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LCD DISPLAYSBy Aravind1

What is an LCD?A liquid crystal display is a flat panel display, electronic visual display or video display that uses light modulating properties of liquid crystals

DISCOVERY Friedrich Reinitzers discovery of liquid crystalline nature of cholesterol extracted from carrots lead to the discovery of LC Displays(1888)That is, two melting points and generation of colors George H. Heilmeier was credited with the invention of LCD(1964)

Heilmeier made the first operational liquid crystal display based on what he called the dynamic scattering mode (DSM).

LCDIt is an electronically modulated optical device made up of any number of segments filled with liquid crystals and arrayed in front of a light source ( backlight ) The most flexible ones use an array of small pixels

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. The managing and control of the data to be displayed is performed by one or more circuits commonly denoted as LCD drivers

TWISTED ALLIGNMENTThe parallel arrangement of liquid crystal molecules along groovesWhen coming into contact with grooved surface in a fixed direction, liquid crystal molecules line up parallel along the grooves.

Construction ofLiquid Crystal Display

Two bounding plates (usually glass slides), each with a transparent conductive coating (such as indium tin oxide) that acts as an electrode; A polymer alignment layer : undergoes a rubbing process as grooves.Spacers to control the cell gap precisely; Two crossed polarizers (the polarizer and the analyzer);Polarizers are usually perpendicular to each other.

ELECTRODE USEDITO(tin doped indium oxide i.e., indium tin oxide ) melting point 1800- 2200 kPale yellow to greenish yellowIt is transparent when thin, but vary from yellow to grey in bulkIto is one of the most widely used transparent conducting oxidesBecause of its electrical conductivity and optical transparency also it can be deposited as thin film

Twisted nematic effect TN effect

HOW IT WORKS?

OFLINEONLINEONLINE12

TFTTFT thin film diodes and thin film transistors, these are non-linear electronic elements in order to allow the addressing of individual picture elements without crosstalk(unintended activation of non addressed pixels)

HOW THE COLOURS ARE PRODUCED?

BACKLIGHT SYSTEMOn most displays, this consists of a cold cathode fluorescent lamp that is situated behind the LCD panel for brighter display.

MATRIX ARRANGEMENT OF PIXELSFor simple displays the pixels needed are very low in number compared to complex displays

ADDRESSING

Display AddressingAddressing is the process by which pixels are turned on and off in order to create an image.

ADDRESSING TYPESThere are two main types of addressing, direct and multiplexing.Direct addressing is convenient for displays where there are only a few elements that have to be activated. With direct addressing, each pixel in the display has its own drive circuit. A microprocessor must individually apply a voltage to each element. A common application of direct addressing is the traditional seven segment liquid crystal display, found in wristwatches and similar devices.

PASSIVE MATRIX ADDRESSINGPassive matrix addressing A pixel in a passive matrix must maintain its state without active driving circuitry until it can be refreshed againPassive addressed matrix do not need the switch component because it has built-in bistability

PASSIVE MATRIX ADDRESSINGonly m+n control signals are required to address a m*n displayThe potential across pixel at selected row and column ViJ=Vsel VonIoff for unselected rows ViJ=Vunsel VonIoff The row signal is select signal and the column signal is video signal

ACTIVE MATRIX ADDRESSINGThis switches individual elements using a silicon based tft for each pixel

OPTICAL RESPONSEElectro-distortional curve is shown as follows :

the electro-distortional response determines the transmission of light through the cell

Alpha-numeric display

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.

PIXEL GEOMETRY

Properties of LCD Display

Light weight (typ. 1/5 of CRT) power consumption (typ. 1/4 of CRT) Completely flat screen - no geometrical errorsCrisp pictures - digital and uniform colors No electromagnetic emission Fully digital signal processing possible Large screens (>20 inch) on desktops High price (presently 3x CRT) Poor viewing angle (typ. 50 degrees) Low contrast and luminance (typ. 1:100) Low luminance (typ. 200 cd/m2)

ADVANTAGES Very compact and light.Low power consumption.No geometric distortion.Little or no flicker depending on backlight technology.Not affected by screen burn-in.Can be made in almost any size or shape.No theoretical resolution limit.

Advantage of LCD over CRT

Smaller sizeAMLCDs occupy approximately 60 percent less space than CRT displaysan important feature when office space is limited. Lower power consumptionAMLCDs typically consume about half the power and emit much less heat than CRT displays. Lighter weightAMLCDs weigh approximately 70 percent less than CRT displays of comparable size. No electromagnetic fieldsAMLCDs do not emit electromagnetic fields and are not susceptible to them. Thus, they are suitable for use in areas where CRTs cannot be used. Longer lifeAMLCDs have a longer useful life than CRTs; however, they may require replacement of the backlight.

DISADVANTAGESLimited viewing angle , causing color, saturation, contrast and brightness to vary, even within the intended viewing angle, by variations in posture.Bleeding and uneven backlighting in some monitors, causing brightness distortion, especially toward the edges.Smearing and ghosting artifacts caused by slow response times (>8 ms) and "sample and hold" operation.Only one native resolution . Displaying resolutions either requires a video scaler , lowering perceptual quality, or display at 1:1 pixel mapping , in which images will be physically too large or won't fill the whole screen.

DISADVANTAGESFixed bit depth , many cheaper LCDs are only able to display 262,000 colors. 8-bit S-IPS panels can display 16 million colors and have significantly better black level, but are expensive and have slower response time.Dead or stuck pixels may occur during manufacturing or through use.Not all LCDs are designed to allow easy replacement of the backlight.Cannot be used with light guns/pens.Loss of contrast in high temperature environments.

APPLICATIONSThey are used in a wide range of applications, including common wrist watch and pocket calculator to an advanced VGA computer screen computer monitors , television , instrument panels, aircraft cockpit displays etc

QUERIES?QUERIES?32

THANK YOU !HAPPY 2012!!! - Aravind-A33