liquid-crystal display

 

A liquid crystal display (commonly abbreviated 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 often utilized in battery-powered electronic devices because it uses very small amounts of electric power.

Overview

Each pixel of an LCD typically consists of a layer of molecules aligned between two transparent electrodes, and two polarizing filters, the axes of transmission of which are (in most of the cases) perpendicular to each other. With no liquid crystal between the polarizing filters, light passing through the first filter would be blocked by the second (crossed) polarizer.

The surface 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, for example, a cloth. The direction of the liquid crystal alignment is then 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 (still the most common liquid crystal device), the surface alignment directions at the two electrodes are perpendicular to each other, and so the molecules arrange themselves in a helical structure, or twist. Because the liquid crystal material is birefringent, 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. Half of the incident light is absorbed by the first polarizing filter, 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 in the center of the layer are almost completely untwisted and the polarization of the incident light is not rotated as it passes through the liquid crystal layer. This light will then be mainly polarized perpendicular to the second filter, and thus be blocked and the pixel will appear black. By controlling the voltage applied across the liquid crystal layer in each pixel, light can be allowed to pass through in varying amounts thus constituting different levels of gray.

The optical effect of a twisted nematic device in the voltage-on state is far less dependent on variations in the device thickness than that in the voltage-off state. Because of this, these devices are usually operated between crossed polarizers such that they appear bright with no voltage (the eye is much more sensitive to variations in the dark state than the bright state). These devices can also be operated between parallel polarizers, in which case the bright and dark states are reversed. The voltage-off dark state in this configuration appears blotchy, however, because of small thickness variations across the device.

Both the liquid crystal material and the alignment layer material contain ionic compounds. If an electric field of one particular polarity is applied for a long period of time, this ionic material is attracted to the surfaces and degrades the device performance. This is avoided either by applying an alternating current or by reversing the polarity of the electric field as the device is addressed (the response of the liquid crystal layer is identical, regardless of the polarity of the applied field).

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.

Specifications

Important factors to consider when evaluating an LCD monitor:

• Resolution: The horizontal and vertical size expressed in pixels (e.g., 1024x768). Unlike CRT monitors, LCD monitors have a native-supported resolution for best display effect.
• Dot pitch: The distance between the centers of two adjacent pixels. The smaller the dot pitch size, the less granularity is present, resulting a sharper image. Dot pitch may be the same both vertically and horizontally, or different (less common).
• Viewable size: The size of an LCD panel measured on the diagonal (more specifically known as active display area).
• Response time: The minimum time necessary to change a pixel's color or brightness.
• Matrix type: Active or Passive.
• Viewing angle: (coll., more specifically known as viewing direction).
• Color support: How many types of colors are supported (coll., more specifically known as color gamut).
• Brightness: The amount of light emitted from the display (coll., more specifically known as luminance).
• Contrast ratio: The ratio of the intensity of the brightest bright to the darkest dark.
• Aspect ratio: The ratio of the width to the height (for example, 4:3, 16:9 or 16:10).
• Input ports (e.g., DVI, VGA, LVDS, or even S-Video and HDMI).

Brief history

• 1888: Friedrich Reinitzer (1858-1927) discovers the liquid crystalline nature of cholesterol extracted from carrots (that is, two melting points and generation of colors) and published his findings at a meeting of the Vienna Chemical Society on May 3, 1888 (F. Reinitzer: Beiträge zur Kenntniss des Cholesterins, Monatshefte für Chemie (Wien) 9, 421-441 (1888)).^[1]
• 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.^[2]
• 1962: Richard Williams of RCA found that liquid crystals had some interesting electro-optic characteristics and he realized an electro-optical effect by generating stripe-patterns in a thin layer of liquid crystal material by the application of a voltage. This effect is based on an electro-hydrodynamic instability forming what is now called “Williams domains” inside the liquid crystal.^[3]
• 1964: In the fall of 1964 George H. Heilmeier, then working in the RCA laboratories on the effect discovered by Williams realized the switching of colors by field-induced realignment of dichroic dyes in a homeotropically oriented liquid crystal. Practical problems with this new electro-optical effect made Heilmeier to continue work on scattering effects in liquid crystals and finally the realization of the first operational liquid crystal display based on what he called the dynamic scattering mode (DSM). Application of a voltage to a DSM display switches the initially clear transparent liquid crystal layer into a milky turbid state. DSM displays could be operated in transmissive and in reflective mode but they required a considerable current to flow for their operation.^[4][5][6]

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).

• 1970: In December 1970, the twisted nematic field effect in liquid crystals was filed for patent by Hoffmann-LaRoche in Switzerland (Swiss patent No. 532 261) with Martin Schadt and Wolfgang Helfrich (then working for the Central Research Laboratories) listed as inventors.^[4] Hoffmann-La Roche then licensed the invention to the Japanese electronics industry which soon produced the first digital quartz wrist watches with TN-LCDs and numerous other products. James Fergason at Kent State University filed an identical patent in the USA 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.
• 1972: The first active-matrix liquid crystal display panel was produced in the United States by T. Peter Brody.^[7]

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A detailed description of the origins and the complex history of liquid crystal displays from the perspective of an insider during the early days has been published by Joseph A. Castellano in "Liquid Gold, The Story of Liquid Crystal Displays and the Creation of an Industry" ^[8].

The same history seen from a different perspective has been described and published by Hiroshi Kawamoto (The History of Liquid-Crystal Displays, Proc. IEEE, Vol. 90, No. 4, April 2002^[9]), This paper is publicly available at the IEEE History Center.

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 'subpixel' structures via the use of phosphors, although the analog electron beam employed in CRTs do not hit exact 'subpixels'.

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 addressed LCDs

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 (DSTN corrects a color-shifting problem with STN), and (CSTN) color-STN (a technology where color is added by using an internal color 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.

Active matrix technologies

Main article: TFT LCD, Active-matrix liquid crystal display

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.

For a more comprehensive description refer to the section on the twisted nematic field effect.

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. This 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.

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. 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.^[10] Currently, though, Samsung adheres to the more restrictive ISO 13406-2 standard.^[11] Other companies have been known to tolerate as many as 11 dead pixels in their policies.^[12] 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.^[13] 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 this example, a 300 mm SVGA LCD has 8 defects and a 150 mm wafer has only 3 defects. However, 134 of the 137 dies on the wafer will be acceptable, whereas rejection of the 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 guarantee" and 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.^[14]

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 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 Cholesteric 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 also demonstrated two new types of Zero Power bistable LCDs based on Zenithal bistable techniques.^[15]

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, LCDs produce crisp images only in their "native resolution" and, sometimes, fractions of that native resolution. Attempting to run LCD panels at non-native resolutions usually results in the panel scaling the image, which introduces blurriness or "blockiness" and is susceptible in general to multiple kinds of HDTV Blur.
• Although LCDs typically have more vibrant images and better "real-world" contrast ratios (the ability to maintain contrast and variation of color in bright environments) than CRTs, they do have lower contrast ratios than CRTs in terms of how deep their blacks are. A contrast ratio is the difference between a completely on (white) and off (black) pixel, and LCDs can have "backlight bleed" where light (usually seen around corners of the screen) leaks out and turns black into gray. Nowadays the very best LCDs actually surpass the best plasmas in terms of delivering a deep black, but most LCDs still lag behind. ^[16]
• Many LCDs cannot "truly" display as many colors as their CRT and plasma counterparts, typically ones that have lower-end panel types (see List of LCD matrices) such as Twisted Nematic panels (TN).
• LCDs typically have longer response times than their plasma and CRT counterparts, especially older displays, creating visible ghosting when images rapidly change. For example, when moving the mouse too fast on an LCD, multiple cursors can sometimes be seen.
• Some LCDs have significant input lag. If the lag delay is large enough, such displays can be unsuitable for fast and time-precise mouse operations (CAD, FPS gaming) as compared to CRT displays or smaller LCD panels with negligible amounts of input lag. Short lag times are sometimes emphasized in marketing.
• LCD panels tend to have a limited viewing angle relative to CRT and plasma displays. This can reduce the number of people able to conveniently view the same image – laptop screens are one example.
• Some LCD monitors can cause migraines and eyestrain problems due to flicker from fluorescent backlights fed at 50 or 60 Hz.
• A small percentage of LCD screens suffer from image persistence, which is similar to screen burn on CRT and plasma displays, though in LCD monitors, this condition can be repaired very easily.
• Many LCDs are incapable of displaying very low resolution screen modes (such as 320x200) due to scaling limitations.
• Consumer LCD monitors tend to be more fragile than their CRT counterparts. The screen may be especially vulnerable due to the lack of a thick glass shield as in CRT monitors.
• Dead pixels are a common occurrence and few manufacturers replace screens with dead pixels for free.
• Horizontal and/or vertical banding is a problem in some LCD screens. This flaw occurs as part of the manufacturing process, and cannot be repaired (short of total replacement of the screen). Banding can vary substantially even among LCD screens of the same make and model. The degree is determined by the manufacture's quality control procedures.
• Color metering is a common problem often not thought about. For a realist image the frequency range of each of the 3 colors should match the color perception (frequency range) of the human eye. CRT monitors generally do a better job than that of LCD screens. (ref:http://www.sencore.com/newsletter/Mar05/Why%20You%20Need%20a%20CP5000.htm)

See also

LCD technologies

• List of LCD matrices
• TFT LCD
• Transreflective liquid crystal display – adaptation to environment brightness
• Active-matrix liquid crystal display (AMLCD)
• Anisotropic Conductive Film
• Backlight
• HDTV Blur

Other display technologies

• Comparison of display technology
• Cathode ray tube (CRT)
• Digital Light Processing (DLP)
• Field emission display (FED)
• Light-emitting diode (LED)
• Liquid crystal on silicon (LCOS)
• Organic light-emitting diode (OLED)
• Plasma display panel (PDP)
• Surface-conduction electron-emitter display (SED)
• Vacuum fluorescent display (VFD)

Display applications

• Television and digital television
• Liquid crystal display television (LCD TV)
• LCD projector
• Computer monitor

Manufacturers

• Acer (company)
• AU Optronics
• Barco
• BenQ
• BOE Hydis
• Casio
• Chi Mei Optoelectronics
• CoolTouch Monitors
• Corning Inc.
• Eizo
• Epson
• Fujitsu
• HP
• International Display Works
• JVC
• LG.Philips LCD
• LXD Incorporated
• Medion
• NEC Display Solutions
• Ocular Inc.
• Optrex America, Inc.
• Pacific Display
• Samsung Electronics
• Sharp Corporation
• S-LCD
• Sony
• Toshiba
• TPO Display Corp.
• Varitronix
• Viewsonic
• Vizio
• Xerox

References

1. ^ Tim Sluckin: Ueber die Natur der kristallinischen Flüssigkeiten und flüssigen Kristalle (The early history of liquid crystals), Bunsen-Magazin, 7.Jahrgang, 5/2005
2. ^ George W. Gray, Stephen M. Kelly: "Liquid crystals for twisted nematic display devices", J. Mater. Chem., 1999, 9, 2037–2050
3. ^ R. Williams, “Domains in liquid crystals,” J. Phys. Chem., vol. 39, pp. 382–388, July 1963
4. ^ ^{a b} Castellano, Joseph A. (2006), "Modifying Light", American Scientist 94 (5): pp. 438-445
5. ^ G. H. Heilmeier and L. A. Zanoni, “Guest-host interactions in nematic liquid crystals. A new electro-optic effect,” Appl. Phys. Lett., vol. 13, no. 3, pp. 91–92, 1968
6. ^ G. H. Heilmeier, L. A. Zanoni, and L. A. Barton, “Dynamic scattering: A new electrooptic effect in certain classes of nematic liquid crystals,” Proc. IEEE, vol. 56, pp. 1162–1171, July 1968
7. ^ Brody, T.P., "Birth of the Active Matrix", Information Display, Vol. 13, No. 10, 1997, pp. 28-32.
8. ^ LIQUID GOLD, The Story of Liquid Crystal Displays and the Creation of an Industry, 2005 World Scientific Publishing Co. Pte. Ltd., ISBN 981-238-956-3
9. ^ Hiroshi Kawamoto: The History of Liquid-Crystal Displays, Proc. IEEE, Vol. 90, No. 4, April 2002
10. ^ Samsung to Offer 'Zero-PIXEL-DEFECT' Warranty for LCD Monitors. Forbes.com (December 30, 2004). Retrieved on 2007-09-03.
11. ^ What is Samsung's Policy on dead pixels?. Samsung (February 5, 2005). Retrieved on 2007-08-03.
12. ^ Display (LCD) replacement for defective pixels - ThinkPad. Lenovo (June 25, 2007). Retrieved on 2007-07-13.
13. ^ What is the ISO 13406-2 standard for LCD screen pixel faults?. Anders Jacobsen's blog (January 4, 2006).
14. ^ EBU – TECH 3320, "User requirements for Video Monitors in Television Production", EBU/UER, May 2007, p. 11.
15. ^ Dr Chidi Uche. Development of bistable displays. University of Oxford. Retrieved on 2007-07-13.
16. ^ David Katzmaier. Flat-panel TVs: plasma and LCD. CNET.com. Retrieved on 2007-06-08.

External links - Tutorials

• History and Physical Properties of Liquid Crystals by Nobelprize.org
• Definitions of basic terms relating to low-molar-mass and polymer liquid crystals (IUPAC Recommendations 2001)
• An intelligible introduction to liquid crystals from Case Western Reserve University
• Liquid Crystal Physics tutorial from the Liquid Crystals Group, University of Colorado
• Introduction to liquid crystals from the Liquid Crystal Technology Group, Oxford University
• Liquid Crystals & Photonics Group - Ghent University (Belgium), good tutorial
• Liquid crystals Liquid Crystals Interactive Online (not updated since 1999)
• Liquid Crystal Institute Kent State University
• Liquid Crystals a journal by Taylor&Francis
• Molecular Crystals and Liquid Crystals a journal by Taylor&Francis
• Hot-spot detection techniques for ic's
• What are liquid crystals? from Chalmers University of Technology, Sweden

General information

• How LCDs are made, an interactive demonstration from AUO (LCD manufacturer).
• Development of Liquid Crystal Displays: Interview with George Gray, Hull University, 2004 – Video by the Vega Science Trust.
• History of Liquid Crystals – Presentation and extracts from the book Crystals that Flow: Classic papers from the history of liquid crystals by its co-author Timothy J. Sluckin
• Display Technology, by Geoff Walker in the September 2001 issue of Pen Computing
• Overview of 3LCD technology, Presentation Technology
• LCD Module technical resources and application notes, Diamond Electronics
• LCD Phase and Clock Adjustment, Techmind offers a free test screen to get a better LCD picture quality than the LCDs "auto-tune" function.
• How to clean your LCD screen? The Walyou Blog

Display technology
Non-video	Electronic paper · Flip-dot display · Split-flap display
Video	Cathode ray tube · Digital light processing · Field emission display · Laser TV · (Organic) light-emitting diode Liquid crystal display · Plasma display · Surface-conduction electron-emitter display · Vacuum fluorescent display
3D display	Stereoscopic · Autostereoscopic · Volumetric · Laser beam
Static media	Hologram · Movie projector · Neon sign · Slide projector

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