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. 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 Westinghouse Electric Company 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.

Operation

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 cathode ray 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. The collector ring is also used to collect electrons being released from the granules in the photoemission process. If the gun is scanning a dark area few electrons would be released directly from the scanned granules, but the rest of the mosaic will also be releasing electrons that will be collected during that time. As a result, the black level of the image will float depending on the average brightness of the image, which caused the iconoscope to have a distinctive patchy visual style. This was normally combatted by keeping the image continually and very brightly lit. This also led to clear visual differences between scenes shot indoors and those shot outdoors in good lighting conditions. As the electron gun and the image itself both have to be focused on the same side of the tube, some attention has to be paid to the mechanical arrangement of the components. Iconocopes were typically built with the mosaic inside a cylindrical tube with flat ends, with the plate positioned in front of one of the ends. A conventional movie camera lens was placed in front of the other end, focused on the plate. The electron gun was then placed below the lens, tilted so that it was also aimed at the plate, although at an angle. This arrangement has the advantage that both the lens and electron gun lie in front of the imaging plate, which allows the system to be compartmentalized in a box-shaped enclosure with the lens completely within the case. As the electron gun is tilted compared to the screen, its image of the screen is not as a rectangular plate, but a keystone shape. Additionally, the time needed for the electrons to reach the upper portions of the screen was longer than the lower areas, which were closer to the gun. Electronics in the camera adjusted for this effect by slightly changing the scanning rates. The accumulation and storage of photoelectric charges during each scanning cycle greatly increased the electrical output of the iconoscope relative to non-storage type image scanning devices. In the 1931 version, the electron beam scanned the granules; while in the 1925 version, the electron beam scanned the back of the image plate.

History

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