PLUMBICON

Consequently, the potential of each element on the beam side approaches the potential of the signal electrode side. It will reach a value that varies with the amount of light falling on the element. On the next scan, the electron stream replaces a number of electrons on each element, just sufficient to return it to the potential of the cathode. Because each element is effectively a small capacitor, a capacitive current is produced in the signal electrode circuit that corresponds to the electrons deposited as the element is scanned. When these electrons flow through the load resistor in the signal electrode circuit, a voltage, which becomes the video signal, is produced. PLUMBICON.— The plumbicon is similar in appearance and operation to the vidicon. It has several advantages over the vidicon. The plumbicon has a more rapid response and produces high-quality pictures at lower light levels. Because of its small size and low power consumption, the plumbicon is well suited for use in transistorized TV cameras. Its simplicity and spectral response to primary colors make it particularly useful in color cameras. A unique feature of the plumbicon is that its color response can be varied by the manufacturer. It is, therefore, available with spectral responses for each of the primary colors. The color response of each tube is identified by the letter R (red), G (green), or B (blue) following the basic number. For example, a plumbicon for a green channel may be designated 55875G. A simplified diagram of a plumbicon target is shown in figure 5-31. The faceplate (view A) has its inner surface coated with tin dioxide. This thin, transparent layer is the signal plate of the target. The tin dioxide itself is a strong N-type semiconductor. Two layers of lead oxide are deposited on the scanning side of the target. The first of these two is almost pure lead oxide. Lead oxide is an intrinsic semiconductor. The second layer of lead oxide is doped to form a P-type semiconductor. As shown in view B of figure 5-31, the three layers form a P-I-N junction. Light from the televised scene passes through the layer of tin dioxide and is focused on the photoconductive lead oxide. Notice in view C of figure 5-31 that each picture element charge acts like a capacitor whose positive plate faces the scanning beam. The target signal plate forms the negative plate. As the low-velocity scanning beam strikes each charged element, it releases electrons that neutralize the capacitors. SECONDARY ELECTRON CONDUCTION (SEC).— This is a vidicon-like tube with a special target that uses secondary electron conduction. In this tube, light is focused on the photocathode that emits electrons into the tube. These electrons are focused to form an image of electron streams that strike the SEC target. The electrons are accelerated to approximately 10,000 electron volts by the time they strike the target. The SEC target intercepts these streams of electrons. A great number of secondary electrons from each Figure 5-31.-Plumbicon target. 5-25