Cathode ray tubes (CRTs) were a common display technology that used electron beams to excite phosphors on the inside of an evacuated glass envelope. The electron beams were generated by electron guns and controlled by deflection plates and coils to scan across the phosphor-coated screen in a raster pattern, refreshing the image rapidly to prevent flickering. Different phosphors determined the persistence of the light emission and were used together with color masks and intensity control to generate color images on CRT displays. Random scan displays stored images as line drawings and refreshed all lines periodically rather than using a raster scan.

1. Introduction to Graphical Hardware
Display Technologies
• CRTs are the most common
display device today
• Evacuated glass bottle (last
remaining vacuum tube)
• Heating element (filament)
• Electrons attracted to focusing
anode cylinder
• Vertical and Horizontal
deflection plates
• Beam strikes phosphor coating
on front of tube
Cathode Ray Tubes (CRTs)

2. Evacuated glass bottle (last remaining vacuum
tube)
Heating element (filament)
Electrons attracted to focusing anode cylinder
Vertical and Horizontal deflection plates
Beam strikes phosphor coating on front of tube

3. Cathode Ray Tube
• The phosphor than emits a small spot of light to
each position contacted by the electron beam.
• The light emitted by the phosphor fades very
rapidly.
• One way to keep the phosphor glowing is to
redraw the picture repeatedly. This type of
display is called a Refresh CRT

5. Cathode Ray Tube (CRT)
5. Anode
connection
6. Mask for
separating
beams for
RGB part
of
displayed
image
7. Phosphor
layer with
RGB zones
8. Close-up of
the phos-
phor-
coated
inner side
of the
screen
1. Electron guns
2. Electron beams
3. Focusing coils
4. Deflection coils

6. Cathode Ray Tube
• Intensity of the electron beam is controlled by
setting voltage levels on the control grid
• Amount of light emitted by the phosphor coating
depends on the number of electrons striking the
screen, we control the brightness of a display by
varying the voltage on the control grid
• Focusing deflection of the electron beam can be
controlled either with electric fields or with
magnetic fields

7. Cathode Ray Tube
• Different kind of phosphors are available for use
in a CRT, besides color a major difference
between phosphors is their persistence: how
long they continue to emit light after the CRT
beam is removed.
• Persistence is defined as the time it takes the
emitted light from the screen to decay to one
tenth of its original intensity.
• Lower persistence phosphors require higher
refresh rates to maintain a picture on the screen
without flicker.

8. Raster Scan Display
• The raster is a series of
horizontal lines crossing the face
of the CRT screen. Each
horizontal line is made up of one
trace of the electron beam from
left to right. The raster starts at
the top left corner of the crt
screen. As each horizontal line is
completed, the blanked electron
beam is rapidly returned or
retraced to the left of the screen.
Vertical deflection moves the
beam down, and the horizontal
sweep repeats. When the vertical
sweep reaches the bottom line of
the raster, a vertical blanked
retrace returns the sweep to the
starting position of the raster,
and the process is repeated.

9. • PICTURE ELEMENTS. - The actual display of
data results from the use of picture elements. A
picture element is a variable dot of light derived
from video signals input to the display monitor.
The picture elements, often called pixels
• Pictures with alphanumeric characters and
graphics can be created and displayed by
varying the intensity or brightness of the picture
element dots. This is done in conjunction with
the phosphor coating on the face of the crt.

10. A 7 by 9 picture element character.
Approximately 640
picture elements per
horizontal line are
required for the display
of an 80 character line.
Therefore, you can
expect 140,000 picture
elements on a raster
scan display screen
(80 alphanumeric
characters per line and
25 lines).

11. Random Scan Display
• Refresh rate on a random scan display depends
on the number of lines to be displayed.
• Picture definition is stored as a set of line
drawing commands in an area of memory
referred to as the refresh display file or the
Refresh Buffer.
• Random scan displays are designed to draw all
the component lines of a picture 30 to 60 times
each second.

12. Random Scan Display
• Random scan system are designed for line
drawing applications and cannot display realistic
shaded scenes.
• As the picture definition is stored as a set of line
drawing instructions and not as a set of intensity
values of all screen points it generally have
higher resolution than raster systems.

13. A CRT monitor contains millions of tiny red, green, and blue
phosphor dots that glow when struck by an electron beam
that travels across the screen to create a visible image. The
illustration below shows how this works inside a CRT.

14. • A beam penetration method for displaying color
pictures has been used with random scan
monitors.
• Shadow-mask
A shadow mask is a thin metal screen filled with
very small holes. Three electron beams pass
through the holes to focus on a single point on a
CRT displays' phosphor surface. The shadow
mask helps to control the electron beams so that
the beams strike the correct phosphor at just the
right intensity to create the desired colors and
image on the display. The unwanted beams are
blocked or "shadowed."