Absolutely Small - Michael D. Fayer [39]
FIGURE 7.3. A schematic of a cathode ray tube (CRT). The hot filament heats the cathode, which “boils” off electrons. The positively charged acceleration grid accelerates the negatively charged electrons. Voltages applied to the control grids steer the electrons to particular points on the screen. The screen is covered with tiny adjacent red, green, and blue spots that glow with their particular color when hit with electrons. By rapidly scanning the electron beam to hit the appropriate colors in a given spot on the screen the image is made.
The electrons then pass between the control grids (see Figure 7.3), which control the direction the electrons go. There is one pair of control grids for the vertical direction (shown in the figure) and an equivalent set for the horizontal direction (not shown). Consider the vertical direction. If a positive voltage is applied to the top control grid and a negative voltage is applied to the bottom control grid, the electrons will be deflected up, as shown in Figure 7.3, because the negatively charged electrons will be attracted toward the positive top grid and repelled from the negative bottom grid. If the polarity of the voltages on the two grids is reversed, the electrons will be deflected downward. If large voltages are applied to the grids, the electrons will be deflected a lot. If small voltages are applied, the electrons will be deflected a small amount. If no voltages are applied, the electrons will go straight ahead. The same thing happens by applying voltages to the horizontal control grids. Once the electrons pass by the control grids, they continue in a straight line. In this manner, the electrons can be aimed just like bullets. This part of the CRT is referred to as an electron gun. Electron guns are used in many scientific devices such as electron microscopes and the device discussed below. So even when there are no more CRTs used as TVs and computer monitors, the basic device described here will still be important.
Because there is no air and gravity is a very weak force, the electrons travel basically as free particles until they hit the screen shown on the right side of Figure 7.3. On the screen are very small and very closely spaced patches of chromophores. Chromophores are chemical species that emit light when excited, that is, when sufficient energy is imparted to them. In this case, the chromophores are excited when the electrons hit them. In each very small region of the screen, there are three chromophores, one red, one green, and one blue. The electron beam can be aimed to hit a particular spot with great accuracy. If at a given location the red chromophore is hit, the screen lights up for an instant with a tiny red dot of light. If the green chromophore is hit, there is a green dot of light, and if the blue chromophore is hit, a blue dot of light is generated,
The electronics that produce the voltages on the control grids sweep the electron beam across the screen horizontally, then move the beam down, and sweep horizontally again. This is continued until the entire screen is swept. The beam returns to the top, and the sweep is repeated. As the beam sweeps, it is directed to hit red, green, or blue chromophores. The patches of three chromophores are so close together horizontally and vertically that your eye cannot distinguish them as individual dots. The