MadSci Network: Engineering

Re: How does the electron gun fire into the picture tube of monitors?

Date: Thu Jun 1 21:19:40 2000
Posted By: Richard Bersin, Other (pls. specify below), Senior Technical Staff Member, Emergent Technologies
Area of science: Engineering
ID: 957423143.Eg

Dear Adam,

The picture tube you refer to is made of glass, and is sealed perfectly 
tight.  All of the air is pumped out of the tube to that the pressure is 
less than 1 millionth of the pressure of the air at the earth's surface.  
Under this high vacuum electrons can move around and now bump into air 
molecules,so they can be made to move in straight lines to form an electron 
beam.  The electrons are generated by heating a thin wire, called a 
filament, to over 1,000 degrees C.  At this temperature the electrons have 
enough energy to come boiling out of the metal filament into the vacuum.  
In a metal there is a "sea" of electrons which can move about in the metal 
very easily.  That is why metals conduct electricity so well-because of 
there free electrons.  However they are trapped below the surface of the 
metal under room temperature conditions.  But if you heat the metal very 
hot the electrons begin to boil out of the metal, just like the water boils 
out of a pot when you heat it to boiling temperature.

As they boil out of the metal electric forces act on the electrons to focus 
them into a beam in much the same way a lens focuses the light that comes 
out of the bulb in a movie projector.  This fine beam of electrons is 
accelerated to very high speed.  The beam direction can be changed by 
applying voltages to metal electrodes surrounding the beam.  The beam is 
directed at the large flat screen in front of the picture tube, and is 
controlled to scan across the tube face from inside the tube very quickly. 
The inner surface of the tube screen is coated with material which glows 
and gives off light when it is struck by the beam of electrons.   The 
composition of the material which glows can be adjusted to give off 
different colors, particularly red, blue, and green.  This material is 
called a phosphor, and we say the phosphor glows when the electrons strike 
it.  The location of the beam is precisely controlled as to where it 
strikes the phosphor, so that the beam can be made to produce 
different colors as it moves across depending upon which phosphor it is 
striking.  The Red/Green/Blue phosphors are deposited in a pattern on the 
screen and the beam is directed to strike the right color as it moves 
across the screen.  There is no reaction with any gases-the colors come 
from the physical bombardment of the phosphors by the electrons.

From the front of the tube you see the glowing phosphors!

I hope you understand this OK.  If you have more question for details 
let me know!

R. Bersin.....

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