US2267823A - Scanning device for television - Google Patents

Description

Dec. 30, 1941. c GOLDMARK 2,267,823

SCANNING DEVICE FOR TELEVISION Filed Dec. 10, 1936 Patented Dec. 30, 194.1

SCANNING DEVICE FOR TELEVISION Peter C. Goldmark, New York, N. Y., assignor to Markia Corporation, New York, N. Y., a corporation of New York Application December 10, 1936, Serial No. 115,114

8 Claims.

This invention relates to a scanning device for use in television, and more particularly to a device comprising a cathode ray tube for scanning an image to be televised.

The invention has for its object generally an improved construction and arrangement of parts for increasing the efficiency of a device which scans an image to be televised by means of a cathode ray tube.

More specifically, an object of the invention is to provide improved means for increasing the strength of the signal impulse from a cathode ray tube which has an electron emitting screen or target on which the image to be televised is projected.

Another object is to provide a cathode ray image scanning tube with improved means for increasing the electrons set free in a cathode ray image scanning tube.

Another object is to provide a cathode ray image scanning tube with built-in electron multiplying means, whereby the need for external amplification of the signal current is reduced.

Still another object is to provide a cathode ray image scanning tube which has a mosaic type of target or screen with internally associated electron multiplying electrodes.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combination of elements and arrangement of parts, which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims,

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing, in which:

Fig. l is a view, partly in section and partly in elevation, showing a cathode ray image scanning I electrodes employed in the arrangement of Fig. 1.

One type of image scanning cathode ray tube now known is provided with a so-called mosaic type of screen or target upon which the cathode rays impinge. Such mosaic screen or target is preferably coated with discrete particles of a photoelectric substance disposed on an insulating layer that overlies a conducting layer. When an image is projected upon the photoelectric material overlying the insulating material, electrons are emitted proportionally to and depending upon the illumination of the particles. Under the infiuence of the projected image, charges accumulate on opposite sides of the insulating layer which may be released by the scanning cathode ray and the impulses produced by the release passed in an external circuit. 'The current so passed varies with the light intensity of the picture elements projected upon the screen or target. A picture of high definition requires that the photoelectric particles be small, and a fortiori, that the area impinged by the cathode ray be also small. Since it is not practical to amplify the signal current beyond a certain degree, practical limitations have heretofore been imposed upon either the definition of the image or the signal current strength that may be employed.

By the present invention, such limitations are avoided by providing an associated electron multiplying means combined with the mosaic grid or target and embodied within the evacuated envelope of the cathode ray tube. In consequence, substantially any practical degree of definition and illumination in the picture to be televised may be employed and the current signals set up may be given an initial magnitude which it is feasible to amplify.

Referring now to the drawing, I denotes an evacuated envelope or tube arranged to house an electrode structure adapted to produce an electron beam for image scanning purposes in a cathode ray tube. This tube is shown as provided with conventional electrode structures for producing the scanning cathode beam or ray. The cathode is diagrammatically shown at I I, while the anode and control electrode structure is shown at I2. Cathode ray deflecting means are also associated with the tube, for example, magnet windings I3 and I3 which are indicated symbolically as having their magnetic axes arranged to be mutually perpendicular and are disposed to deflect the cathode ray in horizontal and vertical planes respectively. Suitable sweepcircuit current generators for supplying these windings are shown symbolically at l4 and I 4. A mosaic type target or screen is shown at I5, disposed in a plane which makes an angle other than a right angle with the axis of the neck of the tube Ill. An optical system, here depicted as comprising a simple lens 16,. is disposed adjacent the tube In for projecting the image to be scanned upon the screen or target l5,

A suitable construction for the screen or target here employed is shown in Fig. 2, where H denotes a perforated metal plate which has a layer of insulation -|8 disposed on the side on which the image is projected. Overlying the insulation l8 are discrete particles IQ of suitable photoelectric material. Granules of any suitable photothe target or screen l and a scanningcathode --ray-impinges thereon, for example, as shown at R. in the drawing, electrons are released from the surface of the target, and, under the acceleratin influence of the potential due to the source 21 and the directing influence of the winding 3|,

will move in straight lines with minor helical de- I partures as shown at a, toward the electron emitresponsive metal may be employed, for example,

granules of silver, the surfaces of which have been oxidized and coated with caesium. The perforations in thetarget I5 are shown at and are relatively close; for example, these perforations maybe of a fineness such as toprovide about nine openings per picture element scanned.

The electron multiplying means provided in accordance with the invention .are associated with the screen or'target within the tube I0. For I this purpose, the tube It has an extension lllr preferably disposed so that itsaxis of symmetry,

is perpendicular to the plane of the'target' I5,

In the extension Wm are arranged a, suitable ce-' ries of electron emitting grids, for example, such- These grids are elec- 'trically connected to the screen or target l5 by as shown at 2|, 22, 23, etc.

means of aconductor '28 which is electrically connected across an external source of D. C. 130- tential 21 that has one pole directly connected by other end of the conductor 25 to the source of potential 2'! preferably has a biasing resistance 2%! in series with. it and has :a connection through a condenser with the external circuit which is adapted to carry the signal current. In order that the electrons emitted by the electrode structure within the tube ||l may be accelerated from the target l5 through a desired path toward the final grid, suitable means are provided. A helical path has been ascertained to be advantageous; accordingly, a focusing coil comprising a winding 3| is disposed externally about the extension lflsc of the tube Ill and energized by means of current from a D. C. source of current 32.

The electron emitting grids shown at 2|, 22, 23, etc. are constructed in any suitable manner that provides the desired electron emitting property. Where a focusing coil, as at 3|, is employed, the spacing of these grids is such that they are equidistant and in conjugate planes. An exemplary construction of the electron emitting grids is shown in Fig. 3, where a wire grid structure is indicated at comprising woven woof and warp elements made of metal. On the side of this woven structure, which is adapted to be impinged with electrons from another electrode, a coating of special electron emitting metal is applied, as indicated at 36 and 36'. This electron emitting coating may be caesium, or a layer of silver that is plated on, then subjected to surface oxidation, and finally coated with caesium. As the final grid of this series, here shown as grid 25, is not required to emit electrons, the coatings 36 and 36' may be omitted therefrom; i. e., in the structure here shown, grid 25 may be of plain woven wire.

In operation, when an image is projected on ting grid 2|.

2|, secondary electrons are emitted which aug- 'ment the primary electrons'which'follow multiple paths,- as shown at I), under the accelerating influence of :the source of potential 21 and the directing influence of the Winding 3|, so as to impinge upon the electron emitting grid 22.

- Here,the process is repeated and further secondary electrons are emitted, the number of electrons being thus repeatedly augmented by secondary electrons-from eachof the grids that are dis- I posed in succession in the extension mm: of the envelope, until all are finally collected by the grid 25. .As:a result, a signal current of relatively high value is passed in the conductor 29' which leads from the end of the electron emitting grid; series. :This signal current which varies in magnitude with the intensity of the image-point at R, is passed to the external signal carrying circuit by means of the condenser 30.

As the ray R is moved to scan the various'elements of the mosaic screen which are illuminated with varying illumination by the image projected thereon by the optical system, a varying amount of primary electrons are released. lhe electron I multiplication effectedby the present invention is seen to be substantially proportional in each case. Consequently, the current in the external 1 circuit. though augmented is substantially proportional to the illumination of the picture -element scanned.

Since certain changes may be made in the above construction and different embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

Having described my invention, what I claim as new and desire to secure by Letters Patent is:

1. In electron tube apparatus for transmitting television signals, the combination which comprises a photoelectric perforated mosaic screen for receiving on one side thereof an image of a scene to be transmitted, an electrode structure for producing an electron scanning beam positioned to cause said scanning beam to impinge on said one side of the mosaic screen, means for causing said beam to scan said mosaic screen to produce an electron signal current from image-points of said screen successively scanned corresponding to the various gradations of the image, a series of perforated electron emissive screens positioned on the side of the mosaic screen opposite said one side and each disposed to receive electrons from its predecessor and adapted to emit secondary electrons to its successor, the first screen of said series being disposed to receive electrons from said mosaic screen, said emissive and mosaic screens being substantially parallel and substantially coaxial, a final screen for collecting electrons from the last screen of said series to form a signal, and means including a source of Voltage and a plurality of resistors for maintaining said mosaic screen, each member of said series of When the electrons following the w path a impinge upon the electron emitting grid screens and said final screen at increasingly positive potentials.

2. In electron tube apparatus for transmitting television signals, the combination which comprises a perforated photoelectric mosaic screen for receiving on one side thereof an image of a scene to be transmitted, an electrode structure for producing an electron scanning beam positioned to cause said scanning beam to impinge on said one side of the mosaic screen, means for scanning said screen with said beam to produce an electron signal current from image-points of said screen successively scanned, an electronmultiplying structure including a series of electron-emissive screens disposed in the path of the said electron signal current on the side of the mosaic screen opposite said one side, said series of screens being substantially parallel to each other and to the mosaic screen, focusing means positioned to direct the electrons from one screen to the next, and means for collecting the multiplied electron signal current.

3. In electron tube apparatus for transmitting television signals, the combination which comprises a perforated photoelectric mosaic screen for receiving on one side thereof an image of a scene to be transmitted, an electrode structure for producing an electron scanning beam positioned to cause said scanning beam to impinge on said one side of the mosaic screen, means for scanning said screen with said beam to produce an electron signal current corresponding to the imagepoints of said image successively scanned, a series of electron-emissive screens disposed in the path of the said electron signal current on the side of the mosaic screen opposite said one side and substantially parallel to each other and to the mosaic screen, said emissive and mosaic screens being arranged substantially coaxially and the series of emissive screens being spaced at substantially equal intervals from the mosaic screen, means for maintaining the screens of said series at successively higher positive potentials with respect to the mosaic screen to multiply the said electron signal current by secondary electron emission from said series of screens, focusing means for directing the electrons from screen to screen of said series, and means for collecting the multiplied electron signal current.

4. In electron tube apparatus for transmitting television signals, in combination, a perforated photoelectric mosaic screen for receiving on one side thereof an image of a scene to be transmitted, an electrode structure for producing an electron scanning beam positioned to cause said scanning beam to impinge on said one side of the mosaic screen, means for scanning said mosaic screen with said scanning beam to produce an electron signal current corresponding to the image-points of said image successively scanned, a series of secondary-electron emissive screens disposed on the opposite side of the mosaic screen from said one side and substantially parallel to each other and to the mosaic screen, said emissive and mosaic screens being arranged substantially coaxially, means for maintaining the screens of said series at successively higher potentials with respect to the mosaic screen to thereby multiply the said electron signal current by secondary electron emission from said series of screens, and means for collecting the multiplied electron signal current.

5. In electron tube apparatus for transmitting television signals, in combination, a perforated photoelectric mosaic screen for receiving on one side thereof an image of a scene to be transmitted, an electrode structure for producing an electron scanning beam positioned to cause said scanning beam to impinge on said one side of the mosaic screen, means for scanning said mosaic screen with said scanning beam to produce an electron signal current corresponding to the image-points of said image successively scanned, and an electron multiplier adapted to multiply said electron signal current including a secondary-electron emissive screen positioned on the side of said mosaic screen opposite said one side and substantially parallel and coextensive with the mosaic screen, said secondary-electron emissive screen being maintained at a higher potential than said mosaic screen to thereby collect the said electron signal current from the mosaic screen.

6. In electron tube apparatus for receiving an optical image and deriving a television signal corresponding thereto, in combination, a photoelectric mosaic screen for receiving an image on one side thereof, means for producing an electron scanning beam positioned and adapted to scan said one side of the mosaic screen and thereby produce an electron signal current corresponding to successively scanned areas of an image on said mosaic screen, and an electron multiplier adapted to multiply said electron signal current including a secondary-electron emissive screen adapted to receive the said electron signal current and positioned on the opposite side of the mosaic screen from said one side and substantially parallel and substantially coaxial therewith.

7. In electron tube apparatus for receiving an optical image and deriving a television signal corresponding thereto, in combination, a perforated photoelectric mosaic screen for receiving an optical image on one side thereof, means for producing an electron scanning beam positioned and adapted to scan said one side of the mosaic screen and thereby produce an electron signal current corresponding to successively scanned areas of an image on said mosaic screen, and an electron multiplier adapted to receive and multiply said electron signal current including a series of spaced perforated secondary-electron emissive screens positioned on the side of said mosaic screen opposite said one side, the screens of said series being substantially parallel and coextensive with each other and with the mosaic screen.

8. In an electronic tube for transmitting television signals, the combination which comprises an electron-emissive mosaic screen for receiving an image of a scene to be transmitted, a source of an electron scanning beam for scanning one side of said mosaic screen to cause an electron signal current to be emitted from successively scanned areas of said mosaic screen, and an electron multiplier for multiplying said electron signal current including a secondary-electron emissive screen positioned near said mosaic screen on the side thereof opposite said one side, said secondary-electron emissive screen being substantially parallel and substantially coextensive with said mosaic screen and maintained at a potential sufiiciently high with respect to said mosaic screen to draw said electron signal current toward said secondary-electron emissive screen.

PETER C. GolDMARK,

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