 - A cathode ray tube (CRT) is a specialized 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.
A cathode ray tube consists of several basic
components, as illustrated below. The electron gun generates a
narrow beam of electrons. The anodes accelerate the electrons.
Deflecting coils produce an extremely low frequency electromagnetic field that allows
for constant adjustment of the direction of the electron beam.
There are two sets of deflecting coils: horizontal and vertical.
(In the illustration, only one set of coils is shown for
simplicity.) The intensity of the beam can be varied. The
electron beam produces a tiny, bright visible spot when it
strikes the phosphor-coated screen.
To produce an image on the screen, complex
signals are applied to the deflecting coils, and also to the
apparatus that controls the intensity of the electron beam. This
causes the spot to race across the screen from right to left, and
from top to bottom, in a sequence of horizontal lines called the
raster. As viewed from the front of the CRT, the spot moves in a
pattern similar to the way your eyes move when you read a
single-column page of text. But the scanning takes place at such
a rapid rate that your eye sees a constant image over the entire
screen.
The illustration shows only one electron gun.
This is typical of a monochrome, or single-color, CRT. However,
virtually all CRTs today render color images. These devices have
three electron guns, one for the primary color red, one for the
primary color green, and one for the primary color blue. The CRT
thus produces three overlapping images: one in red (R), one in
green (G), and one in blue (B). This is the so-called RGB color
model.
In computer systems, there are several display modes, or sets
of specifications according to which the CRT operates. The most
common specification for CRT displays is known as SVGA (Super
Video Graphics Array). Notebook computers typically use liquid crystal display.
The technology for these displays is much different than that for
CRTs.
| LAST UPDATED: |
03 Dec 2000
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