Image dissector

An image dissector is a camera tube that creates an "electron image" of a scene from photocathode emissions (electrons) which pass through a scanning aperture to an anode, which serves as an electron detector.Among the first to design such a device were German inventors Max Dieckmann and Rudolf Hell,who had titled their 1925 patent application Lichtelektrische Bildzerlegerröhre für Fernseher (Photoelectric Image Dissector Tube for Television).The term may apply specifically to a dissector tube employing magnetic fields to keep the electron image in focus,an element lacking in Dieckmann and Hell's design, and in the early dissector tubes built by American inventor Philo FarnsworthDieckmann and Hell submitted their application to the German patent office in April 1925, and a patent was issued in October 1927.Their experiments on the image dissector were announced in the May 1928 issue of the magazine Popular Radio.Hell also claimed in 1951 that he had made a tube but could not get it to function properly, since at the time there was an insufficient knowledge of "electron optics" – the manipulation of an electron beam by electric or magnetic fields.

       In January 1927, Farnsworth applied for a patent for his Television System that included a device for "the conversion and dissecting of light". Its first moving image was successfully transmitted on September 7 of 1927,and a patent was issued in 1930. Farnsworth quickly made improvements to the device, among them introducing an electron multiplier made of nickel and deploying a "longitudinal magnetic field" in order to sharply focus the electron image.The improved device was demonstrated to the press in early September 1928.The introduction of a multipactor in October 1933 and a multi-dynode "electron multiplier" in 1937 made Farnsworth's image dissector the first practical version of a fully electronic imaging device for television.



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Unfortunately, it had very poor light sensitivity, and was therefore primarily useful only where illumination was exceptionally high (typically over 685 cd/m²). However, it was ideal for industrial applications, such as monitoring the bright interior of an industrial furnace. Due to their poor light sensitivity, image dissectors were rarely used in television broadcasting, except to scan film and other transparencies.

                          In April 1933, Farnsworth submitted a patent application entitled Image Dissector, but which actually detailed a CRT-type camera tube, apparently the first to propose the use of a "low-velocity" scanning beam.However, he never transmitted a clear and well focused image with such a tube.

          The optical system of the image dissector focuses an image onto a photocathode mounted inside a high vacuum. As light strikes the photocathode, electrons are emitted in proportion to the intensity of the light (see photoelectric effect). The entire electron image is deflected and a scanning aperture permits only those electrons emanating from a very small area of the photocathode to be captured by the detector at any given time. The output from the detector is an electric current whose magnitude is a measure of the brightness of the corresponding area of the image. The electron image is periodically deflected horizontally and vertically ("raster scanning") such that the entire image is read by the detector many times per second, producing an electrical signal that can be conveyed to a display device, such as a CRT monitor, to reproduce the image.The image dissector has no "charge storage" characteristic; the vast majority of electrons emitted by the photocathode are excluded by the scanning aperture,] and thus wasted rather than being stored on a photo-sensitive target, as in the iconoscope or image orthicon (see below), which largely accounts for its low light sensitivity.