Computer Graphics Video Display Devices

1
VIDEO DISPLAY DEVICES
Presented by V.Santhi, Assistant
Professor, Department of BCA
2
A display device is an Output device
for presentation of information for
visual reception . The display systems
are often referred to as Video Monitor
or Video Display Unit (VDU) . Display
devices are designed to project , show ,
exhibit or display softcopy information .
3

• Some of the display devices are
• Cathode Ray Tube (CRT)
• Raster Scan Display
• Random Scan Display
• Liquid Crystal Display (LCD)
• Light Emitting Diode (LED)

4
Diagrammatic representation of CRT
5
Primary output Device Video Monitor Video
monitors are based on standard Cathode ray tube
(CRT) design. Refresh Cathode Ray
Tubes- Working ? A beam of electrons (cathode
rays), emitted by an electron gun, passed through
focusing deflecting system that direct the beam
towards specified positions on the phosphor
coated screen.
6
The phosphor emits a small spot of light at each
position contacted by the electron screen. For
maintaining the screen picture glowing redraw the
picture repeatedly by quickly directing the
electron beam back over the same points. This
type of display is called a refresh CRT.
7
Cathode Ray Tube (CRT)

1. Electron Guns
2. Electron Beams
3. Focusing Coils
4. Deflection Coils
5. Anode Connection
6. Shadow Mask
7. Phosphor layer
8. Close-up of the phosphor coated inner side of
   the screen

8
Electron Gun
9
Heated metal cathode Heating Filament causes
vely charged electrons to boiled off and
accelerate towards phosphor coated screen by
a high ve voltage.
10
Accelerating Anode The accelerating voltage can
be generated with a vely charged metal coating
on the inside of CRT envelop near the phosphor
screen, or we can say Accelerating anode can be
used..
11
Focusing System
12
The focusing system is needed to force the elec
tron beam to converge into a small spot as it
strikes the phosphor . The electron beam will b
e focused properly only at the
center of the screen . As the beam moves to the o
uter edges of the screen , displayed images
become blurred .
13
Deflection Systems
The deflection system contains a set of coils
mounted at the neck of the tube .
There are two sets of deflecting plates .
One pair of plates is mounted horizontally to
control the vertical deflection ,
and the other pair is mounted vertically to
control horizontal deflection .
14
Spots of light

• Persistence The time it takes the emitted light
  from the screen to decay to one-tenth of
  its original intensity .
• Low persitance require high refresh rate
• for eg animations.
• High persistance is used for complex,
• static pictures, monitors in the range from
• 10 to 60 microseconds.
• Resolution The maximum number of points that can
  be displayed without overlap on a CRT .

15
Aspect Ratio This numbers gives the ratio of
vertical points to horizontal points necessary
to produce equal length lines in both directions
on the screen . Refresh CRT One way to keep
the phosphor glowing is to redraw the picture
repeatedly by quickly directing the electron
beam back over the same points .
16
Raster Scan Display
In a raster scan system , the electron beam is
swept across the screen , one row at a time fro
m top to bottom . As the electron beam moves
across each row , the beam intensity is turned o
n and off to create a pattern of illuminated sp
ots . Picture definition is stored in a memory are
a called the refresh buffer or frame buffer
. Refresh buffer or frame buffer This
memory area holds the set of intensity values
for all the screen points .
17
Stored intensity values then retrieved from
refresh buffer and painted on the screen one
row (scan line) at a time .
18
On a black and - white system with one bit per
pixel , the frame buffer is called bitmap
. For system with multiple bits per pixel ,
the frame buffer is called pixmap
. Refreshing on raster scan displays is carried o
ut at the rate 60 to 80 frame per second .
19
Horizontal retrace The return to the left of t
he screen , after refreshing each scan line .
20
Vertical retrace At the end of each frame (
displayed in 1/80th to 1/60th of a second
) the electron beam returns to the top left cor
ner of the screen to begin the next frame .
21
Random Scan Display Random scan monitors draw a
picture one line at a time (Vector display ,
Stroke writing or calligraphic displays)
. Refresh rate depends on the number of lines to b
e displayed . Picture definition is now stored a
s a line-drawing commands an area of
memory referred to as refresh display file (displ
ay list) .
22
The component lines of a picture can be drawn
and refreshed .
23
Random scan displays are designed for
line-drawing applications and can not display
realistic shaded scenes.
24
Differences between Random and Raster Scan Display
RASTER SCAN DISPLAY RANDOM SCAN DISPLAY RASTER SCAN DISPLAY RANDOM SCAN DISPLAY
Picture with better contrast Cannot produce contrast , memory doesnt store intensity value of pixel
Less resolution High resolution
Capable of producing curves better Smooth line drawings
Used in systems to display realistic images Cannot draw realistic shaded scenes
Cost is less Cost is more
Mainly used for point plotting Line drawing , known as vector display
Ex . TV sets Ex . Pen Plotter
RASTER SCAN DISPLAY RANDOM SCAN DISPLAY
Picture with better contrast Cannot produce contrast , memory doesnt store intensity value of pixel
Less resolution High resolution
Capable of producing curves better Smooth line drawings
Used in systems to display realistic images Cannot draw realistic shaded scenes
Cost is less Cost is more
Mainly used for point plotting Line drawing , known as vector display
Ex . TV sets Ex . Pen Plotter
25
Liquid Crystal Displays (LCD)

• Used in small systems , such as calculators ,
  laptop computers .
• Produce a picture by passing polarized light (
  from the surrounding or from an internal light
  source) through a liquid-crystal material that
  can be aligned to either block
  or transmit the light .

26
Two glass plates , each containing a light polariz
er at right angles to the other plate ,
sandwich the liquid crystal materials .
Rows of horizontal transparent conductor column
s of vertical conductors (put into glass plates)
.
27
Polarized light passing through the material is
twisted so that it will pass through the oppos
ite polarizer . The light is then reflected back
to the viewer . Liquid crystal These
compounds have a crystalline arrangement of
molecules , yet they flow like a liquid .
28
On State
Off State
29
Light Emitting Diode (LED) An LED display is a v
ideo display which uses light- emitting diodes
. An LED panel is a small display , or
a component of a larger display .
They are typically used outdoors in store si
gns and billboards . LED panels are sometimes
used as form of lighting , for the purpose of
general illumination , task lighting , or even
stage lighting rather than display .
30
All early devices emitted low-intensity red light
, but modern LEDs are available across the visib
le , ultraviolet and infrared wavelengths ,
with very high brightness . LEDs are based on t
he semiconductor diode . When the
diode is forward biased (switched on) ,
electrons are able to recombine with holes a
nd energy is released in the form of light .
This effect is called electroluminescence and t
he color of the light is determined by the
energy gap of the semiconductor .
31
LEDs present many advantages over traditional li
ght sources including lower energy consumptio
n , longer lifetime , improved robustness ,
smaller size and faster switching . However ,
they are relatively expensive and
require more precise current and heat
management than traditional light sources . Ap
plications of LEDs are diverse .
They are used as low-energy indicators but also
for replacements for traditional light sources
in general lighting and automotive lighting
. The compact size of LEDs has
allowed new text and video displays and sensors
to be developed , while their high switching
rates are useful in communications technology .
32
LED Display
33
Thank You.....