US 3796909 A
Description (OCR text may contain errors)
United States Patent 1191 Chang et al. I
[ ELECTROLUMINESCENT STORAGE DISPLAY lnventors: lfay F. Chang, Mahopac; William B.
Pennebaker, Carmel, both of NY.
21 Appl. No.: 263,038
[451 Mar. 12, 1974 Primary ExaminerCarl D. Quarforth Assistant ExaminerP. A. Nelson Attorney, Agent, or Firm-John A. Jordan  ABSTRACT A bistable storage and display tube using an electrolu-  U.S. Cl. 315/12, 313/92 R, 313/108 B, 1 bl f h 340/166 EL mmescent ayer capa e o storing an on c arge pattern, created by an electron write gun, on the mner  Int. Cl. H011 29/41 surface thereof. An electron flood gun is employed to  Field of Search 315/1 313/92 clam the otential of the inner surface of the electro- 313/92 R, 108 R, 108 B; 340/166 EL P luminescent layer, in the on regions where a charge  References Cited pattern is created, to the potent1al of a collector elec' trode. An a.c. signal applied to the electroluminescent UNITED STATES PATENTS layer, via a transparent conductive layer in contact 3,641,533 2/1972 Sylvander 340/166 EL X with the outer urface theregf acts to develop an 3,673,572 6/1972 Silva et a1. 340/166 EL ternating fi ld across h layer i h regions, to 3,723,977 3/1973 Schaufele" 340/166 EL x thereby produce luminescence thereat 2,905,849 9/1959 Kazan 315/12 X 3,087,086 4/1963 Turner 315/12 X 11 Claims, 2 Drawing Figures 51, T VF FLOOD 'T. /GUN 11 4/1 22 gjere \V 1 13 15 23 27 E: \V K 7/ l n F 20 "X i x T [3 '1' FLOOD 35 7r A: GUN I 33%? E I ELECTROLUMINESCENT STORAGE DISPLAY BACKGROUND OF THE INVENTION The present invention relates to bistable storage and display devices, and more particularly to bistable storage and display tubes which utilize electroluminescence and cathodoluminescence to achieve bright storage display.
One of the difficulties encountered in conventional, two-gun, bistable storage and display tubes, resides in their characteristic low brightness. For example, the typical two-gun bistable storage and display tube utilizing a cathodoluminescent effect suffers from low brightness for long viewing (storage) time. In order to achieve stability of operation in the typical cathodoluminescent bistable tube, it is necessary to operate at relatively low d.c. voltages. On the other hand, in order to obtain tolerable levels of brightness in such tubes, it is necessary to operate at relatively high levels of beam current. However, because of the particular nature of the light emitting mechanism involved in cathodoluminescence, the relatively high levels of beam current required to give practicable levels of brightness, tend to cause the cathodoluminescent material to exhibit a relatively short lifetime. Thus, it can be seen that conventional prior art bistable storage and display devices, and in particular prior art bistable storage and display devices relying upon a cathodoluminescent effect, suffer from low brightness and short life.
SUMMARY OF THE INVENTION In accordance with the principles of the present invention, an improved bistable storage and display device is obtained by employing an electron flood gun to clamp the potential on the inner surface of display material, at those regions where the material is in an on state, to a fixed potential and to allow the potential on the inner surface of the display material at those regions where the material is in an off state to float with an a.c. voltage applied thereto. More particularly, in accordance with the principles of the present invention a bistable storage and display tube exhibiting relatively high brightness and long life is obtained by using electroluminescence, in addition to possible cathodoluminescence, in an arrangement wherein an electroluminescent material is employed as the display surface, and has applied thereto an a.c. voltage. The a.c. voltage creates an alternating field across the electroluminescent material to produce electroluminescence in on regions where the inner surface of the electroluminescent material is held at a fixed potential by an electron flood gun, in response to a charge pattern produced thereon by a high energy electron writing gun. In the off" regions where no charge pattern is formed, the electron flood gun has no effect thereon, and the a.c. voltage fails to produce an alternating field to cause electroluminescence thereat.
It is, therefore, an object of the present invention to provide an improved bistable storage and display device.
It is a further object of the present invention to provide a bistable storage and display tube with improved brightness.
It is yet a further object of the present invention to provide a bistable storage and display tube with long life.
It is still a further object of the present invention to provide a bistable storage and display tube with capability of operating in storage mode and display mode separately, i.e., the a.c. power may be turned off when viewing is not desired, yet the information is still in storage.
It is yet still a further object of the present invention to provide a bistable storage and display tube which utilizes the mechanism of both electroluminescence and cathodoluminescence.
It is another object of the present invention to provide a bistable storage and display tube with readily controllable brightness.
It is yet another object of the present invention to provide an improved bistable storage and display tube which utilizes electroluminescence in a manner to provide improved brightness and long tube life.
It is yet still another object of the present invention to provide a bistable storage and display tube which utilizes an electron flood gun to clamp the potential in the on regions of an electroluminescent display surface so that the a.c. voltage applied across the electroluminescent material therefor generates electroluminescence in the on regions, while the potential in the off regions of the electroluminescent material vary with the a.c. voltage so as to produce no luminescence thereat.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a cross-section of the bistable storage and display tube, in accordance with the present invention.
FIG. 2 shows a plot of typical secondary electron emission ratio characteristics for the electroluminescent display material employed in FIG. 1.
DETAILED DESCRIPTION OF THE DRAWINGS In FIG. 1, there is shown an embodied configuration of the electroluminescent storage and display tube, in accordance with the principles of the present invention. In general, the overall configuration of the storage tube shown in FIG. 1 is analogous to any of the variety of cathode ray type storage tubes, employed in the prior art. For example, conventional two gun cathodoluminescent bistable storage tubes employ a configuration and a structure, somewhat analogous to that shown in FIG. 1. In the conventional cathodoluminescent storage tubes, however, the layer of material shown at 1 is cathodoluminescent while in the arrangement, in accordance with the principles of the present invention, the layer of material at 1 is electroluminescent. In addition, as distinct from conventional cathodoluminescent storage tubes, the arrangement shown in FIG. 1 employs a fine mesh 3 in front of electroluminescent material 1 and a conductive baclc plate 5, behind the electroluminescent material 1. Finally, as distinct from prior art cathodoluminescent storage tubes, the arrangement in FIG. 1 utilizes the application of an a.c. voltage, via a.c. source 7 to the conductive plate 5. The utilization of these distinctions, as will be described in more detail hereinafter, act to provide a mode of operation heretofore unknown in storage tubes.
It is clear, that storage tube 9 may comprise any of a variety of conventional storage tube materials and configurations. Typically, storage tube 9 would be made of glass. In addition, the high energy electron writing gun shown at 11, comprises a conventional configuration, well known to those skilled in the art. In particular, it can be seen that high energy electron source 13 acts to emit high energy electrons through focusing element 115 to the vertical and horizontal fields created by the deflection plates shown at l7, l9 and 21. As can be seen, plates 17 and 19 act to vertically deflect the high energy electron beam while plate 21, V another plate not shown, acts to horizontally deflect the electron beam.
Likewise, in conventional fashion, flood guns 23 and 25 acting through respective focusing elements 27 and 29, serve to flood the entire surface of electroluminescent layer 3 with relatively low energy electrons. Flood guns 23 and 25 are coupled, as shown, to the negative side of respective power supplies 31 and 33, while the positive side of latter supplies are coupled to ground. In like fashion, high energy electron source 13 is coupled to the negative side of power supply 35, while the positive side of latter supply is coupled to ground. Typically, source 13 may be a two kv. high energy electron source. d.c. supplies 31, 33 and 35 may, for example, be set at around 300 volts. However, it is clear that the value of these supplies is not critical, and accordingly, their values may be set anywhere within the range of from 1 to several hundred volts.
As can be seen in FIG. 1, conductive mesh 3 is coupled to ground, and serves to act as the collector for the secondary emission electrons, given off by electroluminescent layer 1. It is understood, however, that this collector may comprise any configuration, such as a ring electrode arrangement, and may be fixed to any potential level, as long as the positive voltage difference relative to the cathode is maintained.
Electroluminescent layer ll may comprise any of a variety of well known electroluminescent materials. Preferably, layer 1 comprises an electroluminescent phosphor, in powder form, embedded in an inorganic dielectric binder. For example, layer 1 may comprise Cu doped ZnS uniformly mixed into a glass binder. Exemplary of the electroluminescent materials that may be employed, and methods for fabricating same, are those described by Blazej et al. in U. S. Pat. No. 3,313,652. An alternative scheme may be employed whereby an electroluminescent layer is coated with another layer having a high secondary emission ratio.
As is evident from the nature of the device being described, conductive layer 5, shown in FIG. 1, must nec essarily be of a transparent nature. In this regard, any of a variety of transparent conductors may be employed, such as, oxides of tin. indium or copper, or etched thin metal layers. Likewise, thin layers of copper iodide may be employed. For fabrication purposes, it is clear that electroluminescent layer I may be deposited upon oxide layer 5, or vice-versa. With layers I and 5 mounted in the envelope of tube 9, the tube is finally sealed with transparent layer 37, which typically may be a relatively thick layer of glass.
Contrary to cathodoluminescent material which exhibits luminescence in response to the impingement of electrons on the surface thereof, electroluminescent material exhibits luminescence in response to an alternating field applied thereacross as known in the art. However, to selectively apply such alternating fields to local regions of electroluminescent material, for display on the storage purposes, proposes some difficulty. In accordance with the principles of the present invention, an alternating field is applied to selected local re gions of electroluminescent layer 1, for purposes of bistable storage and display, by the utilization of the secondary emission characteristics of the electroluminescent material, and the action of the electron flood guns, as shown in FIG. ll. Briefly, by this action, alternating current source 7, designated V acts to apply an alternating field across electroluminescent layer 1 in those regions where writing gun 11 has created a charge pattern, in accordance with the information to be stored and displayed. Thus, it is clear that where the alternating current is developed across layer 1, luminescence is produced. For purposes of explanation, the regions in luminescent layer 1 where a luminescence condition exists, corresponding to the regions where a charge pattern has been written, will be considered in the on" condition. As will be described in more detail, hereinafter, the on condition corresponds to the high potential equilibrium condition of the device. On the other hand, the non-luminescent condition will be con sidered the off condition, or the low potential equilibrium state. Briefly, then, alternating fields are produced across luminescent layer l at regions in the on" condition by clamping the charged surface thereof to the potential of collector electrode 3, via the flood gun beams. Those regions of electroluminescent layer 1 which are uncharged, are in the low potential equilibrium state having no flood beam landing, and are thus free to float with potential variations in the alternating current source 7, as will be described below. It should be noted, that the function of do source 39 (V is to bias electroluminescent layer 1 so as to prevent leakage current and breakdown. Thus, source 39 is selected so as to establish an operating point for layer 1 so that the potential variations thereon will not swing into the breakdown regions thereof.
FIG. 2 shows the typical electron emission characteristics as a function of electron energy, for conventional electroluminescent phosphors. As shown, the horizontal straight line across the characteristic curve corresponds to 6 l, where 8 is the ratio of electrons coming away from the material to the electrons going into the material. In this regard, the electrons coming away from the material are considered the sum of the two secondary electrons plus any back scattered or reflected electrons. As can be seen, where the electron energy is O or negative, corresponding to the region to the left-hand of the ordinate, 8 I. This ratio implies that all electrons entering the vicinity of the material do not have sufficient energy to enter therein to produce secondary electrons, but are rather scattered or reflected on a lzl basis. This corresponds to a condition of equilibrium, and as designated in FIG. 2, at 0 electron energy, a low potential equilibrium condition exists, designated V As the electron energy increases from 0 potential at V the ratio 5 declines. This declination indicates that more electrons are entering the electroluminescent material than are being given off. Thus, the material is accumulating a charge. At V the ratio is unity and thereafter it can be seen that more electrons pass from than enter the material. This point is an unstable point since any deviation in electron energy would either charge down to V or charge up to V The dotted line projecting from the peak of the characteristic curve between V and V represents the normal decline seen in the number of electrons coming away from the material, as the electron energy increases. As shown, a unity ratio is again seen at V As will be explained below, the solid line passing through V represents the effect of collector 3, adjacent electroluminescent material 1. Positioning collector 3 at this potential level has the effect of modifying the ratio in question to establish an equilibrium point at VEQQ.
Accordingly, it can be seen that in order to achieve good secondary emission from electroluminescent material 1, electrons must approach the material with an energy within the range V to V In accordance with the principles of the present invention, in order to establish a constant high potential equilibrium condition within the electron energy range, collector electrode 3 is positioned adjacent electroluminescent layer 1 at a d.c. potential less than, but close thereto. In the operation of the arrangement shown in FIG. 1, assume that initially no charge pattern occurs upon electroluminescent target 1. It is clear, that this corresponds to no information appearing thereon. Under such conditions, none of the electrons from flood guns 23 and 25 are attracted to the surface of the electroluminescent material. Accordingly, the electroluminescent material exhibits no secondary emission, and 8 1, as described above. Such a condition corresponds to the low potential equilibrium condition V as shown in FIG. 2. Since electroluminescent material 1 is not affected by the electron beam from the flood guns to give off secondary emission, the inner surface 1A thereof is electrically free to vary in potential in accordance with the potential applied to back plate 5. Thus, the potential of inner surface 1A of electroluminescent material 1 follows the potential of alternating voltage source 7, applied to plate 5. Under such a condition, no alternating field is effected across the electroluminescent material.
Viewed in somewhat a different manner, it can be seen that with electroluminescent material 1 in its low voltage equilibrium condition V the alternating current source 7 initially commences to operate about the 0 potential ordinate, shown in FIG. 2. With the first positive half cycle, electrons from flood guns 23 and 25 tend to accumulate in electroluminescent material 1, in accordance with the decline in the secondary electron emission characteristic 8, as it moves from 0 potential. Since electroluminescent material 1 is basically a dielectric, the charges accumulated on this and subsequent positive half cycles of the alternating current source tend to build up in the material. This build up acts to produce a self-biasing effect, whereby after several cycles, a net negative d.c. bias equal to the peak voltage of the alternating current signal is produced in the material. Accordingly, with this self-biased condition produced, the alternating voltage source swings between 0 and some negative potential. As can be seen, 8 in the negative electron energy region is l and no secondary electrons are produced.
To achieve an on state or high potential equilibrium condition at selected regions in electroluminescent material 1, writing gun 11 is initially employed to effect a desired charge pattern image on the inner surface 1A thereof. It is clear that this charge pattern image may comprise any of a variety of images, such as an array of alphanumeric characters. As soon as the charge pattern is developed upon the electroluminescent material, the charge acts to attract electrons from flood guns 23 and 25. The attracted electrons, in turn, produce secondary electron emission which is collected by collector 3. Accordingly, at these charge locations, corresponding to the information written,
the electroluminescent material is at the high potential equilibrium condition corresponding to V shown in FIG. 2. The effect of collector 3 collecting the electrons emitted from the electroluminescent material in the regions of the charge image, is to clamp the surface 1A in these regions to the potential of the collector, i.e., ground. With surface 1A in the region corresponding to the charge pattern image clamp to ground, alternating voltage source 7 is able to produce an alternating current thereat. Accordingly, electroluminescence is generated in the electroluminescent material beneath the regions on surface 1A containing the charge pattern.
Thus, it can be seen that flood guns 23 and 25 are employed to clamp the surface 1A of electroluminescent material 1, at regions corresponding to the charge pattern written therein, whereby the alternating current applied to back plate 5 is allowed to develop across the electroluminescent material to produce luminescence. In the regions of surface 1A where no charge pattern has been written, the surface is allowed to vary in potential in accordance in which the manner in which the alternating current source 1 1 varies. These latter regions correspond to the of or low potential equilibrium condition of the electroluminescent material. As stated above, the writing gun 11 acts to produce a beam of much higher landing energy and current density than electrons from flood guns 23 and 25, whereby selected electroluminescent target elements are shifted positive above the first crossover V to V shown in FIG. 2, This constant high potential equilibrium condition is maintained, even with the ac. voltage of alternating current source 7 applied to back plate 5, so long as the condition holds (where J, is the surface current to the on elements, J, is the flood gun current density, C is the capacitance per unit area of the electroluminescent layer, and V is the back plate potential with the dot signifying a time derivative).
In a typical design arrangement, J, can be shown to be greater than CV by choosing parameters and conditions as follows:
Screen size 8 X 10 cm Phosphor Dielectric Constant 25 Phosphor thickness 3 mils V, 200 sin 271' X 500 X t flood gun current 2 ma.
With the above design parameters, then,
J lma/cm 0.025 mA/cm taking 81 to be of order unity, when the energy is less than V then CV 23 /3X25Xl0' X 200 X 211' X 500 0.0148 mA/cm therefore Although a.c. source 7 has been shown as connected to layer for purposes of developing an a.c. field, it is clear that layer 5 may be held at a fixed potential and mesh collector 3 coupled to an a.c. source, as an alternative for developing such a field.
It should be noted that in addition to obtaining electroluminescence at selected regions of electroluminescent layer 1, it is also possible to obtain at these same regions a certain amount of cathodoluminescence. In this regard, many electroluminescent materials also exhibit cathodoluminescence. Thus, it is clear that such materials might be employed, whereby in addition to the luminescence produced by the alternating field across selected regions, electrons from the flood guns, which are attracted to the selected regions, act to provide a certain amount of cathodoluminescence thereat. Alternatively, it is possible that a thin layer of cathodoluminescent material be deposited upon the inner surface 1A of electroluminescent material 1, whereby both electroluminescence and cathodeluminescence would be effected, in accordance with the principles of the present invention.
It should also be noted, that the principles of the present invention whereby selected regions upon a display target are clamped to a fixed potential to allow an alternating field current to be developed thereat, can likewise be applied to targets other than electroluminescent targets. For example, it is possible to employ an array of liquid crystal cells, or gas panel cells, for target layer 1. In accordance with such an arrangement, the inner surface of the target, corresponding to 1A in FIG. 1, would be coated with a layer of secondary electron emission material held in a dielectric binder, as in the present instance. With such an arrangement, a charge pattern could be written upon the secondary emission material, in the manner described above. With the information of the charge pattern, electron flow from the flood guns to the secondary emission material, and the secondary electrons given off and collected by the collector, would act to clamp the inner surface of the liquid crystals or gas cells. Accordingly, an a.c. potential could be developed thereacross to produce a visible change or glow. It is clear that other type display mechanisms might likewise be employed.
It should be recognized that although, in accordance with the present invention, relatively large densities of current are required to produce illumination, the mechanism of producing luminescence by electroluminescent materials is such that these large current densities do not appreciably affect the life of the material. In addition to long life, the arrangement described has the potential for being able to exhibit relatively high levels of brightness.
It should also be recognized that regardless of what arrangement is employed, the electroluminescent material will have a limited life. Accordingly, in order to provide a replaceable electroluminescent layer, rather than replace the whole tube, it should be appreciated that the inner layer of the tube, corresponding to layer 1 in FIG. I, may comprise merely a secondary electron emissive material in a dielectric binder. With such an arrangement. an electroluminescent layer with contact- 6 be made between emissive layer 1 and the outer electroluminescent layer by an array of conductive feedthrough pins, through glass layer 37. Such an arrangement would be analagous to the metal pin arrangement described in the IBM Technical Disclosure Bulletin, Vol. 12, No. 12, May 1970, page 2324, in an article entitled Gas Discharge Display Device by D. M. Hart.
While this invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.
What is claimed is:
1. In a cathode ray type storage display device having a higher energy electron write gun, electron flood gun means and a write surface means capable of retaining a charge pattern written by said write gun, the improvement comprising:
means included in said write surface means to produce a visible change at selected points on the viewing surface thereof according to a charge pattern written by said write gun thereon, with said visible change occurring in response to an a.c. signal applied to the said selected points, said means included in said write surface means also acting to produce secondary electron emission at said selected points in response to electrons from said flood gun means entering said write surface means at said selected points according to said charge pattern;
means for applying an a.c. signal to said write surface means; and collector electrode means positioned adjacent said write surface means for collecting the said secondary electrons emitted from said selected points and causing said selected points to become clamped in potential to the potential of said collector electrode means so as to cause said a.c. signal to be developed thereat to produce said visible change. 2. The display device as set forth in claim 1 wherein means included in said write surface means comprises electroluminescent material held in a dielectric binder to form an insulating layer at said write surface.
3. The display device as set forth in claim 2 wherein said means for applying an a.c. signal to the said insulating layer comprises a transparent conductive layer in contact with the viewing side of said insulating layer.
4. A storage display tube device comprising: storage means including a layer of storage material means having an inner write surface and an outer display surface thereof and being responsive at selected local regions on said inner surface to electrons provided thereto to produce secondary electron emission on at least the said inner surface thereof, said storage material means further being responsive to an a.c. signal applied to the said selected local regions thereof to produce a visible change on the said outer display surface thereof;
means to apply an a.c. signal to said storage material means;
collector electrode means positioned adjacent the said inner surface of said storage material means for collecting electrons;
electron write gun means for selectively writing a charge storage image pattern at said local regions on the said inner surface of said storage material means; and
electron flood gun means for flooding the said charge storage image pattern with electrons to cause said storage material means to produce secondary electron emission at said local regions and clamp the potential thereat to the potential of said collector electrode means such as to cause the said a.c. signal applied to said storage material means to produce the said visible change on the said outer display surface, according to said charge storage image pattern.
5. The storage device as set forth in claim 4 wherein said layer of storage material means comprises a first transparent conductive layer on the said outer display surface thereof and second electroluminescent material layer means in contact with said conductive layer and arranged to provide said secondary emission on the said inner surface thereof in response to said electrons attracted to the charge storage image pattern thereon so as to thereby provide electroluminescence at said selected regions on said display surface in response to said a.c. signal applied to said conductive layer.
6. The device as set forth in claim 4 wherein said layer of storage material means is all within the said tube of said device.
7. The device as set forth in claim 6 wherein said collector electrode means is in the form of a conductive grid positioned adjacent the said inner surface.
8. The device as set forth in claim 7 wherein said means to apply an a.c. signal includes DC bias means arranged to DC bias said electroluminescent material so as to reduce leakage current therefrom.
9. The device as set forth in claim 5 wherein said storage material means is all within the said tube of said device.
10. A method of operating a cathode ray type storage and display tube device having an electron write gun means, electron flood gun means and an insulating layer at the display surface thereof for storing a charge pattern thereon with said write gun means, the improvement comprising: 1
using a secondary electron emissive material in said insulating layer for producing secondary electron emission from said charge pattern in response to electrons being attracted thereto from said electron flood gun means;
employing a collector electrode adjacent said insulating layer for collecting secondary electrons emitted therefrom;
writing a charge pattern over selected regions on the inner surface of said insulating layer with said write gun means so as to cause electrons from said flood gun means to be attracted to said charge pattern and produce secondary electron emission therefrom so as to thereby fix the surface potential of said insulating layer at said regions to the potential of said collector electrode;
applying an alternating signal to the outer surface of said insulating layer to cause an alternating field to be developed between the fixed potential of said regions and said outer surface. 7
1 1. The method as set forth in claim 10 wherein said insulating layer includes an electroluminescent material.