CATHODE-RAY TUBES
Most display devices currently in use employ a
cathode-ray tube (CRT) for the display screen. The
following information is a review of the fictions and
operation of CRTs.
ELEMENTS OF A CRT
The CRT is a large glass envelope that contains
three basic elements: an electron gun, a deflection
system, and a phosphor screen. These elements
convert electronic signals into visual displays. In our
discussion of CRTs, we will first cover monochrome
CRTs then we cover color CRTs.
All the air in the glass tube must be evacuated to
form a vacuum. This is necessary for three reasons:
Air molecules disrupt the electron beam as it
travels from the anode to the cathode,
Gases tend to ionize when subjected to high
voltages and are conductive, which would
short out the CRT, and
Oxygen in the CRT would cause the filament
to burn up.
Figure 1-1 shows the three basic components: the
phosphor screen, the electron gun, and a deflection
system.
Figure 1-1.–A cathode-ray tube (CRT).
1-2
The Phosphor Screen
The inside of the large end, or face, of a CRT is
coated with phosphor. Phosphor is a material that
displays luminescence when excited by electrons or
other sources of radiation. In other words, electrons
(beta radiation) striking the phosphor will cause it to
glow for a short period of time. The length of time or
duration that the display remains on the screen after
the phosphor has been hit with electrons is known as
persistence. When the electrons are formed into a
beam and directed at the phosphor, the beam produces
a dot. The intensity, or brightness, of the dot is
directly proportional to the intensity of the electron
beam.
The Electron Gun
The electron gun is located in the narrow neck of
the CRT. The gun acts as the source of the electron
beam. Figure 1-2 illustrates the components of the
electron gun.
A small ac voltage is applied to the filament to
heat the cathode. Heating the cathode causes vast
Figure 1-2.–A CRT electron gun.
numbers of electrons to be freed from the cathode.
When the voltage of the control grid is more positive
than the cathode, the beam is turned on, or
unblanked, and the electrons are drawn to the anode
(phosphor screen). When the control grid is negative
with respect to the cathode, the beam is turned off, or
blanked. In a monochrome CRT, the beam is either
on or off and has a uniform brightness. In a black and
white CRT that displays varying shades of gray, the
