Iconoscope ) The early electronic camera tubes (like the
image dissector) suffered from a very disappointing and fatal flaw: They scanned the subject and what was seen at each point was only the tiny piece of light viewed at the instant that the scanning system passed over it. A practical camera tube needed a different technological approach, that of what later became known as the
charge storage camera tube. It was based on a physical phenomenon which was discovered and patented in Hungary in 1926 but became widely understood and recognised only from around 1930. An iconoscope is a camera tube that projects an image on a special
charge storage 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. After Hungarian engineer
Kálmán Tihanyi studied
Maxwell's equations, he discovered a new hitherto unknown physical phenomenon, which led to a break-through in the development of electronic imaging devices. He named the new phenomenon as
charge-storage principle. The problem of low sensitivity to light resulting in low electrical output from transmitting or camera tubes would be solved with the introduction of charge-storage technology by Tihanyi in the beginning of 1925. His solution was a camera tube that accumulated and stored electrical charges (
photoelectrons) within the tube throughout each scanning cycle. The device was first described in a patent application he filed in
Hungary in March 1926 for a television system he dubbed Radioskop. and so he applied for patents in the United States. Tihanyi's charge storage idea remains a basic principle in the design of imaging devices for television to the present day. In 1924, while employed by the
Westinghouse Electric Corporation in Pittsburgh, Pennsylvania, Russian-born American engineer
Vladimir Zworykin presented a project for a totally electronic television system to the company's general manager. 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, The first practical iconoscope was constructed in 1931 by Sanford Essig, when he accidentally left a 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". 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 US in 1937. The iconoscope was presented to the general public at 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 40and215
lux (4–20
ft-c). 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.
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
EMI 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 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. 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 result 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, 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 until 1963, when it was replaced by the much better
Plumbicon.
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
photocathode 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). 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 and accelerates in a direction parallel to the target when it scans the image's borders and corners, so that it produces secondary electrons and one gets an image that is well focused in the center but blurry in the borders. Henroteau was among the first inventors to propose in 1929 the use of
low-velocity electrons for stabilizing the potential of a charge storage plate, but Lubszynski and the
EMI team were the first engineers in transmitting a clear and well focused image with such a tube. 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
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; 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's performance was similar to that of the image iconoscope, 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". 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. 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. Image orthicon cameras were still being used by
NASA for capturing Apollo/Saturn rockets nearing orbit, although the television networks had phased the cameras out. 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 was 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 such as the
RCA TK-40/41, where the increased sensitivity of the tube was essential to overcome the very inefficient,
beam-splitting optical system of the camera. The image orthicon tube was at one point colloquially referred to as an Immy.
Harry Lubcke, the then-President of the
Academy of Television Arts & Sciences, decided to have their award named after this nickname. Since the
statuette was female, it was
feminized into
Emmy. The Image orthicon was used until the end of black and white television production in the 1960s.
Operation An image orthicon consists of three parts: a
photocathode with an image store (target), a scanner that reads this image (an
electron gun), and a 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 (nearly 200 or 390 wires per cm), very near (a few hundredths of a 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.
Dark halo 's
Mercury-Atlas 6 liftoff, 1962. The mysterious dark "orthicon halo" around bright objects in an orthicon-captured image (also known as "blooming") is based on the fact that the IO relies on the emission of photoelectrons, but very bright illumination can produce more of them locally than the device can successfully deal with. At a very bright point on a captured image, a great preponderance of electrons is ejected from the photosensitive plate. So many may be 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 instead, much as water splashes in a ring when a rock is thrown into it. Since the resultant splashed electrons do not contain sufficient energy to eject further electrons where they land, they will instead neutralize any positive charge that has been built-up in that region. Since darker images produce 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 visual image due to the
contrast effect. (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 1950 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. Vidicons with these targets are known as Si-vidicons or Ultricons. The vidicon is a storage-type camera tube in which a charge-density pattern is formed by the imaged scene radiation on a
photoconductive surface which is then scanned by a beam of low-velocity
electrons. This surface is on a glass plate and is also called the target. More specifically, this glass plate is covered in a transparent, electrically conductive,
indium tin oxide (ITO) layer, on top of which the photoconductive surface is formed by depositing photoconductive material which can be applied as small squares with insulation between the squares. The photoconductor is normally an insulator but becomes partially conductive when struck by electrons. By using a
pyroelectric material such as
triglycine sulfate (TGS) as the target, a vidicon sensitive over a broad portion of the
infrared spectrum is possible. This technology was a precursor to modern
microbolometer technology, and mainly used in firefighting thermal cameras. vidicon tube, showing the electron gun. Prior to the design and construction of the
Galileo probe to
Jupiter, in the late 1970s to early 1980s
NASA used vidicon cameras on nearly all the unmanned deep space probes equipped with the
remote sensing ability. Vidicon tubes were also used aboard the first three
Landsat earth imaging satellites launched in 1972, as part of each spacecraft's
Return Beam Vidicon (RBV) imaging system. The
Uvicon, a UV-variant Vidicon was also used by NASA for UV duties. Vidicon tubes were popular in 1970s and 1980s, after which they were rendered obsolete by
solid-state image sensors, with the
charge-coupled device (CCD) and then the
CMOS sensor. All vidicon and similar tubes are prone to image lag, better known as ghosting, smearing, burn-in, comet tails, luma trails and luminance blooming. Image lag is visible as noticeable (usually white or colored) trails that appear after a bright object (such as a light or reflection) has moved, leaving a trail that eventually fades into the image. It cannot be avoided or eliminated, as it is inherent to the technology. To what degree the image generated by the Vidicon is affected will depend on the properties of the target material used on the Vidicon, and the capacitance of the target material (known as the storage effect) as well as the resistance of the electron beam used to scan the target. The higher the capacitance of the target, the higher the charge it can hold and the longer it will take for the trail to disappear. The remaining charges on the target eventually dissipate making the trail disappear. Vidicons can be damaged by high intensity light exposure. Image burn-in occurs when an image is captured by a Vidicon for a long time and appears as a persistent outline of the image when it changes, and the outline disappears over time. Vidicons can become damaged by direct exposure to the sun which causes them to develop dark spots. Vidicons often used antimony trisulfide as the photoconductive material. or Epicons, Vidicons using arrays of silicon diodes for the target, were introduced in 1969 for the
Picturephone. They are very resistant to burn-in, have low image lag and very high sensitivity but are not considered suitable for broadcast TV production as they suffer from high image blooming and image non uniformity. The targets in these tubes are made on silicon substrates and require 10 volts to operate, they are made with
semiconductor device fabrication processes. It was demonstrated in 1965 at the
NAB Show. Used frequently in broadcast camera applications, these tubes have low output, but a high
signal-to-noise ratio. They have excellent resolution compared to image orthicons, but lack the artificially sharp edges of IO tubes, which cause 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. Philips received the 1966
Technology & Engineering Emmy Award for the Plumbicon. Targets in Plumbicons have two layers: a pure PbO layer, and a doped PbO layer. The pure PbO is an intrinsic I type semiconductor, and a layer of it is doped to create a P type PbO semiconductor, thus creating a
semiconductor junction. The PbO is in crystalline form. Plumbicons were the first commercially successful version of the Vidicon. They were smaller, had lower noise, higher sensitivity and resolution, had less image lag than Vidicons, Since PbO is not stable in air, the deposition of PbO on the target is challenging. Vistacons developed by RCA and Leddicons made by EEV also use PbO in their targets.
Saticon (1973) Saticon is a registered trademark of
Hitachi from 1973, also produced by
Thomson and
Sony. It was developed in a joint effort by Hitachi and
NHK Science & Technology Research Laboratories (
NHK is The Japan Broadcasting Corporation). Introduced in 1973, 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). Saticon tubes have an average light sensitivity equivalent to that of 64
ASA film. Compared to the Plumbicon it has a less advantageous operating temperature range and has more image lag. It is not considered suitable for broadcast TV production, as it suffers from high image lag. Introduced in 1974, The Newvicon tubes were characterized by high light sensitivity. Its surface consists of a combination of
zinc selenide (ZnSe) 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, such as the HVC-2200 and HVC-2400 models, though Sony also used it in some moderate cost professional cameras in the 1970s and 1980s, such as the DXC-1600 series. 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. Sony later combined the Saticon tube with the Trinicon's RGB color filter, providing low-light sensitivity and superior color. This type of tube was known as the
SMF Trinicon tube, or
Saticon Mixed Field. SMF Trinicon tubes were used in the HVC-2800 and HVC-2500 consumer cameras, the DXC-1800 and BVP-1 professional cameras, as well as the first
Betamovie camcorders. Toshiba offered a similar tube in 1974, and Hitachi also developed a similar Saticon with a color filter in 1981.
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 separator, a technique still in use with
3CCD solid state cameras today. A variation was to add a
fourth image tube that received a black-and-white image to mitigate imperfect registration between the three colored images and achieve a sharper combined image. 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 (i.e. green; green and blue; and green, blue and red) 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 systems 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 independently developed a field-sequential color disk system in Mexico in the early 1940s, for which he requested a patent in Mexico on August 19 of 1940 and in the US in 1941. Gonzalez Camarena produced his color television system in his laboratory Gon-Cam for the Mexican market and exported it to the Columbia College of Chicago, who regarded it as the best system in the world.
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, 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==