CRT,LCD, LED , TV Technologies like Liquid crystal display

7/28/2019 CRT,LCD, LED , TV Technologies like Liquid crystal display

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The experimentation of cathode rays is largely accredited to J. J.

Thomson ,was able to deflect cathode rays, a fundamental function

of the modern CRT. A common CRT used in computer monitors and

television sets.

Cathode ray tubes (CRT's) is a special type of electronic vacuum

tube, in which a device called an electron gun projects a beam ofelectrons onto the fluorescent screen, causing the affected part of

the screen to glow.

Electrically charged metal plates inside the CRT, or electromagnets

outside the CRT, move the beam across the screen.

The beam thus creates a picture on the screen with spots of light.

CRT's are used in electronic equipment to display pictures or other

information. The picture tube of a television set is a CRT.


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Functions of a Cathode Ray Tube

Parts of a CRT

1. Electron Gun:The role of this section is

to produce electrons at a high, fixed, velocity.

This is done through a process knownas thermionic emission. An anode with a high

voltage applied to it accelerates the electrons

towards the screen due to electrostatic

attraction. On the way, the electrons pass

through a series of control grids which

control the brightness of the imageproduced. The more negative the grid, the

darker the image and vice versa.

2. Deflection system:The role of the deflection system is to control the image produced by

controlling the position that the electrons hit the screen. It consists of two perpendicular

sets of electric/magnetic fields. This allows control over both horizontal and vertical axes. Bycontrolling the voltage applied to the fields, it is possible to vary the deflection

through electrostatic force/motor effect.

3. Fluorescent screen:The role of this part is to display where the electrons are hitting the CRT.

It is a screen coated with a material that emits light when struck by electrons. Zinc sulfide orphosphorus are two commonly used materials.


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A CRT/monochromatic cannot be used to

display only one color except black. Each pixel in

monochromatic CRT contain a phosphor dot of

one color.

Whereas,

In color CRT the phosphor dot in each pixel

contains three colors red, green, blue(RGB).


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An electron beam produced at the cathode is

accelerated and focused to strike on the screen.

The screen is covered with a phosphor whichcan emit light of specific colors when excited.

The three type of phosphors used for the dots

emits red , green or blue light respectively when

struck by an electron beam.

Red , green and blue are primary colors and

by the combination of this three colors in a

correct ratio, all the others colors can be

produced.

To generate an image on the screen the electron beam scans across the

phosphor dots according to the information taken from the video signal.The phosphor dots at each position on the screen light up with the correct and

intensity to create the desired image.


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1. CATHODE: A filament heats up the cathode to facilitate the emission of electrons. As the

cathode has a negative voltage is applied to it, electron beam is produced at cathode and

travel towards the screen.

2. CONTROL GRID: the video signal voltage is applied to the control gird. As the voltage

between the cathode and the control grid varies, the intensity of the electron beam varies

accordingly, controlling the brightness of the image on the screen.

3. ACCELERATING ELECTRODE: This positive electrode accelerates the electrons in theelectrons in the electron beam. The baffles inside the accelerating electrode cylinder

restrict the beam to a narrow ray.

4. FOCUSING ANODE: A high voltage is applied to the focusing anode to force the electron

beam into paths that focus on the phosphor screen. The focusing anode is sometimes

referred to as an electrostatic lens as it uses electrostatic means to focus the electronbeam.

5. ELECTRON BEAM: Three beams of electrons are needed to strike the red, green or blue

phosphor respectively. The three beams can either be produced by three electron guns

or by one electron gun fitted with a colour deflection system that separates one electron

beam into three. In this case each electron gun has been given separate color ofphosphor that the beam has to strike. #Electron beam has no colour.


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6. ANODE WALL COATING : This metallic coating is connected to a high voltage of several

thousand volts. Such high voltage further accelerates the electrons in the electron

beam. The anode wall coating is also designed for collecting electrons after they hit the

phosphor screen.

7. SHADOW MASK: The shadow is a thin plate with holes in it to let the electron beam

to pass through. The purpose of the shadow mask is to separate the three electron

beams intended to hit the red, green or blue phosphor dots so that they do not hit

the wrong colour.

8. PHOSPHOR SCREEN: The phosphor screen contains many phosphor dots grouped in

threes. The three dots within each group are mad of different types of phosphor which,

when struck by the electron beam, emit red, blue and green light respectively.

The intensity of the light emitted depends on the intensity of the electron beam striking

the phosphor. The ratio of red, green and blue emitted by each group can thus becontrolled to produce


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A liquid-crystal display (LCD) is a flat

panel display electronic visual display,or video display that uses the light

modulating properties of liquid crystals.

Each pixel of an LCD typically consists of a

layer of molecules aligned between

two transparent electrodes, and

two polarizing filters, the axes oftransmission of which are perpendicular to

each other.

With actual liquid crystal between the

polarizing filters, light passing through the

first filter would be blocked by the second

(crossed) polarizer.


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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 ofthin polymer layer that is unidirectionally rubbed

using a cloth.

The direction of the liquid crystal alignment is then

defined by the direction of rubbing.

Electrodes are made of the transparent

conductor indium tin oxide.The liquid-crystal display is intrinsically a passive

device.

The managing and control of the data to be

displayed is performed by one or more circuits

commonly denoted as LCD drivers.

Before an electric field is applied, the

orientation of the liquid-crystal molecules is

determined by the alignment at the surfaces of

electrodes.

LCD drivers

LCD electrodes


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1. In a colour LCD monitor there are

many minute display units called

pixels used to display an image. The

structure of a pixel of an LCD monitoris shown in the diagram.

2. A pixel is divided into three sub pixels,

covered with red, green and blue

colour filters respectively.

3. Each sub pixel consists of a liquid

crystal layer sandwiched between twopolarizer's.

4. The two polarizer's have axes oriented

perpendicular to each other

5. The pixel is illuminated by a florescent lamp from behind the first polarizer. If the crystal

was absent, light passing though the first polarizer would be polarized in a direction

perpendicular to the axis of the second polarizer. In this case no light could pass

through the second polarizer.

6. The presence of the liquid crystal layer, however, rotates the polarization direction of the

light at a certain angle. Thus light coming out of the liquid crystal is polarized in a direction not

exactly perpendicular to the axis of the second polarizer.As a result , part of the light can passthrough the second polarizer and reach our eyes.


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7. The intensity of light that passes through the second polarizer depends on the amount

of polarization rotation produced by the liquid crystal layer. This is controlled by the

voltage applied to the liquid crystal through a device called a thin film transistor.

8. When each storage capacitor is charged to its maximum amount by the thin film

transistor, a high enough voltages is applied to the liquid crystal layer.9. This makes all the molecules in the liquid crystal align in such a way that they produce

no polarization rotation of the light passing through them. As a result, the lights

polarization direction is not changed and the light is blocked by the second polarizer.

10. As no light comes out of the pixel, the pixel is said to be off.

11. When the storage capacitor is partially charged, a smaller crystal layer causing the

molecules of the liquid crystal to rotate by a certain degree. This in turn rotates the

lights polarization direction. As a result, part of the light can pass through the second

polarizer.

12. The intensity of the light that passes through second polarizer is controlled by the

amount of charge in the storage capacitor. When the storage capacitor is uncharged, no

voltage is applied to the liquid crystal.

13. As the molecular arrangement of the liquid crystal layer twisted by 90 degrees, the

lights polarization direction is rotated by 90 degrees.


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1. Polarizing filter film with a vertical axis to polarize light as it enters.

2. Glass substrate with ITO electrodes. The shapes of these electrodes will determine the

shapes that will appear when the LCD is turned ON. Vertical ridges etched on the

surface are smooth.3. Twisted pneumatics liquid crystal.

4. Glass substrate with common electrode film (ITO) with horizontal ridges to line up

with the horizontal filter.

5. Polarizing filter film with a horizontal axis to block/pass light.

6. Reflective surface to send light back to viewer.


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http://en.wikipedia.org/wiki/File:LED,_5mm,_green_(en).svg


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A light-emitting diode(led) is a semiconductor light source LEDs are

used as indicator lamps in many devices and are increasingly used for

other lighting.

A led is often small in area (less than 1 mm2), and integrated optical

components may be used to shape its radiation pattern

When a light-emitting diode is switched on, electrons are able to

recombine with holes within the device, releasing energy in the form

of photons.

This effect is called electroluminescence and the color of the light(corresponding to the energy of the photon) is determined by the

energy band gap of the semiconductor.

LIGHT EMITTING DIODE:
http://en.wikipedia.org/wiki/File:LED,_5mm,_green_(en).svghttp://en.wikipedia.org/wiki/File:Verschiedene_LEDs.jpg


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The LED consists of a chip of semiconducting material doped with impurities to create

a p-n junction. As in other diodes, current flows easily from the p-side to the n-side but

not in the reverse direction.

Charge carriers electrons and holes flow into the junction from electrodes with different

voltages. When an electron meets a hole, it falls into a lower energy level, and

releases energy in the form of a photon.

OPERATION OF LED:
http://en.wikipedia.org/wiki/File:Diode-IV-Curve.svghttp://en.wikipedia.org/wiki/File:PnJunction-LED-E.svg


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The wavelength of the light emitted, and thus its color depends on the band gap energy

of the materials forming the p-n junction.

In silicon or germanium diodes, the electrons and holes recombine by a non-radiative

transition, which produces no optical emission, because these are indirect band

gap materials.

Most materials used for LED production have very high refractive indices. This means

that much light will be reflected back into the material at the material/air surface

interface.

Thus, light extraction in LEDs is an important aspect of LED production, subject to much

research and development.

Colour Wavelength range(nm) TypicalEfficiency(lm/W)

Red 620 < < 645 72

Red-orange 610 < < 620 98

Green 520 < < 550 93

Cyan 490 < < 520 75

Blue 460 < < 490 37


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There are two primary ways of producing white light-emitting

diodes (WLEDs), LEDs that generate high-intensity white light.

One is to use individual LEDs that emit three primary colorsred , green, and blue and then mix all the colors to form white

light.

The second is to use a phosphor material to convert

monochromatic light from a blue or UV LED to broad-spectrum

white light, much in the same way a fluorescent light bulbworks.

White light

The first blue LEDs using gallium nitride.These devices had too little light output to

be of practical use.

Ultraviolet and blue LEDs :
http://en.wikipedia.org/wiki/File:Blue_light_emitting_diodes_over_a_proto-board.jpghttp://en.wikipedia.org/wiki/File:RGB_LED.jpg


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RGB LEDWhite light can be formed by mixing

differently colored lights; the most common

method is to use red, green, and blue (RGB).

Hence the method is called multi-color white

LEDs . Because these need electronic circuits

to control the blending and diffusion of

different colors, and because the individual

color LEDs typically have slightly different

emission patterns even if they are made as a

single unit, these are seldom used to produce

white lighting.

Nevertheless, this method is particularly

interesting in many uses because of the flexibility ofmixing different colors and, in principle, this

mechanism also has higher quantum efficiency in

producing white light.
http://en.wikipedia.org/wiki/File:RGB_LED.jpghttp://en.wikipedia.org/wiki/File:RGB_LED.jpghttp://en.wikipedia.org/wiki/File:RGB_LED.jpghttp://en.wikipedia.org/wiki/File:RGB_LED.jpghttp://en.wikipedia.org/wiki/File:RGB_LED.jpghttp://en.wikipedia.org/wiki/File:RGB_LED.jpghttp://en.wikipedia.org/wiki/File:RGB_LED.jpghttp://en.wikipedia.org/wiki/File:Red-YellowGreen-Blue_LED_spectra.pnghttp://en.wikipedia.org/wiki/File:RGB_LED.jpg


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These are mostly single-die LEDs used as indicators, and

they come in various sizes from 2 mm to 8 mm, through-hole

and surface mount packages.Common package shapes include round, with a domed or

flat top, rectangular with a flat and triangular or square with

a flat top

There are three main categories of miniature single die

LEDs:

Low-current: Typically rated for 2 ma at around 2 V(approximately 4 mw consumption).

Standard: 20 mA LEDs (ranging from approximately 40 mW

to 90 mW) at around

1. 1.9 to 2.1 V for red, orange and yellow,

2. 3.0 to 3.4 V for green and blue,

3. 2.9 to 4.2 V for violet, pink, purple and white.

Ultra-high-output: 20 mA at approximately 2 V or 45 V,

designed for viewing in direct sunlight.

5 V and 12 V LEDs are ordinary miniature LEDs that

incorporate a suitable series resistor for direct connection to

a 5 V or 12 V supply.

Miniature


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Medium-power LEDs are often through-hole-

mounted and mostly utilized when an output of just a

few lumen is needed.

They sometimes have the diode mounted to four

leads (two cathode leads, two anode leads) for better

heat conduction and carry an integrated lens.

These LEDs are most commonly used in light panels,emergency lighting, and automotive tail-lights.

Due to the larger amount of metal in the LED, they

are able to handle higher currents (around 100 mA).

The higher current allows for the higher light output

required for tail-lights and emergency lighting.

Mid-range:


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High-power

High-power LEDs (HPLED) can be driven at currents

from hundreds of mA to more than an ampere,compared with the tens of mA for other LEDs. Some can

emit over a thousand lumens.

LED power densities up to 300W/cm2 have been

achieved

Some well-known HPLEDs in this category are the

Nichia 19 series, Lumileds Rebel Led, OsramOpto.Semiconductors Golden Dragon, and Cree X-lamp.

Since overheating is destructive, the HPLEDs must be

mounted on a heat sink to allow for heat dissipation. If

the heat from a HPLED is not removed, the device will

fail in seconds. One HPLED can often replace an

incandescent bulb in a flashlight, or be set in an array toform a powerful LED lamp.


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Flashing LEDs are used as attention

seeking indicators without requiring

external electronics. Flashing LEDs

resemble standard LEDs but they containan integrated multi-vibrator circuit that

causes the LED to flash with a typical

period of one second. In diffused lens

LEDs this is visible as a small black dot.

APPLICATION-SPECIFIC VARIATIONS

Bi-color LEDs are two different

LED emitters in one case. There

are two types one type consists

of two dies connected to thesame two leads anti-parallel to

each other. Current flow in one

direction emits one color, and

current in the opposite

direction emits the other color.


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Tri-color LEDs are three different

LED emitters in one case. Each

emitter is connected to a separatelead so they can be controlled

independently. A four-lead

arrangement is typical with one

common lead and an additional lead

for each color.

RGB LEDs are Tri-color LEDs with

red, green and blue emitters, in

general using a four-wire

connection with one common lead.These LEDs can have either

common positive or common

negative leads. Others however,

have only two leads and have a

built in tiny electronic control unit.


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Alphanumeric LED displays are available

in seven-segment and starburst format. Seven-

segment displays handle all numbers and a

limited set of letters. Starburst displays can

display all letters. Seven-segment LED displays

were in widespread use, but rising use of liquid

crystal displays, with their lower power needs

and greater display flexibility, has reduced the

popularity of numeric and alphanumeric LED

displays.


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http://en.wikipedia.org/wiki/Cathode_ray_tube.http://www.hk-phy.org/energy/commercial/office_phy/flash/crt_e.html .

http://ecomputernotes.com/computer-graphics/graphics-device/what-is-

color-crt-display-explain-beam-penetration-and-shadow-mask-method

LCD

https://en.wikipedia.org/wiki/Liquid-crystal_displayhttp://www.hk-phy.org/energy/commercial/office_phy/flash/lcd_e.html

LED

https://en.wikipedia.org/wiki/Light-emitting_diode

http://www.google.co.in/search?q=LED&tbm=isch&tbo=u&source=univ&sa=X

&ei=Za6dUbuUNIqKrgeo7oGwAg&ved=0CDoQsAQ&biw=1280&bih=933

CRT
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k-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://www.hk-phy.org/energy/commercial/office_phy/flash/crt_e.htmlhttp://www.hk-phy.org/energy/commercial/office_phy/flash/crt_e.htmlhttp://www.hk-phy.org/energy/commercial/office_phy/flash/crt_e.htmlhttp://en.wikipedia.org/wiki/Cathode_ray_tube


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CRT,LCD, LED , TV Technologies like Liquid crystal display