US3408530A

US3408530A - Scan conversion storage tube and method of operation

Info

Publication number: US3408530A
Application number: US412713A
Authority: US
Inventors: Jr Charles B Gibson
Original assignee: Tektronix Inc
Current assignee: Tektronix Inc
Priority date: 1964-11-20
Filing date: 1964-11-20
Publication date: 1968-10-29
Anticipated expiration: 1985-10-29

Description

Oct. 29, 1968 c. B. GIBSON, JR

SCAN CONVERSION STORAGE TUBE AND METHOD OF OPERATION 2 SheetsSheet 1 Filed Nov. 20, 1964 CHARLES E. GIBSON JR.

.zm 58 BE; T mxm mw kw w h. PNLPENWW BUC/(HO/P/V, 52095, KLAROU/ST a SPAR/(MAN ATTORNEYS Oct. 29, 1968 c. B. GIBSON, JR 3,408,530

SCAN CONVERSION STORAGE TUBE AND METHOD OF OPERATION Filed Nov. 20, 1964 2 Sheets-Sheet 2 EQUALIBRIUM POTENTIAL 9 3 w k Q Z 9 P E w 0 m m I o I; Q o

, J I I BUCK/405W, BL OPE, KLAROU/ST 8 SPAR/(MAN ATTORNEYS United States ABSTRACT OF THE DISCLOSURE A scan conversion storage tube is described which writes a charge image on its storage dielectric by electron beam deflection signal modulation and reads out the charge image by uniformly scanning such storage dielectric with an electron beam to produce an electrical readout signal corresponding to such charge image. The tube is operated in a secondary electron redistribution mode during writing and reading to provide a loW potential gradient above the surface of the storage dielectric which enables charge images to be stored closer together. An electron multiplier may be employed within the storage tube to amplify the electrical readout signal and provide it with a high signal to noise ratio. The storage dielectric is preferably of aluminum oxide but can also be a photosensitive material to enable the use of light for writing or erasing of charge images.

The subject matter of the present invention relates generally to electron image storage apparatus, and in particular to scan conversion type storage tubes and methods of operation of such tubes.

Briefly, in the embodiment of the scan conversion storage tube of the present invention a storage target in the form of a layer of dielectric material on a conductive target electrode, is employed along with an electron multiplier so that when the storage dielectric is scanned in a predetermined manner by an electron beam to emit secondary electrons, such secondary electrons are transmitted to the electron multiplier to produce an electrical readout signal of increased amplitude at the output of such multiplier which corresponds to a charge image stored on such dielectric layer. In order to improve the operation of the storage tube, a collector electrode provided Within the tube is maintained at substantially the same DC. voltage as the target electrode, and the storage dielectric is first scanned uniformly by bombarding it with electrons to cause secondary emission until such dielectric is charged substantially uniformly to an equilibrium potential slightly positive with respect to the voltage of the target electrode. Then a writing beam of electrons is directed onto the storage dielectric and deflected in accordance with an input signal to form a charge image thereon by causing the emission of secondary electrons from written portions of the dielectric which are collected by the collector electrode until the potential of the bombarded region of the target comes to a new equilibrium potential. When this condition has been reached, the secondary electrons will then be returned to, and redistributed on the target. In this manner, the charge initially on the written target area is redistributed to other target areas to maintain the average potential of the storage dielectric substantially the same as it was before Writing. After the charge image is Written onto the storage dielectric, a reading beam of lower current density is scanned across the surface of the storage dielectric in a predetermined manner to emit secondary electrons therefrom in amounts varying with the charge distribution, and at least a portion of which are transmitted to the electron multiplier to produce an electrical readout signal corresponding to the charge image atent stored on such dielectric. The above described secondary electron redistribution mode of operation of the storage tube produces lower potential gradients along the surface of the storage dielectric and enables charge images to be stored closer together on such dielectric without being destroyed due to image spreading.

The scan conversion storage apparatus of the present invention has several advantages over conventional storage apparatus. As stated above, there is a lower potential gradient along the surface of the storage dielectric, and this enables a smaller target to be employed for storing the same amount of information because separate charge images can be stored closer together. In addition, by employing an electron multiplier to produce the electrical readout signal, the signal to noise ratio of such readout signal can be increased because the target electrode can then be connected directly to AC. ground without employing a load resistor, and the thermal noise signal ordinarily produced in such resistor is not added to the readout signal. In addition, the electron multiplier produces less noise when amplifying the signal than a conventional external amplifier circuit so that it is possible to transmit the readout signal directly from the output of the multiplier to the z-axis input of a monitor cathode ray tube with less additional amplification than would otherwise be required. Also, a reading electron beam of smaller diameter and less current can be employed to scan the storage target because the electron multiplier can distinguish secondary electron signals of lower amplitude.

It is generally true of a scan conversion storage tube that the diameter of the electron beam employed to Write the charge image on the storage target, can be of smaller diameter than that employed in a direct viewing storage tube, and this enables the use of a smaller diameter target. In addition, the cath-ode-to-target acceleration voltage can be lower in the scan conversion tube because a greater acceleration potential is required in a direct viewing storage tube in order to increase the brightness of the light image produced on its fluorescent screen to an acceptable viewing level. Because of the smaller target size and the simpler structure of the present scan conversion tube, such tube can be made much smaller in diameter and shorter in length than conventional storage tubes. The increased deflection sensitivity of the present storage tube due to the smaller deflection angles and lower acceleration potential, enables the electrostatic deflection plates in such tube to be reduced in size to reduce the capacitance of such plates and to lower the transit time of the electrons passing through such plates so that the tube can operate at higher deflection signal frequencies. In addition, the present scan conversion storage tube can store half tone charge images and may be provided with a continuously variable persistence by varying the current density of the electron beam during reading. Furthermore, the present tube is simpler to operate because the potentials of the target and collector electrodes need not be varied during operation.

It is, therefore, one object of the present invention to provide an improved electron image storage apparatus and method of operation.

Another object of the invention is to provide an improved scan conversion storage tube of small size, simple and inexpensive construction and superior performance.

A further object of the present invention is to provide an improved scan conversion storage tube in which an electron multiplier is employed to increase the signal to noise ratio of the electrical readout signal transmitted from such tube, to enable a lower beam current to be used and longer reading times to be realized.

An additional object of the invention is to provide an improved scan conversion storage tube of greater deflection sensitivity and higher frequency response which is also capable of half-tone storage.

Still another object of the invention is to provide an improved method of operation of a storage tube for storing charge images on the storage target in such tube with a lower potential .gradient along the surface of the storage dielectric to reduce the tendency of the images to spread and to enable the storage of images closer together.

A still further object of the present invention is to provide improved method of operation of a scan conversion storage tube in which the voltage of. a target electrode in the storage target of such tube is maintained substantially constant and equal to the voltage on a collector electrode adjacent such target, so that the storage dielectric of the target can be initially charged to a substantially uniform equilibrium potential slightly more positive than the voltage of the target electrode and a charge image canv be formed on such storage dielectric by secondary electron redistribution while maintaining the average potential of the storage dielectric substantially constant.

Other objects and advantages of the present invention will be apparent from the following detailed description of certain preferred embodiments thereof, and from the attached drawings of which:

FIG. 1 is a schematic diagram of one embodiment of the storage apparatus of the present invention;

FIG. 2 is a schematic view of a second embodiment of a scan conversion storage tube which may be employed in place of the storage tube in the apparatus of FIG. 1;

FIG. 3 is a schematic view of a third embodiment of the storage tube in which a photo-cathode is employed to produce the charge image on the storage target, and which may also be substituted for the storage tube in the apparatus of FIG. 1; and

FIG. 4 is a diagram of the potential gradient across a portion of the storage dielectric employed in the tubes of FIGS. 1 to 3, and the current of the electrical readout signal produced by scanning such portion of the storage dielectric with an electron beam.

One embodiment of the scan conversion storage apparatus of the present invention is shown in FIG. 1 and includes a storage tube which has an evacuated envelope 12 of glass or other insulating material, containing a storage target 14 mounted at one end of such envelope and an electron gun 16 mounted adjacent the other end of such envelope. The storage target 14 may be in the form of a metal backplate 18 having a layer 20 of storage dielectric material coated on one side of such backplate and exposed to the electron beam emitted by the electron gun 16. The storage dielectric employed in layer 20 may be made of any suitable insulating material, such as aluminum oxide (A1 0 having a high dielectric constant and a large value of resistivity, as well as a high secondary electron emission ratio and good thermal conductivity. When aluminum oxide is used as the storage dielectric, the dielectric layer 20 has a substantially uniform thickness which may be any thickness below approximately 20,000 angstroms down to about angstroms. For special applications it might be desirable to employ a photo-conductive dielectric material which changes its resistivity upon illumination of the target surface with' light to write the image on such target or to erase a previously written image from such target. A photo-emissive material can also be employed as the storage dielectric or as the target electrode to enable a charge image to be written directly on a storage target by illumination of such target with a light image to cause photo-electrons to be emitted from such dielectric or such electrode. This limit image could be a graticule scale or the geographical background seen on the viewing screen of a radar apparatus.

The electron gun 16 includes an indirectly heated cathode 22 which may be connected to a negative D.C. voltage source of l300 volts, and a control grid electrode 24 connected to the movable contact of a selector switch 26. The selector switch 26 may be a two-position switch which connects the control grid to the movable contacts of a pair of

potentiometers

27 and 29 each having their end terminals connected between negative DC. voltage sources of 1300 volts and 1350 volts. The setting of the potentiometer '29, connected to the Scan position of switch 26 is usually at a morenegative voltagethan that of potentiometer 27 in the Read position of such switch in order to reduce the current density of the electron beam emitted by cathode 22 during the readout operation of the storage tube. The electron gun also includes a first anode 28 which may be connected to a positive D.C. voltage source of +175 volts, a focus electrode 30 which may be connected to the movable arm of a potentiometer 32 whose opposite end terminals are connected to negative DC. voltage sources of l000 volts and l200 volts, respectively, and a second anode 34 connected to the movable contact of a potentiometer 36 whose opposite end terminals are connected to positive DC. voltage sources of +200 and volts, respectively. A pair of horizontal deflection plates 38 and a pair of vertical deflection plates 40 are mounted within the envelope 12 between the electron gun 16 and the storage target 14. These deflection plates, as well as the electrodes of the electron gun, may be mounted between a pair of insulator support plates 42 of mica or other suitable insulating material, in order to form a rigid box-like structure which maintains such electrodes in axial alignment. In addition, a plurality of metal pins 44 are provided through one end of the envelope for electrical connection to the electrodes within such envelope.

The storage target 14 is mounted within the envelope 12 on a metal support ring 46 which extends through the side of the envelope and connects the backplate or target electrode 18 to the common terminal of a pair of series

voltage divider resistors

48 and 50. The other terminals of

resistors

48 and 50 are connected to a positive DC. voltage source of +200 volts and to ground, respectively, so that the DC. voltage applied to such target electrode is volts. A collector electrode 52 in the form of a hollow metal cylindrical member or coating on the inner surface of the envelope, is positioned immediately adjacent the storage dielectric layer 20 of the target 14 and connected to a positive DC. voltage of +175 volts which is made equal to the DC. voltage applied to the target electrode for reasons hereafter described. Thus, the storage target and collector electrodes are sufliciently positive with respect to the voltage of the cathode 22 that the secondary emission ratio of the dielectric layer 18 is greater than unity so that more than one secondary electron is emitted for each primary electron bombarding such dielectric layer. A field corrector electrode 54 in the form of a hollow metal cylinder or wall coating may be provided between the collector electrode 52 and the vertical deflection plates 40 in order to provide a more uniform electrical field along the surface of the dielectric layer 20 of the storage target. The field corrector electrode is connected to the movable contact of a potentiometer 55 whose end terminals are connected between posi tive DC. voltage sources of +225 volts and +175 volts.

The horizontal deflection plates 38 are connected through a two position selector switch 56 to a horizontal sweep signal generator 58 in the Write position of such switch and to a raster signal generator 60 in the Scan position of such switch. The vertical deflection plates 40 are connected through another two position selector switch 62 whose movable contact is ganged to that of switch 56, to a vertical amplifier 64 in the Write position of such switch and to the raster signal generator in the Scan position of such switch. The vertical amplifier 64 is connected to an input terminal 56 to which the input signal to be stored on target 14 is applied. A portion of this input signal may be transmitted to the horizontal sweep generator to trigger such sweep generator and in that event the vertical amplifier circuit 64 would be provided with a delay line to enable the input signal to be applied to the storage tube at the same time as the horizontal sweep signal generated thereby.

The raster signal generator 60 produces ramp voltages of different frequency to cause the electron beam emitted from electron gun 16 to scan the surface of the storage dielectric layer 20 on the target in a predetermined manner, such as the rectangular pattern of a television raster. It should be noted that appropriate blanking circuitry is provided for the storage tube to apply negative voltage blanking pulses to the control grid during the retrace or flyback portion of the raster signal, as well as during other operations of the tube, in a conventional manner which has not been shown here. Raster signals related in frequency to the raster signals applied to the storage tube, are applied to the horizontal and vertical deflection plates of a monitor cathode ray tube 72. The electrical readout current produced in the storage target electrode is transmitted through the

load resistors

48 and 50 to generate a readout signal voltage which is transmitted through a coupling capacitor 74 to a wide band video frequency amplifier 76. The output of the amplifier 76 is connected to control grid or cathode z-axis input of the monitor tube 72 in order to modulate the intensity of the beam within such monitor tube in accordance with the readout signal to reproduce the waveform of the charge image stored on dielectric layer 20 of the storage target of scan conversion storage tube 12 as a light image on the fluorescent screen of the monitor tube.

The operation of the storage apparatus of FIG. 1 and the secondary electron redistribution mode mentioned previously, will now be described in greater detail. Before a charge image is produced on the dielectric layer 20 of the storage target, such dielectric layer is initially charged substantially uniformly to an equilibrium potential slightly positive with respect to the DC. voltage applied to the target electrode 18. This may be accomplished by moving selector switches 26, 56 and 62 to the Scan position to scan the electron beam across the surface of the dielectric layer 20. The equilibrium potential of the dielectric is about +178 volts, or 3 volts positive with respect to the target electrode. Since the collector electrode 52 operates more efficiently with respect to the secondary electrons emitted from dielectric layer 20 in those regions around the periphery of such dielectric layer more closely adjacent the collector electrode than it does with respect to secondary electrons emitted from the centrally located portions of the dielectric, the field of the dielectric layer tends to be non-uniform. However, the effect of the field corrector electrode 54 compensates for this to provide a substantially uniform potential gradient along the surface of the target layer 20 during the charging step.

After the uniform charging step, a charge image is written on the dielectric layer of the storage target by moving the ganged

selector switches

26, 56 and 62 to the Write position. This transmits the input signal applied to input terminal 56 to the vertical deflection plates 40 of the storage tube and causes a time base sweep signal to be transmitted to the horizontal deflection plates 38 so that a charge image of such input signal is formed on the dielectric layer 20 of the storage target by an electron beam of different current density.

As shown in FIG. 4, a curve 78 representing the potential gradient across the storage tube adjacent a written portion of the dielectric layer 20 of the storage target increases from zero to about +3 volts at the edge of the storage target and remains at this equilibrium potential until a portion 80 of the charge image stored on the dielectric layer is reached. While FIG. 4 is highly simplified, the potential of the charge image portion 80 is a positive peak rising above the equilibrium potential and target areas immediately surrounding the charge image are slightly negative with respect to such equilibrium potential, as indicated by valley portions 82 and 84. These valley portions are produced by the collection of secondary electrons emitted by the charge image portion of the target, on the dielectric layer due to positive potential of such layer with respect to the collector electrode 52. Of course, some of the secondary electrons emitted by the written portions of the dielectric layer are collected by unwritten target portions more remote from the charge image, but this does not appreciably affect their potential. As a result of this secondary electron redistribution of the charge, the average potential of the storage dielectric layer is maintained substantially the same after writing as it was before the writing step. Thus, it can be seen that the potential gradient along the dielectric layer 20 is very low after the charge image has been written on such layer in the secondary electron redistribution mode of operation of the present invention. This low potential gradient enables charge images to be stored closer together on the dielectric layer 20 without destroying such images due to charge spreading so that the target is capable of storing more information per unit area than previously.

After writing, the charge image may be stored on the dielectric layer of the storage target for a limited time, which can be greater than one hundred hours. It should be noted that the torage time or persistence of the charge image depends upon the current density of the writing beam and may be varied by changing the setting of the movable contact of potentiometer 27 connected to switch 26 in the Write position of such switch.

The stored charge image is converted to an electrical readout signal by a reading step in which the selector switches 26, 56 and 62 are moved to the Scan position in order to change the cur-rent density of the electron beam to the appropriate value and to scan such beam over the dieelctric layer of the storage target by applying Taster signals to the horizontal and vertical deflection plates to produce an electrical readout signal on the conductor lead connected to the target electrode 18. As shown in FIG. 4, the readout current waveform corresponds to the charge image stored on such dielectric layer so that such current decreases to form a negative going valley portion 87 when the beam crosses the charge image portion 80. This electrical readout signal is transmitted through the coupling capacitor 74 and the video amplifier 76 to z-axis input of the monitor tube 72. Since the raster signal generator 60 is also connected to the horizontal and vertical deflection plates of the monitor tube, a television picture of the charge image stored on the dielectric layer of the storage target 14 is reproduced on the fluorescent screen of such monitor tube.

Of course, the screen of the monitor tube 72 can be much larger than the storage target 14 in order to magnify the image of the input signal stored on such target. Also it is possible to scan only a portion of the charge image on such target in order to magnify such image portion and present it over the entire screen of the monitor tube. In addition, separate additional electron gun (not shown) may be provided to accomplish the reading step simultaneously with the writing step described above. This may be accomplished by mounting the addition electron gun on the same side of the storage target as electron gun 16, or by mounting such additional electron gun on the opposite side of the storage target from gun 16. When the reading gun is mounted on the same side of the storage target, its axis will be skewed slightly with respect to the axis of the writing gun so that there will be some distortion in the readout signal due to the registration error of the reading beam. This trapezoidal error may be corrected electronically, or alternatively, the reading gun may be mounted on the opposite side of the target from the Writing gun with its axis in alignment with that of such writing gun. However, this has the disadvantage that the target electrode 18 must be changed to a mesh electrode and the dielectric layer 20 must be provided in the form of a thin self-supporting member of aluminum oxide, or other dielectric material, similar to that shown in FIG. 3. Such a storage dielectric member is extremely fragile and can be destroyed quite easily by the heating action of the electron beam. For these reasons the preferred embodiment of the present invention employs the same electron gun for reading and writing since a strong storage target structure can be employed and there is no registration error between reading and writing beams.

If the readingstep is carried out a sufiicient number of times, the stored charge image will eventually be destroyed and the potential of the dielectric layer will become substantially uniform at a voltage equal to the equilibrium potential of the target. Therefore, in order to erase the charge image and to simultaneously charge the dielectric layer to a uniform equilibrium potential, the selector switches 26, 56, and 62 are maintained in the Scan position and the potentiometer 29 is set at voltage level which will make the current density of the scanning beam sufficient to cause the image to be completely erased during readout. Thus, the setting of potentiometer 29 controls the persistence of the charge image and the electrical readout signal as well as the erase time of the tube.

It should be noted that half-tone charge images can be stored successfully by reducing the current density of the electron beam during the writing step so that the voltage of the dielectric layer does not quite reach equilibrium potential before a charge image is Written the desired number of times onto such layer. When halftone charge images are stored, the electrical readout signal will be linear with respect to the potential of different portions of the charge image.

In order to increase the signal to noise ratio of the electrical readout signal transmitted from the scan conversionstorage tube, the modified storage tube 10 of FIG. 2 can be employed in place of the storage tube 10 in the apparatus of FIG. 1. The storage tube of FIG. 2 is similar to the previously described tube and differs from it primarily by employing an electron multiplier 86 within the tube envelope on the opposite side of the storage target 14 from the electron gun 16. Other tube components are similar to the storage tube 10 and are identified by the same reference numbers as such tube. The electron multiplier 86 consists of a plurality of annular first, second, third and fourth dynode stages 88, 90, 92 and 94, and an anode 96 between the third and fourth dynodes which is connected to an output terminal 98 through the coupling capacitor 74. It should be noted that the electrons emitted from the third dynode stage 92 are transmitted through the anode 96 due to its mesh-like structure and strike the fourth dynode stage 94 to emit secondary electrons therefrom which are transmitted back to such anode. Also, the anode 96 is connected to a DC. voltage source of about +500 volts across

voltage divider resistors

48 and 50 which function as the load resistance of the multiplier. The electron multiplier also includes a collector mesh 100 which is connected to a positive DC. voltage source of about +250 volts which is positive with respect to the target electrode 18 and collector electrode 52 so that such collector mesh attracts the secondary electrons emitted from the dielectric layer 20 of the storage target into the first dynode stage 88 of the multiplier.

The scan conversion storage tube of FIG. 2 operates in a similar manner to that of FIG. 1 except that during the reading step the secondary electrons emitted from the dielectric layer of the storage target as it is being scanned by the reading beam, are transmitted through the dynode stages of the electron multiplier and amplified to a large current which is transmitted to output terminal 98 as a readout signal of much greater amplitude than the readout signal of the tube of FIG. 1. Since the readout current is greatly amplified by the electron multiplier before it is transmitted through the

voltage divider resistors

48 and 50, the signal to noise ratio of the electrical readout signal is much greater than that of the readout signal of the tube of FIG. 1. This is due to the fact that in FIG. 1 the

load resistors

48 and 50 are directly connected to the target electrode 18 and such load resistors cause noise to be introduced into the output signal before it is amplified by the video amplifier 76. Also the electron multiplier itself introduces less the video amplifier, and the readout signal transmitted to output terminal 98 of FIG. 2 can be applied to the z-axis input of the monitor cathode ray tube with less amplification than the tube of FIG. 1. Thus, the scan conversion storage tube of FIG. 2 has several distinct advantages over conventional scan conversion storage tubes. In addition, the electron multiplier permits longer storage times because a reading beam of lower current density can be employed. Also better image resolution can be obtained because of the smaller spot size and current densities which can be employed for the electron beam during writing, and lower cathode to target accelerating voltages can be employed. At this point it is perhaps worth mentioning that electron multipliers have been previously used in television camera tubes of the image orthicon type in which the primary electrons are repelled away from a mesh storage target and returned as the input signal to the multiplier. This is entirely different from the use of such multiplier to amplify secondary electron current emitted from a non-foraminous storage target in the manner of the scan conversion tube of the present invention.

Another embodiment of the present scan conversion storage tube is shown in FIG. 3. The modified storage tube 10" of FIG. 3 employs an electron multiplier 86 having its anode 96 connected to the output terminal 98 in a similar manner to the tube of FIG. 2. However, the electron multiplier in the tube of FIG. 3 is positioned on the same side of the storage target 14' as the electron gun 16. In addition a photocathode 102 is provided in the tube as a coating of photo-emissive material, such as antimony with a cesium activator therein, on the inner surface of the face plate portion of the tube envelope. The storage target 14 is formed by a self-supporting member 20' of aluminum oxide or other dielectric material mounted within an annular metal support ring 104 which also supports a mesh electrode 106 spaced a short distance from the surface of the dielectric member on the side of such member adjacent the photocathode. The mesh electrode is connected through the support frame 104 to a positive DC. voltage source of +175 volts, which is substantially the same as that of the collector ring 52 on the opposite side of the dielectric member so that such mesh electrode functions in the manner of the target electrode 18 in the storage tubes of FIGS. 1 and 2 to maintain the voltage on the dielectric member at the equilibrium potential before writing the charge image thereon. In addition, such mesh electrode 106 also functions as a collector electrode for collecting the secondary electrons emitted from the right side of the dielectric member 20' in FIG. 3 due to bombardment of such dielectric member by the photoelectrons emitted from photocathode 102. The photo cathode emits an electron image when it is exposed to a visible light image.

In order to focus the electron image emitted by photocathode 102, onto the dielectric member 20' of the storage target 14', a focusing electrode 108 which in the form of an annular metal coating on the inner surface of the side walls of the glass envelope 12, is provided between such photocathode and the target. The photocathode 102 may be connected to ground and the focusing electrode 108 connected to the movable contact of a potentiometer 110 whose end terminals are connected between volts and ground in order to vary the voltage applied to such focusing electrode. It should be noted that in the scan conversion tube of FIG. 3, the electron beam emitted by cathode 22 functions primarily as a reading beam to scan the dielectric member 20 since the charge image is noise into the readout signal than Written on the opposite side of such dielectric member by the electrons emitted from photocathode 102. However, the electron gun 16 may also be employed to write a charge image on the dielectric member in a similar manner to the tube of FIG. 2. Also as stated previously, a second electron gun may be employed on the opposite side of the storage target 14' from the electron gun 16, in place of the photocathode 102 if it is desired to employ separate electron guns for performing the reading and writing steps in the operation of the storage apparatus of the present invention.

It will be obvious to those having ordinary skill in the art that various changes may be made in the details of the above described embodiments of the present invention without departing from the spirit of the invention. For example switches 26, 56 and 62 may be vacuum tubes or transistors controlled by suitable circuits for automatic operation of the storage apparatus. Therefore, the scope of the present invention should only be determined by the following claims.

I claim:

1. A scan conversion type electron image storage apparatus comprising:

an evacuated envelope;

a storage target mounted Within said envelope, said target including a storage element of dielectric material provided on a target electrode;

a collector electrode supported within said envelope adjacent said target;

means connecting said target electrode to substantially the same DC voltage as said collector electrode;

writing means for deflecting an electron beam across the storage element in accordance with an input signal to produce a charge image on said storage element by secondary electron redistribution; and

scanning means for scanning the storage element substantially uniformly with a beam of electrons to initially charge said element to an equil brium potential of at least as positive a voltage as said collector elec- .trode prior to Writing, and to produce an electrical readout signal corresponding to the charge image stored on the storage element.

2. A scan conversion type electron image storage apparatus comprising:

an evacuated envelope;

a storage target mounted within said envelope, said target including a layer of dielectric material provided over a target electrode;

a collector electrode supported within said envelope adjacent said target;

means connecting said target electrode to substantially the same DC. voltage as said collector electrode;

means for bombarding the dielectric layer substantially uniformly with electrons to initially charge the bombarded surface of said dielectric to an equilibrium potential at least as positive as the voltage on said collector electrode;

Writing means for deflecting an electron beam across the dielectric layer in accordance with an input signal to produce a charge image on said layer by secondary electron redistribution which includes causing secondary electrons to be emitted from written portions of said layer and collected by unwritten portions of said layer; and

reading means for scanning the dielectric layer with a beam of electrons in a predetermined manner to cause secondary electrons to be emitted from said layer to produce an electrical readout signal corresponding to the charge image stored on the dielectric layer.

3. A scan conversion type electron image storage apparatus comprising:

an evacuated envelope;

a storage target mounted within said envelope, said target including a storage element of dielectric material supported adjacent a target electrode;

a collector electrode supported within said envelope adjacent said target;

an electron multiplier mounted within said envelope adjacent said collector electrode and said target in position to receive secondary electrons emitted by the dielectric element;

Writing means for bombarding the dielectric element with electrons to produce a charge image on said element by secondary electron emission; and

reading means for scanning the dielectric element with a beam of electrons in a predetermined manner to cause secondary electrons to be emitted from said element and transmitted to the electron multiplier to produce an electrical readout signal at the output of said multiplier corresponding to the charge image stored on the dielectric element.

4. A scan conversion type electron image storage apparatus comprising:

an evacuated envelope;

a storage target mounted within said envelope, said target including a layer of dielectric material on one side of a non-foraminous target electrode;

a collector electrode supported within said envelope adjacent said target;

an electron multiplier mounted within said envelope adjacent said collector electrode and said target in position to receive secondary electrons emitted by the dielectric layer;

writing means for bombarding the dielectric layer with electrons to produce a charge image on said layer by causing secondary electrons to be emitted from written portions of said layer and collected by unwritten portions of said layer; and

reading means for scanning the dielectric layer with a beam of electrons in a predetermined manner to cause secondary electrons to be emitted from said layer and transmitted to the electron multipler to produce an electrical readout signal at the output of said multiplier corresponding to the charge image stored on the dielectric layer.

5. A method of operating a scan conversion type storage tube having a setorage target which includes an element of dielectric material provided over a target electrode, comprising the steps of:

applying substantially the same D.C. supply voltage to said target electrode and to a collector electrode within the tube adjacent the dielectric element;

bombarding the dielectric element with electrons to initially charge said dielectric element substantially uniformly to an equilibrium potential slightly more positive than the voltage of said target electrode and to cause secondary electrons emitted from the dielectric element to be returned to and redistributed upon said element;

deflecting an electron beam across the dielectric element in accordance with an input signal to produce a charge image on said dielectric element by secondary electron distribution after such element has been charged to said equilibrium potential; and

scanning the dielectric element with a beam of electrons in a predetermined manner to produce an electrical readout signal corresponding to said charge image.

6. A method of operating a scan conversion type storage tube having a storage target which includes an element of dielectric material provided on a target electrode comprising the steps of:

applying substantially the same D.C. supply voltage to said target electrode and to a collector electrode within the tube adjacent the dielectric element, said voltage being sufiiciently positive with respect to a cathode in said tube that the secondary electron emission ratio of the dielectric element is greater than one; scanning the dielectric element with a beam of electrons to initially charge said dielectric element substantially uniformly to an equilibrium potential slightly more positive than the voltage of said target electrode; deflecting an electron beam across the dielectric element in accordance with an input signal to write a charge image on portions of said element by causing secondary electrons to be emitted from the written portions of said element to increase the voltage of said written portions above said equilibrium potential, and causing said secondary electrons to be collected by other portions of the dielectric element so that the initial charge of said written portions is redistributed to said other portions and the average potential of the dielectric element remains substantially constant; and scanning the dielectric element with a beam of electrons in a predetermined manner when the charge image is on said element to produce an electrical readout signal corresponding to said charge image. 7. A method of operating a scan conversion type storage tube having a storage target which includes a layer of dielectric material on a non-foraminous target electrode, comprising the steps of:

applying substantially the same DC. voltage to said target electrode and to a collector electrode within the tube adjacent the dielectric layer, said voltage being sufiiciently positive with respect to a cathode in said tube that the secondary electron emission ratio of the dielectric layer is greater than one; scanning the dielectric layer with a beam of electrons to initially charge said dielectric layer substantially uniformly to an equilibrium potential slightly more positive than the voltage of said target electrode; deflecting a beam of electrons across said dielectric layer in accordance with an input signal to write a charge image of such input signal on portions of said layer by causing secondary electrons to be emitted from the Written portions of said layer by causing secondary electrons to be emitted from the written portions of said layer to increase the voltage of said written portions above said equilibrium potential, and causing said secondary electrons to be collected by other portions of the dielectric layer so that the initial charge on said written portions is redistributed to said other portions and the average potential of the storage dielectric layer remains substantially constant; scanning the dielectric layer with a beam of electrons in a predetermined manner to produce an electrical readout signal on said target electrode corresponding to the stored charge image; and transmitting said readout signal to a display device to reproduce said charge image on said display device. 8. A method of operating a scan conversion type storage tube having a storage target which includes a layer of dielectric material on a non-foraminous target electrode, comprising the steps of:

applying substantially the same DC. voltage to said target electrode and to a collector electrode within the tube adjacent the dielectric layer, said voltage being sufficiently positive with respect to a cathode in said tube that thesecondary electron emission ratio of the dielectriclayer is greater than one;

scanning the dielectric layer with priming beam of electrons to initially charge said dielectric layer substantially uniformly to an equilibrium potential slightly more positive than the voltage of said target electrode;

deflecting a beam of electrons across the dielectric layer in accordance with an input signal to write a charge image on portions of said layer by causing secondary electrons to be emitted from the written portions of said layer to increase the voltage of said Written portions above said equilibrium potential, and causing said secondary electrons to be collected by other portions of the dielectric layer so that the initial charge on said written portions is redistributed to said other portions and the average potential of the storage dielectric layer remains substantially constant;

scanning the dielectric layer with a reading beam of electrons in a predetermined manner to emit readout signal of secondary electrons from said layer corresponding to said charge image; and

transmitting said readout signal through an electron multiplier within said storage tube to amplify the readout signal.

9. A storage apparatus in accordance with claim 2 in which the storage dielectric material is photosensitive.

10. A storage apparatus in accordance with claim 2 in which the storage dielectric material is aluminum oxide.

11. A storage apparatus in accordance with claim 2 which also includes an electron multiplier mounted within the envelope in position to recieve secondary electrons emitted from said dielectric materia 12. A method in accordance with claim 5 in which the dielectric element is made of photosensitive material and which also includes directing a light image onto the dielectric element to form ianother charge image on said dielectric element.

13. A method in accordance with claim 5 in which the dielectric element is made of photosensitive material and which also includes directing light onto .a dielectric element to erase at least a portion of the charge image.

14. A method in accordance with claim 5 which also includes transmitting the electrical readout signal through an electron multiplier within the storage tube to amplify said readout signal.

References Cited UNITED STATES PATENTS 2,857,551 10/1958 Siegfried Hansen 315-12 RODNEY D. BENNETT, Primary Examiner. J. P. MORRIS, Assistant Examiner.