors conceived electrooptical effects as alternatives to twisted nematic field effect LCDs (TN- and STN- LCDs). One approach was to use interdigital electrodes on one glass substrate only to produce an electric field essentially parallel to the glass substrates (Abstract). To take full advantage of the properties of this In-Plane Switching (IPS) technology further work was needed. After thorough analysis, details of advantageous embodiments are filed in Germany by Guenter Baur et al. and patented in various countries (Abstract). The Fraunhofer Institute in Freiburg, where the inventors worked, assigns these patents to Merck KGaA, Darmstadt, the world's leading supplier of LC substances.
- 1992: Shortly thereafter, engineers at Hitachi work out various practical details of the IPS technology to interconnect the thin-film transistor array as a matrix and to avoid undesirable stray fields in between pixels (Abstract). Hitachi also improves the viewing angle dependence further by optimizing the shape of the electrodes (Super IPS).
- NEC and Hitachi become early manufacturers of active-matrix addressed LCDs based on the IPS technology. This is a milestone for implementing large-screen LCDs having acceptable visual performance for flat-panel computer monitors and television screens.
- 1996 Samsung
The Samsung Group is a South Korean multinational conglomerate corporation headquartered in Samsung Town, Seoul, South Korea...
develops the optical patterning technique that enables multi-domain LCD. Multi-domain and In Plane Switching subsequently remain the dominant LCD designs through 2010.
- 2007: In the 4Q of 2007 for the first time LCD televisions surpass CRT units in worldwide sales.
- 2008: LCD TVs become the majority with a 50% market share of the 200 million TVs forecast to ship globally in 2008 according to Display Bank.
- In October 2011, Toshiba
is a multinational electronics and electrical equipment corporation headquartered in Tokyo, Japan. It is a diversified manufacturer and marketer of electrical products, spanning information & communications equipment and systems, Internet-based solutions and services, electronic components and...
has announced 2560x1600 pixels on an 6.1-inch LCD panel, suitable for use in a tablet computerA tablet computer, or simply tablet, is a complete mobile computer, larger than a mobile phone or personal digital assistant, integrated into a flat touch screen and primarily operated by touching the screen...
, especially for Chinese character display.
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 until 1991 has been published by Hiroshi Kawamoto, available at the IEEE History Center.
Illumination
As LCD panels produce no light of their own, they require an external lighting mechanism to be easily visible. On most displays, this consists of a cold cathode
fluorescent lampA fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to excite mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that then causes a phosphor to fluoresce, producing visible light. A fluorescent lamp converts electrical power into useful...
that is situated behind the LCD panel. Passive-matrix displays are usually not backlit, but active-matrix displays almost always are, with a few exceptions such as the display in the original Gameboy Advance.
Recently, two types of
LED backlit displaysAn LED-backlight LCD television is an LCD television, flat panel display that uses LED backlighting instead of the cold cathode backlighting used in other LCD televisions. While not an LED display, it is called "LED TV" by some manufacturers and suppliers...
have appeared in some televisions as an alternative to conventional backlit LCDs. In one scheme, the LEDs are used to backlight the entire LCD panel. In another scheme, a set of red, green and blue LEDs is used to illuminate a small cluster of pixels, which can improve contrast and black level in some situations. For example, the LEDs in one section of the screen can be dimmed to produce a dark section of the image while the LEDs in another section are kept bright. Both schemes also allows for a slimmer panel than on conventional displays.
Passive-matrix and active-matrix addressed LCDs
Monochrome passive-matrix LCDs were standard in most early laptops (although a few used plasma displays) and the original Nintendo
Game BoyThe , is an 8-bit handheld video game device developed and manufactured by Nintendo. It was released in Japan on , in North America in , and in Europe on...
until the mid-1990s, when color active-matrix became standard on all laptops. The commercially unsuccessful Macintosh Portable (released in 1989) was one of the first to use an active-matrix display (though still monochrome).
Passive-matrix LCDs are still used today for applications less demanding than laptops and TVs. In particular, portable devices with less information content to be displayed, where lowest power consumption (no
backlightA backlight is a form of illumination used in liquid crystal displays . As LCDs do not produce light themselves , they need illumination to produce a visible image...
), low cost and/or readability in direct sunlight are needed, use this type of display.
Displays having a passive-matrix structure are 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) in which color is added by using an internal filter.
STN LCDs have been optimized for passive-matrix addressing. They exhibit a sharper threshold of the contrast-vs-voltage characteristic than the original TN LCDs. This is important, because pixels are subjected to partial voltages even while not selected. Crosstalk between activated and non-activated pixels has to be handled properly by keeping the RMS voltage of non-activated pixels below the threshold voltage, while activated pixels are subjected to voltages above threshold. STN LCDs have to be continuously refreshed by alternating pulsed voltages of one polarity during one frame and pulses of opposite polarity during the next frame. Individual pixels are addressed by the corresponding row and column circuits. This type of display is called
passive-matrix addressedPassive matrix addressing is an addressing scheme used in earlier LCD displays, and may be used in future LCD displays. This is a matrix addressing scheme meaning that only m + n control signals are required to address a m × n display...
, 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. Slow response times and poor
contrastContrast in visual perception is the difference in appearance of two or more parts of a field seen simultaneously or successively ....
are typical of passive-matrix addressed LCDs with too many pixels.
New
zero-power (bistable) LCDs do not require continuous refreshing. Rewriting is only required for picture information changes. Potentially, passive-matrix addressing can be used with these new devices, if their write/erase characteristics are suitable.
High-
resolutionThe display resolution of a digital television or display device is the number of distinct pixels in each dimension that can be displayed. It can be an ambiguous term especially as the displayed resolution is controlled by all different factors in cathode ray tube , flat panel or projection...
color displays, such as modern LCD
computer monitorsA monitor or display is an electronic visual display for computers. The monitor comprises the display device, circuitry, and an enclosure...
and televisions, use an
active matrixActive matrix is a type of addressing scheme used in flat panel displays. The term describes a method of switching individual elements of a flat panel display, using a CdSe or Silicon-based thin-film transistor for each pixel...
structure. A matrix of
thin-film transistorA thin-film transistor is a special kind of field-effect transistor made by depositing thin films of a semiconductor active layer as well as the dielectric layer and metallic contacts over a supporting substrate. A common substrate is glass, since the primary application of TFTs is in liquid...
s (TFTs) is added to the electrodes in contact with the LC layer. Each pixel has its own dedicated
transistorA transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
, allowing each column line to access one pixel. When a row line is selected, all of the column lines are connected to a row of pixels and voltages corresponding to the picture information are driven onto all of the column lines. The row line is then deactivated and the next row line is selected. All of the row lines are selected in sequence during a
refreshThe refresh rate is the number of times in a second that a display hardware draws the data...
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 crystals that twist and untwist at varying degrees to allow light to pass through. When no voltage is applied to a TN liquid crystal cell, polarized light passes through the 90-degrees twisted LC layer. In proportion to the voltage applied, the liquid crystals untwist 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 that aligns the liquid crystals in a plane parallel to the glass substrates. In this method, the electrical field is applied through opposite electrodes on the same glass substrate, so that the liquid crystals can be reoriented (switched) in the same plane. This requires two transistors for each pixel instead of the single transistor needed for a standard thin-film transistor (TFT) display. Before
LGLG may refer to:*LG Corp., a South Korean electronics and petrochemicals conglomerate*LG Electronics, an affiliate of the South Korean LG Group which produces electronic products* Lawrence Graham, a London headquartered firm of business lawyers...
Enhanced IPS was introduced in 2009, the additional transistors resulted in blocking more transmission area, thus requiring a brighter backlight and consuming more power, making this type of display less desirable for notebook computers. This newer, lower power technology can be found in the Apple iMac, iPad, and iPhone 4, the Hewlett-Packard
EliteBookHP EliteBook is a brand of premium business-class notebooks and mobile workstations made by Hewlett-Packard. The EliteBook series, which slots above the lower-end ProBook series, was introduced in August 2008.-Features:...
mobile workstations and the
Nokia 701The Nokia 701 is a multi-touch Symbian Belle smartphone released in Q4 2011. It has a very bright and robust 3.5" IPS-LCD display with 640 x 360 pixels. The brightness of the display is 1000 nits and the used Gorilla Glass makes the display very robust. Nokia announced the phone on 24 August...
. Currently Panasonic is using an enhanced version eIPS for their large size LCD-TV products as well as Hewlett-Packard in its WebOS based TouchPad tablet.
IPS LCD vs AMOLED
LG claimed the smartphone LG Optimus Black with an IPS LCD (LCD NOVA) has the brightness up to 700 nits, while the competitor has only IPS LCD with 518 nits and tumble an
Active-matrix OLEDAMOLED is a display technology for use in mobile devices and televisions. OLED describes a specific type of thin-film display technology in which organic compounds form the electroluminescent material, and active matrix refers to the technology behind the addressing of pixels.As of 2011, AMOLED...
(AMOLED) display with 305 nits. LG also claimed the NOVA display to be 50 percent more efficient than regular LCDs and to consume only 50 percent of the power of AMOLED displays when producing white on screen. When it comes to contrast ratio, AMOLED display still performs best due to its underlying technology, where the black levels are displayed as pitch black and not as dark grey. On August 24, 2011, Nokia announced the Nokia 701 and also made the claim of the world's brightest display at 1000 nits. The screen also had Nokia's Clearblack layer, improving the contrast ratio and bringing it closer to that of the AMOLED screens.
Advanced fringe field switching (AFFS)
Known as fringe field switching (FFS) until 2003,
advanced fringe field switching is similar to IPS or S-IPS offering superior performance and color gamut with high luminosity. AFFS was developed by Hydis Technologies Co., Ltd, Korea (formally Hyundai Electronics, LCD Task Force).
AFFS-applied notebook applications minimize color distortion while maintaining a wider 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 2004, Hydis Technologies Co.,Ltd licensed AFFS to Japan's Hitachi Displays. Hitachi is using AFFS to manufacture high-end panels. In 2006, HYDIS licensed AFFS to Sanyo Epson Imaging Devices Corporation.
Shortly thereafter, Hydis introduced a high-transmittance evolution of the AFFS display, called HFFS (FFS+).
Hydis introduced AFFS+ with improved outdoor readability in 2007. AFFS panels are mostly utilized in the cockpits of latest commercial aircraft displays.
Vertical alignment (VA)
Vertical alignment displays are a form of LCDs in which the liquid crystals naturally align vertically to the glass substrates. When no voltage is applied, the liquid crystals remain perpendicular to the substrate creating a black display between crossed polarizers. When voltage is applied, the liquid crystals shift to a tilted position allowing light to pass through and create a gray-scale display depending on the amount of tilt generated by the electric field.
Blue Phase mode
Blue phase mode LCDs have been shown as engineering samples early in 2008, but they are not in mass-production yet. The physics of blue phase mode LCDs suggest that very short switching times (~1 ms) can be achieved, so time sequential color control can possibly be realized and expensive color filters would be obsolete. For details refer to
Blue Phase Mode LCDA Blue Phase Mode LCD is a liquid crystal display technology that uses highly twisted cholesteric phases in a blue phase. It was first proposed in 2007 to obtain a better display of moving images with, for example, frame rates of 100–120 Hz to improve the temporal response of liquid crystal...
.
Quality control
Some LCD panels have defective
transistorA transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
s, 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 transistors are usually still usable. Manufacturers' policies for the acceptable number of defective pixels vary greatly. At one point, Samsung held a zero-tolerance policy for LCD monitors sold in Korea. As of 2005, though, Samsung adheres to the less restrictive
ISO 13406-2ISO 13406-2 is an ISO standard, with the full title "Ergonomic requirements for work with visual displays based on flat panels -- Part 2: Ergonomic requirements for flat panel displays". It is best known to end consumers for defining a series of flat-panel display "classes" with different numbers...
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 13406-2ISO 13406-2 is an ISO standard, with the full title "Ergonomic requirements for work with visual displays based on flat panels -- Part 2: Ergonomic requirements for flat panel displays". It is best known to end consumers for defining a series of flat-panel display "classes" with different numbers...
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. For 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. In recent years, quality control has been improved. 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.
LCD panels also have defects known as
clouding (or less commonly
muraMura is traditional general Japanese term for unevenness, irregularity or inconsistency in physical matter or human spiritual condition. It is also a key concept in performance improvement systems such as the Toyota Production System. Mura is one of the three types of waste . Waste reduction is an...
), which describes the uneven patches of changes in
luminanceLuminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
. It is most visible in dark or black areas of displayed scenes.
Zero-power (bistable) displays
The zenithal bistable device (ZBD), developed by
QinetiQQinetiq is a British global defence technology company, formed from the greater part of the former UK government agency, Defence Evaluation and Research Agency , when it was split up in June 2001...
(formerly
DERAThe Defence Evaluation and Research Agency was a part of the UK Ministry of Defence until July 2, 2001. At the time it was the United Kingdom's largest science and technology organisation...
), 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.
Kent Displays has also developed a "no power" display that uses polymer stabilized
cholesteric liquid crystalA cholesteric liquid crystal is a type of liquid crystal with a helical structure and which is therefore chiral. Cholesteric liquid crystals are also known as chiral nematic liquid crystals. They organize in layers with no positional ordering within layers, but a director axis which varies with...
(ChLCD). Kent has recently demonstrated the use of a ChLCD to cover the entire surface of a
mobile phoneA mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
, allowing it to change colors, and keep that color even when power is cut off.
In 2004 researchers at the
University of OxfordThe University of Oxford is a university located in Oxford, United Kingdom. It is the second-oldest surviving university in the world and the oldest in the English-speaking world. Although its exact date of foundation is unclear, there is evidence of teaching as far back as 1096...
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, e.g., BiNem® technology, are based mainly on the surface properties and need specific weak anchoring materials.
Specifications
Important factors to consider when evaluating an LCD:
- Resolution versus range: Fundamentally resolution is the granularity (or number of levels) with which a performance feature of the display is divided. Resolution is often confused with range or the total end-to-end output of the display. Each of the major features of a display has both a resolution and a range that are tied to each other but very different. Frequently the range is an inherent limitation of the display while the resolution is a function of the electronics that make the display work.
- Spatial performance: LCDs come in only one size for a variety of applications and a variety of resolutions within each of those applications. LCD spatial performance is also sometimes described in terms of a "dot pitch". The size (or spatial range) of an LCD is always described in terms of the diagonal distance from one corner to its opposite. This is an historical remnant from the early days of CRT television when CRT screens were manufactured on the bottoms of glass bottles, a direct extension of cathode ray tubes used in oscilloscope
An oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time,...
s. The diameter of the bottle determined the size of the screen. Later, when televisions went to a more square format, the square screens were measured diagonally to compare with the older round screens.
The spatial resolution of an LCD is expressed in terms of the number of columns and rows of pixels (e.g., 1024×768). This had been one of the few features of LCD performance that was easily understood and not subject to interpretation. Each pixel is usually composed of a red, green, and blue sub pixel. However there are newer schemes to share sub-pixels among pixels and to add additional colors of sub-pixels. So going forward, spatial resolution may be more subject to interpretation.
One external factor to consider in evaluating display resolution is the resolution of your own eyes. For a normal person with 20/20 vision, the resolution of your eyes is about one minute of arc. In practical terms that means for an older standard definition TV set the ideal viewing distance was about 8 times the height (not diagonal) of the screen away. At that distance the individual rows of pixels merge into a solid. If you were closer to the screen than that, you would be able to see the individual rows of pixels. If you are further away, the image of the rows of pixels still merge, but the total image becomes smaller as you get further away. For an HDTV set with slightly more than twice the number of rows of pixels, the ideal viewing distance is about half what it is for a standard definition set. The higher the resolution, the closer you can sit to the set or the larger the set can usefully be sitting at the same distance as an older standard definition display.
For a computer monitor or some other LCD that is being viewed from a very close distance, resolution is often expressed in terms of
dot pitchDot pitch is a specification for a computer display, computer printer, image scanner, or other pixel-based device that describes the distance, for example, between dots of the same color on the inside of a display screen...
or pixels per inch. This is consistent with the printing industry (another form of a display). Magazines, and other premium printed media are often at 300 dots per inch. As with the distance discussion above, this provides a very solid looking and detailed image. LCDs, particularly on mobile devices, are frequently much less than this as the higher the dot pitch, the more optically inefficient the display and the more power it burns. Running the LCD is frequently half, or more, of the power consumed by a mobile device.
An additional consideration in spatial performance are viewing cone and aspect ratio. The
Aspect ratioThe aspect ratio of a shape is the ratio of its longer dimension to its shorter dimension. It may be applied to two characteristic dimensions of a three-dimensional shape, such as the ratio of the longest and shortest axis, or for symmetrical objects that are described by just two measurements,...
is the ratio of the width to the height (for example, 4:3, 5:4, 16:9 or 16:10). Older, standard definition TVs were 4:3. Newer High Definition televisions (HDTV) are 16:9, as are most new notebook computers. Movies are often filmed in much different (wider) aspect ratios, which is why there will frequently still be black bars at the top and bottom of an HDTV screen.
The
Viewing Angle-Viewing direction:When a visual display with non-vanishing size is seen by an observer, every point of the display area is seen from a different direction as illustrated in fig. 1. No two spots on the display are seen from the same direction...
of an LCD may be important depending on its use or location. The viewing angle is usually measured as the angle where the contrast of the LCD falls below 10:1. At this point, the colors usually start to change and can even invert, red becoming green and so forth. Viewing angles for LCDs used to be very restrictive however, improved optical films have been developed that give almost 180 degree viewing angles from left to right. Top to bottom viewing angles may still be restrictive, by design, as looking at an LCD from an extreme up or down angle is not a common usage model and these photons are wasted. Manufacturers commonly focus the light in a left to right plane to obtain a brighter image here.
- Temporal/timing performance: Contrary to spatial performance, temporal performance is a feature where smaller is better. Specifically, the range is the pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
response time of an LCD, or how quickly you can change a sub-pixel's brightness from one level to another. For LCD monitors, this is measured in btb (black to black) or gtg (gray to gray). These different types of measurements make comparison difficult. Further, this number is almost never published in sales advertising.
Refresh rateThe refresh rate is the number of times in a second that a display hardware draws the data...
or the temporal resolution of an LCD is the number of times per second in which the display 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. High-end LCD televisions now feature up to 240 Hz refresh rate, which requires advanced digital processing to insert additional interpolated frames between the real images to smooth the image motion. However, such high refresh rates may not be actually supported by pixel response times and the result can be visual artifacts that distort the image in unpleasant ways.
Temporal performance can be further taxed if it is a
3DA 3D television is a television set that employs techniques of 3D presentation, such as stereoscopic capture, multi-view capture, or 2D-plus-depth, and a 3D display – a special viewing device to project a television program into a realistic three-dimensional field.- History :In the late-1890's,...
display. 3D displays work by showing a different series of images to each eye, alternating from eye to eye. For a 3D display it must display twice as many images in the same period of time as a conventional display and consequently the response time of the LCD becomes more important. 3D LCDs with marginal response times, will exhibit image smearing.
It is actually greater in your black and white vision (
rod cellRod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...
s) than in color vision (
cone cellCone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...
s). You are more able to see flicker or any sort of temporal distortion in a display image by not looking directly at it as your rods are mostly grouped at the periphery of your vision.
- Color performance: There are many terms to describe color performance of an LCD. They include color gamut which is the range of colors that can be displayed and color depth which is the color resolution or the resolution or fineness with which the color range is divided. Although color gamut can be expressed as three pairs of numbers, the XY coordinates within color space
A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components...
of the reddest red, greenest green, and bluest blue, it is usually expressed as a ratio of the total area within color space that a display can show relative to some standard such as saying that a display was "120% of NTSC". NTSC is the National Television Standards Committee, the old standard definition TV specification. Color gamut is a relatively straight forward feature. However with clever optical techniques that are based on the way humans see color, termed color stretch, colors can be shown that are outside of the nominal range of the display. In any case, color range is rarely discussed as a feature of the display as LCDs are designed to match the color ranges of the content that they are intended to show. Having a color range that exceeds the content is a useless feature.
- Color depth or color support is sometimes expressed in bits, either as the number of bits per sub-pixel or the number of bits per pixel. This can be ambiguous as an 8-bit color LCD can be 8 total bits spread between red, green, and blue or 8 bits each for each color in a different display. Further, LCDs sometimes use a technique called dithering which is time averaging colors to get intermediate colors such as alternating between two different colors to get a color in between. This doubles the number of colors that can be displayed; however this is done at the expense of the temporal performance of the display. Dithering is commonly used on computer displays where the images are mostly static and the temporal performance is unimportant.
When color depth is reported as color support, it is usually stated in terms of number of colors the LCD can show. The number of colors is the translation from the base 2-bit numbers into common base-10. For example, 8-bit, in common terms means 2 to the 8th power or 256 colors. 8-bits per color or 24-bits would be 256 x 256 x 256 or over 16 Million colors. The color resolution of the human eye depends on both the range of colors being sliced and the number of slices; but for most common displays the limit is about 28-bit color. LCD TVs commonly display more than that as the digital processing can introduce color distortions and the additional levels of color are needed to ensure true colors.
There are additional aspects to LCD color and color management, such as
white pointA white point is a set of tristimulus values or chromaticity coordinates that serve to define the color "white" in image capture, encoding, or reproduction. Depending on the application, different definitions of white are needed to give acceptable results...
and
gamma correctionGamma correction, gamma nonlinearity, gamma encoding, or often simply gamma, is the name of a nonlinear operation used to code and decode luminance or tristimulus values in video or still image systems...
, which describe what color white is and how the other colors are displayed relative to white. LCD televisions also frequently have facial recognition software, which recognizes that an image on the screen is a face and both adjust the color and the focus differently from the rest of the image. These adjustments can have important effects on the consumer, but are not easily quantifiable; people like what they like and everyone does not like the same thing. There is no substitute for looking at the LCD one is going to buy before buying it. Portrait film, another form of display, has similar adjustments built in to it. Many years ago, Kodak had to overcome initial rejection of its portrait film in Japan because of these adjustments. In the U.S., people generally prefer a more colorful facial image than in reality (higher color saturation). In Japan, consumers generally prefer a less saturated image. The film that Kodak initially sent to Japan was biased in the wrong direction for Japanese consumers. Television monitors have their built-in biases as well.
- Brightness and contrast ratio: Contrast ratio
The contrast ratio is a property of a display system, defined as the ratio of the luminance of the brightest color to that of the darkest color that the system is capable of producing...
is the ratio of the brightness of a full-on pixel to a full-off pixel and, as such, would be directly tied to brightness if not for the invention of the blinking backlight (or burst dimmingBurst dimming is a method to control dimming of cold cathode fluorescent lamps by using pulse width modulation at approximately 100-300 Hz which is supposed to be above the noticeable flicker limit for the human eye....
). The LCD itself is only a light valve, it does not generate light; the light comes from a backlight that is either a florescent tube or a set of LEDsLEDS can be initials for:* Law Enforcement Data System* Link Eleven Display System* Low Energy Dislocation Structure* Land Electronic Defence System * LEDs * Life-Events and Difficulties Schedule...
. The blinking backlight was developed to improve the motion performance of LCDs by turning the backlight off while the liquid crystals were in transition from one image to another. However, a side benefit of the blinking backlight was infinite contrast. The contrast reported on most LCDs is what the LCD is qualified at, not its actual performance. In any case, there are two large caveats to contrast ratio as a measure of LCD performance.
The first caveat is that contrast ratios are measured in a completely dark room. In actual use, the room is never completely dark, as one will always have the light from the LCD itself. Beyond that, there may be sunlight coming in through a window or other room lights that reflect off of the surface of the LCD and degrades the contrast. As a practical matter, the contrast of an LCD, or any display, is governed by the amount of surface reflections, not by the performance of the display.
The second caveat is that the human eye can only image a contrast ratio of a maximum of about 200:1. Black print on a white paper is about 15–20:1. That is why viewing angles are specified to the point where they fall below 10:1. A 10:1 image is not great, but is discernable.
BrightnessBrightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target...
is usually stated as the maximum output of the LCD. In the CRT era,
TrinitronTrinitron is Sony's brand name for its line of aperture grille based CRTs used in television sets and computer display monitors. One of the first truly new television systems to enter the market since the 1950s, the Trinitron was announced in 1966 to wide acclaim for its bright images, about 25%...
CRTs had a brightness advantage over the competition, so brightness was commonly discussed in TV advertising. With current LCD technology, brightness, though important, is usually the same from maker to maker and is consequently not discussed much except for notebook LCDs and other displays that will be viewed in bright sunlight. In general, brighter is better, but there is always a trade-off between brightness and battery life in a mobile device.
Military use of LCD monitors
- LCD monitors have been adopted by the United States military, instead of CRT
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
displays, because they are smaller, lighter and more efficient, although monochrome plasma displays are also used, notably for the M1 Abrams tank. For use with night vision imaging systems a U.S. military LCD monitor must be compliant with MIL-STD-3009 (formerly MIL-L-85762A). These LCD monitors go through extensive certification so that they pass the standards for the military. These include MIL-STD-901D – High Shock (Sea Vessels), MIL-STD-167B – Vibration (Sea Vessels), MIL-STD-810F – Field Environmental Conditions (Ground Vehicles and Systems), MIL-STD-461E/F – EMI/RFI (Electromagnetic Interference/Radio Frequency Interference), MIL-STD-740B – Airborne/Structureborne Noise, and TEMPEST – Telecommunications Electronics Material Protected from Emanating Spurious Transmissions.
Advantages and disadvantages
In spite of LCDs being a well proven and still viable technology, as display devices LCDs are not perfect for all applications.
Advantages
- Very compact and light.
- Low power consumption.
- No geometric distortion.
- Little or no flicker depending on backlight technology.
- Not affected by screen burn-in.
- Can be made in almost any size or shape.
- No theoretical resolution limit.
Disadvantages
- Limited viewing angle
In display technology parlance, viewing angle is the maximum angle at which a display can be viewed with acceptable visual performance. In a technical context, this angular range is called viewing cone defined by a multitude of viewing directions....
, causing color, saturation, contrast and brightness to vary, even within the intended viewing angle, by variations in posture.
- Bleeding and uneven backlighting in some monitors, causing brightness distortion, especially toward the edges.
- Smearing and ghosting artifacts caused by slow response times (>8 ms) and "sample and hold" operation.
- Only one native resolution
The native resolution of a LCD, LCoS or other flat panel display refers to its single fixed resolution. As an LCD display consists of a fixed raster, it cannot change resolution to match the signal being displayed as a CRT monitor can, meaning that optimal display quality can be reached only when...
. Displaying resolutions either requires a video scalerA video scaler is a device for converting video signals from one size or resolution to another: usually "upscaling" or "upconverting" a video signal from a low resolution to one of higher resolution A video scaler is a device for converting video signals from one size or resolution to another:...
, lowering perceptual quality, or display at 1:1 pixel mapping1:1 pixel mapping is a video display technique used in some devices, such as LCD monitors. A monitor that has been set to 1:1 pixel mapping will try to display an input source without scaling it, such that each pixel received is mapped to a single native pixel on the monitor...
, in which images will be physically too large or won't fill the whole screen.
- Fixed bit depth, many cheaper LCDs are only able to display 262,000 colors. 8-bit S-IPS panels can display 16 million colors and have significantly better black level, but are expensive and have slower response time.
- Low bit depth results in images with unnatural or excessive contrast.
- Input lag
Display lag is a phenomenon associated with some types of LCD displays, and nearly all types of HDTVs, that refers to latency, or lag measured by the difference between the time a signal is input into a display and the time it is shown by the display. This lag time has been measured as high as...
- Dead or stuck pixels
Defective pixels are pixels on a liquid crystal display that are not performing as expected. The ISO standard ISO 13406-2 distinguishes between three different types of defective pixels, while hardware companies tend to have further distinguishing types. Similar defects can also occur on CCD or...
may occur during manufacturing or through use.
- In a constant-on situation, thermalization may occur, which is when only part of the screen has overheated and looks discolored compared to the rest of the screen.
- Not all LCDs are designed to allow easy replacement of the backlight.
- Cannot be used with light guns/pens.
- Loss of contrast in high temperature environments.
See also
- LCD classification
There are various classifications of the electro-optical modes of liquid crystal displays .- LCD operation in a nutshell :The operation of TN, VA and IPS-LCDs can be summarized as follows:...
- LCD projector
An LCD projector is a type of video projector for displaying video, images or computer data on a screen or other flat surface. It is a modern equivalent of the slide projector or overhead projector...
- List of liquid crystal display manufacturers
External links
General information