US2250283A - Electron discharge device - Google Patents

Description

July 22, 1941. 'T I 2,250,283

ELECTRON DISCHARGE DEVICE Filed Feb. 21, 1939 IN VENT OR By a K TEAL A TTORNEY Patented July 22, 1941 UNITED STATES PATENT OFFICE ELECTRON DISCHARGE DEVICE Gordon K. Teal, New York, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application February 21, 1939, Serial No. 257,634

7 Claims.

This application relates to electron discharge devices and more specifically to electron camera tubes suitable for television.

In systems employing the well-known iconoscope or other electron camera tubes using the electron storage principle, the image produced at the receiving station fromsignals generated at the transmitting station by one or the other of these electron camera tubes has been diffused due to the emission of secondary electrons from one element of a mosaic target to other more positively charged elements. The present invention is directed to an electron camera tube in which this deleterious effect is substantially reduced if not entirely eliminated.

It is an object of this invention to provide an improved electron camera tube of the type utilizing the storage of electrostatic charges.

It is another object of this invention to provide an improved electron camera tube in which the intensity of secondary electrons is controlled by stored charges to produce an image current.

Other and ancillary objects will be apparent from the description below and from the appended claims.

In accordance with one embodiment of this invention, an electron camera tube is provided comprising an envelope enclosing an electron gun, a composite target comprising a perforated insulating plate having a discontinuous or globular photoemissive surfaceand a semitransparent metallic signal plate capable of emitting secondary electrons mounted very close to'each other, a collector electrode which may comprise part of the electron gun, and two pairs of electrostatic deflecting plates. Radiations from an object or field of view pass through the semitransparent signal plate to the photosensitized perforated insulating plate to cause photoelectric emission therefrom to the signal plate. The insulating plate is scanned by a beam of electrons from the electron gun and some of these electrons pass.

through the perforations therein to strike the signal plate. Secondary electrons are driven from the signal plate, passing through the apertures in the insulating plate to thecollector electrode, the number of secondary electrons passing through each perforation depending upon the charge on the adjacent photoemissive globule. The image current is obtained from the signal plate which is resistance coupled to a television amplifier. Due to the fact that the insulating plate and the secondary emitting signal plate are close to each other, the photoelectrons are not allowed to pass from one photoemissive'element to adjacentmore positively charged ones, and thus-a receivedpicture built up from the signal generated by the camera tube of this invention is not diffused aswas the case in the priorart television systems mentioned above.

In a second embodiment of this invention the perforated insulating plate has a continuous photoemissive layer thereon and the plate opposite the perforated plate has a discontinuous layer of secondary emitting material thereon. Also the signal resistance maybe connected to the continuous layer of photoemissive material on the perforated plate. As a modification, the perforated plate may be of metal instead of insulating material.

A feature of the present invention is that the perforated plate is mounted on a cylinder which makes slidable contact with and thus becomes electrically a part of another cylinder forming part of the electron gun system and with the interior of the tube envelope so that the perforated glass plate may be adjusted with respect to the secondary emitting plate by force exerted from without the tube.

The invention will be more readily understood from the following description taken in connection with the accompanying drawing forming a part thereof in which:

Fig. 1 shows an electronic camera tube such as is used in television transmission embodying the principles of this invention; v

Fig. 2 is an enlarged view of the target assembly within the tube of Fig. 1; and

Fig. 3 shows 2 modified target assembly.

Referring more particularly to the drawing, Fig. 1 shows an electronic camera tube used as a pickup device in television transmission comprising an evacuated container I!) in which is sealed a smaller container l l, the two containers enclosing an electron gun assembly indicated generally by the reference character l2 for producing a moving beam of electrons and for accelerating this beam towards a screen or target assembly T at' the end of the-tube remote fromthe electron gun, and means such as the deflecting plates I3, 13' and I4, I l for causing the beam to scan the various elemental areas of the screen or target T in turn. Radiations from an object or field of view 0 without the tube are focussed by a suitable lens system indicated generally by the single lens L upon the target assembly T.

The electron gun assembly [2 comprises a cathode lfi'which is heated by a heating member 16, a first anode ll and a second anode comprising an apertured diaphragm 8, a cylinder IS, a cylinder iii and a slidable cylinder 2|. The elements I8, IQ, is and 2| comprising the second anode member are all placed at ground potential. The cathode I5 is placed at a potential which is fromlOOO to 1500 volts negative with respect to ground potential and the first anode member I! is placed at a potential of from 600 to 800 volts above the cathode potential. These potentials are produced by any suitable means (not shown).

The various elements of the electron gun syspairs of electrostatic deflecting plates i3, i3 and i l4, [4, the normals to whose surfaces are located at right-angles to each other, are provided. To the deflecting plates 13, I3 are applied deflecting voltages of line scanning freouency (5760 cycles per second, for example) and of sawtoothed wave form to produce the horizontal defiection while" deflecting voltages of framing frequency (24 cycles per second, for example) and of saw-toothed waveform are applied to the deflecting plates M, M to produce vertical deflection of the beam. Any appropriate sweep circuits (notshown) may be used to generate the: horizontal. and vertical deflection Voltages. For example, reference may be made to Patent 2,178,- 464, issued October 31, 1939 to M. W. Baldwin, Jr. whichdiscloses suitable balanced sweep circuits forlthis. purpose. For a more full description of the advantages of balanced sweep circuits for use withcathocle ray television tubes, reference may befma'de tothe above-mentioned Baldwin patent andzalso to Patent 2,209,199, issued July 23, 1940 tol 'rank Gray.

Thetarget assembly T comprises (see Fig. 2) a photoemissive mosaic. 22 mounted on an in- Sl11atingLplate-23 (of any suitable material such as glass or mica) having a multiplicity of perforationsnfl. therein, adjacent the signal plate comprising a semitransparent layer 26 of a suitable; metallic material which has a ratio of sec- Qniiary electrons emitted therefrom to primary electrons impinging thereon of greater than one to one; The layer 2.6 is mounted on a glass plate 25. The glassplate 23 is by way of example 4 inches square. and. hasin it 4225 holes per square inch. Thisplate is preferably prepared by coatis-one surface. of a .glass plate with a plurality qiwax: dots through. a. meshscreen, removing the. 5.0113511, melting the wax so that only small spacespf. the .plate. between the dots are left uncove ed dippingtheplate in an etchingsolutionacidgalmost etches through to the side w:notcovered .bythe wax, etching from the, otherside .until. the .acid etches through to tl'mapertures, and then washing the plate. For a 1 9.13 complete description of glassplatesof this q ltllfuCt-er .ZtILdQf a method 0f..preparing them, refer,ence, m&y.be..made to. Patent 2,217,334, issued October .8, 19.40 vto.B. A. Diggory and G. K. Tea f a A la er f. silver is deposited Onthis glass by evaporation to insure its condensation on only one side, The, conducting film. thus attained is their. rendered discontinuous and non-conducting by hea ting to a temperature above 500 C. in air or vacuum until discreteglobules of silver are formed. The glass, plate is then mounted on the endof the sliding cylindrical. collector electrode any. suitable, means, such as spring clips.

il li 'llifir 2!. isheld in position within the tube andin slidable contact with the cylindrical memby means of tl:1..e. sliding contact members 21 The signal plate25 to which an amplifier 28 I is coupled throughthesignal.resistance 29 is a ei es eer eig educiin film 26 of pla in m, f i mplad iqq ii d the fiatLglass disc .25

. silver layer and the platinum electrode are adjacent eachother and about .0015 inch apart, the

distance being set by mica spacers 33 fastened to the member 23 by small clips (not shown) In the making of the tube previous to sealing it onto the vacuum station at the side tube 34, the cylindrical electrode 2| supporting the perforated glass plate 23 is moved back towards the electron gun l2. The tube is thoroughly evacuated, baked at 400 C. for two hours and cleaned up with hydrogen. On cooling to room temperature about 2 millimeters pressure of oxygen is introduced into it and oxidation of the silver particles on the face of a plate 23 to the first order blue interference color is then carried out by use of an electrodeless discharge created in the oxygen of the tube by means of a high frequency spark coil outside the tube. The oxygen is then removed by evacuation from the tube, hydrogen gas is introduced and the tube is baked to reduce the silver oxide. The oxidation-reduction cycle is carried out to insure even oxidation of the silver mosaic. The mosaic is then oxidized again. After evacuation of the oxygen, the oathode l5 of the electron gun is treated in a manner known to sensitize it with an electron emitting coating of BaO-SrO; Caesium vapour is then introducedthroug'n the side tube 34 into. the space between the silver mosaic on the glass plate 23 andthe signal plate 26, and the tube is baked at 200- to 225 C. for a short time after which .it is sealed off from the vacuum station. The caesium photosensitizes the. silver mosaic on the glass plate 23 to form photoemissive globules 22.

In theoperation of the tube shown in Fig. 1' the sensitizedmosaic 22 is moved up close to the platinum signal plate 26 by force exerted from. outside the tube. An optical image. is focussed on the photosensitized mosaic 22 by means of thelens L. The perforated plate 23 is scanned by means of the high velocity cathode ray beam generated by the. gun l2, the electrons in the beam striking the back of the glass plate 23. and also going through the perforations 24 to strike the platinum signal plate 26 to cause the emission of secondary. electrons which pass through the apertures 24. to the collector electrode 2|. The numberof secondary electrons passing througheach aperture is dependent upon the charge on the adjacent photoemissive globules 22 caused by the. emission of photoelectrons. The signal current due to secondary electronspa-sses through the coupling resistance 29 and. the dropinpotential across thisresistance is amplified by the television amplifier 28. The amplified signal, is transmitted to a suitable receiving station (not shown) where it may be used to reproduce an image of the object O. 7

It was found in tests of the tube described above that an optical image focussed on the mosaic 22 could bemoved sideways fairly'rapidly. with no appearance of lag in the received image. In these tests signals were obtained with the collectorelectrode 22 either slightly positive or negative with respect to the signal plate 26 and alsogood signals were obtained when the source 30 were removedentirely.-

The transmitter tube shown in Figs. 1 and 2 gives storage of charge so as to form a large image current which is larger than it Would ordinarily be because of the amplification of the signal by means of the grid action of the stored charges on the photoemissive globules 22. This grid action occurs with respect to the secondary electrons emitted from the platinum layer 26 by the action of the primary electrons in the high velocity scanning beam. By the present method of operation of this device, the photosensitized silver globules 22 are charged several volts negative with respect to the signal plate 26 by the secondary electrons caused to be emitted by the action of the scanning beam from nearby points on the platinum film 26. Between times of scanning of an element of the film 26, the photoelectrons are emitted continuously from the corresponding element of the photosensitized silver mosaic 22 in proportion to the intensity of the light falling on the element during the scanning cycle. In this time interval the element of the mosaic 22 becomes less negative but on the return of the scanning beam to a point on the signal plate 26 just opposite it is quickly charged to its original negative potential. This gives a current in the coupling resistance 29 proportionate to the light energy falling on the element during the entire scanning cycle. Amplification of the image current results from the grid-like action of the stored charge on the secondary current directed through the perforations in the direction of the collector electrode. As a modification, the coupling resistance 29 may be placed between the collector 2| and ground instead of as shown in Fig. 2.

The arrangement of Fig. l is very efficient (theoretically more efficient than some existing well-known camera tubes) for several reasons; (1) the scanning beam charges an element of the mosaic 22 to a negative potential with respect to the signal plate 26 by means of the emission of secondary electrons from the signal plate 26, which allows immediate emission of the photoelectrons to the signal plate 26; (2) the photoelectrons are emitted under fair conditions of voltage saturation due to the nearness of the signal plate 26; (3) the photoelectrons are not emitted from one element of the mosaic 22 to adjacent ones due to the closeness of the signal plate 26; (4) the secondary electrons escaping through the apertures 24 are not likely to return to nearby or distant elements since the normal potential of the mosaic 22 is negative with respect to the platinum layer 26; (5) the excess of secondary electrons necessary to establish uniform negative equilibrium potential on the mosaic 22 is suppressed and caused to return to the platinum film 26 rather than to be emitted in all directions and return to distant less negative elements as in the case of familiar types of camera tubes; and (6) the grid action of the stored charges on the mosaic elements 22 acting on the secondary electrons escaping to the collector electrode 2| adds to the amplification of signal in the tube.

It is obvious that a tube of this construction can be made to operate with other photoelectron emitters than those described above, subject to the condition that the light must not cause the emission of appreciable numbers of photoelectrons from the secondary emitter 26. It seems probable that the tube would operate even though this last condition is not fulfilled providing the secondary emitter has a sufficiently high secondary emission.

In the embodiment shown in Fig. 3, a continuous photoemissive layer 3| is laid down by evaporation or sputtering, on the surface of the perforated glass plate 23 and a discontinuous secondary emitter mosaic 32 is laid down on the surface of the glass plate 25 by sputtering and subsequent heat treatment, the spacing between the mosaic 3| and the secondary emitting surface 32 being about the same as in Fig. 2. The signal resistance 29 is connected between the photoemitter 3| and the collecting electrode 2|, a variable source 30 being used between the members 29 and 3| if desired. In a modification, the perforated screen 23 may be made of metal as all elements of the photoemitter 3| are in contact with each other. In this case the metal screen 23 must be insulated from the collector 2|. Metal screens with 160,000 holes per square inch are available for this purpose.

The arrangement shown in Fig, 3 operates as follows: Radiations from the object O are projected upon the photoemitting surface 3| by means of a suitable lens system indicated generally by the lens L. Photoelectrons are given off from each elemental area proportional to the intensity of the light-tone value of the corresponding elemental area of the object and these photoelectrons are drawn to the secondary emitting surface 32 causing the various elemental globules thereof to become less positively charged in varying degrees. Once each scanning cycle the beam generated by the electron gun |2 scans the secondary emitter 32 through the perforations 24 in the plate 23 and changes the mosaic elements to an equilibrium positive potential so that they again may receive photoelectrons. During this reestablishment of an equilibrium potential on an element, secondary electron currents flow to 2| and 3|. Due to the grid action of 3! on the current from an element of the mosaic, an image current is produced in the coupling resistance 29. The coupling resistance may be placed between the collector 2| and ground instead of as shown in Fig. 3. The signal in the signal resistance 29 is amplified by the amplifying device 28 and connected by suitable wire or carrier means to a television receiving station where an image is reproduced from the received signals.

Various modifications may be made in the invention as above disclosed, the scope of which is indicated by the appended claims.

What is claimed is:

1. In combination, a supporting plate having a multiplicity of apertures therethrough and a photosensitive surface thereon, a semitransparent metallic secondary emitting member closely adjacent the photosensitive surface of said supporting means, means for impressing radiations from an object or field of view upon said photo sensitive surface, means for scanning said semitransparent member. through the apertures in said supporting member, said supporting plate and said metallic secondary emitting member being spaced by a distance of two one-thousandths of an inch or less.

2. In combination, a supporting member having a multiplicity of apertures therethrough and a continuous photosensitive surface thereon, a semitransparent discontinuous metallic secondary emitting member closely adjacent the photosensitive surface of said supporting member, means for impressing radiations from an object emitting member closely adjacent the-photosen- 'sitive'sur-face of said-insulating member, means for impressing radiations from an object or field of view upon said photosensitive surface through said secondary emitting member, and means for scanning said semitransparent member with a beam of electrons through the apertures in said insulating member, said insulating plate being separated fromsaid metallic secondary emitting member by a distance of the order of two onethousandths of an inch or less.

4. Electro-optical apparatus comprising an insulating grid element, a coating of photoelectric material on said grid element, said coating being adapted to receive light, an element having a surface-capable of emitting electrons all portions of which are equally spaced from a face of said grid element having a photoelectric coating by 5 a distance of the order of two one-thousandths of an inch or less, means for causing electrons to be emitted from said surface and to maintain the potential of said surface positive with respect to'said coating, and an anode adjacent the side of said grid element opposite said surface for receiving the electrons emitted from said surface through the interstices of said grid, whereby when'the coating of said grid element is unequally illuminated the electrons emitted from said coating as the result of said illumination pass to said electron emitting surface and are not returned to some portion of said coating.

5. In an electron discharge device comprising a container having a cylindrical insulating wall,

a cylindrical conducting member within said insulating wall, an electrode supporting element in part at least within said cylindrical conducting member, spring supporting'and contact elements carried by said supporting element in slidable engagement with said cylindrical conducting member under such tension that said supporting element can be moved by shaking said container, a second electrode within said container, and an insulating stop between said two electrodes to I limit said movement to correctly position said supporting element with respect to said second electrode.

6. An electronic tube television camera comprising therein a plate-likeelement of insulating material having a multiplicity of apertures therethrough, photoemissive material on one face of said element between and around said apertures in the form of discrete separated particles, a second element on one side of said platelike element having a face parallel to and closely adjacent said face of said plate-likeelement carrying said photoemissive material, said second element being capable of emitting secondary electrons under impact of'a beam of electrons, the space between said faces being free of matter whereby it affords an'unobstructed path for both electrons and light, means on the opposite side of said plate-like element for "generating a beam of electrons, means for causing said beam to scan said face of said second element through said apertures and release secondary electrons from said face of said second element, means for forming an'irnage of an object upon said face carrying said photoemissive material whereby said discrete particles will emit photoelectrons and thus raise the potential of said particles in accordance with the quantity of light thereon, a'collecting electrode on the same side of said plate-like element as said beam generating means for collecting secondary electrons passing through said apertures from said second element, whereby said apertured member acts as a grid to'control the number of secondary electrons passing through each of said apertures in accordance with the potential of the surrounding particles of photoemissive material, and conductive means extending from said collecting electrode through the tube wall for permitting connection to be made to a translating device to be controlled by the electrons collected by said collecting electrode.

' '7. An electronic tube television camera comprising therein a plate-like element having a multiplicity of apertures therethrough, a continuous coating of photoemissive material on one face of said element between and around said apertures, a second element on one side of said plate-like element having a face parallel to and closely adjacent said face of said plate-like element carrying said photoemissive material, said second element being capable of emitting secondary electrons under impact of a beam of electrons and being capable of maintaining discrete electric charges on the face thereof near the face of said first member carrying the photoemissive material, the space between said faces being free of matter whereby it affords an unobstructed path for both electrons and light, means on the opposite side of said plate-like element for generating a beam of electrons, means for causing said beam to scan said face of said second element through said apertures and release secondary electrons from said face of said second element, means for forming an image of an object upon said face carrying said photoemissive material'whereby photoelectrons are emitted to said face of said second element and lower the potential of various portions of said face'in accordance with the quantity of light on corresponding portions of the face of "said plate-like element carrying photoemissive material, a collecting electrode on the 'sameside of said plate-like element as said beam generating means for collecting secondary electrons passing through said apertures from said second element, whereby the potential difference between each of the various portionsof-said face of said second member and the corresponding portion of the face of said plate-like member carrying 'photoemissive material acts to controlthe number of secondary electrons passing through the corresponding apertures, and conductive means extending from said collecting electrode through the tube wall for permitting connection to be made to a translating device to 'be controlled by the electrons collected by said'collecting electrode.

5 GORDON K. TEAL.

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