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Description: | EMI (makers t6rade name on side of camera). E.M.I. ELECTRONICS LTD HAYES MIDDX VIDICON CAMERA TYPE 201/7 MADE IN ENGLAND No. H318 (on the makers trade plate fixed to the chasis under the cover). E.M.I. ELECTRONICS LTD HAYES MIDDX VIEWFINDER HOOD TYPE 858000 MADE IN ENGLAND No. 413 (makers trade plate on the viewfinder hood). VIDITAL 5cm T 1.5 MADE BY TAYLOR< TAYLOR & DOBSON LTD England. Pat App. For. (makers mark stencilled on the lens). MANU E.M.I. Electronics Limited. circa 1958 Hayes, Middlesex, England. DES after GEISSLER,Johann,Heinrich,Wilhelm. 1855 Bonn, Germany, DES after PLUCKER,Julius 1859 Bonn, Germany, DES after CROOKES,William,Sir. 1870 London, England. This is an E.M.I. Vidicon 201/7 television camera based on a Vidicon image fube in conjunction with an electrostatic CRT tube as a view screen. The image focussing was achieved by rotating the large control on the side of the camera this surprisingly moved the the entire image tube and control coil assembly and not the lens.
The vidicon tube was a development of the American RCA Iconoscope tube that was a derivative of the Zworykin Iconoscope and its predecessor the dissector tube invented by Philo T. Farnsworth. It was developed in America by RCA in the early 1950's. The early tubes Iconoscope tubes were the fusion of earlier ideas on television transmission based on the original gas discharge tube work by Geissler and others.
In early black and white television scanning tubes the electrons impinged on a photosensitive target anode, which absorbed the light falling on its surface, that is the light collected by the lens system of the camera. This tube is controlled by electromagnetic coils . The first coil controls the focussing of the cathode or "electron gun" . The second and third set of coils adjust the vertical and horizontal scan of the of the electrons. This is done by applying a series of differing voltages, to the coils, controlled by a time base.
In this tube the anode is coated with a phtosensitive coating of antimony trisulphide. The controlling coils, surrounding the electron beam allow the beam to scan the target or anode. The tube was designed for black and white telvision transmissions.
The cathode voltage is probably quite low circa 5 volts and the postively charged annular ring surrounding the anode mops up unfocussed, secondary or back scattered electrons that have escaped the tube lining. The low cathode voltage supresses the production of x-rays and secondary electrons. The target or signal plate end of the tube i.e. the collector for the television signal generated by the signal plate. There is usually a lead in posively charged wire designed to capture secondary and back scattered electrons trapped by a coating on the inner surface of the tube. The coating is probably metal or metal oxide based and is there to collect any unwanted back scattered and secondary electrons from the target mozaic. There is probably a capacitor to bridge the gap between the signal plate and the electron collector effectively preventing induction between the two coatings and hence supressing interference.
The television camera transmitting tube is an electronic device that converts an optical image into a television picture signal. Four types of black and white television camera tubes were used until quite recently, they were the, orthicon, emitron, photicon, and the vidicon. They have been or are being replaced by solid state devices and miniature tubes for digital imaging. These older devices differed slightly but all had these principal components:-
a. A photosensitive surface, on which a picture was optically focussed by the camera lens and that converted the light into a corresponding electric charge forming an electronic image.
b. A means of storing the electrical image, by the means of a photochemical coating, that is, a way of causing the charge to accumulate during the interval of several hundredths of a second between each successive line scan of the image. The British standard set in 1936 was originally 405 lines in a full scan.
c. An electron beam that was formed by a cathode, often called the electron gun, that directed electrons over the electrical image in a scanning pattern.
d. A capablility to extinguish the scanning beam during its retrace motions, so that no television signal was then generated. N.B. A television pickup device that does not store the electrical image is called a flying spot scanner.
The iconoscope was a type of camera tube that was not as sensitive as the later image orthicon. Its images were also subject to uneven shading and flare. Within the wide end of the iconoscope tube was a flat sheet of mica that had a uniform metallic coating, called the signal plate or anode, on the rear surface of the mica. The front surface of the mica was covered with a mosaic composed of many hundreds of thousands of tiny droplets of silver that looked like a continuous coating to the naked eye. The mosaic of droplets was then treated with cesium vapour and oxygen, so that each droplet surface was coated with a thin layer of cesium oxide. This combination of materials provides a sensitive photoelectric surface or photomultiplier that converts photons of light to a proportionally larger electric charge on the surface of each droplet of silver. Since the droplets of silver were insulated from each other and from the signal plate or anode by the sheet of mica, the loss of electrons under illumination caused the coating to create a positive charge.
The scanning spot of the iconoscope was formed by a beam of low energy electrons generated at the cathode. Low energy electrons were less likely to create either secondary electrons or x-rays as all discharge tubes do. On its way to the mosaic, this beam passed between the electromagnetic fields of three sets of electromagnet coils. The first coil, which could be adjusted, focussed the electrons into a narrow spot on the surface of the signal plate, the second and third coils had varying voltages, controlled by a time base, that caused the beam to be deflected horizontally and vertically at a predetermined rate and in a predetermined sequence. In old television sets where the process is reversed this was equivalent to the horizontal and vertical hold controls. This pattern, traced out by the electron beam on the mosaic, was rectangular.
As each droplet of silver was hit by the beam of electrons, it underwent a drop in postive electrical charge, the amount of the change being proportional to the light that had fallen on it. The change in potential of the droplet was transferred through the mica support to the signal plate behind it by induction as in a dielectric capacitor. As the electron beam passed over the droplets of silver lying along a given scanning line in the pattern it created a succession of voltages called the picture signal. This signal which was about two hundred thousand times the original charge, because of the photomultiplying effect of the oxide coatings, was itself amplified before transmission as a radio frequency wave.
This type of test card generating tube was a derivative of the orthicon camera tube that had a mosaic made up of squares of photosensitive material, formed by evaporation of the material onto an optically transparent support. An optically transparent metal coating on the reverse side served as the signal plate or anode. The optical image was focussed on the signal plate, passing through it and the support to the mosaic, where it created and stored a positive electrical charge image, in the same way as the iconoscope. The electron beam passed over the mosaic in the standard scanning pattern. The negative electrons, in the beam, land on the mosaic of photosensitive squares, neutralizing the stored positive charge and ultimately building up a negative charge. The electrons in the beam were then repelled and returned to the opposite end of the tube, where they are removed by a positive collecting electrode that surrounded the cathode. The change in electrical potential was transferred by induction, as in the iconoscope, to the signal plate creating the picture signal.
In this gray scale test card generator the parts of the mozaic are blanked off in a predetermined pattern which can be seen through the space between the two layers of aquadag coating. By varing the spacing of the blanked elements of the mozaic a pattern of varying shades of grey, between white and black, was formed. A uniform potential was applied to the mozaic via the end cap in the tube which mimicked an even optical illumination and when scanned the potential was transferred by induction to the signal plate as in a normal camera tube. | License: | http://www.hmag.gla.ac.uk/spirit/rights/ | Publisher: | Hunterian Museum and Art Gallery, University of Glasgow | Rights holder: | Hunterian Museum and Art Gallery, University of Glasgow | Subjects: | SCIENTIFIC COLLECTION : | Source: | Hunterian Museum | Creator: | Hunterian Museum and Art Gallery, University of Glasgow | Identifier: | http://www.huntsearch.gla.ac.uk/cgi-bin/... | Language: | en-GB | Go to resource |
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