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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. |
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Figure 1: Click on the thumbnail at the right to enlarge the working diagram of the cathode ray tube (CRT) in a separate window. |
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Figure 2: Click on the thumbnail at the right to see how changing voltages around the cathode modulates electron beam intensity. |
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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.
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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:
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- 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.
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- Cathode -- The cathode is a grounded metal plate that is
super-heated so that electrons are literally jumping off the surface.
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- 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).
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- 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.
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- 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.
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- 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.
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- 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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