Some video cameras use a form of electron tube that converts visible light into varying electric currents. The two most common types of camera tube are the vidicon and the image orthicon.
Functional diagram of a vidicon
In the vidicon, a lens focuses the incoming image onto a photoconductive screen. An electron gun generates a beam that sweeps across the screen as a result of the effects of deflecting coils, in a manner similar to the operation of an electromagnetic CRT. The sweep in the vidicon is synchronized with any CRT that displays the image.
As the electron beam scans the photoconductive surface, the screen becomes charged. The rate of discharge in a certain region on the screen depends on the intensity of the visible light striking that region. A simplified cutaway view of a vidicon tube is shown in above figure .
The main advantage of the vidicon is its small physical size and mass. A vidicon is sensitive, but its response can be sluggish when the level of illumination is low. This causes images to persist for a short while, resulting in poor portrayal of fast-motion scenes.
Functional diagram of an image orthicon
Another type of camera tube, also quite sensitive but having a quicker response to image changes, is the image orthicon. It is constructed much like the vidicon, except that there is a target electrode behind the photocathode (above figure). When a single electron from the photocathode strikes the target electrode, multiple secondary electrons are emitted as a result. The image orthicon thus acts as a video signal amplifier, as well as a camera.
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 electron gun. Areas of the target electrode with the most secondary electron emission produce the greatest return beam intensity, and regions with the least emission produce the lowest return beam intensity. The greatest return beam intensity corresponds to the brightest parts of the video image. The return beam is modulated as it scans the target electrode and is picked up by a receptor electrode.
One significant disadvantage of the image orthicon is that it produces considerable noise in addition to the signal output. But when a fast response is needed and the illumination ranges from dim to very bright, the image orthicon is the camera tube of choice.
A photomultiplier is a vacuum tube that generates a variable current depending on the intensity of the light that strikes it. It multiplies its own output, thereby obtaining high sensitivity. Photomultipliers can be used to measure light intensity at low levels.
The photomultiplier consists of a photocathode, which emits electrons in proportion to the intensity of the light striking it. These electrons are focused into a beam, and this beam strikes an electrode called a dynode. The dynode emits several secondary electrons for each electron that strikes it.
The resulting beam is collected by the anode. A photomultiplier can have several dynodes, resulting in high gain. The extent to which the sensitivity can be increased by cascading dynodes is limited by the amount of background electron emission or dark noise from the photocathode.
A dissector, also known as an image dissector, is a form of photomultiplier in which the light is focused by a lens onto a translucent photocathode. This surface emits electrons in proportion to the light intensity. The electrons from the photocathode are directed to a barrier containing a small aperture. The vertical and horizontal deflection plates, supplied with synchronized scanning voltages, move the beam from the photocathode across the aperture. The electron stream passing through the aperture is modulated depending on the light and dark nature of the image.
The image resolution of the dissector tube depends on the size of the aperture. The smaller the aperture, the sharper the image, down to a certain limiting point. However, there is a limit to how small the aperture can be, while still allowing enough electrons to pass, and avoiding the generation of interference patterns. The image dissector tube produces very little dark noise, and this allows for excellent sensitivity.