US2213176A - Television transmitting tube - Google Patents

Description

Aug. 27, 1940. A. ROSE TELEVISION TRANSMITTING TUBE Filed June 6, 1939 WNO - INVENTOR. AL BERT ROSE A TTORNE Y.

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Patented Aug. 27, 1940 PATENT OFFICE 2,213,176 TELEVISION TRANSMITTING TUBE Albert Rose,East Orange, N. J., assignor to Radio Corporation of'America, a corporation of Delaa Wavre Application June 6, 1939, serial No. 277,582

`Claims.

My invention=re1ates to television"transmitting tubes Vand is'fmoreparticularly directed to tubes utilizing lowzveloci-ty electron scanning beams.

:In the operation of a television transmitting tube of the low Velocity electron beam scanning type including :a target. or mosaic electrode with an oppositelyv disposed, electron gun anda combinationy magnetic-electrostatic deflection system vincluding electrostatic plates wholly immersed in a uniform axial magnetic field, theelectrons of the scanningbeam which do not reach the target or mosaic e-lectrodebecause of their low velocity return-to acollecting electrode at thesurface of which theyl may. produce secondary electrons which travel toward the target. These secondaryelectrons*v modify the charges on the target or mosaic yelectrode.thereby producing distortionaleiects. ,Y

'Iheprincipal object of my invention is to pro- Videsuch La television transmitting tube which will generate signals representative of an optical image without the production of distortional effects-due. to secondary electronsreturning to the target'electrode. It is a further object of my invention to provide such a ytube by-vvhich a television. imagecan: be transmitted with equal idelityy over its entire areatwithout distortional effects being-produced.`

Iniaccordance with my invention, an electrostatic image correspondingin electrostatic energy distribution toA an opticalv image is formedA on a target or mosaic electrodewhich is scanned by a W Velocity electron beam from an electron gun, the scanning` of the beam over the target being accomplished in .a combination magnetic-electrostatic field.v A-portion of the lovv velocity electron beam neutralizesthe electrostatic image, and the remainder' of the beam returns from thevtarget to a collecting electrode which is so constructed and/positioned that most of the secondary electronsliberated from it bythe electrons returning fromthetarget are prevented from reaching the target. Further in accordance vvwith my invention',zI also provide means for intercepting the few secondary electrons which otherwise might reach the target electrode. Y

VA better understanding of my invention will be obtained and other objects, features and advantages Willwappear from the following description taken in connection with the accompanying drawing in which,

Fig. lis la longitudinal sectional View ofV a tele-y visiontransmitting tube embodying my invention and Fig.. 2 is a cross section of the tube shown i-nuFig; 1 taken lalong the lines 2-.2.

(Cl. Z50- 153) In general, the tube incorporating my invention comprises an evacuated envelope having a target preferably of the photosensitive mosaic type at one end and an `electron `gunandan electroncollecting. electrode at the` opposite end of the tube.. The target, if` of the mosaic type, is providedfon-its frontsurface with an extremely large numberv of mutually insulated photosensitive particles and isso positioned that it may be scanned by an electronbeamirom the gun and may also vhave focuseduponit an optical image of the object of which a picture is-to be transmitted. AIntermediate the'collecti'ng electrode and the target I provide twoor more -electrostatic deflection plates Wholly immersed in amagnetic field coaxialw-ith the center line of thegun and the target t'o scan the electrons fromthe gun over the mosaic electrode.` The potentials betweenthe electron gun. and .target are. so `adjustedthat the electron beam is projected. at relatively low velocity and impinges upon the target atzextremely low or substantially zero velocity, that is, with a velocity approaching zero at: the, point of impact. Inoperation,.th'e electrostatic image consisting-of charges proportional.- to the intensity of the incident' light, iszneutralized by the low Velocity beam to `generate electrcal'signals-representative of the optical image.

, Referring specirlcallvy to my tube structune shown in the drawing, the tube comprises an evacuated' envelope I enclosing at one end a target or mosaicvelectrode 2. and. at Atheopposite end an electron gun assembly adapted to project electrons toward. the mosaic electrode.

The electron guny assemblyfis of the convenf tional type and comprises a cathode 3 from which electrons may be drawn, a control electrode 4 connected to the usual bias battery and a i'lrst anode 5 maintained' positive'with respect to the cathode 3. The Aelectron stream leaving thle cathode v3 is accelerated by the first anode 5 and. further accelerated at relatively low velocity and concentrated into an electron' scanning beam focusing'on lthe front surface of the mosaic electrode by a second anode 6 which is preferably an apertured tubular member partially surrounding'the firstV anode 5. The first anode 5 and second anode 5 are; maintained at: the desired positive potentials with respect to the cathode by a battery 1.

The mosaic electrode 2 which faces the electron gun preferably comprises a substantially transparent sheet of insulation such as the mica sheet 9: having on its rear surface a translucent or semitransparent electrically conducting illm I0, the opposite or front surface of the mica sheet facing the electron gun being provided with an exceedingly great number of mutually separated photosensitive particles II. In Inaking the mosaic electrode I select a mica sheet 9 of the desired area having a uniform thickness of approximately .002 inch and as a first step coat,one side of the sheet with a lm of metal of sufficient thinness as to be substantially transparent so that an optical image such as represented by the arrow I2 may be focused on the photosensitive particles II by a lens system I3. The mosaic of mutually separated particles may be made by depositing on the front surface of the mica sheet 9 finely `divided silver oxide which is reduced to provide a surface of individually separated silver particles or globules II which are subsequently oxidized and sensitized with caesium or other alkali metal during the evacuation process. Such an electrode structure and a method of sensitization is disclosed by S. F. Essig in U. S. Patents 2,020,305 and 2,065,570.

Intermediate the electron gun assembly and the mosaic electrodevI provide deection plates I4-l5 which are curved to provide a uniformly increasing and decreasing electrostatic deflection eld. 'I'he plates I4-I5 are connected to ground through a center-tapped resistance of one to ten megohms andto a source of deflection potential. To produce thedesired deflection of the electron beam I provide means to wholly immerse the deflection platse in a uniform magnetic field which is preferably generated by a magnetic coil 8 of slightly larger diameter than the envelope I extending over and beyond the space between the electron gun and the mosaic electrode 2. The plates Irl-I5 in combination with the field produced by the coil 8` deflect the beam in a plane midway between the platesrandthrough the center line of the gunand target. Deflection'of the electron beam in a direction normal to that produced by the `plates I4-I5 is accomplished by a pair of deflection coils IB-I'I. This defiection ispreferably the frame or vertical deflection, and since in 'standard television systems the frame ydeflection is of lower frequency than the horizontal line deection the magnetic deflection coils IB-Il Vshould preferably be operated atthe lower of the' two frequencies. The

. deeetion coils I6--I1 may, of course, be replaced by a second pair of deiiection plates. However, a substitution of magnetic ymeans for the electrostatic deflection means, such as the plates Ill-I5, does not render the device operative as envisaged by :my invention because if this undesired structure is used the electrons returning frorn the mosaic electrode are directed along the same approximate paths over the entire trajectory between the mosaic electrode and the electron gun, and many of these returning electrons are directed toward the cathode which is at a negative potential and incapable of collecting them, whereupon they return to the mosaic electrode and produce a spurious signal effective over the entire picture area.

In accordancewith my invention I provide between theelectro'n gun and the deflection plates I4-I5 an electron collecting electrode so constructed and positioned as to intercept electrons returning from the mosaic electrode. Referring to the drawing, the vcollecting electrode I8 is so positioned with respect to the center line of the electron gun and'mosaic-electrode that few, if any, secondary electrons emitted from the collecting electrode byimpactof the'impinging electrons returning from the mosaic electrode are redirected toward the mosaic electrode.

More specifically, the electrode I8 comprises an apertured metal sheet of extended surface having low secondary electron emitting properties, the aperture I9 being axially aligned with the longitudinal axis of the tube and the axis of the electron gun, the electrode I8 being located in a plane which is inclined at an oblique angle with respect to the longitudinal axis. The electrode I8 may be of nickel or other metal coated on its surface facing the mosaic electrode with carbon or other material having low secondary emission properties. Further in accordance with my invention I provide between the collecting electrode I8 and the mosaic electrode 2, two slotted electron intercepting electrodes -2 l, one on either side of the deflection plates Ill-I5 in a direction along the tube axis, to intercept secondary electrons which may be emitted by the collecting electrode I8 notwithstanding its low secondary emission properties and its positioning with respect to the tube axis. The intercepting electrode 20 is connected to ground and is provided with an elongated slot 22 in axial alignment with the aperture I9 in the collecting electrode I8, the length of the slot 22 being parallel with a plane through the tube axis and midway between the deflection plates Ill-I5. The intercepting electrode 2| is located near the ends of the deflection plates opposite the electrode 20 and is likewise connected to ground and is provided with an elongated slot 23 parallel with the slot 22 in the intercepting electrode 20.

The collector electrode AI8 `may be operated atsecond anode or ground potential and the signal representative of the optical image focused on the mosaic electrode may be taken from the sernitransparent electrically conducting film yIII. With this arrangement I prefer to connect the film I0 to the battery I at a point close to cath-i ode potential through an output impedance 24 and to a-translating device 25 `which ampliiies the signals, whereupon after further amplification, the signals may be applied to a transmitting network as well known in the art. If. desired, however, the output impedance 24 and the translating device 25 may be connected in the circuit of the collecting electrode to ground, although it has been found that with this arrangement somewhat greater distortion is produced than with the connections shown in the drawing.

I have found that electrons liberated from the cathode 3, when accelerated and 'projectedI toward the mosaic electrode 2, follow a-spiral path while under the influence of the axial magnetic field generated by the coil I5. The amplitude of these spiral paths is relatively small and is apparently due to the initial transverse velocities with which the electrons are emitted at the cathode. A portion of the electron beam will neutralize the electrostatic charge of the particles on the mosaic electrode at which the beam is directed during the scanning cycle, and the remainder of the electron beam is reflected from the mosaic and directed toward the electron gun. If the electrons are not collected by the mosaic they return toward the electron gun endtof the tube, following similar spiral paths' of small amplitude occasioned by their transverse emission velocities from the cathode. Since the amplitude of these spiral paths is low, the electrons pass unimpeded through the elongated slots 22-23 in the intercepting electrodes 20-2I and 751 are A'collected' .over the length of the collecting electrode i8. These electrons while approaching themos'aic electrode 2 vlost their velocity by reason ofthe low electrostatic potential on the particles with respect to the cathode 3, but upon returning toward the collecting electrode i8, these electrons receive energy from the deflection'plates litl5 and other electrode structure so that they impinge upon the collecting elec trode with a velocity su'icient to liberate secondary electrons from the collecting electrode it' Most orf-the secondary electrons emitted by they collecting electrode I8 will be emitted nor` mal to the plane of the collecting electrode, and since, in accordance with my invention, the collecting electrode I8 is positioned in a plane at an" oblique `angle with respect to the tube axis and'is consequently at an oblique angle with respect tothe helical paths of the returning electrons,` the secondary electrons will have large components of velocity transverse to the tube axis and transverse to the axial magnetic field. Because of their large transverse velocities these secondary electrons follow helical paths of great amplitude. These electrons starting on what might be another round trip between the collecting electrode and the mosaic electrode are intercepted by the intercepting electrode 2li, while those which may have somewhat lower transverse velocity and therefore less helical amplitude pass through the slot 22 and are collected-'by thel -intercepting electrode 2|. It is therefore impossible for these electrons to again approach the mosaic electrode and possible impinge thereon, thereby causing distortion in the signal representative of the light intensity on the various mosaic elements.

In addition to intercepting electrons the electrodes 2li-2l maintain a substantially field-free space within the electrostatic deflection region of the plates M-IS and at the same time prevent fringing of the electrostatic field between the plates. Thus any variations in electrostatic potential distribution on the mosaic electrode or any slight potential variations applied to the electron gun electrodes produce little or no distortion in the deflection space. In order to further insure a field-free space between the deflection plates lil-I5, I have found it desirable to coat the inner wall of the envelope l with a film 26 of electrically conductive material so that electrons are prevented from collecting on the bulb wall, thereby introducing an electrostatic eld within the space separating the intercepting electrodes 2li-2|,

In the operation of tubes made in accordance with my invention I have found that the operating parameters such as the size of the electrodes, the separation of electrodes and operating potentials, may be varied over wide limits. However, to limit the structure to practical proportions I have found that the potential applied between the cathode 3 and the anode 6 is preferably between 50 and 250 volts. Tubes operating with a potential below 50 volts have shown decreased beam focusing properties, and the use of potentials below this value necessitates greater accuracy in the alignment of the electron gun electrodes and the deflection plates. Certain tubes operating with potentials above 250 volts have given unstable operation. The axial magnetic field is preferably between 50 and 150 gausses. Below 50 gausses some distortion due to stray electrostatic fields may be produced, and the use of a field stronger than 150 gausses necessitates increased deflecting power and use of heavier coils and results in poorer focus of the electron beam near the edges of the target electrode. The width of the target or mosaic electrode is preferable made between 5 and 7 centimeters, since the width of the plates must be of the same or greater width, and greater width vof the target would necessitate a tube of larger overall diameter. The length of the deflection plates is preferably between and 20 centimeters, for when using a length less than l0 centimeters, some distortion and defocusing of the beam is produced. Lengths of the deflection plates above 20 centimeters unduly increase the tube length and the length of the focusing coil.

While I have indicated the preferred embodiments of my invention of which I am now aware and have indicated the specific application as directed to cathode ray transmitting tubes having target relectrodes of the photosensitive mosaic type, it will be apparent that my invention is by no means limited to the purpose of television transmission, to the exact forms illustrated, or to its use in cathode ray tubes incorporating target electrodes of the mosaic type, but that many variations may be made in the particular structure used, such as b-y replacing the mosaic electrode with target electrodes of the fluorescent, photoconductive, or photovoltatic type, without departing from the scope of the invention as set forth in the appended claims.

I claim:

1. A cathode ray tube having an electron source, an oppositely disposed electron receiving target, an anode adjacent said source to form and direct an electron beam toward said target, means to generate a magnetic field having lines of force parallel with the center line between said anode and said target, means to generate electron beam deflection fields at least one of which is an electrostatic field to deflect the beam o-ver said target in mutually perpendicular directions, and apertured means adjacent said anode and interposed between said anode and said means to generate beam deflection fields to collect electrons not reaching said target between said anode and said deflection means, said apertured means lying in a plane oblique to the center line of said anode and said target and surrounded by said first-mentioned means.

2. A cathode ray tube having a cathode to emit' electrons, an anode adjacent said cathode to form the emitted electrons into a beam, a target electrode in the path of said beam, means to generate a magnetic field having lines of force which are substantially parallel with the center line between said anode and said target and extending over the space between said anode and said target, means wholly immersed in said magnetic field and between said anode and said target for electrostatically deflecting the beam over said target, two.electron intercepting electr'odes, one on either side of said deflection means, and an electron collecting electrode between said anode and the intercepting electrode nearer said anode to collect electrons returning from said target electrode in the direction of said anode, said collecting electrode having an extended surface which is oblique to said axis and said intercep-ting electrodes intercepting the secondary electrons emitted by said electron collecting electrode.

3. A cathode ray tube having a target, an oppositely disposed electron gun including an anode to direct a beam of electrons toward said target,

an electron focusing coil surrounding the space u between said gun and said target to generate a magnetic iield having lines of force extending from said gun to said target, a pair of oppositely disposed beam deiiection plates between said gun and said target to deflect, in combination with the magnetic field, the electron beam over said target, two slotted electron intercepting electrodes positioned with their slotted portions parallel one to the other and parallel with the plane of deflection of said beam within said deiiection plates, and an apertured electron collecting electrode aligned coaxially with said intercepting electrodes and said electron gun inclined at an oblique angle to the center line of said electron gun and target to collect electrons of said beam not reaching said target and said intercepting electrodes intercepting secondary electrons emitted by said collecting electrode.

4. In combination with a cathode ray tube having means to generate a uniform magnetic eld, an electron receiving target, and an electron gun having a center line to said target parallel with the lines of force of said l'ield, said gun being adapted to generate an electron beam and direct said beam along the lines of force of said magnetic eld toward said target, an electrostatic electron beam deflection structure comprising a pair of plates on opposite sides of the path between said gun and target and Wholly immersed in said field, an electron collecting electrode adjacent the electron gun and between said gun and said deection structure to collect electrons of said beam not reaching said target, and a pair of slotted electron intercepting electrodes to intercept secondary electrons emitted by said collecting electrode, one on either side of the said deflection structure in the direction between said gun and target aligned with their slotted portions in the plane of deflection of the electron beam between said deection structure, and in planes substantially perpendicular to the direct path between said gun and target.

5. In combination with a cathode ray tube having means to generate a uniform magnetic eld, an electron gun to generate an electron beam, an electron receiving target of the mosaic type in the path of the electrons from said gun, an electrostatic electron beam deflection structure Wholly immersed in said eld comprising a pair of plates on opposite sides of the path between said gun and target, a pair of electron intercepting electrodes having slots aligned in the plane of deiiection of the electron beam between said plates and in planes substantially perpendicular to the direct path between said gun and target, one on either side of the said plates in the direction between said gun and said target, and an electron collecting electrode having an extended surface lying in a plane at an oblique angle with the plane of deflection to collect electrons which return from said target, said intercepting electrodes intercepting secondary electrons emitted by said collecting electrode.

ALBERT ROSE.

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