Video camera tube
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In older video cameras, prior to 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.
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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 200 foot-lamberts, but it was ideal for high light levels such as when engineers wanted to monitor the bright, hot interior of an industrial furnace. Due 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 caesium oxide. When light strikes caesium oxide, the material emits electrons, somewhat like a mirror that reflects an image made of electrons, rather than light. This invisible electron reflection is aimed at a small detector circuit which captures the electrons, so that at a given instant electrons from a single small point in the image pass through the aperture to an electron multiplier; this is similar in principle to the image orthicon. Scanning currents, very much like those of other cameras, pass through the external deflection coils. In this case the entire electron image is deflected, rather than a narrow beam of electrons. The portion of the image which is deflected into the aperture produces the video at a given instant. Electrons emitted from the remaining portion of the tube are wasted, rather than stored on the target as in the image orthicon. The image dissector has no storage characteristic, which accounts in part for its low sensitivity (approximately 3000 lux).
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.
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.
Vidicon.png
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.
Orthicon
The image orthicon tube or simply orthicon tube was common until the 1960s. It replaced the iconoscope, which required a great deal of light to work adequately. A properly constructed image orthicon could take television pictures by candlelight due 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 acround 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".
Plumbicon
Saticon
Pasecon
Newvicon
External links
- Orthicon brief history, description and diagram (http://www.acmi.net.au/AIC/IMAGE_ORTHICON.html)
- Camera tubes (http://members.chello.nl/h.dijkstra19/page4.html)