US2754449A - Cathode ray tube and system - Google Patents

Description

y 10, 1956 P. T. FARNSWORTH 2,754,449

CATHODE RAY TUBE AND SYSTEM 4 Sheets-Sheet '1 Filed Nov. 25, 1950 INVHVTOR. PH/LO T FARNSWORTH ATTORNEY y 1956 P. T. FARNSWORTH 2,754,449

CATHQDE RAY TUBE AND SYSTEM Filed Nov. 25, 1950 INVENTOR. PHILO 7'. FARNSWORTH ATTORNEY July 10, 1956 P. T. FARNSWORTH 2,754,449

CATHODE RAY TUBE AND SYSTEM Filed Nov. 25, 1950 4 Sheets-Sheet 3 PH/LO T. FARNSWORTH ATTORNEY July 10, 19 P. T. FARNSWORTH 2,754,449

CATHODE RAY TUBE AND SYSTEM Filed Nov. 25, 1950 4 Sheets-Sheet 4 INVENTOR. PHILO T FAR/V5 WORTH /Wad;

ATTORNEY CATHODE RAY TUBEAND SYSTEM Phi lo T. Earnsworth, FortWayne, Ind, assignor to International Telcphone andTelegraph Corporation, a corporation of Maryland Application November-25, 1%0, Serial-No. 197,612

22; (Elaims. (Cl. 315-42) This invention relates to cathode ray tubes and circuits and more particularly to such tubes and circuits which serve to provide an electron image amplification.

In my Patent No. 2,228,388, granted January 1, 1941, entitled Cathode Ray Amplifier a cathode ray tube with image amplification is disclosed. According to this patent, amplification of an electron image is accomplished by means of a cathode ray tube provided with two electron guns mounted to emit electrons in generally the same direction. In the path of the beams from these guns is mounted a perforate grid lattice or mesh strueture consisting of a metal screen the wires of which are surrounded by insulation. An anode electrode main tained at a higher positive potential than the grid is mounted on the side of the grid structure away from the guns. One of the guns emits electrons at aslow velocity and focussed to cover all of the area of the grid structure. The other gun emits a beam of electrons focussed to cover only an elemental area of the grid structure, and of a sufi'iciently high velocity to cause greater than unity secondary emission from the insulating covering of the grid structure. If picture signals are applied to the second gun tovary the electron velocity or to the conduc tor of the grid structure to vary the potential difierence of the second gun cathode relative thereto, while the beam from this gun is scanned over the grid structure, a positive charge image is produced on the screen varying in accordance with the picture signals. As a con sequence, varying the flow of electrons from the first or flood gun through the grid structure will result producing in eifect an amplified electron image. This may be applied to a viewing screen or other utilization device.

The system as described above requires a relatively high velocity in the second or scanning gun beam so that greater than unitysecondary emission is assured. Furthermore, the degree of image charge that can be produced on the grid structure depends largely; on the velocity of the scanning beam and the nature of the secondary emissive surface of the grid. The tube works onlyin the positive region serving only to reduce the number of electrons passing through the grid by attracting electrons to it by the positive charges. Moreover, leakage of the impressed charges must take place through the insulating coating on the grid structure condoctors; this can be controlled only by the thickness of the insulator coating and the dielectric properties thereof.

Another proposed recording tube construction, based on the use of a storage screen previously proposed byapplicant, is disclosed in the Proceedings of theI. R. E., July 1950, on page 740 et seq; In this construction a storage screen is provided of a metallic mesh coated on the surface facing away from an electron gun with a dielectric material. A combined collector-reflectorelectrode is positioned facing toward the insulating surface. In operation this collector reflector electrode is maintained at anegative potential during application of the image signals to the storage screen, while the screen supporting. the dielectric material is. at a positive poten- States atent Patented July 19, F356 tial. Electronsfrorn the gun penetrate through the apertures of thescreen and because of the reflecting electrode are caused to turn back and bombard the insulating coating knocking out secondary electrons so that a positive charge is made in the insulating coating, dependent upon the variations imparted to the beam by picture signals. Thus, as the beam is scanned a positive electrostatic image is stored on the insulating coating. For reproduction of this image the collector electrode is rendered positive so that the beam as it is scanned over the charged screen willvary in strength in accordance with the stored charge to reproduce the picture. In one arrangement proposed in this article the beam may be defocussed during this reproduction period so that the entire area of the storage screen will be covered by the beam and the picture may be reproduced in toto without scanning.

It; willbe noted in this instance that the electrostatic charge must be in a positive direction. Thus the efiective control grid action is limited to the positive spectre. Furthermore, this construction requires that the insulating coating be arranged to face away from the electron gun requiring relatively complex control arrangement.

It is an object of this invention to provide an image amplifying tube of the type wherein an image charge is impressed upon a perforate grid structure by a scanning electron beam and to control the flow of electrons provided from a flood gun source through the apertures, wherein the grid structure is operated at all times in the negative region. The scanning gun beam may be of low velocity so as to build up a negative image charge on the grid, the grid comprising a conductive screen the surface toward the scanning beam source, only, being coated with a secondary emissive dielectric. In this case the conductive screeen is biased positively. If a scanning beam of higher velocity is used so that the dielectric tends to become more positive a negative bias is used on the conductive screen to preserve the negative region operating characteristics of the dielectric surface with respect to the flood gun cathode.

In accordance with a feature of my invention the gun structure may be energized so that two electron beams are provided, one of which is concentrated to cover an elemental area, and the other to cover a substantially greater or the entire area of a dielectric screen covered onthe side toward the guns with a quartz for example. The conductor of this composite screen may be maintained-at a potential slightly more positive than the flood gun cathode, the scanning or elemental gun cathode being negative with respect to the flood gun. If the scanning beam is made of higher velocity so that it causes secondary emission tending positively to charge the insulating surface, then the screen is maintained at a potential slightly negative with respect to the flood gun cathode. Preferably separate guns are used to produce the flood beam and scanning beam, either simultaneously or alternately operated. Alternatively, a single gun may be used with variable controls to change its operating characteristics alternately to serve as a scanning and flood gun.

Means is provided to scan the elemental beam over the composite screen or image storage grid in timed relation withthe image signals to be stored and reproduced. At the same time variations in voltage corresponding with the image signals are produced between the scanning beam source and the image storage grid. Leakage of the stored charge takes place through the insulating coating and by surface leakage around the edges of this coating to the metal mesh, the latter being the predominate effect. The second, or flood gun, may be scanned over the area of the storage grid if the entire area is not covered by the beam. In view of the difliculty in obtaining a beam of uniform cross-section over the entire area, it may be preferable to use a beam which does not cover the entire surface and scan it to achieve greater uniformity of average coverage.

In order to provide a uniform virtual cathode or electron source adjacent the storage grid a collector electrode may be mounted, spaced from the insulated surface thereof. This collector electrode may be in the form of an open mesh, preferably coarser than the mesh of the storage screen. A predetermined small potential difference will exist between these elements dependent upon biasing potentials and the electrons introduced into the space by the guns.

The utilization device for the electron image formed by electrons passing through the storage may be a luminescent screen, such as a fluorescent screen. Since the electrons passing through the control storage screen mesh cover a substantial area of the luminescent screen a greater average brightness is obtained without excessively high current densities than when a beam of elemental cross section is used. Accordingly a brighter image suitable for projection may be readily obtained. Moreover, a rela tively small storage charge controls, a much larger electron flow from the virtual cathode, resulting in electron amplification.

' The above and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, in which: a

Fig. 1 is a perspective view partly broken away, showing an image tube in accordance with this invention;

Fig. 2 is a diagrammatic illustration of a cathode gun structure as shown in Fig. 1;

Fig. 3 is a schematic circuit diagram for a tube such as shown in Fig. 1;

Fig. 4 is a schematic illustration showing the operation of the system illustrated in Fig. 3;

Fig. 5 is a typical characteristic operating curve for the structure of Fig. 3;

Figs. 6 and 7 are diagrams of alternative tube constructions to that illustrated in Fig. 1;

Fig. 8 is a schematic circuit digaram illustrating a system for alternate operation of the flood gun and scanning gun in accordance with my invention.

Fig. 9 is a schematic diagram of a modified arrangement for operation of the electron guns alternately similar to showing of Fig. 8;

Fig. 10 is a schematic diagram of a modified cathode ray tube in accordance with my invention, and

Fig. 11 is a detailed diagram of the cathode gun structure used in the circuit of Fig. 10.

Turning now to the drawings, a typical tube in accordance with this invention, as constructed for test purposes is illustrated. The tube comprises an envelope 1 having a portion 2 of relatively small diameter and an other .portion 3 of larger diameter. At one end of the envelope is mounted an electron gun assembly 4 for providing two electron beams, one of which is concentrated to provide coverage of an elemental area on an image storage screen, the other providing a flood of electronsover a substantial area thereof. Connections for the various gun electrodes are brought out through base 5. A conductive coating 6 is provided internally of the envelope portion 2 for the purposes of beam control as generally provided in cathode ray tubes. At the opposite end of the envelope 1 is provided an image reproducing screen 7 which may, for example, be a fluorescent screen having a coating of phosphor material which will fluoresce under the impact of the electrons thereon. A Covar ring 3 serves to seal the fluorescent screen 7 to the body portion 3 of the envelope. A fine open mesh screen 9 is mounted substantially mid-way between the ends of portion 3. This screen or mesh may be termed as the collector electrode and is conveniently made by an electro-plating method. This screen may be, for example,

approximately 600 mesh per inch screen. Mounted within the tube and spaced between screen 9 and fluorescent screen 7 is provided the image storage mesh or screen 10. This perforate screen may have 1,000,000 perforations or more per square inch. In an actual tube construction a 1,000 mesh screen was used in which the openings constituted approximately 60% of the screen surface. On the surface of screen 10 facing the collector electrode 9 is provided a thin film of insulating material which may be coated thereon, by evaporation. This may be of any desirable dielectric, for example quartz. The thickness of the dielectric film is determined by the requirement that the electro-static capacity be relatively high, of the order of .7 10 farads to 7 l0 farads per square cm. This will provide a thickness of quartz of 5 microns to .5 micron (5.0 l0 cm. to 0.5 10 cm.). These elements 9 and 10 are supported by support and lead out conductors 11 from the shoulder between portions 2 and 3 of the envelope. In order to control the focusing of the beams onto the screens 9 and 10 and the scanning of the beams over these screens magnetic focusing coil 12 and deflection coils 13, 14 may be provided. An additional focusing coil 15 may be provided about the envelope portion 3 between the electrode 10 and the fluorescent screen 7. The inner surface of this portion of the envelope is also preferably provided with a conductive coating 16 for the same purposes as the coating 6.

Since it is desired that the gun provide two beams one of which is concentrated in an elemental area and the other of which covers a larger area, a typical electron gun construction for this purpose is illustrated in Fig. 2. For the elemental beam gun there is provided an indirectly heated cathode 17 in front of which is mounted an accelerating electrode 18 and a focussing electrode 19. Elements 18 and 19 are provided with narrow apertures 20 and 21 through which the electron beam from cathode 17 'is directed. Mounted within the shell of element 19 is provided a second indirectly heated cathode 22 which may consist of a coated annulus having an opening 23 aligned with apertures 20 and 21 so that the narrow beam from the cathode 17 may pass through the center. A heating filament 24 may be provided for heating the cathode 23 and an accelerating electrode 25 is provided to direct the emission from cathode 22 outwardly from the gun. Output leads for electrodes 18, 19 and 25 are shown at 26, 27 and 28 respectively, Electrons from cathode '17 will be focussed into a very narrow beam which may be restricted to cover an elemental picture area. The electrons from cathode 22 however, will be projected so as to cover a relatively large area in the space between screens 9'and 10.

p In Fig. 3 is shown a typical circuit arrangement for operation of a tube such as shown in Figs. 1 and 2. Input video frequency signals may be applied over condenser 29 to'collector electrode 9. A negative biasing battery 30 may be connected to the gun cathode. Electrode 19 may be at ground potential as shown. The cathode and the electron gun portions 17 and 13 are negative with respect to ground and Will produce a relatively low velocity elemental electron beam. The second cathode 23 will, be also at ground potential. Electrode 25 is maintained at a potential positive with respect to cathode 19,

by means of the battery 31. The internal wall coating 6 of the tube is maintained at a positive potential less than that of'electrode 25 for the purpose of picking up stray electrons. Electrode 9 is maintained at a potential somewhat positive with respect to cathode 23 but of lower positive potential than elements 25 or 6. Storage electrode 10 which consists of the metal grid portion 32 and insulated coating 33 has the metallicelement maintained at a potential slightly positive with respect to the potential of element 9 by means of the adjustable source shownv as battery 34. This source may be made adjustable to provide any desired potential difference between these two electrodes for the purpose of controllingthe. field r bc wnt A Pos t v ta e s app ed 2 i he. l o ti Al rn t v i an; dditiona Q F IQSlQ: positively polarised may be, usedto aceelerat any-c1ec-. rom a s n h ou h e. r ron o1L en. The variousfocus sing and deflecting coils are diagrarnmatiq ly lu t at 3 14 nd. 55-.

In order more fully to explain the operation of this, tube. reference is-rnade to Fig; 4 along with Fig. 3. In, Fig. 4 is illustrated an enlarged showing of; a part of the storage control electrode 10, having the conducting and insulating portions 31 and 33; respectiyely, and a portion of the collector screen eleetrode 9 in Fig. 3, The control or scanning beam is indicatedby. the-heayy lines- 35 and; theelectrons from the flood gun are indicated by the light; slanting lines 36. We may first consider the. action of the control or scanning. beam 35, as though the flood beam'36 wereturned off. "As the beam 35. is scanned over symbolically indicated by dotted resistor 37; The time,

constantof the composite screen may be defined as RLCF. where R1 is the leakage resistance through 37 and C1; is the capacitance between the two surfaces of 33. For. good dielectrics the leakage through the coating is small with respect to the surface leakage and can be neglected. The proper value of this time constant varies for different applications, and might be for example about .1 sec. for television.

It may-be assumed first that the control beam 35 strikes the screen 33 with a velocity suflicient to eject more than, one secondary per primary electron. When. this control beam is. bombarding an insulating element, current flows into or out of the element until the voltage of theelement is changed by just a required amount to make the current into the element, including leakage current, equal to. the current out. Thus a balance of current is, obtained for somedefinite value of potential difference between the insulator and the collector. The collector electrode 9 will always be maintained somewhat more positive than the insulator element 33. Some of the control beam electrons will pass, through the openings in the mesh during scanning but this effect is negligible and. has no appreciable effect on the image produced on the fluorescentv screen.

An idea of the approximate value of potential difference between the collector 9 and the insulating element 33 for a particular value of beam current 35: may be. had by applying Langmuirs three-halves-power law. Since the secondary electrons which leave theinsulators diverge. as they leave the element the geometry. of the electrostatic fieldmay be assumed to lie somewhere between that of. concentric spheres" and that of parallel plane geometry; Simple calculations based on both types of geometry indicate:

(1) That the equilibrium potential difference E between. an insulating element and. the collector screen, is given by the relationship where i is the current into and out of the insulating element. That is, it is the component of control beam current which strikes the insulating element.

(2) The constant A in. Equation 1 is determined by the geometry of the structure.

(3). For a current i of microamps the equilibrium potential E appearsto lie. between 5 and volts.

It may be assumed now that the beam current is such that a constant potential of exactly 5 volts exists. between the equilibrium potential of the insulator; and. the collector. If the potentialv of the colleetor isvaried by the video voltages as the control beam, is scanned over the insulating film in a scanning raster pattern, a charge; pattern: will be built up which is an electrostatic image of the picture defined by the input video signals. By. making the electrostatic capacity of the insulating film 33 to the metal support 32 of approximately the correct value, about l0 farads, the charges will approach closely to their equilibrium value of 5 volts. Under these conditions the insulator element which is being bombarded at any given instant by the control beam will assume a potential 5 volts negative with respect to the collector voltage at that instant.

The flood gun, energized from the cathode 22, establishes a space current charge immediately adjacent the perforations of the insulated storage grid 10, thus electrons from this space charge produced by beam 36 will flow through the apertures of electrode 10 in an amount dependent 'upon the picture charge voltages establ'ished' over this surface. If the electron or current flow through the apertures of the insulated grid 10 is plotted against the potential of the insulator element 33 a characteristic curve 3 8 as, shown in Fig. 5 will be obtained. It should be noted, curve 39, that the flood gun current will be collected by the insulator elements when the element is negative, by less than about 0.5 volt. This collection of electrons can be avoided by confining the swing of the gridelements 10 to voltages more negative than -.5 volt. It appears desirable to make the operating part of the characteristic 38 illustrated in Fig. 5 as long as possible. The D. C, voltage on the base 32 of electrode 10 should be as high as possible without drawing electrons from the insulator to it. This potential may be, for example, 540 volts positive with respect to the voltage of collector electrode 9.

The electron beam 35 which is used to control the charge distribution of theinsulating surface 33 must be focussed into a spot of element size, for example, .002 inch (.005 cm.) in diameter. The beam current is relatively small for example 10 microamps, so this required accuracy of focus is readily obtainable. The focussing of this beam is conveniently obtained by using a coil surrounding the neck of the tube.

The function of the flood gun is to establish a virtual cathode source of electrons between the collector screen and the perforated insulator on electrode 10. The area covered by the virtual cathode should extend over a substantial part of the area of the insulator. The flood beam may be scanned over the insulator simultaneously with the control beam. This serves to control the uniformity of the resulting final image. It is unnecessary however that the flood gun scan the insulator in the same sequence as the control beam nor even that it be deflected at all. In some applications, for example, it may be advantageous to scan the flood beam in one direction only.

It may be useful in some instance to have the extent of area covered by the flood beam controllable. Referring to Fig. 2, the actual area of the screen coverage may be changed by changing the size of cathode 22, the geometry of the gun structure or the voltages employed. In a given tube the size of the flood beam may be changed either by using a different focussing coil system or by changing the order of the magnetic focus used. A wide variation of gun structures may be devised, the structure shown in Fig. 2 being only considered as an example.

While I have described above one method of establishing the electrostatic charge image on the dielectric film of electrode 10 there are other alternative methods which may be used. In one other such method the control beam source cathode 17 may be operated only slightly more negative (l5 to -20 volts) than the flood cathode 2 2. The electron beam 35 then strikes the insulator with a velocity which is insuflicient to eject more than one secondary per primary electron. The control beam, therefore, always imparts a negative charge on the insulating element. This control beam may be modulated with the video signals preferably by varying the potential on the cathode 17. A negative charge image pattern is' thus placed on the insulator. This charge will leak off, at least in part between scansions, to the backing plate 35. The time required for such leak-off is directly proportional to R1, C1, and inversely to the potential difference between the insulator 33 and its support 32.. In this type of operation the insulator backing plate 32 is operated normally slightly positive with respect to the flood cathode. The value of this positive voltage determines the average brightness of the image.

A third method of operation may be used which is similar to the second method described above except that the control beam voltage of cathode 17 is operated sufficiently more negative than the flood cathode to permit a secondary emission ratio at the insulator greater than unity. The beam then charges the insulator element positively instead of imparting a negative charge and the polarity of the electrostatic image is reversed. In this case it is necessary to maintain the backing plate 32. at a slightly negative potential with respect to the flood cathode 23. The charge pattern, however, leaks otf to the backing plate between scansions in a manner similar to that of the second method described. When the insulator backing plate is maintained at a negative potential with respect to the flood cathode, it becomes necessary to maintain a much larger positive potential on the wall coating 16 to pull electrons through the grid apertures.

The conversion of the charge pattern into an electron image by drawing electrons from the virtual cathode through the apertures is the same for both of these methods of applying the electrostatic charge as was described in the first instance.

While the tube structure is described above as contemplating the use of electromagnetic focussing of the electron images, it is clear that other types of focussing may be used if desired. In Fig. 6, is indicated a tube structure similar to that of Fig. 1 wherein electrostatic focussing of the amplified electron image is used in place of the electromagnetic focussing. and storage electrode 18' are preferably made concave toward the fluorescent screen 7. An extension 44) is made to the backing part of electrode 10. Wall coating 41 is at a higher positive potential than 4%), and a still higher positive potential is applied to an electrode 42 provided with a central aperture 43. As is well known in the art this electrode arrangement, with properly adjusted Voltages, will focus an inverted electron image from screen It? onto the luminescent screen 7.

A simplified version of the electrostatic focussing arrangement is shown in Fig. 7. in this figure the electrostatic focussing constitutes simply an arrangement of the fluorescent screen 7 with relatively close spacing to the storage electrode so that the amplified electron image does not have sufiicient time to spread out after passing through the apertures. Thus the image will be properly impressed on fluorescent screen 7. If desired a positive potential may be applied to ring 3 as shown at 44.

In all of the system so far described a common dual gun together with arrangements for simultaneously applying energy to the control gun and the. flood gun have been shown. It is clear however that applicants system may be used with arrangements wherein the flood gun and the screen gun are alternately energised. A schematic circuit arrangement for a tube utilizing this type of operation is illustrated in Fig. 8. In this figure an envelope 4 5 is provided with two branches at one end as indicated at 46 and 47. In 46 is arranged a flood gun which may comprise an indirectly heated cathode 48, an anode 49 and a control grid electrode 50. The flood gun is so arranged as to project the beam over all or a substantial portion of electrodes 9 and 10. In branch 46 of the envelope is provided the normal scanning gun structure having in addition a switching control electrode 51. Electrostatic plates 52, 53 are provided to furnish the In this case collector electrode 9 8 normal line and frame scanning raster for the image signals. The received signals are applied to a receiver 54 and from this receiver the video signals may be applied through video amplifier 55 and resistor 56, to the collector electrode 9. The synchronizing signals are applied through synchronizing signal separator 57 to the line and frame synchronizing generators shown at 58 and 59. An output signal from synchronizing generator 59 may be applied to the switching source 60 so as alternately to render the scanning gun and flood gun operative. The scanning gun will have its grid 51 rendered positive for a period sufficient to scan the entire raster and produce the electrostatic charge on electrode 10. Thereafter grid 51 will be biassed to cut off and grid 50 will be made positive so that the flood gun may project electrons through electrode 10 onto image screen 7. 7

Turning now to Fig. 9, there is illustrated schematically a part of the tube structure and circuit showing an alternative operating system similar in some respects to that shown in Fig. 8. In this arrangement the scanning gun 60 and the flood gun 23 are shown in axial alignment as was shown .in connection with Fig. 2. The collector screen 9 and the image storage screen 10 are arranged in the same manner as previously described. However, the metallic backing portion 32 of screen 10 is maintained negative by battery source 61 and similarly screen 9 may be maintained negative by battery source 62. The input video signals shown at 63 together with the synchronizing pulses 64 may be applied over condenser 65 to the metallic backing of electrode 10 and over a condenser 66 to electrode 9. Switches 67 and 68 are provided so that the signal may be applied over one or both of electrodes 9 and 10. Batteries 61 and 62 are of such a value that the electrodes 9 and 10 are normally maintained sufliciently negative to prevent electrons from flood gun 23 from reaching these electrodes. At the same time, a control grid 69 in front of cathode 60 is maintained at a positive potential so as to permit the electrons from gun 60 to impinge upon the insulator coating 33 at electrode 10 and to knock out secondary electrons therefrom so that they tend to go in a positive direction in accordance with the variation in picture signals 63 as applied to the screen. The output signals from video amplifier 7!} may be applied through the synchronizing signal separator and an inverter 72 to grid 69. The separated inverted signals 73 are of suflicient negative amplitude as to bias grid 69 to cut-off. At the same time, the synchronizing pulses 64 applied in positive polarity to electrodes 10 and 9 are of sufficient positive value to overcome the effect of the negative biasing batteries thus permitting the electrons from the flood gun to flow through the apertures of the screens to the reproducing device. If it is desired, the synchronizing pulses may be applied to the cathode of flood gun 60 instead of to a control grid. In this case positive pulses will have to be applied to effect the desired cut-off. The biasing potential on electrode 10 is such that even though secondary emission occurs the insulating covering 33 is still operated in the negative region so that no grid current is drawn. With this arrangement modulation may be applied directly to the storage screen instead of to other electrodes as previously described. It will also be noted that because of the potentials used, the time constant of the storage screen is not critical so long as it is not short and a leakage discharge of the storage coating occurs.

In Fig. 10 is shown a schematic diagram of an alternative construction and circuit arrangement in accordance with this invention which has been found to provide good performance. In this arrangement scanning beam cathode 74 is provided with a narrow aperture 75 and an additional cathode 76 is mounted within the cathode 74 adjacent this aperture. Cathodes 74 and 76 may be heated by the same heating filament.

the flood gun electrons is shown M77. The accelerating electrode 78 is provided for cathodes 74 and 76 and A third cathode furnishing 9 another accelerating electrode 79 is furnished forall of these guns. The collector screen 9 is preferably main-. tained' at substantially the same potential as electrode 79. Cathode 76 is maintained snfiiciently negative with respect to cathode 77 so that it will always eject more than one secondary electron from the covering 33 of electrode 10 for each electron impinging thereon. Cathode 74 is maintained only slightly more negative than the flood gun cathode 77 so that it will contribute a negative charge tothe insulator covering 33. The flood gun cathode may be operated at ground potential.

To explain the principle of operation of this system it may be considered first that the insulator cover 33 is.

initially charged to a negative potential of a few volts with. respect to ground. Under these conditions only the electrons from cathode 76 can strike the insulator. Since the electrons from the cathode 76 strike with sufficient velocity to eject more than one secondary per primary electron and since these secondary electrons will be drawn away from the insulator to the collector screen grid 9, the insulator will begin to charge toward a positive, direction and will continue to do so until it has reached a point substantially equal to the instantaneous potential of the cathode 74. When the potential of the insulator reaches that of cathode 74 electrons from this cathode will impinge upon the insulator and will prevent it from becoming more positive than the instantaneous voltage of this cathode. The potential of cathode '74 however is not constant but varies with the input video signals applied over condenser 30 and resistor 81. Thus as the beam from cathodes 74 and 76 is secured over the screen the charge pattern on the insulator will be changed each scansion to correspond to the instantaneous variations in the video scanning. It will be evident that the beam current from cathode 74, should be greater than the actual current which flows out of the insulator as a result of the impact from the cathode 76, including the leakage current. This condition will generally be met by making the beam from cathode 74 at least as great as. that from cathode 76. It will thus be seen that an image charge will be placed on coating 33 which will control the flow of electrons from cathode 77 onto the utilisation circuit 7 in accordance with the image signals. Here again the time constant of the electrode 10 is not critical as long as it is not so short that excessive leakage will occur.

In Fig. 11 an actual constructive arrangement of the electron gun assembly of Fig. 10 is shown. Cathode 74 is made in the form of a sleeve having its end provided with aperture 75 and coated with a properly. emissive substance. Cathode 76 is mounted inside of the sleeve of cathode 74 adjacent aperture 75 and a common heating filament 82 is provided to heat this assembly. Preferably a negative voltage of from 30-50 voltsis applied to cathode 76 from the battery 83 and a battery having a negative potential for example, of 1.5 volts, isconnected to cathode 74. The anode of insulating electrode 78 may be in the form of a sleeve shielding; the cathode assembly 74 and 76 having an aperture 84 aligned with aperture 75. The flood cathode 77 may be mounted within a shell 85 maintained substantially at ground potential while anode 79 may be maintained at a positive potential of 100300 volts. In order to prevent interaction between the electron beams from.

all of the methods of operation described the flood electrons never actually strike the insulating coating on the storage control grid as under every condition of operation this grid is maintained negative.

This type of op:

eration is; unique in the present tube so. far, as;applieant. is aware.

It is clear that while certain embodiments of the in vention have been described with a degree of particularity many modifications and variations thereof will occur to those skilled in the art.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

What is claimed is:

1. In a cathode ray tube system a perforate electrostatic storage electrode including a conductor covered on: one surface only, with a dielectric material, means forproviding an electron cloud serving as a virtual cathode adjacent said dielectric surface of said electrode, means, for establishing a predetermined potential on said conductor, means for establishing a mean negative potential bias on said surface of a magnitude such that the surface is always to some degree negative with respect to said virtual cathode during operation, means for applying an electrostatic potential pattern on said dielectric which. is

negative with respect to said virtual cathode even at its most positive excursion, means for applying a potential field to attract electrons from said virtual cathode through the perforations in said electrode to produce an electron image, corresponding with said pattern, and a utilization device for said electron image.

2. A cathode ray tube system comprising a cathode ray tube having a first and a second cathode ray beam source, a perforate screen mounted in said tube transversely of said beam, said screen having an insulating covering on its surface facing said beam sources, means for focusing said first beam to cover an elemental area of said screen, means for establishing a mean negative potential bias on said surface of a magnitude such that said surface is always to some degree negative with respect to said second cathode ray beam source, means for scanning said first beam over said screen to produce an electrostatic charge on said insulating surface, said charges being negative with respect to said second cathode ray beam source at their most positive excursion, means for modifying the action of said first beam and screen in accordance with a signal in synchronism with the scanning to cause said charge to be an electrostatic image charge of said signal on said screen, means for producing a virtual cathode adjacent the insulator surface of said screen in response to electrons from said second source, and means for attracting electrons from said. virtual cathode through the perforations of said screen to produce an electron image corresponding to said electrostatic charge. image.

3. A cathode ray tube according to claim 2, wherein said first beam source comprises a first emitter means for maintaining said emitter slightly negative with respect to said second source, said first emitter having an aperture in one end, a second emitter positioned to ernit electrons through said. aperture, and means for maintaining said second emitter negative with respect to said first emitter, whereby it will cause secondary emission upon impinging onto said insulated covering, and said means for modifying the action of said beams comprises means for applying image signal voltage variations to said first emitter.

4. A cathode ray tube system comprising an envelope, a perforated conductive screen positioned within said envelope, an insulating coating on one surface of said screen, a first electron beam source directed toward said surface, focusing means for focusing said first electron beam onto an elemental area of said surface, scanning means for scanning said first electron beam over said surface, means for establishing an electrostatic image signal charge on said insulated coating in response to applied signals, means for establishing a virtual electron source adjacent said surface including a second electron source, directed toward said insulated surface for providing a second electron beam emission covering a substantial area of said surface, means for establishing a mean negative potential bias on said surface of a magnitude such that said surface is always to some degree negative with respect to said virtual electron source during operation, means for attracting electrons from said virtual source through the perforations of said screen whereby an electron image is established through the action of said charge, and a utilization device for said electron image.

5. A cathode ray tube system comprising an envelope, a perforated conductive screen positioned within said envelope, an insulating coating on one surface of said screen, a first electron beam source directed toward said surface, focusing means for focusing said first electron beam onto an elemental area of said surface, scanning means for scanning said first electron beam over said surface, means for establishing an electrostatic image signal charge on said insulated coating in response to applied signals, means for establishing a virtual electron source adjacent said surface including a second electron source directed toward said insulated surface for providing a second electron beam emission covering a substantial area of said surface, said virtual electron source being positive with respect to said electrostatic charge, means for attracting electrons from said virtual source through the perforating of said screen whereby an electron image is established through the action of said charge, and a utilization device for said electron image, said scanning means being positioned to also scan said second beam over said surface.

6. A system according to claim 4, further comprising means for alternately rendering said first and second sources effective.

7. A cathode ray tube according to claim 6, wherein said means for establishing the image signal charge comprises means for normally maintaining said conductive screen negative with respect to said second source and positive with respect to said first source, and means for applying image signals to said screen, and said means for alternately rendering said first and second sources ef fective comprises means responsive to received synchronizing signals for rendering said conductive screen positive with respect to said second source, and means responsive to said synchronizing signals for blocking emission from said first source.

8. A system according to claim 4, wherein said utilization device comprises a luminescent screen and means for accelerating said electrons passed through said perforations toward said screen.

9. A system according to claim 4, wherein said means for establishing the image signal charge comprises means for modulating said first electron beam.

10. A system according to claim 4, further comprising a collector screen mounted between said storage screen and said guns. I

11. A system according to claim 10, wherein said means for establishing the image signal charge comprises means for varying the voltage of said collector screen in response to image signals. 7

12. A cathode ray tube system comprising an envelope, a perforated conductive screen positioned within said envelope and provided with a reference voltage terminal extending outside said envelope, an insulating covering on one surface of said screen, a collector conductive screen mounted within said envelope facing said insulating covering and provided with a terminal extending out side said envelope, a first electron beam source directed toward said surface, focusing means for focusing said electron beam onto an elemental area of said surface, scanning means for scanning said electron beam over said surface, whereby an electrostatic image charge is established over said surface in response to the scanning of said electron beam, a second electron source directed toward said insulated surface to cover a substantial area of said surface, whereby an electron space charge between said surface and said collector screen is produced, means for establishing a mean negative potential bias ,on said surface of a magnitude such that said surface is always to some degree negative with respect to said second electron source during operation; means for establishinga variation in said electrostatic image charge in response to applied signals within the limits that said charge is always to some degree negative with respect to. said second electron source, whereby an amplified electron image is produced in the space beyond said perforate conductive screen by electrons passed through the perforations, and a utilization device for said amplified electron image.

13. A cathode ray tube system comprising an envelope, a perforated screen having a predetermined number of perforations per unit positioned with said envelope and provided with a reference voltage terminal extending outside said envelope, an insulating covering on one surface of said screen, a collector conductive screen having a different number of perforations per unit area than said first screen mounted Within said envelope facing the covering of said first screen and provided with a terminal extending outside said envelope, a first electron beam directed through said control screen toward said surface, focusing means for focusing said electron beam onto an 7 elemental area of said surface, scanning means for scanning said electron beam over said surface wherebyan electrostatic image charge is established over said surface in response to the scanning of said electron beam, a second electron source directed through said control screen toward said insulated surface for providing an electron emission covering a substantial area of said surface, to produce an electron space current charge between said surface and said control screen, means for establishing a mean negative potential bias on said surface of a magnitude such that said surface is always to some degree negative with respect to said second electron source during operation, means for producing a variation in said electrostatic image charge in response to applied signals within the limits that said charge is always to some degree negative with respect to said second electron source, whereby an amplified electron image is established in the space beyond said perforate conductive screen by electrons passed through the perforations, and a utilization device forsaid amplified electron image.

14. In a cathode ray tube system having a perforate conductive electrode covered on one side only, with a dielectric material, a method of operation comprising providing a virtual cathode source adjacent said dielectric surface, establishing a negative mean operating potential on said surface which is of a magnitude sutficient to maintain said surface negative to some degree throughout the'entire range of operation of said system, applying a potential pattern which is at all times negative to some degree with respect to said virtual cathode source to said dielectric, and providing a potential field to attract electrons from saidvirtual cathode source through perforations in saidelectrode, to produce an electron image.

15. A method of operation according to claim 14, wherein an electron beam of elemental area is provided, the method of applying said potential pattern which comprises scanning said beam over said surface and varying the current of said beam in accordance with said potential pattern.

'16. A method of operation according to claim l4,

wherein an electron beam of elemental area is provided directed toward said surface and a collector screen is positioned between said source and said surface, the method of applying said potential pattern which comprises scanning said beam over said surface and simultaneously varying the potential on said screenin accordance with said potential pattern.

17. A method according to claim 16 further comprising maintaining said electrode positive with respect to the electron beam source.

18. A method according to claim 17, further comprising giving to said electron beam a velocity insufficient to cause secondary emission from said surface.

19. A method according to claim 17, further comprising giving to said electron beam a velocity sufficient to cause emission of electrons from said surface.

20. A method of operation according to claim 14, wherein an electron beam of elemental area is provided directed toward said surface, and an electron beam of relatively large cross-section and of lower velocity is directed toward said surface to provide said virtual source, the step including alternately rendering beams from said sources effective.

21. A method according to claim 20, wherein the method of applying said potential pattern to said screen, comprises scanning said elemental beam over said sur face, and simultaneously varying the potential on said electrode in accordance with said potential pattern.

References Cited in the file of this patent UNITED STATES PATENTS 2,122,095 Gabor June 28, 1938 2,259,507 Iams Oct. 21, 1941 2,280,191 Hergenrother Apr. 21, 1942 2,532,339 Schlesinger Dec. 5, 1950 2,547,638 Gardner Apr. 3, 1951

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