US2856559A

US2856559A - Picture storage tube

Info

Publication number: US2856559A
Application number: US295768A
Authority: US
Inventors: Knoll Max
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1952-06-26
Filing date: 1952-06-26
Publication date: 1958-10-14
Anticipated expiration: 1975-10-14

Description

Oct. 14,1958

M. KNo| PICTURE STORAGE TUBE a sheeTsf-sheet 1 Filed lJune 26, 1952 IN1/Tyrol? X Knall Get. l14, 1958 M. KNoLL 2,856,559

` PICTURE STORAGE TUBE Filed' June 2e, 1952 2 sheets-sheet 2 United States atent PICTURE STORAGE TUBE Knoll, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 26, 1952, Serial No. `295,768

18 Claims. (Cl. 315-12) This vinvention is directed to a viewing tube, and more particularly to a charge storage device having a fluorescent screen to provide a visual display of information.

This application is a continuation-impart of the copending application Serial No. 254,999, filed November v6, 1951, now abandoned.

There is a class of electron discharge tubes which utilize a fine mesh screen upon which can be established a pattern of charges corresponding to information to be reproduced for visual inspection. Within the tube a source of electron emission provides an electron discharge directed through the storage screen. On the opposite side of the screen there is mounted a fluorescent screen to receive the electron discharge and to vprovide a visual display. The storage screen controls the amount of the electron discharge passing through it from point to point, so that the luminescent display provided by the uorescent screen is varied from point to point, in accordance with the charge pattern on the storage screen. The pattern on the storage screen may be established by either a photocathode providing an electron image discharge or by an electron gun, whose beam is modulated in accordance with incoming signals, while it is being scanned over the surface of the storage screen. Such storage viewing tubes provide a greater intensified visual display than .is possible with asingle modulated electron beam.

However, it has been diicult to provide satisfactory visual displays on the fluorescent screen of Vsuch tubes described above. The electrons penetrating through the storage screen tend to spread and diffuse to a point that the visual display or picture los-es resolution.

Itis therefore an object of my invention to provide an improved charge control storage tube.

It is a further object of my invention to provide a charge control storage tube in which the resolution of the visualgpicture `is greatly improved.

In particular the invention is directed to a charge controlled storage tube with an electron optical `system consisting of a critical spacing of a tine mesh screenfrom the uorescent screen of the tube, whereby the electrons penetrating through the screen are brought to a well .focused dot raster on the fluorescent screen. This provides 'an optimum resolution of the resulting picture. The conditions for providing an electron lens raster is preferably that in which a high potential eld gradient exists between the uorescent screen and the storage grid and a relatively low potential eld, as compared with 4the fOrmeriieId, is maintained on the vother side of the storage grid. Y

Fig. 1 is a sectional view of a 'charge controlled storage tubein accordance vwith my invention.

Figs. A2 and g3 are graphical representations of conditions 'existing during operation of the tube of Figure l.

VFigure 4.s a sectional Aview of an image tube using a charge lcontrolled storage screen, .in .accordance with my invention.

Figures 5 and 6 are graphical illustrations of the secondary emission characteristics of magnesium 'uoi'ide and silica respectively.

Fig. 1 discloses a charge control storage tube consisting of an evacuated envelope 10 having two neck portions respectively indicated as 12 and 14. Within the envelope neck 12 is an electron gun 16 which may b'e considered as the viewing gun, and which may also be used for erasing. Within the neck 14 is a second electron gun structure 18 for providing a modulated beam of electrons which is accelerated down the neck 14 into the envelope portion 10. Gun 18 is considered as the writing gun but may lalso be used for erasing. Mounted at the large end of the envelope portion is a target assembly 20 consisting of (a) ja glass support sheet 22 having, on its surfacefacing the electron guns 16 and 18, a ythin lm lof transparent conductive material 24, which may be formed of metal or a metallic compound such as tin oxide for example. On top of conductive film 24, is formed a uorescent screen 26 which is of a phosphor material which willuoresce under electron bombardment. Closely spaced from the surface of iluorescent scr-een 26, and at a distance in the order of several millimeters, is (b) la ne metal mesh storage and focusing screen 28, and (c) a second metal mesh screen 30 is' spaced `beyond screen 28 at a distance in the order of 10 mm. Screen 30 Vmay be of a woven stainless steel mesh of 'neness in the order of 230 mesh per inch, while screen 28 may be a metal screen having a fineness in the order of 200 mesh per inch. Storage screen 28 is provided on lthe gun side with a coating 32 of dielectric insulating material, such as silica or magnesium uoride, and having a thickness in the order of several microns'.

Screen 20 is mounted on a ring 21 of insulating ma'- terial fixed within the tube envelope 10, adjacent to the large opening or face plate 11 ofthe tube. Fixed to the insulating mounting ring 21 is an annular support ring 23, supporting, intermediate itsl ends, the glass target sheet 22. The fine metal mesh screen `28 is iixed across the open end of the metal vsupportring 23 which faces the electron guns `16 and 18. Screen 28 may be fastened to support ring 23 in any manner such as by welding, for example. Also, mounted on the insulating mounting n'ng 21 is a second annular metal support ring 25 across the open end of which is mounted a metal mesh screen 30, as shown in Figure v1. The Aconductive target film 24 of the uorescent screen is insulated from the metal support ring 23 by the glass support sheet 22, as show-ri in Figure l. The conductive film .24 is connected asis shown by a lead 27 to a source of potential outside the tube envelope 10. Furthermore, the metal mesh screen 28 is provided with a potential during tube operation by connecting alead 29 to the metal support ring 23, to which metal mesh 28 is attached. In a similar manner, metal mesh 30 is `connected to a source of `potential by a lead 31 attached to metal support ring 25. Normally the electrodes 30 and v24 are maintained during tube operation at a constant potential. However, the metal mesh electrode 28, which is the storage screen, is connected by a variable .adjustment to a voltage divider 33 so that the voltage of screen 28 may be varied from a voltage somewhat more negative than 20 volts, relative to ground, to a potential in the order of 1,000 volts positive relative to ground.

Electron beam 38 is :scanned 'over the surface ofthe storage screen electrode 28 in any well known manner, as for example, by two .pairs of

electrostatic deilection plates

40 and 42, respectively. As -is well known, the pairs of

plates

40 and 42 may be each connected to sources of saw-tooth voltages for providing vline and :frame scanning of the beam over the surface of the mesh screen 28.

The viewing gun 16 consists of a cathode electrode 7 having an electron emitting'coating on its surface facing screen 20. A control electrode 13, a rst accelerating electrode 15, and a second accelerating electrode 17 are mounted successively along the axis of gun 16 and between cathode 7 and screen 20. These electrodes are maintained, during tube operation, at appropriate voltages to form the electron emission from cathode 7 into a wide beam or spray 44 of electrons. The inner surface of the envelope 10 is coated by a conductive coating 34 of colloidal graphite adjacent to the electron gun 16. Coating 34 may be maintained at the same potential as the second accelerating electrode 17 of gun 16. A second wall coating 36 extends from a point spaced from but adjacent to coating 34 over the bulb wall enclosing the target assembly 20.` This coating 36 is operated at a different potential than that of coating 34 and in this mannerprovides a collimating electron lens between the two coat- `ings to align the electrons of the spray beam 44, so that they are directed on to the target 20 substantially normal thereto.

Electron gun 18, in like manner, consists of a cathode electrode 19 and a control grid electrode 35 enclosing cathode 19. Spaced alongrthe gun axis toward the target are in order,1a iirst accelerating electrode 37 and a second accelerating electrode 39. The wall coating 34 extends into the neck 14 of the writing gun and forms a third accelerating electrode for focussing the electrons of gun 18 as a beam 38 onto the target '20. Incoming signals may be applied to the control grid 35 by any appropriate circuit means 43.

The voltages of the several electrodes disclosed in Figure 1 are illustrative of voltage values which can be used during operation of the tube of the type described. However, these voltages are not limiting. For example, mesh grid may be operated between 200vvolts and 2,000 volts positive relative to ground. The conductive coating 24 may be operated within a range of 2,000 volts to'20,000 volts positive relative to ground, while storage and focusingscreen 28 may be varied between a minus 100 volts relative to ground to 2,000 volts positive to ground.

As shown in Figure 1, the metal mesh grid 30 is operated at around 1,000 volts positive with respect to ground or with respect to the cathode potential of the viewing gun 16. Also, the conductive coating 24 of the fluorescent screen is operated at close to 8,000 volts positively with respect to ground or viewing gun cathode potential. The potential of the metal mesh 28 supporting the insulating coating 32` is operated, in the tubeV described and shown in Figure 1, within a voltage range varying from a negative voltage in the order of 20 volts with respect to ground to a voltage in the order of 1,000 volts positive with respect to ground.

In accordance with the invention, the tube of Figure l may be operated as follows:

To prepare the target for storing a charge on the insulating coating 32, it is necessary to establish a` uniform potential over all of the surface of ilm 32. This is done by turning on the viewing gun 16. The electrons of beam 44 are accelerated withenergies up to 1,000 volts through the screen 30. The potential of screen 28 is set with. adjustable lead 29 to a sufficiently positive potential that the electrons of beam 44 will strikethe insulating surface of 32 at velocities or energies to initiate a secondary electron emission from all portions of film 32. If screen 28 is set below the rst cross-over potential, or above the second cross-over potential, of the secondaryemission curve of the insulating material forming lm 32, then. the surfaceof film 32 will be driven toa uniform potential approaching viewing gun cathode potential. lf the potential of screen 32 is set between the rst and second cross-over points of the secondary emission curve of the material of film 32, the surface 32 will be driven posi-I tively to a potential approaching that of collector screen 30.

In the tube described and shown in Figure 1, film 32 may be silica and electrons striking a silica llm of this type with energies above 50 Volts will initiate a secondary emission greater than unity or in which the number of secondary electrons leaving any point of the dielectric surface 32 is greater than the number of electrons of beam 44 striking that point. Depending upon the potential at which screen 28 is set, electrons of beam 44 will drive the insulating surface 32 to a uniform potential. A fraction of a second only is required to drive all of the surface to a uniform potential.

The viewing gun is then switched off and the focusing and storage screen 28 is set at a voltage in the neighborhood of 20 volts negative with respect to the viewing gun cathode potential, or ground. The viewing gun is then turned on again. However, the surface of the insulating lm 32, because of its thinness, is closely coupled by capacity to the metal screen 28 and assumes substantially the same `potential as the screen 28. The electrons of beam 44, which are accelerated through the positive collector screen 30 now enter a retarding field adjacent the surface of film 32 and they are turned back to the collector screen 30 by the negative potential of the film. However, the positive field of the metal lm 24, which is ysubstantially 8,000 volts positive with respect to viewing gun cathode potential, will tend to extend through the interstices of the coated mesh 28 and draw electrons of beam 44 to the fluorescent screen 26. By adjusting the potential of screen 28 suciently negative, it is possible to just prevent any electrons of beam 44 from `penetrating through screen 28.

The beam 38 of the writing gun is then turned on and scanned over the surface of the insulating film 32 simultaneously while the beam is being modulated `by incoming signals applied to .the control grid 35 by the input circuit 43. The writing beam 38 strikes the insulating coating at around 3,000 volts which is between the first and the second crossover points on the secondary emission curve of the silica lm 32. In this manner, beam 38 initiates secondary emission from the silica surfacegreater than the number of electrons striking the surface from beam 38, and in those areas, where the beam strikes, the silica surface is driven positively by the secondary emission from its potential `of a -20 volts toward the` viewing gun cathode potential or ground. However, no point of the surface of lm 32 can be driven by beam 38 more positively than ground as low velocity electrons of beam 44 will land on the lm at that point and drive it back to ground potential or slightly below. In those areas of the silica film 32 which are driven positively by beam 38 toward the viewing gun cathode potential, the positive field of the uorescent screen 24-26 will now penetrate and draw the low velocity electrons of beam 44 through the screen 28. These electrons of beam 44 which pass through mesh 28, are highly accelerated by the intense positive field between screen 28 and film 24 and will strike the uorescent screen 26 at high velocities to cause luminescence of the screen. This luminescence will only appear from portions of screen 26 adjacent those areas of the storage screen 28 struck by the writing beam 38. Where beam 38 does not strike lm 32, the lm remains negative and the positive eld of the uorescent screen can not penetrate through screen 28 and draw beam electrons 44 through to the fluorescent target surface.

Thisvtype of writing provides a picture on the fluorescent screen, which is dark in the unwritten areas and light or bright in the written areas. The background of the picture will be black and the written areas will be white. Theoretically, once a signal has been Written on the charge storage screen 28, it may be seen indefinitely on the luminescent screen 26 since with the mode of tube operation as described above, the charged areas are never discharged by the low velocity beam 44. Actually, however, the stored charge on film 32 .is gradually dissipated by spurious ions in the tube. Y

The stored signal on the insulating film 32 may be erased and the surface of the film rewritten on. Erasing may be done as described above by .shifting the potential of mesh screen 28 in a positive direction until the electrons of the spray beam 44 strike film 32 with energies between the first and second crossover points of the secondary emission curve of the insulating material. At this point, .all of the surface of the .insulating film 32 is driven positively toward the potential of the collector screen 30. Then, the screen 28 is reset to its negative potential and a new signal may be written down on the surface of the storage screen, in the mannerdescribed.

The tube shown in Figure 1 may also be operated in a manner to produce a black writing on a white background. This may be done by first preparing the target surface for writing, as described above, by setting screen 28 positively to erase any previously written signal or any unequal charge distribution on the surface of film 32. Screen 28 then is reset toa negative potential inthe order of `two volts below the viewing gun cathode potential or ground. The spray beam 44 is then turned on and the voltage of screen 28 is adjusted yto a point where ythe positive field of the fluorescent screen 26 penetrates the openings through mesh 28 to draw the low velocityr electrons through mesh 28 onto the fluorescent screen. This is indicated, in the absence of the writing vbeam 38, by a white raster on the fluorescent screen 26. The writing beam is then turned on and the cathode 19 of the writing beam 18 is set at a potential so that beam 38 will strike the silica film 32 at either below the first crossover point or above the second crossover point of the secondary emission ratio curve for .the material of vfilm 42. The electrons of beam 38, at either of these velocities, will initiate a secondary emission which is less .thanunity and `thus drive the surface of film 32 negatively. vIf the beam 38 is modulated by incoming signals from source 43 simultaneously While it is being scanned across the surface of film 32, there will be established on the surface of the insulating film a negative charge pattern. In thoseareas of the film 32 struck by beam 38, the negative Acharge established will be sufficient to neutralize the positive field of the fluorescent screen r26 which penetrates through the screen 28. Thus in those areas Vof film A32 charged negatively by beam 38, the low velocity electrons of beam 44 will not see the positive field of zthe fluorescent screen but will be reflected and turned `back-to the collecting screen 30. In this manner ythen the writing beam 38 will effect a black signal pattern on vthe fluorescent screen 26 since no electrons of beam 44 Will penetrate through screen 28 in the Written areas. I

Also in accordance with the invention, I haveprovided an electron lens or focusing raster, in amanner that Ythe viewing beam electrons passing through thestorage screen r 28 are brought to points of focus on the'uorescent screen. This provides an optimum resolution of the picture formed on the fluorescent screen. Without the provision of a focusing system, the electrons passing through storage grid 28 will become diffused, and the areas in which they strike, will overlap to provide poor resolution in the visual picture formed on fluorescent screen 26. By providing appropriate spacing between fiuorescent screen 26 and grid 28, as well as appropriate fieldstrength on both sides of screen 28, there is formed an electron lens adjacent each aperture of screen 28 which focuses the electrons passing through the aperture to a limited spot on fluorescent screen 26.

Fig. 3 shows a partial enlarged view of the region between

screens

28 and 26 of the target structure 20 of Fig. 1. As shown in Fig. 3, the spray beam 44 passing through screen 28 is converged to focal points 45, for example, lying substantially in a common plane. The electrons of beams 44 passing through these focal points diverge and strike .the tiuorescentfilm 26 `in small Yareas ordots47. It is found in accordance with the invention that by varying several factors, it is possible to control the size of the luminescent dots 47 and to maintain the spray electrons 44 as tiny narrow beams as they emerge through the aperture 28. The conditions at which dots 47 are just tangent to one another may be consideredthe optimum focusing condition.

To provide the optimum focussing conditions for a bright viewing tube, and conveniently low viewing gun voltage, it is essential that the field gradient between the focussing mesh 28 and the fluorescent screen 26 be con' siderably greater than the field gradient between the collector screen 30 and the focussing grid 28. For example, `in the tube described in Fig. 1 the difference of potential, Aduring tube operation, between collector screen 3ft and the focussing vgrid y28vis in order of 1,000 volts. The-distance between these two screens is approximately l cm., so that the field gradient between the screensy 30 and 28 is substantially 1,000 volts per cm. On theother hand, the potential difference between the focussing grid 28 .and fluorescent screen 26 is in the order of 8,000 volts, and the distance between

screens

28 and 26 in the order of 0.3 cm. This space provides then a field between

screens

28 and 26 in the order of 25,000 volts per cm. The ratio of the high potential gradient between

screens

28 and 26, and low potential gradient between

screens

30 and 28 is in this

case

25,00021000 or 25. Lower field gradient ratios are applicable, if brightness or size of viewing beam power supplies are ynot essential.

`Furthermore, it has been shown that the individual beams emerging through the focussing grid -28 remain distinctly separated from each other within a certain v'range of potentials of -screen 28, near the cathode potential of the viewing beam gun. It has also been found ythat if the focussing screen 28 is near the potential of lwhen screen 28 is negative, the beams are brought respectively to crossover points45 just beyond the focussing screen 28. The electrons, however, leaving crossover points 45 do not become Wider at the fluorescent screen 26, but also become narrower with increasing negative potential of the focussing screen '28. The reason for this effect is that the action of the negative field around each Vaperture of screen 28 cuts off the outside orfringe lsectrons of each electronbundle passing through screen A positive screen 28 is an advantage, if the tube'of the type described in Fig. 1 is to be'used merely asnonstoring, post acceleration kinescope image intensifier.

For the given spacing of 0.3 cm. between

screens

28 and 26 and for the given field strength ratio of 25, dots 47 formed by beams 44 become tangent to each other. However, if screen 26 were spaced farther from screen 28, with the same field strength ratio, spots 47 would overlap, and the resolution of the picture would suffer seriously. "Spacing screen 26 closer to screen 28, with the same field strength conditions, would not deteriorate picture resolution. But, a closer spacing of screens V26 and 28 is undesirable due to the possibility of voltage breakdown caused by the shorter insulating path between the two screens. If brighter pictures are desired by increasing the potential of screen 26, it would be advisable for better insulation between the screens to

space screens

28 and 26 at greater distances at the same time. However, for optimum focussing conditions, it would be necessary to use a morer'negative potential on the focussing screen 28, or it would vbe 'even possible to .go to a 'posianzidetto "tive potential for grid 28 if beam modulation and storage is not a consideration as mentioned above.

A condition `for optimum beam focus has been described above as that in which the field strength ratio is l5. This is not limiting as thefield strength ratio is not 'critical and may be varied between 2 and 50, for eX- ample. For any given value of the ratio of the field strengths, there is a range of potentials at which the focussing grid 28 may be set with a corresponding change in the potential of screen 26 to provide a conditionof optimum focussing. It is only necessary, as described above, to adjust the voltages of

screens

28 and 26 for the given spacing of

screens

28 and 26, to aect the condition of tangent spot condition. It has furthermore been found that the aperture opening in screen 28 is also a determining factor for producing optimum beam focus. Larger apertures in screen 28 require a correspondingly more negative potential on the screen 28 during tube operation for the same potential on screen 26, or in other words, optimum beam focus is produced with larger hole diameters in screen 28 by using a smaller potential on screen 28.

Accordingly, as fully described above, and in accordance with the invention, it is possible to provide a picture beam tube in which the visual picture is greatly intensified and is also one of high resolution. The result is produced, as described above, by controlling the electrons falling upon the fluorescent screen by a focussing grid or a lens raster which not only modulates the electrons passing through, but also directs them in well defined beams on to the fluorescent screen to provide a high luminescent picture of good definition. Furthermore, it is possible to provide a luminous picture for a period of time by the storage of signal charges on the control grid or focussing lens raster whereby certain information may be retained visibly to an observer for a desired period of time.

Figure 4 discloses a type of image tube using an electron lens raster system in accordance with my invention. The tube of Figure 4 consists of an evacuated envelope 50 having a fluorescent target electrode 52 mounted at one end of a metal cylinder 54. The target electrode 52 consists of a supporting sheet of glass 56, a fluorescent film 58 having on its exposedsurface a thin film 60 of conductive material, such as an electron permeable film of aluminum. On the end of the tube envelope is a photocathode coating electrode 62 formed on a conductive transparent film 61. Spaced between the photocathode and target electrode 52 are a collector screen 64 and a storage screen 66, forming an electron lens raster system. On the surface of screen 66 facing photocathode 62 is a coating 67 of insulating material such as silica or magnesium fluoride, for example, and having its secnd crossover potential ofthe secondary emission curve somewhat below 10,000 volts, in this example. `The mesh screens 64 and 66 correspond in function respectively to

screens

30 and 28 of Figure l. The aluminum coating 60 of the fluorescent screen is grounded as is shownQwhile screens 64 and 66 are connected by leads to

terminals

68 and 70, respectively.

Terminals

68 and 70 can be connected by a movable switching device 72 respectively to

several terminals

1, 2, 3 and 1', 2' and 3, which in turn are connected respectively by leads to sources of different potential supplied by a voltage divider 7S, for example. l

The operation of the device of Figure 4 is that in which the switching device 72 is first positioned so as to connect

terminals

70 and 68 to terminals 1 and 1 respectively. This is the position for erasure of the target in preparation for signal storage thereon. Screen 66, which may be considered a backplate of film 67, and screen 64 are now connected to a voltage of minus 9,000 volts, relative to ground. Conductivefilm 61 is connected to a minus 9,400 volts, relative to ground, during tube operation. The photocathode coating 62 is now flooded with light from closely positioned light sources 74.` Photoelectrons from the photocathode coating. 62 pass through the accelerator grid 64 with energies of around 400 volts and strike the insulating surface 67 of grid 66 above rst crossover potential of thesecondary emission curve for the material of lm 67. The insulating surface 67 is driven to the, potential of screen 64 or substantially a minus,9,000 volts, since the secondary electrons from coating67 are driven back to coating 67 bythe negative collecting field of screen 66. This procedure erases any charge on insulating ,coating 67 and establishes a uniform charge over all of coating 67 near to the potential of screen 66.

The floodlights 74 are turned olf, and the switch 72 is moved to terminals 2 and 2. This sets screen 66 at a potential of a minus 6,000 volts relative to ground, and screen 64 at a potential of a minus94l0 volts relative to ground or some 10 volts negative to photocathode potential. This produces a high accelerating field of some 3,400 volts between

screens

34 and 36. This accelerating field will penetrate through the interstices of negative screen 64 and will accelerate any electrons leaving photocathode 62 through screen 64 onto the insulating coating 67. Furthermore, the penetrating of the accelerating field through the negativescreen 64 produces an electron lens field distribution at each interstice of the screen 64. All electrons then passing through each interstice of screen 64 are converged and brought to a focus at well defined spots on the insulating coating 67.

If now an image of an object to be viewed is focused by any appropriate lens system, indicated by 76, upon the photocathode 62, photoelectrons will be emitted from each portionof the photocathode 62 in numbers corresponding to the amount of light falling on that portion. These photoelectrons penetrate through negative screen 64 and are focused on coating 67, as described, by lens fields formed in each interstice of screen 64. The photoelectrons will strike the dielectric, coating 67 with energies around 3,400 volts and .will initiate a secondary emission, which is repelled by the negatively charged screen 64. However, the intensely positive 'field of some 6,000 volts ybetween storage grid 66 and aluminum film 60 penetrates through the apertures ot storage screen 66 and draws the secondary emission from coating 67 through and onto the fluorescent screen 58. Also, in the manner described above for the operation of screen 28 of Figure 1, storage grid 66 provides a lens raster so that the secondary electrous passing through the apertures of screen 66 are focused to well-defined dots on the luminescent screen 58. Thus, there is established on the fluorescent screen a visual picture which corresponds to the optical image focused upon the photocathode 62 by the lens system 76.

The preferred operation of the modificationof Figure 4 is to flash the optical picture` or image on the photocathode 62 for a very short time and thus write or store such an image electrically on grid 66. The electrically stored image can then be reproduced visually on the fluorescent screen 60 for any desired length of time to provide a storage viewing of the picture. This is done by first erasing any charge pattern on the storage screen 66 in the manner described above. Lights 74 are turned off and switch 72 is moved to position 2--2 for writing" a charge image on the storage` screen. A picture is optically projected on the photocathode 62 and a corresponding charge pattern is established on coating 67 in the manner described above. The picture may be flashed on for only an instant or for longer before being cut off. To view the stored information on coating 67, the switching device 72 is moved next to

terminals

3 and 3 respectively. This connects accelerator screen 64 to a potential of minus 8,800 volts relative to ground which is 600 volts positive relative to photocathode potential. The `storage screen 66 is also connected to a potential of minus 9,420 volts which is 2O volts negative to photocathode potential.` Simultaneously, the insulating coating 67 is 59 by .grid 66 to substantially the lsame potential negative to the photocathode potential. The photocathode 62 is now illuminated by light from sources 74. yA uniform photoemission now leaves all portions of photoca'thode 62 and is accelerated through screen 64 "toward the storage screen 66. In those portions of the vstorage screen which were charged positively during the writing operation, the positive field from the target 52 will penetrate through screen 66 and draw the photoelectrons through to the luminescent screen in amounts proportional to the positive charge established in each area of coating 67. However, in those areas of coating l67 which were uncharged during the Writing operation, '.fphotoelectrons will be repelled back 'to the collector .screen 64 by the negative potential of coating 67. The Iphotoelectrons, which do penetrate Vthrough screen 66 rWill-be focused onto the fluorescent layer, in the manner described above for the tube of Figure 1, and will rproduce :a visual picture or display in accordance to the distribution of positive charges established on the insulated surface 67 of the storage screen 66. Furthermore, this display will remain for a period of time as the potential of screen 66 is adjusted until all portions of the storage screen 66 are negative to photocathode potential and no :photoelectrons land on the target to discharge the charge pattern. In this manner, there is thus provided a stored .picture on the fluorescent screen which lis much brighter `than the directly viewed picture. Due to the focusing action of the storage grid, the viewed picture is of good definition. Resolutions up to 200 limit per linear inch of the luminescent screen are easy to obtain.

The optical picture may be pulsed or gated and the .pattern produced can be'observed between pulses. Such an application would be of value in observing stroboyrscopically rapid periodic motions. Also, such applications would be of value in X-ray observations where the patient cannot be safely exposed to X-rays for long periods of time.

The tube of Figure 4 has been described as operating with a positive charge pattern on storage coating 67. However, the potential of screen 66 may be set during writing so that the photoelectrons released by the optical picture will strike coating 67 with energies above second crossover potential to produce in the light areas a negative charge pattern. This will produce on the fluorescent screen 58 a negative picture showing luminescence in the dark areas.

The image tube of Figure 4 depends for picture definition upon the lens raster which provides focusing of the electron passing through each aperture of screen 66 upon thefluorescent screen 58. The requisites for proper focusing conditions are similar to those described for the tube of Figure 1. For example, the tube of Figure 4 provides a-low potential gradient between the accelerating screen 64 and screen 66 and a relatively high Vpotential gradient between screen 66 and the fluorescent screen.

.The potentials described at which the tube of Figure 4 is operated are illustrative and knot meant to be limiting. For example, each of the electrodes of the tube of Figure 4 may be operated at half the values of the respective potentials set forth above, to produce similar results.

The tubes of Figures l and 4 have been described as storage devices in which the focusing screens respectively y28 and 66 are coated with a dielectric lllmfor .the storage ofcharges. My invention is not limited to storage tubes, but also includes viewing tubesin which screen electrodes ZSand 66 are-not coated and provide merely focusing of the yelectrons `on the fluorescent screen. VSuch beam intensifier tubes may, in the case of Figure l have only one (scanning) gun, and represent a post deflection acceleration oscilloscope with excellent contrast because no landing of secondary electrons stemming .from the acfcelerati'on assembly at the luminescent screen takes place. Inthe icase of Figure 4, Ian image vintensifier 'for larger -ytures can be made which is4V relatively short and may Abe Agated by applying low voltage electrical pulses to the `electron lens raster grid near the 'luminescent screen.

What I claim is:

l. An electron discharge device comprising an electron source, a fluorescent screen spaced fromr said electron source, a mesh screen electrode positioned between said electron source and said fluorescent screen and closely spaced. from said fluorescent screen, and a second mesh screen electrode positioned between said first mesh screen and said fluorescent screen, said second mesh screen electrode having an insulating coating on the surface thereof facing said first mesh screen electrode for storing a charge pattern, means establishing a first potential gradient between said frst and second mesh screens and a higher potential gradient relative to said first potential gradient between said second mesh screen and said fluorescent screen whereby electrons from said source and passing through said screens are focused on said fluorescent screen.

2. An electron discharge device comprising, an electron source, a fluorescent screen spaced from said electron source, a mesh screen electrode positioned between said electron source and said fluorescent screen and closely spaced from said fluorescent screen, and a second mesh screen electrode positioned between said first mesh screen electrode and said fluorescent screen, said second mesh screen electrode having an insulating coating on the surface thereof facing said first mesh screen electrode for storing a charge pattern, means establishing a first potential gradient between said first and second mesh screens and a higher potential gradient relative to said first potential gradient between said second mesh screen electrode and said iluorescent screen, said means including potential sources respectively connected to said electron source and said second mesh for maintaining said second mesh screen and said electron source respectively at potentials differing only in the range from 0-100 volts.

3. An electron discharge vdevice comprising, an electron source, a fluorescent screen spaced from said electron source, a mesh screen electrode positioned between said electron source and said fluorescent screen and closely spaced from said fluorescent screen, and a second mesh electrode screen positioned between vsaid first mesh electrode screen andsaid fluorescent screen, said second mesh screen electrode having an insulating coating on the surface thereof facing said first mesh screen electrode for storing a charge pattern, means establishing a first potential gradient between said first and second mesh screens and a high potential gradient relative to said first potential gradient between-said second mesh screen and said fluo- .rescent screen, said means including potential sources respectively connected to said electron source and said second mesh screen for maintaining said second mesh screen, respectively at a few volts negative relative to the potential of said electron source.

4. An electron discharge device comprising, a target assembly including a fluorescent screen, a focussing screen electrode and collector screen electrode, said screen electrodes mounted-in parallel spaced relation to a surface of said fluorescent screen', said second mesh screen electrode having an insulating coating on the surface thereof facing said vfirst 'mesh-screen electrode for storing a charge pattern, and means `for providing a flow of electrons through said screen electrodes onto said fluorescent screen.

5. An electron discharge device comprising, a target assembly including a fluorescent screen, a focussing screen electrode and collector screen electrode, said screen electrodesl mounted in parallel spaced relation to a surface of said fluorescent screen with said focussing `screen electrode between said collector screen electrode and said fluorescent screen, an insulating film on the surface of said focussing screen electrode facing said collector electrode, and means for providing a modulated stream of electrons through said collector-screen' onto said insulatl1 ing film, and means for providing a second'stream of electrons through said screen electrodes onto said fluorescent screen.

6. An electron storage device comprising, a fluorescent screen, a collector screen spaced substantially parallel to one surface of said fluorescent screen, a focussing screen electrode positioned between said collector and fluorsecent screens, said second mesh screen electrode having an insulating coating on the surface thereof facing said first mesh screen electrode for storing a charge pattern, means for establishing a potential gradient on both sides of said focussing screen with the ratio of the field strength between said focussing and fluorescent screens to the field strength between said focussing and collector screens being in the rang from 2 to 50, and means including said focussing screen for providing a modulated flow of electrons through said focussing screen onto said fluorescent screen.

7. An electron storage device comprising, a fluorescent screen', a collector screen spaced substantially parallel to one surface of said fluorescent screen, a focussing screen electrode positioned between said collector and fluorescent screens said second mesh screen electrode having an insulating coating on the surface thereof facin'g said first mesh screen electrode for storing a charge pattern, means for establishing a potential gradient on both sides of said focussing screen with the ratio of the field strength between said focussing and fluorescent screens to the field strength between said focussing and collector screens being in the range from to 30, and means for providing a flow of electrons through said focussing screen onto said fluorescent screen.

8. An electron storage device comprising, a fluorescent screen, a collector screen spaced substantially parallel to one surface of said fluorescent screen, a focussing screen electrode positioned between said collector and fluorescent screens, said secon'd mesh screen electrode having an insulating coating on the surface thereof facing said first mesh screen electrode for storing a charge pattern, means for establishing a potential gradient on both sides of said focussing screen' with the ratio of the field strength between said focussing and said fluorescent screens to the field strength between said focussing and collector screens being equal to 25, and means including said focussing screen for providing a modulated stream of electrons through said focussing screen onto said fluorescent screen.

9. An electron storage device comprising, a fluorescent screen, a collector screen spaced substantially parallel to one surface of said fluorescent screen, a focussing screen electrode positioned between said collector and fluorescent screens, said focussing screen having a plurality of uniform apertures therethrough said second mesh screen electrode having an insulating coating on the surface thereof facing said first mesh screen electrode for storing a charge pattern, means forestablishing a potential gradient on both sides of said focussing screen with the ratio of the field strength between said focussing and fluorescent screens to the field strength between' said focussing and collector screens being in the range of 10 to 30, and means including said focussing screen electrode for providing a'modulated flow of electrons through said focussin'g screen and said fluorescent screen.

l0. An electron discharge device comprising, a target assembly including a fluorescent screen, a storage grid electrode and a collector screen electrode, said electrodes being mounted in parallel spaced relation to a surface of said fluorescent screen with said storage grid electrode between said fluorescent screen an'd said collector screen, said second mesh screen electrode having an insulating coating on the surface thereof facing said first mesh screen electrode for storing a charge pattern, means including an electron source for providing low velocity electrons directed toward said target assembly and means 12 including a second electron source for providing high velocity electrons directed at said target assembly.

1l. An electron discharge device comprising, a target assembly including a fluorescent screen, a storage `grid electrode and a collector screen' electrode, said electrodes mounted in parallel spaced relation to a surface of said fluorescent screen with said storage grid electrode between said fluorescent screen and said collector screen, a first electron gun for providing a low velocity electron beam along a path intercepting said target assembly, an'd a second electron gun for providing a high velocity electron beam directed at said target assembly.

12. An electron discharge device comprising, a target assembly including a fluorescent screen, a storage grid electrode and a collector screen electrode, said electrodes mounted in parallel spaced relation to a surface of `said fluorescent screen with said storage grid electrode between said fluorescent screen and said collector screen, a rst electron gun for providing a low velocity electron beam along a path intercepting said target assembly, and a second electron gun for providing a high velocity electron beam directed at said target assembly, and an insulating coating on the surface of said storage grid electrode facing away from said fluorescent screen surface.

13. An electron discharge device comprising, a target assembly including a fluorescent screen', a storage screen electrode and a collector screen electrode, saidelectrodes mounted in parallel spaced relation to a surface of said fluorescent screen with said `storage screen electrode between said fluorescent screen and said collector screen, a first electron gun for providing low `velocity electrons directed at said target assembly, and a second electron gun for providing a high velocity electron beam along a path intercepting` said target assembly, and an insulating coating on the surface of said storage screen electrode facing away from said fluorescent screen surface, means for establishing a potential gradient on both sides of said ystorage screen electrode with the ratio of the field strength between said storage screen and fluorescent screen to the field *strength between said storage screen and collector screen being in the range from 2 to 50, and means for providing a modulated stream of electrons through said storage screen electrode onto said fluorescent screen.

14. An electron discharge device comprising, a target assembly including a fluorescent screen, a storage mesh screen electrode and a collector mesh screen' electrode,

said electrodes mounted in parallel spaced relationto a surface of said fluorescent screen with said storage' screen electrode between said fluorescent screen and said4 collector screen, a first electron gun includingan electron' source for providing low velocity electrons directed at -said target assembly, and a second electron gun for providing a high velocity electron beam directed at said target assembly, and an insulating coatin'g on the surface of said storage screen electrode facing away from said fluorescent screen surface, means establishing a first potential gradient between said collector screen and storage `screen and a high potential gradient relative to said first potential gradient between said storage screen and said fluorescent screen, said means including potential sources respectively connected to said electron source and said storage screen' for maintaining said electron source and said storage screen respectively at potentials differing only in the range of zero to volts.

15. An electron discharge device comprising, a target Yassembly including a fluorescent screen, a plurality of screen electrodes mounted in parallel spaced relation to a surface of said fluorescent screen', photoemissive means for providing a modulated stream of electrons through said screen electrodes onto said fluorescent screen,` and means for connecting said fluorescent screen,` said photoemissive means and said electrodes to sources of potential to provide a high potential gradient between xsaid fluorescent screen and the adjacent one of said screen electrodes and a high potential gradient between said screen electrodes whereby electrons passing through said screen electrodes are focused on said adjacent screen electrode and saiduorescent screen.

16. An electron discharge device comprising, a targetl assembly including a fluorescent screen, a focussing screen electrode and a collector screen electrode, said screen electrodes mounted in parallel spaced relation to a surface of said fluorescent screen, .an insulating coating on said focussing screen electrode, photoemissive means vfor providing a stream of electrons through said collector and focussing screen.

17. The method of operating an electron discharge device consisting of a uorescent screen an'd a mesh screen electrode mounted in parallel spaced relationship 18. The method of operating an electron dischargev device consisting of a iluorescent screen and a mesh screen electrode mounted in parallel spaced relationship to one surface of said fluorescent screen, said method comprising the steps of, establishing an electrostatic potential eld gradient on both sides of said mesh screen electrode with the ratio of the eld strength between the lfluorescent screen and said mesh screen electrode to the ield strength on' the opposite side of said mesh screen electrode being in `the range from 10 to 30, and providing a modulated electron ow through said mesh electrode onto said fluorescent screen.

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