In operation, the scene to be televised is focused on the photocathode, which emits electrons proportional to the intensity of the light striking each of its areas. The electron streams are focused on the target and cause secondary electrons to be emitted by the glass disc. These secondary electrons are collected by the adjacent wire mesh, leaving the photocathode side of the disc with a pattern of positive charges corresponding exactly to the image being televised. Since the glass is quite thin, a similar charge pattern is set up on its opposite side.
The glass is scanned by a low-velocity electron beam produced by an electron gun and deflected by external electromagnetic coils. This beam is made to decelerate and to approach the glass vertically at essentially zero velocity by the potentials applied to the decelerator grid (No.5) and the field mesh. Some of the electrons are deposited on the glass to neutralize its positive charge while others are repelled to form a return beam. The return electron beam, then, is modulated by the positive charge pattern on the target and hence in accordance with the original light image.
On coming back to the electron gun area, the return beam passes through a five-stage electron multiplier similar to that employed in photomultiplier tubes, developing an output video signal. The dynodes in the multiplier section may amplify the modulated beam by 500 times or more, with the result that the image orthicon is basically more sensitive than the human eye in picking up faint light images.
The monoscope is a special type of camera tube. Its basic principle of operation is similar to that of other CRT’s, for it incorporates essentially the same type of electron gun and deflection systems. However, it is fitted with a permanently installed fixed pattern – such as a TV test pattern – and develops only a repetitive video signal.
Special CRTs. In addition to the cathode-ray tubes we’ve discussed, there are a number of special types which depend on electron beams for their operation. Among these are a variety of discharge and demonstration tubes used for classroom study and laboratory experiments, but by far the most common type is the X-ray tube.
The basic X-ray tube consists of two principal electrodes: an electron source (cathode) , and a target anode. The anode is of dense metal and set at an angle with respect to the electron source. In operation, extremely high voltages are applied to the two electrodes, accelerating the electron stream to tremendous velocities. On striking the target anode, the electron beam excites the metal atoms, causing them to emit ultra-short electromagnetic radiation – X rays. Since the target is set at an angle, the X rays are radiated out through the side of the tube’s glass envelope, where they can be photographed and used to trace in outline the interior make-up of solid matter.