Computer Graphics | Cathode ray tube (video display device)

Computer Graphics has become a common element in today’s modern world. Be it in user interfaces, or data visualization, motion pictures etc, computer graphics plays an important role. The primary output device in a graphics system is a video monitor. Although many technologies exist, but the operation of most video monitors is based on the standard Cathode Ray Tube (CRT) design.

Cathode Ray Tubes (CRT) –
A cathode ray tube (CRT) is a specialized vacuum tube in which images are produced when an electron beam strikes a phosphorescent surface.It modulates, accelerates, and deflects electron beam(s) onto the screen to create the images. Most desktop computer displays make use of CRT for image displaying purposes.

Image source – physics.usyd

Construction of a CRT –

  1. The primary components are the heated metal cathode and a control grid.
  2. The heat is supplied to the cathode (by passing current through the filament). This way the electrons get heated up and start getting ejected out of the cathode filament.
  3. This stream of negatively charged electrons is accelerated towards the phosphor screen by supplying a high positive voltage.
  4. This acceleration is generally produced by means of an accelerating anode.
  5. Next component is the Focusing System, which is used to force the electron beam to converge to small spot on the screen.
  6. If there will not be any focusing system, the electrons will be scattered because of their own repulsions and hence we won’t get a sharp image of the object.
  7. This focusing can be either by means of electrostatic fields or magnetic fields.

Types Of Deflection:

  1. Electrostatic Deflection –
    The electron beam (cathode rays) passes through a highly positively charged metal cylinder that forms an electrostatic lens. This electrostatic lens focuses the cathode rays to the center of the screen in the same way like an optical lens focuses the beam of light. Two pairs of parallel plates are mounted inside the CRT tube.

  2. Magnetic Deflection –
    Here, two pairs of coils are used. One pair is mounted on the top and bottom of the CRT tube, and the other pair on the two opposite sides. The magnetic field produced by both these pairs is such that a force is generated on the electron beam in a direction which is perpendicular to both the direction of magnetic field, and to the direction of flow of the beam. One pair is mounted horizontally and the other vertically.

Now as this highly energetic beam strikes the surface of the screen, these electrons are stopped and their kinetic energy is absorbed by the phosphor screen (atoms). Some energy is wasted in heat also, but majority of the kinetic energy gets transferred to the phosphor atoms. As these atoms receive this huge amount of energy, they get excited to a higher energy level.

After a short time, these atoms start returning to their original energy level. The original level is at a lower energy level than the excited one, hence the atoms release some energy while coming down. This extra energy is dissipated in the form of small quantums of light. Thus the de-excitation results in a bright coloured spot on the screen. The frequency (color) of the spot depends on the difference between the two energy levels (excited level and ground state level).

Different kinds of phosphors are used in a CRT. The difference is based upon the time for how long the phosphor continues to emit light after the CRT beam has been removed. This property is referred to as Persistence. Basically persistence means how much time is taken by the emitted light to reduce to one-tenth of its original intensity. Now, phosphors with lower persistence require higher refresh rates to maintain a picture on the screen without any flicker.

Reference: Computer Graphics by Pauline Baker

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