The cathode of the electron gun in the CRT is required not only to emit electrons, but also to concentrate emitted electrons into a tight beam. The cathode of the CRT consists of a small diameter nickel cap. The closed end of the cap is coated with emitting material.  Because of this type of construction, electrons can only be emitted in one direction. If these electrons were allowed to strike the screen, the whole screen would glow. Since the object of the electron gun is to concentrate the electrons into a tight beam, a special grid must be used. This special grid is in the form of a solid metal cap with a small hole in the center. The grid is placed over the emitting surface of the cathode and charged negatively in relation to the cathode. Since all emitted electrons leave the cathode), their paths can be identified. The curving electron paths are due to the negative potential of the grid coupled with the high positive potential of the anode. The potential of the anode attracts electrons out of the cathode-grid area past toward the screen. The grid potential may be varied to control the number of electrons allowed to go through the control-grid opening. Since the brightness or intensity of the display depends on the number of electrons that strike the screen, the control grid is used to control the brightness of the CRT.

The proper name, BRIGHTNESS CONTROL, is given to the potentiometer used to vary the potential applied to the control grid. The control grid actually serves as an electron lens. It is this electronic lens that you adjust when you turn up the brightness control on your TV set. After passing, the electrons start to spread out, or diverge, again. Therefore, it becomes necessary to provide some additional focusing to force the electrons into a tight beam again. The first electrode is commonly called the FOCUSING ANODE. Generally, the focusing anode is charged a few hundred volts positive with respect to the cathode. Electrons emitted by the cathode are attracted to the focusing anode. This is the reason that they travel through the small hole in the grid. The second electrode, called the ACCELERATING ANODE, is charged several thousand volts positive in relation to the cathode. Any electrons approaching the focusing anode will feel the larger electrostatic pull of the accelerating anode and will be bent through the opening in the focusing anode and will travel into the area. Thus, an electron is pulled in all directions at once, forcing the electron to travel down the center of the tube. Then, the electron is accelerated into the accelerating anode. Once it passes the mid-point, it feels the electrostatic attraction from the front wall of the accelerating anode, which causes it to move faster toward the front.

THE CRT SCREEN

The inside of the large end of a CRT is coated with a fluorescent material that gives off light when struck by electrons. This coating is necessary because the electron beam itself is invisible. The material used to convert the electrons' energy into visible light is a PHOSPHOR. Many different types of phosphor materials are used to provide different colored displays and displays that have different lengths of PERSISTENCE (duration of display).

In order to reach the screen, electrons from the cathode are accelerated to relatively high velocities. When these electrons strike the screen, they dislodge other electrons from the material of the screen. If these secondary emission electrons are allowed to accumulate, they will form a negatively-charged barrier between the screen and the electron beam, causing a distorted image on the CRT screen. The method used to control secondary emission, which you are already familiar with, i.e., a suppressor grid, is not practical in CRT's. Instead, a special coating called an AQUADAG COATING is applied to the inside of the tube. This coating is composed of a conductive material, such as graphite, and has the same high-positive potential applied to it that is applied to the accelerating anode. This allows the aquadag to perform two functions. First, since the aquadag coating is positive, it attracts the secondary emitted electrons and removes them. Second, because the aquadag is operated at a high-positive potential and is mounted in front of the accelerating anode, it aids in the acceleration of electrons toward the screen.

Before going on, let's review what you have already learned about CRT operation.

Electrons are emitted from a specially constructed cathode and move toward the front of the CRT. The number of electrons that leave the area of the cathode is determined by the cap-shaped grid. In addition, the grid concentrates the emitted electrons into a beam. The electron beam is focused and accelerated toward the screen by two electrodes: the focusing anode and the acceleration anode. The electron beam strikes the screen and causes a bright spot to appear at the point of impact. Any electrons released by secondary emission are removed from the tube by the aquadag coating.

At this point, you have a bright spot in the center of the CRT screen.