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Liquid-crystal display

A liquid-crystal display (LCD) is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers. Liquid crystals do not emit light directly[1] but instead use a backlight or reflector to produce images in color or monochrome.[2]

"LCD" redirects here. For other uses, see LCD (disambiguation).

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: preset words, digits, and seven-segment displays (as in a digital clock) are all examples of devices with these displays. They use the same basic technology, except that arbitrary images are made from a matrix of small pixels, while other displays have larger elements.


LCDs can either be normally on (positive) or off (negative), depending on the polarizer arrangement. For example, a character positive LCD with a backlight will have black lettering on a background that is the color of the backlight, and a character negative LCD will have a black background with the letters being of the same color as the backlight.


LCDs are used in a wide range of applications, including LCD televisions, computer monitors, instrument panels, aircraft cockpit displays, and indoor and outdoor signage. Small LCD screens are common in LCD projectors and portable consumer devices such as digital cameras, watches, calculators, and mobile telephones, including smartphones. LCD screens have replaced heavy, bulky and less energy-efficient cathode-ray tube (CRT) displays in nearly all applications. The phosphors used in CRTs make them vulnerable to 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 do not have this weakness, but are still susceptible to image persistence.[3]

CCFL: The LCD panel is lit either by two fluorescent lamps placed at opposite edges of the display or an array of parallel CCFLs behind larger displays. A diffuser (made of PMMA acrylic plastic, also known as a wave or light guide/guiding plate[85][86]) 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.[87] The thickness of the inverter transformers also limits how thin the display can be made.

cold cathode

EL-WLED: The LCD panel is lit by a row of white LEDs placed at one or more edges of the screen. A light diffuser (light guide plate, LGP) is then used to spread the light evenly across the whole display, similarly to edge-lit CCFL LCD backlights. The diffuser is made out of either PMMA plastic or special glass, PMMA is used in most cases because it is rugged, while special glass is used when the thickness of the LCD is of primary concern, because it doesn't expand as much when heated or exposed to moisture, which allows LCDs to be just 5mm thick. Quantum dots may be placed on top of the diffuser as a quantum dot enhancement film (QDEF, in which case they need a layer to be protected from heat and humidity) or on the color filter of the LCD, replacing the resists that are normally used. 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[88] 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[89]), 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.

[85]

WLED array: The LCD panel is lit by a full array of white LEDs placed behind a diffuser behind the panel. LCDs that use this implementation will usually have the ability to dim or completely turn off the LEDs in the dark areas of the image being displayed, effectively increasing the contrast ratio of the display. The precision with which this can be done will depend on the number of dimming zones of the display. The more dimming zones, the more precise the dimming, with less obvious blooming artifacts which are visible as dark grey patches surrounded by the unlit areas of the LCD. As of 2012, this design gets most of its use from upscale, larger-screen LCD televisions.

RGB-LED array: Similar to the WLED array, except the panel is lit by a full array of . 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.

RGB LEDs

Monochrome LEDs: such as red, green, yellow or blue LEDs are used in the small passive monochrome LCDs typically used in clocks, watches and small appliances.

Mini-LED: Backlighting with Mini-LEDs can support over a thousand of Full-area Local Area Dimming (FLAD) zones. This allows deeper blacks and higher contrast ratio.

[90]

Since LCDs produce no light of their own, they require external light to produce a visible image.[81][82] In a transmissive type of LCD, the light source is provided at the back of the glass stack and is called a backlight. Active-matrix LCDs are almost always backlit.[83][84] Passive LCDs may be backlit but many are reflective as they use a use a reflective surface or film at the back of the glass stack to utilize ambient light. Transflective LCDs combine the features of a backlit transmissive display and a reflective display.


The common implementations of LCD backlight technology are:


Today, most LCD screens are being designed with an LED backlight instead of the traditional CCFL backlight, while that backlight is dynamically controlled with the video information (dynamic backlight control). The combination with the dynamic backlight control, invented by Philips researchers Douglas Stanton, Martinus Stroomer and Adrianus de Vaan, simultaneously increases the dynamic range of the display system (also marketed as HDR, high dynamic range television or FLAD, full-area local area dimming).[91][92][88]


The LCD backlight systems are made highly efficient by applying optical films such as prismatic structure (prism sheet) to gain the light into the desired viewer directions and reflective polarizing films that recycle the polarized light that was formerly absorbed by the first polarizer of the LCD (invented by Philips researchers Adrianus de Vaan and Paulus Schaareman),[93] generally achieved using so called DBEF films manufactured and supplied by 3M.[94] Improved versions of the prism sheet have a wavy rather than a prismatic structure, and introduce waves laterally into the structure of the sheet while also varying the height of the waves, directing even more light towards the screen and reducing aliasing or moiré between the structure of the prism sheet and the subpixels of the LCD. A wavy structure is easier to mass-produce than a prismatic one using conventional diamond machine tools, which are used to make the rollers used to imprint the wavy structure into plastic sheets, thus producing prism sheets.[95] A diffuser sheet is placed on both sides of the prism sheet to distribute the light of the backlight uniformly, while a mirror is placed behind the light guide plate to direct all light forwards. The prism sheet with its diffuser sheets are placed on top of the light guide plate.[96][85] The DBEF polarizers consist of a large stack of uniaxial oriented birefringent films that reflect the former absorbed polarization mode of the light.[97] 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.[98] The combination of such reflective polarizers, and LED dynamic backlight control[88] 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.[99][100]

Quality control[edit]

Some LCD panels have defective transistors, causing permanently lit or unlit pixels which are commonly referred to as stuck pixels or dead pixels respectively. Unlike integrated circuits (ICs), LCD panels with a few defective transistors are usually still usable. Manufacturers' policies for the acceptable number of defective pixels vary greatly. At one point, Samsung held a zero-tolerance policy for LCD monitors sold in Korea.[131] As of 2005, Samsung adheres to the less restrictive ISO 13406-2 standard.[132] Other companies have been known to tolerate as many as 11 dead pixels in their policies.[133]


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, which was made obsolete in 2008 with the release of ISO 9241, specifically ISO-9241-302, 303, 305, 307:2008 pixel defects. However, not every LCD manufacturer conforms to the ISO standard and the ISO standard is quite often interpreted in different ways. LCD panels are more likely to have defects than most ICs due to their larger size.[134]


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 commonly have a defect known as clouding, dirty screen effect, or, less commonly, mura, which involves uneven patches of luminance on the panel. It is most visible in dark or black areas of displayed scenes.[135] As of 2010, most premium branded computer LCD panel manufacturers specify their products as having zero defects.

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 among pixels and add Quattron which attempt to efficiently increase the perceived resolution of a display without increasing the actual resolution, to mixed results.

sub-pixels

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 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.

dot pitch

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.

[138]

Color performance: There are multiple terms to describe different aspects of color performance of a display. 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.[139] 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.

Color gamut

Brightness and contrast ratio: is the ratio of the brightness of a full-on pixel to a full-off pixel. The LCD itself is only a light valve and does not generate light; the light comes from a backlight that is either fluorescent or a set of LEDs. Brightness is usually stated as the maximum light output of the LCD, which can vary greatly based on the transparency of the LCD and the brightness of the backlight. Brighter backlight allows stronger contrast and higher dynamic range (HDR displays are graded in peak luminance), but there is always a trade-off between brightness and power consumption.

Contrast ratio

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.

[140]

Little heat emitted during operation, due to low power consumption.

No geometric distortion.

The possible ability to have little or no flicker depending on backlight technology.

Usually no refresh-rate flicker, because the LCD pixels hold their state between refreshes (which are usually done at 200 Hz or faster, regardless of the input refresh rate).

Sharp image with no bleeding or smearing when operated at .

native resolution

Emits almost no undesirable (in the extremely low frequency range), unlike a CRT monitor.

electromagnetic radiation

Can be made in almost any size or shape.

No theoretical resolution limit. When multiple LCD panels are used together to create a single canvas, each additional panel increases the total resolution of the display, which is commonly called stacked resolution.

[141]

Can be made in large sizes of over 80-inch (2 m) diagonal.

LCDs can be made , but they cannot emit light without a backlight like OLED and microLED, which are other technologies that can also be made flexible and transparent.[142][143][144][145]

transparent and flexible

Masking effect: the LCD grid can mask the effects of spatial and grayscale quantization, creating the illusion of higher image quality.

[146]

Unaffected by magnetic fields, including the Earth's, unlike most color CRTs.

As an inherently digital device, the LCD can natively display digital data from a or HDMI connection without requiring conversion to analog. Some LCD panels have native fiber-optic inputs in addition to DVI and HDMI.[147]

DVI

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.

Flat-panel display

FPD-Link

Hitachi HD44780 LCD controller

LCD classification

LCD projector

LCD television

List of liquid-crystal-display manufacturers

/ Remarkable (tablet)

Boogie board (product)

Raw monitor

Smartglasses

– Video by the Vega Science Trust.

Development of Liquid Crystal Displays: Interview with George Gray, Hull University, 2004

Timothy J. Sluckin , a presentation and extracts from the book Crystals that Flow: Classic papers from the history of liquid crystals.

History of Liquid Crystals

David Dunmur & Tim Sluckin (2011) Soap, Science, and Flat-screen TVs: a history of liquid crystals, ISBN 978-0-19-954940-5.

Oxford University Press

Artamonov, Oleg (January 23, 2007). . X-bit labs. Archived from the original on May 16, 2008. Retrieved May 17, 2008.

"Contemporary LCD Monitor Parameters: Objective and Subjective Analysis"

Presentation Technology

Overview of 3LCD technology

Animations explaining operation of LCD panels

on YouTube

LCD Monitor Teardown – engineerguyvideo

History and Physical Properties of Liquid Crystals by Nobelprize.org

from Newhaven Display

What's an IPS Display

Archived March 8, 2021, at the Wayback Machine

How TFT-LCDs are made, by AUO

Archived June 6, 2021, at the Wayback Machine

How LTPS (Low Temperature Poly Silicon) LCDs are made, by AUO