Typically, the primary output
device in a graphics system is a video monitor.The operation of most video
monitors is based on the standard cathode-ray* tube (CRT) design, but several
other technologies exist and solid-state* monitors may eventually predominate.
REFRESH CATHODE-RAY TUBES
1.
Figure
2-2 illustrates the basic operation of a CRT. A beam of electrons (cathode
rays), emitted by an electron gun, passes through focusing and deflection
systems that direct the beam toward specified positions on the phosphor-coated
screen.
2.
The
phosphor then emits a small spot of light at each position contacted by the
electron beam. Because the light emitted by the phosphor fades very rapidly,
3.
Some
method is needed for maintaining the screen picture. One way to do this is to
store the picture information as a charge distribution within the CRT.
4.
This
charge distribution can then be used to keep the phosphors activated. However,
the most common method now employed for maintaining phosphor glow is to redraw
the picture repeatedly by quickly directing the electron beam back over the
same screen points.
5. This type of display is called a refreshCRT,
and the frequency at which a picture is redrawn on the screen is referred
to as the refresh rate. The primary components of an electron gun in a
CRT are the heated metal cathode and a control grid (Fig. 2-3).
6. Heat is supplied to the cathode
by directing a current through a coil of wire, called the filament, inside the
cylindrical cathode structure. This causes electrons to be “boiled off” the hot
cathode surface.
7. In the vacuum inside the CRT
envelope, the free, negatively charged electrons are then accelerated toward
the phosphor coating by a high positive voltage. The accelerating voltage can
be generated with a positively charged metal coating on the inside of the CRT
envelope near the phosphor screen, or an accelerating anode, as in Fig. 2-3,
can be used to provide the positive voltage.
8. Sometimes the electron gun is
designed so that the accelerating anode and focusing system are within the same
unit.
