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Liquid-crystal display
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Display that uses the light-modulating properties of liquid crystals
"LCD" redirects here. For other uses, see LCD (disambiguation).
Not to be confused with LED.
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Reflective twisted nematic liquid crystal display
1. Polarizing filter film with a vertical axis to polarize light as it
enters.
2. Glass substrate with ITO electrodes. The shapes of these electrodes
will determine the shapes that will appear when the LCD is switched
ON. Vertical ridges etched on the surface are smooth.
3. Twisted nematic liquid crystal.
4. Glass substrate with common electrode film (ITO) with horizontal
ridges to line up with the horizontal filter.
5. Polarizing filter film with a horizontal axis to block/pass light.
6. Reflective surface to send light back to viewer. (In a backlit LCD,
this layer is replaced or complemented with a light source.)
A liquid-crystal display (LCD) is a flat-panel display or other
electronically modulated optical device that uses the light-modulating
properties of liquid crystals combined with polarizers. Liquid crystals
do not emit light directly^[1] but instead use a backlight or reflector
to produce images in color or monochrome.^[2] LCDs are available to
display arbitrary images (as in a general-purpose computer display) or
fixed images with low information content, which can be displayed or
hidden. For instance: preset words, digits, and seven-segment displays,
as in a digital clock, are all good examples of devices with these
displays. They use the same basic technology, except that arbitrary
images are made from a matrix of small pixels, while other displays
have larger elements. LCDs can either be normally on (positive) or off
(negative), depending on the polarizer arrangement. For example, a
character positive LCD with a backlight will have black lettering on a
background that is the color of the backlight, and a character negative
LCD will have a black background with the letters being of the same
color as the backlight. Optical filters are added to white on blue LCDs
to give them their characteristic appearance.
LCDs are used in a wide range of applications, including LCD
televisions, computer monitors, instrument panels, aircraft cockpit
displays, and indoor and outdoor signage. Small LCD screens are common
in LCD projectors and portable consumer devices such as digital
cameras, watches, digital clocks, calculators, and mobile telephones,
including smartphones. LCD screens are also used on consumer
electronics products such as DVD players, video game devices and
clocks. LCD screens have replaced heavy, bulky cathode-ray tube (CRT)
displays in nearly all applications. LCD screens are available in a
wider range of screen sizes than CRT and plasma displays, with LCD
screens available in sizes ranging from tiny digital watches to very
large television receivers. LCDs are slowly being replaced by OLEDs,
which can be easily made into different shapes, and have a lower
response time, wider color gamut, virtually infinite color contrast and
viewing angles, lower weight for a given display size and a slimmer
profile (because OLEDs use a single glass or plastic panel whereas LCDs
use two glass panels; the thickness of the panels increases with size
but the increase is more noticeable on LCDs) and potentially lower
power consumption (as the display is only "on" where needed and there
is no backlight). OLEDs, however, are more expensive for a given
display size due to the very expensive electroluminescent materials or
phosphors that they use. Also due to the use of phosphors, OLEDs suffer
from screen burn-in and there is currently no way to recycle OLED
displays, whereas LCD panels can be recycled, although the technology
required to recycle LCDs is not yet widespread. Attempts to maintain
the competitiveness of LCDs are quantum dot displays, marketed as SUHD,
QLED or Triluminos, which are displays with blue LED backlighting and a
Quantum-dot enhancement film (QDEF) that converts part of the blue
light into red and green, offering similar performance to an OLED
display at a lower price, but the quantum dot layer that gives these
displays their characteristics can not yet be recycled.
Since LCD screens do not use phosphors, they rarely suffer image
burn-in when a static image is displayed on a screen for a long time,
e.g., the table frame for an airline flight schedule on an indoor sign.
LCDs are, however, susceptible to image persistence.^[3] The LCD screen
is more energy-efficient and can be disposed of more safely than a CRT
can. Its low electrical power consumption enables it to be used in
battery-powered electronic equipment more efficiently than a CRT can
be. By 2008, annual sales of televisions with LCD screens exceeded
sales of CRT units worldwide, and the CRT became obsolete for most
purposes.
[ ]
Contents
* 1 General characteristics
* 2 History
+ 2.1 Background
+ 2.2 1960s
+ 2.3 1970s
+ 2.4 1980s
+ 2.5 1990s
+ 2.6 2000s-2010s
* 3 Illumination
* 4 Connection to other circuits
* 5 Passive-matrix
* 6 Active-matrix technologies
+ 6.1 Twisted nematic (TN)
+ 6.2 In-plane switching (IPS)
o 6.2.1 Super In-plane switching (S-IPS)
+ 6.3 M+ or RGBW controversy
+ 6.4 IPS in comparison to AMOLED
+ 6.5 Advanced fringe field switching (AFFS)
+ 6.6 Vertical alignment (VA)
+ 6.7 Blue phase mode
* 7 Quality control
* 8 "Zero-power" (bistable) displays
* 9 Specifications
* 10 Advantages and disadvantages
+ 10.1 Advantages
+ 10.2 Disadvantages
* 11 Chemicals used
+ 11.1 Environmental impact
* 12 See also
* 13 References
* 14 External links
+ 14.1 General information
General characteristics[edit]
An LCD screen used as a notification panel for travellers
Each pixel of an LCD typically consists of a layer of molecules aligned
between two transparent electrodes, often made of Indium-Tin oxide
(ITO) and two polarizing filters (parallel and perpendicular
polarizers), the axes of transmission of which are (in most of the
cases) perpendicular to each other. Without the liquid crystal between
the polarizing filters, light passing through the first filter would be
blocked by the second (crossed) polarizer. Before an electric field is
applied, the orientation of the liquid-crystal molecules is determined
by the alignment at the surfaces of electrodes. In a twisted nematic
(TN) 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 induces the rotation of the
polarization of the incident light, and the device appears gray. If the
applied voltage is large enough, the liquid crystal molecules in the
center of the layer are almost completely untwisted and the
polarization of the incident light is not rotated as it passes through
the liquid crystal layer. This light will then be mainly polarized
perpendicular to the second filter, and thus be blocked and the pixel
will appear black. By controlling the voltage applied across the liquid
crystal layer in each pixel, light can be allowed to pass through in
varying amounts thus constituting different levels of gray.
The chemical formula of the liquid crystals used in LCDs may vary.
Formulas may be patented.^[4] An example is a mixture of
2-(4-alkoxyphenyl)-5-alkylpyrimidine with cyanobiphenyl, patented by
Merck and Sharp Corporation. The patent that covered that specific
mixture expired.^[5]
Most color LCD systems use the same technique, with color filters used
to generate red, green, and blue subpixels. The LCD color filters are
made with a photolithography process on large glass sheets that are
later glued with other glass sheets containing a TFT array, spacers and
liquid crystal, creating several color LCDs that are then cut from one
another and laminated with polarizer sheets. Red, green, blue and black
photoresists (resists) are used. All resists contain a finely ground
powdered pigment, with particles being just 40 nanometers across. The
black resist is the first to be applied; this will create a black grid
(known in the industry as a black matrix) that will separate red, green
and blue subpixels from one another, increasing contrast ratios and
preventing light from leaking from one subpixel onto other surrounding
subpixels.^[6] After the black resist has been dried in an oven and
exposed to UV light through a photomask, the unexposed areas are washed
away, creating a black grid. Then the same process is repeated with the
remaining resists. This fills the holes in the black grid with their
corresponding colored
resists.^[7]^[8]^[9]^[10]^[11]^[12]^[13]^[14]^[15]^[16]^[17]^[18]^[19]^
[20] Another color-generation method used in early color PDAs and some
calculators was done by varying the voltage in a Super-twisted nematic
LCD, where the variable twist between tighter-spaced plates causes a
varying double refraction birefringence, thus changing the hue.^[21]
They were typically restricted to 3 colors per pixel: orange, green,
and blue.^[22]
LCD in a Texas Instruments calculator with top polarizer removed from
device and placed on top, such that the top and bottom polarizers are
perpendicular. As a result, the colors are inverted.
The optical effect of a TN 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, TN displays with low information
content and no backlighting 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). As most of 2010-era LCDs are used in television sets, monitors
and smartphones, they have high-resolution matrix arrays of pixels to
display arbitrary images using backlighting with a dark background.
When no image is displayed, different arrangements are used. For this
purpose, TN LCDs are operated between parallel polarizers, whereas IPS
LCDs feature crossed polarizers. In many applications IPS LCDs have
replaced TN LCDs, particularly in smartphones. 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).
A Casio Alarm Chrono digital watch with LCD
Displays for a small number of individual digits or fixed symbols (as
in digital watches and pocket calculators) can be implemented with
independent electrodes for each segment.^[23] In contrast, full
alphanumeric 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.
LCDs, along with OLED displays, are manufactured in cleanrooms
borrowing techniques from semiconductor manufacturing and using large
sheets of glass whose size has increased over time. Several displays
are manufactured at the same time, and then cut from the sheet of
glass, also known as the mother glass or LCD glass substrate. The
increase in size allows more displays or larger displays to be made,
just like with increasing wafer sizes in semiconductor manufacturing.
The glass sizes are as follows:
LCD-Glass-sizes-generation
Generation Length [mm] Height [mm] Year of introduction References
GEN 1 200-300 200-400 1990 ^[24]^[25]
GEN 2 370 470
GEN 3 550 650 1996-1998 ^[26]
GEN 3.5 600 720 1996 ^[25]
GEN 4 680 880 2000-2002 ^[25]^[26]
GEN 4.5 730 920 2000-2004 ^[27]
GEN 5 1100 1250-1300 2002-2004 ^[25]^[26]
GEN 5.5 1300 1500
GEN 6 1500 1800-1850 2002-2004 ^[25]^[26]
GEN 7 1870 2200 2003 ^[28]^[29]
GEN 7.5 1950 2250 ^[25]
GEN 8 2160 2460 ^[29]
GEN 8.5 2200 2500 2007-2016 ^[30]^[31]
GEN 8.6 2250 2600 2016 ^[31]
GEN 10 2880 3130 2009 ^[32]^[33]
GEN 10.5 (also known as GEN 11) 2940 3370 2018^[34] ^[35]
Until Gen 8, manufacturers would not agree on a single mother glass
size and as a result, different manufacturers would use slightly
different glass sizes for the same generation. Some manufacturers have
adopted Gen 8.6 mother glass sheets which are only slightly larger than
Gen 8.5, allowing for more 50 and 58 inch LCDs to be made per mother
glass, specially 58 inch LCDs, in which case 6 can be produced on a Gen
8.6 mother glass vs only 3 on a Gen 8.5 mother glass, significantly
reducing waste.^[31] The thickness of the mother glass also increases
with each generation, so larger mother glass sizes are better suited
for larger displays. An LCD Module (LCM) is a ready-to-use LCD with a
backlight. Thus, a factory that makes LCD Modules does not necessarily
make LCDs, it may only assemble them into the modules. LCD glass
substrates are made by companies such as AGC Inc., Corning Inc., and
Nippon Electric Glass.
History[edit]
The origins and the complex history of liquid-crystal displays from the
perspective of an insider during the early days were described by
Joseph A. Castellano in Liquid Gold: The Story of Liquid Crystal
Displays and the Creation of an Industry.^[36] 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.^[37] A description of Swiss contributions to LCD developments,
written by Peter J. Wild, can be found at the Engineering and
Technology History Wiki.^[38]
Background[edit]
Main articles: Liquid crystal and Thin-film transistor
In 1888,^[39] Friedrich Reinitzer (1858-1927) discovered 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:
Beitraege zur Kenntniss des Cholesterins, Monatshefte fuer Chemie
(Wien) 9, 421-441 (1888)).^[40] In 1904, Otto Lehmann published his
work "Fluessige Kristalle" (Liquid Crystals). In 1911, Charles Mauguin
first experimented with liquid crystals confined between plates in thin
layers.
In 1922, Georges Friedel described the structure and properties of
liquid crystals and classified them in three types (nematics, smectics
and cholesterics). In 1927, Vsevolod Frederiks devised the electrically
switched light valve, called the Freedericksz transition, the essential
effect of all LCD technology. In 1936, the Marconi Wireless Telegraph
company patented the first practical application of the technology,
"The Liquid Crystal Light Valve". In 1962, the first major English
language publication Molecular Structure and Properties of Liquid
Crystals was published by Dr. George W. Gray.^[41] In 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 are now called "Williams
domains" inside the liquid crystal.^[42]
The MOSFET (metal-oxide-semiconductor field-effect transistor) was
invented by Mohamed M. Atalla and Dawon Kahng at Bell Labs in 1959, and
presented in 1960.^[43]^[44] Building on their work with MOSFETs, Paul
K. Weimer at RCA developed the thin-film transistor (TFT) in 1962.^[45]
It was a type of MOSFET distinct from the standard bulk MOSFET.^[46]
1960s[edit]
In 1964, George H. Heilmeier, then working at 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 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.^[47]^[48]^[49]^[50] George H. Heilmeier was inducted
in the National Inventors Hall of Fame^[51] and credited with the
invention of LCDs. Heilmeier's work is an IEEE Milestone.^[52]
In the late 1960s, pioneering work on liquid crystals was undertaken by
the UK's Royal Radar Establishment at Malvern, England. The team at RRE
supported ongoing work by George William Gray and his team at the
University of Hull who ultimately discovered the cyanobiphenyl liquid
crystals, which had correct stability and temperature properties for
application in LCDs.
The idea of a TFT-based liquid-crystal display (LCD) was conceived by
Bernard Lechner of RCA Laboratories in 1968.^[53] Lechner, F.J.
Marlowe, E.O. Nester and J. Tults demonstrated the concept in 1968 with
an 18x2 matrix dynamic scattering mode (DSM) LCD that used standard
discrete MOSFETs.^[54]
1970s[edit]
On December 4, 1970, the twisted nematic field effect (TN) in liquid
crystals was filed for patent by Hoffmann-LaRoche in Switzerland,
(Swiss patent No. 532 261) with Wolfgang Helfrich and Martin Schadt
(then working for the Central Research Laboratories) listed as
inventors.^[47] Hoffmann-La Roche licensed the invention to Swiss
manufacturer Brown, Boveri & Cie, its joint venture partner at that
time, which produced TN displays for wristwatches and other
applications during the 1970s for the international markets including
the Japanese electronics industry, which soon produced the first
digital quartz wristwatches with TN-LCDs and numerous other products.
James Fergason, while working with Sardari Arora and Alfred Saupe at
Kent State University Liquid Crystal Institute, filed an identical
patent in the United States on April 22, 1971.^[55] In 1971, the
company of Fergason, ILIXCO (now LXD Incorporated), produced 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. Tetsuro Hama and Izuhiko Nishimura of Seiko received a US
patent dated February 1971, for an electronic wristwatch incorporating
a TN-LCD.^[56] In 1972, the first wristwatch with TN-LCD was launched
on the market: The Gruen Teletime which was a four digit display watch.
In 1972, the concept of the active-matrix thin-film transistor (TFT)
liquid-crystal display panel was prototyped in the United States by T.
Peter Brody's team at Westinghouse, in Pittsburgh, Pennsylvania.^[57]
In 1973, Brody, J. A. Asars and G. D. Dixon at Westinghouse Research
Laboratories demonstrated the first thin-film-transistor liquid-crystal
display (TFT LCD).^[58]^[59] As of 2013^[update], all modern
high-resolution and high-quality electronic visual display devices use
TFT-based active matrix displays.^[60] Brody and Fang-Chen Luo
demonstrated the first flat active-matrix liquid-crystal display (AM
LCD) in 1974, and then Brody coined the term "active matrix" in
1975.^[53]
In 1972 North American Rockwell Microelectronics Corp introduced the
use of DSM LCDs for calculators for marketing by Lloyds Electronics
Inc, though these required an internal light source for
illumination.^[61] Sharp Corporation followed with DSM LCDs for
pocket-sized calculators in 1973^[62] and then mass-produced TN LCDs
for watches in 1975.^[63] Other Japanese companies soon took a leading
position in the wristwatch market, like Seiko and its first 6-digit
TN-LCD quartz wristwatch, and Casio's 'Casiotron'. Color LCDs based on
Guest-Host interaction were invented by a team at RCA in 1968.^[64] A
particular type of such a color LCD was developed by Japan's Sharp
Corporation in the 1970s, receiving patents for their inventions, such
as a patent by Shinji Kato and Takaaki Miyazaki in May 1975,^[65] and
then improved by Fumiaki Funada and Masataka Matsuura in December
1975.^[66] TFT LCDs similar to the prototypes developed by a
Westinghouse team in 1972 were patented in 1976 by a team at Sharp
consisting of Fumiaki Funada, Masataka Matsuura, and Tomio Wada,^[67]
then improved in 1977 by a Sharp team consisting of Kohei Kishi,
Hirosaku Nonomura, Keiichiro Shimizu, and Tomio Wada.^[68] However,
these TFT-LCDs were not yet ready for use in products, as problems with
the materials for the TFTs were not yet solved.
1980s[edit]
In 1983, researchers at Brown, Boveri & Cie (BBC) Research Center,
Switzerland, invented 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,^[69] U.S. Patent 4,634,229
and many more countries. In 1980, Brown Boveri started a 50/50 joint
venture with the Dutch Philips company, called Videlec.^[70] Philips
had the required know-how to design and build integrated circuits for
the control of large LCD panels. In addition, Philips had better access
to markets for electronic components and intended to use LCDs in new
product generations of hi-fi, video equipment and telephones. In 1984,
Philips researchers Theodorus Welzen and Adrianus de Vaan invented a
video speed-drive scheme that solved the slow response time of
STN-LCDs, enabling high-resolution, high-quality, and smooth-moving
video images on STN-LCDs.^[71] In 1985, Philips inventors Theodorus
Welzen and Adrianus de Vaan solved the problem of driving
high-resolution STN-LCDs using low-voltage (CMOS-based) drive
electronics, allowing the application of high-quality (high resolution
and video speed) LCD panels in battery-operated portable products like
notebook computers and mobile phones.^[72] In 1985, Philips acquired
100% of the Videlec AG company based in Switzerland. Afterwards,
Philips moved the Videlec production lines to the Netherlands. Years
later, Philips successfully produced and marketed complete modules
(consisting of the LCD screen, microphone, speakers etc.) in
high-volume production for the booming mobile phone industry.
The first color LCD televisions were developed as handheld televisions
in Japan. In 1980, Hattori Seiko's R&D group began development on color
LCD pocket televisions.^[73] In 1982, Seiko Epson released the first
LCD television, the Epson TV Watch, a wristwatch equipped with a small
active-matrix LCD television.^[74]^[75] Sharp Corporation introduced
dot matrix TN-LCD in 1983.^[63] In 1984, Epson released the ET-10, the
first full-color, pocket LCD television.^[76] The same year, Citizen
Watch,^[77] introduced the Citizen Pocket TV,^[73] a 2.7-inch color LCD
TV,^[77] with the first commercial TFT LCD.^[73] In 1988, Sharp
demonstrated a 14-inch, active-matrix, full-color, full-motion TFT-LCD.
This led to Japan launching an LCD industry, which developed large-size
LCDs, including TFT computer monitors and LCD televisions.^[78] Epson
developed the 3LCD projection technology in the 1980s, and licensed it
for use in projectors in 1988.^[79] Epson's VPJ-700, released in
January 1989, was the world's first compact, full-color LCD
projector.^[75]
1990s[edit]
In 1990, under different titles, inventors conceived electro optical
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.^[80]^[81] 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.^[82]^[83] The Fraunhofer Institute ISE in Freiburg, where
the inventors worked, assigns these patents to Merck KGaA, Darmstadt, a
supplier of LC substances. In 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.^[84]^[85]
Hitachi also improved 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. In 1996, Samsung developed the optical
patterning technique that enables multi-domain LCD. Multi-domain and In
Plane Switching subsequently remain the dominant LCD designs through
2006.^[86] In the late 1990s, the LCD industry began shifting away from
Japan, towards South Korea and Taiwan,^[78] which later shifted to
China.
2000s-2010s[edit]
In 2007 the image quality of LCD televisions surpassed the image
quality of cathode-ray-tube-based (CRT) TVs.^[87] In the fourth quarter
of 2007, LCD televisions surpassed CRT TVs in worldwide sales for the
first time.^[88] LCD TVs were projected to account 50% of the
200 million TVs to be shipped globally in 2006, according to
Displaybank.^[89]^[90] In October 2011, Toshiba announced 2560 * 1600
pixels on a 6.1-inch (155 mm) LCD panel, suitable for use in a tablet
computer,^[91] especially for Chinese character display. The 2010s also
saw the wide adoption of TGP (Tracking Gate-line in Pixel), which moves
the driving circuitry from the borders of the display to in between the
pixels, allowing for narrow bezels.^[92] LCDs can be made transparent
and flexible, but they cannot emit light without a backlight like OLED
and microLED, which are other technologies that can also be made
flexible and transparent.^[93]^[94]^[95]^[96] Special films can be used
to increase the viewing angles of LCDs.^[97]^[98]
In 2016, Panasonic developed IPS LCDs with a contrast ratio of
1,000,000:1, rivaling OLEDs. This technology was later put into mass
production as dual layer, dual panel or LMCL (Light Modulating Cell
Layer) LCDs. The technology uses 2 liquid crystal layers instead of
one, and may be used along with a mini-LED backlight and quantum dot
sheets.^[99]^[100]^[101]^[102]^[103]^[104]
Illumination[edit]
Since LCDs produce no light of their own, they require external light
to produce a visible image.^[105]^[106] In a transmissive type of LCD,
the light source is provided at the back of the glass stack and is
called a backlight. Active-matrix LCDs are almost always
backlit.^[107]^[108] Passive LCDs may be backlit but many use a
reflector at the back of the glass stack to utilize ambient light.
Transflective LCDs combine the features of a backlit transmissive
display and a reflective display.
The common implementations of LCD backlight technology are:
18 parallel CCFLs as backlight for a 42-inch (106 cm) LCD TV
* CCFL: The LCD panel is lit either by two cold cathode fluorescent
lamps placed at opposite edges of the display or an array of
parallel CCFLs behind larger displays. A diffuser (made of PMMA
acrylic plastic, also known as a wave or light guide/guiding
plate^[109]^[110]) then spreads the light out evenly across the
whole display. For many years, this technology had been used almost
exclusively. Unlike white LEDs, most CCFLs have an even-white
spectral output resulting in better color gamut for the display.
However, CCFLs are less energy efficient than LEDs and require a
somewhat costly inverter to convert whatever DC voltage the device
uses (usually 5 or 12 V) to ~=1000 V needed to light a CCFL.^[111]
The thickness of the inverter transformers also limits how thin the
display can be made.
* EL-WLED: The LCD panel is lit by a row of white LEDs placed at one
or more edges of the screen. A light diffuser (light guide plate,
LGP) is then used to spread the light evenly across the whole
display, similarly to edge-lit CCFL LCD backlights. The diffuser is
made out of either PMMA plastic or special glass, PMMA is used in
most cases because it is rugged, while special glass is used when
the thickness of the LCD is of primary concern, because it doesn't
expand as much when heated or exposed to moisture, which allows
LCDs to be just 5mm thick. Quantum dots may be placed on top of the
diffuser as a quantum dot enhancement film (QDEF, in which case
they need a layer to be protected from heat and humidity) or on the
color filter of the LCD, replacing the resists that are normally
used.^[109] As of 2012, this design is the most popular one in
desktop computer monitors. It allows for the thinnest displays.
Some LCD monitors using this technology have a feature called
dynamic contrast, invented by Philips researchers Douglas Stanton,
Martinus Stroomer and Adrianus de Vaan^[112] Using PWM (pulse-width
modulation, a technology where the intensity of the LEDs are kept
constant, but the brightness adjustment is achieved by varying a
time interval of flashing these constant light intensity light
sources^[113]), the backlight is dimmed to the brightest color that
appears on the screen while simultaneously boosting the LCD
contrast to the maximum achievable levels, allowing the 1000:1
contrast ratio of the LCD panel to be scaled to different light
intensities, resulting in the "30000:1" contrast ratios seen in the
advertising on some of these monitors. Since computer screen images
usually have full white somewhere in the image, the backlight will
usually be at full intensity, making this "feature" mostly a
marketing gimmick for computer monitors, however for TV screens it
drastically increases the perceived contrast ratio and dynamic
range, improves the viewing angle dependency and drastically
reducing the power consumption of conventional LCD televisions.
* WLED array: The LCD panel is lit by a full array of white LEDs
placed behind a diffuser behind the panel. LCDs that use this
implementation will usually have the ability to dim or completely
turn off the LEDs in the dark areas of the image being displayed,
effectively increasing the contrast ratio of the display. The
precision with which this can be done will depend on the number of
dimming zones of the display. The more dimming zones, the more
precise the dimming, with less obvious blooming artifacts which are
visible as dark grey patches surrounded by the unlit areas of the
LCD. As of 2012, this design gets most of its use from upscale,
larger-screen LCD televisions.
* RGB-LED array: Similar to the WLED array, except the panel is lit
by a full array of RGB LEDs. While displays lit with white LEDs
usually have a poorer color gamut than CCFL lit displays, panels
lit with RGB LEDs have very wide color gamuts. This implementation
is most popular on professional graphics editing LCDs. As of 2012,
LCDs in this category usually cost more than $1000. As of 2016 the
cost of this category has drastically reduced and such LCD
televisions obtained same price levels as the former 28" (71 cm)
CRT based categories.
* Monochrome LEDs: such as red, green, yellow or blue LEDs are used
in the small passive monochrome LCDs typically used in clocks,
watches and small appliances.
* Mini-LED: Backlighting with Mini-LEDs can support over a thousand
of Full-area Local Area Dimming (FLAD) zones. This allows deeper
blacks and higher contrast ratio.^[114] (Not to be confused with
MicroLED.)
Today, most LCD screens are being designed with an LED backlight
instead of the traditional CCFL backlight, while that backlight is
dynamically controlled with the video information (dynamic backlight
control). The combination with the dynamic backlight control, invented
by Philips researchers Douglas Stanton, Martinus Stroomer and Adrianus
de Vaan, simultaneously increases the dynamic range of the display
system (also marketed as HDR, high dynamic range television or FLAD,
full-area local area dimming).^[115]^[116]^[112]
The LCD backlight systems are made highly efficient by applying optical
films such as prismatic structure (prism sheet) to gain the light into
the desired viewer directions and reflective polarizing films that
recycle the polarized light that was formerly absorbed by the first
polarizer of the LCD (invented by Philips researchers Adrianus de Vaan
and Paulus Schaareman),^[117] generally achieved using so called DBEF
films manufactured and supplied by 3M.^[118] Improved versions of the
prism sheet have a wavy rather than a prismatic structure, and
introduce waves laterally into the structure of the sheet while also
varying the height of the waves, directing even more light towards the
screen and reducing aliasing or moire between the structure of the
prism sheet and the subpixels of the LCD. A wavy structure is easier to
mass-produce than a prismatic one using conventional diamond machine
tools, which are used to make the rollers used to imprint the wavy
structure into plastic sheets, thus producing prism sheets.^[119] A
diffuser sheet is placed on both sides of the prism sheet to make the
light of the backlight, uniform, while a mirror is placed behind the
light guide plate to direct all light forwards. The prism sheet with
its diffuser sheets are placed on top of the light guide
plate.^[120]^[109] The DBEF polarizers consist of a large stack of
uniaxial oriented birefringent films that reflect the former absorbed
polarization mode of the light.^[121] Such reflective polarizers using
uniaxial oriented polymerized liquid crystals (birefringent polymers or
birefringent glue) are invented in 1989 by Philips researchers Dirk
Broer, Adrianus de Vaan and Joerg Brambring.^[122] The combination of
such reflective polarizers, and LED dynamic backlight control^[112]
make today's LCD televisions far more efficient than the CRT-based
sets, leading to a worldwide energy saving of 600 TWh (2017), equal to
10% of the electricity consumption of all households worldwide or equal
to 2 times the energy production of all solar cells in the
world.^[123]^[124]
Due to the LCD layer that generates the desired high resolution images
at flashing video speeds using very low power electronics in
combination with LED based backlight technologies, LCD technology has
become the dominant display technology for products such as
televisions, desktop monitors, notebooks, tablets, smartphones and
mobile phones. Although competing OLED technology is pushed to the
market, such OLED displays do not feature the HDR capabilities like
LCDs in combination with 2D LED backlight technologies have, reason why
the annual market of such LCD-based products is still growing faster
(in volume) than OLED-based products while the efficiency of LCDs (and
products like portable computers, mobile phones and televisions) may
even be further improved by preventing the light to be absorbed in the
colour filters of the LCD.^[125]^[126]^[127] Such reflective colour
filter solutions are not yet implemented by the LCD industry and have
not made it further than laboratory prototypes. They will likely be
implemented by the LCD industry to increase the efficiency compared to
OLED technologies.
Connection to other circuits[edit]
A pink elastomeric connector mating an LCD panel to circuit board
traces, shown next to a centimeter-scale ruler. The conductive and
insulating layers in the black stripe are very small.
A standard television receiver screen, a modern LCD panel, has over six
million pixels, and they are all individually powered by a wire network
embedded in the screen. The fine wires, or pathways, form a grid with
vertical wires across the whole screen on one side of the screen and
horizontal wires across the whole screen on the other side of the
screen. To this grid each pixel has a positive connection on one side
and a negative connection on the other side. So the total amount of
wires needed for a 1080p display is 3 x 1920 going vertically and 1080
going horizontally for a total of 6840 wires horizontally and
vertically. That's three for red, green and blue and 1920 columns of
pixels for each color for a total of 5760 wires going vertically and
1080 rows of wires going horizontally. For a panel that is 28.8 inches
(73 centimeters) wide, that means a wire density of 200 wires per inch
along the horizontal edge.
The LCD panel is powered by LCD drivers that are carefully matched up
with the edge of the LCD panel at the factory level. The drivers may be
installed using several methods, the most common of which are COG
(Chip-On-Glass) and TAB (Tape-automated bonding) These same principles
apply also for smartphone screens that are much smaller than TV
screens.^[128]^[129]^[130] LCD panels typically use thinly-coated
metallic conductive pathways on a glass substrate to form the cell
circuitry to operate the panel. It is usually not possible to use
soldering techniques to directly connect the panel to a separate
copper-etched circuit board. Instead, interfacing is accomplished using
anisotropic conductive film or, for lower densities, elastomeric
connectors.
Passive-matrix[edit]
Prototype of a passive-matrix STN-LCD with 540 *270 pixels, Brown
Boveri Research, Switzerland, 1984
Monochrome and later color passive-matrix LCDs were standard in most
early laptops (although a few used plasma displays^[131]^[132]) and the
original Nintendo Game Boy^[133] 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 in the 2010s for applications less
demanding than laptop computers and TVs, such as inexpensive
calculators. In particular, these are used on portable devices where
less information content needs to be displayed, lowest power
consumption (no backlight) and low cost are desired or readability in
direct sunlight is needed.
A comparison between a blank passive-matrix display (top) and a blank
active-matrix display (bottom). A passive-matrix display can be
identified when the blank background is more grey in appearance than
the crisper active-matrix display, fog appears on all edges of the
screen, and while pictures appear to be fading on the screen.
Displays having a passive-matrix structure are employing super-twisted
nematic STN (invented by Brown Boveri Research Center, Baden,
Switzerland, in 1983; scientific details were published^[134]) 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 as
discovered by Peter J. Wild in 1972,^[135] while activated pixels are
subjected to voltages above threshold (the voltages according to the
"Alt & Pleshko" drive scheme).^[136] Driving such STN displays
according to the Alt & Pleshko drive scheme require very high line
addressing voltages. Welzen and de Vaan invented an alternative drive
scheme (a non "Alt & Pleshko" drive scheme) requiring much lower
voltages, such that the STN display could be driven using low voltage
CMOS technologies.^[72]
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 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. Slow response
times and poor contrast are typical of passive-matrix addressed LCDs
with too many pixels and driven according to the "Alt & Pleshko" drive
scheme. Welzen and de Vaan also invented a non RMS drive scheme
enabling to drive STN displays with video rates and enabling to show
smooth moving video images on an STN display.^[71] Citizen, amongst
others, licensed these patents and successfully introduced several STN
based LCD pocket televisions on the market^[137]
How an LCD works using an active-matrix structure
Bistable LCDs do not require continuous refreshing. Rewriting is only
required for picture information changes. In 1984 HA van Sprang and
AJSM de Vaan invented an STN type display that could be operated in a
bistable mode, enabling extremely high resolution images up to 4000
lines or more using only low voltages.^[138] Since a pixel may be
either in an on-state or in an off state at the moment new information
needs to be written to that particular pixel, the addressing method of
these bistable displays is rather complex, a reason why these displays
did not made it to the market. That changed when in the 2010
"zero-power" (bistable) LCDs became available. Potentially,
passive-matrix addressing can be used with devices if their write/erase
characteristics are suitable, which was the case for ebooks which need
to show still pictures only. After a page is written to the display,
the display may be cut from the power while retaining readable images.
This has the advantage that such ebooks may be operated for long
periods of time powered by only a small battery.
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 electrodes in contact with the LC
layer. Each pixel has its own dedicated transistor, 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
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.
Sharp produces bistable reflective LCDs with a 1-bit SRAM cell per
pixel that only requires small amounts of power to maintain an
image.^[139]
Segment LCDs can also have color by using Field Sequential Color (FSC
LCD). This kind of displays have a high speed passive segment LCD panel
with an RGB backlight. The backlight quickly changes color, making it
appear white to the naked eye. The LCD panel is synchronized with the
backlight. For example, to make a segment appear red, the segment is
only turned ON when the backlight is red, and to make a segment appear
magenta, the segment is turned ON when the backlight is blue, and it
continues to be ON while the backlight becomes red, and it turns OFF
when the backlight becomes green. To make a segment appear black, the
segment is always turned ON. An FSC LCD divides a color image into 3
images (one Red, one Green and one Blue) and it displays them in order.
Due to persistence of vision, the 3 monochromatic images appear as one
color image. An FSC LCD needs an LCD panel with a refresh rate of
180 Hz, and the response time is reduced to just 5 milliseconds when
compared with normal STN LCD panels which have a response time of 16
milliseconds.^[140]^[141]^[142]^[143] FSC LCDs contain a Chip-On-Glass
driver IC can also be used with a capacitive touchscreen.
Samsung introduced UFB (Ultra Fine & Bright) displays back in 2002,
utilized the super-birefringent effect. It has the luminance, color
gamut, and most of the contrast of a TFT-LCD, but only consumes as much
power as an STN display, according to Samsung. It was being used in a
variety of Samsung cellular-telephone models produced until late 2006,
when Samsung stopped producing UFB displays. UFB displays were also
used in certain models of LG mobile phones.
Active-matrix technologies[edit]
A Casio 1.8 in color TFT LCD, used in the Sony Cyber-shot DSC-P93A
digital compact cameras
Structure of a color LCD with an edge-lit CCFL backlight
Main articles: Thin-film-transistor liquid-crystal display and
Active-matrix liquid-crystal display
See also: List of LCD matrices
Twisted nematic (TN)[edit]
See also: Twisted nematic field effect
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
gray level or transmission can be achieved.
In-plane switching (IPS)[edit]
Main article: IPS panel
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) essentially in the same plane, although fringe fields
inhibit a homogeneous reorientation. This requires two transistors for
each pixel instead of the single transistor needed for a standard
thin-film transistor (TFT) display. The IPS technology is used in
everything from televisions, computer monitors, and even wearable
devices, especially almost all LCD smartphone panels are IPS/FFS mode.
IPS displays belong to the LCD panel family screen types. The other two
types are VA and TN. Before LG Enhanced IPS was introduced in 2001 by
Hitachi as 17" monitor in Market, 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. Panasonic Himeji G8.5 was using an enhanced
version of IPS, also LGD in Korea, then currently the world biggest LCD
panel manufacture BOE in China is also IPS/FFS mode TV panel.
Close-up of a corner of an IPS LCD panel
Super In-plane switching (S-IPS)[edit]
Super-IPS was later introduced after in-plane switching with even
better response times and color reproduction.^[144]
M+ or RGBW controversy[edit]
In 2015 LG Display announced the implementation of a new technology
called M+ which is the addition of white subpixel along with the
regular RGB dots in their IPS panel technology.^[145]
Most of the new M+ technology was employed on 4K TV sets which led to a
controversy after tests showed that the addition of a white sub pixel
replacing the traditional RGB structure would reduce the resolution by
around 25%. This means that a 4K TV cannot display the full UHD TV
standard. The media and internet users later called this "RGBW" TVs
because of the white sub pixel. Although LG Display has developed this
technology for use in notebook display, outdoor and smartphones, it
became more popular in the TV market because the announced 4K UHD
resolution but still being incapable of achieving true UHD resolution
defined by the CTA as 3840x2160 active pixels with 8-bit color. This
negatively impacts the rendering of text, making it a bit fuzzier,
which is especially noticeable when a TV is used as a PC
monitor.^[146]^[147]^[148]^[149]
IPS in comparison to AMOLED[edit]
In 2011, LG claimed the smartphone LG Optimus Black (IPS LCD (LCD
NOVA)) has the brightness up to 700 nits, while the competitor has only
IPS LCD with 518 nits and double an active-matrix OLED (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.^[150] 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 gray. 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.
This pixel-layout is found in S-IPS LCDs. A chevron shape is used to
widen the viewing cone (range of viewing directions with good contrast
and low color shift).
Advanced fringe field switching (AFFS)[edit]
Known as fringe field switching (FFS) until 2003,^[151] 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).^[152] 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/gray
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.
However, it is no longer produced as of February
2015.^[153]^[154]^[155]
Vertical alignment (VA)[edit]
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. It has
a deeper-black background, a higher contrast ratio, a wider viewing
angle, and better image quality at extreme temperatures than
traditional twisted-nematic displays.^[156] Compared to IPS, the black
levels are still deeper, allowing for a higher contrast ratio, but the
viewing angle is narrower, with color and especially contrast shift
being more apparent.^[157]
Blue phase mode[edit]
Main article: Blue phase mode LCD
Blue phase mode LCDs have been shown as engineering samples early in
2008, but they are not in mass-production. 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.^[citation needed]
Quality control[edit]
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 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.^[158] As of 2005, though, Samsung adheres to the less
restrictive ISO 13406-2 standard.^[159] Other companies have been known
to tolerate as many as 11 dead pixels in their policies.^[160]
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,^[161] which was made
obsolete in 2008 with the release of ISO 9241, specifically
ISO-9241-302, 303, 305, 307:2008 pixel defects. 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 whole 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.^[according to whom?]
Some manufacturers, notably in South Korea where some of the largest
LCD panel manufacturers, such as LG, are located, now have a
zero-defective-pixel guarantee, which is an extra screening process
which can then determine "A"- and "B"-grade panels.^[original
research?] 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 mura),
which describes the uneven patches of changes in luminance. It is most
visible in dark or black areas of displayed scenes.^[162] As of 2010,
most premium branded computer LCD panel manufacturers specify their
products as having zero defects.
"Zero-power" (bistable) displays[edit]
See also: Ferroelectric liquid crystal display
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 manufactured both grayscale and color ZBD devices. Kent
Displays has also developed a "no-power" display that uses polymer
stabilized cholesteric liquid crystal (ChLCD). In 2009 Kent
demonstrated the use of a ChLCD to cover the entire surface of a mobile
phone, allowing it to change colors, and keep that color even when
power is removed.^[163]
In 2004, researchers at the University of Oxford demonstrated two new
types of zero-power bistable LCDs based on Zenithal bistable
techniques.^[164] Several bistable technologies, like the 360DEG 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[edit]
* Resolution The resolution of an LCD is expressed by the number of
columns and rows of pixels (e.g., 1024 *768). Each pixel is usually
composed 3 sub-pixels, a red, a green, and a blue one. This had
been one of the few features of LCD performance that remained
uniform among different designs. However, there are newer designs
that share sub-pixels among pixels and add Quattron which attempt
to efficiently increase the perceived resolution of a display
without increasing the actual resolution, to mixed results.
* Spatial performance: For a computer monitor or some other display
that is being viewed from a very close distance, resolution is
often expressed in terms of dot pitch or pixels per inch, which is
consistent with the printing industry. Display density varies per
application, with televisions generally having a low density for
long-distance viewing and portable devices having a high density
for close-range detail. The Viewing Angle of an LCD may be
important depending on the display and its usage, the limitations
of certain display technologies mean the display only displays
accurately at certain angles.
* Temporal performance: the temporal resolution of an LCD is how well
it can display changing images, or the accuracy and the number of
times per second the display draws the data it is being given. LCD
pixels do not flash on/off between frames, so LCD monitors exhibit
no refresh-induced flicker no matter how low the refresh
rate.^[165] But a lower refresh rate can mean visual artefacts like
ghosting or smearing, especially with fast moving images.
Individual pixel response time is also important, as all displays
have some inherent latency in displaying an image which can be
large enough to create visual artifacts if the displayed image
changes rapidly.
* Color performance: There are multiple terms to describe different
aspects of color performance of a display. Color gamut is the range
of colors that can be displayed, and color depth, which is the
fineness with which the color range is divided. Color gamut is a
relatively straight forward feature, but it is rarely discussed in
marketing materials except at the professional level. Having a
color range that exceeds the content being shown on the screen has
no benefits, so displays are only made to perform within or below
the range of a certain specification.^[166] There are additional
aspects to LCD color and color management, such as white point and
gamma correction, which describe what color white is and how the
other colors are displayed relative to white.
* Brightness and contrast ratio: Contrast ratio is the ratio of the
brightness of a full-on pixel to a full-off pixel. The LCD itself
is only a light valve and does not generate light; the light comes
from a backlight that is either fluorescent or a set of LEDs.
Brightness is usually stated as the maximum light output of the
LCD, which can vary greatly based on the transparency of the LCD
and the brightness of the backlight. Brighter backlight allows
stronger contrast and higher dynamic range (HDR displays are graded
in peak luminance), but there is always a trade-off between
brightness and power consumption.
Advantages and disadvantages[edit]
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Some of these issues relate to full-screen displays, others to small
displays as on watches, etc. Many of the comparisons are with CRT
displays.
Further information: Comparison of CRT, LCD, Plasma, and OLED
Advantages[edit]
* Very compact, thin and light, especially in comparison with bulky,
heavy CRT displays.
* Low power consumption. Depending on the set display brightness and
content being displayed, the older CCFT backlit models typically
use less than half of the power a CRT monitor of the same size
viewing area would use, and the modern LED backlit models typically
use 10-25% of the power a CRT monitor would use.^[167]
* Little heat emitted during operation, due to low power consumption.
* No geometric distortion.
* The possible ability to have little or no flicker depending on
backlight technology.
* Usually no refresh-rate flicker, because the LCD pixels hold their
state between refreshes (which are usually done at 200 Hz or
faster, regardless of the input refresh rate).
* Sharp image with no bleeding or smearing when operated at native
resolution.
* Emits almost no undesirable electromagnetic radiation (in the
extremely low frequency range), unlike a CRT
monitor.^[168]^[169]^[better source needed]
* Can be made in almost any size or shape.
* No theoretical resolution limit. When multiple LCD panels are used
together to create a single canvas, each additional panel increases
the total resolution of the display, which is commonly called
stacked resolution.^[170]
* Can be made in large sizes of over 80-inch (2 m) diagonal.
* Masking effect: the LCD grid can mask the effects of spatial and
grayscale quantization, creating the illusion of higher image
quality.^[171]
* Unaffected by magnetic fields, including the Earth's, unlike most
color CRTs.
* As an inherently digital device, the LCD can natively display
digital data from a DVI or HDMI connection without requiring
conversion to analog. Some LCD panels have native fiber optic
inputs in addition to DVI and HDMI.^[172]
* Many LCD monitors are powered by a 12 V power supply, and if built
into a computer can be powered by its 12 V power supply.
* Can be made with very narrow frame borders, allowing multiple LCD
screens to be arrayed side by side to make up what looks like one
big screen.
Disadvantages[edit]
* Limited viewing angle in some older or cheaper monitors, causing
color, saturation, contrast and brightness to vary with user
position, even within the intended viewing angle.
* Uneven backlighting in some monitors (more common in IPS-types and
older TNs), causing brightness distortion, especially toward the
edges ("backlight bleed").
* Black levels may not be as dark as required because individual
liquid crystals cannot completely block all of the backlight from
passing through.
* Display motion blur on moving objects caused by slow response times
(>8 ms) and eye-tracking on a sample-and-hold display, unless a
strobing backlight is used. However, this strobing can cause eye
strain, as is noted next:
* As of 2012, most implementations of LCD backlighting use
pulse-width modulation (PWM) to dim the display,^[173] which makes
the screen flicker more acutely (this does not mean visibly) than a
CRT monitor at 85 Hz refresh rate would (this is because the entire
screen is strobing on and off rather than a CRT's phosphor
sustained dot which continually scans across the display, leaving
some part of the display always lit), causing severe eye-strain for
some people.^[174]^[175] Unfortunately, many of these people don't
know that their eye-strain is being caused by the invisible strobe
effect of PWM.^[176] This problem is worse on many LED-backlit
monitors, because the LEDs switch on and off faster than a CCFL
lamp.
* Only one native resolution. Displaying any other resolution either
requires a video scaler, causing blurriness and jagged edges, or
running the display at native resolution using 1:1 pixel mapping,
causing the image either not to fill the screen (letterboxed
display), or to run off the lower or right edges of the screen.
* Fixed bit depth (also called color depth). Many cheaper LCDs are
only able to display 262144 (2^18) colors. 8-bit S-IPS panels can
display 16 million (2^24) colors and have significantly better
black level, but are expensive and have slower response time.
* Input lag, because the LCD's A/D converter waits for each frame to
be completely been output before drawing it to the LCD panel. Many
LCD monitors do post-processing before displaying the image in an
attempt to compensate for poor color fidelity, which adds an
additional lag. Further, a video scaler must be used when
displaying non-native resolutions, which adds yet more time lag.
Scaling and post processing are usually done in a single chip on
modern monitors, but each function that chip performs adds some
delay. Some displays have a video gaming mode which disables all or
most processing to reduce perceivable input lag.
* Dead or stuck pixels may occur during manufacturing or after a
period of use. A stuck pixel will glow with color even on an
all-black screen, while a dead one will always remain black.
* Subject to burn-in effect, although the cause differs from CRT and
the effect may not be permanent, a static image can cause burn-in
in a matter of hours in badly designed displays.
* In a constant-on situation, thermalization may occur in case of bad
thermal management, in which part of the screen has overheated and
looks discolored compared to the rest of the screen.
* Loss of brightness and much slower response times in low
temperature environments. In sub-zero environments, LCD screens may
cease to function without the use of supplemental heating.
* Loss of contrast in high temperature environments.
Chemicals used[edit]
Several different families of liquid crystals are used in liquid
crystal displays. The molecules used have to be anisotropic, and to
exhibit mutual attraction. Polarizable rod-shaped molecules (biphenyls,
terphenyls, etc.) are common. A common form is a pair of aromatic
benzene rings, with a nonpolar moiety (pentyl, heptyl, octyl, or alkyl
oxy group) on one end and polar (nitrile, halogen) on the other.
Sometimes the benzene rings are separated with an acetylene group,
ethylene, CH=N, CH=NO, N=N, N=NO, or ester group. In practice, eutectic
mixtures of several chemicals are used, to achieve wider temperature
operating range (-10..+60 DEGC for low-end and -20..+100 DEGC for
high-performance displays). For example, the E7 mixture is composed of
three biphenyls and one terphenyl: 39 wt.% of
4'-pentyl[1,1'-biphenyl]-4-carbonitrile (nematic range 24..35 DEGC), 36
wt.% of 4'-heptyl[1,1'-biphenyl]-4-carbonitrile (nematic range
30..43 DEGC), 16 wt.% of 4'-octoxy[1,1'-biphenyl]-4-carbonitrile
(nematic range 54..80 DEGC), and 9 wt.% of
4-pentyl[1,1':4',1-terphenyl]-4-carbonitrile (nematic range
131..240 DEGC).^[177]
Environmental impact[edit]
See also: Electronic waste
The production of LCD screens uses nitrogen trifluoride (NF[3]) as an
etching fluid during the production of the thin-film components. NF[3]
is a potent greenhouse gas, and its relatively long half-life may make
it a potentially harmful contributor to global warming. A report in
Geophysical Research Letters suggested that its effects were
theoretically much greater than better-known sources of greenhouse
gasses like carbon dioxide. As NF[3] was not in widespread use at the
time, it was not made part of the Kyoto Protocols and has been deemed
"the missing greenhouse gas".^[178]
Critics of the report point out that it assumes that all of the NF[3]
produced would be released to the atmosphere. In reality, the vast
majority of NF[3] is broken down during the cleaning processes; two
earlier studies found that only 2 to 3% of the gas escapes destruction
after its use.^[179] Furthermore, the report failed to compare NF[3]'s
effects with what it replaced, perfluorocarbon, another powerful
greenhouse gas, of which anywhere from 30 to 70% escapes to the
atmosphere in typical use.^[179]
See also[edit]
* Transflective liquid-crystal display
* Flat-panel display
* FPD-Link
* LCD classification
* LCD projector
* LCD television
* List of liquid-crystal-display manufacturers
* Boogie board (product) / Remarkable (tablet)
* Raw monitor
* Smartglasses
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External links[edit]
Wikimedia Commons has media related to Liquid crystal displays.
* LCD Monitor Teardown - engineerguyvideo on YouTube
* History and Physical Properties of Liquid Crystals by
Nobelprize.org Archived August 30, 2009, at the Wayback Machine
* Definitions of basic terms relating to low-molar-mass and polymer
liquid crystals (IUPAC Recommendations 2001)
* An intelligible introduction to liquid crystals from Case Western
Reserve University
* Liquid Crystal Physics tutorial from the Liquid Crystals Group,
University of Colorado
* What's an IPS Display from Newhaven Display
* Molecular Crystals and Liquid Crystals a journal by Taylor and
Francis
* How TFT-LCDs are made, by AUO Archived March 8, 2021, at the
Wayback Machine
* How LTPS (Low Temperature Poly Silicon) LCDs are made, by AUO
Archived June 6, 2021, at the Wayback Machine
General information[edit]
* Development of Liquid Crystal Displays: Interview with George Gray,
Hull University, 2004 - Video by the Vega Science Trust.
* Timothy J. Sluckin History of Liquid Crystals, a presentation and
extracts from the book Crystals that Flow: Classic papers from the
history of liquid crystals.
* David Dunmur & Tim Sluckin (2011) Soap, Science, and Flat-screen
TVs: a history of liquid crystals, Oxford University Press
ISBN 978-0-19-954940-5.
Oleg Artamonov (January 23, 2007). "Contemporary LCD Monitor
Parameters: Objective and Subjective Analysis". X-bit labs. Archived
from the original on May 16, 2008. Retrieved May 17, 2008.
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