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Video camera tube
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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). Several types were in use from the 1930s to the 1980s. These tubes are a type of cathode ray tube.
Some clarification of terminology is in order. Any vacuum tube which operates using a focused beam of electrons is called a cathode ray tube or CRT. However, in the popular lexicon CRT is usually used to refer to the type of tube used as a television or computer monitor picture tube.
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Encyclopedia
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). Several types were in use from the 1930s to the 1980s. These tubes are a type of cathode ray tube.
Some clarification of terminology is in order. Any vacuum tube which operates using a focused beam of electrons is called a cathode ray tube or CRT. However, in the popular lexicon CRT is usually used to refer to the type of tube used as a television or computer monitor picture tube. The proper term for these display tubes is actually kinescope. Kinescopes are simply one of many types of cathode ray tubes. Others include the types of display tubes used in oscilloscopes, radar displays, and the camera pickup tubes described in this article. (In the interest of avoiding further confusion it will be noted that the word "kinescope" has an alternate meaning as it has become the popular name for a television film recording made by focusing a motion picture film camera onto the face of a kinescope cathode ray tube as was commonly done before the invention of video tape recording.)
Image dissector
The image dissector was invented by Philo Farnsworth, one of the pioneers of electronic television, in 1927. It is a type of cathode ray tube occasionally employed as a camera in industrial television systems. The image dissector had very poor light sensitivity, and so was useful only where scene illumination was high (typically over 685 cd/m˛); it was therefore ideal for high light levels such as in monitoring the bright, hot interior of an industrial furnace. Owing to its lack of sensitivity, the image dissector was rarely used in TV broadcasting, except to scan film and other transparencies. It represents, however, the beginning of the electronic TV age.
The image dissector views the outside world through a glass lens which focuses an image through the clear glass wall of the tube onto a special plate that is coated with a layer of caesium oxide. When light strikes caesium oxide, the material emits electrons in proportion to the intensity of the light (see photoelectric effect). These electrons are aimed and accelerated by electric and magnetic fields onto the dissector's electron detector so that only a small portion of the electron image reaches the detector at any given moment. The output from the detector is an electric current whose magnitude is a measure of the brightness of the corresponding point in the image. As time passes, the electron image is deflected back and forth and up and down so that the entire image, portion by portion, is read by the detector, which consequently produces a time-varying video signal.
The image dissector has no "storage characteristic": electrons that do not hit the single detector are wasted (rather than being stored on the target as in the image orthicon, described below), which accounts in part for its low sensitivity (approximately 3000 lux).
The iconoscope
In 1931, five years after Kálmán Tihanyi's electronic camera tube in 1926, Vladimir Zworykin 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. 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 down to about 75 lux. 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.
Image Orthicon
The image orthicon tube (often abbreviated as IO) was common in American broadcasting from 1946 until 1968. A combination of Farnsworth's image dissector and RCA's orthicon technologies, it replaced the iconoscope/orthicon, which required a great deal of light to work adequately.
The image orthicon tube was developed by Dr. Albert Rose, Paul K. Weimer, and Harold B. Law in the employ of the RCA. 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 orthicon cameras 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 IO 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, IO 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 IO 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 IO consists of three parts: an image store ("target"), a scanner that reads this image (an electron gun), and a multiplicative amplifier. In the image store, light falls upon a photosensitive plate, and is converted into an electron image (borrowed from Farnsworth's image dissector). These electrons ("rain") are then accelerated towards the target, causing a "splash" of electrons to be discharged (secondary electrons). Each image electron ejects, on average, more than one "splash" electron, and these excess electrons are soaked up by a positively-charged mesh very near and parallel to the target (the image electrons also pass through this mesh, whose positive charge also helps to accelerate the image electrons). 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 then scanned over the back side of the target. The electrons are slowed down just before reaching the target so that 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 rather than absorbed. These reflected electrons return down the cathode ray tube toward an electron detector (multiplicative amplifier) surrounding the electron gun. The number of reflected electrons is a measure of the target's original positive charge, which, in turn, is a measure of brightness.
In analogy with the image dissector, this beam of electrons is scanned around the target so that the image is read one small portion at a time.
Multiplicative amplification is also performed via the splashing of electrons: a stack of charged pinwheel-like disks surround the electron gun. As the returning electron beam hits the first pinwheel, it ejects electrons exactly like the target. These loose electrons are then drawn toward the next pinwheel back, 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 pinwheel. 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.
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-focussed 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 (sometimes called a hivicon 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 PK Weimer, SV Forgue and RR 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.
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, NASA used Vidicon camera on most of their unmanned deep space probes equipped with the remote sensing ability
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.
Compared to Saticons, Plumbicons had much higher resistance to burn in, and coma 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.
The company says, "In comparison to other image tube technologies, Plumbicon tubes offer high resolution, low lag and superior image quality."
http://www.nimaging.com/about/history.html
http://www.nimaging.com/products/tubes/index.html
http://www.nimaging.com/products/tubes/plumbicon_broadcast.html
Surface: PbO — Lead Oxide.
Saticon
Saticon is a registered trademark of Hitachi also produced by Thomson and Sony. Its surface consists of SeAsTe — Selenium Arsenic Tellurium.
Pasecon
Pasecon is a registered trademark of Heimann. Its surface consists of CdSe — Cadmium selenide.
Newvicon
Newvicon is a registered trademark of Matsushita. The Newvicon tubes were characterized by high light sensitivity. Its surface consists of ZnSe, ZnCdTe — Zinc Selenide, 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.
http://www.labguysworld.com/Sony_DXC-1600.htm for a more detailed explanation of the Trinicon tube.
Technological obsolescence
For television camera uses, the vidicon has been technologically superseded by the CCD and CMOS.
See also
External links
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