A fine beam of electrons, emitted from the electron gun, scans the target electrode.The secondary electrons cause some of this beam to be reflected back toward the elec-tron gun. Areas of the target electrode with the most secondary-electron emission pro-duce the greatest return beam intensity; regions with the least emission produce thelowest return beam intensity. The greatest return beam intensity corresponds to thebrightest parts of the video image. The return beam is modulated as it scans the targetelectrode in a pattern just like the one in the TV receiver CRT. The return beam ispicked up by a receptor electrode.The main disadvantage of the image orthicon is that it produces significant noise inaddition to the signal output. But when a fast response is needed (when there is a lot ofaction in a scene) and the light ranges from very dim to very bright, the image orthiconis the camera tube of choice. It is common in commercial broadcasting.Traveling-wave tubesA traveling-wave tube is a form of electron-beam tube that is useful at ultra-high fre-quencies (UHF) and microwave frequencies. There are several variations on thistheme; the two most common are the magnetron and the klystron.The magnetronMost magnetrons contain a cathode at the center, and a surrounding plate as shown inFig. 29-10. The plate is divided into sections, or cavities, by radial barriers. The outputis taken from an opening in the plate and passes into a hollow waveguide that servesas a transmission line for the UHF or microwave energy.The cathode is connected to the negative terminal of a high-voltage source, and theanode is connected to the positive terminal. Therefore, electrons flow radially outward.A magnetic field is applied lengthwise through the cavities. As a result, the electronsmove outward in spirals from the cathode to the anode, rather than in straight lines.The electric field produced by the high voltage, interacting with the longitudinal mag-netic field and the effects of the cavities, causes the electrons to bunch up into clouds.The swirling movement of the electron clouds causes a fluctuating current in the anode.Traveling-wave tubes54929-9Simplified cutaway viewof an image orthicon.