Page 25 - DCAP313_LAB_ON_COMPUTER_GRAPHICS

Unit 2: Graphics Systems

2.2.1 Cathode-ray-tubes Notes
A CRT is a displaced glass tube. An electron gun at the rear of the tube produces a beam of
electrons which is directed towards the front of the tube (screen). The internal side of the screen
is coated with phosphor material which gives off light when it is stroked by electrons. It is
possible to control the point at which the electron beam hits the screen, and thus the position of
the dot upon the screen, by deflecting the electron beam. The fig below shows the electrostatic
deflection of the electron beam in a CRT. The deflection system of the cathode-ray-tube consists
of two pairs of parallel plates, referred to as the vertical and horizontal deflection plates. The
voltage applied to vertical plates controls the vertical deflection of the electron beam and voltage
applied to the horizontal deflection plates controls the horizontal deflection of the electron beam.
There are two techniques used for producing image on the CRT screen.

Figure 2.2: Cathode-ray-tube (CRT)

A cathode ray tube (CRT) is a dedicated vacuum tube in which images are produced when an
electron beam strikes a phosphorescent surface. Most desktop computer displays make use of
CRTs. The CRT in a computer display is similar to the “picture tube” in a television receiver.

All CRT’s have three main elements: an electron gun, a deflection system, and a screen. The
electron gun provides an electron beam, which is a highly concentrated stream of electrons. The
deflection system positions the electron beam on the screen, and the screen displays a small spot
of light at the point where the electron beam strikes it.

Basic Operation of a CRT
The basic operation of CRT is as follows:
Electron Gun: The primary parts of an electron gun in a CRT are the heated metal cathode and a
control grid. The cathode is heated by an electric current passed through a coil of wire called the
filament. This causes electrons to be boiled off the hot cathode surface. In the vacuum inside the
CRT envelope, 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 can be used. Sometimes the electron gun is built to hold the accelerating anode and
focusing system within the same unit.

Focusing System: The focusing system is used to produce a clear picture by focusing the electrons
into a narrow beam. Otherwise, electrons would repel each other and beam would spread out
as it reaches the screen. Focusing is accomplished with either electric or magnetic fields.
Deflection System: Deflection of the electron beam can be controlled by either electric fields or
magnetic fields. In case of magnetic field, two pairs of coils are used, one for horizontal deflection

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