Video camera
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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 experimental broadcasts through the 1930s. Allelectronic designs based on the cathode ray tube, such as Vladimir Zworykin's
Iconoscope and Philo T. Farnsworth's Image dissector, supplanted the Baird system by
the 1940s and remained in wide use until the 1980s, when cameras based on solid-state
image sensors such as CCDs (and later CMOS active pixel sensors) eliminated common
problems with tube technologies such as burn-in and made digital video workflow
practical.
Video cameras are used primarily in two modes. The first, characteristic of much early
television, is what might be called a live broadcast, where the camera feeds real time
images directly to a screen for immediate observation; in addition to live television
production, such usage is characteristic of security, military/tactical, and industrial
operations where surreptitious or remote viewing is required. The second is to have the
images recorded to a storage device for archiving or further processing; for many years,
videotape has been the primary format used for this purpose, but optical disc media, hard
disk, and flash memory are all increasingly used. Recorded video is used not only in
television and film production, but also surveillance and monitoring tasks where
unattended recording of a situation is required for later analysis.
Modern video cameras have numerous designs and uses, not all of which resemble the
early television cameras.
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Professional video cameras, such as those used in television and sometimes film
production; these may be studio-based or mobile. Such cameras generally offer
extremely fine-grained manual control for the camera operator, often to the
exclusion of automated operation.
Camcorders, which combine a camera and a VCR or other recording device in
one unit; these are mobile, and are widely used for television production, home
movies, electronic news gathering (including citizen journalism), and similar
applications.
Closed-circuit television cameras, generally used for security, surveillance, and/or
monitoring purposes. Such cameras are designed to be small, easily hidden, and
able to operate unattended; those used in industrial or scientific settings are often
meant for use in environments that are normally inaccessible or uncomfortable for
humans, and are therefore hardened for such hostile environments (e.g. radiation,
high heat, or toxic chemical exposure). Webcams can be considered a type of
CCTV camera.
Digital cameras which convert the signal directly to a digital output; such cameras
are often extremely small, even smaller than CCTV security cameras, and are
often used as webcams or optimized for still-camera use. These cameras are
sometimes incorporated directly into computer or communications hardware,
particularly mobile phones, PDAs, and some models of laptop computer. Larger
video cameras (especially camcorders and CCTV cameras) can also be used as
webcams or for other digital input, though such units may need to pass their
output through an analog-to-digital converter in order to store the output or send it
to a wider network.
Special systems, like those used for scientific research, e.g. on board a satellite or
a spaceprobe, or in artificial intelligence and robotics research. Such cameras are
often tuned for non-visible light such as infrared (for night vision and heat
sensing) or X-ray (for medical and astronomical use).
In 1931, five years after Kálmán Tihanyi's electronic camera tube in 1926, Vladimir
Zworykin filed for a patent on a camera tube that projected an image on a special plate on
which was set a mosaic of photosensitive material, a pattern comparable to the receptors
of the human eye. Emission of photoelectrons from each granule in proportion to the
amount of light received 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 image
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
Farnsworth 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. The
iconoscope was the primary camera tube used in American broadcasting from 1936 until
1946, when it was replaced by the image orthicon tube.[1][2]
[edit] Operation
The image entered through the series of lenses at upper right, and was projected onto a
photosensitive surface. The mosaic of photosensitive elements emitted an electric charge
in variance with the amount of light hitting them. The cathode ray at the right swept the
image plate, discharging the electrostatic charges. The successive discharges from the
image plate were carried out the left side of the tube and amplified.
[edit] Image Orthicon
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Schematic of image orthicon tube.
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.
The image orthicon tube was developed by Dr. Albert Rose, Paul K. Weimer, and Harold
B. Law in the employ of the RCA. It represented a considerable advance in the television
field, and after further development work, RCA created original models about 1939–
1940. Recognizing the merit of the tube, the National Defense Research Council entered
into a contract with RCA whereby NDRC bore the expense of further development.
RCA's development of the more sensitive image orthicon tube was sufficiently advanced
at the end of 1943 to allow the execution of a production contract with the U.S. Navy,
and the first tubes under the contract were delivered in January of 1944.[3][4] RCA began
production of image orthicon cameras for civilian use in the second quarter of 1946.[5]
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 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, 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 (an anomaly
referred to as "blooming" in the broadcast industry when IO tubes were the standard).
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".
[edit] 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 hitting 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.
[edit] 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-focussed that 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.
[edit] Vidicon
A vidicon tube (sometimes called a hivicon 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 PK Weimer, SV Forgue and RR 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.
Schematic of vidicon tube.
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.
Prior to the design and construction of Galileo probe to Jupiter in the late 70s, NASA
used Vidicon camera on most of their unmanned deep space probes equipped with the
remote sensing ability
[edit] Plumbicon
Plumbicon is a registered trademark of Philips for its Lead Oxide 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 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.
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.
The company says, "In comparison to other image tube technologies, Plumbicon tubes
offer high resolution, low lag and superior image quality."
http://www.nimaging.com/about/history.html
http://www.nimaging.com/products/tubes/index.html
http://www.nimaging.com/products/tubes/plumbicon_broadcast.html
Surface: PbO — Lead Oxide.
[edit] Saticon
Saticon is a registered trademark of Hitachi also produced by Thomson and Sony. Its
surface consists of SeAsTe — Selenium Arsenic Tellurium.
[edit] Pasecon
Pasecon is a registered trademark of Heimann. Its surface consists of CdSe — Cadmium
selenide.
[edit] Newvicon
Newvicon is a registered trademark of Matsushita. The Newvicon tubes were
characterized by high light sensitivity. Its surface consists of ZnSe, ZnCdTe — Zinc
Selenide, Zinc Cadmium Telluride.
[edit] Trinicon
Trinicon is a registered trademark of Sony. It uses a vertically striped RGB color filter
over the faceplate of the 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.
http://www.labguysworld.com/Sony_DXC-1600.htm for a more detailed explanation of
the Trinicon tube.
[edit] Technological obsolescence
For television camera uses, the vidicon has been technologically superseded by the CCD
and CMOS.