Display Systems

Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

Display Systems

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Imaging Science FundamentalsChester F. Carlson Center for Imaging

Science

In This Section...

• We will explore how display systems work.– Cathode Ray Tube

• Television

• Computer Monitor

– Flat Panel Display• Liquid Crystal Display


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Cathode Ray Tube

• A common device used in televisions, and computer monitors.

• The tube directs electrons to form an image on a screen.


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Cathode Ray Tube

A cathode ray tube (CRT) is a special kind of vacuum tube.


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Cathode Ray Tube

One end of the tube is supplied with electronsby a high voltage power supply.

This part of the tube is called the cathode.

Power supply+-

Cathode


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Cathode Ray Tube

The electrons are drawn at high speed toward aplate of metal, called an anode, that is positively charged.

The anode accelerate the electrons to very high speed. The electrons hit the anode and return to the power supply

Power supply+-

AnodeCathode


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Cathode Ray Tube

This is how all vacuum tubes work. The thing that makes a CRT tube special is that the anode has a hole in it.

Power supply+-

AnodeCathode

Hole


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Cathode Ray Tube

Electron beam, or cathode ray

The hole allows most of the high speed electrons miss the anode. These electrons continue to fly toward the other end of the tube as an electron beam (or e-beam). The e-beam is also called a cathode ray.

Power supply+-

Cathode Anode


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Cathode Ray Tube

This part of the CRT is called the electron gun.

Electron beam

Electron Gun

Power supply+-

Cathode Anode


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Cathode Ray Tube

Deflectors use electric fields to bend the e-beam in a desired direction.

(Some CRTs use magnetic coils as deflectors.)

Deflectors

Electron beam

Electron Gun

Power supply+-


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Cathode Ray Tube

The e-beam collides with a phosphor screen causing it to temporarily glow and become viewable.

The electrons then return to the power supply through a wire.

Phosphor screen

Deflectors

Electron beam

Electron Gun

Power supply+-

Light


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Cathode Ray Tube

By controlling the deflectors, the e-beam writes on the phosphor screen just like a pencil writing on a piece of paper.

Phosphor screen

Deflectors

Electron beam

Electron Gun

Power supply+-

Light


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Cathode Ray Tube

This is a general CRT for ablack-and-white television system.

Phosphor screen

Deflectors

Electron beam

Electron Gun

Power supply+-

Light


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Cathode Ray Tube• Color Modifications

– 1 or 3 Electron Guns

– Additional E-beam Guide• Shadow Mask

• Aperture Grill

– 3 Types of Phosphorson 1 Screen

• Red (R)

• Green (G)

• Blue (B)


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CRT - ColorSupplemental Guide

The supplemental guide (shadowmask, aperture grill, etc.) for color is placed just before the phosphor screen.

Phosphor screen

Deflectors

Electron beam

Electron Gun

Power supply+-

Light


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Additive Color

• The three primary colors of red, green, and blue combine to form other colors.

• The idea is similar to that of pointillism in certain Impressionistic paintings:– Tiny dots or lines are

placed closely next to each other

– When the viewer is far enough away, the dots blur

– The primaries add together to form other colors


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Additive Color Mixing

The additive primary colors used are RED, GREEN, and BLUE.


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Overlapping red, green, and blue light, creates yellow, cyan, and magenta light.


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The combination of the three additive primaries gives white light (R + G + B = WHITE).


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CRT - Color• Shadowmask CRT

– Three e-beams are shot by 3 electron guns to a phosphor screen.• RGB phosphor dots are arranged on the screen in triads making up the

corners of equilateral triangles; each dot < .4 mm.

• The guns are also arranged at the corners of an equilateral triangle.

– The shadowmask is a metal sheet with a single hole for each triad.• Placed just before the screen the shadowmask has the same shape.

• The holes limit the beam so it hits the correct color phosphor dot.

triad

Phosphor Screen

Shadowmask

E-guns


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Progressive Scanning

• The method by which electron beam(s) scan over a phosphor screen. – The electron beam is systematically moved across the screen

• Raster lines: The horizontal lines that make up an image.– Left to Right (from viewing position)

– Top to Bottom

– Refresh Rate:• A group of scanned lines forms a picture.

• Refresh Rate = # pictures per second (pps)

• Refresh Rate < 30 pps is seen asmany individual pictures

• Refresh Rate > 30 pps is seen asconstant motion


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Interlacing

• Fields– Two fields per picture

• First, all odd lines are scanned - Odd field

• Then, all even lines are scanned - Even field– Measured as fields per second (fps)

123456789

101112131415

– Doubles the refresh rate• 30 pps yields 60 fps

• At > 60 fps no flicker is detected between frames

• US Standards (NTSC)• 525 lines per picture• 60 fps (30 pps)

• Great Britain Standards (PAL)• 625 lines per picture• 50 fps (25 pps)


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Movies on Television

• Compare refresh rates:– TVs display at 30 pps.– Movies display at 24 pps.

• How are movies adjusted for TV?– Every 5th picture is doubled and some ‘dark’

time is inserted for each frame to increase the refresh rate to 30 pps.

• The rate is still too fast for the human eye to detect these changes.


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Flat Panel Display

• Flat panel displays, or flat screens, are used for systems that have limited space.

– 2 Widely Used Types• Liquid Crystal Displays (LCDs)

• Light-emitting Diodes (LEDs)

– Applications• Laptop computers

• Calculators

• Hand-held organizers

• Digital clocks

• VCR/Stereo displays

• And so on...


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Liquid Crystal Display

• What is a liquid crystal?– A material that exists between the liquid and solid

phases of matter.

– When an electric field is applied to a liquid crystal the optical properties of the matter change.

• Causes light to passes through at varying brightness levels.


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• Liquid crystals are flattened between two glass plates.– The crystal layer is a few

microns thick.

Polarizer

Liquid CrystalLayer

Glass Layer

Conductor

• A transparent electrical conductor is placed on the inner sides of the glass.

• Perpendicularly oriented polarizers are placed over the outer sides of each glass plate

Liquid Crystal Device

•Direction of polarization


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• How does it work?– When the voltage is off:

• The liquid crystals are in a relaxed state and therefore they are aligned (i.e., arranged parallel to one another).

• Polarized light that has passed through the first polarizer is unaffected by the aligned crystals and is blocked by the second (perpendicular) polarizer.

No Light Transmitted


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Liquid Crystal Display• How does it work?

– When the voltage is on:• the conductors transfer an electric field that twists the crystals

• when the liquid crystals are forced to twist, so does the direction of polarization of the light

• some or all of the light can pass through the second polarizer.

Light Transmitted


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• How does it work?– The amount of voltage controls the orientation of the

crystal, or how much they will twist:• The maximum amount of light is transmitted when the first and

final liquid crystals are perpendicular to each other.

• The minimum amount of light is transmitted when the first and final liquid crystals are parallel to each other.

• Any intermediate amount of light can be transmitted when the first and final crystals are oriented at other angles.


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• Screens– An individual liquid crystal device is called a cell.

– A two-dimensional matrix of cells forms a screen.

– Wires are connected to the cells to transfer voltages.• Certain voltages go to specific cells to control the amount of

light through each cell.

– Color filters are placed over each cell for color screens.• The color patterns and shapes change for different displays.

– Back-lighting• The initial light source needs to be bright enough to pass

through the LCD cells and be detected by a viewer’s eye.

Documents

Display Systems