Figure 5-33.-Color picture tube

Figure 5-32.-SEC camera tube target operation. electron striking the target are emitted and stored (fig. 5-32). The photoelectrons possess sufficient energy to penetrate the thin, metallic signal and support plate. As they travel through the porous potassium-chloride layer, many secondary electrons are emitted as the beam strikes the interlined particles. These secondary electrons either escape to the positive collector screen or they travel through the spaces of the porous layer to the positive collector plate. This loss of electrons produces a positive charge on the scanned side of the target. Several hundred secondary electrons are emitted for each incident electron, producing a substantial gain at the target. The video signal is developed from the target by the scanning beam discharging the positively charged areas of the target in the same manner as in a vidicon tube. This charging current, flowing out of the signal plate connection, is then amplified by an external amplifier. Figure 5-33.-Color picture tube. The SEC tube has applications in extremely low-light, nighttime military TV systems where a high internal amplification and a fast speed or response to moving images are important. Picture Tubes A monochrome picture tube is a specialized form of the cathode-ray tube. An electron gun in the tube directs a beam of electrons toward a fluorescent material on the screen. The screen glows when struck by the electrons. Between the gun and the screen are deflection coils that deflect the beam horizontally and vertically to form a raster. The brightness of the picture is controlled by varying the grid-bias voltage with respect to the cathode voltage. This bias can be changed by varying either the cathode voltage or the grid voltage. Color picture tubes operate on the same basic principle as monochrome picture tubes. The difference between the two systems is the types of phosphors that coat the screen. The different types of phosphors produce colors when struck with the electron beam. Three basic or primary colors are used in combination to produce all the other desired colors. These primary colors are red, green, and blue. In a three-gun color picture tube (fig. 5-33), there is a separate gun for each of the color phosphors. The tube’s screen consists of small, closely spaced phosphor dots of red, green, and blue. The dots are arranged so a red, green, and blue dot form a small triangle. The shadow mask provides a centering hole in the middle of the triangle of dots. The convergence electrode causes the three separate electron beams to meet and cross at the hole in the shadow mask. Figure 5-34.-Color picture tube. A. Side view. B. Front view. 5-26