An image dissector is video camera tube in which photocathode emissions create an "electron image" which is then scanned to produce an electrical signal representing the actual image.[1]
The first working Image Dissector was designed and built by American television pioneer Philo Farnsworth.
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Operation
An Image Dissector focuses an 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 periodically manipulate the resulting electron image horizontally and vertically before a small aperture leading to a positively-charged detector or "anode". The aperture permits only those electrons emanating from a very small area of the electron image, representing a similarly small area of the actual image, to pass through to the anode. The entire image is scanned several times per second to produce an electrical signal suitably representative of a moving image.
History
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.[2][3] By September 3, 1928, Farnsworth had developed the system sufficiently to hold a demonstration for the press,[3] the first such successful demonstration of a fully electronic television system. In 1929 Farnsworth eliminated a motor generator from the system, so it then had no mechanical parts. Further developments that year included improvements in image clarity and an increase in the number of lines of resolution, such that it exceeded that of the mechanical television systems.[4]
Also in 1929, Farnsworth transmitted the first live human images with his system, including a three and a half-inch image of his wife Elma ("Pem") with her eyes closed (possibly due to the bright lighting required).[5]
On August 25, 1934, Farnsworth gave the world's first public demonstration of a complete, all-electronic television system at the Franklin Institute in Philadelphia, Pennsylvania.[6][7]
Improvements
Since the electrons emitted within the Image Dissector were collected by the anode only during the very brief time an area of the "electron image" passed over the aperture, the bulk of the electrons were lost. Thus the original Image Dissector was very inefficient, and extremely bright illumination was required for it to be used effectively. Farnsworth overcame this difficulty with the invention of the "multipactor", a unique device he began work on in 1930, and demonstrated in 1931.[8][9] This small tube reportedly could amplify a signal to the 60th power or better,[10] and showed great promise in other fields of electronics. An initial problem with the multipactor was that it wore out at an unsatisfactorily rapid rate.[11] However, as the first-generation Farnsworth Image Dissector was commercially produced for applications where extremely bright illumination existed, such as monitoring the interior of industrial furnaces, second generation dissectors equipped with Electron Multipliers, as they came to be known, were in demand by RCA for their unique technology. RCA leased the technology from Farnsworth in 1939 and incorporated it into their future television systems.[12][13]
Farnsworth submitted a patent application for a second-generation Image Dissector in 1933, based on what he learned working with his original designs and the multipactor.[14] The new Image Dissector — actually a cathode-ray tube (CRT), and the first to use a low velocity electron scanning beam[14] — would become the ancestor of the Image Orthicon tube, introduced in 1946, long after Farnsworth and RCA had established a mutual business relationship,[15] and may have significantly influenced the design of RCA's Iconoscope, introduced in 1934.[citation needed]
References
- ^ Horowitz, Paul and Winfield Hill, The Art of Electronics, Second Edition, Cambridge University Press, 1989, pp. 1000-1001. ISBN 0521370957.
- ^ Postman, Neil, "Philo Farnsworth", The TIME 100: Scientists & Thinkers, TIME.com, 1999-03-29, retrieved 2009-07-28.
- ^ a b "Philo Taylor Farnsworth (1906-1971)", The Virtual Museum of the City of San Francisco, retrieved 2009-07-15.
- ^ Abramson, Albert, Zworykin, Pioneer of Television, p. 226.
- ^ The Philo T. and Elma G. Farnsworth Papers
- ^ "New Television System Uses 'Magnetic Lens'", Popular Mechanics, Dec. 1934, p. 838–839.
- ^ Burns, R. W. Television: An international history of the formative years. (1998). IEE History of Technology Series, 22. London: IEE, p. 370. ISBN 0-85296-914-7.
- ^ Abramson, Albert (1987), The History of Television, 1880 to 1941. Jefferson, NC: Albert Abramson. p. 148. ISBN 0-89950-284-9.
- ^ Everson, George (1949), The Story of Television, The Life of Philo T. Farnsworth New York, NY: W. W. Norton & Co,. ISBN 978-0405060427, pages 137-141.
- ^ Everson, George (1949), The Story of Television, The Life of Philo T. Farnsworth New York, NY: W. W. Norton & Co,. ISBN 978-0405060427, page 139.
- ^ Everson, George (1949), The Story of Television, The Life of Philo T. Farnsworth New York, NY: W. W. Norton & Co,. ISBN 978-0405060427, page 141.
- ^ Abramson, Albert (1987), The History of Television, 1880 to 1941. Jefferson, NC: Albert Abramson. p. 254. ISBN 0-89950-284-9.
- ^ The Cathode Ray Tube Site - Scroll To Image Orthicon, Electron Multiplier Hilighted
- ^ a b Abramson, Albert (1987), The History of Television, 1880 to 1941. Jefferson, NC: Albert Abramson. p. 196. ISBN 0-89950-284-9.
- ^ Schatzkin, Paul (1977, 2001), "Who Invented What -- and When??", The Farnsworth Chronicles. Retrieved 2009-11-29.
External links
- The Farnovision - history of Philo Farnsworth and invention of the Image Dissector
- The Philo T. and Elma G. Farnsworth Papers
- Farnsworth's Image Dissector
- First U.S. patent ever issued for a television system. Filed in 1927, issued 1930.
- Patent for Multipactor device. Filed in 1930, issued 1934.
- Dissector Target. Filed in 1930, issued 1933.
- Second Generation Image Dissector, First ever patent on a low velocity system. Precursor to Iconoscope and Image Orthicon systems. Filed in 1933, issued 1937.
See also
Further reading
- [Abramson, Albert (1987), The History of Television, 1880 to 1941. Jefferson, NC: Albert Abramson. p. 195. ISBN 0-89950-284-9.]
- [Everson, George (1949), The Story of Television, The Life of Philo T. Farnsworth New York, NY: W. W. Norton & Co,. ISBN 0-405060-42-7, 266 pages.]
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