A liquid crystal display (LCD) is a
thin, flat display device made up of any number of color or monochrome
pixels arrayed in front of a light source or reflector. It is prized by
engineers because it uses very small amounts of electric power, and is
therefore suitable for use in battery-powered electronic devices.
Liquid
crystal displays are one of the reasons laptop computers have been so
successful. Without this creation, we could not have the compactness for
portable computers. Some of the earlier portable computers included a
small CRT monitor and were rather bulky. In the future, LCDs will be
used more, not only for computers but also for HD televisions. As
technology and production becomes less expensive, the cost of a flat
screen computer monitor or a HD television will keep going down. It is
quite possible that eventually LCDs will completely replace the
traditional CRT, just as the transistor replaced the vacuum tube.
Overview
Each
pixel of an LCD consists of a layer of liquid crystal molecules aligned
between two transparent electrodes, and two polarizing filters, the
axes of polarity of which are perpendicular to each other. With no
liquid crystal between the polarizing filters, light passing through one
filter would be blocked by the other.
The surfaces of the
electrodes that are in contact with the liquid crystal material are
treated so as to align the liquid crystal molecules in a particular
direction. This treatment typically consists of a thin polymer layer
that is unidirectionally rubbed using a cloth (the direction of the
liquid crystal alignment is defined by the direction of rubbing).
Before
applying an electric field, the orientation of the liquid crystal
molecules is determined by the alignment at the surfaces. In a twisted
nematic device (the most common liquid crystal device), the surface
alignment directions at the two electrodes are perpendicular, and so the
molecules arrange themselves in a helical structure, or twist. Because
the liquid crystal material is birefringent (i.e. light of different
polarizations travels at different speeds through the material), light
passing through one polarizing filter is rotated by the liquid crystal
helix as it passes through the liquid crystal layer, allowing it to pass
through the second polarized filter. The first polarizing filter
absorbs half of the light, but otherwise the entire assembly is
transparent.
When a voltage is applied across the electrodes, a
torque acts to align the liquid crystal molecules parallel to the
electric field, distorting the helical structure (this is resisted by
elastic forces since the molecules are constrained at the surfaces).
This reduces the rotation of the polarization of the incident light, and
the device appears gray. If the applied voltage is large enough, the
liquid crystal molecules are completely untwisted and the polarization
of the incident light is not rotated at all as it passes through the
liquid crystal layer. This light will then be polarized perpendicular to
the second filter, and thus be completely 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, correspondingly illuminating the pixel.
With a
twisted nematic liquid crystal device it is usual to operate the device
between crossed polarizers, such that it appears bright with no applied
voltage. With this setup, the dark voltage-on state is uniform. The
device can be operated between parallel polarizers, in which case the
bright and dark states are reversed (in this configuration, the dark
state appears blotchy).
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 by applying either an alternating current,
or by reversing the polarity of the electric field as the device is
addressed (the response of the liquid crystal layer is identical,
regardless of the polarity of the applied field).
When a large
number of pixels is required in a display, it is not feasible to drive
each directly since then each pixel would require independent
electrodes. Instead, the display is multiplexed. In a multiplexed
display, electrodes on one side of the display are grouped and wired
together (typically in columns), and each group gets its own voltage
source. On the other side, the electrodes are also grouped (typically in
rows), with each group getting a voltage sink. The groups are designed
so each pixel has a unique, unshared combination of source and sink. The
electronics, or the software driving the electronics then turns on
sinks in sequence, and drives sources for the pixels of each sink.
Important
factors to consider when evaluating an LCD monitor include resolution,
viewable size, response time (sync rate), matrix type (passive or
active), viewing angle, color support, brightness and contrast ratio,
aspect ratio, and input ports (e.g. DVI or VGA).
Brief history
1904: Otto Lehmann publishes his work "Liquid Crystals"
1911: Charles Mauguin describes the structure and properties of liquid crystals.
1936:
The Marconi Wireless Telegraph Company patents the first practical
application of the technology, "The Liquid Crystal Light Valve."
1962:
The first major English language publication on the subject "Molecular
Structure and Properties of Liquid Crystals," by Dr. George W. Gray.
Pioneering
work on liquid crystals was undertaken in the late 1960s by the UK's
Royal Radar Establishment at Malvern. The team at RRE supported ongoing
work by George Gray and his team at the University of Hull who
ultimately discovered the cyanobiphenyl liquid crystals (which had
correct stability and temperature properties for application in LCDs).
The
first operational LCD was based on the Dynamic Scattering Mode (DSM)
and was introduced in 1968 by a group at RCA in the United States,
headed by George Heilmeier. Heilmeier founded Optel, which introduced a
number of LCDs based on this technology.
In December 1970, the
twisted nematic field effect in liquid crystals was filed for patent by
M. Schadt and W. Helfrich, then working for the Central Research
Laboratories of Hoffmann-LaRoche in Switzerland (Swiss patent No.
CH532261). James Fergason at Kent State University filed an identical
patent in the U.S. in February 1971.
In 1971 the company of
Fergason ILIXCO (now LXD Incorporated) produced the first LCDs based on
the TN-effect, which soon superseded the poor-quality DSM types due
improvements of lower operating voltages and lower power consumption.
In the United States in 1972, T. Peter Brody produced the first active-matrix liquid crystal display panel.
In
2005 Mary Lou Jepsen developed a new type of LCD display for the One
Laptop Per Child project to reduce power consumption and manufacturing
cost of the Children's Machine. This display uses a plastic diffraction
grating and lenses on the rear of the LCD to illuminate the colored
subpixels. This method absorbs very little light, allowing for a much
brighter display with a lower powered backlight. Replacing the backlight
with a white LED allows for reduced costs and increased durability as
well as a wider color gamut.
Color displays
In
color LCDs, each individual pixel is divided into three cells, or
subpixels, which are colored red, green, and blue, respectively, by
additional filters (pigment filters, dye filters and metal oxide
filters). Each subpixel can be controlled independently to yield
thousands or millions of possible colors for each pixel. Older CRT
monitors employ a similar method.
Color components may be arrayed
in various pixel geometries, depending on the monitor's usage. If
software knows which type of geometry is being used in a given LCD, this
can be used to increase the apparent resolution of the monitor through
subpixel rendering. This technique is especially useful for text
anti-aliasing.
Passive-matrix and active-matrix
LCDs
with a small number of segments, such as those used in digital watches
and pocket calculators, have a single electrical contact for each
segment. An external dedicated circuit supplies an electric charge to
control each segment. This display structure is unwieldy for more than a
few display elements.
Small monochrome displays such as those
found in personal organizers, or older laptop screens have a
passive-matrix structure employing supertwist nematic (STN) or
double-layer STN (DSTN) technology (DSTN corrects a color-shifting
problem with STN). Each row or column of the display has a single
electrical circuit. The pixels are addressed one at a time by row and
column addresses. This type of display is called a passive matrix
because the pixel must retain its state between refreshes without the
benefit of a steady electrical charge. As the number of pixels (and,
correspondingly, columns and rows) increases, this type of display
becomes less feasible. Very slow response times and poor contrast are
typical of passive-matrix LCDs.
High-resolution color displays
such as modern LCD computer monitors and televisions use an "active
matrix" structure. A matrix of thin-film transistors (TFTs) is added to
the polarizing and color filters. Each pixel has its own dedicated
transistor, allowing each column line to access one pixel. When a row
line is activated, all of the column lines are connected to a row of
pixels and the correct voltage is driven onto all of the column lines.
The row line is then deactivated and the next row line is activated. All
of the row lines are activated in sequence during a refresh operation.
Active-matrix displays are much brighter and sharper than passive-matrix
displays of the same size, and generally have quicker response times,
producing much better images.
Active matrix technologies
Twisted nematic (TN)
Twisted
nematic displays contain liquid crystal elements that twist and untwist
at varying degrees to allow light to pass through. When no voltage is
applied to a TN liquid crystal cell, the light is polarized to pass
through the cell. In proportion to the voltage applied, the LC cells
twist up to 90 degrees changing the polarization and blocking the
light's path. By properly adjusting the level of the voltage almost any
grey level or transmission can be achieved.
3LCD Display Technology
3LCD
is a video projection system that uses three LCD microdisplay panels to
produce an image. It was adopted in 1995 by numerous front projector
manufacturers and in 2002 by rear projection TV manufacturers for its
compactness and image quality.
3LCD is an active-matrix, HTPS
(high-temperature polysilicon) LCD projection technology. It inherits
sharp images, brightness and excellent color reproduction from its
active matrix technology. Deeper blacks are contributed by the HTPS
technology.
The 3LCD website describes the technology in detail
and is supported by various companies including 3LCD manufacturers and
vendors.
In-plane switching (IPS)
In-plane
switching is an LCD technology that aligns the liquid crystal cells in a
horizontal direction. In this method, the electrical field is applied
through each end of the crystal, but this requires two transistors for
each pixel instead of the one needed for a standard thin-film transistor
(TFT) display. Before Enhanced IPS (e-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. Following the
introduction of e-IPS, other forms of IPS were developed, including
S-IPS, H-IPS, and P-IPS, with even better response times and color
reproduction. Currently, IPS panels are generally considered the best
overall LCD technology for image quality, color accuracy, and viewing
angles.
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, LCD panels with a few
defective pixels are usually still usable. It is also economically
prohibitive to discard a panel with just a few defective pixels because
LCD panels are much larger than ICs. Manufacturers have different
standards for determining a maximum acceptable number of defective
pixels.
LCD panels are more likely to have defects than most ICs
due to their larger size. In this example, a 12-inch SVGA LCD has eight
defects and a six-inch wafer has only three defects.
The location
of defective pixels is important. A display with only a few defective
pixels may be unacceptable if the defective pixels are near each other.
Manufacturers may also relax their replacement criteria when defective
pixels are in the center of the viewing area.
LCD panels also
have defects known as clouding (or lmura), which describes the uneven
patches of changes in luminance. It is most visible in dark or black
areas of displayed scenes.
Zero-power displays
The
zenithal bistable device (ZBD), developed by QinetiQ (formerly DERA),
can retain an image without power. The crystals may exist in one of two
stable orientations (Black and "White") and power is only required to
change the image. ZBD Displays is a spin-off company from QinetiQ who
manufacture both grayscale and color ZBD devices.
A French
company, Nemoptic, has developed another zero-power, paper-like LCD
technology that has been mass-produced in Taiwan since July 2003. This
technology is intended for use in low-power mobile applications such as
e-books and wearable computers. Zero-power LCDs are in competition with
electronic paper.
Kent Displays has also developed a "no power"
display that uses Polymer Stabilized Cholesteric Liquid Crystals
(ChLCD). The major drawback to the ChLCD display is slow refresh rate,
especially with low temperatures.
Drawbacks
LCD technology still has a few drawbacks in comparison to some other display technologies:
- While CRTs are capable of displaying multiple video resolutions without introducing artifacts, LCD displays produce crisp images only in their "native resolution" and, sometimes, fractions of that native resolution. Attempting to run LCD display panels at non-native resolutions usually results in the panel scaling the image, which introduces blurriness or "blockiness."
- LCD displays have a lower contrast ratio than that on a plasma display or CRT. This is due to their "light valve" nature: some light always leaks out and turns black into gray. In brightly lit rooms the contrast of LCD monitors can, however, exceed some CRT displays due to higher maximal brightness.
- LCDs have longer response time than their plasma and CRT counterparts, older displays creating visible ghosting when images rapidly change; this drawback, however, is continually improving as the technology progresses and is hardly noticeable in current LCD displays with "overdrive" technology. Most newer LCDs have response times of around 8 milliseconds.
- Overdrive technology on some panels can produce artifacts across regions of rapidly transitioning pixels (e.g. video images) that looks like increased image noise or halos. This is a side effect of the pixels being driven past their intended brightness value (or rather the intended voltage necessary to produce this necessary brightness/color) and then allowed to fall back to the target brightness in order to enhance response times.
- LCD display panels have a limited viewing angle, thus reducing the number of people who can conveniently view the same image. As the viewer moves closer to the limit of the viewing angle, the colors and contrast appear to deteriorate. However, this negative has actually been capitalized upon in two ways. Some vendors offer screens with intentionally reduced viewing angle, to provide additional privacy, such as when someone is using a laptop in a public place. Such a set can also show two different images to one viewer, providing a three-dimensional effect.
- Some users of older (around pre-2000) LCD monitors complain of migraines and eyestrain problems due to flicker from fluorescent backlights fed at 50 or 60 Hz. This does not happen with most modern displays which feed backlights with high-frequency current.
- LCD screens occasionally suffer from image persistence, which is similar to screen burn on CRT and plasma displays. This is becoming less of a problem as technology advances, with newer LCD panels using various methods to reduce the problem. Sometimes the panel can be restored to normal by displaying an all-white pattern for extended periods of time.
- Some light guns do not work with this type of display since they do not have flexible lighting dynamics that CRTs have. However, the field emission display will be a potential replacement for LCD flat-panel displays since they emulate CRTs in some technological ways.
- Some panels are incapable of displaying low-resolution screen modes (such as 320 by 200 pixels). However, this is due to the circuitry that drives the LCD rather than the LCD itself.
- Consumer LCD monitors are more fragile than their CRT counterparts, with the screen especially vulnerable. However, lighter weight makes falling less dangerous, and some displays may be protected with glass shields.