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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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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:
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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:
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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 :
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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
http://en.wikipedia.org/wiki/Cathode_ray_tubehttp://www.hk-phy.org/energy/commercial/office_phy/flash/crt_e.htmlhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttps://en.wikipedia.org/wiki/Liquid-crystal_displayhttp://www.hk-phy.org/energy/commercial/office_phy/flash/lcd_e.htmlhttps://en.wikipedia.org/wiki/Light-emitting_diodehttp://www.google.co.in/search?q=LED&tbm=isch&tbo=u&source=univ&sa=X&ei=Za6dUbuUNIqKrgeo7oGwAg&ved=0CDoQsAQ&biw=1280&bih=933http://www.google.co.in/search?q=LED&tbm=isch&tbo=u&source=univ&sa=X&ei=Za6dUbuUNIqKrgeo7oGwAg&ved=0CDoQsAQ&biw=1280&bih=933http://www.google.co.in/search?q=LED&tbm=isch&tbo=u&source=univ&sa=X&ei=Za6dUbuUNIqKrgeo7oGwAg&ved=0CDoQsAQ&biw=1280&bih=933http://www.google.co.in/search?q=LED&tbm=isch&tbo=u&source=univ&sa=X&ei=Za6dUbuUNIqKrgeo7oGwAg&ved=0CDoQsAQ&biw=1280&bih=933https://en.wikipedia.org/wiki/Light-emitting_diodehttps://en.wikipedia.org/wiki/Light-emitting_diodehttps://en.wikipedia.org/wiki/Light-emitting_diodehttp://www.hk-phy.org/energy/commercial/office_phy/flash/lcd_e.htmlhttp://www.hk-phy.org/energy/commercial/office_phy/flash/lcd_e.htmlhttp://www.hk-phy.org/energy/commercial/office_phy/flash/lcd_e.htmlhttps://en.wikipedia.org/wiki/Liquid-crystal_displayhttps://en.wikipedia.org/wiki/Liquid-crystal_displayhttps://en.wikipedia.org/wiki/Liquid-crystal_displayhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-methodhttp://ecomputernotes.com/computer-graphics/graphics-device/what-is-color-crt-display-explain-beam-penetration-and-shadow-mask-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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Thank you