The iconoscope

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.In Tihanyi's version of 1928, the electron beam scanned the granules,while in Zworykin's version of 1925, the electron beam scanned the back of the image plate.

         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 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 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. Two year later, in 1928, he applied for a patent for a refined "Television Apparatus" that is essentially an iconoscope.


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            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, 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 USA in 1937.



The iconoscope was presented 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.Unlike the Farnsworth image dissector, the Zworykin iconoscope was much more sensitive, useful with an 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.