Video camera tube
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.
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 kinescope. Kinescopes are 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. (The word "kinescope" has also 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
Alan Archibald Campbell-Swinton was maybe the first person in describing an all-electronic video camera tube in 1908.[1] One of the first all-electronic cameras was invented by Edvard-Gustav Schoultz in 1921 (patent issued in 1922).[2]
The image dissector was nevertheless the first practical all-electronic video camera tube filed for patent by Dieckmann and Hell in 1925 (patent issued in 1927)[3] and filed again for patent and demostrated by Philo Farnsworth in 1927 (patent issued in 1930)[4]. 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/m²); 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 Vladimir Zworykin, head of television development at Radio Corporation of America (RCA), 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.[6] 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.[7][8]
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.[9] 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.[10][11] RCA began production of image orthicon cameras for civilian use in the second quarter of 1946.[12]
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: 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-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.
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.
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.
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.
References
- ^ A. A. CAMPBELL SWINTON (1908) Distant Electric Vision
- ^ Edvard-Gustav Schoultz (filed 1921, patented 1922). "Procédé et appareillage pour la transmission des images mobiles à distance". Patent No. FR 539,613,. Office National de la Propriété industrielle. Retrieved 2009-07-28.
{{cite web}}: Check date values in:|date=(help)CS1 maint: extra punctuation (link) - ^ Dr. Max Dieckmann und Dipl. Ing. Rudolf Hell (filed 1925, patented 1927). "Lichtelektrische Bildzerlegerroehre fuer Fernseher". Patent No. DE 450,187,. Deutsches Reich Reichspatentamt. Retrieved 2009-07-28.
{{cite web}}: Check date values in:|date=(help)CS1 maint: extra punctuation (link) - ^ Philo T. Farnsworth (filed 1927, patented 1930). "Television System". Patent No. 1,773,980,. United States Patent Office. Retrieved 2009-07-28.
{{cite web}}: Check date values in:|date=(help)CS1 maint: extra punctuation (link) - ^ "Kálmán Tihanyi’s 1926 Patent Application 'Radioskop'", Memory of the World, United Nations Educational, Scientific and Cultural Organization (UNESCO), 2005, retrieved 2009-01-29.
- ^ "Kálmán Tihanyi (1897-1947)", IEC Techline, International Electrotechnical Commission (IEC), 2009-07-15.
- ^ "R.C.A. Officials Continue to Be Vague Concerning Future of Television", The Washington Post, November 15, 1936, p. B2.
- ^ Albert Abramson, The History of Television, 1942 to 2000, McFarland, 2003, p. 18. ISBN 0786412208.
- ^ Albert Abramson, The History of Television, 1942 to 2000, McFarland, 2003, p. 124. ISBN 9780786412204.
- ^ Remington Rand Inc., v. U.S., 120 F.Supp. 912, 913 (1944).
- ^ Albert Abramson, The History of Television, 1942 to 2000, McFarland, 2003, p. 7–8. ISBN 0786412208.
- ^ Abramson, p. 18.