US2240186A - Electron discharge device - Google Patents

Description

April 29, 1941.

H. A. IAMS ELECTRON DISCHARGE DEVICE Filed Nov. 30, 1938 J wmfib 8 1: hlm TA MI. 4 R E T- WA w m R A .Q m Y B A TTORNEY.

Patented Apr. 29, 1941 V .by .mesneassiginnents, to Radio Corporation of America, New York, N. 'Y;, a corporation of Delaware Application. November 30, 1338, Seri al No. 243,159

. 4 Claims. (Cl; 250*153) My invention relates to image amplifying tubes and is concerned-primarily with electronic devices and their method of operation-for generating television signals representative of an optical image which" may' be recreated as a replica of the image ata-distance.

It has been customaryto -provideelectronic image transmitting-tubes of either the storage or non-storagetypeand to use eitherthe-resultant electrostatic charges produced from storage or to use the current image signals from the non-storage type-to develop signalling impulses for transmission of an optical image replica. While th storage type of tube offersconsiderable increase in the magnitude of-the. output signal energy-obtained when the storage device is scanned with an'electron'beam,itehas been found that in-m-any instances the signal to noise ratio is not as high as might be desired for thetransmissionof images having lowvalues of light intensity. Manysuch storage devices incorporate a mosaic electrode of the photosensitive type which when scanned by a suitableelec'tron beamdevelops local potential distributions which vary from point to point over the surface ofthe mosaic of photosensitive particles and since-the potential of all parts of themosaicis not'the sameeven in darkness; the problemof collecting a uniform sccondary emission current is rendered difificult; The effect of local potential distributions on the mosaic-is to generate spurioussignals which are usually referred to as dark spot signals in that they generate, upon re-creation-of'the optical image, anon-uniform gradation of light corresponding, not to 'thedesired distribution of light and shade of the optical image, but to random areas of light and shade which do not correspond to those of the desired image.

Since the field utilized'for collecting secondary electrons emitted by a mosaic electrode under scansicn is modified by the electron scanning beam and consequently is of unequal intensity or effect over the scanned area'of the electrode, it has been found difficult to uniformly collect the secondary electron emission from the surface of the mosaic electrode whereby it might be'amplified such as by secondary emission amplification. It is, therefore, diflicult, in tubes of the prior art to generate signals representative of anoptical image and simultaneously'amplify the signals within the tube injwhich they are generated.

It is -an object of my invention to-obtain a light translating and scanning device whichwill simplify translation ofoptical eflects -into signallingimpulses and simultaneously amplify the resultant signallingimpulses directly within the scanning device, Another object of myinvention is to providearnplification of the normal si nal generated in a cathoderay scanning tube byapurely electronic device.

ity is a; further objector my invention to provide an electronic. device suitable forgenerating television signals capable of generating a uniform electronpcollecting ,fieldwhereby a more linear response. ofgradationsof light and shade of an optical image may be obtained.

Inaccordance with my invention, I Vprovidean electronic device suitable for generating television signalling impulses wherein the, optical image is-iocusedwdirectly upon anew and novel electrodeto generate-an electrostatic imagelcorresponding inintensity and distribution to the optical image, which electrostatic. imageis used to; control the. collection or rejection of electrons in a time and spacesequence determined by,v an

analyzinglbeam such as, a? moving cathode ray beam.;l may control the electrons of the beam -.if ;of;low :velocity: directly with the g electrostatic imageor use the beam if of high velocity to provducevsecondan electrons which are controlled ,bythe image; Further ,inaccordance with my invention IJsubJ'ect the electrons used to produce '-the signalling impulses to thecontrolof, the .electrostatic image only during thestep ,by step process of; analyzing the electrostatic image. electrostatic image, by controlling the. collection of: electrons, serves effectively as a multi-element The electronic valve whereby considerable current am- Figure 1.is a longitudinal view. of anelectron discharge device. and 1 circuit embodying. my. invention;

Figure 2 isa perspective plan viewshowing'in enlarged detail a portion of: the electrode structure s'h'o-w'n in Eigurie 1;

.Figure 3-is.a sectional] view of the electrode shown in Figure 2 taken :along the linesi;3+3; and,

- Figure 4- is a schematic diagram showing a 'modifid form 'of the circuit shown in Figure l.

ln the illustrative embodiment of my invention, tasls'hown. in Figure; 1,: the discharge device orv'tubelcomprises a v highly evacuated glass en- :velope .orbulb l of --frusto-conical"shape with a tubular arm or neck section enclosing a conventional type electron gun. Since the envelope is highly evacuated, it contains substantially no ionizable gas. The bulb l encloses a flat target or mosaic electrode 2 of a new and novel design to obtain the objects of my invention, symmetrically positioned in the frusto-conical portion of the envelope so that it is substantially perpendicular to the longitudinal axis of the bulb I whereby one of its surfaces may be scanned by a beam of electrons from the electron gun and may also have projected on the opposite surface from that scanned, that which is optical image torbe transmitted. Since the image projected on the mosaic electrode is produced by light from an object situated outside of the tube, a portion of the tube, such as the transparent window 3, which is optically uniform and preferably curved so that it may better withstand atmospheric pressure is provided so that the image to be transmitted may be projected upon the electrode 2 with a minimum of distortion by a lens system 4.

I'he electron gun assembly is of the conventional type and comprises a cathode 5 from which an electron stream may be drawn, a control electrode 6 connected to the usual biasing battery, and a first anode l maintained positive with respect to the cathode 5. The electron stream leaving the first anode I is accelerated and concentrated into an electron scanning beam fo- 0 cused on the surface of the target or mosaic electrode 2 facing the electron gun by a second anode 8 which is preferably a conductive coating on the inner surface of the envelope I, over a portion of the neck and frusto-conical sections. The first anode l and the second anode 8 are maintained at the desired positive potentials by a potential source, such as the battery 9. The second anode 8 is preferably divided into two sections 8a and 8b, the section 8b being a continuation of the section 6a but insulated therefrom and is preferably about one-half inch wide and at its closest edge about one inch from the mosaic electrode 2. The purpose of this construction will be referred to in connection with the description of Figure 4. Conventional deflection means, such as deflection coils l0 and II, may be used to sweep the beam in horizontal and vertical planes respectively to scan the mosaic electrode 2. It is obvious that conventional electroa static deflection plates may be substituted for either one or both of the deflection coils if desired.

In accordance with one teaching of my invention, the optical image of an object of which a picture is to be transmitted is optically focused on a mosaic of mutually separated particles insulatingly supported by an apertured foundation member capable of emitting secondary electrons when bombarded or scanned by an electron beam of relatively high velocity as hereinafter described. The mutually separated photosensitized particles surround the apertures in the electrode foundation and are of a much greater number than the apertures in the foundation. Under the influence of the optical image, the photosensitive particles acquire electrostatic charges proportional in intensity to the incident light thereby forming an electrostatic image of variable intensity over its area which produces in or adjacent the apertures in the electrode an electron accelerating or retarding field of an intensity directly proportional to the electrostatic charge. In following the first method of operation hereinafter referred to, the surface of the ,tween the window 3 and the mosaic electrode 2.

Further in accordance with my invention and following a second method of operation, instead of controlling the secondary electrons produced by the bombardment of the scanning beam, I control the passage of the electrons of the scaning beam through the apertures. This method of operation may be effected by utilizing a low velocity scanning beam which is scanned over the mosaic electrode and sequentially collected or rejected in accordance with the intensity of the electrostatic charges on the elementary areas formed in response to the optical image focused on the mosaic electrode.

The photosensitive mosaic electrode 2, as best shown in Figures 2 and 3, is formed from an apertured foundation l2, such as an apertured metal sheet or a closely woven wire mesh screen, having a number of interstices or apertures l3 which are provided in sufiicient quantity to satisfy the requirements for which the device is designed. More specifically, the foundation [2 comprises a perforated metal sheet which, when designed for use in television transmitting tubes, may to advantage have -200 or more apertures I3 per linear inch if the tube is to be suitable for reproducing a television image in good detail. The apertures are preferably square or rectangular in shape. The foundation [2 may to advantage be rolled and treated with acid to increase the area of the interstices or apertures l3 as described by W. H. Hickok in U. S. Patent 2,047,369. The foundation is provided on one side with a coating of insulation M such as enamel, on which the photosensitive mosaic of mutually separated and insulated particles [5 of metal such as silver is deposited. The choice of insulating material depends on whether the tube is to be used in applications wherein the scene to be transmitted varies slowly or rapidly in degrees of brilliance or whether the scene to be transmitted is moving or not. In astronomical work, for instance, in which a minute or longer might be allowed for changing the picture, the resistance of the insulation M on the one side of the foundation may be very high. For television work, however, the resistance may be made lower to assist in the dissipation of the charges acquired in response to a focused optical image so that most of this charge can be dissipated within the time of scanning one picture frame which, in conventional systems, is approximately one-thirtieth second.

Prior to insulating the foundation I find it advantageous to clean it thoroughly and, if made of oxidizable metal, to then oxidize it slightly by heating it in air, and if made of nickel, until it assumes a greenish color probably due to a film of nickel monoxide. The foundation is then coated on one side only with a coating of insulation M, such as enamel or other vitreous material, having the desired electrical resistance. The usual enamels for ferrous metals may be used although for a high resistance enamel for use with a foundation of perforated nickel I have found advantageous a vitreousenamel in which the percentages of alkali and silica are somewhat higher, and the. percentage of boric anhydride is lower than in the usual enamels. Such an enamel is. described in copending application, Serial No. 115,192, filed by John Gallup September 10, 1936 now U. S- Patent 2,178,232. In spraying the foundation care should be taken that the enamel does not flow through the interstices or apertures ity to the opposite side of the screen. I therefore prefer to dothis spraying with the perforated foundation sheet horizontal and the spray gun below thefoundation so that the enamel is prevented by gravitation from overflowing the upper surface of. the foundation. The uncoated side of the foundation should be wiped clean with a dry cloth to remove any particles of the enamel which may have fallen back on this surface. The sprayed foundation is then fired in a furnace at about 900 C. in air to fuse the enamel into a smooth, glassy coating. I prefer to build the enamel coating up to a thickness of approximately 3 mils on the coated surface of the foundation and to a thickness of to 1 mil on the walls of the apertures by applying the enamel in several thin coats and firing the foundation after each coat is applied. I find that in this way a foundation well insulated on one side and highly suitable for the production of mosaic electrodes for use in my new and improved tubes, can be made with such a perforated sheet of metal. Instead of applying a vitreous enamel to the foundation by the above process, it may be advantageous to deposit the coating of insulation on the foundation by an evaporation method. Materials such as quartz, cryolite or calcium fluoride may be applied to a tungsten filament in close proximity to the foundation to be insulated and the filament flashed to vaporize the insulating material which condenses as a relatively uniform coating on the foundation. Such a method of depositing insulating material on a base member is disclosed by W. H. Hickok in his copending application Serial No. 133,899 filedv March 30, 1937. Whether the insulating material is sprayed or condensed from the vapor phase or otherwise applied to the foundation, it is desirable to maintain the shape of the apertures I3 substantially square or rectangular so that the photosensitive particles which are later applied to the coating of insulation produce a maximum effect during the operation of the tube. Following the formation of the coating of insulation [4 on one side and on the walls of the apertures of the foundation, I provide the insulated surface with a mosaic of a great number of mutually separated and insulated metal particles l5. These particles may be deposited by a number of methods, such as by dusting an easily reducible compound of the metal which, if silver, may be silver oxide on the insulated surface and reducing the compound to the metal. Another method of providing the individually separated metal particles l5 is to deposit, such as by vaporizing, a thin film of metal, such as silver, on the insulated side of the foundation and break this film up into the particles l5 by suitable heat treatment. These methods for forming small metal particles are disclosed in U. S. Patentv 2,065,570.

Following the formationof the mosaicof metal particle n. the insulated ide. thefoundation,

til

apertures and-referring. again to F i'gure l, the foundation is suitably supported withinthe frusto-conical portion of the envelope as" shown in Figure 1 in such a' position that the uninsulatedsurface of the foundation may be scanned byithe electron beam generated by the electron gun in the neck portion of the tube. An electron collecting electrode it is supported adjacent the insulated side of the foundation. This electrode l6 may be in the 'form of a wire mesh screen of coarseweave, such as four wires to the linear inch, and collects both the'photoelectrons liberated by the mosaic and secondary electrons from the scanned side of the mosaic electrode which pass through the L3. The positioning of the mosaic electrode 2 and the collecting electrode l6 within the envelope is facilitated by sopositioning them prior 'tothe sealing of the window 3 to the envelope. Care should be exercised to ensure the planes of the two electrodes are substantially parallel and under normal operating conditions a spacing of one inch between these electrodes should be found satisfactory. While I prefer to use a wire mesh collecting electrode it'is obvious that this electrode may take the form of a narrow band of metal or other conductor on the 'Wallof the tube between the mosaic electrodeZ and the window 3.

Following the sealing of the component parts within the bulb l, the tube is exhausted and baked to remove any residual gasses that may be adsorbed in the component parts following which oxygen is admitted to the tube and the mosaic of metal particles 15 oxidized such as by the method set forth by S. F. Essig in U. S. Patent 2,020,305. Excess oxygen is removed from the tube whereupon caesium or other light sensitizing material, such as another alkali, metal, is deposited on the metal particles I5 which are thereby photosensitized to complete the mosaic electrode. The various steps, such as disclosed in the above mentioned patents for photosensitizing mosaic electrodes, are well known and it is believed unnecessary to point out in detail the specific methods followed.

If the tube shown in Figure 1 is to be suitable for following my method of operation hereinbefore referred to as my first method of operation the side of the mosaic electrode opposite that carrying the coating of insulation I4 and the mosaic particles; 15 is coated with a film ll of material capable of producing high secondary electron emission when bombarded by electrons. Therefore, duringthe sensitizing process of the mosaic the alkali metal, such as caesium, is intentionally deposited upon the uncoated side of the mosaic electrode to form a, film H which is desirable in that high secondary electron emission may be obtained from a metal surface coated with caesium or other alkali metal. It is very desirablethat the ratio of secondary electrons emitted by theuninsulated side of the-mosaic electrode foundation'to incident electrons from the electron beam be as high as possible for the reasons to be set forth in connection'with the operation of the device. However, if the tube is designed for practicing my second disclosed method of operation the uninsulated side of the foundation is left uncoated of alkali metal because secondary emission from this surface is not desired in practicing this method. I have found that" some of the alkali metal. incidentally condenses on the uncoatedside of the foundation when the mosaic particles. 15; are .photosensitizedlbnt, that. this is not detrimental when the tube is operated with a low velocity electron beam.

Referring again to Figure 1, the tube may be made ready for operation in accordance with my first method of operation by connecting the mosaic electrode foundation l3 through a source of potential, such as the battery Hi, to ground and the second anode 8, the potential source being such that the foundation is operated slightly negative with respect to the anode 8. I have found that the potential of the battery l8 may be ap roximately 6 volts irrespective of the voltage applied by the battery 9 to the second anode 8 which is usually in the neighborhood of 1000 volts positive with respect to the cathode 5. The electron collecting electrode is connected to an output impedance I9 and to the input electrode of the translation device 20, such as a thermionic amplifying tube whose cathode is connected through the usual biasing battery 2| to the opposite side of the output impedance and the second anode 8. The high potential difference between the cathode 5 and the foundation l2, such as approximately 994 volts for the value of the batteries 9 and I8 as given above, produces a high velocity electron beam which is scanned over the mosaic electrode 2 by applying deflection currents of sawtooth form to the deflection coils l and H. Under bombardment of the high velocity electron beam secondary electrons are liberated from the uninsulated side of the mosaic electrode.

In accordance with my second method of operation, I prefer to connect the tube as shown in Figure 4, the section 8a of the anode 8 being operated at 150 volts positive with respect to the cathode and the first anode I at about 800 to 1000 volts positive with respect to the cathode 5 by a battery 9. The second section 8b of the anode 8 is operated at low positive potential, such as volts, with respect to the cathode 5 by the battery 22. The mosaic electrode foundation is connected to the battery 23 which maintains this electrode at approximately 1 volt negative with respect to the cathode 5 and the collecting electrode IE to a battery 24 to maintain it at approximately 25 volts positive with respect to the cathode 5. Although I have shown the battery 24 between the collecting electrode 16 and the impedance l9, it is preferable to connect this battery between the lower end of the impedance 1!! and ground as shown in dotted lines. With the voltages given above, the electron beam is accelerated and focused upon the mosaic electrode but decelerated to a very low velocity as it approaches this electrode. The method of operation of the device and circuit shown in Fig. 4 are described more fully in my copending application Serial No. 372,077, filed December 28, 1940, being a continuation in part of this application. Another form of electron gun and deflection system suitable for producing a low velocity electron beam is disclosed by Albert Rose in his copending application Serial No. 222,153 filed July 30, 1938 now U. S. Patent 2,213,174.

While I do not wish to be limited to any particular theory of operation of my new and improved device, it seems probable that when the mosaic electrode is illuminated, such as by an optical image projected thereon, there is formed an electrostatic field in and surrounding the individual apertures of the mosaic electrode, which field is directly proportional in intensity to the light'intensity of the optical image. Under the first mode of operation the secondary elec= trons liberated, from the scanned surface of the mosaic electrode under bombardment of a high velocity electron beam are entrained by this field and are propelled toward and collected by the electron collecting electrode 16. In this manner, the secondary electrons passing through the apertures 13 in the mosaic electrode are proportional to the intensity of the light incident upon the particles of the mosaic electrode surrounding each and every aperture and are collected by the collecting electrode in a time sequence determined by the rate of scanning. However, in my second method of operation utilizing a low velocity scanning beam the beam does not impinge on the mosaic electrode under normal conditions because the foundation is approximately 1 volt negative with respect to the electron source. When, however, an optical image is projected or'focus'ed upon the particles l5, photoelectrons are liberated from the particles l5 thereby producing an electrostatic image which is a replica of the optical image consisting of discrete positive charges on the particles 15 between the apertures of the electrode. These charges effectively look, through the apertures of the electrode and entrain the electrons of the low velocity beam to an extent proportional to the intensity of the charges surrounding any single aperture. As the beam is scanned over the electrode, the instantaneous current from the electron beam collected by the collecting electrode IB is proportional to the intensity of the electro static image and is collected in a time sequence determined by the rate of scanning.

'The mosaic electrode 2 and the collecting electrode I6 may be likened unto a great plurality of individual thermionic triodes wherein the uncoated surface of the mosaic electrode in the case of a high velocitybeam or the cathode 5 in the case of a low velocity beam corresponds to the common cathode of the triodes, the photosensitive particles I5 surrounding the individual apertures l3 of the mosaic electrode to the control grids of the triodes and the collecting electrode [6 to the common anode of the triodes. By this analogy it may be seen that the current collection to the collecting electrode of the secondary electrons liberated from the uncoated side of the mosaic during scansion is controlled by the electrostatic charges developed on the individual particles by the incident light and that, as a result of this control action, considerable amplific-ation and a resultant high signal to noise ratio is obtained in the output circuit of the device. I have made cathode ray tubes in accordance with my invention having the foundation provided with square and rectangular apertures 13 and. these tubes have been satisfactory for television transmission, I believe that the particles [5 when charged electrostatically such as by photoemission under, the influence of an optical image, have a greater control eifect upon the electrons passing through the apertures l3 if these apertures are square or rectangular shaped. This is probably due to making the distance from the edges of apertures to the points emitting secondary electrons as uniform as possible. I also prefer to focus the beam to a spot size at the uninsulated side of the mosaic electrode so that the spot diameter is substantially equal to the width of the elementary line into which the optical image is subdivided for transmission and re-creation. Thus, for a system in which an optical image of four inches in height is subdivided into 441 lines the diameter of the beam at the mosaic electrode should be effectively 0.0905 inch in diameter. Where the number of apertures per linear inch is high the beam may cover several of the apertures at any single instant during the scanning of the electrode but this is advantageous in that a more uniform and lifelike image can be transmitted.

From the above it will be evident that I have provided a new method of operating a television transmitting tube wherein an electron fiow of elemental picture area is controlled by an electrostatic image formed of elemental charges proportional in intensity to elemental areas of an optical image. in accordance with this method the controlled electron flow of elemental area is collected from an area substantially equivalent. to the area of the optical image in a time sequence determined by the analysis of the optical image, such as by scanning. My method of operating a television device is not limited to the particular apparatus and structure disclosed but may be followed in many modifications thereof. The electron :beam may thus be formed in a number of ways in addition to the use of the electron gun which I disclosed or that referred to as being shown in the Rose application. Instead of scanning the apertured foundation with an electron beam the foundation might be made photosensitive and scanned with a light beam to provide the flow of electrons which is controlled by the electrostatic image. Therefore, while I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated only one specific application for which my invention may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

I claim:

1. A cathode ray tube for generating signals representative of elemental areas of an optical image comprising a highly evacuated envelope containing substantially no ionizable gas, said envelope including a window forming one wall thereof adapted to transmit an optical image, a perforated target electrode comprising a perfor ted conductive foundation having a mosaic of mutually insulated photosensitive particles on one side facing said window and an uninsulated bare secondary electron emitting side facing away from said window, a single electron source within said envelope exposed to the secondary electron emitting side .of said target, means including an anode adjacent said source to develop a beam of electrons having sufiicient ,velocity to liberate secondary electrons in excess of the number of beam electrons from the bare secondary electron emitting side of said foundation, means to scan saidelectron beam over successive elemental areas of the secondary electron emitting surface of said target electrode, and a light permeable electron collecting electrode adjacent said mosaic, further removed from said said target electrode and said window.

2. A cathode ray tube for generating electrical signals representative of elemental areas of light and shade of an optical image comp-rising a highly evacuated envelope including a transparent window at one end thereof, a target electrode having a perforated metallic foundation covered on only the side facing said window with a mosaic of mutually insulated photosensitized particles, the exposed foundation side facing away from said window, a cathode ray gun ineluding an electron source and an anode to generate and direct on the exposed foundation side of said target an electron beam having a cross-sectional area substantially equivalent to the area of said elemental areas of the optical image, and means to scan said beam over successive elemental areas of the secondary electron emitting material of said target, and a light permeable electron collecting electrode between said window and said target electrode and adjalcent said target electrode exposed to said plurality of particles.

3. A television transmitting tube including a highly evacuated envelope including an optically transparent window, a mosaic electrode comprising an apertured base member of electrically conducting material having one side facing said Window, a coating of electrically insulating material on one side only and on the side of said base member facing said window leaving the opposite side of said base member exposed, a mosaic of mutually separated photosensitive partlcles on said coating of insulating material, means between said electron gun and said mosaic electrode to scan the electrons emitted by said last-mentioned means over the said exposed side of said base, and optically permeable means to draw a portion of the generated electron emission through the apertures of said member and collect that portion passing through the apertures, said means being positioned between said mosaic electrode and said windcw.

4. In combination a television transmitting tube including an envelope with a transparent Window, a photosensitive mosaic electrode comprising a multi-apertured metallic foundation of metal having the property of emitting secondary electrons when bombarded by an electron beam, a plurality of photosensitive particles on one side of and between the apertures of said foundation, said particles being insulated from said foundation and exposed to said window leaving the metal of said foundation exposed on the opposite side from said particles, means including an electron gun opposite the exposed metal side of said foundation and out of View of the side of said foundation bearing said photosensitive particles to produce secondary electron emission from the said exposedside of said foundation, means including an optical system onthe opposite side of said foundation from said electron gun to form an electrostatic image on the photosensitive particles representative of an optical image and means permeable to light from said optical system and between said electrode and said window to draw a portion of saidsercondary electrons through said apertures in accordance with the intensity of said electrostatic image and collect said portion of secondary electrons.

HARLEY A. IAMS.

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