US2788467A - Direct-viewing storage tube - Google Patents

Description

April 9, 1957 H. M. SMITH DIRECT-VIEWfNG STORAGE TUBE 2 Sheets-Sheet 1.

Filed Feb. 9, .1954

. INVENTOR. A i/Viy M Jul/7%,

April 9, 1957. H. M. SMITYHI DIRECT-VIEWING STORAGE TUBE 2 Sheets-Sheet 2 Filed Feb. 9, 1954 5444 fa an:

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' nmncr-vlnwmosronaonroen H enry h lillard smithg-Pacific Palisades, Califi, assignor to Hughes Aircraft Company, Culver City, aiif., a cor poratiomohnclarva're; i A

1Mgllli'caiioll'v Febr ary .5, swarm. 409,245 13: Claims (Cli-3 15--1 2) This invention relates to direct-viewing. storage tubes, and more particularly. to an. apparatus. constituting; an improved. storage grid assembly for-such tubes and a m'etlrod for In aliirigthemn The storage grid assembly hereinafter described is of the same general type as the target element disclosed in a cop'en'ding Uhited States patent application by Siegfried Hansen, SerialNo. 7 299,363,..filed July 17,. 1952,..and entitled, Direct YiewingStorageTube. The target element disclosed inrthe Hansen application. comprises. a contrast control grid, a layer of dielectric material disposed uniformly o'vcnt'he contract control grid and a collector grid) disposed in actual contact: with thelayer of dielectric material; Thestorage surface of the target element is provided by the exposed surface of the dielectric material within thedntersticesofi the collector g-rid'.

In. theoperation of this target element, the collector grid ismaintained positive with respect to a reference potential level so that it may collect secondary electrons firo'mthe storage surface. Fl'ood electrons, emanating from a source maintai ned at thereference potential level, aredirected uniformly over the entire area of the target element to: initially charge its storage surface to'the reference potential, thus producing a positive potential gradient fronrthe storage surface to the collector grid; The contrast control grid is maintained at a sufiiciently -negativepotential-to allow the Hood electrons to penetrate only through the" interstices within positively charged areas of the flanges element'to a viewing screen disposed contiguous thereto and maintained at a high positive potential.

Positively charged areas are produced onthe storage surface-by directing amodi1l ate'd electron writing beam over the target element in a mannerconsistent with, the chargepattern. thatiti's desired to produce. Each high energy-electron incident on the storage surface releases numerous electrons which are attracted to the collector grid by tlie potential gradientto charge the storage surface in a positive direction; Subsequent action of the flood electrons then charges those portions of the storage surface, which the writing-beam made more positive than the critical potentialof the dielectric material, to the potential of the collector grid and maintains these portions atthis' potential fora period of time dependent on the applied potentials.

A positive charge on the storage surface aids field penetration throughthestoragegrid to the viewing screen. This enables flood electrons incident on the positively charged portions of the storage surface to penetrate through the positively charged interstices to the viewing screen reproduce a visual image of the positively charged portions of the storage surface.

'In its actual operation, the target element in the aforementioned Hansen application was found to have disadvantages inthat extensive areas of dielectric, not exposed to the flood electrons, accumulated a positive chargedue to positive ion bombardment. This positive charging allowed some flood electrons to penetrate through the target elementeven when the storage surface was at the 'ice reference potential, thereby substantially decreasing the contrast. in the visual image. by; raising the. overall: illu-' Further; the. different pitchaof the collectorandcontrast control grids produced a varying density in these flood electrons penetrating mination of the viewing screen.

through the interstices: of the: target: element which in tern produced a series of: light and dark regions on the viewing screen similar to a moire pattern... The above disadvantages are rather objectionable. in most. of thep'ractical applicationsof the; tube.

The storage grid assembly of the present inventionrepre ents" an. improvement over the aforementioned target element disclosed in the, Hansen. application in. that it improves contrastl and: produces a; moire pattern: of sub- In the; first instance, the contrast is improved by evaporating a conductive coating on the surface areas of. the dielectric material adjacent: to the contrast control grid. These-areas were not normally illuminated by the Writing beam and the flood: electrons, hence there was no. available. way to control. the charge thereon. By having. aconductive coating on,

stantially decreased intensity".

these unillurninatedl areas, their potential is determined by the voltage impressed onthe contrast control grid; in. the. secondinstance, the intensity of the moire pattern,

caused by themeshesiof the contrast control and collector grids: not being in register, is substantially decreased by orienting the meshes of the collector grid with those of' the contrast control grid at an: angle which produces a. minimum of such pattern on the viewing screen.

In addition. to the above, the invention discloses the method for makingithe. storage grids, the method avoid-- ing the. difficulties encountered with the prior methods. More particularly, these. difiicultiesresided in making the: collector screen and disposing it on the storage surface material in such a manner that it does not become damaged and. is retained in close; permanent mechanical and. electrical contactover the entire area of the storage grid? assembly.

It is therefore an object of this invention to provide an improved. storage grid assembly for a direct-viewing: storage tube.

Another objectof this invention isto provide astoragegrid assembly for a. direct-viewing. storage tube incorpo rating apparatus for. producing an image of improved contrast over that heretofore available with storage grid assemblies. of this type.

Still another object of this invention is toprovide a storage grid assembly that produces-a less intense moire pattern as compared tOiti'lOSC. heretofore produced by prior art. assemblies of this. type;

A further object. of this invention is to provide. a method of fabricating the aforementioned improved storage" grid assembly. 7

A still further object of this. invention is to provide a method of making a thin metallic screen. and disposing it inintimate contact with the dielectric. storage surface material of the storage grid assembly.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advan tages thereof, will be better understood from the following description considered in connection with: the accompanying drawings in which an embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the. drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of. the invention.

Fig. 1 shows across sectional schematic representation of the storage grid assembly ofthe present invention;

Figs. 2 and' 3 are enlarged cross section. and planv views, respectively, of a portion of the. grid of Fig. 1;

Fig. 4 is'a sectional schematic view of the storage grid 2,788,467 Patented? Apr... 9",. 13957 assembly as employed in a direct-viewing storage tube;

Fig. 5 shows apparatus for spraying a dielectric coating on a grid;

Fig. 6 is a partially completed sectional view of the storage grid assembly;

Fig. 7 shows apparatus for applying a plastic coating to a glass pane;

Figs. 8 and 9 are cross section and plan views, respectively, of a mask;

Figs. 10 and 11 show metal evaporating apparatus; and

Fig. 12 shows apparatus for placing thin conductive screen in position.

Before proceeding with the description of the storage grid assembly, a brief description of the storage tube will be given first for a better understanding of this invention Referring to Fig. 4, there is shown a schematic diagram of a direct-viewing storage tube of the type disclosed in the aforementioned Hansen application. This directviewing storage tube comprises an evacuated envelope 10 having a neck portion, disposed as shown in the drawing, for housing an electron gun 12 which produces an electron writing beam, deflecting means 14 for the writing beam, and a flood gun 16 for producing a flood beam of electrons. Deflecting means 14 is disposed axially about the path of the writing beam and is energized by appropriate beam deflection control signals to cause the elec tron beam to trace a desired pattern on a storage grid assembly 17 disposed adjacent to and in front of a viewing screen 18 at the opposite extremity of envelope 10. Conductive coatings 20 and 22 are disposed concentrically about the inner surface of envelope 10 in the intervening space between flood gun 16 and storage grid assembly 17.

In the preferred mode of operation of the direct-viewing storage tube, flood gun 16 has a cathode 24 which is maintained at a reference potential such as, for example, ground. The electron gun 12 has a cathode 26 which is maintained at a potential of the order of 3000 volts with respect to ground, and an intensity grid 28 which is maintained at from 50 to -100 volts negative with respect to cathode 26 by means of a connection thereto through a resistor 30 and an adjustable source of potential 32. The electron beam produced by electron gun 12 is intensity-modulated by varying the potential impressed on intensity grid 28, this being effected by impressing an input signal on grid 28 through a capacitor 34 which is connected thereto. Conductive coatings 20 and 22 are maintained at +200 volts and at from +50 to +100 volts, respectively. The viewing screen 18 comprises a transparent conductive coating and a phosphor coating disposed in the order named on the inner surface of envelope 10. A voltage of the order of from +5000 to +10,000 volts with respect to ground is impressed on the transparent conductive coating of viewing screen 18. The storage grid assembly 17 of the present invention is disposed adjacent to and in a plane parallel with the viewing screen 18.

Proceeding now with a more detailed description of the storage grid assembly 17, it is shown in Figs. 1, 2 and 3. Referring to these figures, the grid assembly 17 comprises a metal ring 40 welded to one side of the periphery of an electroformed nickel screen 42, the screen 42 serving as a contrast control grid. Screen 42 is of the order of 0.0005 to 0.001 inch thick and has 250 meshes per inch with a concomitant light transparency of 40 percent. A coating 44 of the order of 0.002 inch thick of dielectric material possessing secondary electron emission characteristics is disposed uniformly over the same side of the nickel screen 42 with due precautions being taken to keep the interstices of the screen from becoming clogged. This is accomplished by forcing air through the interstices of screen 42 while spraying it with a suspension of the dielectric material.

An apparatus for depositing the dielectric coating 44 on screen 42 is shown in Fig. 5. It comprises a chamber 60 having a tubular member 62 of a diameter substantially equal to that of the storage grid assembly 17. Member 62 extends inwards, from one wall of chamber 60 to about 2 inches fromthe opposite wall. Openings 64 and 66 are provided in the wall of chamber 60 directly opposite the ends of tubular member 62. Air pressure within the chamber 60 is provided by means of a blower 68 which is disposed concentrically within a cylindrical extension 70 attached to chamber 60. Turbulence in the chamber is decreased by a bafile 72 disposed over and spaced from the entrance of extension 70 into chamber 60.

A spray gun 74 fits into a small aperture provided in a transparent plastic plate 76 slidably mounted over opening 66. The side of screen 42 which is to be coated with the dielectric material faces the spray gun, and a dielectric coating is applied to screen 42 by operating the gun. A continuous stream of air is forced through the interstices of nickel screen 42 for preventing clogging of the holes with the dielectric material.

Talc has been found to be a satisfactory dielectric material for providing the storage surface of grid as sembly 17. A quantity of talc is first ball-milled for twenty-four hours and then is employed in the preparation of a suspension having the following proportions:

The binder such as, for example, collodion, nitrocellulose, polystyrene, or potassium silicote, is added to the mixture to hold the tale together when applied to the nickel screen.

The above mixture is loaded in the spray gun 74, its pressure regulated to 20 pounds per square inch, and the nozzle adjusted to give a slightly fan-shaped spray. The nickel screen 42 is then sprayed with this mixture while the pressure in chamber 60 forces an air stream through its interstices. After spraying several passes of the talc mixture over screen 42, its position at the opening of tubular member 62 is reversed and a solvent such as, for example, acetone, is sprayed over its back side to clean the exposed nickel and to open any holes that may have become clogged.

The position of the nickel screen 42 at the opening of tubular member 62 is again reversed and an additional several passes of the dielectric mixture sprayed over the side of the screen to be coated with dielectric. This process is repeated until the thickness of the talc layer on screen 42 is of the order of 0.002 inch. The light transmission of screen 42, with a 0.002 inch layer of talc. applied in this manner, is of the order of from 20 to 23 percent as measured by a photometer. Subsequent to coating screen 42 with the tale, it is placed in an oven and subjected to a temperature of 500 C. for 15 minutes.

Referring now to Figs. 2 and 3, a thin layer of metal 46 is disposed over the screen 42 and those portions of the surface of coating 44 which overhang the interstices thereof so as to maintain the potential of these surfaces at the same potential as that of screen 42. Metal layer 46 is applied by evaporating a metal, such as aluminum, on the side of screen 42 opposite to the side coated with dielectric material. In order to have a sharply defined line of demarcation at the edges of layer 46 within each interstice, it is necessary for the metal to be evaporated in a high vacuum so that the metal molecules will be directed at normal incidence towards the screen 42. In addition to the above, it is also necessary to prevent the formation of a cloud of the metal molecules about their source in that this cloud constitutes a virtual source of metal molecules. This is accomplished by evaporating aluminum from a source six inches in length and spaced approximately ten inches from screen 42. Evaporation of the metal is effected by heating it to an appropriate temperature in a chamber having a residual pressure of the order of 5X10 millimeters of mercury.

.A collector electrode v48 includes an annular electrode .49 and a thin conductive screen .50. Annular electrode 49 is disposed about the periphery of screen 42 just inside of ring 40 and has its edge nearest screen 42 bent under and in to form a narrow ledge'51. Thin conductivescreen 50 has of the order of from 100 to 200 meshes perinchand is disposed over the surface of the coating 44 of .the dielectric material and overlaps ledge 51 on the edges thereby effecting electrical contact to annular electrode 49. The exposed portions of the surface of coating 44 within the interstices of the thin conductive screen .50 provide the storage surface on the storage grid assembly 17. Further, the meshes of the thin conductive screen 50 are disposed at an acute angle with the meshes of screen 42 to minimize the moire effect on the flood electrons. The. exactanglethat produces optimum minirnization of the moire effect must be found empirically and will vary with the number of meshes per inch in thin conductivescreenSti relative tothe meshes per inch in the screen .42. The optimum anglemay be found for any two meshes by mounting the screens contiguous to each otherand adjacent to. a frosted glass viewing screen. Light is .then directed through the screens on to the frosted glass viewing screen simultaneously with the rotating of one of the screens with respect to the other to find the angle at which there is a minimum of light and dark regions. For example, when there are 112 meshes per inch in the conductive screen 50 and 250 meshes per inch in the screen 42, the optimum angle for minimum moire effectis 37. On the other hand, when there are 176. meshes per inch in conductive screen 59 for the same screen 42, the optimum angle for minimum moire effectreducesto from 25 to 30.

In .the completed storage grid assembly 17, annular electrode 49 is. held in position by several supporting reds, two of which are illustrated byway of example as rods 53,, 5,4. Rods 53, 54 each comprise two separate metal rods joined together by glass beads 55, 56, respectively, thereby electrically insulating one extremity from the other. Rods 53, 54 arewelded to the sides of annular electrode49and to the metal ring 49, as shown in Fig. 1, to hold .them in a fixed relation with respect to each other.

Infabricating the storage grid-assembly 17 of the present invention, the electroformed nickel screen 42is welded to metalring 4.0, and the coating 44 of talc applied thereto as previouslydescribed. Rods 53, 54 are then welded to ring'40 and the annular electrode 49 clamped in place withitsledge 51 spaced.0.040 inch from the top of the coating 44 of talc as illustrated in Fig. 6. In this figure, a clamp 80 is shown, by way of example, in position for holding rod .53 .fastenedto annular electrode 49.

The next phase of .the process constitutes the fabrication of thin conductivescreeu 50 and disposing it on the surface of the coating 44 of talc. For ease of handling, screen 50 is formed on a plastic coated pane $2 of glass. Referring to .Fig. 7, pane 82 of glass is immersed into a container .84 containing a plastic solution of the following proportions:

Grams Polysterene 1 Ethylene dichloride 100 Subsequent to ,the immersion of the pane 32 into the above. solution, it is removed and allowed to dry. A thin plastic film is thus produced over the entire surface of pane 82.

In the subsequent steps of the process for making storage. grid assembly 17, the thin conductive screen is evaporated on the plastic coating on pane 32. This comprises evaporating a series of narrow metal parallel lines on the plastic coating of pane 82 through a mask 56 consisting of a succession of tightly strung steel wires. The pane fi2 is then turned at right angles to its initial position behind ,the mask 86 and an additional series of narrow metal parallel lines evaporated thereon.

Referring to Figs. 8 and :9, there is illustrated end and plan views, respectively, of a preferred embodiment of mask 86. Mask 86 comprises a heavy rectangular piece of metal 88 having a channel sufliciently large to accommodate pane 82 machined out of its center portion, as shown in the figures. Scmicircular grooves are provided along each side of the channel to accommodate metal rods 90, 92 which are threaded with a number of turns per inch as it is desired to have meshes per inch in the conductive screen 50. in the event that it is desired to have 112 meshes per inch, steel piano wire94, having a diameter of 0.008 inch, is then wrapped tightly around the piece of metal 88 in a manner such that succeessive turns of wire fall in successive grooves between the threads on rods 90, 92. The portion of the wire 94 strung over the channel in the piece of metal 88 provides a mask for the pane 82.

The next step in the process of making storage grid assembly 17 constitutes the evaporating of thin conductive .screen 50 on the plastic coating of pane 82 with the .aid of mask 86. In order to produce a clearly defined screen, it is necessary for the evaporated metal to penetrate through the mask to the plane 82 in a direction substantially normal thereto. This requires that the mean free path of the evaporated metal molecules be of sub stantial length and that the metal vapor molecules. emanate from a point or line source disposed in a planethat could be rotated about said point or line source so as to pass through any Wire of the mask. Anydeviation from a pointor line source, such as a virtual source formed about the point or line source by a cloud of the metal vapor, will tend to defocus the mask on thepane 82.

An apparatus for evaporating metal inthe afore' mentioned desired manner is shown in Fig. 10. Referring to the figure, apparatus 100 comprises an evacuated chamber 102 which includes means for reducing the pressure in the chamber to 5 10 millimeters of mercury.

A device 104 for melting -a six inch length of metal wire is incorporated in apparatus 100 in order that the evaporated metal molecules emanate from-a line source rather than a point source so as to avoid, as far as possible, the formation of a vapor cloud about the molten: metal which would constitute a large virtual source of the metal molecules, as previously-mentioned.

As viewed in the drawing, heating device 104 appears at the bottom of evacuated chamber 102 in Fig. 10.. Also, for the purpose of explanation, an enlarged plan view of device 104 is shown in Fig. 11. Referring to this figure, heatingdeviceltM comprises two lengths of tungsten wire 105, 106 Which-arc mounted contiguous to each other in metal blocks 107 and 108. An clectrica'l circuit is formed by the metal block 167, the two parallel lengths of tungsten wire 105, 106 and metal block 108. This circuit is connected across an adjustable po tential source 110 to enable the tungsten wires 105, 106 to be heated to a desired temperature.

Returning to Figs. 8 and 10, the plastic coated glass pane 82 is inserted behind the wires 94 of the mask 86- which is then mounted approximately ten inches above the heating device 104. Aluminum is preferably used for the metal to be evaporated although metals such as gold, silver, or copper are satisfactory. Evaporation of a series of parallel metal stripes on the plastic coating of pane- 82 is accomplished by placing a six inch length of 0.060 inch diameter aluminum wire in the grove formed by the tungsten wires 165, 106; evacuating chamber 102 to 5 10* millimeters of mercury; and regulating potential source 110 so as to completely evaporate all of the aluminum in from three to four minutes. The pane 82 is then removed from the mask 86 and reinserted at right angles to its former position and the process repeated.

Thin conductive screen 59 has now been evaporated on the plastic coating of pane 82. It now remains to transpose the conductive screen 50 from pane 82 on to the tale coating 44 of the storage grid assembly 17 as it exists in Fig. 6. The manner in which this is accomplished is illustrated in Fig. 12. The assembly 17, as shown in Fig. 6, is submerged in a vat 112 filled with water. An area corresponding to the internal dimensions of annular electrode 49 is scribed on plastic coating of pane 82. The pane 82 is then slowly immersed in vat 112 while being held in a nearly vertical position, which causes the scribed area of plastic coating to float free. This plastic coating is then made to settle in place on top of the tale coating 44 and overlapping the ledge 51 of annular electrode 49 of the assembly 17. The storage grid assembly 17 is then removed from the vat 112 and baked in an oven for minutes at a temperature of 400 C. in order to evaporate the plastic coating and leave thin conductive screen 50 on the tale coating 44 overlapping the ledge 51. In order to reduce the tensile stress in the screen 50 at points where it overlaps ledge 51, the annular electrode is lowered from 0.040 inch to 0.010 inch of the top of the talc coating 44. The rods represented by rod 53 are then welded to the annular electrode 49 and the clamps removed. The storage grid assembly 17 is now in the form shown in Fig. 1 and described in connection therewith.

Returning again to Fig. 4, in the preferred mode of operation of the direct-viewing storage tube, the collector grid 48 is maintained at a potential of the order of +175 volts with respect to ground and the nickel screen 42, which provides the contrast control grid, is maintained at a potential of the order of 10 volts with respect to ground.

The functioning of the direct-viewing storage tube in this preferred mode of operation is the same as previously described for the tube disclosed in the aforementioned Hansen application. As previously mentioned, however, the storage grid assembly 1'7 has minimum moire effect on the flood electrons penetrating therethrough. Further, the assembly 17 more completely controls the flow of flood electrons in that substantially all surface areas of dielectric material not exposed to the action of the flood and writing electrons are covered with the thin layer of metal 46, thereby maintaining these areas at the same potential as that of the contrast control grid 42.

What is claimed as new is:

1. In a direct-viewing storage tube, a storage grid assembly comprising a first conductive screen; a coating of dielectric material disposed uniformly over one side of said first screen and overhanging the interstices thereof; a thin layer of metal disposed on the portions of the surface of said coating of dielectric material within the interstices of said first screen visible from the other side thereof; and a second conductive screen disposed in contact with those portions of the surface of said coating of dielectric material which are farthest from first screen and in insulative relationship therewith, whereby the portions of the surface of said coating Within the interstices of said second screen provide storage surface, said second screen being substantially thinner than said first screen and the meshes thereof being at an acute angle of from 20 to 40 with the meshes of said first screen to produce a minimum moire elfect.

2. In a direct-viewing storage tube, a storage grid assembly as defined in claim 1 wherein said dielectric material is talc.

3. In a direct-viewing storage tube, a storage grid assembly comprising an electroformed nickel screen having a light transparency of the order of 40%; a coating of talc disposed uniformly over one side of said nickel screen and overhanging the interstices thereof, said coating of talc constituting a dielectric screen, whereby the light transparency of said dielectric screen is of the order of 23%; a thin layer of aluminum disposed on the portions of the surface of said dielectric screen within the interstices of said nickel screen visible from the other side.

thereof; and an aluminum screen of greater light transparency than 40% disposed in contact with those portions of the surface of said dielectric screen which are farthest from said nickel screen whereby the portions of the surface of said coating within the interstices of said aluminum screen provide storage surface, said aluminum screen being substantially thinner than said nickel screen and the meshes thereof being at an acute angle of from 20 to 40 with the meshes of said nickel screen to produce minimum moire effect.

4. In a direct-viewing storage tube, a storage grid assembly comprising a first conductive screen; a coating of dielectric material disposed uniformly over one side of said first screen coextensive with the meshes thereof; and a second conductive screen disposed in contact with those portions of the surface of said dielectric material farthest from said first screen on said one side thereof whereby the portions of the surface of said coating within the interstices of said second screen provide storage surface, said second mesh being substantially thinner than said first screen and the meshes thereof being oriented at an acute angle of from 20 to 40 with the meshes of said first screen to produce a minimum moire effect.

5. In a direct-viewing storage tube, a storage grid assembly comprising a first conductive screen of a predetermined light transparency; a coating of dielectric material having secondary electron emission characteristics disposed uniformly over one side of said first screen and overhanging the interstices thereof whereby said predetermined light transparency is reduced; and a second conductive screen having greater light transparency than said predetermined light transparency disposed in contact with those portions of the surface of said dielectric material farthest from said first screen on said one side thereof, whereby the portions of the surface of said coating within the interstices of said second screen provide storage surface, said second screen being substantially thinner than said first screen and the meshes thereof being at an acute angle with the meshes of said first screen to produce a minimum moire effect.

6. in a direct-viewing storage tube, the storage grid assembly as defined in claim 5 wherein said first conductive screen has 2.25 meshes per each mesh in said second conductive screen and said acute angle is equal to from 35 to 40.

7. In a direct-viewing storage tube, the storage grid assembly as defined in claim 5 wherein said first conductive screen has 1.42 meshes per each mesh in said second conductive screen and said acute angle is equal to from 25 to 30.

8. In a direct-viewing storage tube, a storage grid assembly comprising an electroformed metal screen for providing a contrast control grid; a metal ring attached to one side of said electroformed screen about the outer periphery thereof for supporting said electroformed screen; a coating of dielectric material disposed uniformly over said one side of said electroformed metal screen and overhanging the interstices thereof; a thin layer of metal disposed on the portions of the surface of said coating of dielectric material visible from the other side of said electroformed screen for maintaining said portions at the same potential as the potential of said electroformed screen: an annular electrode disposed about the periphery of said electroformed mesh inside of and in insulative relationship with said metal ring, said electrode having an inner ledge on the edge nearest said electroformed screen; an additional metal screen disposed in contact with those portions of the surface of said dielectric material farthest from said electroformed screen on said one side thereof and overlapping the ledge of said annular electrode, whereby the portions of the surface of said coating within the interstices of said additional metal screen provide storage surface and said annular electrode in conjunction with said additional screen provide a collector electrode, said additional screen being substantially thinner than said electroformed screen and the meshes thereof being at an angle of from 20 to 40 with the meshes of said electroformed screen.

9. A direct-viewing storage tube comprising a storage grid assembly including a first conductive screen for providing a contrast control grid, a coating of dielectric material disposed uniformly over one side of said first screen and overhanging the interstices thereof, said dielectric material having secondary electron emission characteristics, a thin layer of metal disposed on the portions of the surface of said coating of dielectric material within the interstices of said first screen visible from the other side thereof, and a second conductive screen disposed in contact with those portions of the surface of said coating of dielectric material farthest from said first screen on said one side thereof for providing a collector grid, the portions of the surface of said coating within the interstices of said second screen providing storage surface, said second screen being substantially thinner than said first screen and the meshes thereof being at an angle from 20 to 40 with the meshes of said first screen; means for maintaining a positive potential gradient from said storage surface to said collector grid; and means for bombarding selected portions of said storage surface with high energy electrons to liberate numerous electrons with in said selected portions, whence said liberated electrons are attracted to said collector grid by said potential gradient to establish positive charges on said selected portions of said storage surface.

10. A process for the production, with the aid of a mask having a series of uniformly spaced parallel slits, of a thin metallic screen, said process including the steps of coating a support surface with a plastic film; evaporating metal through said mask on to said plastic film in a first position to produce a first uniform series of metallic stripes thereon; evaporating metal through said mask on to said plastic film disposed at right angles to said first position to produce a second uniform series of metallic stripes thereon at right angles to said first series, said first and second series of metallic stripes constituting the thin metallic screen; removing the plastic film on which the thin metallic screen has been evaporated from the support surface; and vaporizing the plastic from the screen.

11. The process of producing, with the aid of a mask having a series of uniformly spaced parallel slits, a thin metallic screen utilized as an element of a storage grid assembly, said process comprising the steps of dipping an object having a flat support surface into a plastic solution to leave a plastic film on the fiat surface; evaporating metal through said mask on to said surface maintained in a first position to produce a first uniform series of metallic stripes thereon; evaporating metal through said mask on to said surface maintained in a second position at right angles to said first position to produce a second uniform series of metallic stripes thereon at right angles to said first series, said first and second series of metallic stripes constituting the thin metallic screen; scribing a configuration on the plastic film corresponding to the desired dimensions for said metallic screen; removing the plastic film on which the thin metallic screen has been evaporated from said support surface and placing it in position on the storage grid assembly while submerged in a liquid; and vaporizing the plastic from the screen.

12. A storage grid assembly for a cathode ray tube, said assembly comprising a dielectric screen having a first transparency; a first metallic screen having one mesh per each mesh of said dielectric screen disposed in register with and partially imbedded in one side of said first screen whereby an exposed surface protrudes out of the surface of said first screen, the transparency of said first metallic screen being greater than the transparency of said dielectric screen; a metallic coating on said exposed surface and extending over a portion of the surface of dielectric screen to make the transparency of said metallic coating equal to the transparency of said dielectric screen; and a second metallic screen disposed in contact with the surface of said dielectric screen on the other side thereof, the transparency of said second screen being greater than the transparency of said first metallic screen, whereby the surface portions of said dielectric screen within the interstices of said second screen constitute storage surface.

13. In an electronic storage tube, a storage grid assembly comprising a first conductive screen; a dielectric material deposited over one side of said first screen to form a dielectric screen superimposed on said one side of said first screen, said dielectric screen having a transparency less than the transparency of said first screen; a layer of metal disposed on the portions of the surface of said dielectric screen from the line defining the narrowest point through each interstice thereof to the function of said dielectric screen with said first screen; a second conductive screen disposed in contact with that portion of the surface of said dielectric screen which is farthest away from said first conductive screen, the meshes of said second conductive screen being disposed at an acute angle with the meshes of said first conductive screen to produce a minimum moire efiiect.

References Cited in the file of this patent UNITED STATES PATENTS 2,164,961 Strubig July 4, 1939 2,237,681 McGee et a1. Apr. 8, 1941 2,240,186 Iams Apr. 29, 1941 2,259,507 Iams Oct. 21, 1941 2,544,755 Johnson Mar. 13, 1951 2,667,596 Szegho et a1. Jan. 26, 1954 2,687,492 Szegho et al Aug. 24, 1954

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