US3421041A

US3421041A - Charge image storage method and apparatus

Info

Publication number: US3421041A
Application number: US599862A
Authority: US
Inventors: Charles B Gibson Jr
Original assignee: Tektronix Inc
Current assignee: Tektronix Inc
Priority date: 1966-12-07
Filing date: 1966-12-07
Publication date: 1969-01-07
Anticipated expiration: 1986-01-07

Description

Jan. 1969 c. B. GIBSON, JR 3,421,041

CHARGE IMAGE STORAGE METHOD AND APPARATUS Filed Dec. 7, 1966 Sheet LIGHT D 12 +5oov 56 4o 34 SWEEP 62 58 GENERATOR 3 9o) 94) SQUARE SQUARE Vd Ve Vf TARGET POTENTIAL E 5 2 CHARLES B.G|BSON JR.

INVENTOR 8) SUCK/0571; BLORE, KLAROU/ST 8 SPAR/(MAN Arromsxs' 1969 c. B. GIBSON, JR 3,421,041

CHARGE IMAGE STORAGE METHOD AND APPARATUS Filed Dec. 7, 1966 Sheet 2 of 2 l ---WRITING THRESHOLD 1 I T] A; E

I I I l 1 I ll t2 t3 t4 Vmin. i I I a f I CHARLES B. GIBSON JR.

INVENTOI? BUC/(HOHM SL095 KLAROU/ST 8 SPAR/(MAN ATTORNEYS United States atent 3,421,041 CHARGE IMAGE STORAGE METHOD AND APPARATUS Charles B. Gibson, Jr., Portland, reg., assignor to Tektronix, Inc., Beaverton, Oreg., a corporation of Oregon Filed Dec. 7, 1966, Ser. No. 599,862 U.S. Cl. 31512 Int. Cl. H01j 29/41 18 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a charge image storage method and apparatus, and particularly to such a method and apparatus for providing improved input sensitivity.

A conventional bistable storage tube includes a dielectric storage target for storing information in the form of a charge pattern. A first positive potential level of the target represents a stored condition, while a second or negative potential level indicates the absence of stored information. The stored information may take the form of an oscillographic waveform or it may be a pictorial, that is in the form of an intensity pattern. In either of these cases, the input information is delivered to the storage dielectric by means of the tubes electron gun. The dielectric storage target may also be rendered photosensitive, making possible the storage of an input light pattern directed toward the exposed target.

A tube of the foregoing type is conventionally provided with one or more flood guns for bombarding the dielectric target with a broad beam of electrons and forcing substantially all portions of the target to assume one of the two stable potentials. It is the action of these flood guns tha gives the tube its bisable properties. If an area of the dielectric target is driven above a predetermined threshold potential by input information, then the flood of electrons will maintain such area at a first or positive potential level. I

The same flood beam of electrons will maintain all other areas of the target at the second or negative potential level indicating non-storage.

The negative or non-written potential level of the target is set within a stable range of target potential between a minimum retention level, below which storage is not possible, and the writing threshold, above which the entire storage target would be driven uniformly positive by flood electrons. While this negative potential level must be greater than the retention level, it cannot be too close to the writing threshold, or unwanted information in the form of background illumination or electron bombardment may be written from time to time, resulting in erroneous longterm storage. The negative potential level is usually empirically determined for best overall operation and is normally appreciably below the aforementioned writing threshold. Unfortunately, a desired input signal must raise an area of the target above the writing threshold if storage is to result. Rapid waveform transients or intensity signals which do not reach a given level of electron bombardment will not then produce a written image on the target.

It is therefore an object of the present invention to provide an improved charge image storage method and apparatus whereby input information to be stored can more easily attain the writing threshold thereby rendering the apparatus more sensitive to input information.

It is another object of the present invention to provide an improved charge image storage method resulting in a sensitive condition of a charge image storage target such that transient or low intensity information may be more easily detected and stored.

Another object of the present invention is to provide an improved charge image storage method and apparatus whereby a storage target is rendered selectable sensitive during specific time periods for storing input information that otherwise would not write on the storage target.

It is another object of the present invention to provide a more sensitive integration mode of operation for a charge image storage apparatus.

According to the present invention, a charge image storage apparatus emits a writing electron beam and a flood beam of electrons towards a dielectric target. The writing electron beam is capable of writing information thereon by bombardment of electrons causing an area of the target to exceed a writing threshold level. Meanwhile the flood beam maintains the target either at a first stable potential, for those areas written by the electron beam, or at a second stable potential for those areas not written by the electron beam.

For rendering the target more sensitive to written information, substantially the entire target dielectric available for storage is brought through the aforementioned threshold level from a written state towards the targets negative stable potential level. Then, when the target dielectric reaches a level between the aforementioned threshold and its negative stable potential level, the flood of electrons from the flood guns is discontinued to retain the target in a sensitive condition near the threshold level. This condition is maintained for a predetermined period of time. During this time, the target is especially sensitive to input information which might not otherwise be capable of Writing on the target. After such predetermined period of time, the flood of electrons from the flood guns is resumed and the input information received during the sensitive period is bistably stored on the target.

In accordance with an embodiment of the present invention, an erase pulse generating means is connected to the target. This means first applies a positive signal portion to the target for writing substantially the entire storage surface of the target, followed by a negative signal portion for uniformly charging the surface towards a negative stable potential level. When the target dielectric reaches a point between its writing threshold level and the negative stable potential level, timing means operate to discontinue emission of flood electrons whereby the target is retained in a sensitized condition. Subsequently, timing means cause resumption of the flood of electrons and continue storage of the input information provided.

During the period of time in which the flood of electrons is discontinued, the target is in an integration mode, and transient signals as well as repetitive high frequencysignals are not hindered in their writing ability by the tendency of the flood electrons to return unwritten portions of the target to the negative stable potential level. It is preferred that the target according to the present invention have photosensitive properties whereby the same can receive input information in the form of a light image as Well as an image provided by an electron beam. The sensitivity of the tar-get to a light image will be enhanced in the same manner as information provided via an electron beam.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of operation together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements and in which:

FIG. 1 is a schematic diagram of an apparatus according to the present invention including a charge image storage device and circuitry for operating the same;

FIG. 2 is a plot of target secondary emission ratio versus target otential for a charge image storage apparatus in accordance with the present invention;

FIG. 3 is a plot of a waveform for an erase pulse employed in the apparatus according to the present invention;

FIG. 4 is a plot of the target voltage waveform for the usual or prior art erase operation of charge image storage apparatus receiving the FIG. 3 erase pulse; and

FIG. 5 is a plot of target potential for the charge image apparatus according to the present invention illustrated in timed relation with respect to the FIG. 3 and FIG. 4 waveforms.

Referring to FIG. 1, a charge image storage apparatus according to the present invention includes a storage tube envelope formed of insulating material housing a principal electron gun including a filament 12, a cathode 14 connected to a high negative voltage source, a control grid 16, and a focusing and accelerating structure 18. The electron beam produced by the principal electron gun is deflected horizontally by means of horizontal deflection plates 22 and vertically by means of Vertical deflection plates 24. Horizontal deflection plates 22 are suitably connected to a horizontal sweep generator 62 for causing periodic or triggered horizontal deflection of beam 20. The beam is in general directed towards a target 46 at the opposite end of the tube. The storage tube is additionally provided with one or more flood type electron guns 26 each having a cathode 28, a control grid 30, and an anode 32, and which are supported inside the envelope 10 adjacent the end of the vertical deflection plates 24 closest the target. Cathodes 28 are conveniently maintained at the zero volt level or ground level through relay contacts 100, while cathodes 30 are suitably connected to a minus 25 volts. Electrons emitted from the flood guns diverge into a Wide beam which is substantially uniformly distributed towards the target 46.

A plurality of electrodes are also provided on the inner surface of envelope 10 beyond the flood guns. A first electrode 34, connected to the midpoint of a voltage divider comprising resistors 36 and 38 coupled between a plus 250 volts and ground, acts to provide a more uniform electric field to collimate electrons. A collector electrode 40 near the target end of the tube is connected at the midpoint of a voltage divider, including

resistors

42 and 44, coupled betwen a plus 500 volts and ground. This electrode can perform the additional function of collecting secondary electrons as will hereinafter become more evident.

Storage target 46 is disposed on the inner side of glass endplate 48 and includes a transparent storage target electrode 50 over which is disposed a photosensitive dielectric 52, suitably an integral layer of P1 type phosphor. Target electrode 50 is a thin transparent conductive coating such as tin oxide or the like and is connected to the midpoint of a voltage

divider comprising resistors

56 and 58 disposed between a plus 500 volts and ground, as well as to resistor 84 connected between the cathode of tube 68 and ground. The voltages and resistances are selected to result in the desired secondary emission characteristic of the tube as illustrated in FIG. 2, wherein secondary emission ratio is plotted against target potential. The voltage on target electrode 50 is positive with respect to the cathodes of flood guns 26.

Information may be written on the storage target via electron beam 20. This information may be in the form of a waveform applied to terminal 60 while the beam is scanned horizontally with sweep generator 62, or intensity information may be applied to grid 16 while the beam is scanned both vertically and horizontally.

During operation, the tube polarities are such that beam 20 has a relatively high velocity for writing and is capable of producing secondary electrons when it strikes storage dielectric 52. Secondary electrons are then suitably collected by collector 40 in which case the potential of collector electrode 40 may be adjusted to be just slightly higher than the potential of electrode 50. The storage dielectric 52 may alternatively have a suiiiciently porous structure to enable secondary electrons emitted from the bombarded surface of dielectric 52 to be transmitted therethrough and be collected. In any case an elemental area of the target can be driven positive or Written as a result of the secondary emission produced by electron beam 20.

The written area is retained at a relatively positive potential after beam 20 has passed such elemental area because of the action of flood guns 26. Flood guns 26 produce relatively low velocity electrons which strike the target but which ordinarily have insufficient velocity for writing information. When the electrons from flood guns 26 strike areas of the target upon which a positive charge has not been written, these flood electrons tend to maintain such areas at the relatively negative potential of the flood guns, e.g., at ground level or zero volts. This is the stable negative potential level of the target. However, the flood gun electrons are attracted by positive elemental areas and obtain a high velocity with respect to these areas for producing continued secondary emission therefrom. Therefore these areas are maintained relatively positive or near the potential of target electrode 50 and collector electrode 40. This comprises the stable positive potential level of the target. The target thus has bistable properties and is capable of retaining information written thereon, with the flood beam of electrons driving target areas towards one of two stable potentials depending upon the information Written thereon with beam 20.

The manner in which this storage tube operates will be further described with reference to FIG. 2, a plot of secondary emission vs. target potential for the side of the target bombarded by electron beam 20. Examining this curve, we see two points at which the secondary emission ratio for the target is equal to one. At V 6:1, because the target, and specifically the inside surface of dielectric 52, has collected sufficient electrons to charge a few tenths of a volt negative with respect to the flood gun cathode, thereby rejecting all electrons. At V the accelerating potential is high enough for the material on the target dielectric surface to emit secondary electrons and at V the target dielectric surface has charged a few volts higher than the collector and all secondary electrons in excess of primary electrons are returned to the target. V and V are the stable potentials. If the target begins to rise above V the target collects electrons, the secondary emission being less than one, and the target dielectric charges negatively restoring the target dielectric to V If we bombard the target with a high energy electron beam 20, and allow it to charge by secondary emission to any potential just under V it will return under the action of the flood guns to V However, if we allow it to charge more positive than V due to the action of beam 20, the secondary emission caused by the flood electrons will discharge the target dielectric positively until it reaches V If it passes V the secondary emission ratio becomes less than one and any electrons arriving attempt to charge the target negatively. V is described as the first cross over voltage of the secondary emission characteristic or the minimum voltage necessary for storage.

The voltage level at V is also the writing threshold level, above which electron beam 20 must bring an elemental area of the target in order for the flood beams to take over and retain such elemental area at a stable positive potential substantially equal to the potential of electrode 50, that is the positive stable potential level of the target. All areas which have not been raised above this writing threshold by electron beam 20, or by some other means, will be retained by the flood beam electrons at a voltage close to the potential of the flood gun cathode, i.e., zero volts or the stable negative potential level for the target.

Since the phosphor storage dielectric 52 is photosensitive, a light image directed onto the storage dielectric, through end plate 48 and transparent electrode 50, produces an image on the storage dielectric corresponding to such light image. The light image produces charge carriers in the dielectric such that illuminated areas of the target dielectric become more conductive and tend towards the more positive potential of electrode 50. A light image of suflicient intensity directed towards target 46 will be maintained through the action of the flood guns in the same manner as the image written on the target with electron beam 20. In order to enhance the photosensitive characteristics of the phosphor storage dielectric, photoconductive material such as cadmium sulphide or zinc oxide may be uniformly mixed throughout the phosphor layer.

When it is desired to remove or erase a stored image from the target, an erase pulse is applied to the target electrode and this erase pulse is derived from an erase pulse generator illustrated in simplified form within dashed lines at 64 in FIG. 1. In this circuit, switch 66 is normally in the lower position shown but may be momentarily operated to its upper position with subsequent substantially automatic return to the lower position. When the switch 66 in its lower position, the grid of tube 68 is coupled through grid resistor 70 to the movable tap on potentiometer 72, the latter forming a part of a voltage divider also including resistor 74. This divider is interposed between +500 volts and +100 volts. Capacitor 76, connected between the movable tap and the +100 volt terminal, assists in maintaining the voltage at the tap at a selected quiescent value for causing moderate conduction in tube 68. The grid is also coupled through capacitor 78 in series with resistor 80 to the movable contact of switch 66, while the lower fixed contact of switch 66 is connected via resistor 82 to ground level. The cathode of tube 68 is returned to ground through cathode resistor 84 and the cathode is also connected to electrode 50 in the storage tube. In this lower position of switch 66, the proper positive voltage is applied through this connection to electrode 50 for maintaining normal storage operation.

Now if switch 66 is momentarily raised to its upper position, the movable contact will connect to the switch terminal connected to the junction between resistor 86 and capacitor 88, disposed in that order between +500 volts and ground. As a result, a positive pulse is instantaneously applied to the grid of tube 68 causing the voltage across cathode resistor 84 also to rise. The waveform applied to the target electrode is illustrated in FIG. 3, wherein the voltage on the target electrode with respect to the flood gun cathodes is plotted. The time at which switch 66 is moved to its upper position is indicated at 1 At this time the target electrode voltage is caused to change from A to B. The time between 1 and t is the period during which switch 66 is maintained in its upper position. This time is not critical so long as the dielectric 52 has time to discharge in a positive direction and reach its positive stable potential level, after crossing the writing threshold level.

The voltage across dielectric 52 is illustrated in FIG. 4, where the voltage across the dielectric is shown as dropping from the maximum voltage to the minimum voltage present across the dielectric. The latter condition corresponds to the positive stable voltage level of the bistable target. The target is thus caused to fade positive or assume a completely written condition between t and t Now at t switch 66 is allowed to return to its lower position. At this time, the voltage applied to the grid of tube 68 is relatively negative-going since the voltage across capacitor 78 cannot change immediately. A resultant negative-going signal is applied to the target electrode as indicated in FIG. 3. The voltage at the electrode substantially instantaneously changes from C to D, and then rises along an RC time constant curve to E as capacitor 78 charges. The negative signal portion of the erase pulse is long enough and of suflicient potential to bring the target dielectric back across the threshold level in a nega tive direction.

Referring to FIG. 4 the voltage across the dielectric 52 normally charges during the time from t to Z back to its maximum value corresponding to the negative stable potential of the target. After t new information may be written on the target either by means of the electron beam 20 or by light impinging upon the target through glass endplate 48. Any such information written on the target must then raise the bombarded side of the dielectric in a positive direction by an amount suflicient to exceed the writing threshold level as hereinbefore defined, this threshold level being indicated by the dashed line in FIGS. 3, 4 and 5. Thus, referring specifically to FIG. 4, new input information in order to write must decrease the voltage diflerence across the dielectric until the writing threshold is crossed.

The erase pulse comprising a positive signal followed by negative signal is preferred because a uniform erasure occurs and the entire target ends up at substantially the same potential. However, the erasure pulse is a vehicle, according to the present invention, for bringing the target substantially uniformly through the threshold potential in a negative direction towards the bistable targets negative stable potential. According to the present invention, the flood guns are disabled when the bombarded dielectric reaches a potential level between the writing threshold and the negative stable potential. At this time, the bombarded surface of the dielectric layer is close in potential to the writing threshold, but is slightly more negative, such that a small additional input either from the electron beam or from the light image will be required for an area of the target to exceed the writing threshold. Since this operating level is close to the writing threshold, an input may be stored which would otherwise not be capable of exceeding the writing threshold. Moreover, in this condition with the operation of the flood guns discontinued, the apparatus is in an integration mode whereby a fast transient signal is capable of exceeding the writing threshold for storage, even though such transient could not be stored during ordinary operation. Furthermore, a high frequency repetitive signal, or the like, may be stored by allowing successive repetitions to build up and exceed the writing threshold. Buildup is possible inasmuch as the flood guns are turned off and the dielectric will not be returned to the targets negative stable potential by the flood guns between repetitions of the input signal.

The FIG. 5 waveform is illustrative of operation according to the present invention. Here, again, an erase pulse is applied at time t as in FIG. 3. The positive signal portion thereof, beginning at t causes the voltage across the dielectric to change from maximum value, corresponding to the targets negative stable potential, towards a minimum value, corresponding to the targets positive stable potential. In so doing, the bombarded surface of the dielectric crosses the writing threshold such that the entire target is thus written or raised to a uniform positive potential as hereinbefore discussed. Now, the following negative signal portion of the erase pulse causes the dielectric potential level to change and cross the writing threshold in a negative direction. In acordance with the present invention, as soon as the writing threshold is crossed, -i.e. at time t the flood guns are disabled and hence the dielectric is driven no further in the direction of the negative stable potential. This condition may be maintained for a desired period of time during which the target is said to be operating in an integration mode and input information may build up and cause recrossing of the writing threshold. Since the operating level is reasonably close to the writing threshold the target is rendered quite sensitive and a small change in potential, delivered by either the electron beam or by means of a light image, may cause crossing the writing threshold when such input value would not be capable of establishing crossing of this threshold during normal operation. The target is retained in the sensitive condition for a predetermined period of time, the flood guns being reenergized at time t At the end of this integration period, when the flood beams are reenergized those portions written during the integration period will be driven by the flood beam towards the positive stable potential of the target. The flood beams should not be left oif for an unnecessarily long period, or for longer than necessary to apply the desired input or for allowing desired input integration on the target. Otherwise the target may become written due to the buildup of various unwanted stray signals and ambient light and erroneous storage patterns on the target may result. The flood guns are thus reenergized at time r for returning the target dielectric to its normal operating potentials.

The operation of the storage target is additionally illustrated in FIG. 3, where the change in target dielectric potential during the negative signal portion of the erase pulse is shown in dashed lines between C and E. The dielectric waveform is shown superimposed on the FIG. 3 erase pulse for comparison purposes. When the flood guns are deenergized at time t the potential of the target is maintained at a level just under the writing threshold as indicated by the dotted line.

The invention is further illustrated by means of the FIG. 1 circuit diagram disclosing circuitry for deenergizing the flood guns in correct timed relation with the negative going signal portion of the erase pulse, that is at time 1' Referring to FIG. 1, a first square wave generator 90 is triggered by the negative going signal portion of the erase pulse from generator 64. The square wave generator 90, which may conveniently comprise an accurately calibrated sweep circuit and gate generator, produces an output pulse 92 of duration equaling the time from t to I Thus, square wave 92 has a duration long enough to permit the uniform lowering of the potential of the target dielectric to a level just slightly more negative than the writing threshold. Square wave generator 94 receives pulse 92 and is triggered by the negative-going or concluding portion thereof to provide a second square wave 96. This square wave operates relay coil 98 for opening normally-closed relay contacts 100 in the cathode circuit of flood guns 26 of the storage tube. Therefore the emission of flood electrons is discontinued at this time. The duration of square wave 96 is equal to the selected integration period and at the end of square wave 96, contacts 100 reclose and the emission of flood electrons is continued whereby the target is returned to the respective bistable operating potentials. The circuit is thus constructed and arranged to accomplish the procedure outlined in connection with FIG. 5.

While I have shown and described a preferred embodiment of my invention, it will be clear to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects. I therefore intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

1 claim:

1. Charge image storage apparatus comprising:

a dielectric storage target for storing a charge pattern,

first electron emitting means for bombarding a selected area of such target with a first beam of electrons capable of causing secondary emission from such selected area to raise the potential thereof,

second electron emitting means for bombarding the storage target with a lower velocity flood beam of electrons acting to drive areas of said target towards one of two stable potentials on either side of a writing threshold, said flood beam acting to maintain at a positive stable potential an area of target raised by said first beam above said threshold, while maintaining other areas at a negative stable potential below said threshold.

third means for altering the potential of said target including means for first applying a relatively positive signal to the target for raising the potential of said target towards said positive stable potential, and for then applying a negative signal to said target for lowering said target below said writing threshold, and

fourth means for disabling said flood beam when said target is lowered to a level between said writing threshold and said negative stable potential and for at least temporarily retaining said target in a sensitive condition between said writing threshold and said negative stable potential.

2. The apparatus according to claim 1, wherein said third means for altering the potential of said target comprises an erase pulse generator providing a waveform, including said positive signal, immediately followed by said negative signal wherein said positive signal has a duration for fading the target into a written condition and wherein said negative signal is applied for a period sufficient for lowering said target below said writing threshold,

said fourth means comprising switching means for effectively disconnecting said second electron emitting means in timed relation with the initiation of said negative signal such that said target will reach a predetermined level between said writing threshold and said negative stable level at the conclusion of said erase pulse.

3. The apparatus according to claim 1, wherein said target is retained in said sensitive condition by said fourth means at a level closer to said writing threshold than to said negative stable potential.

4. The apparatus according to claim 1, wherein said fourth means is also operated in predetermined timed relation after the application of said negative signal to said target for restoring the operation of said flood beam.

5. The apparatus according to claim 1, wherein a repetitive input is applied to said target via said first electron emitting means while said target is retained in a sensitive condition between said writing threshold and said negative stable potential, and wherein the input by repetition builds up an image on said target such that portions of said target are raised above said threshold, after which said fourth means is operated to restore said flood beam.

6. The apparatus according to claim 1, including means for applying a transient input signal to said first electron emitting means while said target is retained in a sensitive condition between said writing threshold and said negative stable potential such that said first beam is able to raise portions of said target above said writing threshold despite the duration and amplitude of said input signal,

said fourth means operating to reenergize said flood 9 beam at a predetermined time for storing said input signal on said target.

7. Charge image storage apparatus according to claim 1, wherein said dielectric storage target is also photosensitive for receiving a light pattern, said target receiving such light pattern while in said sensitive condition between said writing threshold and said negative stable potential,

said light pattern acting to discharge certain areas of said target causing the same to rise above said threshold level in said sensitive condition,

said fourth means being operated in timed relation with said negative signal to restore said flood beam and store said light pattern on said target.

8. Charge image storage apparatus comprising:

a storage target including a dielectric layer,

means for establishing a charge pattern on said target from input information,

a flood emitter for bombarding said surface of said dielectric layer with relatively low velocity electrons directed at substantially the entire dielectric layer available for storage, said low velocity electrons acting to drive portions of said dielectric layer toward a first positive stable potential when a predetermined threshold level is exceeded by input information establishing said charge pattern, while normally driving other portions of said dielectric layer towards a negative stable potential by charging said dielectric layer,

means for charging substantially the entire dielectric layer available for storage in a negative direction from a level more positive than said threshold level, through the value of said threshold level, and toward said negative stable potential, and

means for at least temporarily discontinuing the operation of said flood emitter when said target is charged to a level more negative than said threshold level, but not as negative as said negative stable potential, for enhancing the sensitivity of said target.

9. The apparatus according to claim 8, wherein said target further includes a target electrode located on the surface of said dielectric layer opposite from the bombarded surface thereof, said electrode being connected to a potential normally more positive than said flood emitter, and

wherein the said means for charging substantially the entire dielectric layer comprises circuit means for applying an erase pulse to said electrode having a positive going portion for fading the target positive, prior to a negative going portion charging substantially the entire dielectric layer available for storage through the value of said threshold level in a negative direction.

10. The apparatus according to claim 9, wherein said means for at least temporarily discontinuing the operation of said flood emitter operates in timed relation to the initiation of said negative going portion so that said target reaches said level more negative than said threshold level but not as negative as said negative stable potential before the operation of said flood emitter is discontinued, and further including means for resuming the operation of said flood emitter at a predetermined time after the discontinuance thereof.

11. The charge image storage apparatus according to claim 10, wherein the dielectric of said storage target is also photoconductive such that a light pattern directed thereon will have the effect of discharging selected portions of said dielectric for writing information thereon.

12. The charge image storage apparatus according to claim 8 wherein said flood emitter comprises a flood gun having a cathode and wherein said means for at least temporarily discontinuing the operation of said flood emitter comprises switching means for disconnecting said cathode.

13. A method for operating a charge image storage apparatus having a dielectric storage target and an electron gun for directing electrons at said target, comprising:

directing a flood of electrons at the target to establish a writing threshold thereof, said flood of electrons operating by secondary emission to retain a portion of said target at a first stable potential more positive than said writing threshold when said portion of said target has been raised above such threshold by input information, while normally driving other portions of said target toward a second stable potential more negative than such writing threshold,

applying a positive potential to said target for raising the target to a potential more positive than said writing threshold,

then .lowering said target to a potential more negative than said writing threshold, and

discontinuing the said flood of electrons when said target is lowered between said writing threshold and said second stable potential and before it reaches such second stable potential, for enhancing the writing sensitivity of said target.

14. The method according to claim 13, wherein the target is lowered to a potential more negative than said writing threshold immediately after the application of said positive potential to said target for the purpose of erasing information on said target, and wherein the electron flood operation is discontinued in timed relation to the lowering of said target to a potential more negative than said writing threshold such that said target reaches said potential before said flood of electrons is discontinued.

15. The method according to claim 14, wherein the said flood of electrons is resumed in the timed relation to the discontinuance thereof for storing information on said target which may be written after the discontinuance of said flood electrons.

16. The method according to claim 13, further including impinging a light image on said dielectric, said dielectric having photoconductive properties so that a light image has the effect of discharging portions of said dielectric.

17. A method for operating a charge image storage apparatus having a dielectric storage target and an electron gun for directing electrons at said target, comprising:

directing a flood of electrons at the target to establish a writing threshold thereof, said flood of electrons operating by secondary emission to retain a portion of said target at a first stable potential on one side of said writing threshold when said portion of said target has been raised above such threshold by input information, while normally driving other portions of said target toward a second stable potential on the other side of such writing threshold,

lowering the potential of said target through said writing threshold to a sensitive level between said writing threshold and said second stable potential,

maintaining said target at said sensitive level for a predetermined period of time,

applying input information to said target during said predetermined period of time, and

then lowering the potential of said target to the level of said second stable potential.

18. Charge image storage apparatus comprising:

a storage target including a dielectric layer,

means for establishing a charge pattern on said target representing input information,

a flood emitter for bombarding said surface of said dielectric layer with relatively low velocity electrons directed at substantially the entire dielectric layer available for storage, said low velocity electrons acting to drive portions of said dielectric layer toward a first positive stable potential when a predetermined threshold level is exceeded by input information establishing said charge pattern, while normally driving other portions of said dielectric layer towards 1 1 l 2 a negative stable potential by charging said dielectric tivity of said target to input information during such layer, period of time. means for charging substantially the entire dielectric References Cited layer available for storage in a negative direction from a level more positive than said threshold level 5 UNITED STATES PATENTS through the value of said threshold level, and 3,144,579 8/1964 Holsinger et a1.

means for maintaining said layer at a level between said threshold level and said negative stable potential RODNEY BENNETT P'lmary Examme' for a limited period of time for enhancing the sensi- D. C. KAUFMAN, Assistant Examiner.