Aviation Electronics Technician 1 - Historic Naval Ships Association

Consequently, the potential of each element on thebeam side approaches the potential of the signalelectrode side. It will reach a value that varies withthe amount of light falling on the element. On thenext scan, the electron stream replaces a number ofelectrons on each element, just sufficient to return itto the potential of the cathode. Because eachelement is effectively a small capacitor, a capacitivecurrent is produced in the signal electrode circuitthat corresponds to the electrons deposited as theelement is scanned. When these electrons flowthrough the load resistor in the signal electrodecircuit, a voltage, which becomes the video signal,is produced.plate. As the low-velocity scanning beam strikes eachcharged element, it releases electrons that neutralizethe capacitors.SECONDARY ELECTRON CONDUCTION(SEC).— This is a vidicon-like tube with a specialtarget that uses secondary electron conduction. In thistube, light is focused on the photocathode that emitselectrons into the tube. These electrons are focused toform an image of electron streams that strike the SECtarget. The electrons are accelerated to approximately10,000 electron volts by the time they strike the target.The SEC target intercepts these streams of electrons.A great number of secondary electrons from eachPLUMBICON.— The plumbicon is similar inappearance and operation to the vidicon. It hasseveral advantages over the vidicon. Theplumbicon has a more rapid response and produceshigh-quality pictures at lower light levels. Becauseof its small size and low power consumption, theplumbicon is well suited for use in transistorizedTV cameras. Its simplicity and spectral response toprimary colors make it particularly useful in colorcameras.A unique feature of the plumbicon is that its colorresponse can be varied by the manufacturer. It is,therefore, available with spectral responses for eachof the primary colors. The color response of eachtube is identified by the letter R (red), G (green), or B(blue) following the basic number. For example, aplumbicon for a green channel may be designated55875G.A simplified diagram of a plumbicon target isshown in figure 5-31. The faceplate (view A) has itsinner surface coated with tin dioxide. This thin,transparent layer is the signal plate of the target. Thetin dioxide itself is a strong N-type semiconductor.Two layers of lead oxide are deposited on thescanning side of the target. The first of these two isalmost pure lead oxide. Lead oxide is an intrinsicsemiconductor. The second layer of lead oxide isdoped to form a P-type semiconductor. As shown inview B of figure 5-31, the three layers form a P-I-Njunction.Light from the televised scene passes through thelayer of tin dioxide and is focused on thephotoconductive lead oxide. Notice in view C offigure 5-31 that each picture element charge acts likea capacitor whose positive plate faces the scanningbeam. The target signal plate forms the negativeFigure 5-31.-Plumbicon target.5-25