Video camera tubes were devices based on the cathode ray tube that were used in television cameras to capture television images, prior to the introduction of charge-coupled device (CCD) image sensors in the 1980s.[1][2][3] Several different types of tubes were in use from the early 1930s, and as late as the 1990s.
Any vacuum tube which operates using a focused beam of electrons, originally called cathode rays, is known as a cathode ray tube (CRT). These are usually seen as display devices as used in older (i.e., non-flat panel) television receivers and computer displays. The camera pickup tubes described in this article are also CRTs, but they display no image.[5]
Recording Video In The Era Of CRTs: The Video Camera Tube
An image dissector is a camera tube that creates an "electron image" of a scene from photocathode emissions (electrons) which pass through a scanning aperture to an anode, which serves as an electron detector.[21][22] Among the first to design such a device were German inventors Max Dieckmann and Rudolf Hell,[23][24] who had titled their 1925 patent application Lichtelektrische Bildzerlegerröhre für Fernseher (Photoelectric Image Dissector Tube for Television).[25] The term may apply specifically to a dissector tube employing magnetic fields to keep the electron image in focus,[22] an element lacking in Dieckmann and Hell's design, and in the early dissector tubes built by American inventor Philo Farnsworth.[23][26]
In April 1933, Farnsworth submitted a patent application also entitled Image Dissector, but which actually detailed a CRT-type camera tube.[44] This is among the first patents to propose the use of a "low-velocity" scanning beam and RCA had to buy it in order to sell image orthicon tubes to the general public.[45] However, Farnsworth never transmitted a clear and well focused image with such a tube.[46][47]
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.[48][49]
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. (further information: 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 the Hungarian engineer Kálmán Tihanyi in the beginning of 1925.[50] 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.[51] After further refinements included in a 1928 patent application,[50] Tihanyi's patent was declared void in Great Britain in 1930,[52] 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.
The iconoscope was presented to the general public at a press conference in June 1933,[63] and two detailed technical papers were published in September and October of the same year.[64][65] 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.[citation needed] The iconoscope was the primary camera tube used by RCA broadcasting from 1936 until 1946, when it was replaced by the image orthicon tube.[66][67]
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.[71][72][73]
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.[70] It was used for an outside broadcast by the BBC, for the first time, on Armistice Day 1937, when the general public could watch in a television set how the King laid 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 neighboring buildings.[74]
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.[47][90] 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,[91] but Lubszynski and the EMI team were the first engineers in transmitting a clear and well focused image with such a tube.[46] 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.[85]
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 still being used by NASA for capturing Apollo/Saturn rockets nearing orbit, although the television networks had phased the cameras out. Only they could provide sufficient detail.[105][failed verification]
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.[106] Image orthicons were used extensively in the early color television cameras, where the increased sensitivity of the tube was essential to overcome the very inefficient, beam-splitting optical system of the camera.[106][107]
The vidicon is a storage-type camera tube in which a charge-density pattern is formed by the imaged scene radiation on a photoconductive surface which is then scanned by a beam of low-velocity electrons. The fluctuating voltage coupled out to a video amplifier can be used to reproduce the scene being imaged. The electrical charge produced by an image will remain in the face plate until it is scanned or until the charge dissipates. By using a pyroelectric material such as triglycine sulfate (TGS) as the target, a vidicon sensitive over a broad portion of the infrared spectrum[114] is possible. This technology was a precursor to modern microbolometer technology, and mainly used in firefighting thermal cameras.[115]
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.[116] 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.[117][118][119] The Uvicon, a UV-variant Vidicon was also used by NASA for UV duties.[120]
Plumbicon is a registered trademark of Philips from 1963, for its lead(II) oxide (PbO) target vidicons.[122] 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.[123][124][125]Philips received the 1966 Technology & Engineering Emmy Award for the Plumbicon.[126] 2ff7e9595c
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