"Computer Graphics" 2.1.1-Refresh CRT

A graphics system typically uses a display as its output device. Most video monitor operations are based on standard cathode ray tube (Cathode-ray tube,crt), but other technologies have emerged, and solid state monitors will eventually dominate. 2.1.1 Refresh type crt[Figure 2.2] Figure 2.2 shows the basic workings of the CRT. The electron beam emitted by an electron gun (cathode rays) is directed at a specified position on a screen coated with a fluorescent layer through a focusing system and a deflection system. In the electron beam red remembers each position, the fluorescence layer will produce a small bright spot. Since the light emitted by the fluorescence layer will decay quickly, there must be some way to maintain the screen image. One method is to store the graphic information as a charge distribution on the CRT. This charge distribution is used to keep the crown points in the active state. But now the use of more to maintain the phosphor brightness is to quickly control the electron beam repeatedly re-painting images. This type of display is called a refreshed display (refresh CRT), and the frequency at which the paint repeats on the screen is called the refresh rate. The main components of the CRT electron gun are the metal cathode and the control gate which are heated to excite (Fig 2.3). By adding a hot cathode to the coil that is called the filament, the electrically heated electrons "boil out" to the cathode surface. In a vacuum in the CRT package, a negatively charged free electron accelerates toward the phosphor screen under the action of a higher positive voltage. The acceleration voltage can be generated by a metal coating that is charged with a positive charge near the screen in the CRT package, or with an accelerated anode (Figure 2.3). What's the matter, in the electronic gun structure, the accelerating anode and the focusing system are placed in the same part. [FIG. 2.3] The strength of the electron beam is controlled by a voltage level set on the control grid. The control gate is a metal cylinder that is mounted next to the cathode. If a higher negative voltage is added to the control gate, the electron activity is organized to intercept the electron beam and stop it from passing through the keyhole at the end of the control gate. The lower voltage applied to the control gate only reduces the number of electrons passed. Because the intensity of the light emitted by the fluorescence layer depends on the amount of electrons in the bombardment stock, the actual light intensity can be controlled by changing the voltage of the control gate. We use graphical software commands to set the brightness level of each screen position. (Conclusion: The control gate voltage determines the screen brightness) the CRT's focusing system is used to control the electron beam converging to a small point during the bombardment of the phosphor. Otherwise, as electrons repel each other, the electron beam spreads out as it approaches the screen. The focus can be achieved either by electric fields or by magnetic fields. For attraction focus, the electron beam is the same as the positively charged Yingshu shown in 2.3, which forms an electrostatic lens. The function of the electrostatic lens is to focus the electron beam on the center of the screen, just as the optical lens focuses the beam on the specified focal length. A lens-like focusing effect can be accomplished by a magnetic field formed by a coil enclosed outside the CRT package. The magnetic focusing lens produces the smallest possible size on the screen. In high-precision systems, or with additionalFocused hardware to keep the electron beam focused on all screen positions. Since the diameter of most CRT parts is larger than the distance from the focus system to the center of the screen, the electron beam is different from the poly when the screen does not power on. Therefore, the electron beam can only focus on the center of the screen. When the electron beam moves to the screen border, the displayed image becomes blurred. The system can compensate for this defect by adjusting the focus to the screen position of the electron beam. The deflection of an electron beam is controlled by an electric field or magnetic field. The CRT is now usually equipped with a magnetic deflection coil mounted on the external CRT wind, shown in 2.2. Using two pairs of coils, they are mounted in pairs on the neck of the CRT package, a pair mounted on the top and bottom of the neck, and the other pair mounted on both sides of the neck. The magnetic field generated by the unrealized voucher causes transverse deflection force, which is also perpendicular to the direction of the electron beam. A stack of coils is horizontally deflected and the other pair is vertically deflected. Adjust the current through the coil to get the appropriate deflection amount. When electrical deflection is applied, two pairs of parallel plates are installed in the CRT package. The pair is placed horizontally, controls the vertical deflection, and the other pair is vertically positioned to control the horizontal deflection (Figure 2.4). [Figure 2.4] by transferring the energy of the CRT electron beam to the fluorescence layer, a bright spot can be formed on the screen. The kinetic layer of the electron beam is absorbed by the fluorescence layer when the electron strikes the fluorescence layer to stop the motion. Part of the electron beam energy is converted into thermal energy due to matcha, and the remainder leads to a higher two-character energy level in the fluorescence layer of the courtyard. After a short period of time, the "activated" fluorescence layer electrons release the smaller quantum energies and begin to fall back to their stable state. What we see on the screen is the combined effect of all electronic light emission: the glow point decays quickly as all activated fluorescence layers are transferred to their basic levels. The frequency (or color) of the light emitted by the fluorescence layer is proportional to the energy differential between the activated quantum state and the basic state. The fluorescent layers used by the CRT have different types. In addition to the color, the main difference between these fluorescence layers is their afterglow (persistence) Time: After the CRT electron beam is removed, they will continue to glow (that is, the activation of electrons into the basic state) for how long. Afterglow time is defined as the time from the screen glow to the attenuation of one-tenth of its original brightness. Shorter afterglow time of the fluorescence layer, the need for a higher refresh rate to keep the screen graphics do not blink. The fluorescent layer of the short afterglow is used for animation, while the long afterglow fluorescence layer is used to display the static figure of the complexity. Although some fluorescence layer afterglow time is greater than one second, but for the graphics monitor, the afterglow time is usually made of 10~60us material. Figure 2.5 shows the two-point distribution of the one or two points on the screen. The two-point center position has the highest brightness and is attenuated by the Gaussian distribution to the edge of the two point. [Figure 2.5] The maximum number of points that the CRT displays without overlap is called resolution (resolution). [Figure 2.6]

Computer Graphics 2.1.1-Refresh CRT