A
cathode ray tube (CRT) is a specialized vacuum tube in which images are
produced when an electron beam strikes a phosphorescent surface. Most desktop
computer displays make use of CRTs. The CRT in a computer display is similar to the "picture
tube" in a television receiver.
All CRT's have three main elements: an
electron gun, a deflection system, and a screen. The electron gun provides an
electron beam, which is a highly concentrated stream of electrons. The
deflection system positions the electron beam on the screen, and the screen
displays a small spot of light at the point where the electron beam strikes it.
Refresh
CRT
A
beam of electrons (cathode rays), emitted by an electron gun, passes through
focusing and deflection systems that direct the beam towards specified position
on the phosphor-coated screen. The phosphor then emits a small spot of light at
each position contacted by the electron beam. Because the light emitted by the
phosphor fades very rapidly, some method is needed for maintaining the screen
picture. One way to keep the phosphor glowing is to redraw the picture repeatedly
by quickly directing the electron beam back over the same points. This type of
display is called a refresh CRT.
Basic
Operation of a CRT
The
basic operation of CRT is shown in figure below:
Electron
Gun
The
primary components of an electron gun in a CRT are the heated metal cathode and
a control grid. The cathod is heated by an electric current passed through a
coil of wire called the filament. This causes electrons to be boiled off the
hot cathode surface. In the vacuum inside the CRT envelope, negatively charged
electrons are then accelerated toward the phosphor coating by a high positive
voltage. The accelerating voltage can be generated with a positively charged
metal coating on the in side of the CRT envelope near the phosphor screen, or
an accelerating anode can be used. Sometimes the electron gun is built to
contain the accelerating anode and focusing system within the same unit.
Focusing
System
The
focusing system is used to create a clear picture by focusing the electrons
into a narrow beam.
Deflection
System
Deflection(
means abnormal way of acting) of the electron beam can be controlled by either
electric fields or magnetic fields. In case of magnetic field, two pairs of
coils are used, one for horizontal deflection and other for vertical
deflection. In case of electric field, two pairs of parallel plates are used,
one for horizontal deflection and second for vertical deflection as shown in
figure above.
CRT
Screen
The
inside of the large end of a CRT is coated with a fluorescent material that
gives off light when struck by electrons. When the electrons in the beam is
collides with phosphor coating screen, they stopped and their kinetic energy is
absorbed by the phosphor. Then a part of beam energy is converted into heat
energy and the remainder part causes the electrons in the phospor atom to move
up to higher energy levels. After a short time the excited electrons come back
to their ground state. During this period, we see a glowing spot that quickly
fades after all excited electrons are returned to their ground state.
Persistence
It
is defined as the time they continue to emit light after the CRT beam is
removed. Persistence is defined as the time it take 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. A phosphor with low persistence is useful for animation ; a
high-persistence phosphor is useful for displaying highly complex, static
pictures. Although some phosphor have a persistence greater than 1 second,
graphics monitor are usually constructed with a persistence in the range from
10 to 60 microseconds.
Resolution
The
number of points per centimeter that can be used be plotted horizontlly and
vertically. Or Total number of points in each direction.
The
resolation of a CRT is depend on
§ type of phosphor
§ intensity to be displayed
§ focusing and deflection system
Aspect
Ratio
It
is ratio of horozontal to vertical points.
Example:
An aspect ratio of 3/4 means that a verticle line plotted with three points has
same lenght as horizontal line plotted with four points.
Raster Scan Systems !
It
is the most common type of graphics monitor based on television technology. In
a raster scan system, the electron beam is swept across the screen, one row at
a time from top to bottom. When electron beam moves across each row the beam
intensity is turned ON and OFF to craete a pattern of illuminated spots. Picture
definition is stored in a memory called frame buffer which holds the set of
intensity values, which are then retrieved from the frame buffer and pointed on
the screen one row at a time as shown in figure below:
At
the end of each line the beam must be turned off and redirect to the left hand
side of the CRT, this is called Horizontal Retrace. At the end of each frame,
the electrin beam return to top left corner of the screen to begin the next
frame called Verticle Retrace as shown in figure below:
Advantages
§ produce realistic images
§ also produced different colors
§ and shadows scenes.
Disadvantages
§ low resolution
§ expensive
§ electron beam directed to whole screen
Random Scan Systems !
In
Random Scan System, an electron beam is directed to only those parts of the
screen where a picture is to be drawn. The picture is drawn one line at a time,
so also called vector displays or stroke writing displays. After drawing the
picture the system cycles back to the first line and design all the lines of the
picture 30 to 60 time each second.
Advantages
§ Produced smooth line drawings
§ High resolution
Disadvantages
§ Designed only for line drawing applications.
§ Can't display relastistic images.
Grey
Shades !
The intensity of a phosphor dot is proportional to the number
of electrons colliding on it. If this number is controlled electronically, the
dots could be excited to different energy ststes. Then they would emit
different radint energy in their process of returning to the normal quantu
level. This phenomenon generates different intensity level or grey shedes as
perceived by the viewer. If the number of electrons impinging on phosphor dots
is fixed to a particular value, the viewer perceives only a single intensity
level which considered as ON state of the pixel while others are regarded as
OFF state.
If the phosphor dots are excited at 64 or 256 different
intensity levels, the monitors are grey shades viedo monitors.
Interlacing
!
Interlace is a technique of improving the picture quality of
a video signal primarily on CRT devices without consuming extra bandwidth. In
this technique, each frame is displayed in two passes using interlacing. In the
first pass, the beam sweeps across every other scan line from top to bottom. In
the second pass, the beam sweeps out the remaining scan lines.
Color CRT Monitors !
A
color CRT monitor displays color picture by using a combination of phosphors
that emit different colored light. By combining the emitted light a range of
colors can be generated. Two basic methods for producing color displays are:
§ Beam Penetration Method
§ Shadow-Mask Method
Beam Penetration Method
Beam Penetration Method
Random
scan monitors use the beam penetration method for displaying color picture. In
this, the inside of CRT screen is coated two layers of phorphor namely red and
green. A beam of slow electrons excites ony the outer red layer, while a beam
of fast electrons penetrates red layer and excites the inner green layer. At
intermediate beam speeds, combination of red and green light are emitted to
show two addtional colors- orange and yellow.
Advantages
§ Less expensive
Disadvantages
§ Quality of images are not good as comparatable with other
methods
§ Four colors are allowed only
Shadow
Mask Method
Raster
scan system are use shadow mask methods to produced a much more range of colors
than beam penetration method. In this, CRT has three phosphor color dots. One
phosphor dot emits a red light, second emits a green light and third emits a
blue light. This type of CRT has three electrons guns and a shadow mask grid as
shown in figure below:
In
this figure, three electrons beams are deflected and focused as a group onto
the shadow mask which contains a series of holes. When three beams pass through
a hole in shadow mask they activate dot triangle as shown in figure below:
The
colors we can see depends on the amount of excitation of red, green and blue
phosphor. A white area is a reasult of all three dots with equal intensity
while yellow is produced with green and red dots and so on.
Advantages
§ produce realistic images
§ also produced different colors
§ and shadows scenes.
Disadvantages
§ low resolution
§ expensive
§ electron beam directed to whole screen
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