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As the name implies, CRTs operate by sending rays of electrons from a cathode inside a tube down to a screen of phosphors on the inside of the front face. The phosphors convert the energy of the electrons into visible light. The device has integral coils to focus the beam of electrons, and either electromagnetic coils or electrostatic plates that are used to steer the elec- trons to a series of selected locations on the phosphor screen.

The electrons come from a device called an electron gun (refer to Figure 8.4 ). This is a device that, when heated, emits a stream of electrons. It has a con- trol grid at its muzzle, which controls the rate of electron beam production. That is, the grid can fully open the gates and let out a large number of elec- trons per second, which gives a bright spot on the phosphor screen; or it can shut it down, giving virtually no light emission. It is capable of a full range of intensities between these extremes.

The beam can be defl ected independently in the vertical and horizon- tal directions very rapidly, and as this happens, it moves the spot of light to proper locations on the screen. In many devices, the horizontal defl ec- tion signal moves the beam steadily across the screen from one side to the other, and then drops back very rapidly to the initial setting. So the beam

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seems to move from left to right, and then disappear, only to reappear on the left again and repeat the sweep. The wave form of this signal is called a sawtooth. In waveform display devices, such as oscilloscopes, there is no preprogrammed vertical defl ection. This is driven by the input signal. In computer monitors and televisions, there is a preprogrammed vertical defl ec- tion signal. Traditionally the spot starts in the upper left-hand corner of the display screen and moves across to the upper right-hand corner. Then it disappears and reappears on the left again, but just below its fi rst sweep point. Each sweep from left to right is called a row, and each transition from right to left is called a fl yback.

In color displays, there are three electron guns, and three beams move down the tube to the screen. The screen is coated with spots of red, green, and blue phosphor material interspersed by a sieve, or shadow mask, that separates the individual spots. This ensures that the red phosphor is acti- vated only by the red intended beam, and so on. The spots appear in groups of three: red, green, and blue (one of each). The most common arrangements

FIGURE 8.4 The Cathode Ray Tube. The overall schematic is shown at the top. Key parts of it are shown in blow-ups below and to the right. At the base of the tube is the electron gun, which liberates electrons, forms them into a beam, and sends them down the tube. At the neck of the tube this drawing shows a coil that causes the electron beam to move across the face of the tube. Some CRTs use electrostatic defl ection plates instead of a coil. The blow up of the face of the tube indicates the triads of spots of phosphor and the black shadow mask.

are small, closely spaced vertical bars, or circular spots in a triangle. Each triad of spots constitutes a pixel, and each of the elements within the pixel is a subpixel. As in all of digital imaging, the spacing of the pixels per unit of length, or per screen dimension, indicates the device resolution. Since CRTs have a fi xed pattern, the maximum resolution is this fi xed value. The device can be programmed to work at integer fractions of this value — ½,

¹⁄_³, ¼, and so on — but not at higher values. CRTs can be made with rather high resolutions, but in large formats they become prohibitively expensive, cumbersome in size, and consume huge amounts of power. CRTs produce their light essentially at the screen face, so they can accommodate a wide viewing angle. CRTs are not as suitable for projection as some alternatives, like DLP and LCD, since these have a light source separate from the screen. Conversely, when used as a direct-view device, they — like Plasma and OLED devices — are relatively bright.

The phosphors can “ burn in ” over time if certain parts of the screen are used repeatedly to produce high levels of output. This causes a ghost image and is the reason behind the “ screen saver. ” Also, the shadow mask can become magnetized. This will cause noticeable and fi xed patterns in the image. Most of the newer CRTs have a built-in demagnetizer to reduce any magnetization every time the device is started up. The phosphors have a relatively short persistence. That is, they do not continue to produce sig- nifi cant amounts of light after a few milliseconds. The persistence time is set to coincide with the time required to produce an entire frame. There is a very low level of afterglow that can be seen when a TV set is turned off. The result is that CRTs have a relatively good contrast ratio.