US2518434A

US2518434A - Electron discharge device such as a television transmitting tube

Info

Publication number: US2518434A
Application number: US777975A
Authority: US
Inventors: Lubszynski Hans Gerhard
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1945-07-12
Filing date: 1947-10-04
Publication date: 1950-08-08
Anticipated expiration: 1967-08-08

Description

k 1950 v H a. LUBSIZYNSKI 2,518,4

ELECTRbN DISCHARGE DEVICE SUCH AS TELEVISION TRANSMITTING TUBES Filed Oct. 4, 1947 INVENTOR HANS 'GERHARD Luaszwvskl ATTORNEY Patented Aug. 8, 1950 Hans Gerhard Lubszynskij Northwood, England, assignor to Electric & Musical Industries Lim- UNITED, STATE 1 ited, Hayes, Middlesex, England, a company of GreatBritaimg Application cause 4, 194i, Serial No.l77,9"75

' I In Great Britain July12, 1945 Sectiond, Public. Law 690, August 8, 1946 Patent expires July 12,1965

' scr ams. (01. 250-464) I This invention relates to electron discharge devices 'such as television transmitting tubes of the kind comprising a double-sided target electrode, a photo electric cathode so arranged that photo-electrons can be projected on to said tara mosaic screen formed on an insulated conducting mesh the apertures of which are provided with conducting elements. If no such mosaic screen is employed and the target electrode is formed in a different manner, then the recurring patternis usually reintroduced in the form of a signal plate consisting of a mesh disposed in close proximity to the target electrode.

The object of the present invention is to provide an improved electron discharge device of the kind described which avoids the use of a structure havingja regularly recurring pattern, such as a grid or grill, in or close to the target electrode.

According to thefpresentinvention there is provided an electron discharge device of the kind described wherein said target electrode comprises a sh'eetpf .semi 'conducting material provided with an insulating film on theside from which the said electron image is projected, and on said insulating film a conducting layer is provided, the arrangement being such that photo-electrons from said photo-cathode can: be (projected through said conducting layeiz-and" insulating film so as to charge said sheet of semi-conducting .material, and that said sheet can then be dischargedsby scanning with said beam of electrons from said other side thereof, scanning of said side serving to generate signals capacitively in said conducting layer.

The expression fsheetof sem' conducting material used herein and in the claims is intended to mean a sheet of material having a very high specific resistivity and such that the resistance between opposite points on the surfaces of the sheet is small in relation to the resistance between centres of adjacent elemental areas of the surfaces, the size of these elemental areas being effectively determined by the spot size of the from.2.10 to 10 ohm ems.

scanning beam of electrons. In practice satisfactory results are produced if the distance between the centres is more than twice the dis tance between the surfaces, and if the specific resistivity lies between a lower limit such that no considerable decrease in image resolution occurs due to lateral leakage of electrons between elemental areas, and an upper limit such that potential differences between corresponding areas of the opposite surfaces become effectively equalised Within the time taken by the electron beam to scan the screen, i. e., during a frame scanning period.

For example, in a television transmitting tube arranged to have a scanning period of of a second, and having 202 lines in each scan, alternate scans being interlaced, resistivities giving satisfactory results lie within therange The range ma however, vary for difierent'applications of the invention.

In order that the said invention may be clearly understood and readily carried into effect, the same will now be more fully described with reference to the accompanying drawing which illustrates diagrammatically one example of the invention applied to a television transmitting tube.

In this example the double-sided target electrode of the tube comprises a thin sheet I of semi-conducting material of very high specific resistivity. In this example the material emconducting glass is approximately 0.001-0.002"

in thickness and has a resistivity within the range from 2.10 to 10 ohm cms. On one side of the magnesium, or by evaporating an insulator, such as a borate or a fluoride. On the exposed surfaceof the insulating film a thin conducting layer '3 is deposited by evaporation of a suitable metal such as aluminium, beryllium or molybdenum having a thickness of 5 to Angstroms, or any other suitable conducting layer may be employed. Due to the insulating film 2, the conducting layer 3 is capacitatively associated with the sheet of conducting glass 2.

The television transmitting tube illustrated is of the kind having a photo-electric cathode and an optical system, indicated diagrammati cally at 6, is arranged to focus an optical image of the object for transmission on to the cathode 5. The cathode emits a photo-electron image corresponding to the optical image, and this photo-electron image is then focussed by the electrode 1 and accelerated to such .a velocity that the electrons can penetrate the thin conducting layer 3 and the insulating film 2, and reach the conducting glass sheet I. Instead of employing the electrode 1 the photo-electron image may be focussed magnetically, or the photo-electric cathode may be disposed so close to the thin conducting layer that no separate focussing means is required. In the transmitting tube, on the side of the sheet of conducting glass remote from the photo-electric cathode 5 is arranged an electron gun structure 3, Which generates a beam 9 which can be caused to scan the surface of the conducting glass remote from the cathode 5 by magnetic deflecting coils indicated diagrammatically at I!) and H, the, aXes of the coils being disposed at right angles to the axis of the tube. The deflecting coils are supplied in known manner with sawtooth waveform currents of line and frame frequency respectively from conventional scanning generators indicated in 'block form at. I3 and i4 and they serve to deflect the beam 8 in mutually Perpendicular directions. The beam .is accelerated by applying appropriate potentials to the second anode Ill so that the beam has a velocity of the order of 1000 volts when scanning the conducting glass.

The screenmay, for example, be scanned 50 times per. second, each scan having 202 lines and alternate scans beinginterlaced. Representative potentials such as may be applied to the electrodes or" the tube are indicated in the drawing.

By suitable choice of potentials applied to the photo-electric cathode 5, the thin conducting layer 3., and the sheet of conducting glass jl it tively. The elemental areas of the conducting glass are restored to an equilibrium potential as 'a result of the action of the fast scanning beam, the change of potential which results during scanning causing signals 'to be developed in 'the thin conducting layer 3.

This .layer '3 thus forms the signal plate of the screen and is connectedto a resistance I 6 across which picture signals are developed.

By virtue ;ofthe construction of electron discharge devices described it will be appreciated that there is no grid structure such as is usually necessary .in double-sided target electrodes and,

moreover, since bright areas of the image charge the conducting glass more negatively, the Well known tilt efiiect which is experienced in some form of television transmitting tubes may possibly be reduced.

What I claim is:

1. An electron discharge device such as a television transmission tube comprising a target electrode, a photoelectric cathode disposed at one :side of said target electrode, means disposed at the other side of .said target electrode for generating a beam of electrons for scanning said target electrode, said target electrode comprisingasheet-of semi-conducting material provided with an insulating film on the side disposed towards said photoelectric cathode and a conductive layer on said insulating film, and means for setting up a potential field between said photoelectric cathode and said conducting layer to cause photoelectrons released from said photoelectric cathode to be projected through said conductinglayer and said insulating said conducting layer being capacitively associated with said sheet of semi-conducting material, whereby scanning of said .sheet from said other side thereof will generate signals capac'itively in said conducting layer.

.2. .An electron discharge device according to claim 1, comprising an acceleratingelectrode between said photoelectric cathode and said target electrode.

3. Television transmitting apparatus comprising a television transmission tube including a target structure, ,a photoelectric cathode at one side of said target structure, an electron gun at the, other side of said target structure, said target structure comprising .a sheet of semiconducting material provided with an insulating film on the side disposed towards, said photoelectric cathode and a conducting layer on said insulating film, the apparatus. further comprising means for establishing an accelerating potential field between Said photoelectric cathode and said conducting layer to cause photoelectrons released from said photoelectric cathode tofibe projected through said conducting layer and said insulating him on to said sheet of semiconducting material to charge said sheet negatively, means for energizing said electrongun .to cause the generation of a fast scanning beam of electrons, and means for deflecting said beam of electrons to scan said target and periodically discharge said sheet of semiconducting material .by the release of secondary electrons therefrom, whereby signals are capacitively generated in said conducting layer. HANS GEHHARD 'LUBSZYNSKI.

REFERENCES CITED The following references are of record in the ,file ,of this patent: