The Cathode Ray Tube

The CRT: A vacuum tube device in which an electron beam can be focused onto a small spot on a fluorescent screen at the opposite end of the structure; used in televisions and Oscilloscopes to form the display.

The Root of all Evil- The Electron Gun

At the heart of your monitor is the electron gun. The actual electron gun itself is really an internal component of the cathode ray tube (CRT), but it has fallen into common practice that both terms refer to the same device.

In essence, the electron gun is really nothing more than a big, powerful diode, passing electrons from one point to another (but not vice versa). The electron gun emits a stream of electrons at a screen made up of a matrix of photoconductive and photo-resistant layers. The beam of energy strikes the phosphor screen, which excites each individual phosphor and causes it to give off energy, making it glow. This makes up the visible display you are looking at right now. The device itself is amazingly complex, and yet is very reliable, offering many thousands of hours of operation without fail or error.

The Yolk

As previously stated, the reason why a CRT is so reliable is because the device contains very few moving parts. The yolk (also called the "deflection coil", "or deflection yoke") is positioned at the end of the electron gun's emitter. The yolk is an electronically controlled device that generates a strong magnetic field. The application or removal of current to yolk generates a magnetic field of varying intensity. The electron beam itself is aimed by electronic manipulation of that strong magnetic field. As the yolk's field interacts with the electron beam, it is deflected to make it sweep across the screen.

The problem here is that the beam can only be deflected in a limited range, so when the beam strikes a flat surface, the left and right edges of the image produced will look "warped". This occurs because the beam itself misses its target slightly, only partially energizing the phosphor it's supposed to hit.

Manufacturers remedy this problem by adding a slight curve to the screen surface to make it match the "sweep radius" of the electron gun so that it would hit it's mark more precisely. This of course, makes the screen more curved, but allows the image to match the dimension of the screen itself. This procedure is repeated many times, over and over again (525 times for television, varied for monitors based on current resolution)