LOVELY INSTITUTE OF TECHNOLGY

TERM PAPER OF

PHYSICS

TOPIC- LIQUID CRYSTAL DISPLAY(LCD)

SUBMITTED TO: MISS PRIYA

SUBMITTED BY: RAGHAV MAHAJAN REGISTRATION NO: 10802416 ROLL NO: R713A03 COURCE: B. TECH(C.S.E)-145

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Acknowledgement

Gratitude cannot be seen or expressed. It can only be felt in heart and is

beyond description. Often words are inadequate to serve as a model of

expression of one’s feeling, specially the sense of indebtedness and gratitude

to all those who help us in our duty.

It is of immense pleasure and profound privilege to express my

gratitude and indebtedness along with sincere thanks to Sir Varun Mishra,

lecturer of PHYSICS of Lovely Professional University for providing me the

opportunity to work for a project on “LCD ”.

I am beholden to my family and friends for their blessings and

encouragement.

Yours obediently,

RAGHAV MAHAJAN

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LCD (LIQUID CRYSTAL DISPLAY)

A liquid crystal display (LCD) is an electro-optical amplitude modulator realized as 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.

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CONTENTS

INTRODUCTION SPECIFICATIONS

HISTORY

HOW LCD WORKS?

CLASSIFICATION

o Passive-matrix addressed LCD

o Active-matrix addressed LCD

BLUE PHASE MODE LCD COLOR DISPLAYS

QUALITY CONTROL

ZERO POWER DISPLAYS

DISPLAY APPLICATION AND USES

COMPARISON WITH OTHER DISPLAY TECHNOLOGIES

DRAWBACKS

CONCLUSION

REFERENCES

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INTRODUCTION

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. Electrodes are made of a transparent conductor called Indium Tin Oxide(ITO).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 reasonably transparent.LCD with top polarizer removed from device and placed on top, such that the top and bottom polarizers are crossed.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 grey.

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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. LCD with top polarizer removed from device and placed on top, such that the top and bottom polarizers are parallel. 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 variations of thickness 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 are needed in a display, it is not technically possible 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.

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SPECIFICATIONS

Important factors to consider when evaluating an LCD monitor:

Resolution: The horizontal and vertical size expressed in pixels (e.g., 1024x768). Unlike monochrome 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 in 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. Response time is also divided into rise and fall time. For LCD Monitors, this is measured in btb (black to black) or gtg (gray to gray). These different types of measurements make comparison difficult.

Refresh rate: The number of times per second in which the monitor draws the data it is being given. A refresh rate that is too low can cause flickering and will be more noticeable on larger monitors. Many high-end LCD televisions now have a 120 Hz refresh rate (current and former NTSC countries only). This allows for less distortion when movies filmed at 24 frames per second (fps) are viewed due to the elimination of telecine (3:2 pulldown). The rate of 120 was chosen as the least common multiple of 24 fps (cinema) and 30 fps (TV).

Matrix type: Active TFT or Passive. Viewing angle: (coll., more specifically known as viewing direction).

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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, DisplayPort, or even S-Video and HDMI).

History of Liquid Crystal Displays - LCD

In 1888, liquid crystals were first discovered in cholesterol extracted from carrots by Austrian botanist and chemist, Friedrich Reinitzer. In 1962, RCA researcher Richard Williams generated 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. According to the IEEE, "Between 1964 and 1968, at the RCA David Sarnoff Research Center in Princeton, New Jersey, a team of engineers and scientists led by George Heilmeier with Louis Zanoni and Lucian Barton, devised a method for electronic control of light reflected from liquid crystals and demonstrated the first liquid crystal display. Their work launched a global industry that now produces millions of LCDs." Heilmeier's liquid crystal displays used what he called DSM or dynamic scattering method, wherein an electrical charge is applied which rearranges the molecules so that they scatter light. The DSM design worked poorly and proved to be too power hungry and was replaced by an improved version, which used the twisted nematic field effect of liquid crystals invented by James Fergason in 1969. Inventor, James Fergason holds some of the fundamental patents in liquid crystal displays filed in the early 1970's, including key US patent number 3,731,986 for "Display Devices Utilizing Liquid Crystal Light Modulation" In 1972, the International Liquid Crystal Company (ILIXCO) owned by James Fergason produced the first modern LCD watch based on James Fergason's patent. In the 4Q of 2007 for the first time LCD surpassed CRT in worldwide sales. In 2008, LCD TVs are the main stream with 50% market share of the 200 million TVs forecast to ship globally in 2008 according to Display Bank

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How an LCD Works

According to a PC world article, liquid crystals are liquid chemicals whose molecules can be aligned precisely when subjected to electrical fields, much in the way metal shavings line up in the field of a magnet. When properly aligned, the liquid crystals allow light to pass through.

A simple monochrome LCD display has two sheets of polarizing material with a liquid crystal solution sandwiched between them. Electricity is applied to the solution and causes the crystals to align in patterns. Each crystal, therefore is either opaque or transparent, forming the numbers or text that we can read.

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CLASSIFICATION OF LCD

1) Passive-matrix addressed LCD

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

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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.

2)Active matrix technologies

An active matrix liquid crystal display (AMLCD) is a type of flat panel display, currently the overwhelming choice of notebook computer manufacturers, due to light weight, very good image quality, wide color gamut, and response time. The term was first used in 1975 by Dr T. Peter Brody to describe a method of switching individual elements of a flat panel display, using a CdSe TFT for each pixel

a) 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.

b) 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

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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.

c) 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 Mode LCD

In blue phase based LC-displays for TV applications it is not the selective reflection of light according the lattice pitch (Bragg reflection), but an electric field deforms the lattice which results in anisotropy of the refractive indices of the layer, followed by a change of transmission between crossed polarizers.

Developed with a look at cost-efficiency, blue phase mode LCDs do not require liquid crystal alignment layers, unlike today’s most widely used LCD modes such as Twisted Nematic (TN), In-Plane Switching (IPS) or Vertical Alignment (VA) modes. The blue phase mode can make its own alignment layers, eliminating the need for any mechanical alignment and rubbing processes. This reduces the number of required fabrication steps, resulting in savings on production costs. Additionally it has been claimed that blue phase panels will reduce sensitivity of the LC-layer to mechanical

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pressure which can impair the lateral uniformity of display luminance.Overdrive circuits that are currently applied to many LCD panels with 120 Hz frame frequency for improvement of the display of moving images in premium LCD TVs will become obsolete since the blue phase mode features a superior response speed, allowing images to be reproduced at 240 Hz frame rate or higher without the need for any overdrive circuit.

CONTROL PLATES

A subpixel of a color LCD

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

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one point, Samsung held a zero-tolerance policy for LCD monitors sold in Korea. Currently, though, Samsung adheres to the less restrictive ISO 13406-2 standard. Other companies have been known to tolerate as many as 11 dead pixels in their policies. Dead pixel policies are often hotly debated between manufacturers and customers. To regulate the acceptability of defects and to protect the end user, ISO released the ISO 13406-2 standard. However, not every LCD manufacturer conforms to the ISO standard and the ISO standard is quite often interpreted in different ways.

Examples of defects in LCDs

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 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. 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.

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Zero-power (bistable) displays

The zenithal bistable device (ZBD), developed by QinetiQ, 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 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.

See Ferro Liquid Display for more information about ferro fluid based bistable displays.

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DISPLAY APPLICATIONS AND USES

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

Aircraft Instrumentation displays

COMPARISON OF LCD WITH OTHER DISPLAY TECHNOLOGIES

Different display technologies have vastly different temporal characteristics, leading to claimed perceptual differences for motion, flicker etc.

Sketch of some common display technologies' temporal behaviour

The figure shows a sketch of how different technologies present a single white/gray frame. Time and intensity is not to scale. Notice that some have a fixed intensity, while the illuminated period is variable. This is a kind of pulse-width modulation. Others can vary the actual intensity in response to the input signal. Single-chip DLPs use a kind of "chromatic multiplex" in which each color is presented serially. The intensity is varied by modulating the "on" time of each pixel within the time-span of one color. Multi-chip DLPs are not represented in this sketch, but would have a curve identical to the plasma display. LCDs have a constant (backlit) image, where the intensity is varied by blocking the light shining through the panel. CRTs use an electron beam, scanning the display, flashing a lit image. If interlacing is used, a single full-resolution image results in two

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"flashes".Plasma displays modulate the "on" time of each sub-pixel, similar to DLP.Movie theaters use a mechanical shutter to "flash" the same frame 2 or 3 times, increasing the flicker frequency to make it less perceptible to the human eye

Drawbacks

Two IBM ThinkPad laptop screens viewed at an extreme angle.

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. Many LCDs are incapable of displaying very low resolution screen modes (such as 320x200) due to these scaling limitations.

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

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around corners of the screen) leaks out and turns black into gray. However, as of December 2007, the very best LCDs can approach the contrast ratios of plasma displays in terms of delivering a deep black.

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 quickly on an LCD, multiple cursors can sometimes be seen.

Some LCD TVs have significant input lag due to slow video processing. 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 other LCD panels with negligible amounts of input lag. Some LCD TVs have a "game mode" (the term used by Sony) that reduces both the amount of video processing and the amount of input lag.

LCD panels using TN tend to have a limited viewing angle relative to CRT and plasma displays. This reduces the number of people able to conveniently view the same image – laptop screens are a prime example. Usually when looking below the screen, it gets much darker; looking from above makes it look lighter. Many displays based on thin film transistor variants such as IPS, MVA, or PVA, have much improved viewing angles; typically the color only gets a little brighter when viewing at extreme angles.

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 can occur when the screen is damaged or pressure is put upon the screen; few manufacturers replace screens with dead pixels under warranty.

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 manufacturer's quality control procedures.

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The cold cathode fluorescent bulbs typically used for back-lights in LCDs contain mercury. LED backlit LCD displays are mercury-free.CONCLUSIONLCD is one of finest finest and advanced technology used for many display purposes. Lcd technology is much better than other display technologies like CRT. It consumes very less electric energy in comparison to other display technologies. It also consumes very less space and are very thin in size. It is quite nicely used in mobiles, watches, television screens and computer monitors. The display quality of LCD is also very nice.

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REFERENCES

WEBSITES

www.about.com www.answer.com www.britanica.org www.educypedia.be

BOOKS University physics

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