US2217168A - Electron discharge device - Google Patents

Description

Oct. 8, 1940. .1. R. Han-ELE r-:r A1. 2,27,168

ELECTRON DISCHARGE DEVICE Filed Feb. 19. 1938 @y @MMM ArmRA/Er Patented Oct. 8, 1940 invita 2,217,168 I ELEc'rnoN DISCHARGE DEvioE .lohn R. Hefele, Yonkers, and Gordon K. Teal, New York, N. Y., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application February 19, 1938, Serial No. 191,410

15 Claims.

This invention relates to electron discharge devices and more specically to electronic pickup devices for the transmission of television images.

In television operation With a cathode ray type of pick-up device at the transmitter and a cathode ray receiving tube for reproducing the image, trouble was encountered in the form of a diffused dark area in the approximate center of A' the receiving tube screen. An experimental investigation of this phenomenon has resulted in the determination of its cause and the provision of means for eliminating this effect. The present invention is based upon the discovery that this diffused dark area results from a spray of secondary electrons surrounding a focussed beam in the cathode ray pick-up device at the transmitter.

It is an object of this invention to provide electrode arrangements for cathode ray pick-up tubes which prevent the above-mentioned spray of secondary electrons around the focussed beam.

In the single embodiment of this invention which is herein described by way of example, a cathode ray tube is provided comprising an electron gun arrangement for generating, accelerating and focussing a beam of electrons upon a photosensitive target which comprises a dielectric element which may be of glass and which may have a coating on the surface thereof remote from the beam of suitable metallic material and a globular layer of photosensitized silver elements on the surface thereof near said electron gun, and two pairs of deilecting plates for causing the beam to scan in turn the respective elemental areas of a field on the target. The electron gun arrangement preferably comprises a cathode, a first anode adjacent the cathode cornprising a cylindrical portion having one or more 9- metallic apertured diaphragms therein, and a f to that of the cathode, that is of the order of 3000 volts, While the second anode is placed at a potential which is positive with respect to that of the cathode but which is substantially negative with respect to the potential of the first anode, as, for example, from 800 to 900 volts with respect to the potential of the cathode. Due to the fact that the second anode is at a negative potential with respect to that of the first anode, ther secondary electrons emitted by the primary r beam impinging upon one or the other of the diaphragrns of the rst anode are repelled by the second anode. Inasmuch as the electrons in the primary beam are of high velocity, the electrons thereof are decelerated by the second anode but are not prevented from reaching the (Cl. Z50-,27)

screen or target. Between the rst anode and the cathode is preferably arranged a control element for varying the intensity of the primary electron beam, which element in the preferred embodiment constitutes a cylindrical member having a metal cap in which is located a circular aperture. This cylinder is placed directly around the cathode.

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

Fig. 1 shows schematically a cathode ray pickup device for television transmission embodying the principles of this invention; and

Fig. 2 is a schematic diagram of the electron lens system of the tube shown in Fig. 1.

Referring more particularly to the drawing, Fig, 1 shows a cathode ray tube I0 and associated apparatus and circuits to render the tube suitable for a pick-up device for the transmission 'of television images to a receiving station. The device l preferably comprises a gas-tight con-v tainer II enclosing an electron gun for producing a moving beam of electrons and for focussing and accelerating this beam toward a screen or target I2 which is mounted in the end of the tube remote from the electron gun, and means such as two pairs of defiecting plates I3, I4 and I5, I6 for delecting the beam along a predetermined path of the screen I2, which path successively covers the elemental areas of a field on the screen. 'External to the tube I0 is mounted a suitable lens system, which is represented by way of example as a single convex lens il, for focussing an image of an object or field of view il upon a surface of the screen or target I2, which Will be hereinafter designated the front surface.

The electron gun arrangement comprises a cathodevlll, a first anode I9, a second anode 20, and a control element 2| directly surrounding the cathode I8.

The cathode I8 is heated by a heater element 22 which is supplied with heating current from a source of direct current potential 23. A portion of the cathode I8 near the control element 2I is preferably coated with a thin layer of electron emissive material.

The control element 2| preferably comprises a cylindrical member having a cap 24 with an aperture 25 therein opposite the portion of the cathode It having the electron emissive coating. The control element 2l is placed preferably at a negative potential with respect to that of the cathode I8 by means whichl will be described more fully below. The purpose of the control element is to vary the intensity and the size of the electron beam.

The first anode I9 comprises portions of two cylinders of different diameters joined by a ring- Cil shaped connecting member 28. The cylindrical portion 21 is of larger diameter than and eX- tends up closely to the control element 2l while the cylindrical portionr 28 is of approximately the same diameter as that of the control element 2l and extends toward the screen I2. The portion 28 preferably encloses two apertured diaphragms 29 and 30, to assist in the focussing of the electron stream generated by the cathode I8. rThe second anode 20 preferably' comprisesl a metallic cylindrical member of `approximately the same diameter as the cylindrical portion 21 of The member 2 ilA extendsfrom'.

the first anode I9. a point near the end of the first anode I9 toward the screen I2. "If desired, it may be connect'ed to a conducting coating 3|` located onthe inside wallsy of the tube I0.. This conducting coating may be of any material suitable v.for the purpose such as, for example, aluminum, platinum or gold. The coating may be in they form ofv a cylinder of thin sheet aluminum. or. othermetahor it` may be an evaporated nlm of ithe aluminum, platinum or gold.

In order. to cause the. electron beamgenerated bythe electron` gunapparatus described above, to scan every elemental area of the image or eld of view on the screen or. target I2, suitable defiecting means, suchv as, for example,.two pairs of deecting plates i3, lll and I5, -6,.the axes of which are located at right angles to each other,` are provided. To the deflecting plates I5,4 IB are applied dei'iectingy voltages of framing frequency and having asaw-tooth wave form to produce the vertical deflection of kthe beam while deflecting voltages of line scanning frequency and of sawtcoth. wave form are applied to the deflecting plates i3, I d to produce the horizontal deflection of the beam. Any suitable sweep circuits (not` shown) may be used to generate these horizontal and. vertical deecting voltages. For example., reference may be made to Patent 2,178,464, issued. October 31,1939 to M. W. Baldwin, Jr., whichl discloses appropriate balanced sweep circuits f'or this purpose.. Connectionsmay be made from thebalanced sweep. circuits. to the pairs of plates I3, I4 and I5, I6 by means of coupling condensers. 32, 33,.and 34,. 35 respectively. High coupling. resistors 36 and 31 are respectively connected across the pairs-of plates I 3, i4 and I5, I6. The mid-points ofthe resistors 36 and 31 are connectedfto the second anode 2i) in order thatthe average. potential of the. dei'lecting plates is at all times substantially equal to the potential of the second anode 2B. For a full description of the advantages of balanced sweep circuits, reference may be made to the above-mentioned Baldwin patent and also to Patent 2,209,199, issued July 23, 1940 to Frank Gray.

The direct current vpotentials for biasing the different elements of the electron gun with respect to the cathode are derived from an alterhating current oscillator 4), the outputvof which is rectified and filtered by a device represented schematically by the block 4I, which has. in its Voutput circuit a potentiometer resistor 42 having aplurality of taps from which the various voltages may be taken. An alternating current ground is. provided for two spaced-apart terminals of the potentiometer resistor. 42 by means of condensers LlS- and. 44 which are connected between these terminals and ground'. An inner tap. 45 of the, potentiometerresistorV 4Z is connected to ground and also to the second anode 20. A second tap 415; representing a potential which. is. approximately 800 to 900l volts negative with respect to that of the tap 45, is connected to the cathode I8, and an adjustable tap 41, which represents a potential which may bevaried' within a region of from Goto -90 volts'w'ith respect to that of the cathode I8, is connected to the control element 2l. Tap 43 is connected to the first anode i9 and represents a potential which is approximately 3G60 volts positive with respect to the potential of the cathode I3, that is, with respect to'the potential or' the tap 45, and a potential which'is'2100't'o 2200 volts positive with respect to that of ground and'with respect to the potential of. the.. second anode 20. It will be understood that the above values are given only by way of example and that the invention is not limited thereto.

The screen or'. target I2 preferably-comprises adielectric` member 50 of glass or micahaving a conducting backing 5I of a suitable material such as silver or platinum and a globular layer 52 ofsilver which has been oxidized and photosensitized with photoelectric material, such as caesium; For a description of a. method.- of preparing photosensitized screens ortargets similar to that described above, reference maybe made to. Patent 2,173,923, issuedSeptemberv 26, 1939 to Gordon K. Teal.

Between the metallic backing plate5lVv of the target I2 and the second accelerating anode 20 is connecteda signal resistor 53v which may also be placed in the input circuit of an amplifying device (not shown). If desired, abiasing. battery, (not shown) may be connected between the resistance 53 and the second accelerating anode 20 although the device is operative. withoutthis battery.

The operation of thedevice shown in Fig. 1 is as follows: Light from al suitable source (not shown) and reflected fromthe object or eldof view [lis focussed upon the photosensitive mosaic surface. 52 of the target or screen` I2 by means of the optical system represented by the lens I1. The photoemissive` material 52 on the mosaic screen emits electrons, the degreey of emission from` any` elemental area thereof bei-ng. dependent upon the intensity of light projected on-the-front surface. of the screen from the object. Each photosensitive globule l thus becomesv chargedV because. of the condenser action. of. the dielectric member 5l] and". the. metallic coating. 5l and the photoemissive surface 52 on its opposing surfaces,

the degreeof the charge for any particular globule corresponding to the light-tone value of. the corresponding elemental. area of theobject. Electrons are emit-ted from the cathode. I 3 and. accelerated toward the anode I9 because ofthe high positive potential'. applied` thereto with respect to the potential ofthe cathode. With reference to Fig. 2, it will be seen that there is an electron lens. action between. the cathode land" the control element 2`I` and between the control element 2l and the rst anode I9`which causes the formation of a converging electron beam which would be focussed in a fine spot. at-point F if it were not for the. action of' the diverging electron lens action, between. the rst. anoder I9. and the second anodev 20.y Due to thenegative potential applied to the second anode ZIJl with respect to that of the r-st anode I9, the speed of the electrons in-the beam is reduced and the electron beam is rendered diverging totheextent that instead of focussing` into a spot at the-point F,. it. is. forcussedl into. a spot oni the. screen; or target I2. This electron beam. i's'caused; to. scan every elementalarea in turn'of the photosensitivev` surface 52 by means of the saw-toothed voltages applied between the pairs of deflecting plates I3, I4 and i5, I6.

This scanning is repeated about twenty-four times a second so that advantage can be taken at the receiving station of the phenomenon of persistence of vision. As the electron beam scans the mosaic surface, it passes over Aeach element in turn, releasing the charge it has acquired and returning it to equilibrium. Due to the fact that each element is coupled by capacity to thebacking plate 5l, the sudden change of charge of the elements will induce a change of charge on the backing plate 5l and result in a current .pulse in the signal resistor 53 in the external circuit. The succession of pulses constitutes the image current. For a more complete disclosure of a tube employing photosensitive mosaics together with a more complete description of a method of operation thereof, reference may be made to an article entitled The iconoscope by V. K. Zworykin in the January, 1934 Proceedings of the Institute of Radio Engineers, pages 16 to 32, inclusive, and to an article by the same author in the July, 1936 R. C. A. Review page 60, entitled Iconoscopes and kinescopes in television.

In certain well-known electron gun arrangements of the prior art the primary electrons constituting the beam of electrons are surrounded by` a halo of secondary electrons which because of their small velocity and because they are inside the focus of the electron lens formed by the first and second anodes are not focussed on the screen by the lens between the rst and second ancdes. In the arrangement of this invention, however, while secondary electrons may be generated by the impingement of primary electrons upon the diaphragms 29 and 3B of the rst anode i9, they are caused to return to this anode due to the negative potential of the second anode with respect to the potential of the rst anode le. This arrangement has been found to be very satisfactory in practice to prevent the ne spray of secondary electrons from reaching the photosensitive target and at the same time makes possible a sharp focus of primary beam electrons upon the photosensitive screen. The picture received from such a pick-up device the screen of a receiving tube shows no trace of the spurious image usually appearing in the field when wellknown electron gun systems of the prior art are used. It was observed that the magnitude of the received-signal was considerably greater than heretofore obtained and the resolution of the system seemed improved.

While the invention has been described in connection with a tube employing a target comprising photoemissive material, it is obvious that the invention is not limited to the use of this type of target. For example, the invention is equally applicable to a tube including a target comprising a metal backing plate having a layer of high resistance photoconducting material thereon. Such a target and the use thereof in a pick-up tube is described in Patent 2,150,160, issued March 14, 1939 to Frank Gray.

Various other modifications may obviously be made without departing from the spirit of the invention, the scope of which is dened by the appended claims.

What is claimed is:

1. A cathode ray device comprising a target having a photosensitive portion upon which radiations from an object are applied and which is adapted to be scanned with a beam of electrons to produce image signals, said device being adapted for use in connection with areceiving member on the screen of which an image of said object is reproduced, an electron gun for generating a beam of electrons to impinge upon said target, said electron gun having means normally generating secondary electrons by the impact of the primaryelectrons upon said means, which secondary electrons will, if they are not suppressed, reach the photosensitive portion of said target to thereby cause the generation of spurious signals which in turn cause the formation of a diffused dark Spot over a large portion of the screen area of said receiving member, and means adjacent to or forming a portion of said electron gun for substantially preventing the flow of said secondary electrons to said target and the formation of said dark spot.

`2. A cathode ray device comprising a target having, a photosensitive portion upon which radiations from an object are applied and which l is adapted to be scanned with a beam of electrons to produce image signals, said device being adapted for use in connection with a receiving member on the screen of which an image of said object is reproduced, la cathode for emitting a stream of primary electrons, a rst electrode member between said cathode and said target, means for placing said first electrode member at a potential which is positive with respect to the potential of said cathode, said first electrode having a portion thereof in the path of some of the electrons of said stream, which portion tends to give off secondary electrons when struck by said primary electrons, which secondary electrons will, if they are not suppressed, reach the photosensitive portion of said target to thereby.

cause the generation of spurious signals which in turn cause the formation of a diffused dark Spot over a large portion of the screen area of said receiving member, a second electrode member, and means for placing the second electrode member .at a potential Which is sufficiently negative with respect to the potential of said first electrode member to prevent, at least in large measure, the passage of said secondary electrons to said target and the formation of said dark spot.

3. A cathode ray device comprising a target having a photosensitive portion upon which radiations from an object are applied and which is adapted to be scanned with a beam of electrons to produce image signals, said device being adapted for use in connection with a receiving member on the screen of which an image of said object is reproduced, a cathode for emitting a stream of primary electrons, an anode adjacent said cathode for accelerating said electrons towards said target, the structure of said anode being such that secondary electrons are generated when the primary electrons impinge upon portions thereof, which secondary electrons will, if they are not suppressed, reach the photosensitive portion of said target to thereby cause the generation of spurious signals which, in turn, cause the formation of a diffused dark spot over a large portion of the screen area of said receiving member, and electrode means between said anode and said target to focus said primary electrons into a spot on said target, said electrode means being placed `at such a negative potential with respect to the anode that said secondary electrons are substantially prevented from reaching said target and the formation of said dark spot is substantially obviated.

having a photoemissive portion upon which` radi.-

ations from an object are applied and which isl adapted to be scanned with a beamof electrons to produce image signals, said device being adapted for use in connectionwith a receiving member'.

on the screen of` which an image of said objectk is reproduced, an electron gun for generating a beam of electrons to impinge upon said target, said electron gun having means normallyy generating secondary electrons by the impact' of the primary electrons upon said means, which secondary electrons will, ifthey are not suppressed, reach the photoemissive portion of saidtargetto thereby cause the generationV of spurious signals which iii-turn cause the formation of a diffused dark spot over a large portion of the screen area of said receiving member, and means including aV portion of saidrelectron gun for substantially preventing the flow of said secondary electronsV to said target and the formation of said dark spot.

5i. A cathode ray device comprising a target having a photosensitive portion upon which radiations from an object are applied and which is adapted to be scanned with a beam ofV electrons to produce image signals, said device being adapted for use in connection with a receiving member ony the screen of which an image of said object is reproduced, a cathode for emitting a stream of primary electrons, a first anode comprising a cylindrical member having at least one apertured diaphragm therein which emits secondary electrons when impinged upon by abeam of primary electrons from said cathode, which secondary electrons will, if they are not suppressed, reach the photosensitiveportion of said target to thereby cause the generation of spurious signals which, in turn, cause the formation of a diiTused dark spot over a large portion of the screen area of said receiving member, a second anode comprising a cylindrical metallic member of larger diameter than the cylinder of Ysaid first anode, means for placing said rst anode at a relatively high posi tive. potential with respect to. said cathode, and means for placing said second anode at a` potential which is positive with respect toV said cathode but which is negative with respect to said rst anode by an amount sufcient toy substantially suppress said secondary electrons and, at least in large measure, to prevent the formation of said dark spot.

6. A cathode ray device comprising a target having a photosensitive portionupon which radiations from, an object are applied and which is adapted tob-e scannedV with a beam of, electrons to produce image signals, said. device being adapted for use in connection with a receiving member on the screenY of which an. image of said object is reproduced, a cathode for emitting a stream of primary electrons, a control member placed around the cathode, a first anode comprising a cylindrical member having at least one apertured diaphragm therein which emits secondary electrons, when impinged upon bly-a beam of primary electrons from said cathodewhich secondary electrons will, if they are not suppressed, reach the photosensitive portion of said target to thereby cause the generation of spurious signalsy which, in turn, cause the formation of a diffusedl dark spot over a large portion of' the screen area of said receiving member, a second anode comprising a cylindrical metallic member of larger diameter thanthe cylinder of said rst anode, means for placing said rst anode at a 4. A cathode ray device comprising a targeti relatively high positive potential with respect to said4 cathode, and means for placing said second anode at a potential which is positive with respect to, said cathode but which is negative with respect tosaid rst anode by an amount suiilcient to substantially'suppress said secondary electrons and, at: leastl in large measure, to prevent the formation of saiddark spot.

'7'. Ina. television transmitter tube having a target' comprising photoelectric material adapted tohavean imageof an object formed thereon and a. primary beam generating electron gun therein, said tube being adapted for use in connection with a receiving member on the screen of which anV imageof said object is reproduced, the method of preventing, in large measure, secondary electrons emitted by a portion of the electron gun upon which the primary beam strikes from reaching the target and thereby causing the generation of spurious signals which, in turn, causes the formation. of a diffused dark spot over a large portion of the screen area of said receiving memberv which comprises the step of forming a retarding field between the target and the portion of the gun giving rise to the secondary electrons to substantially suppress the flow of said secondary electrons toward said target and, in large measure, obviate the formation of said dark spot.

8'. A cathode ray device comprising a target having a photosensitive portion upon which radiations from an object are applied and which is adapted' to be scanned with a beam of electrons to,` produce image signals, a cathode for emitting primary electrons, a rst electrode member between said cathode and said target, means for placing said rst electrode at a potential which is highly positive with respect to said cathode, said first electrode comprising at least one apertured metallic diaphragm in the path of the electrons, the aperture in which diaphragm has a smaller cross-sectional area than the stream of electrons at the plane of the diaphragm whereby primary electrons strike said diaphragm and cause the emission of secondary electrons which secondary electrons would, if they were not suppressed, reach the photosensitive portion of said target in a ne spray over at least the major portion thereof which spray would cause the generation of spurious signals, a second electrode between said rst electrode and said target and closely adjacentl Said iirst electrode member, means for placing the second electrode at a potential which is positive with respect to that of said cathode but which is less than half the potential ofy said first electrode with respect to said cathode and which potential bears the proper relation to the potential of the iirst electrode to cause the stream of electrons to be focussed to a une. spot at the target,the potential of said second electrode member being suii'iciently negative with respect to that of said first electrode member to substantially prevent the passage of said secondary electrons to said target, and means for causing the focussed spot to scan every elemental area in turn of a eld on said photosensitive portion of the target.

9. A cathode ray device comprising a target having, a photosensitive portion upon which radiations from an object are applied and which is adapted to be scanned with a beam of electrons to produce image signals, a cathode for emitting primary electrons, a first electrode member between said cathode and said target, means for placing said first electrode at a potential which is highly positive with respect t0 said cathode, said first electrode comprising at least one apertured metallic diaphragm in the path of the electrons, the aperture in which diaphragm has a smaller cross-sectional area than the stream of electrons at the plane of the diaphragm whereby primary electrons strike said diaphragm and cause the emission of secondary electrons which secondary electrons would, if they were not suppressed, reach the photosensitive portion of said target in a fine spray over at least the major portion thereof which spray would cause the generation of spurious signals, a second ,electrode between said rst electrode and said target and closely adjacent said first electrode member, means for placing the second electrode at a potential .which is positive with respect to that of said cathode but which is less than half the potential of said first electrode with respect to said cathode and which potential bears the proper relation to the potential of the rst electrode to cause the stream of electrons to be focussed to a fine spot at the target, the potential of said second electrode member being sufciently negative with respect to that of said first electrode member to substantially prevent the passage of said secondary electrons to said target, means for causing the focussed spot to scan every elemental area in turn of a field on said photosensitive portion of the target, said target being placed in a space the potential of which is substantially equal to that of the second electrode member.

l0. A cathode ray device comprising a target having a photosensitive portion upon which radiations from an object are applied and which is adapted to be scanned with a beam of electrons to produce image signals, a cathode for emitting primary electrons, a first electrode member between said cathode and said target, means for placing said first electrode at a potential which is highly positive with respect to said cathode, said first electrode comprising at least one apertured metallic diaphragm in the path of the electrons, the aperture in which diaphragmhas a smaller cross-sectional area than the stream of electrons at the plane of the diaphragm whereby primary electrons strike said diaphragm and cause the emission of secondary electrons which secondary electrons would, if they were not suppressed, reach the photosensitive portion of said target in a fine spray over at least the major portion thereof which spray would cause the generation of spurious signals, a second electrode between said first electrode and said target and closely adjacent said rst electrode members means for placing the second electrode at a potential which is positive with respect to that of said cathode but which is less than half the potential of said rst electrode with respect to said cathode and which potential bears the proper relation to the potential of the first electrode to cause the stream of electrons to be focussed to a fine spot at the target, the potential of said second electrode member being sufficiently negative with respect to that of said rst electrode member to substantially prevent the passage of said secondary electrons to said target, a third electrode member a portion of which is adjacent said target, means for placing said third electrode member at the same potential as said second electrode member, and means for causing the focussed spot to scan every elemental area in turn of a field on said photosensitive portion of the target.

11. A cathode ray device comprising a target having a photosensitive portion upon which radiations from an object are applied and which is adapted to be scanned with a beam of electrons to produce image signals, a cathode for emitting primary electrons,` a first electrode member' between said cathode and said target, means for placing said first electro-de at a potential which is highly positive with respect to said cathode, said first electrode comprising at least one apertured metallic diaphragm in the path of the electrons, the aperture in which diaphragm has a smaller cross-sectional area than the stream of electrons at the plane of the diaphragm whereby primary electrons strike said diaphragm and cause the emission of secondary electrons which secondary electrons would, if they were not suppressed, reach the photosensitive portion of said target in a fine spray over at least the major portion thereof which spray would cause the generation of spurious signals, a second electrode between said first electrode and said target and closely adjacent said first electrode member, means for placing the second electrode at a potential which is positive with respect to that of said cathode but which is less than half the potential of said first electrode with respect to said catho-de and which potential bears the proper relation to th-e potential of the first electrode to cause the stream of electrons to be focussed to a fine spot at `the target, the potential of said second electrode member being sufliciently negative with respect to that of said rst electrode member to substantially prevent the passage of said secondary electrons to said target, a third electrode member a portion of which is adjacent said target, means for placing said third electrode member at the same potential as said second electrode member,' and electrostatic means located completely within a space the potential of which is substantially equal to that of said seco-nd electrode member for causing the focussed spot to scan every elemental area in turn of a field on said photosensitive portion of the target.

12. A cathode ray device comprising a target having a photosensitive portion upon which radiations from an object are applied and which is adapted to be scanned with a beam of electrons to produce image signals, a cathode for emitting primary electrons, a first electrode member between said cathode and said target, means for placing said rst electrode at a potential which is highly positive with respect to said cathode, said first electrode comprising at least one apertured metallic diaphragm in the path of the electrons, the aperture in which diaphragm has a smaller cross-sectional area than the stream of electrons at the plane of the diaphragm whereby primary electrons strike said diaphragm and cause the emission of secondary electrons which secondary electrons would, if they were not suppressed, reach the photosensitive portion ofA said target in a fine spray over atleast the major portion thereof which spray would cause the generation of spurious signals, a second electrode between said rst electrode and said target and closely adjacent said first electrode member, means for placing the second electrode at a potential which is positive with respect to that of said cathode but which is less than half the potential of said first electrode with respect to said cathode and which potential bears the proper relation to the potential ofthe first electrode to cause the stream of electrons to be focussed to a ne spot at the target, the potential of said second electrode member being sufliciently negative with respect to that of said first electrode member to substantially prevent the passage of said secondary electrons to said target, a third electrode member a portion of which is adjacent said target, means for placing said third electrode member at the same potential as'said second electrode member, electrostatic means located completely within a space the potential of which is substantially equal to that of said second electrode member for caus ing the focussed spot to scan every elemental area in turn of a field on said photosensitive portion of the target, and means for making the average potential of said electrostatic means substanially equal tothe potential of said second electrode.

13. A cathode ray device comprising a target having a photosensitive portion upon which radiations from an object are applied and Which is adapted to be scanned with a beam of electrons to produce image signals, a cathode for emitting primary electrons, a first electrode member between said cathode and said target, a cylindrical member around said cathode, means for placing said cylindrical member at a negative potential with respect to said cathode to concentrate the electrons from said cathode to a point within said first electrode member, means for placing said first electrode at a potential which is highly positive with respect to said cathode, said iirst electrode comprising at least one apertured metallic diaphragm in the path of the electrons, the aperture in which diaphragm has a smaller cross-sectional area than the stream of electrons at the plane of the diaphragm whereby primary electrons strike said diaphragm and cause the emission of secondary electrons which secondary electrons would, if they were not suppressed, reach the photosensitive portion of said target in a fine spray over at least the major portion thereof which spray would cause the generation of spurious signals, a second electrode between said first electrode and said target and closely adjacent said rst electrode member, means for placing the second electrode at a potential which is positive with respect to that of said cathode but which is less than half the potential of said rst electrode with respect to said cathode and which potential bears the proper relation to the potential of the first electrode to cause the stream of electrons to be focussed to a fine spot at the target, the potential of said second .electrode mem# ber being suiciently negative with respect to that of said first electrode member to substantially prevent the passage of said secondary electrons to said target, and means for causing the focussed spot to scan every elemental area in turn of a field on said photosensitive portion of the target.

14. A cathode ray device comprisinga target having a photosensitive portion upon which'radiations from an object are applied and which is adapted to be scanned with a beam of electrons to produce image signals, a cathode for emitting primary electrons, a rst electrode member between said cathode and said target, means for placing said rst electrode at a potential which is highly positive with respect to said cathode, said first electrode comprising at least one apertured metallic diaphragm in the path of the electrons, the aperture in which diaphragm has a smaller cross-sectional area than the stream of electrons at the plane of the diaphragm Whereby primary electrons strike said diaphragm and cause the emission of secondary electrons which secondary electrons would, if they were not suppressed, reach the photosensitive portion of said target in a ne spray over at least the major portion thereof which spray would cause the generation of spurious signals, a second electrode between said first electrode and said target and closely adjacent said first electrode member, means for placing the second .electrode at a potential which is positive with respect to that of said cathode but which is less than half the potential of said rst electrode with respect to said cathode and Which potential bears the proper relation to the potential of the rst electrode to cause the stream of electrons to be focussed to a fine spot at the target, the potential of said second electrode member being sufliciently negative with respect to that of. said rst electrode member to substantially prevent the passage of said secondary electrons to said target, a third electrode member a portion of which is adjacent said target, means for placing said third electrode member at the same potential as said second electrode member, and means for causing the focussed spot to scan every elemental area in turn of a eld on said photosensitive portion of the target, said second electrode member comprising a metallic cylinder and said third electrode member comprising a conducting coating on the wall of the device extending from the region of the second electrode tothe region of the target.

15. A cathode ray device comprising a target having a photosensitive portion upon which radiations from an object are applied and Which is adapted to be scanned with a beam of electrons to produce image signals, a cathode for emitting primary electrons, a first electrode member between said cathode and said target, means for placing said first electrode at a potential which is highly positive with respect to said cathode, said first electrode comprising at least one apertured metallic diaphragm in the path of the electrons, the aperture in which diaphragm has a smaller cross-sectional area than the stream of electrons at the plane of the diaphragm whereby primary electrons strike said diaphragm and cause the emission of secondary electrons which secondary electrons would, if they were not suppressed, reach the photosensitive portion of said target in a ne spray over at least the major portion thereof which spray would cause the generation of spurious signals, a second electrode between said first electrode and said target and closely adjacent said first electrode member, means for placing the second electrode at a potential which is positive with respect to that of said cathode but which is less than half the potential of said first electrode with respect to said cathode and which potential bears the proper relation to the potential of the first electrode to cause the stream of electrons to be focussed to a ne spot at the target, the potential of said second electrode member being sufciently negative with respect to that of said rst electrode member to substantially prevent the passage of said secondary `electrons to said target, means for forming a field free space between said second electrode member and said target, and electrostatic means completely within said field free space for causing the focussed spot to scan every elemental area in turn of a field on said photosensitive portion of the target.

JOHN R. HEFELE. GORDON K. TEAL.

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