This diagram shows the nomenclature for the different phase transitions.
The term phase transition (or phase change) is most commonly used to describe transitions between solid, liquid, and gaseousstates of matter, as well as plasma in rare cases. A phase of a thermodynamic system and the states of matter have uniform physical properties.
During a phase transition of a given medium, certain properties of the
medium change, often discontinuously, as a result of the change of some
external condition, such as temperature, pressure, or others. For example, a liquid may become gas upon heating to the boiling point,
resulting in an abrupt change in volume. The measurement of the
external conditions at which the transformation occurs is termed the
phase transition. Phase transitions commonly occur in nature and are
used today in many technologies.
Types of phase transition
Examples of phase transitions include:
The transitions between the solid, liquid, and gaseous phases of a single component, due to the effects of temperature and/or pressure:
A small piece of rapidly melting solid argon simultaneously shows the transitions from solid to liquid and liquid to gas.
Comparison
of phase diagrams of carbon dioxide (red) and water (blue) explaining
their different phase transitions at 1 atmosphere
A eutectic
transformation, in which a two component single phase liquid is cooled
and transforms into two solid phases. The same process, but beginning
with a solid instead of a liquid is called a eutectoid transformation.
A peritectic transformation, in which a two component single phase solid is heated and transforms into a solid phase and a liquid phase.
A spinodal decomposition, in which a single phase is cooled and separates into two different compositions of that same phase.
Transition to a mesophase between solid and liquid, such as one of the "liquid crystal" phases.
The dependence of the adsorption geometry on coverage and temperature, such as for hydrogen on iron (110).
The emergence of superconductivity in certain metals and ceramics when cooled below a critical temperature.
The transition between different molecular structures (polymorphs, allotropes or polyamorphs), especially of solids, such as between an amorphous structure and a crystal structure, between two different crystal structures, or between two amorphous structures.
The breaking of symmetries in the laws of physics during the early history of the universe as its temperature cooled.
Isotope fractionation occurs during a phase transition, the ratio of light to heavy isotopes in the involved molecules changes. When water vapor condenses (an equilibrium fractionation),
the heavier water isotopes (18O and 2H) become enriched in the liquid
phase while the lighter isotopes (16O and 1H) tend toward the vapor
phase.
Phase transitions occur when the thermodynamic free energy of a system is non-analytic for some choice of thermodynamic variables (cf. phases).
This condition generally stems from the interactions of a large number
of particles in a system, and does not appear in systems that are too
small. It is important to note that phase transitions can occur and are
defined for non-thermodynamic systems, where temperature is not a
parameter. Examples include: quantum phase transitions, dynamic phase
transitions, and topological (structural) phase transitions. In these
types of systems other parameters take the place of temperature. For
instance, connection probability replaces temperature for percolating
networks.
At the phase transition point (for instance, boiling point) the two phases of a substance, liquid and vapor,
have identical free energies and therefore are equally likely to exist.
Below the boiling point, the liquid is the more stable state of the
two, whereas above the gaseous form is preferred.
It is sometimes possible to change the state of a system diabatically (as opposed to adiabatically)
in such a way that it can be brought past a phase transition point
without undergoing a phase transition. The resulting state is metastable, i.e., less stable than the phase to which the transition would have occurred, but not unstable either. This occurs in superheating, supercooling, and supersaturation, for example.
Classifications
Ehrenfest classification
Paul Ehrenfest classified phase transitions based on the behavior of the thermodynamic free energy as a function of other thermodynamic variables.
Under this scheme, phase transitions were labeled by the lowest
derivative of the free energy that is discontinuous at the transition. First-order phase transitions exhibit a discontinuity in the first derivative of the free energy with respect to some thermodynamic variable.
The various solid/liquid/gas transitions are classified as first-order
transitions because they involve a discontinuous change in density,
which is the (inverse of the) first derivative of the free energy with
respect to pressure. Second-order phase transitions are continuous in the first derivative (the order parameter,
which is the first derivative of the free energy with respect to the
external field, is continuous across the transition) but exhibit
discontinuity in a second derivative of the free energy. These include the ferromagnetic phase transition in materials such as iron, where the magnetization,
which is the first derivative of the free energy with respect to the
applied magnetic field strength, increases continuously from zero as the
temperature is lowered below the Curie temperature. The magnetic susceptibility,
the second derivative of the free energy with the field, changes
discontinuously. Under the Ehrenfest classification scheme, there could
in principle be third, fourth, and higher-order phase transitions.
Though useful, Ehrenfest's classification has been found to be an
incomplete method of classifying phase transitions, for it does not
take into account the case where a derivative of free energy diverges (which is only possible in the thermodynamic limit). For instance, in the ferromagnetic transition, the heat capacity diverges to infinity. The same phenomenon is also seen in superconducting phase transition.
Modern classifications
In
the modern classification scheme, phase transitions are divided into
two broad categories, named similarly to the Ehrenfest classes:
First-order phase transitions are those that involve a latent heat.
During such a transition, a system either absorbs or releases a fixed
(and typically large) amount of energy per volume. During this process,
the temperature of the system will stay constant as heat is added: the
system is in a "mixed-phase regime" in which some parts of the system
have completed the transition and others have not. Familiar examples are
the melting of ice or the boiling of water (the water does not
instantly turn into vapor, but forms a turbulent
mixture of liquid water and vapor bubbles). Imry and Wortis showed that
quenched disorder can broaden a first-order transition. That is, the
transformation is completed over a finite range of temperatures, but
phenomena like supercooling and superheating survive and hysteresis is
observed on thermal cycling.
Second-order phase transitions are also called continuous phase transitions.
They are characterized by a divergent susceptibility, an infinite
correlation length, and a power-law decay of correlations near
criticality. Examples of second-order phase transitions are the
ferromagnetic transition, superconducting transition (for a Type-I superconductor the phase transition is second-order at zero external field and for a Type-II superconductor
the phase transition is second-order for both normal-state—mixed-state
and mixed-state—superconducting-state transitions) and the superfluid
transition. In contrast to viscosity, thermal expansion and heat
capacity of amorphous materials show a relatively sudden change at the
glass transition temperature which enables accurate detection using differential scanning calorimetry measurements. Lev Landau gave a phenomenologicaltheory of second-order phase transitions.
Apart from isolated, simple phase transitions, there exist transition lines as well as multicritical points, when varying external parameters like the magnetic field or composition.
The liquid–glass transition is observed in many polymers and other liquids that can be supercooled
far below the melting point of the crystalline phase. This is atypical
in several respects. It is not a transition between thermodynamic ground
states: it is widely believed that the true ground state is always
crystalline. Glass is a quenched disorder
state, and its entropy, density, and so on, depend on the thermal
history. Therefore, the glass transition is primarily a dynamic
phenomenon: on cooling a liquid, internal degrees of freedom
successively fall out of equilibrium. Some theoretical methods predict
an underlying phase transition in the hypothetical limit of infinitely
long relaxation times. No direct experimental evidence supports the existence of these transitions.
Characteristic properties
Phase coexistence
A
disorder-broadened first-order transition occurs over a finite range
of temperatures where the fraction of the low-temperature equilibrium
phase grows from zero to one (100%) as the temperature is lowered. This
continuous variation of the coexisting fractions with temperature raised
interesting possibilities. On cooling, some liquids vitrify into a
glass rather than transform to the equilibrium crystal phase. This
happens if the cooling rate is faster than a critical cooling rate, and
is attributed to the molecular motions becoming so slow that the
molecules cannot rearrange into the crystal positions. This slowing down happens below a glass-formation temperature Tg, which may depend on the applied pressure.
If the first-order freezing transition occurs over a range of
temperatures, and Tg falls within this range, then there is an
interesting possibility that the transition is arrested when it is
partial and incomplete. Extending these ideas to first-order magnetic
transitions being arrested at low temperatures, resulted in the
observation of incomplete magnetic transitions, with two magnetic phases
coexisting, down to the lowest temperature. First reported in the case
of a ferromagnetic to anti-ferromagnetic transition,
such persistent phase coexistence has now been reported across a
variety of first-order magnetic transitions. These include
colossal-magnetoresistance manganite materials, magnetocaloric materials, magnetic shape memory materials, and other materials.
The interesting feature of these observations of Tg falling within the
temperature range over which the transition occurs is that the
first-order magnetic transition is influenced by magnetic field, just
like the structural transition is influenced by pressure. The relative
ease with which magnetic fields can be controlled, in contrast to
pressure, raises the possibility that one can study the interplay
between Tg and Tc in an exhaustive way. Phase coexistence across
first-order magnetic transitions will then enable the resolution of
outstanding issues in understanding glasses.
Critical points
In
any system containing liquid and gaseous phases, there exists a special
combination of pressure and temperature, known as the critical point,
at which the transition between liquid and gas becomes a second-order
transition. Near the critical point, the fluid is sufficiently hot and
compressed that the distinction between the liquid and gaseous phases is
almost non-existent. This is associated with the phenomenon of critical opalescence, a milky appearance of the liquid due to density fluctuations at all possible wavelengths (including those of visible light).
Symmetry
Phase transitions often involve a symmetry breaking process. For instance, the cooling of a fluid into a crystalline solid breaks continuous translation symmetry:
each point in the fluid has the same properties, but each point in a
crystal does not have the same properties (unless the points are chosen
from the lattice points of the crystal lattice). Typically, the
high-temperature phase contains more symmetries than the low-temperature
phase due to spontaneous symmetry breaking, with the exception of certain accidental symmetries (e.g. the formation of heavy virtual particles, which only occurs at low temperatures).
Order parameters
An order parameter
is a measure of the degree of order across the boundaries in a phase
transition system; it normally ranges between zero in one phase (usually
above the critical point) and nonzero in the other. At the critical point, the order parameter susceptibility will usually diverge.
An example of an order parameter is the net magnetization in a ferromagnetic system undergoing a phase transition. For liquid/gas transitions, the order parameter is the difference of the densities.
From a theoretical perspective, order parameters arise from
symmetry breaking. When this happens, one needs to introduce one or more
extra variables to describe the state of the system. For example, in
the ferromagnetic phase, one must provide the net magnetization, whose direction was spontaneously chosen when the system cooled below the Curie point.
However, note that order parameters can also be defined for
non-symmetry-breaking transitions. Some phase transitions, such as
superconducting and ferromagnetic, can have order parameters for more
than one degree of freedom. In such phases, the order parameter may take
the form of a complex number, a vector, or even a tensor, the magnitude
of which goes to zero at the phase transition.
There also exist dual descriptions of phase transitions in terms
of disorder parameters. These indicate the presence of line-like
excitations such as vortex- or defect lines.
Relevance in cosmology
Symmetry-breaking phase transitions play an important role in cosmology. It has been speculated by Lee Smolin and Benjamin and Jeremy Bernstein that, in the hot early universe, the vacuum (i.e. the various quantum fields
that fill space) possessed a large number of symmetries. As the
universe expanded and cooled, the vacuum underwent a series of
symmetry-breaking phase transitions. For example, the electroweak
transition broke the SU(2)×U(1) symmetry of the electroweak field into the U(1) symmetry of the present-day electromagnetic field.
This transition is important to understanding the asymmetry between the
amount of matter and antimatter in the present-day universe.
Progressive phase transitions in an expanding universe are
implicated in the development of order in the universe, as is
illustrated by the work of Eric Chaisson and David Layzer.
Critical exponents and universality classes
Continuous phase transitions are easier to study than first-order transitions due to the absence of latent heat,
and they have been discovered to have many interesting properties. The
phenomena associated with continuous phase transitions are called
critical phenomena, due to their association with critical points.
It turns out that continuous phase transitions can be characterized by parameters known as critical exponents. The most important one is perhaps the exponent describing the divergence of the thermal correlation length by approaching the transition. For instance, let us examine the behavior of the heat capacity near such a transition. We vary the temperature T
of the system while keeping all the other thermodynamic variables
fixed, and find that the transition occurs at some critical temperature Tc . When T is near Tc , the heat capacity C typically has a power law behavior,
The heat capacity of amorphous materials has such a behaviour near
the glass transition temperature where the universal critical exponent α
= 0.59 A similar behavior, but with the exponent ν instead of α, applies for the correlation length.
The exponent ν is positive. This is different with α. Its actual value depends on the type of phase transition we are considering.
It is widely believed that the critical exponents are the same
above and below the critical temperature. It has now been shown that
this is not necessarily true: When a continuous symmetry is explicitly
broken down to a discrete symmetry by irrelevant (in the renormalization
group sense) anisotropies, then some exponents (such as , the exponent of the susceptibility) are not identical.
For −1 < α < 0, the heat capacity has a "kink" at the
transition temperature. This is the behavior of liquid helium at the lambda transition from a normal state to the superfluid state, for which experiments have found α
= -0.013±0.003.
At least one experiment was performed in the zero-gravity conditions of
an orbiting satellite to minimize pressure differences in the sample. This experimental value of α agrees with theoretical predictions based on variational perturbation theory.
For 0 < α < 1, the heat capacity diverges at the transition temperature (though, since α
< 1, the enthalpy stays finite). An example of such behavior is the
3D ferromagnetic phase transition. In the three-dimensional Ising model for uniaxial magnets, detailed theoretical studies have yielded the exponent α ∼ +0.110.
Some model systems do not obey a power-law behavior. For example,
mean field theory predicts a finite discontinuity of the heat capacity
at the transition temperature, and the two-dimensional Ising model has a
logarithmic
divergence. However, these systems are limiting cases and an exception
to the rule. Real phase transitions exhibit power-law behavior.
Several other critical exponents, β, γ, δ, ν, and η,
are defined, examining the power law behavior of a measurable physical
quantity near the phase transition. Exponents are related by scaling
relations, such as
.
It can be shown that there are only two independent exponents, e.g. ν and η.
It is a remarkable fact that phase transitions arising in
different systems often possess the same set of critical exponents. This
phenomenon is known as universality. For example, the critical
exponents at the liquid–gas critical point have been found to be
independent of the chemical composition of the fluid.
More impressively, but understandably from above, they are an
exact match for the critical exponents of the ferromagnetic phase
transition in uniaxial magnets. Such systems are said to be in the same
universality class. Universality is a prediction of the renormalization group
theory of phase transitions, which states that the thermodynamic
properties of a system near a phase transition depend only on a small
number of features, such as dimensionality and symmetry, and are
insensitive to the underlying microscopic properties of the system.
Again, the divergence of the correlation length is the essential point.
Critical slowing down and other phenomena
There are also other critical phenomena; e.g., besides static functions there is also critical dynamics. As a consequence, at a phase transition one may observe critical slowing down or speeding up. The large static universality classes of a continuous phase transition split into smaller dynamic universality
classes. In addition to the critical exponents, there are also
universal relations for certain static or dynamic functions of the
magnetic fields and temperature differences from the critical value.
Percolation theory
Another phenomenon which shows phase transitions and critical exponents is percolation.
The simplest example is perhaps percolation in a two dimensional square
lattice. Sites are randomly occupied with probability p. For small
values of p the occupied sites form only small clusters. At a certain
threshold pc a giant cluster is formed and we have a second-order phase transition. The behavior of P∞ near pc is, P∞~(p-pc)β, where β is a critical exponent.
Phase transitions in biological systems
Phase transitions play many important roles in biological systems. Examples include the lipid bilayer formation, the coil-globule transition in the process of protein folding and DNA melting, liquid crystal-like transitions in the process of DNA condensation, and cooperative ligand binding to DNA and proteins with the character of phase transition.
In biological membranes, gel to liquid crystalline phase
transitions play a critical role in physiological functioning of
biomembranes. In gel phase, due to low fluidity of membrane lipid
fatty-acyl chains, membrane proteins have restricted movement and thus
are restrained in exercise of their physiological role. Plants depend
critically on photosynthesis by chloroplast thylakoid membranes which
are exposed cold environmental temperatures. Thylakoid membranes retain
innate fluidity even at relatively low temperatures because of high
degree of fatty-acyl disorder allowed by their high content of linolenic
acid, 18-carbon chain with 3-double bonds.
Gel-to-liquid crystalline phase transition temperature of biological
membranes can be determined by many techniques including calorimetry,
flouorescence, spin labelelectron paramagnetic resonance and NMR
by recording measurements of the concerned parameter by at series of
sample temperatures. A simple method for its determination from 13-C NMR
line intensities has also been proposed.
It has been proposed that some biological systems might lie near critical points. Examples include neural networks in the salamander retina, bird flocks
gene expression networks in Drosophila, and protein folding.
However, it is not clear whether or not alternative reasons could
explain some of the phenomena supporting arguments for criticality.
It has also been suggested that biological organisms share two key
properties of phase transitions: the change of macroscopic behavior and
the coherence of a system at a critical point.
In groups of organisms in stress (when approaching critical
transitions), correlations tend to increase, while at the same time,
fluctuations also increase. This effect is supported by many experiments
and observations of groups of people, mice, trees, and grassy plants.
Polarizing filter film with a vertical axis to polarize light as it enters.
Glass substrate with ITOelectrodes.
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.
Twisted nematic liquid crystal.
Glass substrate with common electrode film (ITO) with horizontal ridges to line up with the horizontal filter.
Polarizing filter film with a horizontal axis to block/pass light.
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. Liquid crystals do not emit light directly, instead using a backlight or reflector to produce images in color or monochrome.
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, such as preset words, digits, and seven-segment displays, as in a digital clock. They use the same basic technology, except that arbitrary images are made up of a large number of small pixels, while other displays have larger elements.
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.
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 CRTs 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.
General characteristics
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 transparentelectrodes, and two polarizingfilters
(parallel and perpendicular), 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. Color LCD
systems use the same technique, with color filters used to generate red,
green, and blue pixels.
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. Note that 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, in particular in smartphones such as iPhones. 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. 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.
History
1880s-1960s
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.
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.
A description of Swiss contributions to LCD developments, written by Peter J. Wild, can be found at the Engineering and Technology History Wiki.
In 1888, 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: Beiträge zur Kenntniss des Cholesterins, Monatshefte für Chemie (Wien) 9, 421–441 (1888)). In 1904, Otto Lehmann published his work "Flüssige 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 3 types (nematics, smectics and cholesterics). In
1927, Vsevolod Frederiks devised the electrically switched light valve, called the Fréedericksz 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 on the subject "Molecular Structure and Properties of Liquid Crystals", by Dr. George W. Gray. 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.
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. George H. Heilmeier was inducted in the National Inventors Hall of Fame and credited with the invention of LCDs. Heilmeier's work is an IEEE Milestone. 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.
1970s
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. Hoffmann-La Roche then licensed the invention to the Swiss manufacturer Brown, Boveri & Cie
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 UniversityLiquid Crystal Institute, filed an identical patent in the United States on April 22, 1971. 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.
In 1972, the first wristwatch with TN-LCD was launched on the market:
The Gruen Teletime which was a four digit display watch. The same year,
the first active-matrixthin-film transistor (TFT) liquid-crystal display panel was prototyped in the United States by T. Peter Brody's team at Westinghouse, in Pittsburgh, Pennsylvania.
In 1973, Sharp Corporation introduced the use of LCD displays for calculators, and then mass-produced TN LCD displays for watches in 1975. Other japanese compagnies soon took a leading position in the market of wristwatch like Seiko
and its first 6-digit TN-LCD quartz wristwatch. A particular type of
color LCD was invented 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, and then improved by Fumiaki Funada and Masataka Matsuura in December 1975. 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, then improved in 1977 by a Sharp team consisting of Kohei Kishi, Hirosaku Nonomura, Keiichiro Shimizu, and Tomio Wada. 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
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, 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.
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.
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.
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. In 1982, Seiko Epson released the first LCD television, the Epson TV Watch, a wristwatch equipped with a small active-matrix LCD television. Sharp Corporation introduced dot matrix TN-LCD in 1983. In 1984, Epson released the ET-10, the first full-color, pocket LCD television. The same year, Citizen Watch, introduced the Citizen Pocket TV, a 2.7-inch color LCD TV, with the first commercial TFT LCD display.
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. Epson developed the 3LCD projection technology in the 1980s, and licensed it for use in projectors in 1988. Epson's VPJ-700, released in January 1989, was the world's first compact, full-color LCD projector.
1990s
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. 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.
The Fraunhofer Institute 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. 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. In the late 1990s, the LCD industry began shifting away from Japan, towards South Korea and Taiwan.
2000s-2010s
In 2007 the image quality of LCD televisions surpassed the image quality of cathode-ray-tube-based (CRT) TVs. In the fourth quarter of 2007, LCD televisions surpassed CRT TVs in worldwide sales for the first time. LCD TVs were projected to account 50% of the 200 million TVs to be shipped globally in 2006, according to Displaybank. In October 2011, Toshiba announced 2560 × 1600 pixels on a 6.1-inch (155 mm) LCD panel, suitable for use in a tablet computer, especially for Chinese character display.
Illumination
Since
LCD panels produce no light of their own, they require external light
to produce a visible image. In a transmissive type of LCD, this light is
provided at the back of the glass stack and is called the backlight.
While passive-matrix displays are usually not backlit (e.g.
calculators, wristwatches), active-matrix displays almost always are.
Over the last years (1990 — 2017), the LCD backlight technologies have
strongly been emerged by lighting companies such as Philips, Lumileds (a
Philips subsidiary) and more.
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 cathodefluorescent lamps
placed at opposite edges of the display or an array of parallel CCFLs
behind larger displays. A diffuser 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. 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 is then used to spread the
light evenly across the whole display. 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 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),
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 the LEDs in the dark
areas of the image being displayed, effectively increasing the contrast
ratio of the display. 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.
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.
The LCD backlight systems are made highly efficient by applying
optical films such as prismatic structure 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), generally achieved using so called DBEF films manufactured and supplied by 3M.
These polarizers consist of a large stack of uniaxial oriented
birefringent films that reflect the former absorbed polarization mode of
the light.
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. The combination of such reflective polarizers, and LED dynamic backlight control
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.
Due to the LCD layer that generates the desired high resolution
images at flashing video speeds using very low power electronics in
combination with these excellent 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 does 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. Although until today such reflective colour filter solutions are not
yet implemented by the LCD industry and did not made it further than
laboratory prototypes, such reflective colour filter solutions still
likely will be implemented by the LCD industry to increase the
performance gap with OLED technologies).
Connection to other circuits
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, click on the image for more
detail.)
A standard television receiver screen, an LCD panel today in 2017,
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 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. These same
principles apply also for smart phone screens that are so much smaller
than TV screens.
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 either adhesive plastic ribbon with conductive
traces glued to the edges of the LCD panel, or with an elastomeric connector,
which is a strip of rubber or silicone with alternating layers of
conductive and insulating pathways, pressed between contact pads on the
LCD and mating contact pads on a circuit board.
Passive and active-matrix
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) and the original Nintendo Game Boy 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)
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 (the
voltages according to the "Alt & Pleshko" drive scheme)
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.
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.
Citizen, amongst others, licensed these patents and successfully
introduced several STN based LCD pocket televisions on the market.
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 extreme high resolution images up to 4000 lines
or more using only low voltages.
Since a pixel however 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, 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
showing still pictures only. After a page is written to the display,
the display may be cut from the power while that information remains
readable. This has the advantage that such ebooks may be operated long
time on just a small battery only. 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.
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)
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.
Before LG
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. 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 and their Chromebook 11.
Comparison to AMOLED
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.
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.
Super In-plane switching (S-IPS)
Super-IPS was later introduced after in-plane switching with even better response times and color reproduction.
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)
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/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.
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. 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.
Blue phase mode
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.
Quality control
Some LCD panels have defective transistors, causing permanently lit or unlit pixels which are commonly referred to as stuck pixels or dead pixels respectively. Unlike integrated circuits
(ICs), LCD panels with a few defective 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 13406-2 standard. Other companies have been known to tolerate as many as 11 dead pixels in their policies.
Dead pixel policies are often hotly debated between manufacturers
and customers. To regulate the acceptability of defects and to protect
the end user, ISO released the ISO 13406-2 standard.
However, not every LCD manufacturer conforms to the ISO standard and
the ISO standard is quite often interpreted in different ways. LCD
panels are more likely to have defects than most ICs due to their larger
size. 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.
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.
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.
"Zero-power" (bistable) displays
The zenithal bistable device (ZBD), developed by Qinetiq (formerly DERA),
can retain an image without power. The crystals may exist in one of two
stable orientations ("black" and "white") and power is only required to
change the image. ZBD Displays is a spin-off company from QinetiQ who
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.
In 2004 researchers at the University of Oxford demonstrated two new types of zero-power bistable LCDs based on Zenithal bistable techniques. Several bistable technologies, like the 360° BTN and the bistable
cholesteric, depend mainly on the bulk properties of the liquid crystal
(LC) and use standard strong anchoring, with alignment films and LC
mixtures similar to the traditional monostable materials. Other bistable
technologies, e.g., BiNem technology, are based mainly on the surface properties and need specific weak anchoring materials.
Specifications
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.
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. 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. In general, brighter is better, but there is always a trade-off between brightness and power consumption.
Advantages and disadvantages
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.
Advantages
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.
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.
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.
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.
Unaffected by magnetic fields, including the Earth's.
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.
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
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, 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. Unfortunately, many of these people don't know that their eye-strain is being caused by the invisible strobe effect of PWM. 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
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 refresh rate. All but a few high-end monitors support no higher than 60 or 75 Hz;
while this does not cause visible flicker due to the LCD panel's high
internal refresh rate, the low input refresh rate limits the maximum
frame-rate that can be displayed, affecting gaming and 3D graphics.
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
Several
different families of liquid crystals are used in liquid crystals. 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 °C for low-end and -20..+100 °C 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 °C), 36
wt.% of 4'-heptyl[1,1'-biphenyl]-4-carbonitrile (nematic range
30..43 °C), 16 wt.% of 4'-octoxy[1,1'-biphenyl]-4-carbonitrile (nematic
range 54..80 °C), and 9 wt.% of 4-pentyl[1,1':4',1-terphenyl]-4-carbonitrile (nematic range 131..240 °C).