Title:
FERROELECTRIC CERAMIC STORAGE DISPLAY TUBE
United States Patent 3792449


Abstract:
A ferroelectric ceramic is mounted within a CRT in a "strain-biased" state, so that it is birefringent. A reflective layer is mounted adjacent the ceramic, and a photoconductive layer is mounted adjacent the reflective layer. With a potential applied across the ceramic-photoconductive layer combination, an image written upon regions of a phosphor target adjacent the photoconductive layer by an electron beam, results in the flow of local charge through corresponding regions of the ceramic, thereby changing the polarization thereat. The flow of polarization charge acts to modulate the birefringence in the ceramic, in accordance with the pattern of the image. A corresponding pattern of brightness is then projected upon an external screen. A "scattering mode" arrangement is also employed.



Inventors:
KAZAN B
Application Number:
05/267750
Publication Date:
02/12/1974
Filing Date:
06/30/1972
Assignee:
IBM,US
Primary Class:
Other Classes:
348/767, 359/262, 365/110, 365/117
International Classes:
H04N5/74; G02B27/50; G02F1/03; G02F1/05; G11C11/23; H01J31/08; H01J31/12; (IPC1-7): G11C11/22
Field of Search:
178/7.5D 340
View Patent Images:



Other References:

IBM Tech. Dis. Bul. Vol. 5, No. 3, Aug. 1962 pp. 54-56. .
"Photostorage System" by R. M. Schaffort..
Primary Examiner:
Fears, Terrell W.
Attorney, Agent or Firm:
Jordan, John A.
Claims:
What is claimed is

1. In a cathode ray-type storage display tube arrangement, the improvement comprising:

2. The display tube arrangement as set forth in claim 1 wherein said means to apply a voltage across the said combination of said layer of ferroelectric ceramic means and said layer of photoconductive material means includes a pair of transparent conductive layers respectively mounted adjacent thereto on each side thereof.

3. The display tube arrangement as set forth in claim 2 wherein said projection means includes a projection screen for projecting thereon the light reflected through said birefringence pattern.

4. The display tube arrangement as set forth in claim 3 wherein said layer of ferroelectric ceramic means comprises lead zirconate-lead titanate ferroelectric ceramic material.

5. In a cathode ray-type storage display tube arrangement, the improvement comprising:

6. The display tube arrangement as set forth in claim 5 wherein said means to apply a voltage across the said combination of said layer of ferroelectric ceramic material means and said layer of photoconductive material means includes a pair of transparent conductive layers respectively mounted adjacent thereto on each side thereof.

7. The display tube arrangement as set forth in claim 6 wherein said rhombohedral-phase lead-lanthanum-zirconate-titinate ferroelectric ceramic material has a grain size of approximately 4 to 5 microns.

Description:
BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to storage display tube arrangements, and more particularly to ferroelectric ceramic storage display tube arrangements wherein the image stored in the tube, which is projected for display purposes onto a viewing screen, may be selectively erased and written locally on the ceramic storage medium thereof.

2. Description of the Prior Art

One of the major difficulties with present day storage display tubes resides in their inability to be selectively erased. In addition, present day storage display tubes suffer from the fact that dynamic or rapidly changing images cannot be displayed without disturbing the stored image. Moreover, present day tubes presently considered most promising for computer terminal applications, suffer from the additional shortcomings of exhibiting low brightness (marginal in many applications), and the inability to store and display half tone information. Typical of such type tubes, is the bistable-phosphor cathode ray storage display tube (CRT).

In addition to storage display devices of the tube-variety, other type storage display devices have been developed in which the input information must be in optical form. Typical of such storage display devices, is that described by J. R. Maldonado et al. in the Proceedings of the I.E.E.E., Vol. 59, No. 3, March 1971, pp. 368 etc., in an article entitled "Strain-Biased Ferroelectric-Photoconductor Image Storage and Display Devices." To address such a device requires an auxiliary cathode-ray tube or a modulated scanning light beam. This results in complex and bulky equipment which may also be expensive.

SUMMARY OF THE INVENTION

In accordance with the principles of the present invention, a ferroelectric ceramic storage display tube is provided, whereby electrical information can be stored and selectively erased, at high speeds. In addition, in accordance with such an arrangement, non-stored images can be superimposed on stored images, and an electrical readout signal may readily be produced.

To achieve this end, the inner surface of the glass faceplate of a CRT is provided with a transparent conductive coating upon which is mounted a ferroelectric ceramic wafer. The surface of the wafer is then coated with a mosaic of reflecting conductive elements, and the mosaic surface of the conductive reflecting elements is, in turn, coated with a photoconductive layer. The photoconductive layer is then coated with a transparent conductive coating, and the latter is, in turn, coated with a thin layer of phosphor. The ceramic wafer is arranged to be "strain-biased," so that it is birefringent. A "write" potential applied across the transparent conductive coatings acts to develop, in the ceramic wafer, a transverse field having an intensity which is modulated by the local resistivity of the photoconductive layer.

Accordingly, when selected areas of the layer of phosphor are bombarded by an electron beam, in accordance with electrical input signals, the phosphor is locally excited, inducing local conductivity in the adjacent photoconductive layer, in accordance with the degree of excitation caused by the beam. The local conductivity, in turn, allows a flow of local polarization charge through corresponding areas of the ceramic layer, thereby modulating the birefringence in these areas, in accordance with the pattern written by the electron beam. Then external plane-polarized light, reflected back after passing through the ceramic wafer, exhibits a phase delay pattern of the two perpendicular components of the polarized light wherein the components vary from area to area across the ceramic wafer, resulting in a component of polarization perpendicular to the original polarization direction in accordance with the polarization pattern produced therein, during writing. Since only the latter components are transmitted through the prism, brightness variations are produced in the reflected light which is projected upon a viewing screen, and a pattern of varying brightness corresponding to the stored image is produced.

As an alternative to the strain-biased mode of operating the ferroelectric ceramic wafer, the wafer may be operated in a scattering mode. In such an arrangement, patterns written upon the photoconductive layer, by the electron beam, act to cause local polarization changes in the ceramic wafer, whereby external light reflected by the mosaic of reflecting elements is scattered at local regions, in accordance with the written pattern. This process whereby scattering is produced in a ferroelectric ceramic is described in the paper "Scattering Mode Ferroelectric -- Photoconductor Image Storage and Display devices," by W. D. Smith and C. E. Land, Applied Physics Letters, Vol. 20, No. 4, Feb. 15, 1972, pp. 169-171.

It is, therefore, an object of the present invention to provide an improved storage display tube.

It is a further object of the present invention to provide an improved storage and display tube, which tube utilizes a ferroelectric ceramic.

It is yet a further object of the present invention to provide an improved storage and display device, whereby information to be stored and displayed may be written therein at high speeds.

It is yet still a further object of the present invention to provide a storage display tube arrangement whereby electrical information can be stored and selectively erased at high speeds.

It is another object of the present invention to provide a storage display tube whereby non-stored images can be superimposed upon stored images.

It is yet another object of the present invention to provide a storage display tube capable of providing an electrical readout signal indicative of the information stored and displayed therein.

It is still another object of the present invention to provide a storage display arrangement whereby the information stored therein may be readily projected upon a viewing screen.

It is yet still another object of the present invention to provide a ferroelectric ceramic storage display arrangement wherein the ferroelectric ceramic is operated either in a strain-biased or scattering mode.

It is a further object of the present invention to provide a ferroelectric ceramic storage and display device wherein the image to be stored and projected thereby may be written by an electron beam, and wherein non-stored images may be superimposed, in a different color, on the stored image.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows, in partial cross-section, one embodiment of the ferroelectric ceramic storage display arrangement in accordance with the principles of the present invention, whereby the ferroelectric ceramic is operated in the "strain-biased" mode.

FIG. 2 shows, in partial cross-section, a further embodiment of the ferroelectric ceramic storage arrangement in accordance with the principles of the present invention, whereby the ferroelectric ceramic is operated in the "scattering" mode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the embodiment of FIG. 1, a ferroelectric ceramic layer or wafer is arranged to operate within the envelope 1 of a cathode ray type storage tube in a strain-biased mode.

The overall configuration of the tube is, in general, analogous to any of the variety of conventional cathode ray type storage and display tube arrangements, heretofore employed in the prior art. Typically, the envelope 1 and faceplate 3 may be made of glass, with the faceplate obviously being transparent. The electron writing gun, shown generally at 5, comprises a conventional configuration for producing a focused electron beam which can be scanned across the target structure of the tube.

In particular, it can be seen, in the arrangements shown, that electron source 7 acts to emit electrons through focusing element 9, and the field created by the vertical deflection plates 11 and 13. As is well known to those skilled in the art, deflection plates 11 and 13 act to control the vertical position of the electron beam. For the sake of simplicity, horizontal deflection plates have not been shown. However, as is understood by those skilled in the art, the omitted pair of horizontal deflection plates act to control the horizontal position of the electron beam. Thus, by appropriate application of signals to both the vertical and horizontal deflection plates, the electron beam can be controlled so that a pattern or image can be formed upon an appropriate write surface, i.e., target, within the tube, in conventional fashion. In the embodiments shown in both FIGS. 1 and 2, this write surface comprises a layer 27 of phosphor, to be described in more detail, hereinafter. Electron source 7 is connected, as shown, to the negative terminal of an appropriate dc source 15. Typically, source 15 may be 5 kv. As understood by those skilled in the art, the intensity of the electron beam is modulated by the control grid of the electron source, i.e., electron gun, in accordance with an input signal (not shown), thereby creating a luminescent pattern on the phosphor layer in accordance with the time varying magnitude or the like of the signal.

The inner surface of glass faceplate 3, is provided with a transparent conductive coating or layer 17. Typically, such a conductive layer would be deposited upon the faceplate. Mounted upon transparent conductive layer 17, is a layer or wafer 19 of ferroelectric ceramic. It is clear, that the ceramic layer 19 may be formed separately, and then cemented to the transparent conductive layer 17. Alternatively, the ceramic layer may be deposited directly upon conductive layer 17 by any of a variety of conventional techniques, such as by vapor deposition or sputtering techniques. Typically, the ceramic layer would be about 1 inch in diameter, and 2 mils thick.

Although not shown in FIG. 1, both the faceplate and the ceramic wafer are maintained in a mechanically-stressed condition so that the polarization direction of the ceramic tends to be in the direction along the surface of the ferroelectric ceramic. Although any of a variety of materials may be employed for ceramic wafer 19, so long as such materials are capable of producing birefringence, for purposes of example for the embodiment of FIG. 1, the wafer may be taken as fabricated from fine-grained lead zirconate-lead titanate ferroelectric ceramic materials.

As shown in FIG. 1, the surface of ceramic layer 19 is coated with a mosaic 21 of conductive reflecting elements, comprising, for example, evaporated aluminum. The reflecting elements are, in turn, coated with a photoconductive layer 23. The photoconductive material may typically comprise CdS, produced, for example, by evaporation or sputtering.

The outer surface, i.e., the surface of the electron beam side, of the photoconductive layer 23 is then coated with a transparent conductive layer or coating 25, akin to transparent layer 17. In this regard, transparent conductive layers 17 and 25 may comprise a thin layer of any of a variety of metals or metal oxides, such as gold or aluminum metal, or metal oxides such as the oxides of tin or indium. Likewise, these materials may be fabricated by any of a variety of conventional fabricating techniques. Finally, after transparent conductive layer 25 is formed, a thin layer 27 of phosphor is formed thereon, as shown. The thin layer of phosphor may comprise, for example, ZnO, or ZnS, either in powder form, or produced as a continuous film by evaporation.

As hereinabove indicated, ceramic layer 19, in the embodiment of FIG. 1, is operated in the strain-biased mode. Strain-biasing ferroelectric ceramics is well known to those skilled in the art. Typical of the techniques for achieving a strain-biased condition, are those described by J. R. Maldonado et al., cited above. Although not shown in detail in FIG. 1, the ferroelectric ceramic layer 19 may typically be strain-biased to produce a uniform strain condition, by bending the faceplate 3. Thus, after the various layers of transparent conductive material and ferroelectric ceramic have been appropriately bonded to one another and to the faceplate, in the sandwich form as shown, the faceplate may be flexed either inwardly or outwardly to produce either tensile or compressive strain in the ceramic.

As is well known to those skilled in the art, straining the ceramic layer or wafer as indicated, produces a state of birefringence that results in part from the photoelastic effect in the ceramic, and in part from the realignment of domains in a direction parallel to the tension axes. Thus, by inducing in the ceramic wafer a uniform strain, a preferred orientation of the polarization in the plane of the wafer is established, whereby the wafer becomes birefringent, with the principle axes of the optical indicatrix along the strain axes. The magnitude of the resultant birefringence can then be controlled by an electric field applied in the thickness direction. Because of the ferroelectric hysteresis of the ceramic, an image may be stored in the ceramic by locally modulating the birefringence therein, in accordance with the desired image, whereby a spatial modulation of birefringence corresponding to the image, is obtained.

Thus, with ferroelectric ceramic wafer 19 in a strain-biased condition so that the wafer thereby becomes birefringent, the tube is ready to write an image, to be stored therein. As shown in FIG. 1, transparent conductive layer 25 is held at ground potential by conductor 29. For the "write" condition, switch 31 is positioned to terminal 33 so that dc potential 35 acts to apply a dc voltage across the photoconductive layer 23 -- ferroelectric ceramic wafer 19 combination, interposed between transparent conductors 17 and 25. Typically, dc source 35 would be of the order of 200 volts. This voltage, as applied to the pair of transparent conductive layer electrodes in question, acts to develop in ceramic wafer 19 a transverse field having an intensity which may be modulated by the photoconductive layer 23.

Initially the ceramic is assumed to be uniformly polarized as a result of previous erasure. When no electron beam is present, the tube is in a "dark" condition and the conductivity of the photoconductor of layer 23 is negligible. Accordingly, no polarization charges flow through the ferroelectric layer interposed between the biased transparent conductive layers 17 and 25. However, when selected regions of the phosphor layer 27 are bombarded by the electron beam from the electron gun, in accordance with the electrical input signals applied to the control grid thereof to create the image to be written, the phosphor is locally excited, causing local conductivity in the adjacent photoconductive layer 23.

This local conductivity in the photoconductive layer results in a flow of local charge through corresponding regions of the ferroelectric wafer, changing its polarization, thereat. The flow of local polarization charge acts to modulate the birefringence in the ceramic wafer, in accordance with the written image. Accordingly, the field applied across the opposite surfaces of the ferroelectric ceramic has an intensity which is modulated by the photoconductive layer 23. When the field is removed, for example by moving switch 31 to terminal 37, the image written on phosphor layer 27 remains stored, as a spatial modulation of the birefringence of the ceramic wafer 19.

With switch 31 in the viewing position corresponding to terminal 37, the applied dc voltage is removed and the image stored in ferroelectric ceramic wafer 19 may then be viewed. In accordance with the principles of the present invention, an optical projection system is employed whereby the mosaic of reflecting elements within the tube, interposed between photoconductive layer 23 and ferroelectric ceramic wafer 19, acts to reflect polarized light, in a polarization pattern corresponding to the stored image. An arrangement somewhat analogous to these projection techniques, is described by G. Marie, in an article entitled "Large-Screen Projection of Television Pictures with an Optical-Relay Tube Based on the Pockels Effect," appearing in the Philips Technical Review, Vol. 30, at pp. 292, etc.

To achieve projection, then, light from an external light source 43 is directed through the polarizing prism 45. Polarizing prism 45 may comprise any of a variety of conventional polarizing prisms. Light entering the polarizing prism 45 in this manner is passed through imaging lens 47, and enters the ferroelectric ceramic wafer 19 as a collimated beam of plane-polarized light. This light is then reflected back, after passing through the ferroelectric ceramic wafer, as shown. The phase delay of the two perpendicular components of the polarized light will vary, from area to area, across the ferroelectric ceramic wafer, in accordance with the polarization pattern produced in the ferroelectric wafer during writing. Accordingly, a projected pattern of varying brightness, corresponding to the stored image, will pass through the polarizing prism 45, reaching projection screen 49. Since viewing does not disturb the polarization pattern in the ferroelectric ceramic during the viewing process, this process may be continued indefinitely.

In order to erase, switch 31 is positioned at terminal 39, whereby a negative dc voltage is applied across the photoconductive layer-ferroelectrical wafer combination. Typically, dc source 41 is of the order of 100 volts. If it is desired to erase selected areas, the phosphor layer 27 is locally bombarded by the electron beam in the regions to be erased, allowing charge to flow in the opposite direction through corresponding regions of the ferroelectric ceramic wafer, thus shifting the polarization back to the initial "off" condition within these regions. Accordingly, light within the erased regions is cut off from the corresponding area of the projection screen 49. In order to erase the entire image, the complete surface of phosphor layer 27 is flooded, or scanned by the electron beam. This, then, acts to cause the ferroelectric ceramic wafer to regain its initial state of uniform birefringence, over the entire wafer.

It is clear that, in accordance with the principles of the present invention, a relatively high resolution can be obtained from a small tube. For example, a 50μ thick ceramic wafer or layer is capable of a limiting resolution of 50 line pairs/mm. With a ceramic plate or wafer 2 cm × 2 cm in area, a total of 1,000 line pairs/mm is possible in both the X and Y directions.

Of additional importance, is the fact that high writing speeds may readily be achieved with the arrangement, in accordance with the principles of the present invention. For example, with a tube target area of 4 cm2, the total polarization charge required for writing or erasing the entire area is about 100μC. With an electron beam of 5 kv accelerating potential, and a phosphor (such as ZnO) with 5 percent efficiency which emits photons of about 2.5 eV energy, each primary electron will produce 100 photons. If these are all absorbed in the photoconductive layer whose quantum gain is, for example, 100, each photon will allow 100 electrons to flow through the photoconductor. In effect, then, each primary beam electron will cause a flow of l04 electrons through the photoconductive layer and ferroelectric ceramic wafer. With a 10μ amp electron beam, this will result in a current of 0.1 A through the ferroelectric ceramic wafer, where the phosphor is bombarded. This current level will produce the required polarization charge of 100μC in approximately 1 millisecond, allowing the writing or erasing of the entire target area, in this time period.

It should be recognized that the arrangement of FIG. 1 is readily capable of a further operating mode. In particular, the arrangement of FIG. 1 may be operated so that the generation of non-stored images may be superimposed, in a different color, on the stored image. Thus, where the external light source 43 provides a blue light, for example, the stored image will thus appear in blue, on the projection screen. On the other hand, if the light emission from the phosphor layer 27 is peaked in the green (matched, for example, to the peak of the absorption curve of CbS), this light may also be seen on the projection screen with appropriate minor changes. Specifically, these minor changes involve replacing the mosaic of opaque reflecting elements interposed between the ferroelectric ceramic wafer 19 and photoconductive layer 23, with a slightly-conducting multilayer thin-film dichroic filter which acts to transmit green light and reflect blue light. Alternatively, this dichroic filter may comprise a mosaic of isolated elements having a relatively high conductivity. In accordance with this latter arrangement, the phosphor may be scanned, as in a conventional CRT, producing dynamic or moving images on the viewing screen in green light, without disturbing the stored image (viewed in blue light). Under such operating conditions, switch 31 is maintained in the viewing position at terminal 37, thus preventing any changes in polarization in the ferroelectric ceramic wafer.

It should also be recognized that the arrangement shown in FIG. 1 is capable of providing an electrical output signal, corresponding to the stored image information. To achieve this end, switch 31 is positioned to the erase condition at terminal 39, after an image has been stored in ferroelectric ceramic wafer 19. Phosphor layer 27 is then scanned with the unmodulated electron beam. This causes the flow of charge through successive areas of the ferroelectric ceramic wafer where writing had previously occurred. An electrical output signal may thereby be obtained, by monitoring the current flow through one of the transparent conductive layers, of the photoconductor-ferroelectric ceramic sandwich.

In summary, then, a storage display tube, about 1 inch in diameter, is capable of extremely high writing and erasing speeds (e.g., 1 millisecond per frame). Such a tube cannot only be selectively erased, but can also be used to present moving or transient images in one color, superimposed on stored images of another color. In addition, an electrical output signal can be obtained, corresponding to the stored information.

In the embodiment of FIG. 2, the configuration of the storage tube shown therein, is essentially the same as that shown in FIG. 1. The basic difference between the arrangements of FIG. 1 and FIG. 2, resides in the fact that the ferroelectric ceramic wafer in FIG. 2 is operated in the "scattering" mode.

In this mode, no polarized light is used. However, when the ceramic has zero remanent polarization in the direction normal to its surface, it is in a clear or transparent state. When the ceramic is locally polarized by charge flow through it, it scatters light passing therethrough. In addition to operating the ceramic in the scattering mode, an optical projection system is employed consistent with the manner in which the scattering mode provides optical information, indicative of the image stored in the ferroelectric ceramic wafer. In FIG. 2, like reference characters have been employed to identify elements which are the same as corresponding elements in FIG. 1.

As can be seen, the various layers positioned on the inner surface of faceplate 3, in FIG. 2, are the same as those shown in FIG. 1. However, rather than have faceplate 3 mounted in a flexed position so as to strain-bias the ferroelectric ceramic wafer, the faceplate is mounted in its normal state so that no strain is produced upon the wafer. As can be seen, a potential and switch arrangement is provided, in the same manner as in FIG. 1.

Typical ceramics exhibiting the scattering mode may comprise rhombohedral-phase lead-lanthanum-zirconate-titanate (referred to as PLZT materials) with the ratio of La, Zr, Ti, about 7:65:35. For proper operation, the grain size of the material should be about 4-5 microns.

In the initial state, it is assumed that ceramic 19 in FIG. 2 is in the state of zero remanent polarization, i.e., the clear state. During writing, switch 31 is maintained in the "write" position at terminal 33 to create a field across the ferroelectric ceramic wafer 19 -- photoconductive layer 23 combination. Thus, when phosphor layer 27 is bombarded with an electron beam, in accordance with the pattern or image to be stored, local regions of conductivity corresponding to said pattern, are created in adjacent photoconductive layer 23, as hereinabove described. The pattern of conductivity in photoconductive layer 23 acts to effect a corresponding polarization in the ferroelectric ceramic wafer, to thereby establish local scattering sites, corresponding to said pattern. These local scattering sites act to diffuse or scatter light, passing through the ceramic.

In order to project the image, as stored in the arrangement of FIG. 2, light from external light source 43 is directed through focusing lens 53 onto a small mirror 55. During reading, switch 31 is maintained at ground read position 37 in order to insure no field is applied across the ferroelectric ceramic. The light reflected from mirror 55 is directed through lens 57, whereby a collimated beam of incident light is directed through faceplate 3, transparent conductive layer 17, ferroelectric ceramic wafer 19 onto the reflective surface of reflective layer 21. The light passing through the polarized regions of the ceramic is then diffused by the local scattering sites formed therein. As shown in FIG. 2, the diffused reflected light from a given scattering site at 51, for example, is focused by lens 57, positioned in the reflection path, to thereby project a light spot or point on projection screen 49, corresponding to the scattering point.

Thus, those areas of the ceramic which are in the erased condition, i.e., do not cause scattering, allow incident light to return to mirror 55, thus not reaching the screen 49. On the other hand, local regions of the ferroelectric where scattering is produced allow a fraction of this light to pass around mirror 55 and therby reach screen 49, where an image is produced.

Thus, it can be seen that for each point in ferroelectric ceramic wafer 19 wherein a scattering site has been induced, a corresponding light point is produced on the screen 49. Accordingly, the image written upon phosphor layer 27 acts to produce a stored pattern of scattering sites in ferroelectric ceramic wafer 19, which sites may be optically transformed into a visual image, on a projection screen.

In order to erase, switch 31 is set to the erase position at terminal 39 whereby a negative potential is applied across the ferroelectric ceramic -- photoconductor combination. It is clear that the pattern or image stored in the ceramic may be selectively erased or completely erased. To erase, selected regions on the phosphor are bombarded with a limited current, thus limiting the amount of reverse current flow through the ceramic and leaving it in the state of zero remanent polarization. Alternatively, the full beam current could be used for erasing, with switch 31 acting to be closed for a limited time so that the reverse charge flow leaves the ceramic in the zero remanent polarization condition. It is evident that a generator producing a negative pulse of controlled width may be used for this latter purpose.

While the invention has been particularly shown and described with reference to preferred embodiments 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.