About Cathode ray tube

1. CATHODE RAY
TUBE
Dr. J. Revathi
Assistant Professor
Department of Biomedical Engineering
Dr. N.G.P Institute of Technology

2. CATHODE RAY TUBE
• The Cathode Ray Tube (CRT) is a vacuum tube containing an electron gun (a source
of electrons or electron emitter) and a fluorescent screen used to view images.
• It has a mean to accelerate and deflect the electron beam onto the fluorescent
screen to create the images.
• The image may represent Electrical waveforms in oscilloscope, pictures
(television, computer monitor), radar targets.
• CRTs have largely superseded by modern technologies such as, LCD, LED, etc,
which offer lower manufacturing and distribution cost.

3. PRINCIPLE OF CRT
• The Cathode Ray Tube (CRT) is the device that displays images upon a
screen using electron beams.
• It is a vacuum sealed tube to avoid air resistance.
• The electrons are first created at the back of the CRT by an electron gun.
• The electrons are shot off the gun at a high velocity and travel through a
magnetic field, which steers them in a certain direction

4. • The electrons then pass through a filter (shadow mask)
and collide with a phosphor screen, to which they transfer their
energy and cause the screen to glow.
• Through specific direction of these electron beams, images are
created and displayed on the screen.

5. CRT

6. THREE MAIN PARTS
• Electron gun
• Deflecting plates
• Fluorsecent screen

7. ELECTRON GUN
The electron gun produces the electron beam that travels through
the CRT.
The electron gun consists of
•cathode
•filament
•accelerating anode

8. FUNCTION OF CATHODE AND
ANODE
• The filament is insulated coil of tungsten located inside the cathode
• The current is applied to this coil to heat it.
• When the cathode is heated, the electrons on the cathode released.
• The cathode creates an electron cloud (emits electrons) whose electrons are
extracted, accelerated and focused into an electron beam.
• The anode serves only to provide the accelerating voltage for the electrons and not as
the electron collector.

9. DEFLECTING PLATES
• The CRT can use either electrostatic or magnetic deflection. so that the
beam of electrons can have its direction altered, allowing the light spot to
appear anywhere on the face of the tube.
• The cathode-ray-tube deflection system is made up of two pairs of parallel
plates known as the vertical and horizontal deflection plates.
• The plates are made up of copper or aluminium.

10. ELECTROSTATIC DEFLECTION
SYSTEM
• Due to this, strong electric field is produced between the
plates.
• When electron beam enters in this field, it is deflected
towards the plate which is positive.
• So the path of beam becomes parabolic.
• Then the beam comes out of this field in straight line and
strikes on the screen at a particular point on the screen.

11. MAGNETIC DEFLECTION SYSTEM
• In this system, electron beam is deflected using external
magnetic field.
• To create alternating magnetic field two pairs of deflection
coils, known as yoke, are fitted on the neck of CRT.
• The pair of vertical deflection coils is connected to y-input of
CRO
• The pair of horizontal deflection coils is connected to
internal sawtooth wave or external signal at x-input.

12. CRT SCREEN
• The CRT screen consists of two main parts: the phosphor screen and the shadow
mask.
• The phosphor screen turns the electron beam into light, and the shadow mask
organizes the beam on the screen.
• It 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 electrons are negative and the screen gives a positive charge so the screen
glows.
•

13. PHOSPHOR PERSISTENCE
• The various phosphors are available depending upon the needs of the measurement or display
application.
• The brightness, color, and persistence of the illumination depends upon the type of phosphor
used on the CRT screen.
• Phosphors are available with persistence's ranging from less than one microsecond to several
seconds.
• For visual observation of brief transient events, a long persistence phosphor may be desirable.
• For events which are fast and repetitive, or high frequency, a short-persistence phosphor is
generally preferable.

14. ADVANTAGES OF CRT
• The cathode rayed tube can easily increase the monitor’s brightness by reflecting the
light.
• They produce more colours.
• The Cathode Ray Tube monitors have lower price rate than the LCD display or Plasma
display.
• The quality of the image displayed on a Cathode Ray Tube is superior to the LCD and
Plasma monitors.
• The contrast features of the cathode ray tube monitor are considered highly excellent.

15. DISADVANTAGES OF CRT
• The electromagnetic fields emitted by CRT monitors constitute a health
hazard to the functioning of living cells.
• CRTs emit a small amount of X-ray band radiation which can result in a
health hazard.
• Constant refreshing of CRT monitors can result in headache.
• CRTs operate at very high voltage which can overheat system or result in
an implosion