A video camera tube used in early television broadcasting.

The Iconoscope was the first electronic video camera tube. It replaced earlier cameras, which required special lighting or spinning disks for television cameras. With this new invention, a much stronger signal was captured that could be used under any well-lit conditions — and it was fully electronic. The name iconoscope comes from the Greek: εἰκών "image" and σκοπεῖν "to look, to see."

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.

An iconoscope is a camera tube that projects an image on a special charge storage plate containing a mosaic of electrically isolated photosensitive granules separated from a common plate by a thin layer of isolating material, somewhat analogous to the human eye's retina and its arrangement of photoreceptors. Each photosensitive granule constitutes a tiny capacitor that accumulates and stores electrical charge in response to the light striking it. An electron beam periodically sweeps across the plate, effectively scanning the stored image and discharging each capacitor in turn such that the electrical output from each capacitor is proportional to the average intensity of the light striking it between each discharge event.

After Hungarian engineer Kálmán Tihanyi studied Maxwell's equations, he discovered a new hitherto unknown physical phenomenon, which led to a breakthrough in the development of electronic imaging devices. He named the new phenomenon a charge-storage principle. (further information: Charge-storage principle) The problem of low sensitivity to light resulting in low electrical output from transmitting or camera tubes would be solved with the introduction of charge-storage technology by the Hungarian engineer Kálmán Tihanyi at the beginning of 1925. His solution was a camera tube that accumulated and stored electrical charges (photoelectrons) within the tube throughout each scanning cycle. The device was first described in a patent application he filed in Hungary in March 1926 for a television system he dubbed Radioskop. After further refinements included in a 1928 patent application, Tihanyi's patent was declared void in Great Britain in 1930, and so he applied for patents in the United States. Tihanyi's charge storage idea remains a basic principle in the design of imaging devices for television to the present day.

In 1923, while employed by the Westinghouse Electric Corporation in Pittsburgh, Pennsylvania, Russian-born American engineer Vladimir Zworykin presented a project for a totally electronic television system to the company's general manager. In July 1925, Zworykin submitted a patent application titled Television System that included a charge storage plate constructed of a thin layer of isolating material (aluminum oxide) sandwiched between a screen (300 mesh) and a colloidal deposit of photoelectric material (potassium hydride) consisting of isolated globules. The following description can be read between lines 1 and 9 on page 2: "The photoelectric material, such as potassium hydride, is evaporated on the aluminum oxide, or other insulating media, and treated so as to form a colloidal deposit of potassium hydride consisting of minute globules. Each globule is very active photoelectrically and constitutes, to all intents and purposes, a minute individual photoelectric cell". Its first image was transmitted in the late summer of 1925, and a patent was issued in 1928. However the quality of the transmitted image failed to impress H.P. Davis, the general manager of Westinghouse, and Zworykin was asked: "to work on something useful". A patent for a television system was also filed by Zworykin in 1923, but this filing is not a definitive reference because extensive revisions were done before a patent was issued fifteen years later and the file itself was divided into two patents in 1931.

The first practical iconoscope was constructed in 1931 by Sanford Essig when he accidentally left a silvered mica sheet in the oven too long. Upon examination with a microscope, he noticed that the silver layer had broken up into a myriad of tiny isolated silver globules. He also noticed that "the tiny dimension of the silver droplets would enhance the image resolution of the iconoscope by a quantum leap". As head of television development at Radio Corporation of America (RCA), Zworykin submitted a patent application in November 1931, and it was issued in 1935. Nevertheless, Zworykin's team was not the only engineering group working on devices that used a charge storage plate. In 1932, the EMI engineers Tedham and McGee under the supervision of Isaac Shoenberg applied for a patent for a new device they dubbed the "Emitron". A 405-line broadcasting service employing the Emitron began at studios in Alexandra Palace in 1936, and patents were issued in the United Kingdom in 1934 and in the US in 1937.

The iconoscope was presented to the general public at a press conference in June 1933, and two detailed technical papers were published in September and October of the same year. Unlike the Farnsworth image dissector, the Zworykin iconoscope was much more sensitive, useful with illumination on the target between 4ft-c (43lx) and 20ft-c (215lx). It was also easier to manufacture and produced a very clear image. The iconoscope was the primary camera tube used by RCA broadcasting from 1936 until 1946 when it was replaced by the image orthicon tube.

Adapted from content published on wikipedia.org
  • Image By Vladimir Zworykin - US patent 2021907, Fig. 1, Public Domain — from wikimedia.org
Last modified on May 20, 2021, 6:53 pm
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