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6 janvier 2011 4 06 /01 /janvier /2011 10:05

A webcam is a video camera which feeds its images in real time to a computer or computer network, often via USB, ethernet or Wi-Fi.Their most popular use is the establishment of video links, permitting computers to act as videophones or videoconference stations      (Dell XPS M1210 Batteryhttp://www.hdd-shop.co.uk .

This common use as a camera for the world wide web gives the webcam its name. Other popular uses include security surveillance and computer vision.

Webcams are known for their low manufacturing costs and flexibility, making them the lowest cost form of videotelephony (Dell Studio XPS 1340 Battery)      .

They have also become the source of security and privacy issues, as some inbuilt webcams can be remotely activated via spyware.


First employed in 1991, a webcam was pointed at the Trojan Room coffee pot in the Cambridge University Computer Science Department  (Dell Studio XPS 1640 Battery)     .

The camera was finally switched off on August 22, 2001. The final image captured by the camera can still be viewed at its homepage.The oldest webcam still operating is FogCam at San Francisco State University, which has been running continuously since 1994  (Dell Studio XPS 1640 Battery)   .

One of the most widely reported-on webcam sites was JenniCam, started in 1996, which allowed Internet users to observe the life of its namesake constantly, somewhat like reality TV series Big Brother, launched three years later. More recently, the website Justin.tv has shown a continuous video and audio stream from a mobile camera mounted on the head of the site's star  (Dell Vostro 1710 Battery)  .

Other cameras are mounted at bridges, public squares, and other public places, their output made available on a public Web page in accordance with the original concept of a "webcam". Aggregator websites have arisen, providing thousands of live video streams or up-to-date still pictures, allowing users to find live video streams based on location or other criteria  (Dell KM958 battery)     .

Around the turn of the 21st century, computer hardware manufacturers began building webcams directly into laptop and desktop screens, thus eliminating the need to use an external USB or Firewirecamera. Gradually webcams came to be used more for communication between two people, or among a few people, than for offering a view on a Web page to an unknown public (Sony VGP-BPS13 battery)   .

The term 'webcam' may also be used in its original sense of a video camera connected to the Web continuously for an indefinite time, rather than for a particular session, generally supplying a view for anyone who visits its web page over the Internet. Some of them, for example those used as online traffic cameras, are expensive, rugged professional video cameras (Sony VGP-BPS13/B battery)  .


Video calling and conferencing

As webcam capabilities have been added to instant messaging, text chat services such as AOL Instant Messenger, and VoIP services such as Skype, one-to-one live video communication over the Internet has now reached millions of mainstream PC users worldwide (Sony VGP-BPS13/S battery)     .

Improved video quality has helped webcams encroach on traditional video conferencing systems. New features such as automatic lighting controls, real-time enhancements (retouching, wrinkle smoothing and vertical stretch), automatic face tracking and autofocus assist users by providing substantial ease-of-use, further increasing the popularity of webcams  (Sony VGP-BPS13A/B battery)   .

Webcam features and performance can vary by program, computer operating system, and also by the computer's processor capabilities. For example, 'high-quality video' is principally available to users of certain Logitech webcams if their computers have dual-core processors meeting certain specifications   (Sony VGP-BPS13B/B battery)    .

Video calling support has also been added to several popular instant messaging programs.

Video security

Webcams are also used as security cameras. Software is available to allow PC-connected cameras to watch for movement and sound, recording both when they are detected  (Sony VGP-BPL9 battery)      ;

these recordings can then be saved to the computer, e-mailed or uploaded to the Internet. In one well-publicised case, a computer e-mailed out images as the burglar stole it, allowing the owner to give police a clear picture of the burglar's face even after the computer had been stolen  (Sony VGP-BPS13B/B battery)   .

Video clips and stills

Webcams can be used to take video and still pictures. Various software tools in wide use can be employed for this, such as PicMaster under Windows, Photo Booth on a Mac, or Cheese under Unix  Sony VGP-BPL15 battery .

Input control device

Special software can use the video stream from a webcam to assist or enhance a user's control of applications and games. Video features, including faces, shapes, models and colors can be observed and tracked to produce a corresponding form of control  Dell Inspiron E1505 battery .

For example, the position of a single light source can be tracked and used to emulate a mouse pointer, a head mounted light would allow hands-free computing and would greatly improve computer accessibility. This can also be applied to games, providing additional control, improved interactivity and immersiveness    Dell Latitude E6400 battery .

FreeTrack is a free webcam motion tracking application for Microsoft Windows that can track a special head mounted model in up to six degrees of freedom and output data to mouse, keyboard, joystick and FreeTrack supported games By removing the IR filter of the webcam HP Pavilion dv6000 Battery ,

IR LEDs can be used, which has the advantage of being invisible to the naked eye, removing a distraction from the user. TrackIR is a commercial version of this technology  SONY VAIO VGN-FZ Battery .

The EyeToy for the PlayStation 2 (The updated PlayStation 3 equivalent is the PlayStation Eye) and similarly the Xbox Live Vision Camera and the Kinect AKA 'Project Natal' for the Xbox 360 and Xbox Live are color digital cameras that have been used as control input devices by some games  SONY VAIO VGN-FZ18 Battery .

Small webcam-based PC games are available as either standalone executables or inside web browser windows using Adobe Flash.


Webcams typically include a lens, an image sensor, and some support electronics. Various lenses are available, the most common in consumer-grade webcams being a plastic lens that can be screwed in and out to set the camera's focus  SONY VAIO VGN-FZ180E Battery .

Fixed focus lenses, which have no provision for adjustment, are also available. As a camera system's depth of field is greater for small imager formats and is greater for lenses with a large f-number (small aperture), the systems used in webcams have sufficiently large depth of field that the use of a fixed focus lens does not impact image sharpness much  SONY VAIO VGN-FZ220E Battery .

Image sensors can be CMOS or CCD, the former being dominant for low-cost cameras, but CCD cameras do not necessarily outperform CMOS-based cameras in the low cost price range. Most consumer webcams are capable of providing VGA-resolution video at a frame rate of 30 frames per second  SONY VAIO VGN-FZ340E Battery .

Many newer devices can produce video in multi-megapixel resolutions, and a few can run at high frame rates such as the PlayStation Eye, which can produce 320×240 video at 120 frames per second.

Support electronics are present to read the image from the sensor and transmit it to the host computer  SONY VAIO VGN-FZ430E Battery .

The camera pictured to the right, for example, uses a Sonix SN9C101 to transmit its image over USB. Some cameras, such as mobile phone cameras, use a CMOS sensor with supporting electronics "on die", i.e. the sensor and the support electronics are built on a single silicon chip to save space and manufacturing costs  SONY VAIO VGN-FZ460E Battery .

Most webcams feature built-inmicrophones to make video calling and videoconferencing more convenient.

The USB video device class (UVC) specification allows for interconnectivity of webcams to computers even without proprietary drivers installed  SONY VAIO VGN-FZ480E Battery .

MicrosoftWindows XP SP2, Linux and Mac OS X (since October 2005) have UVC drivers built in and do not require extra drivers, although they are often installed in order to add additional features.


Many users do not wish the continuous exposure for which webcams were originally intended, but rather prefer privacy SONY VAIO VGN-FZ4000 Battery .

Such privacy is lost when Trojan horse programs allow malicious hackers to activate a camera without the user's knowledge, providing the hackers with a live video and audio feed. Cameras such as Apple's older external iSight cameras include lens covers to thwart this  SONY VAIO VGN-FZ31E Battery .

Some webcams have built-in hardwired LEDindicators that lights up whenever the camera is active.

In mid-January 2005, some search engine queries were published in an on-line forum which allow anyone to find thousands of Panasonic- and Axis-made high-end web cameras, provided that they have a web-based interface for remote viewing SONY VAIO VGN-FZ31B Battery .

Many such cameras are running on default configuration, which does not require any password login or IP address verification, making them visible to anyone.

Some laptop computers have built in webcams which present both privacy and security issues  SONY VAIO VGN-FZ31J Battery ,

as such cameras cannot normally be physically disabled if hijacked by a Trojan Horse program or other similar spyware programs. In the 2010 Robbins v. Lower Merion School District "WebcamGate" case, plaintiffs charged that two suburban Philadelphia high schools secretly spied on students   SONY VAIO VGN-FZ31M Battery

—by surreptitiously remotely activating iSight webcams embedded in school-issued MacBook laptops the students were using at home—and thereby infringed on their privacy rights. School authorities admitted to secretly snapping over 66,000 photographs, including shots of students in the privacy of their bedrooms, including some with teenagers in various state of undress SONY VAIO VGN-FZ31Z Battery .

The school board involved quickly disabled their laptop spyware program after parents filed lawsuits against the board and various individuals.

Effects on modern society

Webcams allow for inexpensive, real-time video chat and webcasting, in both amateur and professional pursuits  SONY VAIO VGN-FZ38M Battery .

They are frequently used in online dating. YouTube is a popular website hosting many videos made using webcams. News websites such as the BBC can also produce professional live news videos.

Webcams encourage the phenomenon of telecommuting, where people work from home over the internet, rather than having to travel to their office     SONY VGP-BPS8 Battery .

On March 23, 2007, a man named Kevin Whitrick committed suicide live on the internet in front of viewers in a chat room website.

Sign language communications via webcam

Main articles: Video Relay Service, a telecommunication service for deaf, hard-of-hearing and speech-impaired (mute) individuals communicating with hearing persons at a different location SONY VGP-BPS13 Battery ,

andVideo Remote Interpreting, used where deaf/hard-of-hearing/mute persons are in the same location as their hearing parties

One of the first demonstrations of the ability for telecommunications to help sign language users communicate with each other occurred when AT&T'svideophone (trademarked as the 'Picturephone') was introduced to the public at the 1964 New York World's Fair –two deaf users were able to freely communicate with each other between the fair and another city  SONY VGP-BPS13/S Battery .

Various other organizations, including British Telecom's Martlesham facility and several universities have also conducted extensive research on signing via videotelephony. The use of sign language via videotelephony was hampered for many years due to the difficulty of using it over regular analogue phone lines coupled with the high cost of better quality data phone lines  SONY VGP-BPS13A/B Battery ,

factors which largely disappeared with the advent of high-speed ISDN and IP Internet services in the last decade of the 20th Century.

A deaf or hard-of-hearing person at his workplace using a VRS to communicate with a hearing person in London  SONY VGP-BPS13B/B Battery .

(Courtesy: SignVideo)

Using such video equipment, the deaf, hard-of-hearing and speech-impaired can communicate between themselves and with hearing individuals using sign language  SONY VGP-BPS13A/S Battery .

The United States and several other countries compensate companies to provide 'Video Relay Services' (VRS). Telecommunication equipment can be used to talk to others via a sign language interpreter, who uses a conventional telephone at the same time to communicate with the deaf person's party       SONY VGP-BPS13AS Battery .

Video equipment is also used to do on-site sign language translation via Video Remote Interpreting (VRI). The relative low cost and widespread availability of 3G mobile phonetechnology with video calling capabilities have given deaf and speech-impaired users a greater ability to communicate with the same ease as others. Some wireless operators have even started free sign language gatewaysDell Inspiron 1320n Battery .

Sign language interpretation services via VRS or by VRI are useful in the present-day where one of the parties isdeaf, hard-of-hearing or speech-impaired (mute). In such cases the interpretation flow is normally within the same principal language, such as French Sign Language (FSL) to spoken French      Dell Inspiron 1464 Battery ,

Spanish Sign Language (SSL) to spoken Spanish, British Sign Language (BSL) to spoken English, and American Sign Language (ASL) also to spoken English (since BSL and ASL are completely distinct), and so on       Dell Inspiron 1564 Battery .

A Video Interpreter (V.I.) assisting an on-screen client. (Courtesy: SignVideo)

Multilingual sign language interpreters, who can also translate as well across principal languages (such as to and from SSL, to and from spoken English), are also available, albeit less frequently     Dell Inspiron 1764 Battery .

Such activities involve considerable effort on the part of the translator, since sign languages are distinct natural languageswith their own construction, semantics and syntax, different from the aural version of the same principal language.

With video interpreting, sign language interpreters work remotely with live video and audio feeds, so that the interpreter can see the deaf or mute party, and converse with the hearing party, and vice versa  Dell Studio 1450 Battery .

Much like telephone interpreting, video interpreting can be used for situations in which no on-site interpretersare available. However, video interpreting cannot be used for situations in which all parties are speaking via telephone alone. VRI and VRS interpretation requires all parties to have the necessary equipment  Dell Studio 1457 Battery .

Some advanced equipment enables interpreters to remotely control the video camera, in order to zoomin and out or to point the camera toward the party that is signing.

Further information: Sign Language and Sign language interpreting Dell Latitude D610 Battery

Videotelephony descriptive names & terminology

Videophone calls (also: 'videocalls' and 'video chat') , differ from videoconferencing in that they expect to serve individuals, not groups      Toshiba NB100 Battery .

However that distinction has becoming increasingly blurred with technology improvements such as increased bandwidth and sophisticated software clients that can allow for multiple parties on a call. In general everyday usage the term videoconferencing is now frequently used instead of videocall for point-to-point calls between two units   Toshiba Satellite M65 battery .

Both videophone calls and videoconferencing are also now commonly referred to as a 'video link'.

Webcams are popular, relatively low cost devices which can provide live video and audio streams via personal computers, and can be used with many software clients for both video calls and videoconferencing       Toshiba Satellite M60 battery .

A videoconference system is generally higher cost than a videophone and deploys greater capabilities. A videoconference (also known as a videoteleconference) allows two or more locations to communicate via live, simultaneous two-way video and audio transmissions       Dell Latitude D830 Battery .

This is often accomplished by the use of a multipoint control unit (a centralized distribution and call management system) or by a similar non-centralized multipoint capability embedded in each videoconferencing unit. Again, technology improvements have circumvented traditional definitions by allowing multiple party videoconferencing via web-based applications Dell Latitude D620 Battery .

A separate webpage article is devoted to videoconferencing.

A telepresence system is a high-end videoconferencing system and service usually employed by enterprise-level corporate offices     Dell Inspiron Mini 10 Battery .

Telepresence conference rooms use state-of-the art room designs, video cameras, displays, sound-systems and processors, coupled with high-to-very-high capacity bandwidth transmissions.

Typical uses of the various technologies described above include videocalling or videoconferencing on a one-to-one Sony VGN-FW11S Battery ,

one-to-many or many-to-many basis for personal, busines s, educational, deaf Video Relay Service and tele-medical, diagnostic and rehabilitative use or services. New services utilizing videocalling and videoconferencing, such as personal videocalls to inmates incarcerated in penitentiaries  Sony VGN-FW11M Battery ,

and videoconferencing to resolve airline engineering issues at maintenance facilities, are being created or evolving on an on-going basis.

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    Sony VGN-FW139E/H battery .

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. All-electronic designs based on the cathode ray tube, such as Vladimir Zworykin's Iconoscope and Philo T       Dell Latitude E5400 Battery .

Farnsworth's Image dissector, supplanted the Baird system by the 1940s and remained in wide use until the 1980s, when cameras based on solid-stateimage 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  Dell Latitude E4200 Battery .

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. A few cameras still serve live television production  Dell Vostro A840 Battery ,

but most live connections are for 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  Dell Inspiron 300M Battery ,

videotape was the primary format used for this purpose, but optical disc media, hard disk, and flash memory are all increasingly used. Recorded video is used in television and film production, and more often surveillance and monitoring tasks where unattended recording of a situation is required for later analysis     Dell Studio 1737 battery .

Modern video cameras have numerous designs and uses, not all of which resemble the early television cameras.

  • Professional video cameras, such as those used in television and sometimes film production; these may be studio-based or mobile      Dell Inspiron E1505 battery .
  • Such cameras generally offer extremely fine-grained manual control for the camera operator, often to the exclusion of automated operation       Dell RM791 battery .
  • 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. Some digital ones are        Dell XPS M1530 battery
    • Pocket video cameras.
  • Closed-circuit television (CCTV) cameras, generally used for security, surveillance, and/or monitoring purposes. Such cameras are designed to be small, easily hidden, and able to operate unattended      Dell XPS M2010 battery ;
  • 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)       Acer Aspire One battery .
    • Webcams are video cameras which stream a live video feed to a computer. Larger video cameras (especially camcorders and CCTV cameras) can be similarly used, though they may need ananalog-to-digital converter in order to store the output on a computer or digital video recorder or send it to a wider network  Toshiba Satellite P10 Battery .
  • Digital cameras which convert the signal directly to a digital output; such cameras are often small, even smaller than CCTV security cameras, and are often used as webcams or optimized for still-camera use. The majority are incorporated directly into computer or communications hardware, particularly mobile phones   SONY VGN-FZ210CE Battery .
  • 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 radiation for Infrared photography (for night vision and heat sensing) or X-ray (for medical and video astronomy use)      Dell Precision M70 Battery .

Video camera tube

In older video cameras, before the mid to late 1980s, a video camera tube or pickup tube was used instead of a charge-coupled device (CCD) for converting a video image into an electrical signal. Several types were in use from the 1930s to the 1980s. The most commercially successful of these tubes were various types of cathode ray tubes or "CRTs"        Toshiba Satellite L305 Battery .

Any vacuum tube which operates using a focused beam of electrons ("cathode rays") is known as a cathode ray tube. However, in the popular lexicon"CRT" usually refers to the "picture tube" in a television or computer monitor. The proper term for this type of display tube is kinescope, only one of many types of cathode ray tubes     Toshiba Satellite T4900 Battery .

Others include the tubes used in oscilloscopes, radar displays, and the camera pickup tubes described in this article. (The word "kinescope" has also become the popular name for a film recording made by focusing a motion picture camera onto the face of a kinescope cathode ray tube, a common practice before the advent of video tape recording      Toshiba PA3399U-2BRS battery . )

Video camera tubes typically had a certain maximum brightness tolerance. If that limit were exceeded, such as by pointing the camera at the sun, sun-reflecting shiny surfaces, or extremely bright point light sources, the tube detecting surface would instantly "burn out" and be rendered insensitive on part or all of the screen. The only remedy was replacing the video tube Toshiba Satellite A200 Battery .

Early steps

In June 1908, the scientific journal Nature published a letter in which Alan Archibald Campbell-Swinton, fellow of the Royal Society (UK), discussed how a fully electronic television system could be realized by using cathode ray tubes (or "Braun" tubes, after its inventor, Karl Braun) as both imaging and display devices     Toshiba Satellite 1200 Battery .

But Campbell-Swinton noted that the "real difficulties lie in devising an efficient transmitter", and that it was possible that "no photoelectric phenomenon at present known will provide what is required". He later expanded on his vision in a presidential address given to the Röntgen Society in November 1911     Toshiba Satellite M300 Battery .

The photoelectric screen in the proposed transmitting device was a mosaic of isolated rubidium cubes.

In a letter to Nature published in October 1926, Campbell-Swinton also announced the results of some "not very successful experiments" he had conducted with G. M. Minchin and J. C. M. Stanton    Sony Vaio PCG-5G2L Battery .

They had attempted to generate an electrical signal by projecting an image onto a selenium-coated metal plate that was simultaneously scanned by a cathode ray beam. These experiments were conducted before March 1914, when Minchin died,  but they were later repeated by H. Miller and J. W. Strange in 1937, observing "very faint signals" from the original selenium-coated metal plate and "strong signals" when the plate was covered with zinc sulphide or selenide        Sony Vaio PCG-5G3L Battery .

Nevertheless, there is no evidence that Campbell-Swinton ever considered using zinc compounds instead of selenium in the transmitting device. A description of a CRT imaging device appeared in a patent application filed by Edvard-Gustav Schoultz in France in August 1921, and published in 1922,although a working device was not demonstrated until some years later Sony Vaio PCG-5J1L Battery .

Image dissector

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   Sony Vaio PCG-5K2L Battery .

Among the first to design such a device were German inventors Max Dieckmann and Rudolf Hell,who had titled their 1925 patent application Lichtelektrische Bildzerlegerröhre für Fernseher (Photoelectric Image Dissector Tube for Television).The term may apply specifically to a dissector tube employing magnetic fields to keep the electron image in focus     Sony Vaio PCG-5J2L Battery ,

an element lacking in Dieckmann and Hell's design, and in the early dissector tubes built by American inventor Philo Farnsworth.

Dieckmann and Hell submitted their application to the German patent office in April 1925, and a patent was issued in October 1927Sony Vaio PCG-5K1L Battery .

In 1951, Hell claimed that he had made a tube but could not get it to function, since at the time there was an insufficient knowledge of "electron optics" – the manipulation of an electron beam by electric or magnetic fields.

In January 1927, Farnsworth applied for a patent for his Television System that included a device for "the conversion and dissecting of light"      Sony Vaio PCG-5L1L Battery .

Its first moving image was successfully transmitted on September 7 of 1927,  and a patent was issued in 1930. Farnsworth quickly made improvements to the device, among them introducing an electron multiplier made of nickel and deploying a "longitudinal magnetic field" in order to sharply focus the electron image     Sony Vaio PCG-6S2L Battery .

The improved device was demonstrated to the press in early September 1928. The introduction of a multipactor in October 1933  and a multi-dynode "electron multiplier" in 1937 made Farnsworth's image dissector the first practical version of a fully electronic imaging device for television     Sony Vaio PCG-6S3L Battery .

Unfortunately, it had very poor light sensitivity, and was therefore primarily useful only where illumination was exceptionally high (typically over 685 cd/m²). However, it was ideal for industrial applications, such as monitoring the bright interior of an industrial furnace       Sony Vaio PCG-6V1L Battery .

Due to their poor light sensitivity, image dissectors were rarely used in television broadcasting, except to scan film and other transparencies.

In April 1933, Farnsworth submitted a patent application entitled Image Dissector, but which actually detailed a CRT-type camera tube, apparently the first to propose the use of a "low-velocity" scanning beam  Sony Vaio PCG-6W1L Battery .

However, he never transmitted a clear and well focused image with such a tube.

The optical system of the image dissector focuses an image onto a photocathode mounted inside a high vacuum. As light strikes the photocathode, electrons are emitted in proportion to the intensity of the light (see photoelectric effect)  Sony Vaio PCG-6W2L Battery .

The entire electron image is deflected and a scanning aperture permits only those electrons emanating from a very small area of the photocathode to be captured by the detector at any given time. The output from the detector is an electric current whose magnitude is a measure of the brightness of the corresponding area of the image   Sony Vaio PCG-6W3L Battery .

The electron image isperiodically deflected horizontally and vertically ("raster scanning") such that the entire image is read by the detector many times per second, producing an electrical signal that can be conveyed to adisplay device, such as a CRT monitor, to reproduce the image     Sony Vaio PCG-7111L Battery .

The image dissector has no "charge storage" characteristic; the vast majority of electrons emitted by the photocathode are excluded by the scanning aperture, and thus wasted rather than being stored on a photo-sensitive target, as in the iconoscope or image orthicon (see below), which largely accounts for its low light sensitivity     Sony Vaio PCG-7112L Battery .

The iconoscope

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     Sony Vaio PCG-7113L Battery .

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     Sony Vaio PCG-7133L Battery .

In Tihanyi's version of 1928, the electron beam scanned the granules, while in Zworykin's version of 1925, the electron beam scanned the back of the image plate    Sony Vaio PCG-7Z2L Battery .

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  Sony Vaio PCG-8Y1L Battery .

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     Sony Vaio PCG-8Y2L Battery .

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,  and a patent was issued in 1928. However the quality of the transmitted image failed to impress to H. P Sony Vaio PCG-8Z1L Battery .

Davis, the general manager of Westinghouse, and Zworykin was asked "to work on something useful".

A patent for a television system was also filed by Zworykin in 1923, but this file is not a reliable bibliographic source because extensive revisions were done before a patent was issued fifteen years later  and the file itself was divided into two patents in 1931Sony Vaio PCG-8Z2L Battery .

In 1926, the Hungarian engineer Kálmán Tihanyi explained in detail that the principle of "storing" electrical charges in proportion to the amount of light received throughout each scanning cycle results in a much more sensitive video camera tube. Two year later, in 1928, he applied for a patent for a refined "Television Apparatus" that is essentially an iconoscope  Sony VAIO PCG-5G2L Battery .

The first practical iconoscope was constructed in 1931 by Sanford Essig, when he accidentally left one 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  Sony VAIO PCG-5G3L Battery .

He also noticed that, "the tiny dimension of the silver droplets would enhance the image resolution of the iconoscope by a quantum leap." As head of television development at Radio Corporation of America (RCA), Zworykin submitted a patent application in November 1931, and it was issued in 1935  Sony VAIO PCG-5J1L Battery .

Nevertheless, Zworykin's team was not the only engineering group working on devices that use a charge stage plate. In 1932, the EMI engineers Tedham and McGee under the supervision of Isaac Shoenberg applied for a patent for a new device they dubbed "the Emitron"       Sony VAIO PCG-5K2L Battery .

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 USA in 1937       Sony VAIO PCG-5J2L Battery .

The iconoscope was presented to the general public in 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 4ft-c (43lx) and 20ft-c (215lx)     Sony VAIO PCG-5K1L Battery .

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     Sony VAIO PCG-5L1L Battery .


Within the iconoscope, an image was projected onto a plate containing a mosaic of electrically isolated photosensitive granules separated from a common plate by a thin layer of isolating material, each granule constituting a tiny capacitor with the common plate that accumulated and stored electrical charge in response to the light striking it  Sony VAIO PCG-6S2L Battery .

Emission of photoelectrons from each granule in proportion to the amount of light received resulted in a charge image being formed on the mosaic. An electron beam was then swept across the image plate from an electron gun, effectively scanning the stored image and discharging each capacitor in turn such that the electrical output from each capacitor was proportional to the average intensity of the light striking it between each discharge event        Sony VAIO PCG-6S3L Battery .

The accumulation and storage of photoelectric charges during each scanning cycle greatly increased the electrical output of the iconoscope relative to non-storage type image scanning devices       Sony VAIO PCG-6V1L Battery .

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      Sony VAIO PCG-6W1L Battery .

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     Sony VAIO PCG-6W2L Battery .

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 new video camera tube developed by Lubszynski, Rodda and McGee in 1934 was dubbed "the super-Emitron"  Sony VAIO PCG-6W3L Battery .

This tube is a combination of the image dissector and the Emitron. It has an efficientphotocathode 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  Sony VAIO PCG-7111L Battery .

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         Sony VAIO PCG-7112L Battery ,

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      Sony VAIO PCG-7113L Battery .

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.It was used for an outside broadcasting by the BBC, for the first time, on Armistice Day 1937, when the general public could watch in a television set how the King lay a wreath at the Cenotaph  Sony VAIO PCG-7133L Battery .

This was the first time that anyone could broadcast a live street scene from cameras installed on the roof of neighbor buildings.

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 results of the collaboration  Sony VAIO PCG-7Z1L Battery .

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 was not affected by the patent war between Zworykin and Farnsworth  Sony VAIO PCG-7Z2L Battery ,

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, two years before Farnsworth did the same in the United States       Sony VAIO PCG-8Y1L Battery .

The image iconoscope (Superikonoskop) became the industrial standard for public broadcasting in Europe from 1936 until 1960, when it was replaced by the vidicon and plumbicon tubes. Indeed it was the representative of the European tradition in electronic tubes competing against the American tradition represented by the image orthicon   Sony VAIO PCG-8Y2L Battery .

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 to 1958       Sony VAIO PCG-8Z1L Battery .


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  Sony VAIO PCG-8Z2L Battery ,

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       Sony VGP-BPS21 Battery ,

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  Sony VGP-BPS21A Battery .

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)      Sony VGP-BPS21B Battery .

The image iconoscope 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. Therefore, secondary electrons are emitted from the surface of the isolating material when the electron image reaches the target    Sony VGP-BPS21/S Battery ,

and the resulting positive charges are stored directly onto the surface of the isolated material.

Orthicon and CPS Emitron

The original iconoscope was very noisy due to the secondary electrons released from the photoelectric mosaic of the charge storage plate when the scanning beam swept it across  Sony VGP-BPS21A/B Battery .

An obvious solution was to scan the mosaic with an electron beam, which velocity and energy were so low in a neighborhood of the plate, that no secondary electrons were emitted at all. That is, an image is projected onto the photoelectric mosaic of a charge storage plate, so that positive charges are produced and stored there due to photo-emission and capacitance, respectively  SONY VGP-BPS21 Battery .

These stored charges are then "gently" discharged by a low-velocity electron scanning beam, preventing the emission of secondary electrons. Not all the electrons in the scanning beam may be absorbed in the mosaic, because the stored positive charges are proportional to the integrated intensity of the scene light       SONY VGP-BPS21A Battery .

The remaining electrons are then deflected back into the anode, captured by a specialgrid, 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      SONY VGP-BPS21B Battery .

However, there are serious problems as well, because the electron beam "spreads" when it scans the image's borders and corners, so that one gets an image that is well focused in the center but blurry in the borders. Another improvement is the use of a semitransparent charge storage plate        SONY VGP-BPS21/S Battery .

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    SONY VGP-BPS21A/B Battery .

In 1928, Tihanyi filed for a patent for a television apparatus where an electron beam scanned the mosaic of a charge storage plate and was deflected into the anode following a V-path. Five years latter, in 1933, Farnsworth filed for a patent where he explained for the first time that the electrons in the scanning beam must arrive with zero-velocity to the charge storage plate  WD passport essential (500GB/640GB) ,

so as to prevent the emission of secondary electrons. Nevertheless, neither Tihanyi nor Farnsworth considered the use of a semitransparent plate, and they never transmitted clear images with the devices described in their patents, because they were not aware that the beam alignment had to be perpendicular to the plate in order to produce a well focused image  WD passport essential (250GB/320GB) .

The first fully-functional low-velocity scanning beam tube, the CPS Emitron, was invented and demonstrated by the EMI team under the supervision of Isaac Shoenberg. 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     WD  passport essential SE (750GB/1TB)  ,

a negative (or slightly positive) grid lay very close to the plate, and a positive one was placed further away. The velocity and energy of the electrons in the scanning beam were reduced to zero by the decelerating electric field generated by this pair of grids, and so a low-velocity scanning beam tube was obtained       WD  passport elite(250GB/320GB) .

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  WD passport elite(500GB/640GB) .

The industrial production and commercialization of the CPS Emitron had to wait until the end of the second world war.

On the other side of the ocean, the RCA team led by Albert Rose began working in 1937 on a low-velocity scanning beam device they dubbed the orthicon  WD  passport studio for Mac(320GB/500GB) .

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 was the tube used in RCA's television demonstration at the 1939 New York World's Fair      WD  passport studio for Mac(500GB/640GB) ,

its performance was sim

ilar to the image iconoscope's one, 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".

Image orthicon

The image orthicon, (sometimes abbreviated IO) was common in American broadcasting from 1946 until 1968 WD  Elements series(250GB/320GB) .

A combination of the image dissector and the orthicon technologies, it replaced the iconoscope and the orthicon, which required a great deal of light to work adequately.

The image orthicon tube was developed at RCA by Albert Rose, Paul K. Weimer, and Harold B. Law  WD  Elements SE(500GB/640GB).

It represented a considerable advance in the television field, and after further development work, RCA created original models between 1939 and 1940. The National Defense Research Council entered into a contract with RCA where the NDRC paid for its further development       WD Elements SE(750GB/1TB) .

Upon RCA's development of the more sensitive image orthicon tube in 1943, RCA entered into a production contract with the U.S. Navy, the first tubes being delivered in January 1944. RCA began production of image orthicons for civilian use in the second quarter of 1946        WD  Elements desktop(500GB/640GB) .

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         WD  Elements desktop(750GB/1TB) .

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 used for capturing Apollo/Saturn rockets nearing orbit after the networks had phased them out, as only they could provide sufficient detail      WD  Elements desktop(1.5 TB/2TB) .

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         WD  passport essential SE (750GB/1TB)--USB 3.0)    .

However, it tends to flare in bright light, causing a dark halo to be seen around the object; this anomaly is 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, where their increased sensitivity was essential to overcome their very inefficient optical system  WD  passport essential (500GB/640GB) .


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           WD  passport for Mac(320GB/500GB) .

In the image store, light falls upon the photocathode which is a photosensitive plate at a very negative potential (approx. -600V), 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 (0V)  WD  passport for Mac(640GB/1TB) ,

and passes through a very fine wire mesh (near 200 wires per cm), very near (a few hundredths of cm) and parallel to the target, acting as a screen grid at a slightly positive voltage (approx +2V). Once the image electrons reach the target, they cause a "splash" of electrons by the effect of secondary emission        My book essential 4 generation  (640GB/1TB) .

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        WD  My book essential 4 generation( 1.5TB/2TB) .

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. +1500V)  WD  My book elite( 1TB/1.5TB) .

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       WD  My book studio(1TB/2TB) ,

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     WD  My book essential 4 generation( 1.5TB/2TB) .

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)    WD  My book elite(640GB/2TB) .

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  Seagate  expansion portable (320GB/500GB) .

Additional amplification is also performed via secondary emission in the electron multiplier which consists of a stack of charged dynodes (pinwheel-like disks surrounding the electron gun) in progressively higher potentials. As the returning electron beam hits the first dynode, it ejects electrons similarly to the target      Seagate  expansion (1.5TB/2TB) ;

for each electron striking a dynode, many are emitted. These secondary electrons are then drawn toward the next dynode at a higher potential, where the splashing continues for a number of steps. Consider a single, highly-energized electron hitting the first dynode, causing, say, four electrons to be emitted and drawn towards the next dynode  Seagate  Freeagent Desktop (500GB/1TB) .

Each of these might then cause four each to be emitted. Thus, by the start of the third stage, you would have about 16 electrons to the original one. As many as 5 to 10 stages were not unusual, thus the achieved amplification is very important  Seagate  Freeagent Go(250GB/320GB) .

Overall, the amplification at the image front and at the electron multiplier, plus the wise use of secondary emission wherever possible make the Image Orthicon an excellent camera tube, with a typical illumination on photocathode for maximum signal output of 0.01ft-c (0.1lx)        Seagate  Freeagent Go(500GB/640GB) .

This makes it on the order of a thousand times more sensitive than the iconoscope.

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   Seagate  Freeagent Go(750GB/1TB) .

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 Seagate  Freeagent Goflex(250GB/320GB) ,

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      Seagate Freeagent Goflex(500GB/640GB) .

Since darker images result in less positive charge on the target, the excess electrons deposited by the splash will be read as a darkregion 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      Seagate  Freeagent Goflex(750GB/1TB) .

(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   Seagate  Freeagent Goflex Pro(500GB/750GB) .


A vidicon 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 P. K. Weimer, S. V. Forgue and R. R. Goodrich as a simple alternative to the structurally and electrically complex Image Orthicon       Seagate  Freeagent Goflex desktop(1TB/2TB) .

While the initial photoconductor used was selenium, other targets–including silicon diode arrays–have been used.

The vidicon is a storage-type camera tube in which a charge-density pattern is formed by the imaged scene radiation on aphotoconductive surface which is then scanned by a beam of low-velocity electrons  Seagate  Freeagent go for Mac(320GB/640GB) .

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  Samsung  G2 protable (250gb/320GB) .

Prior to the design and construction of the Galileo probe to Jupiter in the late 70s to early 80s, NASA used Vidicon cameras on most of their unmanned deep space probes equipped with the remote sensing ability.

Vidicon tubes are notable for a particular type of interference they suffered from, known as vidicon microphony  Samsung  G2 protable (500GB/640GB) .

Since the sensing surface is quite thin, it is possible to bend it with loud noises. The artifact is characterized by a series of many horizontal bars evident in any footage (mostly pre 1990) in an environment where loud noise was present at the time of recording or broadcast       Samsung  S2 protable (320GB/500GB) .

A studio where a loud rock band was performing or even gunshots or explosions would create this artifact.


Plumbicon is a registered trademark of Philips for its Lead Oxide (PbO) target vidicons. Used frequently in broadcast camera applications, these tubes have low output, but a high signal-to-noise ratio  Samsung S1 Mini (120GB/160GB) .

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   Samsung  S1 Mini (250GB/320GB) .

Schematic of a Plumbicon tube. (This image is schematic, not to scale; a Plumbicon has the same shape as a vidicon.)

Compared to Saticons, Plumbicons had much higher resistance to burn in, and comet and trailing artifacts from bright lights in the shot. Saticons though, usually had slightly higher resolution     Samsung story station (1TB/1.5TB) .

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  Samsung Story station (1.5TB/2TB) .

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   Samsung story station Esata(1TB/1.5TB) .


Saticon is a registered trademark of Hitachi also produced by Thomson and Sony. Its surface consists of Selenium Arsenic Tellurium (SeAsTe).


Pasecon is a registered trademark of Heimann. Its surface consists of Cadmium Selenide (CdSe) Samsung  G3 station (1TB/1.5TB) .


Newvicon is a registered trademark of Matsushita. The Newvicon tubes were characterized by high light sensitivity. Its surface consists of a combination of Zinc Selenide (ZnSe) and Zinc Cadmium Telluride (ZnCdTe)  Maxtor  one touch 4 plus (500GB/750GB) .


Trinicon is a registered trademark of Sony. 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  Maxtor one touch 4 plus (1TB/1.5TB) .

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  Maxtor cool black(640GB/1TB) .

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      Maxtor Black diamond (320GB/500GB) .

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      Hitachi  simple touch (250GB/320GB) .

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       Hitachi simple touch (320GB/500GB) .

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 Hitachi  life studio (320GB/500GB) .

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     Hitachi life studio desk (500GB/1TB) ,

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         Hitachi life studio plus (320GB/500GB) .

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. It was also possible to construct a color camera that used a single image tube     Hitachi life studio plus (320GB/500GB) .

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 repeating across the target     Hitachi X mobile (250GB/320GB) .

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      Hitachi X mobile(320GB/500GB) .

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       Hitachi  XL (1TB/2TB) .

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  Toshiba canvio portable(320GB/500GB) .

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  Toshiba  canvio portable(750GB/1TB) .

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      Toshiba  anvio for Mac(500GB/750GB) .

A field-sequential system developed by Peter Goldmark for CBS was demonstrated to the press on September 4, 1940,[87] and was first shown to the general public on January 12, 1950.[88] Guillermo González Camarena developed a field-sequential color disk system in the early 1940s, for which he received the first US patent for color television in 1942      Toshiba  canvio for Mac(750GB/1TB) .

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