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In older video cameras, before the mid to late 1980s, a video camera tube or pickup tube was used instead of a charge-coupled device (CCD) for converting a video image into an electrical signal. Several types were in use from the 1930s to the 1980s. These tubes are a type of cathode ray tube or "CRT".

vidicon tube (2/3 inch in diameter)


Any vacuum tube which operates using a focused beam of electrons ("cathode rays") is known as a cathode ray tube. However, in the popular lexicon "CRT" usually refers to the "picture tube" in a television or computer monitor. The proper term for this type of display tube is kinescope, only one of many types of cathode ray tubes. Others include the tubes used in oscilloscopes, radar displays, and the camera pickup tubes described in this article. (The word "kinescope" has also become the popular name for a film recording made by focusing a motion picture camera onto the face of a kinescope cathode ray tube, a common practice before the advent of video tape recording.)

Early steps

In June 1908, the scientific journal Nature published an article in which Alan Archibald Campbell-Swinton, a fellow of the Royal Society (UKmarker), discussed how a fully electronic television system could be realized by using cathode ray tubes as both imaging and display devices. But Campbell-Swinton noted that the "real difficulties lie in devising an efficient transmitter", and that it was possible that "no photoelectric phenomenon at present known will provide what is required".A description of a CRT imaging device appeared in a patent application filed by Edvard-Gustav Schoultz in Francemarker in August 1921, and published in 1922, although a working device was not demonstrated until some years later.

Image dissector

An image dissector is 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 Germanmarker 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 Americanmarker inventor Philo Farnsworth.

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 field.

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 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 an "electron multiplier" in 1930 made Farnsworth's image dissector the first practical version of a fully electronic imaging device (or "rasterizer") for television. Unfortunately, it had very poor light sensitivity, and was therefore primarily useful only where illumination was exceptionally high (typically over 685 cd/).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.

The optical network of the image dissector focuses an image on the plate of a photoelectric cell. As light strikes the cell, electrons are emitted in proportion to the intensity of the light (see photoelectric effect). The entire electron image is deflected and the scanning aperture permits only those electrons emanating from a very small area of the plate 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 such that the entire image is read by the detector several time 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 photoelectric cell 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.

Much of the problems in the early Image Dissector system were addressed and fixed in a second generation model, patented in 1933. This system did see wide use, and influenced the design of the Image Orthicon systems discussed here (citations in original Image Dissector article).

The iconoscope

In 1931 Vladimir Zworykin, head of television development at Radio Corporation of America , filed for a patent on a camera tube that projected an image on a special plate on which was set a mosaic of photosensitive material, a pattern comparable to the receptors of the human eye. The design was largely based on the pioneering work of Hungarian engineer Kálmán Tihanyi, whose patents RCA was then in negotiations to acquire, and employed the principle of "storage" of electrical charges throughout each scanning cycle. Emission of photoelectrons from each granule in proportion to the amount of light received resulted in a charge image being formed on the mosaic. Each granule, together with the conductive plate behind the mosaic, formed a small capacitor, all of these having a common plate. An electron beam was then swept across the image plate from an electron gun, discharging the capacitors in succession; the resulting changes in potential at the metal plate constituted the picture signal. Unlike the Farnsworth image dissector, the Zworykin model 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 in American broadcasting from 1936 until 1946, when it was replaced by the image orthicon tube.

Operation

The image entered through the series of lenses at upper right, and was projected onto a photosensitive surface. The mosaic of photosensitive elements emitted an electric charge in variance with the amount of light hitting them. The cathode ray at the right swept the image plate, discharging the electrostatic charges. The successive discharges from the image plate were carried out the left side of the tube and amplified.

Orthicon

The orthicon was one of three important tube types developed at RCA by Albert Rose and his colleagues, along with the image orthicon and the vidicon. The orthicon, developed by Rose in 1937, was the tube used in RCA's television demonstration at the 1939 New York World's Fair.

Image orthicon

Schematic of image orthicon tube.


The image orthicon, (sometimes abbreviated IO) was common in American broadcasting from 1946 until 1968. A combination of the Image Dissector and the orthicon technologies, it replaced the iconoscope and the orthicon, which required a great deal of light to work adequately.

The image orthicon tube was developed at RCA by Albert Rose, Paul K. Weimer, and Harold B. Law. It represented a considerable advance in the television field, and after further development work, RCA created original models between 1939 and 1940. The National Defense Research Council entered into a contract with RCA where the NDRC paid for its further development. Upon RCA's development of the more sensitive image orthicon tube in 1943, RCA entered into a production contract with the U.S. Navy, the first tubes being delivered in January 1944. RCA began production of image orthicons for civilian use in the second quarter of 1946.

While the iconoscope and the intermediate orthicon used capacitance between a multitude of small but discrete light sensitive collectors and an isolated signal plate for reading video information, the image orthicon employed direct charge readings from a continuous electronically charged collector. The resultant signal was immune to most extraneous signal "crosstalk" from other parts of the target, and could yield extremely detailed images. For instance, image orthicon cameras were used for capturing Apollo/Saturn rockets nearing orbit after the networks had phased them out, as only they could provide sufficient detail.

An image orthicon camera can take television pictures by candlelight because of the more ordered light-sensitive area and the presence of an electron multiplier at the base of the tube, which operated as a high-efficiency amplifier. It also has a logarithmic light sensitivity curve similar to the human eye. However, it tends to flare in bright light, causing a dark halo to be seen around the object; this anomaly is referred to as "blooming" in the broadcast industry when image orthicon tubes were in operation. Image orthicons were used extensively in the early color television cameras, where their increased sensitivity was essential to overcome their very inefficient optical system.

Operation

An image orthicon consists of three parts: a photocathode with an image store ("target"), a scanner that reads this image (an electron gun), and a multiplicative amplifier.

In the image store, light falls upon the photocathode which is a photosensitive plate at a very negative potential (approx. -600V), and is converted into an electron image (borrowed from Farnsworth's image dissector). This electron rain is then accelerated towards the target (a very thin glass plate acting as a semi-isolator) at ground potential (0V), and pass through a very fine wire mesh (near 200 wires per cm), very near (a few hundredths of cm) and parallel to the target, acting as a screen grid at a slightly positive voltage (approx +2V). Once the image electrons reach the target, they cause a "splash" of electrons by the effect of secondary emission. On average, each image electron ejects several "splash" electrons (thus adding amplification by secondary emission), and these excess electrons are soaked up by the positive mesh effectively removing electrons from the target and causing a positive charge on it in relation to the incident light in the photocathode. The result is an image painted in positive charge, with the brightest portions having the largest positive charge.

A sharply focused beam of electrons (a cathode ray) is generated by the electron gun at ground potential and accelerated by the anode (the first dynode of the electron multiplier) around the gun at a high positive voltage (approx. +1500V). Once it exits the electron gun, its inertia makes the beam move away from the dynode towards the back side of the target. At this point the electrons lose speed and get deflected by the horizontal and vertical deflection coils, effectively scanning the target. Thanks to the axial magnetic field of the focusing coil, this deflection is not in a straight line, thus when the electrons reach the target they do so perpendicularly avoiding a sideways component. The target is nearly at ground potential with a small positive charge, thus when the electrons reach the target at low speed they are absorbed without ejecting more electrons. This adds negative charge to the positive charge until the region being scanned reaches some threshold negative charge, at which point the scanning electrons are reflected by the negative potential rather than absorbed (in this process the target recovers the electrons needed for the next scan). These reflected electrons return down the cathode ray tube toward the first dynode of the electron detector (multiplicative amplifier) surrounding the electron gun which is at high potential. The number of reflected electrons is a linear measure of the target's original positive charge, which, in turn, is a measure of brightness.

Additional amplification is also performed via secondary emission in the electron multiplier which consists of a stack of charged dynodes (pinwheel-like disks surround the electron gun) in progressively higher potentials. As the returning electron beam hits the first dynode, it ejects electrons similarly to the target. These secondary electrons are then drawn toward the next dynode at a higher potential, where the splashing continues for a number of steps. Consider a single, highly-energized electron hitting the first stage of the amplifier, causing 2 electrons to be emitted and drawn towards the next dynode. Each of these might then cause two each to be emitted. Thus, by the start of the third stage, you would have four electrons to the original one. As many as 5 to 10 stages were not unusual, thus the achieved amplification is very important.

Overall, the amplification at the image front and at the electron multiplier, plus the wise use of secondary emission wherever possible make the Image Orthicon an excellent camera tube, with a typical illumination on photocathode for maximum signal output of 0.01ft-c (0.1lx), what places it in the order of a thousand times more sensitive than the iconoscope.

Dark halo

The mysterious "dark halo" around bright objects in an IO-captured image is based in the very fact that the IO relies on the splashing caused by highly energized electrons. When a very bright point of light (and therefore very strong electron stream emitted by the photosensitive plate) is captured, a great preponderance of electrons is ejected from the image target. So many are ejected that the corresponding point on the collection mesh can no longer soak them up, and thus they fall back to nearby spots on the target much as splashing water when a rock is thrown in forms a ring. Since the resultant splashed electrons do not contain sufficient energy to eject enough electrons where they land, they will instead neutralize any positive charge in that region. Since darker images result in less positive charge on the target, the excess electrons deposited by the splash will be read as a dark region by the scanning electron beam.

This effect was actually "cultivated" by tube manufacturers to a certain extent, as a small, carefully-controlled amount of the dark halo has the effect of "crispening" the viewed image. (That is, giving the illusion of being more sharply-focused than it actually is). The later Vidicon tube and its descendants (see below) do not exhibit this effect, and so could not be used for broadcast purposes until special "detail correction" circuitry could be developed.

Vidicon

A vidicon tube is a video camera tube design in which the target material is a photoconductor. The Vidicon was developed in the 1950s at RCA by P. K. Weimer, S. V. Forgue and R. R. Goodrich as a simple alternative to the structurally and electrically complex Image Orthicon. While the initial photoconductor used was Selenium, other targets—including silicon diode arrays—have been used.

Schematic of vidicon tube.


The vidicon is a storage-type camera tube in which a charge-density pattern is formed by the imaged scene radiation on a photoconductive surface which is then scanned by a beam of low-velocity electrons. The fluctuating voltage coupled out to a video amplifier can be used to reproduce the scene being imaged. The electrical charge produced by an image will remain in the face plate until it is scanned or until the charge dissipates. Pyroelectric photocathodes can be used to produce a vidicon sensitive over a broad portion of the infrared spectrum.

Prior to the design and construction of Galileo probe to Jupiter in the late 70s, NASAmarker used Vidicon camera on most of their unmanned deep space probes equipped with the remote sensing ability.

Vidicon tubes are notable for a particular type of interference they suffered from, known as vidicon microphony. Since the sensing surface is quite thin, it is possible to bend it with loud noises. The artifact is characterized by a series of many horizontal bars evident in any footage (mostly pre 1990) in an environment where loud noise was present at the time of recording or broadcast. A studio where a loud rock band was performing or even gunshots or explosions would create this artifact.

Plumbicon

Plumbicon is a registered trademark of Philips for its Lead Oxide target vidicons. Used frequently in broadcast camera applications, these tubes have low output, but a high signal-to-noise ratio. They had excellent resolution compared to Image Orthicons, but lacked the artificially sharp edges of IO tubes, which caused some of the viewing audience to perceive them as softer. CBS Labs invented the first outboard edge enhancement circuits to sharpen the edges of Plumbicon generated images.

Schematic of plumbicon tube.


Compared to Saticons, Plumbicons had much higher resistance to burn in, and comet and trailing artifacts from bright lights in the shot. Saticons though, usually had slightly higher resolution. After 1980, and the introduction of the diode gun plumbicon tube, the resolution of both types was so high, compared to the maximum limits of the broadcasting standard, that the Saticon's resolution advantage became moot. While broadcast cameras migrated to solid state Charged Coupled Devices, plumbicon tubes remain a staple imaging device in the medical field.

Narragansett Imaging is the only company now making Plumbicons, and it does so from the factories Philips built for that purpose in Rhode Island, USA. While still a part of the Philips empire, the company purchased EEV's (English Electric Valve) lead oxide camera tube business, and gained a monopoly in lead oxide tube production.

Saticon

Saticon is a registered trademark of Hitachi also produced by Thomson and Sony. Its surface consists of Selenium Arsenic Tellurium .

Pasecon

Pasecon is a registered trademark of Heimann. Its surface consists of Cadmium Selenide .

Newvicon

Newvicon is a registered trademark of Matsushita. The Newvicon tubes were characterized by high light sensitivity. Its surface consists of a combination of Zinc Selenide and Zinc Cadmium Telluride .

Trinicon

Trinicon is a registered trademark of Sony. It uses a vertically striped RGB color filter over the faceplate of the imaging tube to segment the scan into corresponding red, green and blue segments. Only one tube was used in the camera, instead of a tube for each color, as was standard for color cameras used in television broadcasting. It is used mostly in low-end consumer cameras and camcorders, though Sony also used it in some moderate cost professional cameras in the 1980s, such as the DXC-1800 and BVP-1 models.

Technological obsolescence

For television camera uses, the vidicon has been technologically superseded by the CCD and CMOS.

See also



References

  1. "Cathode-ray tube", McGraw-Hill Concise Encyclopedia of Science & Technology, Third Ed., Sybil P. Parker, ed., McGraw-Hill, Inc., 1992, pp. 332-333.
  2. Abramson, Albert, The History of Television, 1942 to 2000, McFarland, 2003, p. 26. ISBN 0786412208.
  3. Horowitz, Paul and Winfield Hill, The Art of Electronics, Second Edition, Cambridge University Press, 1989, pp. 1000-1001. ISBN 0521370957.
  4. Farnsworth, Elma, Distant Vision: Romance and Discovery on an Invisible Frontier, Salt Lake City, PemberlyKent, 1989, pp. 108-109.
  5. Abramson, Albert (1987), The History of Television, 1880 to 1941. Jefferson, NC: Albert Abramson. p. 159. ISBN 0-89950-284-9.
  6. "Kálmán Tihanyi (1897-1947)", IEC Techline, International Electrotechnical Commission (IEC), 2009-07-15.
  7. "Kálmán Tihanyi’s 1926 Patent Application 'Radioskop'", Memory of the World, United Nations Educational, Scientific and Cultural Organization , 2005, retrieved 2009-01-29.
  8. "R.C.A. Officials Continue to Be Vague Concerning Future of Television", The Washington Post, 1936-11-15, p. B2.
  9. Abramson, Albert, The History of Television, 1942 to 2000, McFarland, 2003, p. 18. ISBN 0786412208.
  10. Abramson, Albert, The History of Television, 1942 to 2000, McFarland, 2003, p. 124. ISBN 0786412208.
  11. Abramson, Albert, The History of Television, 1942 to 2000, McFarland, 2003, pp. 7–8. ISBN 0786412208.
  12. Remington Rand Inc., v. U.S., 120 F. Supp. 912, 913 (1944).
  13. Narragansett Imaging > about > history.
  14. Narragansett Imaging > products > tubes > index.
  15. Narragansett Imaging > products > tubes > plumbicon_broadcast.
  16. "Sony DXC-1600", LabGuysWorld.com.


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