Iconoscope

The Iconoscope (from the Greek: εἰκών "image" and σκοπεῖν "to look, to see") was the name given to an early television camera tube in which a beam of high-velocity electrons scans a mosaic of photoemissive isolated granules. 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 Vladimir 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 Iconoscope cameraused for the historical TV transmission at the Olympic Games in Berlin 1936.The Iconoscope was the leading camera tube used for broadcasting in the United States from 1936 until 1946, when it was replaced by the image orthicon tube.

                                                              

Operation

               Within the iconoscope, an image was projected onto a plate containing a mosaic of electrically isolated photosensitive granules separated from a common plate by a thin layer of isolating material, each granule constituting a tiny capacitor with the common plate that accumulated and stored electrical charge in response to the light striking it. Emission of photoelectrons from each granule in proportion to the amount of light received resulted in a charge image being formed on the mosaic. An electron beam was then swept across the image plate from an electron gun, effectively scanning the stored image and discharging each capacitor in turn such that the electrical output from each capacitor was proportional to the average intensity of the light striking it between each discharge event.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

            In July 1925, Zworykin submitted a patent application for a "Television System" that includes 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 in page 2: The photoelectric material, such as potassium hydride, is evaporated on the aluminum oxide, or other insulating medium, 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 late summer of 1925,and a patent was issued in 1928. However the quality of the transmitted image failed to impress to H P Davis, the general manager of Westinghouse, and Zworykin was a asked to work on something useful.A patent for a television system was also filed by Zworykin in 1923, but this file is not a reliable bibliographic source because extensive revisions were done before a patent was issued fifteen years laterand the file itself was divided into two patents in 1931.In 1926, the Hungarian engineer Kálmán Tihanyi explained in detail that the principle of "storing" electrical charges in proportion to the amount of light received throughout each scanning cycle results in a much more sensitive video camera tube.Although his 1926 application was never acted upon,two year later, in 1928, Tihanyi applied for a patent for a refined "Television Apparatus" that is essentially an iconoscope.The first practical iconoscope was constructed in 1931 by Sanford Essig, when he accidentally left one 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 use a charge stage plate. In 1932, 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 a patent was issued in the USA in 1937.One year latter, in 1933, Philo Farnsworth also applied for a patent for a device that use a charge storage plate and a low-velocity electron scanning beam, a patent was issued in 1937, but Farnsworth did not know that the low-velocity scanning beam must land perpendicular to the target and he never actually built such a tube.