Video camera tube

In older video cameras, before the 1990s, a video camera tube or pickup tube was used instead of a charge-coupled device (CCD). Several types were in use from the 1930s to the 1980s. They operate in a somewhat similar manner to cathode ray tubes, which display pictures, but are instead used to capture images that are projected onto them through the camera lens system.

Image dissector

The image dissector was invented by Philo Farnsworth, one of the pioneers of electronic television, in 1927. It is a type of cathode ray tube occasionally employed as a camera in industrial television systems. The image dissector had very poor light sensitivity, and was useful only where scene illumination exceeded 685 cd/m², but it was ideal for high light levels such as when engineers wanted to monitor the bright, hot interior of an industrial furnace. Owing to its lack of sensitivity, the image dissector was mainly used only to scan film and other transparencies in TV broadcasting. It was, however, the beginning of the electronic TV age.

The image dissector sees the outside world through a glass lens, which focuses an image through the clear glass wall of the tube onto a special plate which is coated with a layer of cesium oxide. When light strikes caesium oxide, the material emits electrons, somewhat like a mirror that reflects an image made of electrons, rather than light (see photoelectric effect). These electrons are aimed and accelerated by electric and magnetic fields onto the dissector's single electron detector so that only a small portion of the electron image hits the detector at any given moment. As time passes the electron image is deflected back and forth and up and down so that the entire image, portion by portion, can be read by the detector. The output from the detector is an electric current whose magnitude is a measure of brightness at a specific point on the image. Electrons that do not hit the single detector are wasted, rather than stored on the target as in the image orthicon (described below) which accounts in part for its low sensitivity (approximately 3000 lux). It has no "storage characteristic".

The iconoscope

Vladimir Zworykin patented the idea, in May 1931, of projecting an image on a special plate which was covered with a chemical photoemissive mosaic consisting of granules of material, a pattern comparable to the receptors of the human eye. Emission of photoelectrons from each granule in proportion to the amount of light resulted in a charge image being formed on the mosaic. Each granule, together with the conductive plate behind the mosaic, formed a small capacitor, all of these having a common plate. An electron beam was then swept across the face of the plate from an electron gun, discharging the capacitors in succession; the resulting changes in potential at the metal plate constituted the picture signal. Unlike the image dissector the Zworykin model was much more sensitive, to about 75 000 lux. It was also easier to manufacture and produced a very clear image.

Image Orthicon

The image orthicon tube (often abbreviated as IO) was common until the 1960s. A combination of Farnsworth's image dissector and RCA's orthicon technologies, it replaced the iconoscope/orthicon, which required a great deal of light to work adequately. While the iconoscope and the intermediate orthicon used capacitance between a multitude of small but discrete light sensitive collectors and an isolated signal plate for reading video information, the IO employed direct charge readings off of a continuous electronically charged collector. The resultant signal was immune to most extraneous signal "crosstalk" from other parts of the target, and could yield extremely detailed images. For instance, IO cameras were used for capturing Apollo/Saturn rockets nearing orbit long after the networks had phased them out, as only they could provide sufficient detail.

A properly constructed image orthicon could take television pictures by candlelight owing to 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 had a logarithmic light sensitivity curve similar to the human eye, so the picture looked more natural. Its defect was that it tended to flare if a shiny object in the studio caught a reflection of a light, generating a dark halo around the object on the picture. Image orthicons were used extensively in the early color television cameras, where their increased sensitivity was essential to overcome their very inefficient optical system.

An engineer's nickname for the tube was the "immy", which later was feminized to become the "Emmy".

Summary of IO Operation: An IO consists of three parts: an image store ("target"), a scanner that reads this image (an electron gun), and a multiplicative amplifier. In the image store, light falls upon a photosensitive plate, and is converted into an electron image (borrowed from Farnsworth's image dissector). These electrons ("rain") are then accelerated towards the target, causing a "splash" of electrons to be discharged (secondary electrons). Each image electron ejects, on average, more than one "splash" electron, and these excess electrons are soaked up by a positively-charged mesh very near and parallel to the target (the image electrons also pass through this mesh, whose positive charge also helps to accelerate the image electrons). The result is an image painted in positive charge, with the brightest portions having the largest positive charge.

A sharply focused beam of electrons (a cathode ray) is then scanned over the back side of the target. The electrons are slowed down just before reaching the target so that they are absorbed without ejecting more electrons. This adds negative charge to the positive charge until the region being scanned reaches some threshold negative charge, at which point the scanning electrons are reflected rather than absorbed. These reflected electrons return down the cathode ray tube toward an electron detector (multiplicative amplifier) surrounding the electron gun. The number of reflected electrons is a measure of the target's original positive charge, which, in turn, is a measure of brightness. In analogy with the image dissector, this beam of electrons is scanned around the target so that the image is read one small portion at a time.

Multiplicative amplification is also performed via the splashing of electrons: a stack of charged pinwheel-like disks surround the electron gun. As the returning electron beam hits the first pinwheel, it ejects electrons exactly like the target. These loose electrons are then drawn toward the next pinwheel back, where the splashing continues for a number of steps. Consider a single, highly-energized electron hiting the first stage of the amplifier, causing 2 electrons to be emitted and drawn towards the next pinwheel. Each of these might then cause two each to be emitted. Thus, by the start of the third stage, you would have four electrons to the original one.

What causes the 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.

Vidicon

A vidicon tube (sometimes called a hivicon tube) is a video camera tube in which the target material is made of antimony trisulfide (Sb₂S₃).

The terms vidicon tube and vidicon camera are often used indiscriminately to refer to video cameras of any type. The principle of operation of the vidicon camera is typical of other types of video camera tubes.

The vidicon is a storage-type camera tube in which a charge-density pattern is formed by the imaged scene radiation on a photoconductive surface which is then scanned by a beam of low-velocity electrons. The fluctuating voltage coupled out to a video amplifier can be used to reproduce the scene being imaged.

The electrical charge produced by an image will remain in the face plate until it is scanned or until the charge dissipates.

Pyroelectric photocathodes can be used to produce a vidicon sensitive over a broad portion of the infrared spectrum.

Plumbicon

Plumbicon is a registered trademark of Philips. Mostly used in broadcast camera applications. These tubes have low output, but a high signal-to-noise ratio. They had excellent resolution compared to Image Orthicons, but lacked the artificially sharp edges of IO tubes, which caused some of the viewing audience to perceive them as softer. CBS Labs invented the first outboard edge enhancement circuits to sharpen the edges of Plumbicon generated images.

Compared to Saticons, Plumbicons had much higher resistance to burn in, and coma and trailing artifacts from bright lights in the shot. Saticons though, usually had slightly higher resolution. After 1980, and the introduction of the diode gun plumbicon tube, the resolution of both types was so high, compared to the maximum limits of the broadcasting standard, that the Saticon's resolution advantage became moot.

surface: PbO Lead Oxide

Saticon

Saticon is a registered trademark of Hitachi also produced by Thomson and Sony. surface: SeAsTe Selenium Arsenic Tellurium

Pasecon

Pasecon is a registered trademark of Heimann. surface: CdSe Cadmium selenide

Newvicon

Newvicon is a registered trademark of Matsushita. The Newvicon tubes were characterized by high light sensitivity. surface: ZnSe, ZnCdTe Zinc Selenide, Zinc Cadmium Telluride

Trinicon

Trinicon is a registered trademark of Sony. Used a vertical stripe filter.

External links

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