Operation of a Cathode Ray Tube (CRT)

Although it is being slowly replaced by solid-state displays that use multi-colored light-emitting diodes (LEDs), the CRT is still the work-horse of visual displays.  The device beautifully illustrates some basic electromagnetic principles and is a great introduction to the field of electron optics.  A description of how the CRT works is found below, which refers to Figure 1.

Figure 1:  Click on the thumbnail at the right to enlarge the working diagram of the cathode ray tube (CRT) in a separate window.


Figure 2:  Click on the thumbnail at the right to see how changing voltages around the cathode modulates electron beam intensity.


Figure 3:  Click on the thumbnail at the right to learn how screen display functions are tied to the electrical signals within the CRT. Try and guess the functionality.

A CRT works by sweeping an electron beam of varying intensity across a phosphor-coated screen.  The basic components of the CRT are described below:
  • Electron Gun -- The electron gun, which consists of the cathode, choke, accelerator, and lensing region, is the device which generates and focuses the electron beam used to project an image on the phosphor screen.
  • Cathode -- The cathode is a grounded metal plate that is super-heated so that electrons are literally jumping off the surface.
  • Accelerator Plate -- This metal ring is held at a large, positive voltage and is used to "grab" loose electrons from the cathode and hurl them forwards into the lensing chamber (towards the right in the diagram).
  • Choke -- This metal ring is located between the cathode and accelerator plate and held at a slightly negative charge.  The electric fields from the choke help columnate the electrons; they also can be used to quickly modulate the number of electrons in the beam and, thus, the brightness or intensity of the picture.
  • Lensing Region -- The lensing region consists of two adjacent metal tubes that are located just after the accelerator. The two tubes are held at different potentials, causing an electrostatic lens to form at their junction.  The electrons that have jumped off the cathode begin to focus.  Ideally, the focal point will occur at the point when the beam strikes the display, thereby providing pinpoint resolution on the screen.  The last metal tube of the lensing chamber is held at the highest potential of all the electron gun components so that exiting electrons have a very high forward velocity.
  • Steering Magnets -- These two sets of electromagnets are fed the retrace signals that synchronize the drawing of the picture on the screen.  The flux between each pair of magnets will bend the electron beam, one in the horizontal direction and the other in the vertical direction.
  • Phosphor Screen -- If all works well, a pinpoint electron beam strikes the screen with the appropriate intensity and causes the phosphor to fluoresce.  The intensity modulation is synchronized with the horizontal and vertical retraces so that one frame of video is displayed.  The process repeats itself rapidly (24 frames/second for analog television) so that the moving scene appears seamless.

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