Cathode Ray Tube Explanation
A cathode ray tube (CRT) is a vacuum tube in which electrons emitted from an electrode are focused onto a phosphorescent screen. The electrons are aimed so as to create an image on the screen. CRTs are ubiquitous, having been used in televisions and personal computer monitors for decades--although they are losing favor to LED, LCD and xenon plasma screens.
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Electron guns
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Color CRTs use three separate electron guns, or electron sources--one for the red, one for blue and one for green dots on the phosphor screen. If electrons are heated enough, their thermal energy overcomes the binding energy that confines them to an atom. This is called thermionic emission.
Electron focus
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As the electrons travel away from the electron gun, they repel each other. So the electron stream spreads out. They are accelerated toward a positively charged electrode, or anode, in the shape of an annulus. The hole in the annulus is large enough to accommodate the spreading electron beam. The high voltage of the anode overwhelms the repulsive force of the electrons' repelling each other, which is why it is called the focusing anode.
Electron acceleration
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The electrons then accelerate toward a second anode, because it is more positively charged than the first one. That is why it is called the accelerating anode. It too is annulus-shaped to let the electron beam through. Why don't the electrons fly to the positively charged wall of the anode? Because inside a cylinder the electrostatic force is equal in all radial directions, so the electrons don't feel an attractive force to the side walls.
Steering coils
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As there were two anodes, there are also two steering coils. Both are magnets, one steering the electrons vertically, the other steering them horizontally. They are called coils because they are electromagnets made of copper windings. These are the devices that focus the electron beams onto each phospor dot on the screen. Because these coils scan every dot of the screen, they must change the direction of the electron beam very fast.
Screen
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Primary Colors (Courtesy of www.hunternuttall.com)
The screen is scanned horizontally consecutively through each row of phosphor dots. No two dots are scanned simultaneously. A phosphor is any material that sustains a glow after exposure to an energized particle. The high-speed electrons collide with the phosphor, causing it to glow.
The green, red, and blue dots on a phosphor screen are each painted with different metallic compounds to attain the color desired. Primary colors are colors that can be combined in different amounts to produce a wide spectrum of colors. Primary colors trick the eyes into seeing a color that isn't really there. For example, a combination of red and blue appears as magenta.
Green, red, and blue are primary colors; therefore, it is economical for a TV station to send out three signals, one for each of the three colors, to be recombined later at the TV set end. This is akin to printers' using yellow, red, and blue inks as an economical approach to color printing, so that hundreds of inks across the visible spectrum need not be stored.
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References
- Photo Credit USC Dept. of Physics
