Index:

|S. no. |Topic |Page no. |
|1 |Abstract |5 |
|2 |Introduction |6 |
|3 |Methodology |7 |
|4 |Liquid crystals |8 |
|5 |Working Principle and operation |9 |
|6 |Types of LCDs |12 |
|7 |Advantages of LCDs |13 |
|8 |Disadvantages of LCDs |14 |
|9 |Applications of LCDs |16 |
|10 |Discussions |19 |
|11 |Conclusions |20 |
|12 |References |21 |

Abstract:

Today we see items containing an LCD (liquid crystal display) everywhere. They are all around us -- in laptop computers, digital clocks and watches, microwave ovens, CD players and many other electronic devices. From the wrist watch and pocket calculator to an advanced VGA computer screen, the liquid crystal display has evolved into an important and versatile interface. A liquid crystal display consists of an array of tiny segments (called pixels) that can be manipulated to present information. This basic idea is common to all displays, ranging from simple calculators to a full color LCD television. The earliest discovery leading to the development of LCD technology, the discovery of liquid crystals, dates from 1888.Liquid crystals are technological materials whose molecular structure changes with the voltage applied. This allows for control of the amount of light passing through the crystal.LCDs are more energy efficient and offer safer disposal than CRTs. Its low electrical power consumption enables it to be used in battery-powered electronic equipment. With LCD technology it is possible to produce space-saving, flicker-free, low-radiation displays that are easier on the eyes than conventional television or computer monitors. In addition, liquid crystal displays have a sharper picture with greater contrast and they use less power.

Introduction:

Objective: The objective of my term paper is to review comprehensively the principle and the physic in the working of the Liquid Crystal Display and also assessing its advantages and disadvantages of in comparison to other display devices.

In the early 1970's, digital watches started showing up in the marketplace with a new and different type of display-the liquid crystal display or LCD.Liquid crystals were first discovered in the late 19th century by the Austrian botanist, Friedrich Reinitzer, and the term "liquid crystal" itself was coined shortly afterwards by German physicist, Otto Lehmann.

A liquid crystal display is a thin, flat electronic visual display that uses the light modulating properties of crystals. It is an electronically-modulated optical device made up of any number of pixels filled with liquid crystals and arrayed in front of a light source (backlight) or reflector to produce images in color or monochrome. Liquid crystal displays (LCDs) are a passive display technology. This means they do not emit light; instead, they use the ambient light in the environment. This has made LCDs the preferred technology whenever low power consumption and compact size are critical. The LCDs are over 1000 times more efficient at their job than the LEDs. LCD flat full color panels are now challenging the CRT as displays for television and computers. There are also many hybrid systems that use LCD display technology.

Methodology:

Thorough research was done before producing this term paper. Scholarly articles and books on the Liquid Crystal Display with particular focus on the physics behind its operation were reviewed. In addition, books and research papers on various display devices were consulted in order to asses the advantages as well as disadvantages of the LCD.The study examined the key concepts involved in the working of LCD and also its major applications.

Liquid crystals

There are 3 states of matter: solid, liquid, gas.
Solids states can be further categorized into: crystalline which has regular arrangement of molecules; and amorphous where there is no regular structure. In 1888, an intermediate phase was discovered and is known as the crystalline liquid or liquid crystal. This phase is called the nematic phase. For example, 4-n-pentyl-4'-cyano-biphenyl (PCB). Since than, over 20,000 known compounds have been found to have the nematic phase. [2]
The main interest in these types of compound is that the nematic phase compounds with rod-like molecules can be aligned by varying an external electric field. More specifically, they are a liquid with molecules oriented in one common direction (having a long range and repeating pattern).
Most liquid crystals are organic compounds consisting of long rod-like molecules which, in their natural state, arrange themselves with their long axes roughly parallel. It is possible to precisely control the alignment of these molecules by flowing the liquid crystal along a finely grooved surface. The alignment of the molecules follows the grooves, so if the grooves are exactly parallel, then the alignment of the molecules also becomes exactly parallel. [5] Most of the liquid crystal displays (LCDs) produced today uses either the twisted nematic (TN) or super twisted nematic (STN) electro-optical effects.

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Principle of LCD

LCDs are light valves. The principle of the liquid crystal display's operation is radically different from all other display devices. LCDs are light modifiers, not light producers. All the other devices are self-illuminating as they produce their own light. The LCD does not make its own light, but operates by modifying light from other sources. This distinction is very important and is responsible for the low power consumption of the LCD. The external light modified by the LCD may be ambient light or a special light source installed within the device just to supply the LCD some light to modify. LCDs use a reflector, backlight, sidelight, or a combination of a reflector and back/sidelight to display an image. Since most of special light source come from the back of the LCD Panel, hence it is commonly known as back-lit LCD.[1]

Inside a LCD, there are two thin plates of glass with a special liquid crystal solution between them. This liquid crystal solution has an interesting property. If there is no current going through the liquid crystal solution, the solution is clear. But when there is an electrical current going through the solution, the solution crystallizes and goes from clear to dark. The solution acts like a shutter when current is passed through it, changing from clear to dark.

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OPERATION OF LCD

The combination of four facts makes LCDs possible:

• Light can be polarized. • Liquid crystals can transmit and change polarized light. • The structure of liquid crystals can be changed by electric current. • There are transparent substances that can conduct electricity.

An LCD is a device that uses these four facts in a surprising way.

In their natural state, LCD molecules are arranged in a loosely ordered fashion with their long axes parallel. However, when they come into contact with a grooved surface in a fixed direction, they line up in parallel along the grooves. The first principle of an LCD consists of sandwiching liquid crystals between two finely grooved surfaces, where the grooves on one surface are perpendicular (at 90 degrees) to the grooves on the other. If the molecules at one surface are aligned north to south, and the molecules on the other are aligned east to west, then those in-between are forced into a twisted state of 90 degrees. Light follows the alignment of the molecules, and therefore is also twisted through 90 degrees as it passes through the liquid crystals. However, following RCA America's discovery, when a voltage is applied to the liquid crystal, the molecules rearrange themselves vertically, allowing light to pass through untwisted.
The second principle of an LCD relies on the properties of polarising filters and light itself. Natural light waves are orientated at random angles. A polarising filter is simply a set of incredibly fine parallel lines. These lines act like a net, blocking all light waves apart from those (coincidentally) orientated parallel to the lines. A second polarising filter with lines arranged perpendicular (at 90 degrees) to the first would therefore totally block this already polarised light. Light would only pass through the second polarizer if its lines were exactly parallel with the first, or if the light itself had been twisted to match the second polarizer. A typical twisted nematic (TN) liquid crystal display consists of two polarising filters with their lines arranged perpendicular (at 90 degrees) to each other, which, as described above, would block all light trying to pass through. But in-between these polarisers are the twisted liquid crystals. Therefore light is polarised by the first filter, twisted through 90 degrees by the liquid crystals, finally allowing it to completely pass through the second polarising filter. However, when an electrical voltage is applied across the liquid crystal, the molecules realign vertically, allowing the light to pass through untwisted but to be blocked by the second polariser. Consequently, no voltage equals light passing through, while applied voltage equals no light emerging at the other end.
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The crystals in an LCD could be alternatively arranged so that light passed when there was a voltage, and not passed when there was no voltage. However, since computer screens with graphical interfaces are almost always lit up, power is saved by arranging the crystals in the no-voltage-equals-light-passing configuration. An important feature of an LCD monitor is its response time, which is the time it takes for an applied voltage to effect the liquid crystals' alignment and register a change to the screen. This is a value measured in milliseconds (ms), and clearly the lower the value the better for the screen.
Colors of LCDs:

In color LCDs each individual pixel is divided into three cells, or sub pixels, which are colored red, green, and blue, respectively, by additional filters (pigment filters, dye filters and metal oxide filters). Each sub pixel can be controlled independently to yield thousands or millions of possible colors for each pixel through the careful control and variation of the voltage applied; the intensity of each sub pixel can range over 256 shades. Combining the sub pixels produces a possible palette of 16.8 million colors (256 shades of red x 256 shades of green x 256 shades of blue). Color displays take an enormous number of transistors.

[pic]
Types of LCDs:

There are 2 types of Liquid crystal displays, namely active and passive.

Passive Matrix:

In a passive matrix, or FSTN, display a grid of electronic control wires or lines are placed on the front and back glass. A pixel is located at the junction of each row and column control lines. Passive matrix displays use one transistor to address each row and one to address each column of pixels. Pixels are turned on when both row and column lines are energized and off when both control lines are de-energized. This addressing scheme is called multiplexing. The residual electrical current that travels down each control line can cause crosstalk at unselected pixels. [8] Crosstalk partially darkens pixels and lowers the display's overall contrast. This usually appears on a passive matrix PowerBook display as two dark boxes, parallel to each other on the display.

Active Matrix:

The active matrix or Thin-Film Transistor (TFT) display is the latest technology used in computers. Rather than using multiplexing (row and column wires on the glass) techniques to address the matrix of crystals, the active matrix LCD includes a transistor fabricated along with each pixel. Because of the transistors, pixels can be turned on and off at a very fast rate. The transistor at each pixel eliminates the crosstalk phenomenon, which lowers contrast on the passive matrix display. The TFT method eliminates the time dependency associated with multiplexed displays by directly addressing each pixel. Active LCD's can show colors other than black and white. This is due to the pixels that make up a computer monitor, digital clock faces, and other liquid crystal displays. In active LCDs, each of these little pixels can be programmed individually to display color.

However, in passive LCDs only entire lines of pixels can be programmed. So, passive LCDs are not made to display colors. Active matrix has a much wider viewing range than passive matrix. Information displayed on the screen can be seen from a wider side angle on an active matrix display than on a passive matrix display.

Advantages of LCDs:

LCDs have various advantages which make them one of the most widely used display techniques. They are not only lightweight, small in size and compact but are also energy efficient. Because LCDs consist of only two thin plates of glass, they are small enough to fit anywhere. Some of the major advantages of a Liquid Crystal Display are as follows:

1.Sharpness:
The image is perfectly sharp at the native resolution of the panel. LCDs using an analog input require careful adjustment of pixel tracking/phase.

2.Power Consumption :
Energy efficient. Consume less than 1/3 the power of a comparable CRT. Consume less electricity than a CRT and produce little heat.

3.Brightness:
Produces very bright images due to high peak intensity. Very suitable for environments that are brightly lit.

4.Screen Shape:
Screens are perfectly flat.

5.Physical:
Thin and compact. Consume little electricity and produce little heat. LCDs take up about 40% less desk space.

6.Emissions: Produce considerably lower electric, magnetic and electromagnetic fields than CRTs.

7.Geometric Distortion:
Zero geometric distortion at the native resolution of the panel. Minor distortion for other resolutions because the images must be rescaled.

There are no convergence problems with an LCD panel, because each cell is switched on and off individually whereas a CRT has three electron guns whose streams must converge faultlessly in order to create a sharp image.
Disadvantages of LCDs:

Although LCDs are extremely useful, they also have some disadvantages and problems.
1.Resolution (The horizontal and vertical screen size expressed in pixels).Each panel has a fixed pixel resolution format determined at the time of manufacture that can not be changed. All other image resolutions require rescaling, which generally results in significant image degradation, particularly for fine text and graphics. It works well only at the native resolution.
2.Interference :LCDs using an analog input require careful adjustment of pixel tracking/phase in order to reduce or eliminate digital noise in the image. Automatic pixel tracking/phase controls seldom produce the optimum setting. Timing drift and jitter may require frequent readjustments during the day. For some displays and video boards its not possible to entirely eliminate the digital noise.
3.Viewing Angle :It has a limited viewing angle. Brightness, contrast, gamma and color mixtures vary with the viewing angle. Can lead to contrast and color reversal at large angles. Need to be viewed as close to straight ahead as possible. [4]
4. Black-Level, Contrast and Color Saturation: LCDs have difficulty producing black and very dark grays. As a result they generally have lower contrast than CRTs and the color saturation for low intensity colors is also reduced. Not suitable for use in dimly lit and dark environments.
5. White Saturation: The bright-end of the LCD intensity scale is easily overloaded, which leads to saturation and compression. When this happens the maximum brightness occurs before reaching the peak of the gray-scale or the brightness increases slowly near the maximum. It requires careful adjustment of the Contrast control.
6. Color and Gray-Scale Accuracy :The internal Gamma and gray-scale of an LCD is very irregular. LCDs typically produce fewer than 256 discrete intensity levels. For some LCDs portions of the gray-scale may be dithered. Images are pleasing but not accurate because of problems with black-level, gray-scale and Gamma, which affects the accuracy of the gray-scale and color mixtures. Hence, they are generally not suitable for professional image color balancing.
7. Bad Pixels and Screen Uniformity :LCDs can have many weak or stuck pixels, which are permanently on or off. Some pixels may be improperly connected to adjoining pixels, rows or columns. Also, the panel may not be uniformly illuminated by the backlight resulting in uneven intensity and shading over the screen.
8. Motion Artifacts: Slow response times and scan rate conversion result in severe motion artifacts and image degradation for moving or rapidly changing images.
9. Aspect Ratio :LCDs have a fixed resolution and aspect ratio (The ratio of the width to the height). For panels with a resolution of 1280x1024 the aspect ratio is 5:4=1.25, which is noticeably smaller than the 4:3=1.33 aspect ratio for almost all other standard display modes.
10. Cost :Considerably more expensive than comparable CRTs.
The liquid crystals used for display technology are thermotropic liquid crystals; they exhibit desired characteristics over a specific temperature range. Thus LCDs do not operate properly when they are too cold or too warm. If liquid crystals are too cold, they will not twist and the display will not form an image. If the display is too warm, the resistance of the liquid crystal material changes and this alters the properties of the display and performance suffers

However, with today's rate of technological innovation, these problems will probably soon be solved, making LCDs even more dominating in display technology.

Applications of liquid crystal technology:
Liquid crystal technology has had a major effect many areas of science and engineering, as well as device technology. Applications for this special kind of material are still being discovered and continue to provide effective solutions to many different problems. The LCD was commercialized in 1960's for use in calculators and wristwatches, but is now most commonly found in notebook PCs. LCDs have also expanded into desktop monitors along with other innovative and exciting digital applications In the future, the TFT-LCD will emerge as the most widely used display solution, because of its high reliability, viewing quality and performance, compact size and environment-friendly features. Various devices use LCD such as:

Microwaves and ovens

VGA computer screens and laptops

Television screens

Digital watches and clocks

Pocket calculators

Monitors for medical equipment

Car navigation and entertainment systems

CD players

Avionics instrumentation.

However, its major use is seen in the following fields:

A) Thin Film Transistor (TFT) ▪ Constructed on a glass surface using a photolithographic process. ▪ The source and gate are the control electrodes. The drain electrode connects to the liquid crystal pixel. The thin layer of amorphous silicon is the semiconducting material that allows the TFT to function. The capacitor is attached to the pixel electrode, but is not an integral part of the TFT.[7]

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B) Alpha-numeric display ▪ Digital letters can be displayed by blocking the lights in different plates we place. ▪ For applications such as digital watches and calculators, a mirror is used under the bottom polarizer. With no voltage applied, ambient light passes through the cell, reflects off the mirror, reverses its path, and re-emerges from the top of the cell, giving it a silvery appearance. ▪ When the electric field is on, the aligned LC molecules do not affect the polarization of the light. The analyzer prevents the incident light from reaching the mirror and no light is reflected, causing the cell to be dark. When the electrodes are shaped in the form of segments of numbers and letters they can be turned on and off to form an alpha-numeric display.

[pic] [pic]

C) Back lighting systems ▪ Alpha-numeric displays are not very bright because the light must pass through multiple polarizer which severely cut down on the intensity of the light, in addition to the various layers of the display which are only semi-transparent. Therefore a more intense source is employed in the form of a back lighting system.

For brighter displays ▪ Light bulbs mounted behind ▪ At the edges of the display replace the reflected ambient light. ▪ Disadvantage: very power intensive. Back lighting systems are used in more complex displays such as laptop computer screens, monitors, LCD projectors, pda, digital devices such as digital camera and DV.[3]

Future technology developments in the military and in aerospace sectors will also fuel the growth for the TFT-LCD market.
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Discussions:

Liquid Crystal Displays, or LCDs, are quickly becoming the display technology of choice. This simple but extremely useful invention is being used in almost everything we can think of. From the displays of digital watches, to calculators and even computer monitors, LCD's provide a way to display information in a more effective way than we ever could before. The LCD displays used in the early digital watches were very different from the LEDs they replaced. While even a tiny LED display consumes a few mill watts of power, the LCD consumes just microwatts of power. Unlike LED displays, which are usually quite small, LCDs can be fabricated in almost any convenient size. Hence, the LCDs are over 1000 times more efficient at their job than the LEDs.
Since its advent in 1971 as a display medium, liquid crystal displays have moved into a variety of fields, including miniature televisions, digital still and video cameras and monitors and today many believe that the LCD is the most likely technology to replace the CRT monitor. The technology involved has been developed considerably since its inception, to the point where today's products no longer resemble the clumsy, monochrome devices of old.

Conclusions:

The Liquid Crystal Display technology is one of the most popular and widely used display technology at present. A Liquid crystal cell consists of a thin layer of liquid crystal sandwiched between two glass sheets with transparent electrodes deposited on their inside faces.LCDs are categorized as non- emissive display devices. In an LCD, an electric current is used to switch segments of liquid crystals from a transparent phase to a cloudy phase, each segment forming part of a number or letter. The segments can also be in the shape of tiny dots or pixels, and the can be arranged in rows and columns. They are turned on and off individually to either block or allow polarized light to pass through. When the light is blocked, a dark spot is created on the reflecting screen.LCDs are used most frequently to display one or more lines of alpha-numeric information in a variety of devices: fax machines, laptop computer screens, answering machine call counters, scientific instruments, portable compact disc players, clocks, and so forth. The most expensive and advanced type—active matrix displays—are even being used as screens for handheld color TVs.
Hence, I would like to conclude that LCDs have a head start over other flat screen technologies and an apparently unassailable position in notebook, handheld PCs and many other devices.

References:

[1] Joseph A Castellano. (2005) Liquid Gold: The Story of Liquid Crystal Displays and the Creation of an Industry .World Scientific Pub Co Inc.

[2] Birendra Bahadur. Liquid Crystals-applications and uses. North-Holland/Elsevier Science

[3] Miriam A. Drake. (2003) Encyclopedia of library and information science.

[4] Mohammad A. Karim. (1992) Electro-optical displays. PWS KENT Publishing Co, Boston,

[5] Peter J.Collings. (2001) Liquid Crystals Princeton University Press.

[6] Encyclopaedia Britannica. 2010. Encyclopaedia Britannica Online. 20 June. 2010 .

[7] T.J Nelson, J R Wullert. Electronic Information Display technologies. (Bell Communications Res. Inc., USA)

[8] Willem den Boer. (2005) Active matrix Liquid Crystal Display-edition-1.Publisher: Newnes.

[9] Ronald G. Driggers. (2003) Encyclopedia of Optical Engineering. Publisher: Crc Press