The document is a lecture on video display devices, covering various types such as cathode-ray tubes (CRT), raster scan displays, random-scan displays, color CRT monitors, direct-view storage tubes (DVST), and flat-panel displays. CRTs remain the most common output device, and the document details their operation, including electron beam mechanics and image creation methods. It also describes the advantages and disadvantages of newer technologies like flat-panel displays and the functioning of different phosphor-based color generation methods.

1. COMPUTER GRAPHICS
LECTURE 2
VIDEO DISPLAY DEVICES
Dr. Saadaldeen Rashid Ahmed

2. Outline
1. Refresh Cathode-Ray Tube
2. Raster Scan Displays
3. Random-Scan Displays
4. Color CRT Monitors
4.1 Beam-Penetration Method
4.2 Shadow-Mask Method
5. Direct-View Storage Tubes
6. Flat-Panel Displays
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3. 1.Cathode Ray Tube Display (CRT)
The primary output device in a graphic system is a video monitor. CRT is still the most common video
display device.
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4. Cathode Ray Tube Display (CRT)
Abeam of electrons (cathode rays) emitted by an electron gun, passes through focusing and deflection systems that direct
the beam toward specified positions on the phosphor coated screen. The phosphor then emits a small spot of light at each
positioncontactedbytheelectronbeam.Becausethelightemittedbythephosphorfadesveryrapidly,thepictureisredrawn
repeatedlyandquickly.
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5. How CRT works?
Electron Gun:- Heat is supplied to the cathode by current
through filament, which produces electrons.
Control Grid:- Controls the flow of the electrons so that they
do not move freely but only pass by electron beam.
Focusing System:- Used to create a clear picture by focusing
electrons into a narrow beam when contacting phosphor.
Deflection Plates:- It is used to control the direction of the
electron beam.
Phosphor Coated Screen:- It glows and shows the picture
when electron beam hits it.
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6. 2.Random Scan Display
-It uses an electron beam which operates as a pencil to draw a line image on CRT screen.
-The picture is drawn only as straight line segments.
-After drawing the picture, the system cycles back to the first line and designs all of the lines of the
image
-These systems are designed for the line-drawing applications and can’t display realistic shaded
scenes.
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7. Random Scan Display
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8. 3.Raster Scan Display
-In a raster-scan system, the electron beam is swept across the screen, one row at a time from
top to bottom.
-As the electron beam moves across each row, the beam intensity is turned on and off to
create a pattern of illuminated spots.
-Picture definition is stored in a memory area called the refresh buffer or frame buffer used for
redrawn. Each screen point is referred to as a pixel or pel (picture element).
- This memory area holds the set of intensity values for all the screen points.
- Raster scan can draw realistic images but it is costy.
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9. Raster Scan Display
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10. 4.Color CRT Monitors
The CRT Monitor display by using a combination of phosphors. The phosphors are different colors.
There are two popular approaches for producing color displays with a CRT are:
1.Beam Penetration Method
The Beam-Penetration method has been used with random-scan monitors. In this method, the CRT
screen is coated with two layers of phosphor, red and green and the displayed color depends on how
far the electron beam penetrates the phosphor layers. This method produces four colors only, red,
green, orange and yellow
2.Shadow-Mask Method
Shadow Mask Method is commonly used in Raster-Scan System because they produce a much
wider range of colors than the beam-penetration method.
It is used in the majority of color TV sets and monitors.
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11. 5.Direct View Storage Tube (DVST)
-DVST terminals also use the random scan approach to generate the image on the CRT screen.
-The term “storage tube” refers to the ability of the screen to retain the image that has been projected
thus, avoiding the need to rewrite the image constantly.
A direct-view storage device stores the picture information as a charge distribution just behind the
phosphor coated screen. Two electron guns are used, the primary gun is used to store the picture
pattern, the second, the flood gun, maintains the picture display.
An alternative method for maintaining a screen image is to store the picture information inside the CRT
instead of refreshing the screen.
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12. Direct View Storage Tube (DVST)
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13. Direct View Storage Tube (DVST)
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1.Electron guns :Two electron guns are used in DVST : Primary Gun and Flood Gun. Primary gun
is used to store picture pattern. Flood gun is used to maintain picture display on phosphor coated
screen.
2.Phosphor Coated Screen :In DVST the inner surface of CRT is coated with phosphor crystals is
of high persistence that emit light when beam of electrons strike them.
3.Storage Mesh:It is thin and high quality wire that is coated with dielectric and is located just
behind phosphor coated screen. Primary gun deposits pattern of positive charge on this grid and it is
transferred to phosphor coated screen by continuous flood of electrons produced by flood gun. Thus
Storage Mesh stores picture to be displayed in form of positive charge distribution.
4.Collector :This grid is located just behind storage mesh and purpose of this negatively charged
grid is to smooth out flow of flood electrons.

14. Direct View Storage Tube (DVST)
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Conceptually the Direct View Storage Tube (DVST) behaves like aCRT with highly persistent phosphor.
Pictures drawn on there will be seen for several minutes (40-50 minutes) before fading. It is similar to
CRT as far as the electronic gun and phosphor-coated mechanisms are concerned. But instead of
the electron beam directly writing the pictures on the phosphor coated CRT screen, the writing is done
with the help of a fine-mesh wire grid.
Since the phosphor is of a very high persistence quality, the picture created on the CRT screen will be
visible for several minutes without the need for being refreshed.

15. Direct View Storage Tube (DVST)
Advantage:
Because of no refreshing is needed very complex pictures can be displayed at very high
resolutions.
Disadvantage:
It can not display colors, and selected parts of a picture can’t be erased. To eliminate
the selected part of a picture, the entire screen must be erased and redrawn. It is a time
consuming process.
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16. 6.Flat Panel Display
It refers to a class of video devices that have reduced volume, weight, and power requirement
compared to CRT.
Examples:- Laptops, TVs, Calculator, etc.
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17. Flat Panel Display
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The two categories of flat-panel displays are
 Emissive displays: the device which convert electrical energy into light. (Plasma panel),
Thin-Film Electroluminescent Display, light emitting diodes (LED).
 Non-Emissive displays: used optical effects to convert sunlight or light from some other
source into graphic patterns. Liquid crystal devices (LCD).

18. Flat Panel Display
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LED
In an LED, a matrix of diodes is organized to form the pixel positions in the display and picture
definition is stored in a refresh buffer. Data is read from the refresh buffer and converted to
voltage levels that are applied to the diodes to produce the light pattern in the display.
It is used in indicators, sign applications, digital watches, cell phones , etc.

19. Flat Panel Display
LCD are the devices that produce a picture by passing polarized light from the surroundings or from an
internal light source through a liquid-crystal material that transmits the light.
Liquid crystal display is temperature dependent. It is between zero to seventy degree Celsius. It is flat
and requires very little power to operate.
It can be used in calculators, laptops, flat plane TVs, etc.
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21. The End
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