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Cathode ray tube

Cathode ray tube

Envelope or container

Electron gun

Focusing and deflection systems

Display screen

Resources

A cathode ray tube is a device that uses a beam of electrons in order to produce an image on a screen. Cathode ray tubes are also known commonly as CRTs. Cathode ray tubes are still widely used in a number of electrical devices, such as computer screens, television sets, radar screens, and oscilloscopes (signalvizualization tools used in science and engineering).

A cathode ray tube consists of five major parts: an envelope or container, an electron gun, a focusing system, a deflection system, and a display screen.

Envelope or container

Most people have seen a cathode ray tube or pictures of one. The picture tube in a television set is perhaps the most familiar form of a cathode ray tube. The outer shell that gives a picture tube its characteristic shape is called the envelope of a cathode ray tube. The envelope is most commonly made of glass, although tubes of metal and ceramic can also be used for special purposes. The glass cathode ray tube consists of a cylindrical portion that holds the electron gun and the focusing and deflection systems. At the end of the cylindrical portion farthest from the electron gun, the tube widens out to form a conical shape. At the flat wide end of the cone is the display screen.

Air is pumped out of the cathode ray tube to produce a vacuum with a pressure in the range of 10-2 to 10-6 pascal, the exact value depending on the use to which the tube will be put. A vacuum is necessary to prevent electrons produced in the CRT from colliding with atoms and molecules within the tube.

Electron gun

An electron gun consists of three major parts. The first is the cathode, a piece of metal which, when heated, gives off electrons. One of the most common cathodes in use is made of cesium metal, a member of the alkali family that loses electrons very easily. When a cesium cathode is heated to a temperature of about 1,750°F (954°C), it begins to release a stream of electrons. These electrons are then accelerated by an anode (a positively charged electrode) placed a short distance away from the cathode. As the electrons are accelerated, they pass through a small hole in the anode into the center of the cathode ray tube.

The intensity of the electron beam entering the anode is controlled by the grid. The grid may consist of a cylindrical piece of metal to which a variable electrical charge can be applied. The amount of charge placed on the control grid determines the intensity of the electron beam that passes through it.

Focusing and deflection systems

Under normal circumstances, an electron beam produced by the electron gun described above would have a tendency to spread out to form a cone-shaped beam. However, the beam that strikes the display screen must be pencil-thin and clearly defined. In order to form the electron beam into the correct shape, an electrical or magnetic lens, similar to an optical lens, can be created adjacent to the accelerating electrode. The lens consists of some combination of electrical or magnetic fields that shapes the flow of electrons that pass through it, just as a glass lens shapes the light rays that pass through it.

The electron beam in a cathode ray tube also has to be moved about so that it can strike any part of the display screen. In general, two kinds of systems are available for controlling the path of the electron beam, an electrostatic system and a magnetic system. In the first case, negatively charged electrons are deflected by similar or opposite electrical charges, and in the second case, they are deflected by magnetic fields.

In either case, two deflection systems are needed, one to move the electron beam in a horizontal direction, and the other to move it in a vertical direction. In a standard television tube, the electron beam completely scans the display screen about 25 times every second.

Display screen

The actual conversion of electrical to light energy takes place on the display screen when electrons strike a material known as a phosphor. A phosphor is a chemical that glows when exposed to electrical energy. A commonly used phosphor is the compound zinc sulfide. When pure zinc sulfide is struck by an electron beam, it gives off a greenish glow. The exact color given off by a phosphor also depends on the presence of small amounts of impurities. For example, zinc sulfide with silver metal as an impurity gives off a bluish glow and with copper metal as an impurity, a greenish glow.

The selection of phosphors to be used in a cathode ray tube is very important. Many different phosphors are known, and each has special characteristics. For example, the phosphor known as yttrium oxide gives off a red glow when struck by electrons, and yttrium silicate gives off a purplish blue glow.

The rate at which a phosphor responds to an electron beam is also of importance. In a color television set, for example, the glow produced by a phosphor has to last long enough, but not too long. Remember that the screen is being scanned 25 times every second. If the phosphor continues to glow too long, color will remain from the first scan when the second scan has begun, and the overall picture will become blurred. On the other hand, if the color from the first scan fades out before the second scan has begun, there will be a blank moment on the screen, and the picture will appear to flicker.

Cathode ray tubes differ in their details of construction depending on the use to which they will be put. In an oscilloscope, for example, the electron beam has to be able to move about on the screen very quickly and with high precision, although it needs to display only one color. Factors such as size and durability are also more important in an oscilloscope than they might be in a home television set.

In a commercial television set, on the other hand, color is obviously an important factor. In such a set, a combination of three electron guns is needed, one for each of the primary colors used in making the color picture.

In the early 2000s, flat-screen display technologies began to rapidly displace CRTs in most of their applications. While CRT televisions still remained on the market as of 2006 because of their price advantage, most computer makers were ramping down their production of CRT monitors in favor of flatscreen monitors. For example, Apple Corporation stopped shipping computers with CRT monitors in the summer of 2001, and Sony Corporation announced in 2005 that it would cease manufacture of CRT monitors. The global CRT market was forecast to plunge from about $20 billion in 1999 to about $10 billion in 2007.

Key Terms

Phosphor A chemical that emits a glow when struck by a beam of electrons.

Resources

BOOKS

Badano, Aldo. High-Fidelity Medical Imaging Displays. Bellingham, WA: International Society for Optical Engineering, 2004.

Compton, Kenneth. Image Performance in CRT Displays. Bellingham, WA: International Society for Optical Engineering, 2003.

David E. Newton

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