A general purpose alphanumeric LCD, with two lines of 16 characters.
LCDs with a small number of segments, such as those used in digital watches and pocket calculators, have individual electrical contacts for each segment. An external dedicated circuit supplies an electric charge to control each segment. This display structure is unwieldy for more than a few display elements.
Small monochrome displays such as those found in personal organizers, or older laptop screens have a passive-matrix structure employing super-twisted nematic (STN) or double-layer STN (DSTN) technology—the latter of which addresses a color-shifting problem with the former—and color-STN (CSTN)—wherein color is added by using an internal filter. Each row or column of the display has a single electrical circuit. The pixels are addressed one at a time by row and column addresses. This type of display is called passive-matrix addressed because the pixel must retain its state between refreshes without the benefit of a steady electrical charge.
As the number of pixels (and, correspondingly, columns and rows) increases, this type of display becomes less feasible. Very slow response times and poor contrast are typical of passive-matrix addressed LCDs.
High-resolution color displays such as modern LCD computer monitors and televisions use an active matrix structure. A matrix of thin-film transistors (TFTs) is added to the polarizing and color filters. Each pixel has its own dedicated transistor, allowing each column line to access one pixel. When a row line is activated, all of the column lines are connected to a row of pixels and the correct voltage is driven onto all of the column lines. The row line is then deactivated and the next row line is activated. All of the row lines are activated in sequence during a refresh operation.
Active-matrix addressed displays look "brighter" and "sharper" than passive-matrix addressed displays of the same size, and generally have quicker response times, producing much better images.
A Casio 1.8" color TFT liquid crystal display which equips the Sony Cyber-shot DSC-P93A digital compact cameras
Twisted nematic (TN)
Twisted nematic displays contain liquid crystal elements which twist and untwist at varying degrees to allow light to pass through. When no voltage is applied to a TN liquid crystal cell, the light is polarized to pass through the cell. In proportion to the voltage applied, the LC cells twist up to 90 degrees changing the polarization and blocking the light's path. By properly adjusting the level of the voltage almost any grey level or transmission can be achieved.
In-plane switching (IPS)
In-plane switching is an LCD technology which aligns the liquid crystal cells in a horizontal direction. In this method, the electrical field is applied through each end of the crystal, but this requires two transistors for each pixel instead of the single transistor needed for a standard thin-film transistor (TFT) display. These results in blocking more transmission area, thus requiring a brighter backlight, which will consume more power, making this type of display less desirable for notebook computers.
Vertical alignment (VA)
Vertical alignment displays are a form of LC displays in which the liquid crystal material naturally exists in a horizontal state removing the need for extra transistors (as in IPS). When no voltage is applied the liquid crystal cell, it remains perpendicular to the substrate creating a black display. When voltage is applied, the liquid crystal cells shift to a horizontal position, parallel to the substrate, allowing light to pass through and create a white display. VA liquid crystal displays provide some of the same advantages as IPS panels, particularly an improved viewing angle and improved black level.
Blue Phase mode
Blue phase LCDs do not require an LC top layer. Blue phase LCDs are new and very expensive, but provide a higher refresh rate than normal LCDs. Blue phase LCDs are currently and emerging technology, and not many people's household includes one. Normal LCDs are cost efficient and actually provide a better color view and sharper image, but do not provide the high refresh rate.
Quality control
Some LCD panels have defective transistors, causing permanently lit or unlit pixels which are commonly referred to as stuck pixels or dead pixels respectively. Unlike integrated circuits (ICs), LCD panels with a few defective pixels are usually still usable. It is also economically prohibitive to discard a panel with just a few defective pixels because LCD panels are much larger than ICs. Manufacturers have different standards for determining a maximum acceptable number of defective pixels. The maximum acceptable number of defective pixels for LCD varies greatly. At one point, Samsung held a zero-tolerance policy for LCD monitors sold in Korea.[13] Currently, though, Samsung adheres to the less restrictive ISO 13406-2 standard.[14] Other companies have been known to tolerate as many as 11 dead pixels in their policies.[15] 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.[16] However, not every LCD manufacturer conforms to the ISO standard and the ISO standard is quite often interpreted in different ways.
LCD panels are more likely to have defects than most ICs due to their larger size. In the example to the right, a 300 mm SVGA LCD has 8 defects and a 150 mm wafer has only 3 defects. However, 134 of the 137 dies on the wafer will be acceptable, whereas rejection of the LCD panel would be a 0% yield. The standard is much higher now due to fierce competition between manufacturers and improved quality control. An SVGA LCD panel with 4 defective pixels is usually considered defective and customers can request an exchange for a new one. Some manufacturers, notably in South Korea where some of the largest LCD panel manufacturers, such as LG, are located, now have "zero defective pixel guarantees", which is an extra screening process which can then determine "A" and "B" grade panels. Many manufacturers would replace a product even with one defective pixel.
Even where such guarantees do not exist, the location of defective pixels is important. A display with only a few defective pixels may be unacceptable if the defective pixels are near each other. Manufacturers may also relax their replacement criteria when defective pixels are in the center of the viewing area.
LCD panels also have defects known as Mura, which look like a small-scale crack with very small changes in luminance or color. It is most visible in dark or black areas of displayed scenes. Defects in various LCD panel components can cause Mura effect.
Zero-power (bistable) displays
The zenithal bistable device (ZBD) developed by QinetiQ (formerly DERA), can retain an image without power. The crystals may exist in one of two stable orientations (Black and "White") and power is only required to change the image. ZBD Displays is a spin-off company from QinetiQ who manufacture both grayscale and color ZBD devices.
A French company, Nemoptic, has developed another zero-power, paper-like LCD technology which has been mass-produced since July 2003. This technology is intended for use in applications such as Electronic Shelf Labels, E-books, E-documents, E-newspapers, E-dictionaries, Industrial sensors, Ultra-Mobile PCs, etc. Zero-power LCDs are a category of electronic paper.
Kent Displays has also developed a "no power" display that uses Polymer Stabilized Cholesterics Liquid Crystals (ChLCD). The major drawback to the ChLCD is slow refresh rate, especially with low temperatures.
In 2004 researchers at the University of Oxford demonstrated two new types of zero-power bistable LCDs based on Zenithal bistable techniques.
Several bistable technologies, like the 360° BTN and the bistable cholesteric, depend mainly on the bulk properties of the liquid crystal (LC) and use standard strong anchoring, with alignment films and LC mixtures similar to the traditional monostable materials. Other bistable technologies (i.e. Binem Technology) are based mainly on the surface properties and need specific weak anchoring materials.
Problems
Dead pixels
A few LCD monitors are produced with "dead pixels". Due to the desire for affordable monitors, most manufacturers sell monitors with dead pixels. Almost all manufacturers have clauses in their warranties which claim monitors with fewer than some number of dead pixels are not broken and will not be replaced. The dead pixels are usually stuck with the green, red, and/or blue sub-pixels either individually always stuck on or off.
Like image persistence, this can sometimes be partially or fully reversed by using the same method listed below; however the chance of success is far lower than with a "stuck" pixel. It can also sometimes be repaired by physically flicking the pixel; however it is always a possibility for someone to use too much force and rupture the weak screen internals doing this.
Stuck pixels
LCD monitors, while lacking phosphor screens and thus immune to phosphor burn-in, have a similar condition known as image persistence, where the pixels of the LCD monitor can "remember" a particular color and become "stuck" and unable to change. Unlike phosphor burn-in, however, image persistence can sometimes be reversed partially or completely. This is accomplished by rapidly displaying varying colors to "wake up" the stuck pixels.
Phosphor burn-in
Phosphor burn-in is localized aging of the phosphor layer of a CRT screen where it has displayed a static bright image for many years. This results in a faint permanent image on the screen, even when turned off. In severe cases it can even be possible to read some of the text, though this only occurs where the displayed text remained the same for years.
This was once a common phenomenon in single purpose business computers. It can still be an issue with CRT displays when used to display the same image for years at a time, but modern computers aren't normally used this way anymore, so the problem is not a significant issue. The issue seems to have become exaggerated in popular opinion. The only systems that suffered the defect were ones displaying the same image for years, and with these the presence of burn-in was not a noticeable effect when in use, since it coincided with the displayed image perfectly. It only became a significant issue in three situations:
• when some heavily used monitors were reused at home,
• or re-used for display purposes
• In some high-security applications (but only those where the high-security data displayed did not change for years at a time).
Screen savers were developed as a means to avoid burn-in, but are unnecessary for CRTs today, despite their popularity.
Phosphor burn-in can be gradually removed on damaged CRT displays by displaying an all-white screen with brightness and contrast turned up full. This is a slow procedure and is usually effective.
Plasma burn-in
Burn-in re-emerged as an issue with early plasma displays, which are more vulnerable to this than CRTs. Screen savers with moving images may be used with these to minimize localized burn. Periodic change of the colour scheme in use also helps.
Glare
Glare is a problem caused by the relationship between lighting and screen, or by using monitors in bright sunlight. Matte finish LCDs and flat screen CRTs are less prone to reflected glare than conventional curved CRTs or glossy LCDs, and aperture grille CRTs, which are curved on one axis only, are less prone to it than other CRTs curved on both axes.
If the problem persists despite moving the monitor or adjusting lighting, a filter using a mesh of very fine black wires may be placed on the screen to reduce glare and improve contrast. These filters were popular in the late 1980s. They do also reduce light output.
The above will only work against reflective glare; direct glare (such as sunlight) will completely wash out most monitors' internal lighting, and can only be dealt with by use of a hood or transreflective LCD.
Color misregistration
With exceptions of correctly aligned video projectors and stacked LEDs, most display technologies, especially LCD, have an inherent misregistration of the color channels, that is, the centers of the red, green, and blue dots do not line up perfectly. Sub-pixel rendering depends on this misalignment; technologies making use of this include the Apple II from 1976, and more recently Microsoft (Clear Type, 1998) and XFree86 (X Rendering Extension).
Incomplete spectrum
RGB displays produce most of the visible color spectrum, but not all. This can be a problem where good color matching to non-RGB images is needed. This issue is common to all monitor technologies with 3 color channels.
Display interfaces
Computer terminals
Early CRT-based VDUs (Visual Display Units) such as the DEC VT05 without graphics capabilities gained the label glass teletypes, because of the functional similarity to their electromechanical predecessors.
Some historic computers had no modern display, using a teletype, modified electric typewriter, or printer instead.
Composite signal
Early home computers such as the Apple II and the Commodore 64 used a composite signal output to drive a CRT monitor or TV. This resulted in degraded resolution due to compromises in the broadcast TV standards used. This method is still used with video game consoles. The Commodore monitor had S-Video input to improve resolution.
Digital monitors
Early digital monitors are sometimes known as TTLs because the voltages on the red, green, and blue inputs are compatible with TTL logic chips. Later digital monitors support LVDS, or TMDS protocols.
TTL monitors
An amber monochrome computer monitor, manufactured in 2007, which uses a 15-pin SVGA connector just like a standard color monitor.
Monitors used with the MDA, Hercules, CGA, and EGA graphics adapters used in early IBM PC's (Personal Computer) and clones were controlled via TTL logic. Such monitors can usually be identified by a male DB-9 connector used on the video cable. The disadvantage of TTL monitors was the limited number of colors available due to the low number of digital bits used for video signaling.
Modern monochrome monitors use the same 15-pin SVGA connector as standard color monitors. They are capable of displaying 32-bit grayscale at 1024x768 resolutions, making them able to interface with modern computers.
TTL Monochrome monitors only made use of five out of the nine pins. One pin was used as a ground, and two pins were used for horizontal/vertical synchronization. The electron gun was controlled by two separate digital signals, a video bit, and an intensity bit to control the brightness of the drawn pixels. Only four shades were possible; black, dim, medium or bright.
CGA monitors used four digital signals to control the three electron guns used in color CRTs, in a signaling method known as RGBI, or Red Green and Blue, plus Intensity. Each of the three RGB colors can be switched on or off independently. The intensity bit increases the brightness of all guns that are switched on, or if no colors are switched on the intensity bit will switch on all guns at a very low brightness to produce a dark grey. A CGA monitor is only capable of rendering 16 colors. The CGA monitor was not exclusively used by PC based hardware. The Commodore 128 could also utilize CGA monitors. Many CGA monitors were capable of displaying composite video via a separate jack.
EGA monitors used six digital signals to control the three electron guns in a signaling method known as RrGgBb. Unlike CGA, each gun is allocated its own intensity bit. This allowed each of the three primary colors to have four different states (off, soft, medium, and bright) resulting in 64 colors.
Although not supported in the original IBM specification, many vendors of clone graphics adapters have implemented backwards monitor compatibility and auto detection. For example, EGA cards produced by Paradise could operate as an MDA or CGA adapter if a monochrome or CGA monitor was used in place of an EGA monitor. Many CGA cards were also capable of operating as MDA or Hercules card if a monochrome monitor was used.
Single color screens
Display colors other than white were very popular on monochrome monitors in the 1980s. These colors were more comfortable on the eye. This was particularly an issue at the time due to the lower refresh rates in use at the time causing flicker, plus the use of less comfortable color schemes than used with most of today's software.
Green screens were the most popular color, with orange displays also available. 'Paper white'
Modern technology
Analog monitors
Most modern computer displays can show an infinite number of different colors in the RGB color space by changing red, green, and blue analog video signals in continuously variable intensities. These have been almost exclusively progressive scan since the middle 1980s. While many early plasma and liquid crystal displays have exclusively analog connections, all signals in such monitors pass through a completely digital section prior to display.
While many similar connectors (13W3, BNC, etc…) were used on other platforms, the IBM PC and compatible systems long ago standardized on the VGA connector. All of these connectors deliver nearly flawless high resolution video which vastly outclasses that of a TV.
Digital and analog combination
The first popular external digital monitor connectors, such as DVI-I and the various breakout connectors based on it, included both analog signals compatible with VGA and digital signals compatible with new flat-screen displays in the same connector. This made the connector nearly painless for users of both technologies.
Digital monitors
Newer connectors are being made which have digital only video signals. Many of these, such as HDMI and Display Port, also feature integrated audio and data connections. One less popular feature most of these connectors share are DRM encrypted signals, although the HDCP technology responsible for implementing the protection was necessarily rudimentary to meet cost constraints, and was primarily a barrier aimed towards dissuading average consumers from creating exact duplicates without a noticeable loss in image quality.
Configuration and usage
Multiple monitors
More than one monitor can be attached to the same device. Each display can operate in two basic configurations:
• The simpler of the two is mirroring (sometimes cloning,) in which at least two displays are showing the same image. It is commonly used for presentations. Hardware with only one video output can be tricked into doing this with an external splitter device, commonly built into many video projectors as a pass through connection.
• The more sophisticated of the two, extension allows each monitor to display a different image, so as to form a contiguous area of arbitrary shape. This requires software support and extra hardware, and may be locked out on "low end" products by cripple ware.
• Primitive software is incapable of recognizing multiple displays, so spanning must be used, in which case a very large virtual display is created, and then pieces are split into multiple video outputs for separate monitors. Hardware with only one video output can be tricked into doing this with an expensive external splitter device; this is most often used for very large composite displays made from many smaller monitors placed edge to edge.
Multiple video sources
Multiple devices can be connected to the same monitor using a video switch. In the case of computers, this usually takes the form of a "Keyboard Video Mouse switch" (KVM) switch, which is designed to switch all of the user interface devices for a workstation between different computers at once.
Virtual displays
Much software and video hardware supports the ability to create additional, virtual pieces of desktop, commonly known as workspaces.
Other specifications of monitors
Screen size
Diagonal size
For any rectangular section on a round tube, the diagonal measurement is also the diameter of the tube
The size of a display is typically given as the distance between two opposite screen corners. One problem with this method is that it does not distinguish between the aspect ratios of monitors with identical diagonal sizes, in spite of the fact that a shape of a given diagonal span's area decreases as it becomes less square. For example, a 4:3 21" monitor has an area of ~211 square inches, while a 16:9 21" widescreen has an area of only ~188 square inches.
This method of measurement is from the first types of CRT television, when round picture tubes were in common use. Being circular, they only needed to use their diameter to describe their tube size. When round tubes were used to display rectangular images, the diagonal measurement was equivalent to the round tube's diameter. This method continued even when CRT tubes were manufactured as rounded rectangles.
Another historically problematic practice is the direct measurement of a monitor's imaging element as its quoted size in publicity and advertising materials. Especially on CRT displays, a substantial portion of the imaging element is concealed behind the case's bezel or shroud in order to hide areas outside the monitor's safe area due to over scan. Seen as deceptive, widespread consumer objection and lawsuits eventually forced most manufacturers to instead measure viewable size.
Additional features
Power saving
Most modern monitors will switch to a power-saving mode if no video-input signal is received. This allows modern operating systems to turn off a monitor after a specified period of inactivity. This also extends the monitor's service life.
Some monitors will also switch themselves off after a time period on standby.
Most modern laptops provide a method of screen dimming after periods of inactivity or when the battery is in use. This extends battery life and reduces wear.
Integrated accessories
Many monitors have other accessories (or connections for them) integrated. This places standard ports within easy reach and eliminates the need for another separate hub, camera, microphone, or set of speakers. Integrated accessories are often of substandard quality.
Glossy screen
Some displays, especially newer LCD monitors, replace the traditional anti-glare matte finish with a glossy one. While this is ostensibly done to increase saturation and sharpness, its benefits and drawbacks are extremely contentious among consumers.
Directional screen
Narrow viewing angle screens are used in some security conscious applications.
Autopolyscopic screen
A specially designed directional screen which generates 3D images without headgear, distortion or eyestrain.
Touch screen
These monitors use touching of the screen as an input method. Items can be selected or moved with a finger, and finger gestures may be used to convey commands. This does however mean the screen needs frequent cleaning due to image degradation from fingerprints.
Tablet screens
A combination of a monitor with a graphics tablet. Such devices are typically unresponsive to touch, but may offer sensitivity to one or more special tools' pressure, tilt, controls, opposite ends, and multiple tools.
Performance measurements
The performance parameters of a monitor are:
• Luminance, measured in candelas per square meter (cd/m²).
• Viewable image size, measured diagonally. For CRTs the viewable size is typically one inch (25 mm) smaller than the tube itself.
• Display resolution, the number of distinct pixels in each dimension that can be displayed. Maximum resolution is limited by dot pitch.
• Dot pitch, describes the distance between pixels of the same color in millimeters. In general, the smaller the dot pitch (e.g. 0.24 mm), the sharper the picture will appear.
• Refresh rate, the number of times in a second that a display is illuminated. Maximum refresh rate is limited by response time.
• Response time, the amount of time a pixel in a monitor takes to go from active (black) to inactive (white) and back to active (black) again. It is measured in milliseconds (ms). Lower numbers mean faster transitions and therefore fewer visible image artifacts.
• Contrast ratio, the contrast ratio is defined as the ratio of the luminosity of the brightest color (white) to that of the darkest color (black) that the monitor is capable of producing.
• Power consumption, measured in watts (W).
• Aspect ratios, which is the horizontal size compared to the vertical size, e.g. 4:3 is the standard aspect ratio, so that a screen with a width of 1024 pixels will have a height of 768 pixels. A widescreen display can have an aspect ratio of 16:9, which means a display that is 1024 pixels wide will have a height of 576 pixels.
• Viewing angle, the ability to be seen from an angle without excessive degradation to the image, measured in degrees horizontally and vertically.
Comparison
CRT
Pros:
• Very high contrast ratio (20,000:1 or greater, much higher than many modern LCDs and plasma displays.)
• High speed response
• Excellent Additive color, wide gamut and low black level limited only by external environment.
• Can display natively in almost any resolution and refresh rate
• Near zero color, saturation, contrast or brightness distortion. Excellent viewing angle.
• No input lag
• A reliable, proven display technology.
Cons:
• Large size and weight (a 40" unit weighs over 200lbs)
• Geometric distortion in non-flat CRTs
• Older CRTs are prone to burn-in.
• Warm up time required prior to peak luminance and proper color rendering.
• Greater power consumption than similarly sized displays, such as LCD.
• Screened devices are prone to moiré effect at highest resolution (does not apply to triple-tube projection)
• Intolerant of damp conditions, with dangerous wet failure characteristics.
• Small risk of implosion (due to internal vacuum) if the picture tube is broken in aging sets.
• Use under Lower refresh rates causes noticeable flicker
• Internal lethally high voltages
• Flyback transformer produces characteristic high-pitched noise when close to set.
• Increasingly difficult to obtain models at HDTV resolutions, due to consumers' perception of antiquity.
LCD
Pros:
• Very compact and light
• Low power consumption
• No geometric distortion
• Rugged
• Little or no flicker depending on backlight
Cons:
• Low contrast ratio.
• Limited viewing angle. This causes color, saturation, contrast and brightness to vary, even within the intended viewing angle from mere variations in posture.
• Uneven backlighting in some monitors can cause brightness distortion, especially toward the edges.
• Slow response times, which cause smearing and ghosting artifacts (although many modern LCDs have response times of 8ms or less).
• Only has one native resolution. Displaying other resolutions requires a video scalar, which degrades image quality at lower resolutions.
• Fixed bit depth, many cheaper LCDs are incapable of true color.
• Input lag
• Somewhat more expensive than CRT
• Dead pixels are possible during manufacturing
Plasma
Pros:
• Compact and light
• High contrast ratios (10,000:1 or greater)
• High speed response
• Excellent color, wide gamut and low black level.
• Near zero color, saturation, contrast or brightness distortion. Excellent viewing angle.
• No geometric distortion
• Highly scalable, with less weight gain per increase in size (from less than 30 inches wide to the world's largest at 150 inches).
Cons:
• Large pixel pitch means either low resolution or a large screen
• Noticeable flicker when viewed at close range
• High operating temperature
• Somewhat more expensive than LCD
• High power consumption
• Only has one native resolution. Displaying other resolutions requires a video scalar, which degrades image quality at lower resolutions.
• Fixed bit depth
• Input lag
• Older PDPs are prone to burn-in
• Dead pixels are possible during manufacturing
Penetron
Pros:
• See-through for transparent HUDs (although LCDs are also transparent, they are not self-lighting.)
• Very high contrast ratios.
• Extremely sharp.
Cons:
• Color displays are limited to about four tints.
• Orders of magnitude more expensive than the other display technologies listed here.
Display applications
• Television and digital television
• Liquid crystal display television (LCD TV)
• LCD projector
• Computer monitor
• Aircraft Instrumentation displays (see glass cockpit)
Manufacturers
• Acer (company)
• Aoc
• AU Optronics
• Barco
• BenQ
• Casio
• Chi Mei Optoelectronics
• CoolTouch Monitors
• Corning Inc.
• Dell
• Eizo
• Epson
• Fujitsu
• Hansol
• HP
• iiyama
• International Display Works
• JVC
• Kyocera
• Lenovo
• LG Display
• LXD Incorporated
• Medion
• NEC Display Solutions
• Panasonic (Matsushita)
• Polaroid Corporation
• Power light
• Samsung Electronics
• Sharp Corporation
• S-LCD
• Sony
• Soyo
• Toshiba
• Videocon
• View sonic
LCDs with a small number of segments, such as those used in digital watches and pocket calculators, have individual electrical contacts for each segment. An external dedicated circuit supplies an electric charge to control each segment. This display structure is unwieldy for more than a few display elements.
Small monochrome displays such as those found in personal organizers, or older laptop screens have a passive-matrix structure employing super-twisted nematic (STN) or double-layer STN (DSTN) technology—the latter of which addresses a color-shifting problem with the former—and color-STN (CSTN)—wherein color is added by using an internal filter. Each row or column of the display has a single electrical circuit. The pixels are addressed one at a time by row and column addresses. This type of display is called passive-matrix addressed because the pixel must retain its state between refreshes without the benefit of a steady electrical charge.
As the number of pixels (and, correspondingly, columns and rows) increases, this type of display becomes less feasible. Very slow response times and poor contrast are typical of passive-matrix addressed LCDs.
High-resolution color displays such as modern LCD computer monitors and televisions use an active matrix structure. A matrix of thin-film transistors (TFTs) is added to the polarizing and color filters. Each pixel has its own dedicated transistor, allowing each column line to access one pixel. When a row line is activated, all of the column lines are connected to a row of pixels and the correct voltage is driven onto all of the column lines. The row line is then deactivated and the next row line is activated. All of the row lines are activated in sequence during a refresh operation.
Active-matrix addressed displays look "brighter" and "sharper" than passive-matrix addressed displays of the same size, and generally have quicker response times, producing much better images.
A Casio 1.8" color TFT liquid crystal display which equips the Sony Cyber-shot DSC-P93A digital compact cameras
Twisted nematic (TN)
Twisted nematic displays contain liquid crystal elements which twist and untwist at varying degrees to allow light to pass through. When no voltage is applied to a TN liquid crystal cell, the light is polarized to pass through the cell. In proportion to the voltage applied, the LC cells twist up to 90 degrees changing the polarization and blocking the light's path. By properly adjusting the level of the voltage almost any grey level or transmission can be achieved.
In-plane switching (IPS)
In-plane switching is an LCD technology which aligns the liquid crystal cells in a horizontal direction. In this method, the electrical field is applied through each end of the crystal, but this requires two transistors for each pixel instead of the single transistor needed for a standard thin-film transistor (TFT) display. These results in blocking more transmission area, thus requiring a brighter backlight, which will consume more power, making this type of display less desirable for notebook computers.
Vertical alignment (VA)
Vertical alignment displays are a form of LC displays in which the liquid crystal material naturally exists in a horizontal state removing the need for extra transistors (as in IPS). When no voltage is applied the liquid crystal cell, it remains perpendicular to the substrate creating a black display. When voltage is applied, the liquid crystal cells shift to a horizontal position, parallel to the substrate, allowing light to pass through and create a white display. VA liquid crystal displays provide some of the same advantages as IPS panels, particularly an improved viewing angle and improved black level.
Blue Phase mode
Blue phase LCDs do not require an LC top layer. Blue phase LCDs are new and very expensive, but provide a higher refresh rate than normal LCDs. Blue phase LCDs are currently and emerging technology, and not many people's household includes one. Normal LCDs are cost efficient and actually provide a better color view and sharper image, but do not provide the high refresh rate.
Quality control
Some LCD panels have defective transistors, causing permanently lit or unlit pixels which are commonly referred to as stuck pixels or dead pixels respectively. Unlike integrated circuits (ICs), LCD panels with a few defective pixels are usually still usable. It is also economically prohibitive to discard a panel with just a few defective pixels because LCD panels are much larger than ICs. Manufacturers have different standards for determining a maximum acceptable number of defective pixels. The maximum acceptable number of defective pixels for LCD varies greatly. At one point, Samsung held a zero-tolerance policy for LCD monitors sold in Korea.[13] Currently, though, Samsung adheres to the less restrictive ISO 13406-2 standard.[14] Other companies have been known to tolerate as many as 11 dead pixels in their policies.[15] 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.[16] However, not every LCD manufacturer conforms to the ISO standard and the ISO standard is quite often interpreted in different ways.
LCD panels are more likely to have defects than most ICs due to their larger size. In the example to the right, a 300 mm SVGA LCD has 8 defects and a 150 mm wafer has only 3 defects. However, 134 of the 137 dies on the wafer will be acceptable, whereas rejection of the LCD panel would be a 0% yield. The standard is much higher now due to fierce competition between manufacturers and improved quality control. An SVGA LCD panel with 4 defective pixels is usually considered defective and customers can request an exchange for a new one. Some manufacturers, notably in South Korea where some of the largest LCD panel manufacturers, such as LG, are located, now have "zero defective pixel guarantees", which is an extra screening process which can then determine "A" and "B" grade panels. Many manufacturers would replace a product even with one defective pixel.
Even where such guarantees do not exist, the location of defective pixels is important. A display with only a few defective pixels may be unacceptable if the defective pixels are near each other. Manufacturers may also relax their replacement criteria when defective pixels are in the center of the viewing area.
LCD panels also have defects known as Mura, which look like a small-scale crack with very small changes in luminance or color. It is most visible in dark or black areas of displayed scenes. Defects in various LCD panel components can cause Mura effect.
Zero-power (bistable) displays
The zenithal bistable device (ZBD) developed by QinetiQ (formerly DERA), can retain an image without power. The crystals may exist in one of two stable orientations (Black and "White") and power is only required to change the image. ZBD Displays is a spin-off company from QinetiQ who manufacture both grayscale and color ZBD devices.
A French company, Nemoptic, has developed another zero-power, paper-like LCD technology which has been mass-produced since July 2003. This technology is intended for use in applications such as Electronic Shelf Labels, E-books, E-documents, E-newspapers, E-dictionaries, Industrial sensors, Ultra-Mobile PCs, etc. Zero-power LCDs are a category of electronic paper.
Kent Displays has also developed a "no power" display that uses Polymer Stabilized Cholesterics Liquid Crystals (ChLCD). The major drawback to the ChLCD is slow refresh rate, especially with low temperatures.
In 2004 researchers at the University of Oxford demonstrated two new types of zero-power bistable LCDs based on Zenithal bistable techniques.
Several bistable technologies, like the 360° BTN and the bistable cholesteric, depend mainly on the bulk properties of the liquid crystal (LC) and use standard strong anchoring, with alignment films and LC mixtures similar to the traditional monostable materials. Other bistable technologies (i.e. Binem Technology) are based mainly on the surface properties and need specific weak anchoring materials.
Problems
Dead pixels
A few LCD monitors are produced with "dead pixels". Due to the desire for affordable monitors, most manufacturers sell monitors with dead pixels. Almost all manufacturers have clauses in their warranties which claim monitors with fewer than some number of dead pixels are not broken and will not be replaced. The dead pixels are usually stuck with the green, red, and/or blue sub-pixels either individually always stuck on or off.
Like image persistence, this can sometimes be partially or fully reversed by using the same method listed below; however the chance of success is far lower than with a "stuck" pixel. It can also sometimes be repaired by physically flicking the pixel; however it is always a possibility for someone to use too much force and rupture the weak screen internals doing this.
Stuck pixels
LCD monitors, while lacking phosphor screens and thus immune to phosphor burn-in, have a similar condition known as image persistence, where the pixels of the LCD monitor can "remember" a particular color and become "stuck" and unable to change. Unlike phosphor burn-in, however, image persistence can sometimes be reversed partially or completely. This is accomplished by rapidly displaying varying colors to "wake up" the stuck pixels.
Phosphor burn-in
Phosphor burn-in is localized aging of the phosphor layer of a CRT screen where it has displayed a static bright image for many years. This results in a faint permanent image on the screen, even when turned off. In severe cases it can even be possible to read some of the text, though this only occurs where the displayed text remained the same for years.
This was once a common phenomenon in single purpose business computers. It can still be an issue with CRT displays when used to display the same image for years at a time, but modern computers aren't normally used this way anymore, so the problem is not a significant issue. The issue seems to have become exaggerated in popular opinion. The only systems that suffered the defect were ones displaying the same image for years, and with these the presence of burn-in was not a noticeable effect when in use, since it coincided with the displayed image perfectly. It only became a significant issue in three situations:
• when some heavily used monitors were reused at home,
• or re-used for display purposes
• In some high-security applications (but only those where the high-security data displayed did not change for years at a time).
Screen savers were developed as a means to avoid burn-in, but are unnecessary for CRTs today, despite their popularity.
Phosphor burn-in can be gradually removed on damaged CRT displays by displaying an all-white screen with brightness and contrast turned up full. This is a slow procedure and is usually effective.
Plasma burn-in
Burn-in re-emerged as an issue with early plasma displays, which are more vulnerable to this than CRTs. Screen savers with moving images may be used with these to minimize localized burn. Periodic change of the colour scheme in use also helps.
Glare
Glare is a problem caused by the relationship between lighting and screen, or by using monitors in bright sunlight. Matte finish LCDs and flat screen CRTs are less prone to reflected glare than conventional curved CRTs or glossy LCDs, and aperture grille CRTs, which are curved on one axis only, are less prone to it than other CRTs curved on both axes.
If the problem persists despite moving the monitor or adjusting lighting, a filter using a mesh of very fine black wires may be placed on the screen to reduce glare and improve contrast. These filters were popular in the late 1980s. They do also reduce light output.
The above will only work against reflective glare; direct glare (such as sunlight) will completely wash out most monitors' internal lighting, and can only be dealt with by use of a hood or transreflective LCD.
Color misregistration
With exceptions of correctly aligned video projectors and stacked LEDs, most display technologies, especially LCD, have an inherent misregistration of the color channels, that is, the centers of the red, green, and blue dots do not line up perfectly. Sub-pixel rendering depends on this misalignment; technologies making use of this include the Apple II from 1976, and more recently Microsoft (Clear Type, 1998) and XFree86 (X Rendering Extension).
Incomplete spectrum
RGB displays produce most of the visible color spectrum, but not all. This can be a problem where good color matching to non-RGB images is needed. This issue is common to all monitor technologies with 3 color channels.
Display interfaces
Computer terminals
Early CRT-based VDUs (Visual Display Units) such as the DEC VT05 without graphics capabilities gained the label glass teletypes, because of the functional similarity to their electromechanical predecessors.
Some historic computers had no modern display, using a teletype, modified electric typewriter, or printer instead.
Composite signal
Early home computers such as the Apple II and the Commodore 64 used a composite signal output to drive a CRT monitor or TV. This resulted in degraded resolution due to compromises in the broadcast TV standards used. This method is still used with video game consoles. The Commodore monitor had S-Video input to improve resolution.
Digital monitors
Early digital monitors are sometimes known as TTLs because the voltages on the red, green, and blue inputs are compatible with TTL logic chips. Later digital monitors support LVDS, or TMDS protocols.
TTL monitors
An amber monochrome computer monitor, manufactured in 2007, which uses a 15-pin SVGA connector just like a standard color monitor.
Monitors used with the MDA, Hercules, CGA, and EGA graphics adapters used in early IBM PC's (Personal Computer) and clones were controlled via TTL logic. Such monitors can usually be identified by a male DB-9 connector used on the video cable. The disadvantage of TTL monitors was the limited number of colors available due to the low number of digital bits used for video signaling.
Modern monochrome monitors use the same 15-pin SVGA connector as standard color monitors. They are capable of displaying 32-bit grayscale at 1024x768 resolutions, making them able to interface with modern computers.
TTL Monochrome monitors only made use of five out of the nine pins. One pin was used as a ground, and two pins were used for horizontal/vertical synchronization. The electron gun was controlled by two separate digital signals, a video bit, and an intensity bit to control the brightness of the drawn pixels. Only four shades were possible; black, dim, medium or bright.
CGA monitors used four digital signals to control the three electron guns used in color CRTs, in a signaling method known as RGBI, or Red Green and Blue, plus Intensity. Each of the three RGB colors can be switched on or off independently. The intensity bit increases the brightness of all guns that are switched on, or if no colors are switched on the intensity bit will switch on all guns at a very low brightness to produce a dark grey. A CGA monitor is only capable of rendering 16 colors. The CGA monitor was not exclusively used by PC based hardware. The Commodore 128 could also utilize CGA monitors. Many CGA monitors were capable of displaying composite video via a separate jack.
EGA monitors used six digital signals to control the three electron guns in a signaling method known as RrGgBb. Unlike CGA, each gun is allocated its own intensity bit. This allowed each of the three primary colors to have four different states (off, soft, medium, and bright) resulting in 64 colors.
Although not supported in the original IBM specification, many vendors of clone graphics adapters have implemented backwards monitor compatibility and auto detection. For example, EGA cards produced by Paradise could operate as an MDA or CGA adapter if a monochrome or CGA monitor was used in place of an EGA monitor. Many CGA cards were also capable of operating as MDA or Hercules card if a monochrome monitor was used.
Single color screens
Display colors other than white were very popular on monochrome monitors in the 1980s. These colors were more comfortable on the eye. This was particularly an issue at the time due to the lower refresh rates in use at the time causing flicker, plus the use of less comfortable color schemes than used with most of today's software.
Green screens were the most popular color, with orange displays also available. 'Paper white'
Modern technology
Analog monitors
Most modern computer displays can show an infinite number of different colors in the RGB color space by changing red, green, and blue analog video signals in continuously variable intensities. These have been almost exclusively progressive scan since the middle 1980s. While many early plasma and liquid crystal displays have exclusively analog connections, all signals in such monitors pass through a completely digital section prior to display.
While many similar connectors (13W3, BNC, etc…) were used on other platforms, the IBM PC and compatible systems long ago standardized on the VGA connector. All of these connectors deliver nearly flawless high resolution video which vastly outclasses that of a TV.
Digital and analog combination
The first popular external digital monitor connectors, such as DVI-I and the various breakout connectors based on it, included both analog signals compatible with VGA and digital signals compatible with new flat-screen displays in the same connector. This made the connector nearly painless for users of both technologies.
Digital monitors
Newer connectors are being made which have digital only video signals. Many of these, such as HDMI and Display Port, also feature integrated audio and data connections. One less popular feature most of these connectors share are DRM encrypted signals, although the HDCP technology responsible for implementing the protection was necessarily rudimentary to meet cost constraints, and was primarily a barrier aimed towards dissuading average consumers from creating exact duplicates without a noticeable loss in image quality.
Configuration and usage
Multiple monitors
More than one monitor can be attached to the same device. Each display can operate in two basic configurations:
• The simpler of the two is mirroring (sometimes cloning,) in which at least two displays are showing the same image. It is commonly used for presentations. Hardware with only one video output can be tricked into doing this with an external splitter device, commonly built into many video projectors as a pass through connection.
• The more sophisticated of the two, extension allows each monitor to display a different image, so as to form a contiguous area of arbitrary shape. This requires software support and extra hardware, and may be locked out on "low end" products by cripple ware.
• Primitive software is incapable of recognizing multiple displays, so spanning must be used, in which case a very large virtual display is created, and then pieces are split into multiple video outputs for separate monitors. Hardware with only one video output can be tricked into doing this with an expensive external splitter device; this is most often used for very large composite displays made from many smaller monitors placed edge to edge.
Multiple video sources
Multiple devices can be connected to the same monitor using a video switch. In the case of computers, this usually takes the form of a "Keyboard Video Mouse switch" (KVM) switch, which is designed to switch all of the user interface devices for a workstation between different computers at once.
Virtual displays
Much software and video hardware supports the ability to create additional, virtual pieces of desktop, commonly known as workspaces.
Other specifications of monitors
Screen size
Diagonal size
For any rectangular section on a round tube, the diagonal measurement is also the diameter of the tube
The size of a display is typically given as the distance between two opposite screen corners. One problem with this method is that it does not distinguish between the aspect ratios of monitors with identical diagonal sizes, in spite of the fact that a shape of a given diagonal span's area decreases as it becomes less square. For example, a 4:3 21" monitor has an area of ~211 square inches, while a 16:9 21" widescreen has an area of only ~188 square inches.
This method of measurement is from the first types of CRT television, when round picture tubes were in common use. Being circular, they only needed to use their diameter to describe their tube size. When round tubes were used to display rectangular images, the diagonal measurement was equivalent to the round tube's diameter. This method continued even when CRT tubes were manufactured as rounded rectangles.
Another historically problematic practice is the direct measurement of a monitor's imaging element as its quoted size in publicity and advertising materials. Especially on CRT displays, a substantial portion of the imaging element is concealed behind the case's bezel or shroud in order to hide areas outside the monitor's safe area due to over scan. Seen as deceptive, widespread consumer objection and lawsuits eventually forced most manufacturers to instead measure viewable size.
Additional features
Power saving
Most modern monitors will switch to a power-saving mode if no video-input signal is received. This allows modern operating systems to turn off a monitor after a specified period of inactivity. This also extends the monitor's service life.
Some monitors will also switch themselves off after a time period on standby.
Most modern laptops provide a method of screen dimming after periods of inactivity or when the battery is in use. This extends battery life and reduces wear.
Integrated accessories
Many monitors have other accessories (or connections for them) integrated. This places standard ports within easy reach and eliminates the need for another separate hub, camera, microphone, or set of speakers. Integrated accessories are often of substandard quality.
Glossy screen
Some displays, especially newer LCD monitors, replace the traditional anti-glare matte finish with a glossy one. While this is ostensibly done to increase saturation and sharpness, its benefits and drawbacks are extremely contentious among consumers.
Directional screen
Narrow viewing angle screens are used in some security conscious applications.
Autopolyscopic screen
A specially designed directional screen which generates 3D images without headgear, distortion or eyestrain.
Touch screen
These monitors use touching of the screen as an input method. Items can be selected or moved with a finger, and finger gestures may be used to convey commands. This does however mean the screen needs frequent cleaning due to image degradation from fingerprints.
Tablet screens
A combination of a monitor with a graphics tablet. Such devices are typically unresponsive to touch, but may offer sensitivity to one or more special tools' pressure, tilt, controls, opposite ends, and multiple tools.
Performance measurements
The performance parameters of a monitor are:
• Luminance, measured in candelas per square meter (cd/m²).
• Viewable image size, measured diagonally. For CRTs the viewable size is typically one inch (25 mm) smaller than the tube itself.
• Display resolution, the number of distinct pixels in each dimension that can be displayed. Maximum resolution is limited by dot pitch.
• Dot pitch, describes the distance between pixels of the same color in millimeters. In general, the smaller the dot pitch (e.g. 0.24 mm), the sharper the picture will appear.
• Refresh rate, the number of times in a second that a display is illuminated. Maximum refresh rate is limited by response time.
• Response time, the amount of time a pixel in a monitor takes to go from active (black) to inactive (white) and back to active (black) again. It is measured in milliseconds (ms). Lower numbers mean faster transitions and therefore fewer visible image artifacts.
• Contrast ratio, the contrast ratio is defined as the ratio of the luminosity of the brightest color (white) to that of the darkest color (black) that the monitor is capable of producing.
• Power consumption, measured in watts (W).
• Aspect ratios, which is the horizontal size compared to the vertical size, e.g. 4:3 is the standard aspect ratio, so that a screen with a width of 1024 pixels will have a height of 768 pixels. A widescreen display can have an aspect ratio of 16:9, which means a display that is 1024 pixels wide will have a height of 576 pixels.
• Viewing angle, the ability to be seen from an angle without excessive degradation to the image, measured in degrees horizontally and vertically.
Comparison
CRT
Pros:
• Very high contrast ratio (20,000:1 or greater, much higher than many modern LCDs and plasma displays.)
• High speed response
• Excellent Additive color, wide gamut and low black level limited only by external environment.
• Can display natively in almost any resolution and refresh rate
• Near zero color, saturation, contrast or brightness distortion. Excellent viewing angle.
• No input lag
• A reliable, proven display technology.
Cons:
• Large size and weight (a 40" unit weighs over 200lbs)
• Geometric distortion in non-flat CRTs
• Older CRTs are prone to burn-in.
• Warm up time required prior to peak luminance and proper color rendering.
• Greater power consumption than similarly sized displays, such as LCD.
• Screened devices are prone to moiré effect at highest resolution (does not apply to triple-tube projection)
• Intolerant of damp conditions, with dangerous wet failure characteristics.
• Small risk of implosion (due to internal vacuum) if the picture tube is broken in aging sets.
• Use under Lower refresh rates causes noticeable flicker
• Internal lethally high voltages
• Flyback transformer produces characteristic high-pitched noise when close to set.
• Increasingly difficult to obtain models at HDTV resolutions, due to consumers' perception of antiquity.
LCD
Pros:
• Very compact and light
• Low power consumption
• No geometric distortion
• Rugged
• Little or no flicker depending on backlight
Cons:
• Low contrast ratio.
• Limited viewing angle. This causes color, saturation, contrast and brightness to vary, even within the intended viewing angle from mere variations in posture.
• Uneven backlighting in some monitors can cause brightness distortion, especially toward the edges.
• Slow response times, which cause smearing and ghosting artifacts (although many modern LCDs have response times of 8ms or less).
• Only has one native resolution. Displaying other resolutions requires a video scalar, which degrades image quality at lower resolutions.
• Fixed bit depth, many cheaper LCDs are incapable of true color.
• Input lag
• Somewhat more expensive than CRT
• Dead pixels are possible during manufacturing
Plasma
Pros:
• Compact and light
• High contrast ratios (10,000:1 or greater)
• High speed response
• Excellent color, wide gamut and low black level.
• Near zero color, saturation, contrast or brightness distortion. Excellent viewing angle.
• No geometric distortion
• Highly scalable, with less weight gain per increase in size (from less than 30 inches wide to the world's largest at 150 inches).
Cons:
• Large pixel pitch means either low resolution or a large screen
• Noticeable flicker when viewed at close range
• High operating temperature
• Somewhat more expensive than LCD
• High power consumption
• Only has one native resolution. Displaying other resolutions requires a video scalar, which degrades image quality at lower resolutions.
• Fixed bit depth
• Input lag
• Older PDPs are prone to burn-in
• Dead pixels are possible during manufacturing
Penetron
Pros:
• See-through for transparent HUDs (although LCDs are also transparent, they are not self-lighting.)
• Very high contrast ratios.
• Extremely sharp.
Cons:
• Color displays are limited to about four tints.
• Orders of magnitude more expensive than the other display technologies listed here.
Display applications
• Television and digital television
• Liquid crystal display television (LCD TV)
• LCD projector
• Computer monitor
• Aircraft Instrumentation displays (see glass cockpit)
Manufacturers
• Acer (company)
• Aoc
• AU Optronics
• Barco
• BenQ
• Casio
• Chi Mei Optoelectronics
• CoolTouch Monitors
• Corning Inc.
• Dell
• Eizo
• Epson
• Fujitsu
• Hansol
• HP
• iiyama
• International Display Works
• JVC
• Kyocera
• Lenovo
• LG Display
• LXD Incorporated
• Medion
• NEC Display Solutions
• Panasonic (Matsushita)
• Polaroid Corporation
• Power light
• Samsung Electronics
• Sharp Corporation
• S-LCD
• Sony
• Soyo
• Toshiba
• Videocon
• View sonic
