Functions of a Cathode Ray Tube and its Applications

Parts of a CRT

  • The features of a CRT can be split into 3 main sections: The electron gun, the deflection system and the fluorescent screen.

Electron Gun

  • The role of this section is to produce electrons at a high, fixed, velocity.
  • This is done through a process known as thermionic emission.
    • A filament in the cathode is heated to the point where its electrons become loose.
    • An anode with a high voltage applied to it accelerates the electrons towards the screen due to electrostatic attraction.
    • On the way, the electrons pass through a series of control grids which control the brightness of the image produced.
      • The more negative the grid, the darker the image and vice versa.

Deflection system

  • The role of the deflection system is to control the image produced by controlling the position that the electrons hit the screen.
  • It consists of Two PERPENDICULAR sets of Electric/Magnetic fields.
    • This allows control over both horizontal and vertical axes.
    • By controlling the Voltage applied to the fields, it is possible to vary the deflection through Electrostatic force/Motor effect.

Fluorescent screen

  • The role of this part is to display where the electrons are hitting the CRT.
  • It is a screen coated with a material that emits light when struck by electrons.
    • Zinc sulfide or Phosphorus are two commonly used materials.



  • Before LCD or Plasma television, the CRT was used to create a moving image.
  • It used the same principle as a CRT, and for Black and White televisions, that worked fine.
    • B&W TVs were essentially the same thing as a CRT, as all that's needed is the control of the brightness of the beam.
  • A CRT TV works by having the electron beam "scan" the screen at an rate faster than our eyes can perceive.
    • This means that it shoots across the screen like a machine gun, and the images we see are actually made from many fluorescent dots.
    • The fluorescence caused by the beam striking the screen lasts a bit longer so that the next scan can be made without the previous image disappearing.
    • It scans twice each time, first filling in the odd "holes" then the even ones.
    • Each scan is about 1/50 of a second.
  • Colour CRT TVs had 3 electron guns rather than a single one, a shadow mask, and a modified fluorescent screen.
  • The 3 electron guns were needed as there were three primary colours (Red, Green and Blue) that could be adjusted in different amounts to create any colour.
  • The colours are formed as a result of the shadow mask, which is a layer with holes in it that controls the angle of the incoming electron beams.
    • This is because the fluorescent screen is separated into multi-coloured phosphors that are placed adjacent to each other at small intervals.
    • Thus it isn't actually a single coloured pixel, but rather 3 very small pixels that join together to form a larger dot.

Cathode Ray Oscilloscopes

  • A Cathode Ray Oscilloscope (CRO) is a diagnostic device that allows one to "see" voltage.
  • It is essential a Cathode Ray Tube with two perpendicular sets of deflecting electric plates.
  • The vertical set is where an input voltage is plugged in for the oscilloscope to display.
  • However, the horizontal set is connected to a "sweep generator"
    • This is what provides a constant, but adjustable, timebase for the sweeping.
    • It essentially creates a "sawtooth voltage."
    • This is what causes the image to be animated, and measured with a linear scale.