US3325673A

US3325673A - Charge integrating bistable storage tube

Info

Publication number: US3325673A
Application number: US302880A
Authority: US
Inventors: Robert H Anderson
Original assignee: Tektronix Inc
Current assignee: Tektronix Inc
Priority date: 1962-08-06
Filing date: 1963-08-19
Publication date: 1967-06-13
Anticipated expiration: 1984-06-13
Also published as: NL142820B; DE1439716A1; DE1439659A1; GB992178A; NL6409585A; SE317452B; NL296317A; NL144770B; US3214516A; GB1036859A; DE1439634A1; NL298991A; GB992177A; DE1439634C2; DE1439634B2; US3214631A; CH418391A

Description

United States Patent 3,325,673 CHARGE INTEGRATHNG BHSTABLE STORAGE TUBE Robert H. Anderson, Portland, Ureg., assignor to Tektronix, lne., Beaverton, Greg., a corporation of Oregon Filed Aug. 19, 1963, Ser. No. 302,889 9 Ciaims. (Cl. 315-412) The subject matter of the present invention relates generally to electron discharge display devices and in particular to cathode ray storage tubes which receive electrical input signals, store such signals for -an indefinite controllable time and reproduce such signals as visual images for direct viewing or as electrical output signals.

The storage tubes of the present invention may be employed in a cathode ray oscilloscope for recording transient signals, in la radar or sonar display device, as a character writing tube and as a signal delay device to store electrical signals for .a controllable time before producing an electrical output signal which is delayed with respect to such input signal. The present storage tube is an improvement over my previous storage tubes disclosed in the following copending U.S-. patents applications: Ser. No. 180,457, now U.S. Patent 3,293,473, entitled, Electron Discharge Display Revice, filed Mar. 19, 1962; Ser. No. 214,877, now US. Patent 3,214,631, entitled Storage Tube, filed Aug. 6, 1962; and Ser. No. 245,716, now U.S. Patent 3,214,516, entitled, Electrical Readout for Storage Tube, led Dec. 19, 1962, of which the present application is a continuation in part.

Conventional storage tubes employ complex targets including dielectric material deposited on a conducting wire mesh which together with other targets are shown by A. V. Haeff in U.S. Patent No. 2,761,098, issued Aug. 28, 1956, and F. H. Harris, in US. Patent No. 2,839,679, issued June 17, 1958. The storage tube of the present invention is less complicated and less expensive to manufacture than conventional storage tubes because it employs a storage target of such simpler construction. The conducting wire mesh of conventional storage tubes usually has the storage dielectric deposited on one sid`e of the wires of such mesh so that such target exerts grid control on flood electrons which pass through apertures in such dielectric coated mesh to a separate phosphor viewing screen. Such tubes are often referred to in the art as the grid barrier or transmission type of storage tubes. They are characterized Iby the storage of a charge image on the dielectric layer which coats the mesh, and phosphor mesh to the selective transmission of electrons through such viewing screen coated on a glass support plate spaced from such mesh to produce a visible light image corresponding to such charge image. In contrast, the storage target of the present invention may be in the form of simply a thin integral layer of phosphor material supported on the face plate portion of the tube envelope over a target electrode which is a light transparent conductive coating. This phosphor layer serves both of the two purposes of providing storage of the charge image `and production of the visible light image, which formerly required two separate structures in the commercially manufactured transmission types of charge storage tubes.

The integral layer of phosphor forming the target of the present invention is a continuous uniform layer in its gross macroscopic structure, but the individual phosphor particles which make up the layer are deposited so that they are not in good electrical contact with each other, especially at the vacuum side of the target surface. This layer of phosphor has a somewhat porous or permeable structure which enables secondary electrons emitted from one side of the layer to be transmitted through such layer and collected by the target electrode and results in desirable storage characteristics as described in my copending application Ser. No. 180,457. The target layer of the present invention is not continuous in its electrical conductivity in a direction transverse to the tube axis. The term semicontinuous layer will 'be used throughout this description to refer to the structure and electrical properties of the combined viewing screen and storage dielectric layer described herein. Such semicontinuous phosphor layer also has a thickness within a critical range of thicknesses so that it may perform the functions of both charge image storage and light image emission.

ln a sequence of experimental tubes, increasing amounts of phosphor were used, until it was found that there is an upper limit to the phosphor thickness which can be used and still have an adequate stable range of storage voltages. For P-l phosphor, it was found that inadequate stable range of complete loss of storage generally occurs when more than about 31/2 to 41/2 mg. per cm.2 is used. This upper limit of thickness is roughly 1/s to 1/2 of the thickness used in conventional cathode ray tube P-l phosphor viewing screen. Layers of this thickness appear to the eye to be fairly dense, conventional continuous layers. However, displacement tests of the packing density of P-l phosphor layers show that about half of the thickness of such layers is empty space, yand only about half of the volume of such layers is actually occupied by P-l phosphor particles. The porosity of the resulting layers, and the use of thin layers results in the semicontinuous target surface of this invention. This layer is sufliciently discontinuous to prevent spreading of stored images, which appears to be a necessary feature for adequate bistable storage stability.

A mesh electrode may be mounted adjacent to but spaced from the surface of the storage target between such target and the sources of electrons which bombard the target so that positive ions, produced when such electrons ionize the residual gas in the tube, are repelled away from the target to reduce the damage to the phosphor layer caused by ion bombardment. The mesh electrode is connected to a positive D.C. voltage and establishes a substantially uniform potential gradient adjacent the rear surface of the phosphor layer which enables more uniform 4distribution of the flood electrons emitted by the viewing or flood gun in such tube and used to maintain the charge image on such phosphor layer. This prevents bright `areas from appearing in the background illumination of the storage target which were caused by the deflection of such flood electrons by the charge on adjacent phosphor areas of different potential due to a type of coplanar grid effect. It should be noted that the mesh electrode may also function to collect some of the secondary electrons emitted by the storage target.

It has been found that the storage characteristics of a storage tube can be improved by effectively turning off the -iiood gun within such tube during the writing of a charge image on the storage target of such tube so that the low velocity flood electrons do not bombard the target at this time. This technique may be employed to increase the writing rate of the tube in order to store the charge image of a high speed transient signal which previously could not be stored because the flood electrons act to oppose writing The ood electrons drive the potential of the target areas, initial-ly charged to a voltage below the first cross-over voltage on the secondary emission curve of the phosphor storage dielectric, down toward the voltage of the flood gun cathode. Thus, the potential of the charge image of a transient signal is often prevented from building up to the first cross-over voltage, which is the minimum voltage necessary for storage, by the opposing action of the flood electrons. This opposing s action may be eliminated by preventing the ood electrous from striking the storage target during the production of the charge image on such target by the bombardment of such target with the high velocity writing electrons, until the potential of such charge image exceeds the first cross-over voltage.

A simi-lar method of operation is effective for the storage of low level, fast rise time or high frequency repetitive signals in which the iiood electrons are prevented from striking the storage target during the writing of several consecutive signals. Since these signals have the same wave form, the potentials of the superimposed charge images corresponding to such signals are added together, When the total potential of the charge images is greater than the first cross-over voltage, the ood electrons are allowed to bombard the storage target to store such charge image for an indefinite controllable time. This charge image integration involves the charging of the target capacitance in discrete amounts of voltage applied in response to each successive signal.

Another improved method of operation of the direct view storage tube of the present invention increases the image contrast of the light image emitted by the phosphor target layer and may be accomplished by applying contrast enhancement pulses to the flood gun cathode or to the target electrode layer of the storage target. These contrast enhancement pulses may be rectangular pulses having the proper polarity to increase the velocity of the flood electrons bombarding the storage target a small amount for approximately 50% of the flood gun duty cycle. In addition, both the writing speed and the erase speed of the tube may be increased somewhat by shifting the operating level of the storage target electrode to a more positive voltage during writing and to a more negative voltage during erasing, such as by pulsing the target electrode layer.

Briefly, one embodiment of the storage tube of the present invention may include an evacuated envelope, a writing beam producing electron gun structure having horizontal and vertical deection plates, a flood beam producing electron gun structure, a storage target supported on the interior of the face plate portion of the envelope, and a mesh electrode supported between the electron guns and such storage target. The storage target may be in the form of a light transparent substrate body of electrical insu-lative material, such as a flat face plate portion of the envelope, with a light transparent iilm of electrically conductive material supported n one side of such substrate body, and a charge storage dielectric layer of secondary emissive phosphor material supported on such substrate body over such conductive iilm. The storage dielectric layer may be a semicontinuous layer of phosphor having a thickness that is within a critical range of thicknesses over which such phosphor material will store an electrical charge image for an indefinite controllable time in the tube and will emit a visible light image corresponding to such charge image, when bombarded by electrons. The envelope may have a rectangular funnel portion of ceramic material `with a flat rectangular face plate of glass sealed thereto. A plurality of axially spaced conducting wall coatings may be applied to the interior of the funnel portion of the envelope and provided with electrical connections to the exterior of such envelope so that such coatings may function as electrodes to focus, collimate and collect the primary electrons generated by the flood gun structure and, in some cases, to collect the secondary electrons emitted by the storage target. The mesh electrode is supported over the phosphor layer of the storage target spaced from such phosphor layer and may be connected to one of the wall coating electrodes to operate at the same voltage as such wall coating electrode. The wall coating electrodes along with the writing gun, the flood guns and the conductive film of the storage target are all connected to suitable sources of electrical potential so that the storage tube of the present invention functions as a i bistable storage tube in that the target voltage on every area element of the surface of the storage layer tends to be held at one of two stable states of electrical potential.

It is therefore one object of the present invention to provide an improved storage tube which is simple and inexpensive to manufacture and whose structure is resistant to mechanical shock and vibration.

Another object of the invention is to provide an improved direct-viewing type storage tube having a storage target of simplified construction in which a semicontinuous layer of phosphor material having a thickness within a critical range of thickness is employed both as a storage dielectric for storing the electrical charge image without image spreading and as a uorescent material for emitting the visible light image of such direct-viewing storage tube, and having a mesh electrode spaced adjacent such target to repel positive ions away from the target and to more uniformly distribute the ood electrons over the phosphor layer to provide substantially uniform background illumination.

A still further object of this invention is to provide an improved method of operation of a direct-viewing bistable storage tube in which the contrast of the visible light image emitted therefrom is substantially increased.

An additional object of the present invention is to provide an improved method of operating a storage tube in which the low velocity electrons of the viewing gun in such tube are prevented from striking the storage target during the bombardment of such target with high velocity electrons from the Writing gun of such tube, until the potential of the charge image produced on the target exceeds the minimum voltage necessary for storage.

Another object of this invention is to provide an improved method of operation of a bistable storage tube which increases the maximum writing speed of the electron beam emitted by the writing electron gun in such tube `which will result in storage of a charge image produced by such electron beam on a storage target in such tube.

A still further object of the present invention is to provide an improved method of operation of a bistable storage tube by which the erase speed of such tube is increased so that the charge image on the storage target employed in such tube may be removed and such target may be reconditioned for storing another such image in a shorter time.

Additional objects and advantages of the present invention will be apparent from the following detailed description of certain preferred embodiments of the present invention shown in the attached drawings of which:

FIG. 1 is a side view of one embodiment of the storage tube of the present invention with portions broken away to show internal structure;

FIG. 2 is a front view of the storage tube shown in FIG. 1;

FIG. 3 is a fragmentary horizontal section View taken along line 3-3 of FIG. l, showing, on an enlarged scale, one embodiment of the storage target which may be employed in the tube of FIGS. 1 to 3; and

FIG. 4 shows the storage and display characteristics of the phosphor storage target of the present invention.

A preferred embodiment of the storage tube of the present invention is shown in FIGS. 1 to 3 to include an evacuated envelope having a tubular neck portion 10 of glass, a funnel-shaped body portion 12 of ceramic material and a fiat rectangular face plate portion 14 of glass. This envelope structure may be similar to that shown in the copending U.S. application Ser. No. 132,915, now U.S. Patent 3,207,936, entitled Electron Beam Display Device, filed by W. H. Wilbanks et al., on Aug. 21, 1961. As stated in such copending application, the glass neck portion 10 may be sealed to the ceramic funnel portion 12 by a glass frit seal 16 at the small end of such funnel portion While the glass face plate portion 14 may be attached to the larger end of such funnel portion by a similar glass frit seal 18. The neck portion of the envelope may contain an electron gun structure including a writing gun 17 and one or more flood guns 19. Electrical lead pins 2o may extend through the side of the neck portion 1d to the horizontal deflection plates, the vertical deflection plates and the isolation shield of such writing gun and to the focusing electrode and isolation shield of such flood guns through a flame seal which joins two tubular glass members forming the neck portion of the envelope. Other electrical leads to electrodes in the writing gun and the flood guns may extend through a seal (not shown) at the base end of the neck portion of the envelope and connected to pins 22 in a plastic base 24 which is suitably secured to such end of envelope neck portion.

The direct viewing storage tube of the present invention may include a split screen storage target 26 of the type described in my copending application Ser. No. 214,877, now U.S. Patent 3,214,631, which may be positioned and supported on the interior surface of face plate 14 and will be ,described in greater detail with reference to FIGS. 2 and 3. A plurality of separate electrodes may be provided as spaced wall coatings of a conductive material, such as silver, tin oxide, aluminum or graphite, on the interior surface of the funnel portion 12 of the envelope. The first electrode wall coating 2S functions primarily as a focusing electrode for the flood electrons emitted from flood guns. It is connected to a suitable source of electrical potential through a first connector plug 30 which extends through a hole in funnel portion 12 of the envelope. A second electrode wall coating 32 having a greater length than the first electrode 28 is spaced from such first electrode and electrically connected to the exterior of such envelope by a second connector plug 34 so that it can also function as a focusing electrode. A third electrode wall coating 36 is provided on the interior surface of funnel portion 12 and spaced from the second electrode 32,. It is positioned near the storage target 26 and functions primarily as a focusing and collimating electrode for the flood electrons so that such electrons are substantially uniformly distributed over the surface of the storage `target 26 and approach such target at approximately right angles thereto. The third electrode wall coating 36 is also connected to a source of electrical potential by a third plug connector 38. A mesh electrode 40 is supported over the storage target 26 between the second and third wall coatings spaced from such target. This mesh electrode may be in the form of a metal wire screen or a foraminous plate of an extremely open mesh which may be connected to the second wall coating 32. The primary function of the mesh electrode 40 is to repel positive ions of residual gas away from the storage target to prevent the destruction of such target. However, the mesh electrodes also enables more uniform distribution of the flood electrons over the storage target to eliminate bright areas in the background illumination of such target.

lt should be noted that a conventional resistive coating 44 of aquadag or similarly conductive material is provided on the interior of a portion of the envelope neck portion 10 and electrically connected to the isolation shield of the writing gun contained in such neck portion so that it serves as an extension of the second anode in such writing gun. A more conductive coating 46 of silver or the like may be provided over the end of conductive coating 44 spaced from the end of the first electrode coating 28, in order to provide a more uniform electrical field at the end of such conductive coating.

A graticule scale 48 is provided on the interior surface of the envelope face plate portion 1d by applying lines of fused glass frit or white insulator material thereto or by scribing notches therein as shown in FIGS. 2 and 3. This internal graticule may be illuminated by a suitable source of light (not shown) positioned outside of the envelope as shown in the above-mentioned copending US. Patent 3,207,936, so that the light is transmitted through the surrounding edge of face plate 14 to such graticule scale. Storage target 26 extends over the graticule scale d@ on the interior surface of face plate 14 and, as described below, has a pair of spaced upper and lower conducting layers extending to the exterior of the envelope which serve as target electrodes. An electrical connection can be made to each of such target electrodes by spaced connector coatings Sti and 51 of silver or the like on the exterior surface of the envelope over the glass seal 18.

One embodiment of the storage target 26 of the present invention is shown in FlGS. 2 and 3 to include two thin light transparent conductive layers or films N2 and 103 coated in spaced insulated relationship on the interior surface of glass face plate 1d. These transparent conductive films may be made of tin oxide formed from stannous chloride, or other suitable material, and are applied to the surface of the face plate in any conventional manner over the graticule scale dit which may be in the form of fused glass lines. The storage dielectric of storage target 26 is a thin semi-continuous layer of phosphor material 104i, for example, P-l type phosphor having a chemical designation of Zn2SiO4zlA/1n, which is applied over the upper and lower

conductive films

102 and 103 in any suitable manner, such as by water settling on the film or the application of a preformed layer of phosphor to the film by a decalcomania technique, to provide a thin porous integral layer of phosphor having a substantially uniform thickness. Such thickness is within that range of thicknesses over which such phosphor material will store a bistable charge image for an indefinite controllable time, with no substantial spreading or migration of the charge image and will emit a light image corresponding to said charge image. This critical range of phosphor thicknesses will be discussed in greater detail with reference to FlG. 4.

An electrical connection may be provided to the upper and lower

conductive films

102 and 103 from the exterior of the envelope by, for example, extending the conductive films through the glass frit seal 18 so that they may be contacted by the connector coatings 5l) and 51, respectively, on the exterior of such envelope. However, this connection may be made in any number of ways including the provision of a conductive coating of silver or other suitable material on the surface of face plate 1d and extending through the seal 13 into contact with the films. Other alternatives include making seal 1S of a conductive glass frit, providing a metal pin through such seal, or using a connector plug similar to

plugs

30, 34 and 38. One such connector plug 3d is shown in FIG. 3 as extending through an aperture in the funnel portion 12 and making contact with the third electrode wall coating 36. This plug is in the form of a body of ceramic material 105 similar to the ceramic material of funnel envelope portion 12 and provided with a connector coating 108 of silver or other conductive material on the exterior of such plug body so that coating 108 provides an electrical connection from the exterior of the envelope to the third electrode wall coating. The plug connector 38 is sealed in the aperture in the funnel portion 12 of the envelope by means of a ceramic-to-metal seal 110.

The upper conductive film 102. is connected to a DC. target voltage produced across a fixed resistor 11?, by current flowing to the grounded terminal of such resistor from a source of +500 volts through a variable resistor 12d. Thus, the setting of the variable resistor 121i controls the flood gun cathode to target voltage of the upper portion of the split screen storage target 26 so that this portion may be operated in a storage or a nou-storage mode by setting the target voltage above or below the retention threshold voltage and below the fade positive voltage. ln addition, the variable resistor 12) may be employed to erase a charge image produced on the upper portion of the phosphor layer 1M by the writing gun 17 and stored due to the secondary emission caused by the flood electrons emitted by the flood gun 19 striking such phosphor layer. The lower conductive film 1&13 is similarly connected to a D.C. target voltage produced across a fixed resistor 114 by current flowing through a variable resistor 116 whose setting `controls the operation of the lower portion of the storage target. Therefore, the two portions of the phosphor layer overlying the

conductive film

102 and 103 may be operated independently in a storage or non-storage mode so that signal displays may be viewed but not stored on one target portion and then may be moved to the other target portion for storage by adjusting the vertical positioning control of the tube.

The direct viewing storage tube of the present invention may also be provided wit-h electrical readout merely by using the writing gun 17 also as a reading gun to scan the phosphor layer 104 of the storage target and produce electrical readout signals on the

conductive films

102 and 103 corresponding to any charge images stored on such phosphor layer. In this regard, the common connection of resistors 114 and 116 is connected to a rst output terminal 122 through a D.C. blocking capacitor 124, and the common connection of resistors 118 and 120 is connected to a second output terminal 126 through a D.C. blocking capacitor 128. These

output terminals

122 and 126 may be connected to the Z-axis input of a remote television monitor tube (not shown) to vary t-he intensity of the electron beam in such monitor tube in accordance with the electrical readout signals produced on

conductive ilms

102 and 103. Then if related television raster signals `are applied to the horizontal and vertical deflection plates of both the storage tube and the monitor tube, the charge image stored on the storage target 26 will be displayed on the fluorescent screen of the monitor tube in a conventional manner.

The mesh electrode 40 may be supported by spot welding it to an annular spring channel member 130 after such channel member is inserted into an annular notch 132 in the inner surface of the ceramic funnel portion 12 of the envelope. The channel member expands outwardly into engagement with the bottom of the not-ch 132 and is held against longitudinal movement by an annular ridge 134I extending from the inner surface of the funnel portion. A plurality of metal spring clips 135 may be secured to the channel member 132 by welding at spaced positions around such channel member so that such clips resiliently engage the second wall coating 32 to electrically connect the mesh electrode to such wall coating.

As in conventional bistable storage tubes, the electrons forming the writing beam emitted from the writing gun 17 of the tube of the present invention have sufiicient velocity to cause secondary electrons to be emitted from the storage dielectric layer 104 of storage target 26. The result is that the area at the point of impact of such writing beam acquires a net positive voltage above the first cross-over voltage on the secondary emission characteristic of such phosphor material. The ood electrons emitted from flood guns 19 are substantially uniformly distributed over the storage target 26 and, in a stable range, do not have sufficient impact velocity to cause much secondary emission from target areas which have not been struck by the writing beam. Thus, the secondary emission ratio is less than one and the ood electrons tend to negatively charge thoseV areas of phosphor storage layer 104 which have not been struck by the writing beam until such negative charge is suicient to repel most of the ood electrons. This target potential corresponds to, or approaches, the first stable point which is approxi mately equal to the potential of the ood gun cathodes. However, the flood electrons are further accelerated by any positive charge areas on the phosphor storage layer 104 which are present due to the secondary emission caused by bombardment of the writing beam. This additional acceleration provides the flood electrons with sufiicient impact velocity to cause greater secondary emission at those target .areas already having a positive charge :lue to the writing beam. Therefore, the secondary emission ratio is greater than one and the positive charge of these areas is thereby increased tending to drive the target potential to a voltage corresponding to the second stable point approximately equal to the target voltage on the

conductive films

102 and 103. It should be noted that the secondary emission characteristics of the storage target are different for the writing beam and the ood beam since their cathodes are at different voltages.

Some typical operating values for the electrodes in the storage tube of FIGS. 1 to 4 are a writing gun cathode voltage of 3,000 volts, a flood gun cathode voltage of Zero volt, a flood gun grid voltage of -lS volts and a flood gun anode voltage of +200 volts. Anode resistance coating 44 may be operated at +220 volts, the first electrode coating 28 at +200 volts, the second electrode coating 32 at +300 volts, and the third electrode coating 36 at +50 volts. The voltage of the conductive film 102 of the storage target may vary considerably but is typically held at +200 volts. In addition, the total current of both ood gun cathodes may be about l5 milliamperes while the typical total operating current of the conductive film 102 may be from about 2 to 9 milliamperes.

The storage and display characteristics of the storage target of the present invention as a function of the relative thickness of phosphor storage layer 104 are shown in FIG. 4. Briefiy, these characteristics are a stable range curve 136, a light image brightness curve 138 and a light image contrast curve 140. The ordinate of the stable range curve is target voltage stable range and such stable range curve 136 increases from zero volt to a maximum of about to 120 volts at point 142 and then returns to Zero volt at the thickness TS which represents .a critical thickness above which the phosphor layer 104 will no longer store an electrical charge image for an indefinite controllable time. The absolute value of Ts is in the neighborhood of .001 to .003 inch depending upon the type of phosphor used and is approximately one-half to onethird of the thickness Tc of conventional cathode ray tube screens. It should be noted that the storage target of the present invention has an extremely wide stable range of operating voltages up to about volts. In this range the phosphor storage layer will store a charge image without spreading or blurring for an indefinite time so as to provide a bistable storage tube. The stable range of conventional bistable storage tubes is ordinarily defined as that range of collector voltages between the retention threshold voltage where effective bistable storage begins and the fade positive point voltage where such storage stops and the charge becomes uniform over the phosphor layer due to the .action of the flood guns. In the present storage tube the stable range voltages are those established between the cathodes of the flood guns 76 and the conductive lm 102. The 100 volt stable range referred to is the initial range for new tubes and this large range is important in a bistable storage tube because the width of the stable range reduces with use due to deterioration of the storage dielectric caused by electron bombardment so that the lifetime of the tube is determined to a large extent by the width of its initial stable range.

It has been found that the brightness curve 138 increases from zero continuously `and at a rather rapid initial rate as the thickness of the phosphor layer 104 is increased from zero to TS so that adequate image brightness may be obtained with very thin layers far below that corresponding to the maximum stable range at point 142. However, the contrast curve 140 increases in value from Zero at a much lower initial rate than the brightness curve 138 as the phosphor thickness is increased from zero to Ts. Thus, the contrast curve usually determines the lower limit of useful phosphor thicknesses. While the absolute values of brightness and contrast are not indicated, the lower limit thickness at which curve 140 reaches a useful contrast is somewhere to the left of the thickness corresponding to point 142 depending upon the use to which the storage tubes are put. Thus, there is a critical range of thicknesses for the phosphor storage layer 104 beginning above Zero and ending below Ts within which such a layer will store an electrical charge image for an indefinite controllable time and still give adequate light image brightness and contrast. It will be noted that the brightness curve 138 levels off so that no substantial increase in brightness is obtained by increasing the thickness of the phosphor beyond the thickness Tc where the curve is substantially level. Above the thickness Tc, brightness decreases. Thus, the operative range of phosphor thicknesses depends upon both the particular type of phosphor employed and the intended use of the storage tube but, in general, the lower limit of this range is somewhat greater than zero and the upper limit is somewhat below one-half the thickness Tc at which the brightness curve becomes substantially level. For P-l type phosphor this critical range of thicknesses is approximately from .001 to .0025 inch.

It has been found that the contrast of the light image of the storage tube of the present invention can be er1- hanced by applying 35 volt pulses to the cathode of the flood gun 19 or +35 volt pulses to the target electrode film 102 for about 50% of the duty cycle of such flood gun. This reduces the background light emitted by the directviewing storage target 26 since the more energetic electrons from the flood gun during contrast enhancement tend to drive the surface of the storage layer 104 from the +40 volts obtained by the ood beam electrons toward the zero volts potential of the cathodes of the ood guns.

It has also been found that an improved type of operation of the storage tube results in increasing the stored writing speed of such tube by a factor on the order of two times for transient signals and one thousand times for repetitive signals. This increase in writing speed may be accomplished by turning off the flood gun during writing, as discussed above. Thus, the ood electrons are prevented from striking the phosphor layer 104 during the writing action of the writing electron beam as it moves across the surface of such phosphor layer to produce a charge image thereon, until after the voltage of such charge image exceeds the rst cross-over voltage which is the minimum voltage necessary for storage. One way of. accomplishing this is to apply a large negative pulse of about 200 volts to a control grid of the flood gun 19 or a +200 volt pulse to the cathode of such flood gun during the time of such writing action. This pulse may be produced by triggering a suitable pulse generator with a portion of the input signal and transmitting the remainder of such input signal through a delay line before applying it to the vertical detiecti-on plates of the writing gun 17. Another way is to employ a switch 144 as shown in FIG. l, to change such control grid voltage from to 200 volts to cut off the flood gun by moving the movable contact of such switch from the viewing position shown labeled View to the position labeled Integrate This enables fast rise time transient .signals applied to the vertical deflection plates of the writing gun 17 to produce a stored charge image on the storage target which could not previously be stored when flood electrons were allowed to strike such target during writing. As has been previously pointed out, the flood electrons act to oppose writing because they tend to drive the potential of the charge image down toward the voltage of the ood gun cathode in those target areas which are initially charged below the rst cross-over voltages. By turning off the ood gun during Writing this opposing action is eliminated and the writing speed of the storage tube effectively increased. The above technique can also be employed for storing the charge image of an extremely high frequency repetitive signal by maintaining the ood gun in its cut oit condition during several successive cycles of such signal. Thus, the voltages produced on the storage target by the charge images of the successive wave forms of such repetitive signal are added together or integrated because such wave forms are superimposed on the same area of the phosphor layer of such target. If the ood electrons are prevented from striking the phosphor storage dielectric layer until after the total voltage of the charge image exceeds the first cross-over voltage, such charge image will be stored for an Vunlimited time when such flood electrons are subsequently allowed to bombard such phosphor dielectric layer. This charge image integration method of operating a storage tube applies to all secondary emission storage tubes which employ low velocity electrons for holding the charge image, including those using transmission type storage targets.

It has also been discovered that the writing speed and the erase speed of the storage tube may be increased by about 25% if the operating level of the target electrode film 102 is shifted about ten volts positive and negative of normal during writing and erasing, respectively. This is apparently due to the fact that writing speed increases while the erase speed decreases with increased target voltage over the stable range of operating voltages. It should be noted that the mesh electrode 40 isolates the writing gun from changes in target voltage so that the deection sensitivity of such writing gun does not vary during the voltage pulsing of the conductive film electrode 102 for contrast enhancement or for increased writing and erase speeds.

In addition, the storage tube of the present invention may be provided with variable image persistence by lowering the potential on the conductive film electrode 102 to about +5 volts above the retention threshold voltage for the target and allowing the mesh electrode 40 to collect most of the secondary electrons emitted from the phosphor layer 104. In this manner the persistence time of the light image emitted by such phosphor layer can be adjusted so that the image of a previous signal wave form is extinguished progressively immediately in front of the electron beam forming a subsequent signal wave form which is very useful in medical diagnosis and research. This is believed to be due to the fact that the phosphor layer is not exactly uniform in thickness so that the retention threshold varies in different areas and some of these areas are not storing a bistable charge image. Halftone storage is also possible during these conditions.

From the above -it should be obvious to one having `ordinary skill in the art that various changes may be made in the detail of the above described preferred embodiments of the present invention. Therefore, the scope of the present invention should only be determined by the following claims.

I claim:

1. Apparatus for operating a bistable charge image storage device to enable a faster writing rate comprising:

a storage target including a storage dielectric;

rst means for forming a charge image on said storage dielectric of a repetitive input signal applied to said storage device;

second means for bombarding the storage dielectric substantially uniformly with low velocity electrons to enable bistable storage of said charge image by causing secondary electron emission from said storage dielectric; and

third means for preventing the low velocity electrons from bombarding the storage dielectric during the formation of the charge image until after the potential of such charge image exceeds the minimum voltage necessary for bistable storage by causing several successive cycles of said input signal to lie superimposed on said storage dielectric to increase the potential of said charge image above said minimum voltage.

2, Apparatus in accordance with claim 1 in which the first means bombards the storage dielectric with high velocity electrons which are modulated by the input signal to form such charge image.

3. Apparatus in accordance with claim 2 in which the storage device is a cathode ray tube and the charge image is formed by deecting the beam of high velocity electrons in accordance with the input signal applied to said tube so that such charge image is the waveform of said input signal.

4. Apparatus in accordance with claim 3 in which the storage dielectric is phosphor material which emits a light image corresponding to the stored charge image.

5. Apparatus in accordance with claim 1 in which the third means applies a large reverse bias voltage between a ood gun cathode which emits the low velocity electrons and a control grid associated with said ood gun cathode to prevent said low velocity electrons from bombarding the storage dielectric.

6. Apparatus in accordance with claim 5 in which the third means includes a puiser means for applying a reverse bias voltage pulse between the flood gun cathode and the control grid.

'1. Apparatus in accordance with claim 4 which also includes a fourth means for momentarily increasing the velocity of the low velocity electrons for a portion of the time said low velocity electrons bombard the storage dielectric to increase the contrast of the light image.

8. Apparatus in accordance with claim 7 in which the 25 fourth means includes pulser means for applying a plurality of voltage pulses to a target electrode of the storage target to increase the velocity of the low velocity electrons.

9. Apparatus in accordance with claim 4 in which the phosphor storage dielectric is a thin integral layer of phosphor material applied over a light transparent electrically conductive support surface, said layer being sutlciently porous to enable secondary electrons emitted from one side of the layer by bombardment of the low velocity electrons to be transmitted through said layer and collected by said conductive surface on the opposite side of the layer.

References Cited UNITED STATES PATENTS 3,144,579 8/1964 Holsirlger 315-12 X 3,155,869 11/1964 Firmin 315-12 3,165,665 1/1965 Firmin 315-12 3,214,516 10/1965 Anderson 315--12 X` 3,214,631 10/1965 Anderson 315--12 3,259,791 7/1966 Jensen 315--12v OTHER REFERENCES Knoil, M., and B. Kazan, Storage Tubes, John Wiley & Sons, New York, 1952, pp. 24, 53, 77.

Haeif, Andrew V.: A Memory Tube in Electronics, September 1947, pp. 80-83.

DAVID G. REDlNBAUGI-I, Primary Examiner.

T. A. GALLAGHER, Assistant Examiner.

Claims

1. APPARATUS FOR OPERATING A BISTABLE CHARGE IMAGE STORAGE DEVICE TO ENABLE A FASTER WRITING RATE COMPRISING: A STORAGE TARGET INCLUDING A STORAGE DIELECTRIC; FIRST MEANS FOR FORMING A CHARGE IMAGE ON SAID STORAGE DIELECTRIC OF A REPETITIVE INPUT SIGNAL APPLIED TO SAID STORAGE DEVICE; SECOND MEANS FOR BOMBARDING THE STORAGE DIELECTRIC SUBSTANTIALLY UNIFORMLY WITH LOW VELOCITY ELECTRONS TO ENABLE BISTABLE STORAGE OF SAID CHARGE IMAGE BY CAUSING SECONDARY ELECTRON EMISSION FROM SAID STORAGE DIELECTRIC; AND THIRD MEANS FOR PREVENTING THE LOW VELOCITY ELECTRONS FROM BOMBARDING THE STORAGE DIELECTRIC DURING THE FORMATION OF THE CHARGE IMAGE UNTIL AFTER THE POTENTIAL OF SUCH CHARGE IMAGE EXCEEDS THE MINIMUM VOLTAGE NECESSARY FOR BISTABLE STORAGE BY CAUSING SEVERAL SUCCESSIVE CYCLES OF SAID INPUT SIGNAL TO BE SUPERIMPOSED ON SAID STORAGE DIELECTRIC TO INCREASE THE POTENTIAL OF SAID CHARGE ABOVE SAID MINIMUM VOLTAGE.