Difference Between Orthicon And Vidicon
In a black and white camera, an image is focused by a lens on a coating on the inside surface of the face of a vacuum tube called an image orthicon. This coating is called the photocathode and is photoemissive; that is, it emits electrons when struck by light. The electrons coming from the photocathode are drawn to, and continue through, a positively charged target screen. They then hit a very thin glass plate called the target electrode. When the electrons strike the target electrode, they knock loose secondary electrons, which are captured and “removed” by the target screen. Because of the loss of the secondary electrons, the target electrode is left with a positive charge. The strength of the positive charge at each point on the target is directly proportional to the brightness of the light that had struck the corresponding points on the photocathode.
At the end of the image orthicon tube, opposite the photocathode, is an electron gun that emits a stream of electrons, each of which is negatively charged. A narrow beam from the gun is directed to the back of the target electrode. To prevent the beam from releasing secondary electrons from the target electrode, a decelerating ring introduces a negative charge. When it strikes a point on the target, the beam loses electrons until the positive charge on the target is neutralized at that point. The negative voltage near the target electrode results in a return beam being reflected back down the tube. As the beam scans the target, it loses more or fewer electrons according to the amount of positive charge at each point on the target. These cause variations in the return beam: the video signal.
At the rear of the image orthicon tube is an electron multiplier made up of plates called dynodes. These are coated with a material that releases many secondary electrons for each electron hitting it. As the beam is “bounced” from each dynode in turn, emission of secondary electrons greatly strengthens the video signal. The amplified video signal then passes out of the image orthicon tube.
For portable TV cameras, the image orthicon tube is too large and heavy. Two other devices are used instead. One is the vidicon tube. The vidicon is much simpler than the image orthicon. On the inside of the tube’s face is a transparent metal coating that comprises a signal plate. A photoconductive coating is deposited directly on the back of the signal plate. The image is focused on the coating. This material shows a sharp increase in conductivity when light strikes it. Wherever light is focused on the photoconductor, the increased conductivity allows a positive charge from the signal plate to pass through the photoconductor and charge it in turn. The strength of the positive charge at a given point on the back of the photoconductor is proportional to the brightness of the light striking the front side at a corresponding point.
A beam of electrons from an electron gun scans the rear surface of the photoconductor, losing electrons in accordance with the strength of the positive charge that it neutralizes at each point. As a positive charge is neutralized, the resulting change in potential is transferred to the signal plate. This forms the video signal that is sent out of the vidicon tube to be amplified.