Cathode Ray Tube - tu-sofia.bg Ray Tube Gun creates an electron beam and changes its brightness Electron lens focus (A1) and accelerate (A2) electrons

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EE141© 2009, Associate Professor PhD. T.Vasileva

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Displays

Semiconductor

Elements


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Cathode Ray Tube

Basic applications

Oscilloscope

TV

Old monitors

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Idea of Electrostatic Deflection


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Inside an Electrostatic Deflection

Cathode Ray Tube

Gun creates an electron beam and changes its brightness

Electron lens focus (A1) and accelerate (A2) electrons

One set of plates (Y) creates an electric field that moves the electron

beam vertically, while another set (X) moves the beam horizontally.

When the electron beam strikes the phosphor, it illuminates visible light

on the fluorescent screen

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Magnetic Deflection – Basic Idea


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Inside a Magnetic Deflection CRT

One set of coils creates a magnetic field that moves the electron beam

vertically, while another set moves the beam horizontally.

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Color CRTThere are three electron beams (Red, Blue and Green - RGB) that move

simultaneously across the screen. All colors on a TV screen are produced as

a mixture of red, green and blue.

A thin metal screen (shadow mask) has very small holes that are aligned with the

phosphor dots (or stripes) on the screen.


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Flat Displays – LCD Displays

Application

LCD (liquid crystal display) are used in laptop computers, digital clocks,

watches, microwave ovens, CD players etc.

Advantages

They are thinner and lighter and draw much less power than cathode

ray tubes

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Liquid Crystals

Liquid crystals are neither a solid nor a liquid

They require an external light source. Liquid crystal materials emit no

light of their own

Светлина

U


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Liquid Crystal Polarization Twisted nematic (TN), is naturally twisted liquid crystal.

It reacts to electric field in such a way as to control light passage.

If electric field is applied to liquid crystal molecules, they untwist to

varying degrees, depending on the applied voltage.

• When they straighten out, consequently, no light can pass through that area.

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Passive Matrix LCD Uses a simple grid to supply the charge to a particular pixel on

the display

Has slow response time and imprecise voltage control (normally

the pixels around the controlled one also partially untwist, which

makes images appear fuzzy and lacking in contrast)Column electrode

Row electrode

Pixel


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Active Matrix – TFT LCD Thin Film Transistors (TFT)

Sub pixels for colors

Basic colors

and brightness

Ensure ability to influence only one pixel at a time. The MOS capacitor is able to

hold the charge until the next refresh cycle.

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LCD Color Display To can show colors a LCD must have three subpixels with red, green and

blue color filters to create each color.

Through the careful control and variation of the voltage applied, the intensity

of each subpixel can range over 256 shades.

An enormous number of

transistors are needed.

Example

For typical laptop with

resolution 1,024x768

1,024 columns multiplied by

768 rows by 3 subpixels get

2,359,296 transistors etched

onto the glass.


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LCD Disadvantages

Very sensitive to temperature changes

Limited display size by the quality-control problems. To increase display size – more pixels and transistors are needed

and the chance of including a bad transistor also increase.

Because of this about 40% of produced displays are rejected which

directly affect the price of large display.

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Plasma Display Panel (PDP)

The basic idea of a plasma display is to illuminate tiny, colored

fluorescent lights to form an image.

Each pixel is made up of three fluorescent lights - a red light, a

green light and a blue light.

Just like a CRT television, the plasma display varies the

intensities of the different lights to produce a full range of colors.


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Plasma Display – Basic Cell

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Cell Construction

The xenon and neon gas in a plasma television is contained in hundreds of thousands of tiny cells positioned between two plates of glass.

Long electrodes are also sandwiched between the glass plates, on both sides of the cells.

The address electrodes sit behind the cells, along the rear glass plate.

The transparent display electrodes are mounted above the cell, along the front glass plate. They are surrounded by an insulating dielectric material and covered by a magnesium oxide protective layer,


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Basic Grid of Horizontal and

Vertical Electrodes

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Flat-Screen Plasma Display Panel


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Advantages and Disadvantages

Advantages Very wide screen can be produced using extremely thin

materials.

The image is very bright and looks good from almost every angle

Drawback High price.

However, falling prices and advances in technology mean that the plasma display may soon edge out the old CRT sets.

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E-paper, e-Ink Display

Electronic paper, e-paper or electronic ink display is a display technology designed to mimic the appearance of ordinary ink on paper.

Electronic paper

Reflects light like ordinary paper (unlike LCD that uses backlight to illuminate its pixels)

It is capable of holding text and images indefinitely without drawing electricity.

Applications

e-readers, capable of displaying digital

versions of books and e-paper magazines.

Mobile phone/watch displays

electronic billboards

time tables at bus stations

electronic pricing labels, general signage.


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E-Ink Technology

E-paper display consists of titanium dioxide particles (e-ink micro capsules)

which are dispersed in a hydrocarbon oil so that they can rotate freely.

Each particle is a dipole composed of negatively charged black plastic on one

side and positively charged white plastic on the other.

This mixture is placed between two parallel, conductive plates.

Visible images are formed by rearranging charged pigment particles using an

applied electric field.

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Mode of Operation

If a negative electric field is applied at the front (viewing) side of the display, the

white particles move to the top of the microcapsule . The surface appears white at

that location, because the light is reflected back to the viewer.

By reversing this process, the black particles appear at the top of the capsule,

which now makes the surface appear dark at that location, because the incident

light is absorbed by the colored dye.

Reflected –

white color

Absorbed –

Black color

LightLight

When a voltage is applied across the

two plates, the particles will migrate to

the plate bearing the opposite charge

from that on the particles.


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Mode of Operation

The polarity of the voltage applied to each pair of electrodes then determines

whether the white or black side is face-up, thus giving the pixel a white or black

appearance.

If the rear electrode is divided into a number of small picture elements (pixels),

then an image can be formed by applying the appropriate voltage to each region

of the display to create a pattern of reflecting and absorbing regions.

Reflected –

white color

Absorbed –

black color

LightLight

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Advantages and DisadvantagesAdvantages High contrast in direct sunlight

Low-power consumption – need energy only when changing image

Stable image, which does not need to be refreshed constantly

The wider viewing angle

Drawback A very low refresh rate (not applicable for interactive applications like

using fast moving menus, mouse pointers or scrolling).

Gray scale only.

There is ongoing competition among manufacturers to provide full-color capability.

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Cathode Ray Tube - tu-sofia.bg Ray Tube Gun creates an electron beam and changes its brightness Electron lens focus (A1) and accelerate (A2) electrons