Display Devices

The first computer display devices were modified typewriters and Teletype machines. These devices were slow, noisy, and expensive, and could only handle text. For graphic output, an X-Y plotter, a device that pulled a pen over a piece of paper, was used. It shared all the problems of the Teletype machine.

It did not take long before these mechanical machines were replaced with electronic counterparts. The replacement was called a terminal. It consisted of a typewriter-like keyboard, which activated switches, and a display screen, which was a modified television receiver. Thus, the first computer display device was a cathode ray tube or CRT.

Cathode Ray Tubes

A cathode ray tube paints an image on a phosphor screen using a beam of electrons. The concept of the CRT was formulated before the nature of the electron was known. Cathode rays are not rays at all but high-speed streams of particles called electrons. The CRT is a vacuum tube where the processing of electrons takes place in an evacuated glass envelope. If the electron beam passed through air or another gas, the electrons would collide with the molecules of that gas, making it difficult to manipulate the electron beam.

The CRT generates a source of electrons from an electron gun. The electrons are accelerated in a straight line to a very high velocity using a high voltage and then deflected from the straight line using a magnetic field. The beam can be turned on or off with an electrical signal. The screen of the CRT is coated with a phosphorus compound, which gives off light energy when it is struck with high-speed electrons. When the beam hits the face of the CRT, a spot of light results. The beam is scanned from left to right and from top to bottom. The beam is turned on when a light area is to be generated and it is turned off when an area is to be dark. This scanning is clearly visible on both computer monitors and television screens.

There are two basic methods of scanning a picture. The first is called "progressive." Every line is scanned beginning at the top left corner and finishing at the lower right. Another method is called "interlace." The picture is scanned twice with half of the lines being scanned each time. This is done to refresh the picture at twice the actual scan rate and reduce the flicker of the display.

From Monochrome to Color Displays

The first CRTs were one color, also called monochrome; the display color was usually green. The first display devices were designed to replace a mechanical printer, so a single color was sufficient. However, the CRT is not limited to only printing text but is capable of producing images and complex graphics where full color is highly desirable. Again, technology was adapted from the television industry, and color monitors were quick to follow the early monochrome versions.

The color CRT is effectively three monochrome CRTs, all in the same glass envelope. There are three electron guns but each gun is individually controlled. The three electron beams are deflected together and strike the face of the CRT. The electron beams must pass through a screen with hundreds of thousands of small holes, called a shadow mask , before striking the phosphor on the front of the CRT. The holes are arranged in groups of three so that when one electron beam passes through the hole, it strikes a small dot of phosphor that gives off red light. Another beam strikes only a dot of phosphor that gives off green light. The third beam falls on a phosphor dot that gives off blue light.

The mechanics of the color CRT are such that each of the three electron beams produces scanning beams of only one color. These three colors—red, green, and blue, or RGB—are the three additive primary colors. Any color may be generated by using a combination of these three. This shadow mask technology was the first to be used for color television and is still used in most CRTs.

Over the years the shadow mask CRT has been refined. Modern tubes have a nearly flat face, have much improved color, and have very high resolution, which is the ability of a display to show very small detail. One improvement is a shadow mask using stripes rather than round holes. This arrangement is easier to align. These improvements are not only found in computer displays but television receivers as well.

Cathode Ray Tube Disadvantages

Since its inception, the CRT has shown a number of disadvantages. First, the tube is large and heavy. CRT sizes are relative to the diagonal measurement and most CRT displays are deeper than their diagonal measurement. Secondly, the electrons in the tube are accelerated using high voltage. The larger the tube, the higher the accelerating voltage, which reaches to the tens of thousands of volts and requires a large power supply. The tube is made of glass, which is not suited for portable equipment or for applications with significant vibration such as aircraft. Finally, the tube requires significant power.

As hard as it is to believe today, the first "portable" computers actually used CRTs for display devices! In a word, these early portable computers were huge and would have remained so if a suitable replacement for the CRT had not been found. What was needed was a low power display device that had the capability of the CRT yet was small, not as fragile, and required low power and low voltage.

From Cathode Ray Tube to Liquid Crystal Display

The liquid crystal display, or LCD, is a low voltage device. It requires very low power but it was not originally a graphics device or capable of color. The LCD is essentially a light gate, which can be opened to allow light to pass, or closed to shut light off. To use the LCD as a full color graphics display, the display is divided up into picture elements called pixels. Each pixel represents the color and intensity of a small part of the complete picture. Three LCD light gates are required for each pixel: one for red, green, and blue. Behind each gate is a color filter, which is illuminated by a white light source. Behind one LCD light gate is a red filter, behind another is a green filter, and the third a blue. By adjusting the amounts of the three primaries, as in the CRT, the correct intensity and color can be generated for each pixel.

LCD construction is simple. The liquid crystal material is sandwiched between two flat glass plates. Crystalline materials, which usually are not liquid, have very profound effects on light waves. The liquid crystal can affect the manner in which light energy passes through the material, and this can be changed by the application of an electric field. A thin metal electrode is placed over the area where the LCD is to change from dark to light. The electrode is so thin that it is completely transparent and cannot be seen. An electric field is created when a voltage is applied to the electrodes on the LCD glass.

Most LCDs use the rotation of polarized light to change the intensity of the light. The light entering the pixel is plane polarized, meaning that the light waves are in one plane. This is done with a polarizer , which is the same technique used in sunglasses to reduce glare. A simple way to visualize a polarizer is to think about a venetian blind where the separation of the slats is so close that light waves can only pass through in one plane.

On the front of the LCD there is a second polarizer, which is oriented at a right angle to the first. If these two polarizers were placed together with nothing but vacuum or air between them, no light could pass through. This is because the light is polarized by the first polarizer and is incompatible with the second.

Liquid crystal material has the ability to overcome this by rotating the polarization of light waves, but only when an electric field is placed across the liquid crystal. Therefore, if a voltage is placed across the liquid crystal, the light is rotated by 90 degrees and will pass through the front polarizer. The application of a voltage can permit or shut off the light intensity.

In the color LCD display three "sub pixels" are required because the intensity of light from the three primaries must be independently controlled. If one pixel could provide both brightness and color, the LCD could be simplified. An improved LCD display uses a single light valve where the liquid crystal material generates both the color and brightness. This new LCD material is called cholesteric because it was originally derived from animal cholesterol.

Display Device Picture Quality

The number of pixels into which an image is divided will directly affect the quality of the picture. As an example, a conventional television picture is generated with 525 scanning lines (the U.S. standard). Of these, only about 484 lines are visible. The aspect ratio of the television picture is 4:3, which means that the width of the picture is four-thirds the height. If the pixels were square, there would be 484 rows and 660 columns of pixels. Because of the interlace scan, the actual number of rows and columns is half of that, or 242 by 330.

When an image is generated with an insufficient number of pixels, the picture lacks resolution and the pixels are very evident. The individual lines of a television picture are clearly visible, particularly in a large screen television. Common computer displays have resolutions of 340 X 680, 680 X 1760, and so on. Computer monitors can have a better picture than some television receivers.

An improved television standard is set to replace the older 525 line system; this is called high definition television, or HDTV. In addition to the improved resolution or definition, the aspect ratio is 16:9, which is the same as motion pictures. Because HDTV is a digital system and optical disks are used to store video, the relationship between computer monitors and television receivers will grow closer over the years.

Simplifying LCD Display Technology

In the LCD display, each light gate has to be connected to electronic drivers, which activate or deactivate the gate. An LCD graphics display has a very large number of pixels, which poses a serious challenge in running conductors to each LCD light gate. Thin, transparent conductors can hardly be seen but the sheer number of them would make manufacturing LCD displays difficult, at best. One solution is a method of connecting the LCD segments by mounting electronic circuits right on the glass plate. This arrangement is called an "active matrix" and it significantly reduces the number of interconnects required. The transistors used for the active matrix are made from thin films that are so small they are virtually invisible. This is called a thin film transistor active matrix LCD, or TFTAM LCD or AMLCD.

Even though the AMLCD has simplified the LCD graphics display, a large number of light gates, transistors, and interconnections remain. In the manufacturing process, if one pixel fails, the display must be scrapped. In an LCD graphics display, the number of LCD light gates numbers more than one million. The chances are good that one of those LCD gates or the thin film transistors would be defective in the manufacturing process.

The percentage of good products from a factory production run is called the yield. A poor yield is reflected in a high price of a product. Increasing the yield of the LCD production was the major challenge to the LCD industry in producing a cost-effective display product. The cholesteric LCD can be made with one-third the number of pixels and therefore, one-third the number of LCD light gates. This means the cholesteric LCD will have three times the manufacturing yield, which makes the technology potentially much more cost effective than other options.

Lighting Sources for Display Devices

The AMLCD requires a white light source to operate. Some of the more common light sources are not suited for backlighting an LCD display. The incandescent lamp and LEDs are point sources of light whereas a distributed source is desired. These two sources are also not energy efficient, which is an important characteristic required for battery power.

For notebook computers, an electroluminescent panel is used. This device generates a low light level with good energy efficiency. The panel is thin and can be sandwiched easily behind the LCD and the display case.

Some portable devices such as small "palm" computers, cellular telephones, and watches must perform in bright sunlight. Displays that reflect, rather than emit, light are used in these devices. LCD displays are well suited to applications where the display operates in the "reflective" mode. When the ambient light is low, a backlight provides the necessary illumination. When backlighting is provided, the LCD is now operating in the "transmissive" mode. LCD displays that operate in both modes are called "transflective." As of the year 2001, transflective LCDs were not yet capable of providing full color.

If the light intensity falling on the front of a transmissive display is greater than the emitted light, the display contrast will be lost and the display will "wash out." Usually, displays are shielded from very bright light such as sunlight but in some applications this is not possible, such as an aircraft instrument panel. Displays used for these applications are called "sunlight readable." This means the display is visible in full sunlight. In these high brightness applications, a thin, serpentine, fluorescent lamp is used for backlighting. This technique provides a high light output but also generates considerable heat. Providing a very high level of backlighting for a color LCD display has become very common as the LCD is used for computer projectors.

The new cholesteric LCD material will also allow for an LCD display that operates with reflected light and will be completely sunlight readable. Improved resolution will result because the cholesteric LCD requires only one light gate per pixel.

Improving LCD Technology

The modern AMLCD display is one of the best display technologies but it still suffers from some weaknesses. The resolution of a good quality AMLCD is not as good as the better CRTs. The cost of AMLCDs, although dropping, is still higher than the equivalent CRT. The AMLCD, or LCD in general, is not well suited for use in harsh environments because it is negatively affected by low temperatures. The response time of an LCD display under these conditions is increased significantly. This would cause moving images to drag and blur. In very cold temperatures, such as those in which military equipment is often operated, the LCD will quit operating completely and could be damaged by the extreme conditions.

A new display technology in the later stages of development is called the field emission display, or FED. The FED uses an array of small, pointed electrodes mounted close to a dot of phosphor. Like the color CRT, the pointed electrode causes an emission of an electron beam, which excites the phosphor to emit light. Essentially, the FED is a flat CRT where the electron beam is not deflected. The FED has all the advantages of the CRT, including good resolution, bright display, full color capability, and sunlight readability, without the major disadvantages, such as low temperature problems. It is not yet clear what direction this new technology will take, but it is likely that FEDs will be used for aircraft instruments and other sunlight readable applications.

see also Computer System Interfaces; Digital Logic Design.

Albert D. Helfrick

Bibliography

Robin, Michael, and Michel Poulin. Digital Television Fundamentals: Design and Installation of Video and Audio Systems. New York: McGraw-Hill, 2000.

Whitaker, Jerry C. Electronic Displays: Technology, Design and Applications. New York: McGraw Hill, 1994.

——. Video Display Engineering. New York: McGraw-Hill, 2000.

Cathode-Ray Tube

Background

A cathode-ray tube, often called a CRT, is an electronic display device in which a beam of electrons can be focused on a phosphorescent viewing screen and rapidly varied in position and intensity to produce an image. Probably the best-known application of a cathode-ray tube is as the picture tube in a television. Other applications include use in oscilloscopes, radar screens, computer monitors, and flight simulators.

The cathode-ray tube was developed in 1897 by Ferdinand Braun of Strasbourg in what was then the French-German region of Alsace-Lorraine. It was first used as an oscilloscope to view and measure electrical signals. In 1908, A.A. Campbell-Swinton of England proposed using a CRT to send and receive images electronically. It wasn't until the 1920s, however, that the first practical television system was developed. The concept for a color cathode-ray tube was proposed in 1938 and successfully developed in 1949.

Although General Electric introduced their first television set for home use in 1928, commercial television broadcasting remained an experimental technology with only limited range and audience. It took until the late-1940s before television net-works had established themselves sufficiently to start a boom in consumer sales. Black-and-white television sets gave way to the first color sets in the 1960s. In the following decades cathode-ray tubes for televisions got both larger and smaller as manufacturers sought to satisfy consumer wants. Recent developments have included tubes with flatter faces, sharper comers, and higher resolution for better viewing.

A CRT consists of three basic parts: the electron gun assembly, the phosphor viewing surface, and the glass envelope. The electron gun assembly consists of a heated metal cathode surrounded by a metal anode. The cathode is given a negative electrical voltage and the anode a positive voltage. Electrons from the cathode flow through a small hole in the anode to produce a beam of electrons. The electron gun also contains electrical coils or plates which accelerate, focus, and deflect the electron beam to strike the phosphor viewing surface in a rapid side-to-side scanning motion starting at the top of the surface and working down. The phosphor viewing surface is a thin layer of material which emits visible light when struck by the electron beam. The chemical composition of the phosphor can be altered to produce the colors white, blue, yellow, green, or red. The glass envelope consists of a relatively flat face plate, a funnel section, and a neck section. The phosphor viewing surface is deposited on the inside of the glass face plate, and the electron gun assembly is sealed into the glass neck at the opposite end. The purpose of the funnel is to space the electron gun at the proper distance from the face plate and to hold the glass envelope together so that a vacuum can be achieved inside the finished tube.

The CRT used in a color television or color computer monitor has a few additional parts. Instead of one electron gun there are three—one for the red color signal, one for blue, and one for green. There are also three different phosphor materials used on the viewing surface—again, one for each color. These phosphors are deposited in the form of very small dots in a repeated pattern across the screen—red, blue, green, red, blue, green, and so on. The key to a color CRT is a piece of perforated metal, known as the shadow mask, which is placed between the electron guns and the viewing screen. The perforations in the shadow mask are aligned so that the red gun can fire electrons at only the phosphor dots which produce the red color, the blue gun at the blue dots, and the green gun at the green dots. By controlling the intensity of the beam for each color as it scans across the screen, different colors can be produced on different areas of the screen, thus producing a color image. To give an idea of how small the perforations and dots have to be, a 25-inch (63 cm) color television picture tube may have a shadow mask with 500,000 perforations and 1.5 million individual phosphor dots.

Design

The electron gun must be designed for each new application. New screen sizes, new overall glass envelope dimensions, and new image resolution requirements all require a new gun design. Brighter images may require higher power accelerating coils. Finer image resolution may require improved beam focusing coils or plates. While the basic design remains the same, the details are constantly refined.

Likewise the basic design of the phosphor viewing surface is fairly well defined, but the details may change. New image resolution requirements may require a new method of depositing the phosphor dots on the face plate, which in turn may require new material processing techniques. The search for truer colors may result in new material formulations. The amount of time the phosphors emit light, or glow, after being struck by the electron beam is also important and is controlled by the chemical composition of the phosphor. This property is called persistence. In a color television, the electron beam scans the screen 25 times per second. If the persistence is longer than one twenty-fifth of a second (0.04 second), the image would show two scans at the same time and would appear blurred. If the persistence is shorter than this time, the image from the first scan would have disappeared before the second scan came along, and the image would appear to flicker.

Even the glass envelope requires extensive design. Strength, radiation absorption characteristics, temperature tolerance, impact resistance, dielectric properties, and optical clarity are a few of the design criteria used when designing the glass components. Computers may be used to perform finite element analysis to evaluate the stresses in complex envelope shapes. This technique divides the part into a finite number of smaller, more easily definable pieces, or elements, and then performs the calculations for each element to spot unacceptably high stress concentrations. Using the computer, dimensions for contours and wall thickness can easily be adjusted until a satisfactory design is achieved.

Raw Materials

Cathode-ray tubes use an interesting and varied assemblage of raw materials. In many cases, it is the raw materials, not the design or manufacturing process, that determine the performance characteristics of the finished product.

The electron gun is made from a variety of metal pieces. The cathode, or electron emitter, is made from a cesium alloy. Cesium is used as a cathode in many electronic vacuum tube devices because it readily gives off electrons when heated or struck by light. In a CRT, the cathode is heated with a high resistance electrical wire. The accelerating, focusing, and deflection coils may be made from small diameter copper wire. A glass tube protrudes from the rear of the electron gun assembly and is used to evacuate the air from the finished CRT.

The phosphor viewing surface is formed from a continuous layer of a single material in monochromatic CRTs, or is composed of individual dots of three different materials in color CRTs. Zinc sulfide is a common phosphor material. The color is determined by adding a very small amount of material called an activator. Zinc sulfide with 0.01% silver activator emits a blue light. When a 0.001% copper activator is used, it produces a green light. A 50/50 mixture of zinc sulfide and cadmium sulfide with a 0.005% silver activator produces a yellow light. Red light can be produced by adding silver or copper to zinc sulfide mixed with a high percentage of cadmium sulfide. The phosphors are usually ground into a fine powder before they are applied to the inside of the face plate.

The glass envelope uses slightly different raw materials for each of its three component parts. The basic raw material for all of the glass components is silica. Alumina may be added to adjust the flow properties of the molten glass when forming it. Various oxides are used to lower the melting temperature. Barium oxide, strontium oxide, and lead oxide are used to provide radiation protection in the neck and funnel. The face plate, on the other hand, must have a minimum of lead oxide to prevent a discoloration phenomenon known as electron or x-ray browning. Neodymium oxide may be used on the face plate to enhance the contrast of the viewed picture.

In color CRTs, the shadow mask is usually made from a thin sheet of a nickel alloy.

The Manufacturing
Process

The glass envelope or its components are usually formed at a glass manufacturing facility and shipped to the cathode-ray tube manufacturer who forms the phosphor viewing screen, fabricates and assembles the electron gun, and assembles the finished CRT.

Forming the glass envelope

• 1 The glass ingredients are weighed and mixed prior to melting. The glass is melted in gas-fired furnaces about 500-3,000 square feet (46-279 sq m) in size. If this is a continuous process, new ingredients are added to maintain a constant level as the molten glass flows out of the furnace to the forming areas. Before forming, the molten glass must be cooled somewhat and made uniform in temperature throughout.
• 2 The face plate is normally pressed into the desired shape by dropping a gob of molten glass into a mold and pressing on the gob with a plunger. The funnel can be formed either by pressing or by centrifugal casting. In the casting method a gob of molten glass drops into a mold, which then spins rapidly to spread the glass uniformly over the inside surface of the mold. A grooving disk near the top of the mold cuts the soft glass at the desired height so that the excess glass can be removed easily. The neck is made from glass tubing, and one end is flared to facilitate insertion of the electron gun.
• 3 In a monochromatic CRT the three glass components are joined together before they are shipped to the CRT manufacturer. In a color CRT only the neck and funnel are joined, and the face plate is shipped separately for further processing. The glass components are usually joined by heating the mating surfaces to a high temperature with gas jets or electric heaters.

Applying the phosphors

• 4 In monochromatic CRTs the phosphor viewing surface is coated on the inside of the glass face plate. This is done by preparing a liquid suspension of the phosphor and pouring a measured amount into the neck of the glass envelope along with a gelling agent. After about 20 minutes, the coating has set and the excess liquid is poured off. The process for color CRTs is more complicated. First the shadow mask is made by applying a light-sensitive coating to the thin mask material, exposing it to light through a perforated template, and then etching away the exposed coating with an acid to form the millions of holes. The mask is then pressed into a slightly curved shape and attached just behind the face plate. The face plate is placed in a centrifuge and the inside surface is coated with the green phosphor material. The centrifuge spins the face plate to ensure an even coating of phosphor. A strong ultraviolet light is shown through the mask to harden the green phosphor material into hundreds of thousands of dots. The remaining material is then washed off. This process is repeated to form the red and blue phosphor dots, with the ultraviolet light being shifted a small amount each time. When this process is finished, the glass face plate is joined to the funnel. On color tubes, the phosphor dots are sensitive to high temperatures, so instead of using high-temperature gas jets, a mixture of chemical solvent and powdered glass, called a frit, is applied to the joint. This acts like a glass "solder," and the joint can be sealed at a much lower temperature.

Assembling the electron gun

• 5 The metal components of the electron gun are precision formed. If coils are used they are wound from fine copper wire. Some electron guns use metal plates instead of coils, and these plates are stamped and formed. The components are assembled either by hand or with automated machines in a clean environment. The glass tube is sealed into the base, and the base is welded into the gun assembly.

Final assembly and packing

• 6 The inside of the glass envelope neck is lubricated with graphite, and the electron gun is inserted and aligned. The neck is then sealed around the gun. A vacuum pump is attached to the glass tube extending from the rear of the gun, and the inside of the CRT is evacuated of air. When the proper vacuum has been achieved, the glass tube is heated and quickly pinched closed to form a seal.
• 7 The finished CRT is tested for performance and carefully packed to prevent damage. Because the CRT is under a high vacuum, any fracture in the glass envelope could result in an inward explosion known as an implosion.

Quality Control

Although the operating principle of a cathode-ray tube is simple, the manufacturing process requires strict controls and precise alignments. The phosphor materials must be extremely pure to achieve the desired colors. Even a tiny variance in the amount of activator used can result in a significant change in color. Likewise, when you consider that a color television CRT requires the placement of over a million tiny dots side by side on the viewing surface, even a small error in alignment could be disastrous.

Byproducts and Recycling

The principal byproduct of CRT manufacturing is scrap glass. Much of this glass is recycled. Recycled glass with a high content of lead oxide is used to provide radiation protection in CRT funnels and has completely replaced previous sources of lead oxide for this application.

The Future

The worldwide market for cathode-ray tubes was estimated at nearly 400 million units in 1994 and is expected to grow at a 6% annual rate through 2000. The color television market is expected to grow at a 5% annual rate, while the color computer monitor market is expected to grow at a 20% rate. In the television market, the demand for larger television picture tubes with higher image resolution is expected to continue.

One important trend is the development of high definition television (HDTV), which has scanning rates more than twice that of conventional systems. This will require new electron gun designs as well as new glass materials and technologies to handle the doubled radiation rate.

Where To Learn More

Books

Braithwaite, Nicholas and Graham Weaver, eds. Electronic Materials. Butterworths, 1990.

Connelly, J.H. and D.J. Lopata. Engineered Materials Handbook, Volume 4. ASM International, 1991.

Haider, Z. Television Glass Bulb Design and Manufacturing Developments, Glass Production and Technology International. Sterling Publications, Ltd., 1992.

Periodicals

Fleischmann, Mark. "The Big Picture." Popular Science, November 1994, pp. 82-85, 92-95.

Meeks, T. "Inside the CRT: Monitor Technology Explained." PC Novice, July 1993, pp. 40-43.

—Laurel M. Sheppard /

Chris Cavette

Cathode-ray tube

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, also known commonly as CRTs, are widely used in a number of electrical devices such as computer screens, television sets, radar screens, and oscilloscopes used for scientific and medical purposes.

Any 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 (units of pressure), 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 1750°F (approximately 825°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 a 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 an electron gun tends to spread out, forming 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 must be added to the CRT. The lens is similar to an optical lens, like the lens in a pair of glasses. The electrical or magnetic

lens shapes the flow of electrons that pass through it, just as a glass lens shapes the light rays passing through it.

The electron beam in a cathode-ray tube also has to be moved back and forth 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: one uses electrical charges and the other uses a magnetic field. 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.

Oscilloscope

An especially useful application of the cathode-ray tube is an oscilloscope. An oscilloscope measures changes in electrical voltage over time. The plates that deflect the electron beam in a vertical direction are attached to some source of voltage. (For example, they can be connected directly to an electric circuit.) The plates deflecting the electron beam in a horizontal direction are attached to some sort of a clock mechanism.

Wired in this way, the oscilloscope shows the change in voltage in a circuit over time. This change shows up as a wavy line on a screen. As voltage increases, the line moves upward. As it decreases, the line moves downward.

One application of the oscilloscope is troubleshooting an electric circuit. An observer can notice immediately if a problem has developed within a circuit. For example, circuits can be damaged if unusually large voltages develop very quickly. If a circuit is being monitored on an oscilloscope, such voltage surges can be detected immediately. Oscilloscopes also have medical applications. They can be connected to electrodes attached to a person's skin. The electrodes measure very small voltage changes in the person's body. Such changes can be an indication of the general health of the person's nervous system.

Display screen

The actual conversion of electrical energy 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, while zinc sulfide with copper metal as an impurity gives off 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, the screen will go blank briefly, making the picture 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.

cathode-ray tube (CRT) A display device in which a beam of electrons (cathode rays), emitted by an electron gun, is focused and deflected to a series of specific positions on the phosphor-coated screen of the display. The image is generated as the electron beam moves over the screen (see also raster-scan display, vector display, beam deflection). Electrons striking a spot of phosphor on the screen increase the phosphor's energy state so that it becomes excited. The excited phosphor emits light as it returns to its ground state, thus creating a small area of the image. As light is emitted for a short period, it is necessary to provide some mechanism for continually redrawing or refreshing, the display if a constant image is required. Different phosphors emit different colored light. By coating the screen with small areas of red, green and blue phosphors and having three electron guns it is possible to produce a color display (see also RGB color model, shadow-mask cathode-ray tube).

The CRT is the most widely used computer display device, with at least 200 million in use worldwide. It is also used in TV sets whose numbers are not included in the above figure). As costs of flat-panel displays fall they are eventually expected to replace the CRT in most applications. However this prediction has been current for at least the past 25 years so a complete replacement of the CRT may be some way off.