CRTs convert color signals to video images

Color CRTs

Just as the color camera works in a manner similar to the black and white camera, the color CRT works in a manner similar to the black and white CRT. The color CRT has three sets of color phosphor dots laid down on the inside of the CRT. These are laid down in a specific pattern. The exact pattern will differ from manufacturer to manufacturer, but for the sake of explanation, a very common pattern, the triad, will be used. As you can see from the first figure on the right, the triad is made up of one red phosphor dot, one green phosphor dot, and one blue phosphor dot arranged in a triangular pattern.

At the back of the CRT there are three separate electron guns, one each for the red, green, and blue information. Close to the front of the tube is a thin metal mask. This mask has tiny holes in it, arranged in such a way that only the red electron beam can strike a red phosphor, the green beam a green phosphor, and the blue beam a blue phosphor. These phosphors are so small and close together that they can't be seen as separate and distinct unless they are looked at under magnification. Thus, when they are struck by the electron beams, their colors blend together to produce the same color that the camera split up. The red part of a stop sign, for example, would cause only the corresponding red phosphors to be illuminated, while the white letters of the sign would cause all of those corresponding phosphors to glow.

Convergence

The CRT can experience a problem called convergence. If you look at the diagram on the right, you will see that each of the electron guns at the back of the CRT is a different distance from the phosphors. The gun that is mounted at the lowest place in the CRT neck will be farther from the phosphors at the top of the screen than the gun that is mounted at the top of the neck of the CRT. Since the electrons travel at a constant speed, the electrons that have to travel a longer distance will arrive at the phosphors an instant later than electrons from the other guns. This is the convergence problem.

If there is a convergence problem, we could see up to three distinct offset images of different colors. Convergence shouldn't be a problem with most home TVs, but it can be with some large-screen projection units. You can usually make some control adjustments to lessen or minimize convergence problems.

We can see from this discussion that the color video system is considerably more complex than the black and white system. In fact, the color camera might be thought of as three synchronized cameras and the color CRT as three separate CRTs synchronized together.

1.  Color CRT.

2.  Components of a color CRT.

3.  CRT convergence error.