US3433996A - Storage system - Google Patents

Description

March 18, 1969 P. E. CARNAHAN ETAL 3,433,996

STORAGE SYSTEM Filed June 13. 1966 FIG. I

AV/AVAV WITNESSES INVENTORS Paul E. Curnuhan and 4%. Donald C. Brooke ATTORNEY United States Patent 3,433,996 STORAGE SYSTEM Paul E. Carnahan, Colorado Springs, Colo., and Donald C. Brooke, Montour Falls, N.Y., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed June 13, 1966, Ser. No. 557,038

US. Cl. 3l512 Int. Cl. H013 29/41, 31/26 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a storage system and more particularly to an electron tube which includes a storage electrode for storage of optical and electrical information.

A storage tube is a device which may be used for applications when it is desired to store signals of variable intensities and read the information out at a later time. The tube may also be utilized for comparison of a stored image with another input image. It is also desirable in such a tube to provide means of reading out the stored image on the storage electrode without destroying the charge image on the electrode. In this manner, one is able to obtain a plurality of readout images of a given stored image. This is sometimes referred to as multicopy readout. It is also desirable to provide half-tone stored informtaion, that is, signals lying between the maximum and minimum signal applied to the tube and obtained in the readout information.

It is accordingly an object of this invention to provide an improved storage system.

It is another object to provide a storage target capable of storing an optical image and providing multi-copy readout thereof.

It is another object of this invention to provide a storage electrode which is capable of storage ofelectrical information and multi-copy readout thereof.

I It is another object of this invention to provide a storage system in which a first optical or electrical image may be stored on the storage electrode and in which a second optical or electrical image is then directed onto the storage electrode and means provided for reading out the difference in intensity of the first and second images.

It is another object of this invention to provide a storage system in which an optical and electrical image may be stored on a storage electrode simultaneously.

Briefly, the present invention accomplishes the above objects by providing an electronic tube having a storage target consisting of a conductive signal electrode, a photoconductive layer and a mosaic dielectric layer consisting of a plurality of islands of dielectric material. The tube is provided with an input so that an optical image may be projected through the optically transmissive conductive back-plate onto the photoconductive layer. An electron gun is provided for scanning an electron beam over the surface of storage target consisting of the photoconductive layer covered with the dielectric islands. The

method of operation and operating voltages of the system also provide th advantages set forth in the above objects.

These and other objects and advantages of th present invention will become more apparent when considered in view of the following detailed description and drawings, in which:

FIGURE 1 is a storage system partially in section in accordance with the teaching of this invention; and

FIG. 2 is an enlarged sectional view of the target illustrated in FIG. 1.

Referring in detail to FIG. 1, there is illustrated a storage system. An electron tube shown therein includes an evacuated envelope, an electron gun and scanning and deflection systems similar to those used in the conventional vidicon type pickup tube. The vidicon is a wellknown television pickup tube for producing a video signal for transmission. The tube shown in FIG. 1 comprises an envelope 10 of a suitable material such as glass. One end of the envelope 10 is closed by an end wall or face portion 12 through which electromagnetic waves in the form of optical information such as visible light from the scene enters and is directed onto an input screen 14. The end wall 12 is of a suitable material transmissive to the optical radiation directed onto the tube. Glass may be used in the case of visible light.

The interior surface of the viewing window 12 provides the support for the storage member or input screen 14. The storage member 14 includes an electrically conductive coating 16 transmissive to the input optical radiation. A suitable material for the electrically conductive coating 16 is stannic oxide. A layer 13 of a suitable optically sensitive material and more commonly referred to as a photoconductive material is deposited on the electrically conductive coating 16. The layer 13 may be of a suitable light sensitive material such as antimony trisulphide. A mosaic layer 15 is provided on the light sensitive layer 13 and is referred to in the art as a mosaic type layer. The mosaic layer 15 consists of a plurality of spaced islands 17 of a suitable dielectric material such as magnesium fluoride or silicon monoxide. The size of the islands 17 may be about 0.5 1O in. and approximately 10- islands may be provided per square inch.

The conductive coating 16 is the signal or back-plate of the target 14 and is provided with a lead 60 to the exterior of the envelope. The lead 60 is connected to a first terminal of a resistor 62. The second terminal of the resistor 62 is connected to a switch 63. The first terminal of the resistor 62 is also connected through a capacitor 64 to an output circuit 65. The switch 63 provides means of connecting a selected voltage source 67, 69, 71 or 73 to the back-plate 16 to provide the proper D.C. voltage thereto.

The other end of the tube envelope 10 is the base portion of the tube and lead-ins are provided (not shown) through the base to connect suitable potentials to the electrodes therein. The tube is provided with the necessary and Well known components needed to produce a beam of electrons and control the scan of the beam across the storage electrode 14 in a point-by-point manner. An electron gun 17 is provided within the tube adjacent the base portion to generate and form the electron beam. The electron gun 17 may be of any suitable design and includes a cathode 20, a control grid 28, a screen grid 30 and a beam focus electrode 32. The cathode 20 is comprised of a tubular sleeve 22 closed at one end facing the storage electrode 14. The closed end of the sleeve 22 is provided with a suitable thermionic electron emissive material 24 to provide electrons for the electron beam. A heater 26 is provided within the sleeve 22 to heat the coating 24. The electrons emitted from the cathode 20 are formed into an electron beam by the control grid 28 and the 3 screen grid 30. The cathode 20 is connected to a suitable voltage source 34 which may be at ground potential. A source 36 of video signals may be connected to the oathode 20 by a switch 38 in some applications of the invention.

The control grid 28 is supplied with a suitable negative potential of about 45 to 95 volts from a suitable source 40. It may be desirable in some applications to modulate the control grid 28 by connecting the video source 36 to the control grid 28. The screen or accelerator grid 30 is supplied with a positive potential of about 300 volts from a suitable source 42. The beam focus electrode 32 is supplied with a positive potential of about 250 volts from a source 44.

The electrons from the gun 17 are magnetically focused to a small area beam spot at the target 14 by a magnetic field derived from a focus coil 50. Alignment coils 52 may also be provided to correct the misalignment of the electron gun 17. A deflection yoke 54 is provided to deflect and scan the electron beam over the surface of the target 14. During operation, suitable voltages are applied to the deflection system 54 to provide the necessary and desired scan. A fine mesh screen 56 is mounted adjacent to the target 14. The screen 56 may be electrically connected to the electrode 32 and operate at the same potential.

An optical scan illustrated as item 68 is provided in front of the input window 12 and light therefrom is projected through a suitable lens 70 onto the target 14. In addition, a light source 75 with suitable switching means 72 is provided for uniformly illuminating the target 14 at desired intervals. A light shutter means 74 is provided in front of the input window 12 which may also be selectively switched.

The fabrication of the target 14 is accomplished by first providing a suitable glass face plate 12. The coating 16 of electrically conductive material such as stannic oxide may be sprayed onto the face plate 12 in a well known manner. The photoconductive material may then be evaporated onto the layer 16 by well known techniques to provide a layer 13 of a suitable thickness of about 1 micron. The photoconductive material is evaporated in a suitable vacuum to provide a metallic-like deposit. The mosaic coating 15 of a suitable material such as magnesium fluoride is deposited by evaporation onto the photoconductive layer 13 and a convenient way of accomplishing this is depositing through a mesh screen of a desired number of lines per inch which is held in intimate Contact with the conductive layer 16 during the evaporation.

In the operation of the device, the switch 38 is open and ground potential is supplied to the cathode 20 by the source 34. Other suitable voltages are applied to the elements in the tube. The shutter 74 is provided in front of the face plate 12 to prevent transmission of patterned light onto the target 14. Alternatively, the face plate 12 and target 14 may be flooded by a source of uniformly diffused illumination. The backing electrode 16 is connected by means of the switch 63 to the voltage source 67 which is at approximately 250 volts positive with respect to ground. This is about the same potential supplied to the collector mesh 56. The electron gun 17 is now operated in a manner to produce an unmodulated dense high velocity electron beam. The energy is between the first and second crossover potential of the scanned dielectric surface. The electron beam is scanned over the target 14. This phase of operation is commonly referred to as the erase cycle in that all surface areas of the dielectric layer 15 facing the electron gun 17 are charged to the potential of the collector 56. The exposed surface of the photoconructive layer 13 is also at a similar potential. The electron gun 17 is then turned off by suitable techniques such as removing the potential from the cathode 20. The potential on the backing electrode 16 is changed by means of switch 63 to the voltage source 69 which is at a positive potential of about volts. The exposed surface of the dielectric layer 15 follows the backing electrode potential change and therefore due to capacitive coupling is now at a potential of about a positive 10 volts.

The next step in the operation is the priming of the target 14. In this operation, the electron gun 17 is turned on and allowed once again to scan over the full area of the target 14. This time the electron gun 17 will charge all of the exposed surface of the target 14 to cathode potential which is at ground. The energy of the electrons is below first crossover potential. The electron gun 17 is then turned off and the potential on the backing electrode 16 is increased to a positive 20 volts by connecting the switch 63 to the voltage source 71. The shutter 74 is closed during this operation.

The tube is now ready for the write operation and the shutter 74 is opened. The optical image of the scene 68 is directed through the lens 70 onto the target 14. Again, the electron gun 17 is turned on with the cathode 20 at ground potential. The areas of the photoconductive layer 13 that are illuminated with the brighter portion of the optical image have their resistance substantially reduced in comparison to those areas that are illuminated by the darker portions. The lowered resistance reduces the ability, of these photoconductive areas to build up charge, permitting higher change to be built up on the superimposed dielectric islands 17. In the darker and higher resistance areas, the charge will divide between the capacitance of the photoconductive material and that of the dielectric islands 17 on those areas. The net charge on the dielectric islands 17 in the darker areas, then, is less than that which can be built up in the bright areas. The overall result is a charge pattern on the dielectric islands 17 proportional in all areas to the illumination thereon. The range of this pattern is from 10 volts to ground or cathode potential.

The readout operation can be accomplished at any suitable scanning rate and at a time substantially later than the write time. The readout is accomplished by connecting the back-plate 16 to a fourth potential source 73 which is at a positive potential of about 5 volts. The electron gun 17 is again turned on and the target 14 is uniformly illuminated by the light source 75. The light source may be an incandescent lamp to which the photoconductive layer 13 is sensitive. The illumination of the photoconductive layer 13 results in simultaneously lowering the resistance of the entire photoconductive layer 13. The dielectric areas 17 of the least negative charge allow the electrons to pass between the dielectric islands 17 to the lowered resistance photoconductive layer 13 where they are conducted out of the target connection and through the external load resistor 62. The voltage that results across the load resistor 62 is coupled to the output 65. The dielectric areas 17 or more negative charge pass fewer electrons and thus the current flowing through the load resistor 62 when the beam is scanning these areas is of a lesser value than the least negative scanned areas. When the beam is scanning over the whole target area, the voltage appearing across the load resistor 62 is proportional to charge on the target 14 for any instantaneous location of the electron beam. The amplified voltage may be fed to a cathode ray display tube whose beam is swept in synchronism with the beam scanning rate over the target in the storage tube. In this manner, a visual image will appear on the cathode ray tube which is a replica of the optical pattern originally imaged on the target 14 of the storage tube. As all dielectric areas 17 of the dielectric layer 15 are more negative than the cathode potential, in that, the potential is reduced 15 volts on the back-plate when the tube was switched from write to readout then no electrons are able to land on the dielectric islands 17 and accordingly on the stored image. The stored charge which is on the dielectric islands is thus in no way affected by readout of the information. The readout may be continuous and entirely non-destructive. The storage time of the device is limited only by positive ions produced by collisions between the beam electrons and the molecules of residual gases in the tube and bombarding the dielectric islands 17. When the electron beam is not being utilized in readout and the tube is turned off then there will be no ions produced and the storage time of the image on the dielectric islands 17 may remain for a period as long as several weeks.

Another possible modification in the operation of the system is to illuminate the target 14 with a second optical pattern instead of the uniform illumination during readout. The result is that the second optical pattern appears as a charge image on the photoconductive layer 13 as it would in a typical vidicon. The charge image of the second image is superimposed over the stored first image on the dielectric islands 17 which is being read out. This second charge image does not store but is erased each scan during readout. The polarity of the readout stored signal is opposite to that of the non-stored signal. Thus, if the stored pattern and the non-stored pattern are superimposed and are identical, then their outputs tend to cancel each other and no output would appear across the load resistor 62. If they are different then only this difference will appear across the load resistor 62. Thus, the device may be used as a moving target indicator or in a signal comparison system.

The stored image may be erased as previously described whenever desired by raising the backing electrode to collector potential and scanning the dielectric as a whole or by scanning only in that area that it is desired to erase the charge image.

Electrical signals may also be stored on the tube, either simultaneous with the writing due to the excitation of the optical pattern or alone. These electrical signals may be written by application of the signals to either the target 14 or the electron gun 17. For example, the video signal source 36 could be connected by means of the switch 38 to the electron gun 17. Once the information is written onto the target, the backing electrode potential is reduced to bring all dielectric surface areas below cathode potential. The charge pattern may then be readout without destroying charge image throughout continuous reading cycles. Erase, prime, write and read voltages and their sequence of application are about the same as used for optical storage. Electrical signals thus stored are of high quality. The device is capable of high resolution, many grey scales (greater than 8), high signal-to-noise ratio and high signal output level.

The device is also capable of operating as a conventional vidicon. Its characteristics, resolution, sensitivity, shading, etc. will be substantially the same as a vidicon.

Various modifications may be made within the spirit of the invention.

We claim as our invention:

1. A storage system comprising an evacuated envelope and having therein a storage target and an electron gun, said storage target positioned at one end of said envelope for receiving input radiations directed thereon from the exterior of said envelope, said target comprising a continuous photoconductive layer, an electrically conductive coating on one surface of said photoconductive layer transmissive to said input radiations and a plurality of dielectric islands provided on the opposite surface of said photoconductive layer and facing said electron gun.

2. The system described in claim 1 in which said input radiations directed onto said target establish a charge pattern on said dielectric islands and means for scanning an electron beam over the surface of said target to derive a signal from said back plate without affecting the charge pattern on said dielectric islands.

3. A storage system comprising an electronic tube including a storage target and an electron gun, said storage target including a layer of electrically conductive material, a layer of photoconductive material and a plurality of dielectric islands provided on one surface of said photoconductor and facing said electron gun, means for directing optical pattern onto said storage target with said layer of electrically conductive material at a first potential While simultaneously scanning said storage target with said electron beam to establish a charge pattern on said dielectric islands representative of the information within said optical pattern, means for changing the potential on the conductive back-plate from said first potential to a second potential, said second potential being of a less positive value than said first potential and directing an electron beam of predetermined energy onto said storage target in which the number of electrons landing on the photoconductive layer between said dielectric islands is modulated by the amount of charge on said dielectric islands to derive a signal from said electrically conductive layer representative of the charge pattern on said storage target and in which the electrons do not substantially strike said dielectric islands so that said charge pattern is not erased during readout operations.

4. The method of operating a storage device in which a storage target is provided including an electrically conductive layer, a continuous layer of photoconductive material and a mosaic layer of dielectric material on said continuous photoconductive layer and in which an electron beam is provided for scanning a first surface of said target including the exposed surface portion of said photoconductive layer and said dielectric layer, providing a first potential on said conductive layer of suflicient positive potential to permit charging of said first target surface of said target to a first positive charge potential corresponding to the potential of an adjacent grid electrode, providing a second potential on said conductive layer of a lower potential than said first potential to permit charging by said electron beam of said first surface of said target to a second charge potential corresponding to the cathode of said electron gun, providing a third potential on said conductive layer of a higher potential than said second potential and lower than said first potential and directing an optical image onto said target while simultaneously scanning said target with an electron beam to store a charge pattern on said dielectric layer representative of the information within said optical image and providing a fourth potential on said conductive layer of a lower potential than said third potential so that said charge pattern on said dielectric is negative with respect to said cathode of said electron gun and directing uniform illumination onto said target while simultaneously scanning said first surface of said target with the electron beam from said electron gun to derive an output signal from said target representative of said charge pattern on said dielectric layer without substantial electron beam landing on said dielectric layer to provide non-destructive readout of said charge image.

References Cited UNITED STATES PATENTS 2,730,639 1/1956 Johnson 31367 2,839,699 6/1958 Szegho et al. 313-66 2,963,604 12/1960 Weimer 3l366 RODNEY D. BENNETT, Primary Examiner. JEFFREY P. MORRIS, Assistant Examiner.

US. Cl. X.R. 313-66, 91

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