US2777970A - Television camera storage tube - Google Patents

Description

Jan. 15, 1957 P. K. WEIMER TELEVISION CAMERA. STORAGE TUBE 3 Shets-Sheet 1 Filed Oct. 3, 1950 s? x A 5% Jan. 15, 1957 P. K. WEIMER 2,777,970

l TELEVISION vCAMERA STORAGE TUBE Filed Oct. 5. 1950 3 Sheets-Shear 2' I l v um.' fg w ,A 'u

-Fzg I lNvEN-roR i Jan. 15, 1957 P. K. WEIMER 2,777,970

TELEVIsioN CAMERA STORAGE TUBE.' Filed Oct. 3. 1950 3 Sheets-Sheet 3 INVENTOR ffm/'Ai www NEY United States Patent 2,777,970 TELEVISION CAMERA STORAGE TUBE Paul K. Weimer, Princeton, N. J., assignor, by mesne assignments, to the United States of 4America as represented by the Secretary of the Army Application October 3, 1950, Serial No. 188,197 11 Claims. (Cl. 315-11) This invention relates to television camera storage and image tubes, particularly those tubes having two sided targets.

This application 4is a continuation-impart of my copending application Serial No. 608,663, tiled August 3, 1945, now U. S. Patent 2,537,250, issued Ian. 9, 1951. In the above-cited copending application, the invention relates to a television camera and pickup tube in which the tube was operated as a storage tube.

Briefly such a tube comprises an evacuated envelope having an electron gun at one end of the envelope, a dielectric target mounted intermediate the ends of the tube and a photocathode sheet spaced from the target at the other end of the envelope. A scene to be televised is focused upon the photocathode to initiate photoemission from all portions of the photocathode sheet in proportion to the amount of light falling on each portion of the photocathode. This photoemission falls upon the dielectric target and through secondary emission establishes a charge distribution or pattern on the target surface and corresponding to the scene or picture focused on the photocathode. A fine mesh screen, closely spaced from the dielectric target is scanned by the low velocity electron beam. The dielectric target is operated sufficiently negative to repel the beam` from a point closely adjacent to the target surface and to cause the beam to pass to the mesh screen for collection. The charge pattern on the dielectric surface modulated the repelled electron beam by permitting more or less of the beam to be collected by the mesh screen. 'Ihe portion of the beam not collected was returned to a multiplier section to provide a video signal in the output circuit of the tube. This arrangement was such that the electron beam did not discharge the dielectric surface, but was only modulated thereby so that the charge pattern could be maintained on the dielectric target indenitely. In tubes of the type disclosed in the copending application, it is often desirable to amplify the image signals received by the tube in order to provide greater tube sensitivity for low lighted conditions.

It is therefore an object of this invention to use a dielectric target to control emission from a screen electrode to provide image signal amplification.

It is another object of this invention to provide an electron discharge device which has means for amplifying the image signals.

It s a further object of the invention to provide a discharge device having an electron emissive electrode whose emission is controlled by a charge pattern on an adjacent electrode.

It is another object of this invention to provide a storage discharge device in which a signal may be amplified by controlling the emission from an emissive electrode in accordance with the signal.

The novel features of the invention are directed to a discharge tube in which the line mesh screen in front of the dielectric target surface is sensitized to be electron emitting and means are provided for causing electron emis sion from the sensitized mesh which electron emission can be modulated and controlled by a charge pattern on the dielectric surface. A photocathode and an image section or an electron gun is used for establishing a charge pattern on the dielectric target.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawing, in which:

Figure l is a sectional view of a camera tube utilizing my invention as set forth in the above-cited copending application and in accordance with the invention;

Figure 2 is a graph of current produced by electrons landing on a dielectric screen vs. the potentials of the target; l

Figure 3 is an enlarged section through a very small area of the control screen and indicating the equipotential surfaces of the electrostatic iield in the vicinity of the scanned side of the target screen;

Figure 4 is a graph showing the percentage of the beam landing on a collector electrode;

Figure 5 is a longitudinal view of according to my invention;

Figure 6 is a schematic showing of a modification of the mesh screen which may be used in the tube of Figure 5.

a camera pickup tube Figure 7 is another form of a storage tube in accordance with my invention;

Figure 8 is an image tube using my invention.

Figure 1 discloses a tube similar to that disclosed in my copending application above cited. This is a pickup or camera tube comprising an evacuated envelope 1, with an electron gun structure 2 mounted at one end thereof, a thin dielectric target 3, such as glass, mounted intermediate the ends of the tube and a photocathode electrode 4 formed as a coating on the end wall of the envelope. The gun structure 2 is formed of conventional electrodes consisting of a cathode 5, a control grid 6, a rst accelerating electrode 7 and a second accelerating electrode formed as a conductive wall coating 11. When appropriate voltages are applied to the several gun electrodes, an electron beam 9 is formed, directed at the surface of target 3, as is well known. The electron beam 9 is scanned over the surface of the dielectric target 3 by any well known means, such as that indicated at 12 in Figure 1, which comprises a neck yoke consisting of two pairs of coils. Each pair of coils are connected in series respectively to a source of saw-tooth voltages for providing line and frame scansion of beam9 over the surface of target 3. Such dellecting means are well known and are not described in fiuther detail as they do not constitute part of the invention.

A longitudinal focusing coil 15 is mounted to enclose the portion of the tube envelope, as shown, and for focusing the electron beam 9 to a sharply defined point adjacent the surface of the dielectric target 3. Mounted about envelope 1 is an alignment coil 14 for correcting for any misalignment of beam 9 with the tube axis. Mounted immediately in front of the dielectric target 3 and spaced therefrom is a foraminous electrode formed as a fine mesh screen 27, which is maintained at substantially 0.5 volt positive relative to the potential of the gun cathode 5. As will be described below, the dielectric target 3 is maintained at an equilibrium potential negative to the gun cathode S by a line mesh screen 24 mounted on the opposite side of the dielectric target 3 and maintained at a potential of several volts negative to gun cathode 5. A scene or picture to be televised is focused on the photocathode 4 to provide a photoemission from all portions of the photocathode 4 and corresponding to the light falling on each photoemissive area. The photoemission from photocathode 4 is accelerated 3 by ring electrodes 23 and 26 onto the surface of the glass target 3. The field of coil 1S maintains the photoemission from 4 in substantially parallel paths so that the photoelectrons on striking the surface of dielectric target 3 are distributed in accordance withl their pattern of emission from photosurface 4. The photoemission will strike the surface of target 3 at suiiicient velocity to provide a secondary emission at each point greater than unity. The secondary electrons, as will be described below, pass to the tine mesh screen 24 and leave the surface of dielectric target 3 charged with a charge dis tribution or pattern corresponding to the light pattern focused on the photocathode 4.

The several electrodes of the tube are provided with appropriate voltages as indicated in Figure 1. These voltages are not limiting in any sense and are given by way of example only. The electron beam from gun 2 is accelerated toward the target and then slowed down to substantially zero velocity by a decelerating electrode 2S formed as a ring in front of the target 3. The negative potential of target 3 tends to reflect the electrons as a return beam 9' back to a dynode surface 8 of a rst accelerating electrode 7. Secondary electrons from dynode 8 are deflected into an electron multiplier section indicated at 18 and operating as set forth in my copending application to amplify the return beam electrons. The nal stage of collector 20 of the multiplier section 18 is essentially a screen as shown, connected to an amplifie tube 22 to provide an output video signal for the tu e.

In the tube of Figure 1 the electrons from beam 9 do not land on the. target 3 during normal tube operation. The potential pattern thereon does, however, control the landing of beam electrons on the adjacent metal mesh screen 27 and hence modulates the return beam in accordance with the potential pattern. This is accomplished without discharging the chargeimage on the unscanned surface of target 3. The discharge screen 27 is substantially on the surface of glass target 3 but spaced therefrom on the gun side sufficiently to prevent conductive contact. This mesh screen 27 has a potential slightly positive to the cathode of gun 2, and substantially at +05 volt. The collecting screen 24 controls the average potential of the glass target 3 and may be biased from 2 to 6 volts or more negative to the dicharge screen 27.

Figure 2 shows the relationship of the potential of the glass target 3 to the current produced by the electrons landing on the mesh discharge screen 27. For one method of operation, the negative bias of mesh screen 24 is ad- Justed so that the variation in potential due to the charge image may fall on the part A-B of the curve of this figure and for another method, operation may be had on the portion of the curve C-D.

The theory of operation will first be explained when the bias voltage of mesh screen 24 is such that operation is had on the part A-B of the curve of Figure 2.

The electrons emitted from the gun do not all have their energy directed normal to the target. Some electrons may have as much as 0.5 volt transverse energy acquired at the expense of their velocity normal to the target. The electron beam 9 as it approaches target 3 will be slowed down to substantially zero velocity by the decelerating ring 25. However, the electrons of the beam will still retain their energy diiferential upon appreaching the discharge screen 27. Figure 4 discloses the relationship ofthe fraction of the beam landing on a collector electrode as the potential of the collector increases from zero volts. This ligure indicates that as the potential of a collector electrode increases to a +05 volt an increased portion of the beam is collected. Thus, any collector electrode placed in the path of the beam will thus collect all of the electrons of the beam if its poten tial is equal to or greater than 0.5 volt positive.

During tube operation, discharge screen 27 is maintained substantially at a positive 0.5 volt. Also, collector screen 24 is maintained at substantially a minus 3.0

volts relative to the potential of gun cathode'S. If photocathode 4 is entirely illuminated there will be a uniform photoemission therefrom which will strike all portions of the surface of the glassV target 3. The photoelectrons striking the surface of target 3 will provide a uniform secondary emission having an emission ratio greater than one. lf any portion of glass target 3 is more positive than the collector mesh 24, the secondary electron emission from that target portion will be repressed and will fall back onto the target portion to drive it to an equilibrium potential negative to the potential of the collector screen 24. The secondary electrons, from the target portion bombarded by the photoelectrons, have energies ranging from a fraction of a volt to a voltage in the order of magnitude equal to the energies of the bombarding photoelectrons. However, energies of the largest part of the secondary electrons are of a few volts.

If it is assumed that the energy of two thirds of the secondary electrons is two volts or more, then the target portion bombarded by the photoelectrons will assume an equilibrium potential of substantially a minus one volt. At this equilibrium potential 'substantially two thirds of secondary electrons will have enough energy' greater than two volts to pass either to the collector screen 24 or be redistributed to other portions of the surface of target 3. The other third of the secondaryV electrons will have an energy less than two volts and will be repressed by the negative eld of screen 24 and will fall back onto the bombarded target area. In this manner,

any portion of the target 3 bombarded by photoelectrons will tend to assume the equilibrium potential at which as many secondary electrons leave a target portion as there are primary photoelectrons and secondary electrons striking the target portion.

' If, however, a varying light intensity is focused upon photocathode 4, then a charge pattern is set up on the surface of the glass target 3 corresponding to the light distribution focused on photocathode 4. Thus, portions of glass target surface 3 corresponding to the dark areas of the light pattern on photocathode 4 receive little, if any, photoemission from the photocathode. However, these dark areas of the glass surface 3 are driven to substantially a minus 3 volts or to the potential of collector electrode 24 by the redistribution electrons falling back to the glass target 3 from 'the light areas of target 3.

In a similar manner charges on the surfaces of glass target 3 are set up corresponding to grey portions of the picture or light distribution focused on the photocathode 3. These grey charge areas on target 3 will be at a potential between a minus 3 volts which is that of the dark area and the minus 1 volt which is that of the light areas, since the redistribution electrons will keep these areas more'negative than the potential of the llight areas. In this mannenthen, when a scene or light pattern is focused upon the photocathode, a negative charge distribution is correspondingly established on the surface of glass target 3. These charged areas on the photocathode side of target 3 maintain a corresponding negative potential pattern on the other surface of target 3 and which extends beyond target 3 through the openings of the discharge screen 27.

To operate the tube on the portion of the curve A-B of Figure 2, the collector screen electrode 24 is biased at a negative 3 volts relative to the potential of the cathode of gun 2. Figure 3 illustrates schematically the conditions which exist in the region of the glass target 3 during tube operation. Figure 3 shows the equipotential surfaces which exist on the scanned side of the glass target 3. Several conditions have been illustrated in Figure 3,v ranging from those existing on the surface of target 3 due to light striking the photosurface 4 and those conditions on the surface of target 3 due to no light striking the photocathode 4. For example, if a portion of the photocathode 4 is illuminated by a large amount of light, the photoemission is large and will produce a correspondingly large number of secondaries from the surface of the glass target 3. Such a condition is indicated in the region A of Figure 3. The secondary emission from such a region is greater than the incident photoelectrons so that this part of the target surface is driven in a positive direction and, as described above, will reach an equilibrium potential of substantially a minus 1 volt at which time the secondary electrons leaving the target surface will equal both the incident photoelectrons plus secondary electrons raining back on the target surface.

Area C of Figure 3 illustrates those conditions where little or no photoelectrons strike the surface of target 3 due to substantially no light falling on a corresponding area of the photocathode 4. Due to this condition then, very few if any secondary electrons are bombarded from area C but an effectively large number of secondaries from other Vmore positive areas of the target surface tend to fall back or strike the target in the region of area C and drive this area to approximately a minus 3 volts negative, which is the potentialof the collector screen 24. At this potential, any more secondary electrons falling upon area C will tend to drive the area more negative than the collector under which condition all further secondary electrons approaching the area C will be repelled to other portions o'f Athe target surface or to-the collector electrode 24. Area B of Figure 3 indicates the conditions existing when a corresponding portion of the photocathode 4 is only partially illuminated by light. Under this condition, the number of photoelectrons are somewhat less than the number falling upon the area A of the target 3. Thus, the surface of area B is driven positive to a less degree than that of area A.

If a picture or view is focused upon the photocathode 4 and consists essentially of light, and dark,v and grey areas a corresponding charge pattern will be set up on thesurface of electrode 3 and in the manner described above. Such a charge pattern will produce a corresponding potential eld on the other side of target 3. Figure 3 schematically indicates by equipotential surface lines such a potential distribution as would exist from the areas A, YB and C, which are charged by different amounts. The scanning electron beam 9 is shown by dotted lines passing through the equipotental surfaces toward the discharge screen electrode 27.

Due to the negative potential established on the collector screen 24, all areas of target surface 3 are maintained negative to the potential of gun cathode 5. FDischarge srceen 27 is set at a positive 0.5 volt relative to the'potential of the gun cathode 5. The negative charges on the surface of target 3 establish a field which extends through the openings 28 of mesh 27 and has a configuration somewhat as schematically shown in Figure 3. As indicated above in Figure 4, at least some of the electrons of -beam 9 lwill approach any potential greater than zero or cathode potential but all of the electrons of the beam will be repelled or reflected by a potential of zero or less. Thus, in the region of A, where the potential of the target is substantially minus 1 volt to minus 1.5 volts, the negative potential fields extend through openings 28 to repel the beam electrons but they do not extend sufficiently far to prevent the electrons striking the metallic portions of the screen 27. In the region B where the target 3 is at a more negative potential, the potential surfaces vwill extend farther through the mesh of screen 27 to repel more of the beam electrons back to dynode electrode 8 and thus tend to prevent some of the beam electrons from striking the metal portions of the screen 27. In the region C, where the screen has been indicated as being around minus 3 volts negative, the negative potentials extend suticiently far through the openings of screen 27 to effectively block all of the beam electrons from landing on the opaque 6 portions of the screen 27. At this point, then, all of the beam is returned to the multiplierelectrode.

From the above description, it is thus clear that the potential charge pattern on the surface of target 3 will modulate the return beam 9'. Under the conditions described, the electrons landing on screen 27 are directly proportional to the amount of light striking corresponding surfaces of photocathode 4 and accordingly, the current of the return beam 9 is inversely proportional to the amount of light striking the photocathode surface 4 in areas corresponding to the charge pattern on the surface of screen 3. The output or video signal of the tube may be taken from either the collector of the multiplier section 1S, or from the discharge screen 27.

The tube of Figure 1 may also be operated on the portion C-D of the curve of Figure 2 by raising the potential or' the collecting screen v24 by a small voltage to a point where the charge pattern of the electrode 3 varies somewhere between a minus l volt and zero volts. Thus, areas of target 3 at a minus l volt potential well correspond to the darker or unlighted areas of the photocathode 4 while those at zero will correspond to the bright areas of photocathode 4. Figure 3 also indicates the operation of the target under these conditions. When the target surface 3 is at substantially zero volts, the screen 27 will absorb al1 of the beam which strikes the opaque portions of the mesh and that portion of the beam entering the apertures 2S will be reflected by the zero potential field surface. However, where the target surface 3 is substantially a minus 1 volt, as shown in Figure 3, the negative equipotential surfaces extend substantially through apertures 28 to form fields which will focus electrons of the beam away from the openings 28 toward the opaque portions of the screen 27. Under this condition then, screen 27 will collect a larger amount of the beam 9 than in the first case. C-D of the curve of Figure 2. Also, under this condition of tube operation, the video output signal of the tube collected by electrode 20 is directly proportional to the optical picture focused on the photocathode 4. 'I'hat is, there is a maximum beam current for the bright regions of the picture and minimum current for the dark regions.

It will be appreciated that under the conditions described the electrons landing on screen wires 28 do not discharge the charge pattern on the photocathode side of the target 3. However, with a moving scene in successive frames, as the light pattern changes, the secondary electrons emitted by photoelcctron bombardment of the glass target will not all go to the collecting screen 24, but enough will rain down on the elemental areas of the target surface to discharge the previous charge pattern. The discharge 0f the image by means other than the landing of beam electrons on the target areas has the important advantage that surface conditions on the screen electrode 27 may be varied to control the effects of the landing thereon without varying the etect of the charge-image on the beam.

With stationary scenes, the glass target will, of course, not be discharged at all, as the photoelectrons bring the potential up on the elemental areas and counteract the raining down of secondaries from adjacent areas. With stationary pictures, the illumination therefore builds up to saturation; the charges in successive frames being added to those of previous frames. This is an improvement over the usual pickup tube, in which the beam discharges the scanned areas, in that a greater charge-image is obtained through build-up. An important advantage of this action is that, in taking stationary television pictures with very low lighting, the full storage time of the target may be many times greater than a frame time. This permits one to construct a television pickup tube of extremely high sensitivity and time exposures may be taken, as in photography, while continuously transmitting the picture.

In case moving pictures are taken where the average light intensity suddenly decreases, the secondaries pro- This, then accounts for the portion 7 duced by the photoelectrons may not rain down on the surface of target 3 suciently to discharge the charge image in the desired time. In that case, the negative potential bias of screen 24 can be reduced so that some of the electrons of the beam will land and help discharge' the image.

If desired, the signal may be obtained from discharge screen 27 without multiplication and the multiplier structure omitted. This will be particularly feasible as the intensity of the beam current can be increased to bring up the signal value.

In accordance with my invention the tube of Figure 1 is modified and operated in a manner to provide control of secondary emission from the screen electrode Z7. The electron beam 9 should strike the discharge screen 27 with suticient energy to provide an electron emission therefrom. Under these conditions the beam will at the same time penetrate through screen 27 and strike the surface of the glass target 3. For this reason, then, the voltage of the beam is so chosen that the secondary emission ratio from the glass target 3 is unity or nearly so. It the energy of the electron beam 9 is established at substanially 30 volts positive relative to cathode potential, the secondary emission from the surface of the glass target 3 of the type disclosed above will be substantially unity. Furthermore, the secondary emission from the discharge screen 27 will be less than unity. To provide a large enough return beam 9', the primary beam current is then increased. Under such operating conditions, the electron beam 9 will establish a return beam 9' of secondary electrons which will be modulated in the manner described above by the charge pattern established by photoemission from cathode 4 on the target 3. The surface of target 3 scanned by beam 9, however, may be treated in any well known manner to reduce the secondary emission ratio of the glass surface so that the voltage of beam 9 may be raised to provide a secondary emission from the discharge screen 27 greater than unity. In either condition of operation, however, the decelerating ring electrode 25 is maintained at a positive potential of a few volts more positive than that established on the discharge screen 27.

In Figure I have indicated how image amplification may be obtained in accordance with my invention. Tube structures which are identical with those of Figure l are indicated with identical numbers, while similar structures are identitied by prime numbers. Between photocathode iilm 4 and dielectric target 3 are arranged two image amplifying sections consisting respectively of dielectric target sheets 3 and 3" discharge- mesh screens 27 and 27" collector mesh 24' and 24 and accelerating electrodes 23', 23" and 26', 26". These parts correspond in structure and operation respectively to parts 3, 24, 27, 23, and 26 of the tube of Figure l. The surfaces of discharge screens 27 and 27 are photosensitized in any well known manner and during tube operation are illuminated by light 30 from any appropriate light sources. Focusing coil is arranged in sections or withropenings to permit entrance of this light. The voltage sources indicated in Figure 5 will cause dielectric target sheets 3' and 3" to tloat negatively to their respective discharge screens 27' and 27".

In the manner described above, a charge pattern is established on the surface of dielectric sheet 3". This charge pattern on sheet 3" is negative to the potential of discharge screen 27" and will establish a negative potential field patttern which will extend through the openings of discharge screen 2 in the manner illustrated in Figure 3. Since screen 27" of Figure 5 is a photoemitter, a maximum photoemission takes place from the screen in the light areas, less in the grey areas, and substantially no photoemission from the dark areas. Thus the charge pattern on dielectric sheet 3" Will modulate or control the photoemission from screen 27 The photoelectrons leaving the sensitized screen 27 will be greater in number than the photoelectrons landing on the other opposite surface of the glass target 3" and will be proportional to the potential pattern of the charge image produced by the last-mentioned photoelectrons on target 3". Thus, this structure arrangement constitutes an amplifier stage. The amplified photoelectrons emitted by screen target 27" will be accelerated and focused onto another glass target 3' through collecting screen 24 to produce an amplified charge pattern on the surface of target 3' by emission of secondary electrons collected by screen 24. In a similar manner, as described above, the charge pattern on sheet 3 controls the photoelectrons emitted by photosensitized screen 27'. The photoelectrons from the second stage will be accelerated and focused on a third glass target 3 through collecting screen 24 and produce a further amplified charge image. The operation from this point on will be the same as already described for the tube of Figure l. Thus, an amplified charge image can be produced before scanning the beam 9 over the target. ln Figure 5, screen 27 need not be electron emitting as is described for the tube of Figure l, but may be used as is disclosed in my copending application Serial No. 608,663, now Patent No. 2,537,250.

The discharge screens 27 and 27" may be caused to emit electrons by thermal treatment under control of the charge image and the action would be similar to that just described. For example, screens 27 and 27" may consist of a screen shown in Figure 6 in which strands 31 are arranged so that a heating current from a source 32 may be passed through them in parallel, and cause them to emit electrons under control of the potential pattern of an adjacent glass target 3'. In this case, the focusing coil 15' may be of standard construction instead of the form shown in Figure 5. Obviously, the strands 31 of screen may be arranged in series instead of in parallel. ln the image amplifier of Figures 5 and 6 it would be preferable to make glass targets 3, 3' and 3" opaque to prevent scattering of light and reduction of contrast.

Figure 7 shows another application of my invention in which the charge pattern on glass target 3 is established by scanning the target with a high velocity beam produced from a conventional gun structure 40, to provide secondary emission from the target 3. Input signal potentials are applied to a control grid 42 of gun 40 to modulate the beam and establish a charge pattern on target 3. On the other side of the target 3, the tube of Figure 7 constitutes means providing a low velocity scansion and signal multiplication similar to that disclosed in Figures l and S. That is, screen 27, of Figure 7 may be sensitized y for secondary emission according to the invention of Figure l, or for photoemission in accordance with the teaching of Figure 5. V

Figure 8 discloses an image tube utilizing the image amplification disclosed in the tube of Figure 5 to produce a visual picture. Parts corresponding to similar structures in Figure 5 are indicated by the same numerals. In like manner screens 27 and 27 are photosensitized to provide an amplified electron image of the photoemission from cathode 4. In the last stage of the tube, the photoemission from screen 27 is modulated by the charge pattern on glass target 3. This electron image is acceler ated to fall on a collector comprising a tluorescent screen 46 deposited on the end of the tube wall to produce an amplified optical image corresponding to that focused upon the photocathode 4.

The storage tube of Figure 8 need not be limited to a photocathode for an image section for providing a charge pattern on the dielectric target 3". It is obvious that the kinescope gun section of Figure 7 can be provided in the tube of Figure 8 to lay down a modulated charge pattern on the target 3". The resulting tube then would be a picture tube in which the input signal is amplitied by the two stages of grid controlled photoemission, the final,

ammore# 9 picture being visually received on a iluorescent screen similar to 46 of Figure 8.

While certain specific embodiments have been illustrated and described, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

l. An electron discharge device comprising, an envelope, a dielectric target sheet mounted Within said envelope, an electron collector within said envelope and spaced from said dielectric target sheet, an electron emitting foraminous electrode closely spaced from one surface of said dielectric sheet and positioned between said dielectric sheet and said collector electrode for supplying electrons to said collector electrode, means including a cathode electrode within said envelope for establishing a charge pattern on the other surface of said dielectric target sheet by secondary electron emission, an electrode closely spaced from said other surface of said dielectric sheet for collecting the secondary electrons therefrom.

2. An electron discharge device comprising, an envelope, a dielectric target sheet mounted within said envelope, an electron collector Within said envelope and spaced from said dielectric target sheet, an electron emitting discharge screen electrode closely spaced from one surface of said dielectric sheet and positioned between said dielectric sheet and said collector electrode for supplying electrons to said collector electrode, means including a cathode electrode within said envelope for establishing a charge pattern on the other surface of said dielectric target sheet by secondary electron emission, an electrode closely spaced from said other surface of said dielectric sheet for collecting the secondary electrons therefrom, lead means connected to said discharge screen electrode for joining said discharge screen electrode to a source of potential, lead means connected to said collecting electrode for connecting said collecting electrode to a second source of potential to bias said collecting electrode suiciently negative to the potential of said discharge screen whereby said charge pattern will control the electron emission from said discharge screen.

3. An electron discharge device comprising, an envelope, a dielectric target sheet mounted within said envelope, an electron collector within said envelope and spaced from said dielectric target sheet, a foraminous electron emitting discharge screen electrode closely spaced from one surface of said dielectric target sheet and positioned between said dielectric sheet and said collector for supplying electrons to said collector electrode, said discharge screen being photosensitized to provide a photoelectric emission therefrom, means including a cathode electrode within said envelope for establishing a charge pattern on the other surface of said'dielectric target sheet by secondary electron emission, an electrode closely spaced from said other surface of said dielectric sheet for collecting the secondary electrons therefrom, lead means connected to said discharge screen for joining said discharge screen to a source of potential lead means connected to said collecting electrode for connecting said collecting electrode to a second source of potential to bias said collecting electrode suiiiciently negative to the potential of said discharge screen whereby said charge pattern will control the electron emission from said discharge screen.

4. An electron discharge device comprising, an envelope, a dielectric target sheet mounted within said envelope, an electron collector within said envelope and spaced from said dielectric target sheet, an electron emitting foraminous electrode adjacent to and closely spaced from said dielectric target sheet for supplying electrons to said collector electrode, said formaminous electrode being of a material having secondary electron emitting properties, means within said envelope for bombarding said foraminous electrode to provide secondary emission therefrom, means including a cathode electrode within said envelope for establishing a charge pattern on the other surface of said dielectric target sheet by secondary electron emission, an electrode closely spaced from said other surface of said dielectric sheet for collecting the secondary electrons therefrom, lead means connected to said foraminous electrode for joining said foraminous electrode to a source of potential, lead means connected to said collecting electrode for connecting said collecting electrode to a second source of potential to bias said collecting electrode sutliciently negative to the potential of said foraminous electrode whereby said charge pattern will control the electron emission from said discharge screen.

5. An electron discharge device comprising, an envelope, a dielectric target sheet mounted within said envelope, an electron collector within said envelope and spaced from said dielectric target sheet, a thermionic electron emitting discharge screen closely spaced from one surface of said dielectric target sheet and positioned between said dielectric sheet and said collector for supplying electrons to said collector electrode, means including a cathode electrode within said envelope for establishing a charge pattern on the other surface of said dielectric target sheet by secondary electron emission, an electrode closely spaced from said other surface of said dielectric sheet for collecting the secondary electrons therefrom, lead means connected to said discharge screen for joining said discharge screen to a source of potential, lead means connected to said collecting electrode for connecting said collecting electrode to a second source of potential to bias said collecting electrode sufficiently negative to the potential of said discharge screen whereby said ,charge pattern will control the electron emission from said discharge screen.

6. An electron discharge device comprising, an envelope, a dielectric target sheet, an electron gun including a cathode for directing a beam of low energy electrons onto one surface of said dielectric sheet, a secondary electron emitting foraminous electrode closely spaced from said one surface of said dielectric sheet, lead means connected to said foraminous electrode for connecting said foraminous electrode to a source of potential positive to the potential of said gun cathode during tube operation, a screen electrode spaced from the other side of said dielectric sheet, lead means connected to said screen electrode for joining said screen electrode to a source of potential negative to said gun cathode for biasing said dielectric sheet below the potential of said gun cathode, means including a source of electrons within said envelope for establishing a charge pattern on said dielectric target sheet.

7. An electron discharge device comprising, an envelope, a dielectric target sheet mounted within said envelope, an electron collector within said envelope and spaced from said dielectric target sheet, an electron gun including a cathode for directing a beam of low energy electrons onto one surface of said dielectric sheet, anelectron emitting mesh discharge screen closely spaced from said one surface of said dielectric sheet and positioned between said electron gun and said target sheet, a mesh collector screen electrode spaced from the other side of said target sheet, lead means connected to said collector screen for joining said collector screen to a source of potential negative to the potential of said electron gun cathode for biasing said dielectric sheet below the potential of said cathode, means including a photosensitized mesh screen spaced from said other side of said dielectric target for establishing a charge pattern on said dielectric target sheet.

8. An electron discharge device comprising, an envelope, a dielectric target sheet, an electron gun including a cathode for directing a beam of low energy electrons onto one surface of said dielectric sheet, an electron emitting discharge screen closely spaced from said one surface of said dielectric sheet, lead means connected cathode for biasing said dielectric sheet below the potential of said gun cathode, means including a source of electrons within said envelope for establishing a charge pattern on said dielectric target sheet, said last means including a second dielectric sheet spaced from said other side of said rst dielectric sheet, a photosensitized mesh screen closely spaced from said second dielectric sheet and between said two dielectric sheets, and means for providing a 110W of photoelectrons away from said photosensitized mesh screen toward said first dielectric sheet, and means within said enevelope for establishing a charge pattern on said second dielectric sheet to control the ow of electrons to said rst dielectric sheet.

9. An electron discharge device comprising, an envelope, a dielectric target sheet, a fluorescent screen within said envelope spaced from said dielectric sheet, an electron emitting mesh screen closely spaced from one surface of said dielectric sheet and positioned between said uorescentscreen and said sheet, a photo-sensitive coating on said mesh screen, lead means connected to said discharge screen to connect said screen to a voltage source means for providing a ow of photoelectrons from said mesh screen toward said fluorescent screen, a second mesh screen electrode spaced from the opposite side of said dielectric sheet, lead means connected to said second screen for joining said second screen lto a source of potential negative to the potential of said photosensitive screen for biasing said dielectric sheet below the potential thereof, means including a cathode electrode within said envelope for establishing a charge pattern on said dielectric target sheet.

10. An electron discharge device comprising, an envelope, a dielectric target sheet, a fluorescent screen spaced from said dielectric sheet, an electron emitting mesh discharge screen closely spaced from one surface of said dielectric sheet and between said uorescent screen and said dielectric sheet, lead means connected to said discharge screen to connect said screen to a voltage source, a photosensitive coating on said mesh discharge screen, means for providing a flow of photoelectrons from said discharge screen toward said fluorescent screen, a second mesh screen electrode spaced from the opposite side of said dielectric sheet, lead means connected to said second screen for joining said second screen to a source of potential negative to the potential of said photosensitive screen for biasing said dielectric sheet below lthe potential thereof, means including a cathode electrode within said envelope for establishing a charge pattern on said dielectric target sheet, said last means including a second dielectric sheet spaced from said other side of saidfirst dielectric sheet, a photosensitized mesh screen closelyrspaced from said second dielectric sheet and between said two dielectric sheets, and means for providing a ow of photoelectrons away from said photo sensitized mesh screen toward said first dielectric sheet, and means Within said envelope for establishing a charge pattern on said second dielectric sheet to control the ow of photoelectrons to said first dielectric sheet.

11. An electron discharge device comprising, an envelope, a dielectric target sheet, an electron gun ncluding a cathode for directing a beam of low energy electrons onto one surface of` said dielectric sheet, a mesh discharge screen electrode closely spaced from said one surface of said dielectric sheet, lead means connected to said discharge screen electrode for connecting said screen electrode to a source of potential positive to the potential of said gun cathode during tube operation, a second mesh screen electrode spaced from the other side of said` dielectric sheet, lead means connected tov said second screen electrode for joining said second screen electrode to a source of potential negative to said gun cathode for biasing said dielectric sheet below the potential of said gun cathode, means including a source of electrons within said envelope for establishing a charge pattern on said dielectric target sheet, said last means including an electron gun within said envelope for providing a beam of electrons along a path intersecting said other side of said dielectric sheet, and means for scanning the electron beam over said other side of said dielectric sheet.

References Cited in the tile of this patent UNITED STATES PATENTS 2,195,489 Iams Apr. 2, 1940 2,210,987 Schroter Aug. 13, 1940 2,227,015 Schlesinger et al Dec. 31, 1940 2,250,283 Teal July 22, 1941 2,280,191 Hergenrother Apr. 21, 1942 2,280,922 Lubszynski Apr. 28, 1942 2,300,591 Osawa NOV. 3, 1942 2,460,093 Law Ian. 25, 1949 2,470,875 Snyder May 24, 1949 2,498,082 Johnson Feb. 21, 1950 2,537,250 Weimer Jan. 9, 1951 2,544,753 Graham Mar. 13, 1951 2,547,638 Gardner Apr. 3, 1951 2,550,316 Wilder Apr. 24, 1951 2,558,647 Freeman June 26, 1951 2,570,858 Rajchman Oct. 9, 1951 2,687,492 Szegho et al. Aug. 24, 1954

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