Philo Farnsworth Image Dissector Vacuum Tube
January 24, 2012 by staff
Philo Farnsworth Image Dissector Vacuum Tube, An image dissector, also called a dissector tube, is a video camera tube in which photocathode emissions create an “electron image” which is then scanned to produce an electrical signal representing the visual image. The term may apply specifically to a dissector tube employing magnetic fields to keep the electron image in focus, and an electron multiplier to scan the electrons.
Among the first to design such a device were German inventors Max Dieckmann and Rudolf Hell, who had titled their 1925 patent application Lichtelektrische Bildzerlegerr?hre für Fernseher (Photoelectric Image Dissector Tube for Television), and American television pioneer Philo Farnsworth.
An image dissector focuses a visual image onto a layer of photosensitive material, such as caesium oxide, which emits negatively-charged “photoelectrons” proportional to the intensity of the light striking the material. Electrostatic deflecting plates or magnetic fields then periodically manipulate the resulting electron image horizontally and vertically before an electron multiplier, or a small aperture leading to a positively-charged detector or “anode” in the case of the earliest dissector tubes. The electron multiplier or aperture permits only those electrons emanating from a very small area of the electron image, representing a similarly small area of the visual image. The entire image is scanned several times per second to produce an electrical signal suitably representative of a moving visual image.
Dieckmann and Hell submitted their application to the German patent office in April 1925, and a patent was issued in October 1927. In 1951, Hell claimed that he had made a tube but could not get it to function, since at the time there was an insufficient knowledge of “electron optics”, the manipulation of an electron beam by electric or magnetic fields.
Farnsworth submitted a patent application for the image dissector on January 7, 1927. On September 7 of that year, the image dissector successfully transmitted its first image, a simple straight line, at Farnsworth’s laboratory at 202 Green Street in San Francisco. By September 3, 1928, Farnsworth had developed the system sufficiently to hold a demonstration for the press, the first such successful demonstration of a fully electronic television system.
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