Video camera tube
Video camera tubes are devices based on the cathode-ray tube that were used in television cameras to capture television images, prior to the introduction of charge-coupled device image sensors in the 1980s. Several different types of tubes were in use from the early 1930s, and as late as the 1990s.
In these tubes, an electron beam is scanned across an image of the scene to be broadcast focused on a target. This generated a current that is dependent on the brightness of the image on the target at the scan point. The size of the striking ray is tiny compared to the size of the target, allowing 480–486 horizontal scan lines per image in the NTSC format, 576 lines in PAL, and as many as 1035 lines in Hi-Vision.
Cathode-ray tube
Any vacuum tube which operates using a focused beam of electrons, originally called cathode rays, is known as a cathode-ray tube. These are usually seen as display devices as used in older television receivers and computer displays. The camera pickup tubes described in this article are also CRTs, but they display no image.Early research
In June 1908, the scientific journal Nature published a letter in which Alan Archibald Campbell-Swinton, fellow of the Royal Society, discussed how a fully electronic television system could be realized by using cathode-ray tubes as both imaging and display devices. He 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". A cathode-ray tube was successfully demonstrated as a displaying device by the German Professor Max Dieckmann in 1906; his experimental results were published by the journal Scientific American in 1909. Campbell-Swinton later expanded on his vision in a presidential address given to the Röntgen Society in November 1911. The photoelectric screen in the proposed transmitting device was a mosaic of isolated rubidium cubes. His concept for a fully electronic television system was later popularized as the "Campbell-Swinton Electronic Scanning System" by Hugo Gernsback and H. Winfield Secor in the August 1915 issue of the popular magazine Electrical Experimenter and by Marcus J. Martin in the 1921 book The Electrical Transmission of Photographs.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. 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 two different teams in 1937, by H. Miller and J. W. Strange from EMI, and by H. Iams and A. Rose from RCA. Both teams succeeded in transmitting "very faint" images with the original Campbell-Swinton's selenium-coated plate, but much better images were obtained when the metal plate was covered with zinc sulphide or selenide, or with aluminum or zirconium oxide treated with caesium. These experiments would form the base of the future vidicon. A description of a CRT imaging device also 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.
Experiments with image dissectors
An image dissector is a camera tube that creates an "electron image" of a scene from photocathode emissions which pass through a scanning aperture to an anode, which serves as an electron detector. 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. The term may apply specifically to a dissector tube employing magnetic fields to keep the [|electron image in focus], 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 1927. Their experiments on the image dissector were announced in September 1927 issue of the popular magazine Discovery and in the May 1928 issue of the magazine Popular Radio. However, they never transmitted a clear and well focused image with such a tube.
In January 1927, American inventor and television pioneer Philo T. Farnsworth applied for a patent for his Television System that included a device for "the conversion and dissecting of light".
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 using a "longitudinal magnetic field" in order to sharply [|focus the electron image].
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. It had very poor light sensitivity, and was therefore primarily useful only where illumination was exceptionally high. However, it was ideal for industrial applications, such as monitoring the bright interior of an industrial furnace. 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 also entitled Image Dissector, but which actually detailed a CRT-type camera tube. This is among the first patents to propose the use of a "low-velocity" scanning beam and RCA had to buy it in order to sell image orthicon tubes to the general public. However, Farnsworth never transmitted a clear and well focused image with such a tube.
Dissectors were used only briefly for research in television systems before being replaced by different much more sensitive tubes based on the charge-storage phenomenon like the iconoscope during the 1930s. Although camera tubes based on the idea of image dissector technology quickly and completely fell out of use in the field of television broadcasting, they continued to be used for imaging in early weather satellites and the Lunar lander, and for star attitude tracking in the Space Shuttle and the International Space Station.
Operation
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. 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. The electron image is periodically deflected horizontally and vertically such that the entire image is read by the detector many times per second, producing an electrical signal that can be conveyed to a display device, such as a CRT monitor, to reproduce the image.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.
Charge-storage tubes
Iconoscope
The early electronic camera tubes suffered from a very disappointing and fatal flaw: They scanned the subject and what was seen at each point was only the tiny piece of light viewed at the instant that the scanning system passed over it. A practical functional camera tube needed a different technological approach, which later became known as charge storage camera tube. It was based on a new physical phenomenon which was discovered and patented in Hungary in 1926, but became widely understood and recognised only from around 1930.An iconoscope is a camera tube that projects an image on a special charge storage plate containing a mosaic of electrically isolated photosensitive granules separated from a common plate by a thin layer of isolating material, somewhat analogous to the human eye's retina and its arrangement of photoreceptors. Each photosensitive granule constitutes a tiny capacitor that accumulates and stores electrical charge in response to the light striking it. An electron beam periodically sweeps across the plate, effectively scanning the stored image and discharging each capacitor in turn such that the electrical output from each capacitor is proportional to the average intensity of the light striking it between each discharge event.
After Hungarian engineer Kálmán Tihanyi studied Maxwell's equations, he discovered a new hitherto unknown physical phenomenon, which led to a break-through in the development of electronic imaging devices. He named the new phenomenon as charge-storage principle.
The problem of low sensitivity to light resulting in low electrical output from transmitting or camera tubes would be solved with the introduction of charge-storage technology by Tihanyi in the beginning of 1925. His solution was a camera tube that accumulated and stored electrical charges within the tube throughout each scanning cycle. The device was first described in a patent application he filed in Hungary in March 1926 for a television system he dubbed Radioskop. After further refinements included in a 1928 patent application, Tihanyi's patent was declared void in Great Britain in 1930, and so he applied for patents in the United States. Tihanyi's charge storage idea remains a basic principle in the design of imaging devices for television to the present day.
In 1924, while employed by the Westinghouse Electric Corporation in Pittsburgh, Pennsylvania, Russian-born American engineer Vladimir Zworykin presented a project for a totally electronic television system to the company's general manager. 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 sandwiched between a screen and a colloidal deposit of photoelectric material consisting of isolated globules. 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. 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 H.P. 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 filing is not a definitive reference because extensive revisions were done before a patent was issued fifteen years later and the file itself was divided into two patents in 1931.
The first practical iconoscope was constructed in 1931 by Sanford Essig, when he accidentally left a 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. 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, Zworykin submitted a patent application in November 1931, and it was issued in 1935. Nevertheless, Zworykin's team was not the only engineering group working on devices that used a charge storage 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". 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 US in 1937.
The iconoscope was presented to the general public at 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 40and215lux. 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.