Principal of CRT (Cathode ray tube)

Principal of CRT :

  • It consists of an electron gun.
  • The electron gun produce an electron beam.
  • The beam is narrow and is allowed to pass down the tube, and to fall on the screen.
  • The screen is formed by the flat end of glass tube which is coated with the fluorescent material.
  • The point at which the electron beam strikes the screen, a sport is formed.
  • The electron beam passes though two pairs of electrostatic deflection plates , the horizontal and vertical deflecting plates. The voltages which are applied to these plates deflect the beam.
  • Horizontal deflecting plates move the spot in horizontal direction and vertical deflecting plates move the spot on the screen in vertical and horizontal are independent of each other and thus the beam may be displayed anywhere on the screen.
cathode ray tube

cathode ray tube

The main parts of the CRT are:

  • Electron gun assembly
  • Deflection plate assembly
  • Fluorescent screen
  • Glass tube
  • Base

Electron gun assembly

  • An electron gun assembly consists of a heater, focusing anodes and cathode.
  • Cathode emits the electron beam ,this electron beam is accelerated by using the accelerating anodes.
  • The velocity of such an electron beam is very high.
  • When such a high velocity beam strikes the fluorescent screen, a spot is created on the fluorescent screen.

Deflection plate assembly.

  • When the electron beam is accelerated by the accelerating anodes it passes through the deflection plate assembly.
  • The beam can then be positioned anywhere on the screen.
  • The deflection plate assembly of the CRT consists of the two pairs of parallel plates.
  • The plates are called as the vertical deflecting plate and the horizontal deflecting plate.

Fluorescent screen

  • The front end of CRT act as a fluorescent screen. This screen is about 100mm*100mm. An inner side of the screen is coated with phosphor. This consists of pure crystals of phosphor.
  • A phosphor converts the electrical energy to light energy. When an electron beam strikes the phosphor crystals, their energy level is increased. This causes the phosphor crystal are excited and they emit light. This phenomenon is called as fluorescence.

Glass tube

  • The components of a CRT are enclosed in en evacuated glass tube called the envelope.
  • It allows the electrons which are emitted to move freely from one end of the tube to another end.


  • The base is provided to the CRT through which the connections are made to the various parts.

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