Iconoscope

The Iconoscope (from the Greek: εἰκών 'image' and σκοπεῖν 'to look, to see') was the first practical video camera tube to be used in early television cameras. The iconoscope produced a much stronger signal than earlier mechanical designs, and could be used under any well-lit conditions. This was the first fully electronic system to replace earlier cameras, which used special spotlights or spinning disks to capture light from a single very brightly lit spot. The Iconoscope (from the Greek: εἰκών 'image' and σκοπεῖν 'to look, to see') was the first practical video camera tube to be used in early television cameras. The iconoscope produced a much stronger signal than earlier mechanical designs, and could be used under any well-lit conditions. This was the first fully electronic system to replace earlier cameras, which used special spotlights or spinning disks to capture light from a single very brightly lit spot. Some of the principles of this apparatus were described when Vladimir Zworykin filed two patents for a Television system in 1923 and 1925. A research group at RCA headed by Zworykin presented the iconoscope to the general public in a press conference in June 1933, and two detailed technical papers were published in September and October of the same year. The German company Telefunken bought the rights from RCA and built the superikonoskop camera used for the historical TV transmission at the 1936 Summer Olympics in Berlin. The iconoscope was replaced in Europe around 1936 by the much more sensitive Super-Emitron and Superikonoskop, while in the United States the Iconoscope was the leading camera tube used for broadcasting from 1936 until 1946, when it was replaced by the image orthicon tube. The main image forming element in the iconoscope was a mica plate with a pattern of photosensitive granules deposited on the front using an electrically insulating glue. The granules were typically made of silver grains covered with caesium or caesium oxide. The back of the mica plate, opposite the granules, was covered with a thin film of silver. The separation between the silver on the back of the plate and the silver in the granules caused them to form individual capacitors, able to store electrical charge. These were typically deposited as small spots, creating pixels. The system as a whole was referred to as a 'mosaic'. The system is first charged up by scanning the plate with an electron gun similar to one in a conventional television display tube. This process deposits charges into the granules, which in a dark room would slowly decay away at a known rate. When exposed to light, the photosensitive coating releases electrons which are supplied by the charge stored in the silver. The emission rate increases in proportion to the intensity of the light. Through this process, the plate forms an electrical analog of the visual image, with the stored charge representing the inverse of the average brightness of the image at that location. When the electron beam scans the plate again, any residual charge in the granules resists refilling by the beam. The beam energy is set so that any charge resisted by the granules is reflected back into the tube, where it is collected by the collector ring, a ring of metal placed around the screen. The charge collected by the collector ring varies in relation to the charge stored in that location. This signal is then amplified and inverted, and then represents a positive video signal.