Video camera tube
Encyclopedia
In older video camera
s, before the mid to late 1980s, a video camera tube or pickup tube was used instead of a charge-coupled device
(CCD) for converting an optical image into an electrical signal. Several types were in use from the 1930s to the 1980s. The most commercially successful of these tubes were various types of cathode ray tube
s or "CRTs".
Any vacuum tube which operates using a focused beam of electrons ("cathode ray
s") 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.)
Video camera tubes typically had a certain maximum brightness tolerance. If that limit were exceeded, such as by pointing the camera at the sun
, sun-reflecting shiny surfaces, or extremely bright point light sources, the tube detecting surface would instantly "burn out" and be rendered insensitive on part or all of the screen. The only remedy was replacing the video tube.
published a letter in which Alan Archibald Campbell-Swinton
, fellow of the Royal Society
(UK), discussed how a fully electronic television system could be realized by using cathode ray tubes (or "Braun" tubes, after its inventor, Karl Braun) as both imaging and display devices. He 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 cathode ray tube was successfully demonstrated as a displaying device by the German
Professor Max Dieckmann in 1906, his experimental results were published by the journal Scientific American
in 1909. Campbell-Swinton later expanded on his vision in a presidential address given to the Röntgen Society in November 1911. The photoelectric screen in the proposed transmitting device was a mosaic of isolated rubidium cubes. His concept for a fully electronic television system was later popularized by Hugo Gernsback as the "Campbell-Swinton Electronic Scanning System" in the August 1915 issue of the popular magazine Electrical Experimenter.
In a letter to Nature
published in October 1926, Campbell-Swinton also announced the results of some "not very successful experiments" he had conducted with G. M. Minchin and J. C. M. Stanton. They had attempted to generate an electrical signal by projecting an image onto a selenium-coated metal plate that was simultaneously scanned by a cathode ray beam
. These experiments were conducted before March 1914, when Minchin died, but they were later repeated by two different teams in 1937, by H. Miller and J. W. Strange from EMI
, and by H. Iams and A. Rose from RCA
. Both teams succeeded in transmitting "very faint" images with the original Campbell-Swinton's selenium-coated plate, but much better images were obtained when the metal plate was covered with zinc sulphide or selenide, or with aluminum or zirconium oxide treated with caesium. These experiments are the base of the future vidicon. A description of a CRT imaging device also appeared in a patent application filed by Edvard-Gustav Schoultz in France
in August 1921, and published in 1922, although a working device was not demonstrated until some years later.
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 Farnsworth
.
Dieckmann 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 volume 8 (September 1927) of the popular magazine Discovery and in the May 1928 issue of the magazine Popular Radio. However, they never transmitted a clear and well focused image with such a tube.
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. 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 also 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.
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 scan
ning") 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.
" plate containing a mosaic of electrically isolated photosensitive granules separated from a common plate by a thin layer of isolating material, somewhat analogous to the human eye
's retina
and its arrangement of photoreceptors. Each photosensitive granule constitutes a tiny capacitor that accumulates and stores electrical charge in response to the light striking it. An electron beam periodically sweeps across the plate, effectively scanning the stored image and discharging each capacitor in turn such that the electrical output from each capacitor is proportional to the average intensity of the light striking it between each discharge event. In Tihanyi's version of 1928, the electron beam scanned the granules, while in Zworykin's version of 1925, the electron beam scanned the back of the image plate.
In July 1925, Zworykin submitted a patent application titled Television System that included a charge storage plate constructed of a thin layer of isolating material (aluminum oxide) sandwiched between a screen (300 mesh) and a colloidal deposit of photoelectric material (potassium hydride) consisting of isolated globules. The following description can be read between lines 1 and 9 in page 2: "The photoelectric material, such as potassium hydride, is evaporated on the aluminum oxide, or other insulating medium, and treated so as to form a colloidal deposit of potassium hydride consisting of minute globules. Each globule is very active photoelectrically and constitutes, to all intents and purposes, a minute individual photoelectric cell". Its first image was transmitted in late summer of 1925, and a patent was issued in 1928. However the quality of the transmitted image failed to impress to H. P. Davis, the general manager of Westinghouse
, and Zworykin was asked "to work on something useful".
A patent for a television system was also filed by Zworykin in 1923, but this file is not a reliable bibliographic source because extensive revisions were done before a patent was issued fifteen years later and the file itself was divided into two patents in 1931. In 1926, the Hungarian engineer Kálmán Tihanyi
explained in detail that the principle of "storing" electrical charges in proportion to the amount of light received throughout each scanning cycle results in a much more sensitive video camera tube. Two year later, in 1928, he applied for a patent for a refined "Television Apparatus" that is essentially an iconoscope.
The first practical iconoscope was constructed in 1931 by Sanford Essig, when he accidentally left one silvered mica sheet in the oven too long. Upon examination with a microscope, he noticed that the silver layer had broken up into a myriad of tiny isolated silver globules. He also noticed that, "the tiny dimension of the silver droplets would enhance the image resolution of the iconoscope by a quantum leap." As head of television development at Radio Corporation of America (RCA), Zworykin submitted a patent application in November 1931, and it was issued in 1935. Nevertheless, Zworykin's team was not the only engineering group working on devices that use a charge stage plate. In 1932, the EMI
engineers Tedham and McGee under the supervision of Isaac Shoenberg
applied for a patent for a new device they dubbed "the Emitron". A 405-line broadcasting service employing the Emitron began at studios in Alexandra Palace
in 1936, and patents were issued in the United Kingdom in 1934 and in the USA in 1937.
The iconoscope was presented to the general public in a press conference in June 1933, and two detailed technical papers were published in September and October of the same year. Unlike the Farnsworth image dissector, the Zworykin iconoscope 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 by RCA broadcasting from 1936 until 1946, when it was replaced by the image orthicon tube.
, effectively scanning the stored image and discharging each capacitor in turn such that the electrical output from each capacitor was proportional to the average intensity of the light striking it between each discharge event. The accumulation and storage of photoelectric charges during each scanning cycle greatly increased the electrical output of the iconoscope relative to non-storage type image scanning devices.
team under the supervision of Isaac Shoenberg
analyzed how the Emitron (or iconoscope) produces an electronic signal and concluded that its real efficiency was only about 5% of the theoretical maximum. This is because secondary electrons
released from the mosaic of the charge storage plate when the scanning beam sweeps across it may be attracted back to the positively charged mosaic, thus neutralizing many of the stored charges. Lubszynski, Rodda, and McGee realized that the best solution was to separate the photo-emission function from the charge storage one, and so communicated their results to Zworykin.
The new video camera tube developed by Lubszynski, Rodda and McGee in 1934 was dubbed "the super-Emitron". This tube is a combination of the image dissector and the Emitron. It has an efficient photocathode
that transforms the scene light into an electron image; the latter is then accelerated towards a target specially prepared for the emission of secondary electrons
. Each individual electron from the electron image produces several secondary electrons after reaching the target, so that an amplification effect is produced. The target is constructed of a mosaic of electrically isolated metallic granules separated from a common plate by a thin layer of isolating material, so that the positive charge resulting from the secondary emission
is stored in the granules. Finally, an electron beam periodically sweeps across the target, effectively scanning the stored image, discharging each granule, and producing an electronic signal like in the iconoscope.
The super-Emitron was between ten and fifteen times more sensitive than the original Emitron and iconoscope tubes and, in some cases, this ratio was considerably greater. It was used for an outside broadcasting
by the BBC, for the first time, on Armistice Day 1937, when the general public could watch in a television set how the King lay a wreath at the Cenotaph. This was the first time that anyone could broadcast a live street scene from cameras installed on the roof of neighbor buildings.
On the other hand, in 1934, Zworykin shared some patent rights with the German licensee company Telefunken. The "image iconoscope" ("Superikonoskop" in Germany) was produced as a results of the collaboration. This tube is essentially identical to the super-Emitron, but the target is constructed of a thin layer of isolating material placed on top of a conductive base, the mosaic of metallic granules is missing. The production and commercialization of the super-Emitron and image iconoscope in Europe were not affected by the patent war between Zworykin and Farnsworth, because Dieckmann and Hell had priority in Germany for the invention of the image dissector, having submitted a patent application for their Lichtelektrische Bildzerlegerröhre für Fernseher (Photoelectric Image Dissector Tube for Television) in Germany in 1925, two years before Farnsworth did the same in the United States.
The image iconoscope (Superikonoskop) became the industrial standard for public broadcasting in Europe from 1936 until 1960, when it was replaced by the vidicon and plumbicon tubes. Indeed it was the representative of the European tradition in electronic tubes competing against the American tradition represented by the image orthicon. The German company Heimann produced the Superikonoskop for the 1936 Berlin Olympic Games, later Heimann also produced and commercialized it from 1940 to 1955, finally the Dutch company Philips
produced and commercialized the image iconoscope and multicon from 1952 to 1958.
that transforms the scene light into a light-emitted electron image, the latter is then accelerated (and focused) via electromagnetic fields towards a target specially prepared for the emission of secondary electrons
. Each individual electron from the electron image produces several secondary electrons after reaching the target, so that an amplification effect is produced, and the resulting positive charge is proportional to the integrated intensity of the scene light. The target is constructed of a mosaic of electrically isolated metallic granules separated from a common plate by a thin layer of isolating material, so that the positive charge resulting from the secondary emission
is stored in the capacitor formed by the metallic granule and the common plate. Finally, an electron beam periodically sweeps across the target, effectively scanning the stored image and discharging each capacitor in turn such that the electrical output from each capacitor is proportional to the average intensity of the scene light between each discharge event (as in the iconoscope).
The image iconoscope is essentially identical to the super-Emitron, but the target is constructed of a thin layer of isolating material placed on top of a conductive base, the mosaic of metallic granules is missing. Therefore, secondary electrons are emitted from the surface of the isolating material when the electron image reaches the target, and the resulting positive charges are stored directly onto the surface of the isolated material.
and capacitance
, respectively. These stored charges are then "gently" discharged by a low-velocity electron scanning beam, preventing the emission of secondary electrons. Not all the electrons in the scanning beam may be absorbed in the mosaic, because the stored positive charges are proportional to the integrated intensity of the scene light. The remaining electrons are then deflected back into the anode, captured by a special grid
, or deflected back into an electron multiplier
.
Low-velocity scanning beam tubes have several advantages; there are low levels of spurious signals and high efficiency of conversion of light into signal, so that the signal output is maximum. However, there are serious problems as well, because the electron beam "spreads" when it scans the image's borders and corners, so that one gets an image that is well focused in the center but blurry in the borders. Another improvement is the use of a semitransparent charge storage plate. The scene image is then projected onto the back side of the plate, while the low-velocity electron beam scans the photoelectric mosaic at the front side. This configurations allows the use of a straight camera tube, because the scene to be transmitted, the charge storage plate, and the electron gun can be aligned one after the other.
In 1928, Tihanyi
filed for a patent for a television apparatus where an electron beam scanned the mosaic of a charge storage plate and was deflected into the anode following a V-path. Five years latter, in 1933, Farnsworth
filed for a patent where he explained for the first time that the electrons in the scanning beam must arrive with zero-velocity to the charge storage plate, so as to prevent the emission of secondary electrons. Nevertheless, neither Tihanyi nor Farnsworth considered the use of a semitransparent plate, and they never transmitted clear images with the devices described in their patents, because they were not aware that the beam alignment had to be perpendicular to the plate in order to produce a well focused image.
The first fully functional low-velocity scanning beam tube, the CPS Emitron, was invented and demonstrated by the EMI
team under the supervision of Isaac Shoenberg
. In 1934, the EMI engineers Blumlein and McGee filed for patents for television transmitting systems where a charge storage plate was shielded by a pair of special grids
, a negative (or slightly positive) grid lay very close to the plate, and a positive one was placed further away. The velocity and energy of the electrons in the scanning beam were reduced to zero by the decelerating electric field generated by this pair of grids, and so a low-velocity scanning beam tube was obtained. The EMI
team kept working on these devices, and Lubszynski discovered in 1936 that a clear image could be produced if the trajectory of the low-velocity scanning beam was nearly perpendicular (orthogonal) to the charge storage plate in a neighborhood of it. The resulting device was dubbed the cathode potential stabilized Emitron, or CPS Emitron. The industrial production and commercialization of the CPS Emitron had to wait until the end of the second world war.
On the other side of the ocean, the RCA
team led by Albert Rose
began working in 1937 on a low-velocity scanning beam device they dubbed the orthicon. Iams and Rose solved the problem of guiding the beam and keeping it in focus by installing specially designed deflection plates and deflection coils near the charge storage plate to provide a
uniform axial magnetic field. The orthicon was the tube used in RCA's television demonstration at the 1939 New York World's Fair
, its performance was similar to the image iconoscope's one, but it was also unstable under sudden flashes of bright light, producing "the appearance of a large drop of water evaporating slowly over part of the scene".
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 Committee
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.
with an image store ("target"), a scanner that reads this image (an electron gun
), and a multistage electron multiplier.
In the image store, light falls upon the photocathode which is a photosensitive plate at a very negative potential (approx. -600 V), and is converted into an electron image (a principle borrowed from the image dissector). This electron rain is then accelerated towards the target (a very thin glass plate acting as a semi-isolator) at ground potential (0 V), and passes 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 +2 V). 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. +1500 V). 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 multiplier 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 surrounding the electron gun) in progressively higher potentials. As the returning electron beam hits the first dynode, it ejects electrons similarly to the target; for each electron striking a dynode, many are emitted. 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 dynode, causing, say, four electrons to be emitted and drawn towards the next dynode. Each of these might then cause four each to be emitted. Thus, by the start of the third stage, you would have about 16 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
). This makes it on the order of a thousand times more sensitive than the iconoscope.
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.
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 electron
s. 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 photocathode
s can be used to produce a vidicon sensitive over a broad portion of the infrared
spectrum.
Prior to the design and construction of the Galileo probe to Jupiter
in the late 1970s to early 1980s, NASA
used Vidicon cameras on most of their unmanned deep space probes equipped with the remote sensing ability.
for its Lead Oxide (PbO)
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.
also produced by Thomson
and Sony
. It was developed in a joint effort by Hitachi and NHK
(Japan Broadcasting Corporation). Its surface consists of Selenium with trace amounts of Arsenic and Tellurium added (SeAsTe) to make the signal more stable. SAT in the name is derived from (SeAsTe).
.
and Zinc Cadmium Telluride (ZnCdTe).
. It uses a vertically striped RGB color filter over the faceplate of an otherwise standard vidicon 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.
Although the idea of using color stripe filters over the target was not new, the Trinicon was the only tube to use the primary RGB colors. This necessitated an additional electrode buried in the target to detect where the scanning electron beam was relative to the stripe filter. Previous color stripe systems had used colors where the color circuitry was able to separate the colors purely from the relative amplitudes of the signals. As a result the Trinicon featured a larger dynamic range of operation.
, a technique still in use with 3CCD
solid state cameras today. It was also possible to construct a color camera that used a single image tube. One technique has already been described (Trinicon above). A more common technique and a simpler one from the tube construction standpoint was to overlay the photosensitive target with a color striped filter having a fine pattern of vertical stripes of green, cyan and clear filters repeating across the target. The advantage of this arrangement was that for virtually every color, the video level of the green component was always less than the cyan, and similarly the cyan was always less than the white. Thus the contributing images could be separated without any reference electrodes in the tube. If the three levels were the same, then that part of the scene was green. This method suffered from the disadvantage that the light levels under the three filters were almost certain to be different, with the green filter passing not more than one third of the available light.
Variations on this scheme exist, the principal one being to use two filters with color stripes overlaid such that the colors form vertically oriented lozenge shapes overlaying the target. The method of extracting the color is similar however.
s were developed which used synchronized motor-driven color-filter disks at the camera's image tube and at the television receiver. Each disk consisted of red, blue, and green transparent color filters. In the camera, the disk was in the optical path, and in the receiver, it was in front of the CRT. Disk rotation was synchronized with vertical scanning so that each vertical scan in sequence was for a different primary color. This method allowed regular black-and-white image tubes and CRTs to generate and display color images. A field-sequential system developed by Peter Goldmark
for CBS
was demonstrated to the press on September 4, 1940, and was first shown to the general public on January 12, 1950. Guillermo González Camarena
developed a field-sequential color disk system in the early 1940s, for which he received the first US patent for color television in 1942.
and CMOS
.
he found that a longitudinal magnetic field generated by an axial coil can focus an electron beam. This phenomenon was immediately corroborated by J. A. Fleming
, and Hans Busch gave a complete mathematical interpretation in 1926.
Diagrams in this article show that the focus coil surrounds the camera tube; it is much longer than the focus coils for earlier TV CRTs. Camera-tube focus coils, by themselves, have essentially parallel lines of force, very different from the localized semi-toroidal magnetic field geometry inside a TV receiver CRT focus coil. The latter is essentially a magnetic lens; it focuses the "crossover" (between the CRT's cathode and G1 electrode, where the electrons pinch together and diverge again) onto the screen.
The electron optics of camera tubes differ considerably. Electrons inside these long focus coils take helical paths as they travel along the length of the tube. The center (think local axis) of one of those helices is like a line of force of the magnetic field. While the electrons are traveling, the helices essentially don't matter. Assuming that they start from a point, the electrons will focus to a point again at a distance determined by the strength of the field. Focusing a tube with this kind of coil is simply a matter of trimming the coil's current. In effect, the electrons travel along the lines of force, although helically, in detail.
These focus coils are essentially as long as the tubes themselves, and surround the deflection yoke (coils). Deflection fields bend the lines of force (with negligible defocusing), and the electrons follow the lines of force.
In a conventional magnetically deflected CRT, such as in a TV receiver or computer monitor, basically the vertical deflection coils are equivalent to coils wound around an horizontal axis. That axis is perpendicular to the neck of the tube; lines of force are basically horizontal. (In detail, coils in a deflection yoke extend some distance beyond the neck of the tube, and lie close to the flare of the bulb; they have a truly distinctive appearance.)
In a magnetically focused camera tube (there are electrostatically focused vidicons), the vertical deflection coils are above and below the tube, instead of being on both sides of it. One might say that this sort of deflection starts to create S-bends in the lines of force, but doesn't become anywhere near to that extreme.
.
The optical format is defined as the diagonal length of the sensor (in mm) divided by 16. The result is expressed in inches (not converted to inches!). For instance, a 6.4x4.8 mm sensor has a diagonal of 8.0 mm and therefore an optical format of 8.0/16 = 1/2". The reason why it is expressed in inches is historical: a standard size one inch vidicon tube has only 16 mm of useful imaging area. Therefore 1 inch optics became 16 mm, 1/2 inch optics became 8 mm, etc.
Although the video camera tube is now technologically obsolete, the optical format is still used today to express the size of image sensors found in most compact digital cameras. The parameter is also the source of the "Four Thirds" in the Four Thirds system
and its Micro Four Thirds
extension—the imaging area of the sensor in these cameras is identical to that of a 4/3" video camera tube.
Video camera
A video camera is a camera used for electronic motion picture acquisition, initially developed by the television industry but now common in other applications as well. The earliest video cameras were those of John Logie Baird, based on the electromechanical Nipkow disk and used by the BBC in...
s, before the mid to late 1980s, a video camera tube or pickup tube was used instead of a charge-coupled device
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
(CCD) for converting an optical image into an electrical signal. Several types were in use from the 1930s to the 1980s. The most commercially successful of these tubes were various types of cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
s or "CRTs".
Cathode ray
Cathode rays are streams of electrons observed in vacuum tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, the glass opposite of the negative electrode is observed to glow, due to electrons emitted from and travelling perpendicular to the cathode Cathode...
s") is known as a cathode ray tube. However, in the popular lexicon "CRT" usually refers to the "picture tube" in a television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
or computer monitor. The proper term for this type of display tube is kinescope
Kinescope
Kinescope , shortened to kine , also known as telerecording in Britain, is a recording of a television program made by filming the picture from a video monitor...
, only one of many types of cathode ray tubes. Others include the tubes used in oscilloscopes, radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
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.)
Video camera tubes typically had a certain maximum brightness tolerance. If that limit were exceeded, such as by pointing the camera at the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, sun-reflecting shiny surfaces, or extremely bright point light sources, the tube detecting surface would instantly "burn out" and be rendered insensitive on part or all of the screen. The only remedy was replacing the video tube.
Early steps
In June 1908, the scientific journal NatureNature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...
published a letter in which Alan Archibald Campbell-Swinton
Alan Archibald Campbell-Swinton
Alan Archibald Campbell-Swinton, FRS was a Scottish consulting electrical engineer. He described an electronic method of producing television in a 1908 letter to Nature.-Biography:...
, fellow of the Royal Society
Royal Society
The Royal Society of London for Improving Natural Knowledge, known simply as the Royal Society, is a learned society for science, and is possibly the oldest such society in existence. Founded in November 1660, it was granted a Royal Charter by King Charles II as the "Royal Society of London"...
(UK), discussed how a fully electronic television system could be realized by using cathode ray tubes (or "Braun" tubes, after its inventor, Karl Braun) as both imaging and display devices. He 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 cathode ray tube was successfully demonstrated as a displaying device by the German
Germans
The Germans are a Germanic ethnic group native to Central Europe. The English term Germans has referred to the German-speaking population of the Holy Roman Empire since the Late Middle Ages....
Professor Max Dieckmann in 1906, his experimental results were published by the journal Scientific American
Scientific American
Scientific American is a popular science magazine. It is notable for its long history of presenting science monthly to an educated but not necessarily scientific public, through its careful attention to the clarity of its text as well as the quality of its specially commissioned color graphics...
in 1909. Campbell-Swinton later expanded on his vision in a presidential address given to the Röntgen Society in November 1911. The photoelectric screen in the proposed transmitting device was a mosaic of isolated rubidium cubes. His concept for a fully electronic television system was later popularized by Hugo Gernsback as the "Campbell-Swinton Electronic Scanning System" in the August 1915 issue of the popular magazine Electrical Experimenter.
In a letter to Nature
Nature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...
published in October 1926, Campbell-Swinton also announced the results of some "not very successful experiments" he had conducted with G. M. Minchin and J. C. M. Stanton. They had attempted to generate an electrical signal by projecting an image onto a selenium-coated metal plate that was simultaneously scanned by a cathode ray beam
Cathode ray
Cathode rays are streams of electrons observed in vacuum tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, the glass opposite of the negative electrode is observed to glow, due to electrons emitted from and travelling perpendicular to the cathode Cathode...
. These experiments were conducted before March 1914, when Minchin died, but they were later repeated by two different teams in 1937, by H. Miller and J. W. Strange from EMI
EMI
The EMI Group, also known as EMI Music or simply EMI, is a multinational music company headquartered in London, United Kingdom. It is the fourth-largest business group and family of record labels in the recording industry and one of the "big four" record companies. EMI Group also has a major...
, and by H. Iams and A. Rose from RCA
RCA
RCA Corporation, founded as the Radio Corporation of America, was an American electronics company in existence from 1919 to 1986. The RCA trademark is currently owned by the French conglomerate Technicolor SA through RCA Trademark Management S.A., a company owned by Technicolor...
. Both teams succeeded in transmitting "very faint" images with the original Campbell-Swinton's selenium-coated plate, but much better images were obtained when the metal plate was covered with zinc sulphide or selenide, or with aluminum or zirconium oxide treated with caesium. These experiments are the base of the future vidicon. A description of a CRT imaging device also appeared in a patent application filed by Edvard-Gustav Schoultz in France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
in August 1921, and published in 1922, although a working device was not demonstrated until some years later.
Image dissector
An image dissector is a camera tube that creates an "electron image" of a scene from photocathodePhotocathode
A photocathode is a negatively charged electrode in a light detection device such as a photomultiplier or phototube that is coated with a photosensitive compound...
emissions (electrons) which pass through a scanning aperture to an anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
, which serves as an electron detector. Among the first to design such a device were German
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
inventors Max Dieckmann and Rudolf Hell
Rudolf Hell
Rudolf Hell was a German inventor. He was born in Eggmühl, Germany.From 1919 to 1923 he studied electrical engineering in Munich....
, 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 Farnsworth
Philo Farnsworth
Philo Taylor Farnsworth was an American inventor and television pioneer. Although he made many contributions that were crucial to the early development of all-electronic television, he is perhaps best known for inventing the first fully functional all-electronic image pickup device , the "image...
.
Dieckmann 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 volume 8 (September 1927) of the popular magazine Discovery and in the May 1928 issue of the magazine Popular Radio. However, they never transmitted a clear and well focused image with such a tube.
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
Electron multiplier
An electron multiplier is a vacuum-tube structure that multiplies incident charges. In a process called secondary emission, a single electron can, when bombarded on secondary emissive material, induce emission of roughly 1 to 3 electrons...
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
Dynode
A dynode is one of a series of electrodes within a photomultiplier tube. Each dynode is at a more positive electrical potential than its predecessor. Secondary emission occurs at the surface of each dynode...
"electron multiplier" in 1937 made Farnsworth's image dissector the first practical version of a fully electronic imaging device for television. Unfortunately, it had very poor light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
sensitivity, and was therefore primarily useful only where illumination was exceptionally high (typically over 685 cd
Candela
The candela is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function . A common candle emits light with a luminous intensity of roughly one candela...
/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 also entitled Image Dissector, but which actually detailed a CRT
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
-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.
Operation
The opticalOptics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
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
Photoelectric effect
In the photoelectric effect, electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as photoelectrons...
). 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
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
deflected horizontally and vertically ("raster scan
Raster scan
A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image systems...
ning") 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
Video monitor
A video monitor also called a broadcast monitor, broadcast reference monitor or just reference monitor, is a display device similar to a television set, used to monitor the output of a video-generating device, such as playout from a video server, IRD, video camera, VCR, or DVD player. It may or...
, such as a CRT monitor, to reproduce the image.
The image dissector has no "charge storage
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
" 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.
The iconoscope
An iconoscope is a camera tube that projects an image on a special "charge storageCapacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
" plate containing a mosaic of electrically isolated photosensitive granules separated from a common plate by a thin layer of isolating material, somewhat analogous to the human eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
's retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
and its arrangement of photoreceptors. Each photosensitive granule constitutes a tiny capacitor that accumulates and stores electrical charge in response to the light striking it. An electron beam periodically sweeps across the plate, effectively scanning the stored image and discharging each capacitor in turn such that the electrical output from each capacitor is proportional to the average intensity of the light striking it between each discharge event. In Tihanyi's version of 1928, the electron beam scanned the granules, while in Zworykin's version of 1925, the electron beam scanned the back of the image plate.
In July 1925, Zworykin submitted a patent application titled Television System that included a charge storage plate constructed of a thin layer of isolating material (aluminum oxide) sandwiched between a screen (300 mesh) and a colloidal deposit of photoelectric material (potassium hydride) consisting of isolated globules. The following description can be read between lines 1 and 9 in page 2: "The photoelectric material, such as potassium hydride, is evaporated on the aluminum oxide, or other insulating medium, and treated so as to form a colloidal deposit of potassium hydride consisting of minute globules. Each globule is very active photoelectrically and constitutes, to all intents and purposes, a minute individual photoelectric cell". Its first image was transmitted in late summer of 1925, and a patent was issued in 1928. However the quality of the transmitted image failed to impress to H. P. Davis, the general manager of Westinghouse
Westinghouse Electric (1886)
Westinghouse Electric was an American manufacturing company. It was founded in 1886 as Westinghouse Electric Company and later renamed Westinghouse Electric Corporation by George Westinghouse. The company purchased CBS in 1995 and became CBS Corporation in 1997...
, and Zworykin was asked "to work on something useful".
A patent for a television system was also filed by Zworykin in 1923, but this file is not a reliable bibliographic source because extensive revisions were done before a patent was issued fifteen years later and the file itself was divided into two patents in 1931. In 1926, the Hungarian engineer Kálmán Tihanyi
Kálmán Tihanyi
Kálmán Tihanyi , was a Hungarian physicist, electrical engineer and inventor. One of the early pioneers of electronic television, he made significant contributions to the development of cathode ray tubes , which were bought and further developed by the Radio Corporation of America , and German...
explained in detail that the principle of "storing" electrical charges in proportion to the amount of light received throughout each scanning cycle results in a much more sensitive video camera tube. Two year later, in 1928, he applied for a patent for a refined "Television Apparatus" that is essentially an iconoscope.
The first practical iconoscope was constructed in 1931 by Sanford Essig, when he accidentally left one silvered mica sheet in the oven too long. Upon examination with a microscope, he noticed that the silver layer had broken up into a myriad of tiny isolated silver globules. He also noticed that, "the tiny dimension of the silver droplets would enhance the image resolution of the iconoscope by a quantum leap." As head of television development at Radio Corporation of America (RCA), Zworykin submitted a patent application in November 1931, and it was issued in 1935. Nevertheless, Zworykin's team was not the only engineering group working on devices that use a charge stage plate. In 1932, the EMI
EMI
The EMI Group, also known as EMI Music or simply EMI, is a multinational music company headquartered in London, United Kingdom. It is the fourth-largest business group and family of record labels in the recording industry and one of the "big four" record companies. EMI Group also has a major...
engineers Tedham and McGee under the supervision of Isaac Shoenberg
Isaac Shoenberg
Sir Isaac Shoenberg was an electronic engineer born in Russia who was best known for his role in history of television....
applied for a patent for a new device they dubbed "the Emitron". A 405-line broadcasting service employing the Emitron began at studios in Alexandra Palace
Alexandra Palace
Alexandra Palace is a building in North London, England. It stands in Alexandra Park, in an area between Hornsey, Muswell Hill and Wood Green...
in 1936, and patents were issued in the United Kingdom in 1934 and in the USA in 1937.
The iconoscope was presented to the general public in a press conference in June 1933, and two detailed technical papers were published in September and October of the same year. Unlike the Farnsworth image dissector, the Zworykin iconoscope was much more sensitive, useful with an illumination on the target between 4ft-c
Foot-candle
A foot-candle is a non-SI unit of illuminance or light intensity widely used in photography, film, television, conservation lighting, and the lighting industry...
(43lx
Lux
The lux is the SI unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is used in photometry as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface...
) and 20ft-c
Foot-candle
A foot-candle is a non-SI unit of illuminance or light intensity widely used in photography, film, television, conservation lighting, and the lighting industry...
(215lx
Lux
The lux is the SI unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is used in photometry as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface...
). It was also easier to manufacture and produced a very clear image. The iconoscope was the primary camera tube used by RCA broadcasting from 1936 until 1946, when it was replaced by the image orthicon tube.
Operation
Within the iconoscope, an image was projected onto a plate containing a mosaic of electrically isolated photosensitive granules separated from a common plate by a thin layer of isolating material, each granule constituting a tiny capacitor with the common plate that accumulated and stored electrical charge in response to the light striking it. Emission of photoelectrons from each granule in proportion to the amount of light received resulted in a charge image being formed on the mosaic. An electron beam was then swept across the image plate from an electron gunElectron gun
An electron gun is an electrical component that produces an electron beam that has a precise kinetic energy and is most often used in television sets and computer displays which use cathode ray tube technology, as well as in other instruments, such as electron microscopes and particle...
, effectively scanning the stored image and discharging each capacitor in turn such that the electrical output from each capacitor was proportional to the average intensity of the light striking it between each discharge event. The accumulation and storage of photoelectric charges during each scanning cycle greatly increased the electrical output of the iconoscope relative to non-storage type image scanning devices.
Super-Emitron and image iconoscope
The original iconoscope was noisy, had a high ratio of interference to signal, and ultimately gave disappointing results, especially when compared to the high definition mechanical scanning systems then becoming available. The EMIEMI
The EMI Group, also known as EMI Music or simply EMI, is a multinational music company headquartered in London, United Kingdom. It is the fourth-largest business group and family of record labels in the recording industry and one of the "big four" record companies. EMI Group also has a major...
team under the supervision of Isaac Shoenberg
Isaac Shoenberg
Sir Isaac Shoenberg was an electronic engineer born in Russia who was best known for his role in history of television....
analyzed how the Emitron (or iconoscope) produces an electronic signal and concluded that its real efficiency was only about 5% of the theoretical maximum. This is because secondary electrons
Secondary electrons
Secondary electrons are electrons generated as ionization products. They are called 'secondary' because they are generated by other radiation . This radiation can be in the form of ions, electrons, or photons with sufficiently high energy, i.e. exceeding the ionization potential...
released from the mosaic of the charge storage plate when the scanning beam sweeps across it may be attracted back to the positively charged mosaic, thus neutralizing many of the stored charges. Lubszynski, Rodda, and McGee realized that the best solution was to separate the photo-emission function from the charge storage one, and so communicated their results to Zworykin.
The new video camera tube developed by Lubszynski, Rodda and McGee in 1934 was dubbed "the super-Emitron". This tube is a combination of the image dissector and the Emitron. It has an efficient photocathode
Photocathode
A photocathode is a negatively charged electrode in a light detection device such as a photomultiplier or phototube that is coated with a photosensitive compound...
that transforms the scene light into an electron image; the latter is then accelerated towards a target specially prepared for the emission of secondary electrons
Secondary electrons
Secondary electrons are electrons generated as ionization products. They are called 'secondary' because they are generated by other radiation . This radiation can be in the form of ions, electrons, or photons with sufficiently high energy, i.e. exceeding the ionization potential...
. Each individual electron from the electron image produces several secondary electrons after reaching the target, so that an amplification effect is produced. The target is constructed of a mosaic of electrically isolated metallic granules separated from a common plate by a thin layer of isolating material, so that the positive charge resulting from the secondary emission
Secondary emission
Secondary emission in physics is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The primary particles are often charged particles like electrons or ions. If the secondary...
is stored in the granules. Finally, an electron beam periodically sweeps across the target, effectively scanning the stored image, discharging each granule, and producing an electronic signal like in the iconoscope.
The super-Emitron was between ten and fifteen times more sensitive than the original Emitron and iconoscope tubes and, in some cases, this ratio was considerably greater. It was used for an outside broadcasting
Outside broadcasting
Outside broadcasting is the electronic field production of television or radio programmes from a mobile remote broadcast television studio. Professional video camera and microphone signals come into the production truck for processing, recording and possibly transmission...
by the BBC, for the first time, on Armistice Day 1937, when the general public could watch in a television set how the King lay a wreath at the Cenotaph. This was the first time that anyone could broadcast a live street scene from cameras installed on the roof of neighbor buildings.
On the other hand, in 1934, Zworykin shared some patent rights with the German licensee company Telefunken. The "image iconoscope" ("Superikonoskop" in Germany) was produced as a results of the collaboration. This tube is essentially identical to the super-Emitron, but the target is constructed of a thin layer of isolating material placed on top of a conductive base, the mosaic of metallic granules is missing. The production and commercialization of the super-Emitron and image iconoscope in Europe were not affected by the patent war between Zworykin and Farnsworth, because Dieckmann and Hell had priority in Germany for the invention of the image dissector, having submitted a patent application for their Lichtelektrische Bildzerlegerröhre für Fernseher (Photoelectric Image Dissector Tube for Television) in Germany in 1925, two years before Farnsworth did the same in the United States.
The image iconoscope (Superikonoskop) became the industrial standard for public broadcasting in Europe from 1936 until 1960, when it was replaced by the vidicon and plumbicon tubes. Indeed it was the representative of the European tradition in electronic tubes competing against the American tradition represented by the image orthicon. The German company Heimann produced the Superikonoskop for the 1936 Berlin Olympic Games, later Heimann also produced and commercialized it from 1940 to 1955, finally the Dutch company Philips
Philips
Koninklijke Philips Electronics N.V. , more commonly known as Philips, is a multinational Dutch electronics company....
produced and commercialized the image iconoscope and multicon from 1952 to 1958.
Operation
The super-Emitron is a combination of the image dissector and the Emitron. The scene image is projected onto an efficient continuous-film semitransparent photocathodePhotocathode
A photocathode is a negatively charged electrode in a light detection device such as a photomultiplier or phototube that is coated with a photosensitive compound...
that transforms the scene light into a light-emitted electron image, the latter is then accelerated (and focused) via electromagnetic fields towards a target specially prepared for the emission of secondary electrons
Secondary electrons
Secondary electrons are electrons generated as ionization products. They are called 'secondary' because they are generated by other radiation . This radiation can be in the form of ions, electrons, or photons with sufficiently high energy, i.e. exceeding the ionization potential...
. Each individual electron from the electron image produces several secondary electrons after reaching the target, so that an amplification effect is produced, and the resulting positive charge is proportional to the integrated intensity of the scene light. The target is constructed of a mosaic of electrically isolated metallic granules separated from a common plate by a thin layer of isolating material, so that the positive charge resulting from the secondary emission
Secondary emission
Secondary emission in physics is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The primary particles are often charged particles like electrons or ions. If the secondary...
is stored in the capacitor formed by the metallic granule and the common plate. Finally, an electron beam periodically sweeps across the target, effectively scanning the stored image and discharging each capacitor in turn such that the electrical output from each capacitor is proportional to the average intensity of the scene light between each discharge event (as in the iconoscope).
The image iconoscope is essentially identical to the super-Emitron, but the target is constructed of a thin layer of isolating material placed on top of a conductive base, the mosaic of metallic granules is missing. Therefore, secondary electrons are emitted from the surface of the isolating material when the electron image reaches the target, and the resulting positive charges are stored directly onto the surface of the isolated material.
Orthicon and CPS Emitron
The original iconoscope was very noisy due to the secondary electrons released from the photoelectric mosaic of the charge storage plate when the scanning beam swept it across. An obvious solution was to scan the mosaic with an electron beam, which velocity and energy were so low in a neighborhood of the plate, that no secondary electrons were emitted at all. That is, an image is projected onto the photoelectric mosaic of a charge storage plate, so that positive charges are produced and stored there due to photo-emissionPhotoelectric effect
In the photoelectric effect, electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as photoelectrons...
and capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
, respectively. These stored charges are then "gently" discharged by a low-velocity electron scanning beam, preventing the emission of secondary electrons. Not all the electrons in the scanning beam may be absorbed in the mosaic, because the stored positive charges are proportional to the integrated intensity of the scene light. The remaining electrons are then deflected back into the anode, captured by a special grid
Control grid
The control grid is an electrode used in thermionic valves used to modulate the flow of electrons in the cathode to anode or plate circuit.- Operation :...
, or deflected back into an electron multiplier
Electron multiplier
An electron multiplier is a vacuum-tube structure that multiplies incident charges. In a process called secondary emission, a single electron can, when bombarded on secondary emissive material, induce emission of roughly 1 to 3 electrons...
.
Low-velocity scanning beam tubes have several advantages; there are low levels of spurious signals and high efficiency of conversion of light into signal, so that the signal output is maximum. However, there are serious problems as well, because the electron beam "spreads" when it scans the image's borders and corners, so that one gets an image that is well focused in the center but blurry in the borders. Another improvement is the use of a semitransparent charge storage plate. The scene image is then projected onto the back side of the plate, while the low-velocity electron beam scans the photoelectric mosaic at the front side. This configurations allows the use of a straight camera tube, because the scene to be transmitted, the charge storage plate, and the electron gun can be aligned one after the other.
In 1928, Tihanyi
Kálmán Tihanyi
Kálmán Tihanyi , was a Hungarian physicist, electrical engineer and inventor. One of the early pioneers of electronic television, he made significant contributions to the development of cathode ray tubes , which were bought and further developed by the Radio Corporation of America , and German...
filed for a patent for a television apparatus where an electron beam scanned the mosaic of a charge storage plate and was deflected into the anode following a V-path. Five years latter, in 1933, Farnsworth
Philo Farnsworth
Philo Taylor Farnsworth was an American inventor and television pioneer. Although he made many contributions that were crucial to the early development of all-electronic television, he is perhaps best known for inventing the first fully functional all-electronic image pickup device , the "image...
filed for a patent where he explained for the first time that the electrons in the scanning beam must arrive with zero-velocity to the charge storage plate, so as to prevent the emission of secondary electrons. Nevertheless, neither Tihanyi nor Farnsworth considered the use of a semitransparent plate, and they never transmitted clear images with the devices described in their patents, because they were not aware that the beam alignment had to be perpendicular to the plate in order to produce a well focused image.
The first fully functional low-velocity scanning beam tube, the CPS Emitron, was invented and demonstrated by the EMI
EMI
The EMI Group, also known as EMI Music or simply EMI, is a multinational music company headquartered in London, United Kingdom. It is the fourth-largest business group and family of record labels in the recording industry and one of the "big four" record companies. EMI Group also has a major...
team under the supervision of Isaac Shoenberg
Isaac Shoenberg
Sir Isaac Shoenberg was an electronic engineer born in Russia who was best known for his role in history of television....
. In 1934, the EMI engineers Blumlein and McGee filed for patents for television transmitting systems where a charge storage plate was shielded by a pair of special grids
Control grid
The control grid is an electrode used in thermionic valves used to modulate the flow of electrons in the cathode to anode or plate circuit.- Operation :...
, a negative (or slightly positive) grid lay very close to the plate, and a positive one was placed further away. The velocity and energy of the electrons in the scanning beam were reduced to zero by the decelerating electric field generated by this pair of grids, and so a low-velocity scanning beam tube was obtained. The EMI
EMI
The EMI Group, also known as EMI Music or simply EMI, is a multinational music company headquartered in London, United Kingdom. It is the fourth-largest business group and family of record labels in the recording industry and one of the "big four" record companies. EMI Group also has a major...
team kept working on these devices, and Lubszynski discovered in 1936 that a clear image could be produced if the trajectory of the low-velocity scanning beam was nearly perpendicular (orthogonal) to the charge storage plate in a neighborhood of it. The resulting device was dubbed the cathode potential stabilized Emitron, or CPS Emitron. The industrial production and commercialization of the CPS Emitron had to wait until the end of the second world war.
On the other side of the ocean, the RCA
RCA
RCA Corporation, founded as the Radio Corporation of America, was an American electronics company in existence from 1919 to 1986. The RCA trademark is currently owned by the French conglomerate Technicolor SA through RCA Trademark Management S.A., a company owned by Technicolor...
team led by Albert Rose
Albert Rose
Albert Rose was an American physicist, who made major contributions to TV video camera tubes such as the orthicon, image orthicon, and vidicon....
began working in 1937 on a low-velocity scanning beam device they dubbed the orthicon. Iams and Rose solved the problem of guiding the beam and keeping it in focus by installing specially designed deflection plates and deflection coils near the charge storage plate to provide a
uniform axial magnetic field. The orthicon was the tube used in RCA's television demonstration at the 1939 New York World's Fair
1939 New York World's Fair
The 1939–40 New York World's Fair, which covered the of Flushing Meadows-Corona Park , was the second largest American world's fair of all time, exceeded only by St. Louis's Louisiana Purchase Exposition of 1904. Many countries around the world participated in it, and over 44 million people...
, its performance was similar to the image iconoscope's one, but it was also unstable under sudden flashes of bright light, producing "the appearance of a large drop of water evaporating slowly over part of the scene".
Image orthicon
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 lightLight
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
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 Committee
National Defense Research Committee
The National Defense Research Committee was an organization created "to coordinate, supervise, and conduct scientific research on the problems underlying the development, production, and use of mechanisms and devices of warfare" in the United States from June 27, 1940 until June 28, 1941...
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
United States Navy
The United States Navy is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. The U.S. Navy is the largest in the world; its battle fleet tonnage is greater than that of the next 13 largest navies combined. The U.S...
, 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
Logarithmic scale
A logarithmic scale is a scale of measurement using the logarithm of a physical quantity instead of the quantity itself.A simple example is a chart whose vertical axis increments are labeled 1, 10, 100, 1000, instead of 1, 2, 3, 4...
light sensitivity curve similar to the human eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
. However, it tends to flare
Lens flare
Lens flare is the light scattered in lens systems through generally unwanted image formation mechanisms, such as internal reflection and scattering from material inhomogeneities in the lens. These mechanisms differ from the intended image formation mechanism that depends on refraction of the image...
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 photocathodePhotocathode
A photocathode is a negatively charged electrode in a light detection device such as a photomultiplier or phototube that is coated with a photosensitive compound...
with an image store ("target"), a scanner that reads this image (an electron gun
Electron gun
An electron gun is an electrical component that produces an electron beam that has a precise kinetic energy and is most often used in television sets and computer displays which use cathode ray tube technology, as well as in other instruments, such as electron microscopes and particle...
), and a multistage electron multiplier.
In the image store, light falls upon the photocathode which is a photosensitive plate at a very negative potential (approx. -600 V), and is converted into an electron image (a principle borrowed from the image dissector). This electron rain is then accelerated towards the target (a very thin glass plate acting as a semi-isolator) at ground potential (0 V), and passes 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 +2 V). Once the image electrons reach the target, they cause a "splash" of electrons by the effect of secondary emission
Secondary emission
Secondary emission in physics is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The primary particles are often charged particles like electrons or ions. If the secondary...
. 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
Electron gun
An electron gun is an electrical component that produces an electron beam that has a precise kinetic energy and is most often used in television sets and computer displays which use cathode ray tube technology, as well as in other instruments, such as electron microscopes and particle...
at ground potential and accelerated by the anode (the first dynode
Dynode
A dynode is one of a series of electrodes within a photomultiplier tube. Each dynode is at a more positive electrical potential than its predecessor. Secondary emission occurs at the surface of each dynode...
of the electron multiplier
Electron multiplier
An electron multiplier is a vacuum-tube structure that multiplies incident charges. In a process called secondary emission, a single electron can, when bombarded on secondary emissive material, induce emission of roughly 1 to 3 electrons...
) around the gun at a high positive voltage (approx. +1500 V). 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 multiplier 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 surrounding the electron gun) in progressively higher potentials. As the returning electron beam hits the first dynode, it ejects electrons similarly to the target; for each electron striking a dynode, many are emitted. 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 dynode, causing, say, four electrons to be emitted and drawn towards the next dynode. Each of these might then cause four each to be emitted. Thus, by the start of the third stage, you would have about 16 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
Foot-candle
A foot-candle is a non-SI unit of illuminance or light intensity widely used in photography, film, television, conservation lighting, and the lighting industry...
(0.1lx
Lux
The lux is the SI unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is used in photometry as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface...
). This makes it on 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.Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s. The fluctuating voltage coupled out to a video amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
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 photocathode
Photocathode
A photocathode is a negatively charged electrode in a light detection device such as a photomultiplier or phototube that is coated with a photosensitive compound...
s can be used to produce a vidicon sensitive over a broad portion of the infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
spectrum.
Prior to the design and construction of the Galileo probe to Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
in the late 1970s to early 1980s, NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
used Vidicon cameras on most of their unmanned deep space probes equipped with the remote sensing ability.
Plumbicon
Plumbicon is a registered trademark of PhilipsPhilips
Koninklijke Philips Electronics N.V. , more commonly known as Philips, is a multinational Dutch electronics company....
for its Lead Oxide (PbO)
Lead oxide
Lead oxide may refer to:* Lead oxide, PbO, litharge, massicot* Lead oxide, Pb3O4, minium, red lead* Lead dioxide , PbO2Less common lead oxides are:* Lead oxide, Pb2O3, lead sesquioxide...
target vidicons. Used frequently in broadcast camera applications, these tubes have low output, but a high signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
. 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
Rhode Island
The state of Rhode Island and Providence Plantations, more commonly referred to as Rhode Island , is a state in the New England region of the United States. It is the smallest U.S. state by area...
. 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 HitachiHitachi, Ltd.
is a Japanese multinational conglomerate headquartered in Marunouchi 1-chome, Chiyoda, Tokyo, Japan. The company is the parent of the Hitachi Group as part of the larger DKB Group companies...
also produced by Thomson
Thomson SA
Technicolor SA , formerly Thomson SA and Thomson Multimedia, is a French international provider of solutions for the creation, management, post-production, delivery and access of video, for the Communication, Media and Entertainment industries. Technicolor’s headquarters are located in Issy les...
and Sony
Sony
, commonly referred to as Sony, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan and the world's fifth largest media conglomerate measured by revenues....
. It was developed in a joint effort by Hitachi and NHK
NHK
NHK is Japan's national public broadcasting organization. NHK, which has always identified itself to its audiences by the English pronunciation of its initials, is a publicly owned corporation funded by viewers' payments of a television license fee....
(Japan Broadcasting Corporation). Its surface consists of Selenium with trace amounts of Arsenic and Tellurium added (SeAsTe) to make the signal more stable. SAT in the name is derived from (SeAsTe).
Pasecon
Pasecon is a registered trademark of Heimann. Its surface consists of Cadmium Selenide (CdSe)Cadmium selenide
Cadmium selenide is a solid, binary compound of cadmium and selenium. Common names for this compound are cadmium selenide, cadmium selenide, and cadmoselite ....
.
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 (ZnSe)Zinc selenide
Zinc selenide , is a light yellow binary solid compound. It is an intrinsic semiconductor with a band gap of about 2.70 eV at 25 °C. ZnSe rarely occurs in nature...
and Zinc Cadmium Telluride (ZnCdTe).
Trinicon
Trinicon is a registered trademark of SonySony
, commonly referred to as Sony, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan and the world's fifth largest media conglomerate measured by revenues....
. It uses a vertically striped RGB color filter over the faceplate of an otherwise standard vidicon 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.
Although the idea of using color stripe filters over the target was not new, the Trinicon was the only tube to use the primary RGB colors. This necessitated an additional electrode buried in the target to detect where the scanning electron beam was relative to the stripe filter. Previous color stripe systems had used colors where the color circuitry was able to separate the colors purely from the relative amplitudes of the signals. As a result the Trinicon featured a larger dynamic range of operation.
Light Biasing
All the vidicon type tubes except the vidicon itself were able to use a light biasing technique to improve the sensitivity and contrast. The photosensitive target in these tubes suffered from the limitation that the light level had to rise to a particular level before any video output resulted. Light biasing was a method whereby the photosensitive target was illuminated from a light source just enough that no appreciable output was obtained, but such that a slight increase in light level from the scene was enough to provide discernible output. The light came from either an illuminator mounted around the target, or in more professional cameras from a light source on the base of the tube and guided to the target by light piping. The technique would not work with the baseline vidicon tube because it suffered from the limitation that as the target was fundamentally an insulator, the constant low light level built up a charge which would manifest itself as a form of 'fogging'. The other types had semiconducting targets which did not have this problem.Color Cameras
Early color cameras used the obvious technique of using separate red, green and blue image tubes in conjunction with a color separatorDichroic prism
A dichroic prism is a prism that splits light into two beams of differing wavelength . They are usually constructed of one or more glass prisms with dichroic optical coatings that selectively reflect or transmit light depending on the light's wavelength. That is, certain surfaces within the prism...
, a technique still in use with 3CCD
3CCD
A three-CCD camera is a camera whose imaging system uses three separate charge-coupled devices , each one taking a separate measurement of the primary colors, red, green, or blue light. Light coming into the lens is split by a trichroic prism assembly, which directs the appropriate wavelength...
solid state cameras today. It was also possible to construct a color camera that used a single image tube. One technique has already been described (Trinicon above). A more common technique and a simpler one from the tube construction standpoint was to overlay the photosensitive target with a color striped filter having a fine pattern of vertical stripes of green, cyan and clear filters repeating across the target. The advantage of this arrangement was that for virtually every color, the video level of the green component was always less than the cyan, and similarly the cyan was always less than the white. Thus the contributing images could be separated without any reference electrodes in the tube. If the three levels were the same, then that part of the scene was green. This method suffered from the disadvantage that the light levels under the three filters were almost certain to be different, with the green filter passing not more than one third of the available light.
Variations on this scheme exist, the principal one being to use two filters with color stripes overlaid such that the colors form vertically oriented lozenge shapes overlaying the target. The method of extracting the color is similar however.
Field-sequential color system
During the 1930s and 1940s, Field-sequential color systemField-sequential color system
A field-sequential color system is a color television system in which the primary color information is transmitted in successive images, and which relies on the human vision system to fuse the successive images into a color picture. One field-sequential system was developed by Dr. Peter Goldmark...
s were developed which used synchronized motor-driven color-filter disks at the camera's image tube and at the television receiver. Each disk consisted of red, blue, and green transparent color filters. In the camera, the disk was in the optical path, and in the receiver, it was in front of the CRT. Disk rotation was synchronized with vertical scanning so that each vertical scan in sequence was for a different primary color. This method allowed regular black-and-white image tubes and CRTs to generate and display color images. A field-sequential system developed by Peter Goldmark
Peter Carl Goldmark
Peter Carl Goldmark was a German-Hungarian engineer who, during his time with Columbia Records, was instrumental in developing the long-playing microgroove 33-1/3 rpm vinyl phonograph disc, the standard for incorporating multiple or lengthy recorded works on a single disc for two generations...
for CBS
CBS
CBS Broadcasting Inc. is a major US commercial broadcasting television network, which started as a radio network. The name is derived from the initials of the network's former name, Columbia Broadcasting System. The network is sometimes referred to as the "Eye Network" in reference to the shape of...
was demonstrated to the press on September 4, 1940, and was first shown to the general public on January 12, 1950. Guillermo González Camarena
Guillermo González Camarena
Guillermo González Camarena , was a Mexican engineer who was the inventor of a color-wheel type of color television, and who also introduced color television to Mexico....
developed a field-sequential color disk system in the early 1940s, for which he received the first US patent for color television in 1942.
Technological obsolescence
For television camera uses, the vidicon has been technologically superseded by the CCDCharge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
and CMOS
Active pixel sensor
An active-pixel sensor is an image sensor consisting of an integrated circuit containing an array of pixel sensors, each pixel containing a photodetector and an active amplifier. There are many types of active pixel sensors including the CMOS APS used most commonly in cell phone cameras, web...
.
Magnetic focusing in typical camera tubes
The phenomenon known as magnetic focusing was discovered by A. A. Campbell-Swinton in 1896,he found that a longitudinal magnetic field generated by an axial coil can focus an electron beam. This phenomenon was immediately corroborated by J. A. Fleming
John Ambrose Fleming
Sir John Ambrose Fleming was an English electrical engineer and physicist. He is known for inventing the first thermionic valve or vacuum tube, the diode, then called the kenotron in 1904. He is also famous for the left hand rule...
, and Hans Busch gave a complete mathematical interpretation in 1926.
Diagrams in this article show that the focus coil surrounds the camera tube; it is much longer than the focus coils for earlier TV CRTs. Camera-tube focus coils, by themselves, have essentially parallel lines of force, very different from the localized semi-toroidal magnetic field geometry inside a TV receiver CRT focus coil. The latter is essentially a magnetic lens; it focuses the "crossover" (between the CRT's cathode and G1 electrode, where the electrons pinch together and diverge again) onto the screen.
The electron optics of camera tubes differ considerably. Electrons inside these long focus coils take helical paths as they travel along the length of the tube. The center (think local axis) of one of those helices is like a line of force of the magnetic field. While the electrons are traveling, the helices essentially don't matter. Assuming that they start from a point, the electrons will focus to a point again at a distance determined by the strength of the field. Focusing a tube with this kind of coil is simply a matter of trimming the coil's current. In effect, the electrons travel along the lines of force, although helically, in detail.
These focus coils are essentially as long as the tubes themselves, and surround the deflection yoke (coils). Deflection fields bend the lines of force (with negligible defocusing), and the electrons follow the lines of force.
In a conventional magnetically deflected CRT, such as in a TV receiver or computer monitor, basically the vertical deflection coils are equivalent to coils wound around an horizontal axis. That axis is perpendicular to the neck of the tube; lines of force are basically horizontal. (In detail, coils in a deflection yoke extend some distance beyond the neck of the tube, and lie close to the flare of the bulb; they have a truly distinctive appearance.)
In a magnetically focused camera tube (there are electrostatically focused vidicons), the vertical deflection coils are above and below the tube, instead of being on both sides of it. One might say that this sort of deflection starts to create S-bends in the lines of force, but doesn't become anywhere near to that extreme.
Size
The size of video camera tubes are expressed in a strange parameter called the optical formatOptical format
Optical format is a measure of the maximum diagonal size of an imaged object in the focal plane of an optical system. The diagonal size of the imaging sensor and the optical format of the lens system must be matched....
.
The optical format is defined as the diagonal length of the sensor (in mm) divided by 16. The result is expressed in inches (not converted to inches!). For instance, a 6.4x4.8 mm sensor has a diagonal of 8.0 mm and therefore an optical format of 8.0/16 = 1/2". The reason why it is expressed in inches is historical: a standard size one inch vidicon tube has only 16 mm of useful imaging area. Therefore 1 inch optics became 16 mm, 1/2 inch optics became 8 mm, etc.
Although the video camera tube is now technologically obsolete, the optical format is still used today to express the size of image sensors found in most compact digital cameras. The parameter is also the source of the "Four Thirds" in the Four Thirds system
Four Thirds System
The Four Thirds system is a standard created by Olympus and Kodak for digital single-lens reflex camera design and development.The system provides a standard that, with digital cameras and lenses available from multiple manufacturers, allows for the interchange of lenses and bodies from different...
and its Micro Four Thirds
Micro Four Thirds system
The Micro Four Thirds system is a standard created by Olympus and Panasonic, and announced on August 5, 2008, for mirrorless interchangeable lens digital cameras and camcorders design and development...
extension—the imaging area of the sensor in these cameras is identical to that of a 4/3" video camera tube.