Image Storage Device

Abstract

An electro-optic information storage device for converting an optical or electron image into an intensified optical output image and, by the process of optical feedback, continuously refreshing the output image. An input optical image is converted by a photocathode into a corresponding electron image. The electron image is amplified by a multichannel plate, the output electrons of which bombard a layer of cathodoluminescent material, thereby generating an optical image which consists of two parts. One part is projected forward and serves as the optical output image. The second part is projected rearward and impinges upon the photocathode which releases photoelectrons that repeat the aforementioned process, thereby continually refreshing the output image.

Description

BACKGROUND OF THE INVENTION

I. Field of the Invention

This invention relates to a simplified image storage panel, and more particularly, it relates to a high gain image intensifier which both stores an incident photon or electron image and simultaneously provides an intensified image for viewing. This invention finds utility in such applications as an electronic blackboard, computer terminal and the like.

II. Description of the Prior Art

In typical electro-optic image intensifiers, means such as a photocathode provides an electron image, the intensity of which is subsequently increased by an electron multiplier or microchannel plate. The resultant intensified electron image is directed upon a display element such as a phosphor screen to provide a visual image. Disposed upon that surface of the phosphor screen nearest to the microchannel plate is a layer of electrically conductive material such as aluminum, the thickness of which is such that it is permeable to electrons emanating from the microchannel plate. The electrically conductive layer removes the charge which tends to build up as a result of electrons impinging thereon, and it is of sufficient thickness to reflect light generated by the phosphor screen so that a maximum amount of light is directed toward the viewer.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an electro-optic image intensifier which is capable of converting a portion of the output light into an electron image which can be amplified and utilized to refresh the output image.

Briefly, the image storage device of the present invention comprises a multichannel plate having plurality of parallel channels, the inner surfaces of which contain a resistive, secondary electron emissive layer. The openings in the ends of the channels lie on first and second surfaces which constitute input and output ends, respectively of the multichannel plate. At least one photoemissive layer is provided at the input end of the multichannel plate. Means is disposed at the output end of the multichannel plate for generating in response to electrons provided by the microchannel plate an optical image comprising two components. One of the components provides an output image, whereas the other provides feedback light that is transmitted through the channels of the multichannel plate. This feedback light impinges upon the photoemissive layer, thereby generating photoelectrons which are amplified by the multichannel plate and utilized to refresh the output optical image.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration, partly schematic and partly in cross-section, of the image storage device of the present invention.

FIGS. 2, 3 and 4 are cross-sectional views of modifications of a portion of the device of FIG. 1.

FIG. 5 is a simplified diagram of those portions of FIG. 1 which are useful for illustrating the operation of the device of FIG. 1.

FIG. 6 is a simplified diagram of a modification of the device of FIG. 1.

FIG. 7 is a simplified diagram of yet another modification of the device of FIG. 1.

DETAILED DESCRIPTION

Referring to FIG. 1, there is shown an evacuated container 10, having first and second faceplates 12 and 14 which are formed of a material such as transparent glass, which will transmit a photon image. The term phonton image when used herein not only includes images being transmitted by visible light rays, but also includes those transmitted by invisible light rays such as infrared or ultraviolet light and X-rays. The remaining portion of container 10 other than faceplates 12 and 14 may be formed from any suitable material such as metal, glass or the like which can be sealed to faceplates 12 and 14 to provide a structure that can be evacuated to a vacuum level within the range commonly used by standard vacuum tubes, such as approximately 10.sup..sup.-7 Torr. The embodiment illustrated in FIG. 1 utilizes two photocathodes for converting a photon image represented by undulated arrows 16 into an electron image. A first photocathode 18 is disposed upon the surface of a transparent, conductive film 20 which is supported by faceplate 12. Film 20 may consist of a very thin layer of metal, e.g., 200-300 A of aluminum or nichrome, but it preferably consists of a more transparent conductive material such as about 10,000 A of tin oxide. A second photocathode in the form of a photoemissive layer 22 is disposed upon the input end of a microchannel plate 24 which will hereinafter be described in detail. A shutter mechanism 28, which is disposed adjacent to faceplate 12, includes an annular support frame 30 and a movable element 32, which may be actuated from a closed position A wherein movable element 32 is illustrated by dashed lines, to an open position B in order to allow the input photon image to be focused by a lens system (not shown) onto the photocathodes. Surface 33 of movable element 32 may be a light reflecting surface for reasons which will be hereinafter discussed. Since two longitudinally spaced photocathodes are utilized in this embodiment, higher resolution will be obtained if the light is collimated by the optical system. Photon image 16 is converted by photocathode 18 into an electron image, the density pattern of which corresponds to variations in the photon image, arrows 34 illustrating some of the released photoelectrons. Undulated arrows 16, which are illustrated by solid lines, continue on through photocathode 18 as dashed lines 16' illustrating the fact that as much as 70 percent of the input image may pass through photocathode 18. Some portion of the photon image represented by arrows 14' strikes photoemissive layer 22 which also emits electrons in a density pattern corresponding to variations in the photon image. This latter electron image represented by arrow 36 combines with the electron image represented by arrows 34, and it is this combined electron image which is amplified by electron multiplying microchannel plate 24.

A suitable photocathode may readily be fabricated by one familiar with the art by depositing a layer of photoemissive material over one surface of a transparent glass substrate. Photoemissive layers consisting of the alkali metal cesium in combination with silver, bismuth or antimony will respond to wavelengths of light in the range from infrared through the visible and ultraviolet and up to and including X-rays. Photoemissive layers consisting of any of the alkali metals or combinations thereof will respond to wavelengths of light in the range from visible through ultraviolet and up to and including X-rays. Photoemissive layers of most metals will respond to wavelengths of light in the range from ultraviolet up to and including X-rays. The most suitable metals being aluminum, gold or nickel. The electrons emitted by photocathodes 18 and 22 enter channels 26 of microchannel plate 24 which consists of a plurality of tubes 40 each having a secondary emissive surface 42 formed on the inside wall thereof. It is to be noted that the drawings are illustrative only, and are not to scale. The microchannel plate is greatly enlarged with respect to the other portions of the drawings so that the operation of these components may be more readily understood. The microchannel plate may be formed in accordance with the teachings of U.S. Pat. No. 3,341,730 to G. W. Goodrich et al. Microchannel plates for use as electron multipliers are normally formed of tubes with an inside diameter of about 50 microns, and the thickness of the plate or the length of the tubes are normally in the range of 25 to 100 times the tube diameter. The tube inside diameter range and the microchannel plate thickness range heretofore set out are not critical and in some instances it may be desirable to operate outside of these ranges. The inside diameter range and the length range as given represent particularly suitable dimensions for microchannel amplifier plates produced to date. However, new developments in the filed of microchannel amplifiers may result in other more favorable dimensions. Microchannel plate 24 could also be formed as a monolithic structure such as by chemically machining holes in a nonconductive substrate and depositing secondary emissive material on the walls of the holes. Secondary emissive surface 42 comprises a metal oxide such as tin oxide, lead oxide, aluminum oxide and the like, or any other semi-conductive material having good secondary emissive characteristics such as strontium titanate or vanadium phosphate and the like. Layer 42 could also be formed by reducing suitable glasses, such as lead glasses, to form a surface layer of the proper resistivity. The input and output surfaces of the microchannel plate are provided with conductive layers 44 and 46, respectively. Each conductive layer is at least 1,000 A in thickness and may be deposited on the ends of microchannel plate 24 by such methods including but not limited to sputtering and evaporation; said conductive layers may consist of such materials as aluminum, gold, silver, chrome filled alloys or the like.

The negative side of potential source 50 is connected by conductive coating 20 to photocathode 18, the positive side thereof being connected to ground. The negative side of potential source 54, the potential of which is less than that of source 50, is connected through switch 56 to conductive layer 44, the positive side of source 54 being connected to ground. Therefore, a potential or electrical field E.sub.1 will exist between photocathode 18 and conductive layer 44 which will focus and accelerate the electrons emitted by photocathode 18 toward microchannel plate 24. Furthermore, electrical field E.sub.1 causes electrons 36 emitted by photoemissive layer 22 to be focused or repelled into adjacent channels 26 and also increases the energy of these photoelectrons, thereby improving the secondary electron yield resulting from photoelectrons 36 impinging upon secondary emissive surface 42. The electrons traveling between photocathode 18 and microchannel plate 26 may be focused by applying a magnetic field in the region between photocathode 18 and the input end of the microchannel plate 26, but a magnetic field is only useful for focusing electrons and cannot increase the energy thereof, a condition which is necessary for optimum operation of the device. An electrical field E.sub.1 of between 10 volts/0.001 inch and 50 volts/0.001 inch between the photo-cathode and the input surface of the microchannel plate would be sufficient for focusing, and additional magnetic focusing of the electrons would be unnecessary. For example, if the spacing between the two members is about 0.02 inch and a potential or electrical field of 300 volts exists between photo-cathode 18 and conductive layer 44, the electrical field will be about 15 volts/0.001 inch and the electrons will travel in substantially parallel paths without magnetic focusing. Testing indicates that an electrical field of approximately 30 volts/0.001 inch and a spacing of between 0.01 inch and 0.1 inch will produce the most effective focusing.

The negative side of potential source 58, the potential of which is less than that of source 54, is connected to conductive layer 46, the positive side of source 58 being connected to ground. Therefore, a potential will also exist between the input and output surfaces of the microchannel plate. During operation of the microchannel plate, electrons represented by arrows 34 and 36 enter channels 26, impact with secondary emissive surfaces 42 and give rise to secondary electrons represented by arrows 34' and 36' which in turn impact the secondary emissive surfaces further down the channel giving rise to still additional secondary electrons represented by arrows 34" and 36". This cascading process continues for the length of the channel thereby resulting in a large electron gain. Microchannel plate 24 can provide extremely high current gains of around 10.sup.4 to 10.sup.6, and the gain may be adjusted below such range to any appropriate value. Such adjustment may be accomplished by varying the potential that exists between the input and output surfaces of said microchannel plate. This potential is typically around 1,000 volts, and provides the operating potential for the microchannel plate. A layer 60 of cathodoluminescent material such as phosphor, cathodoluminescent glass, or the like is disposed upon the inner surface of faceplate 14, and a thin film 62 of transparent conductive material is disposed upon the surface of layer 60. If cathodoluminescent glass is utilized as the light emitting medium, it may be disposed on the inner surface of faceplate 14 as illustrated in FIG. 1. However, as illustrated in FIG. 2, wherein elements similar to those of FIG. 1 are indicated by primed reference numerals, conductive layer 62' may be disposed upon the inner surface of plate 66 of cathodoluminescent glass which also serves as a wall portion of container 10'.

Conventional image intensifiers employ a layer of cathodoluminescent material backed by a conductive layer which both conducts away charge buildup resulting from bombarding electrons and reflects light from the excited phosphor which would otherwise radiate back toward the electron source. A layer of aluminum about 1,000 A thick has been adequate for reflecting this backward radiating portion of the phosphor light toward the viewer, thereby increasing the brightness of the image. The present invention requires optical feedback from the cathodoluminescent layer to the photoemissive layer and therefore cannot utilize the conventional reflective conductive layer adjacent to the cathodoluminescent layer. A film of aluminum about 300 A thick could be utilized since it would transmit about 30 percent of the phosphor light radiating thereon. However, it is preferred that layer 62 consist of about 300 A of tin oxide which is more transparent than aluminum and yet provides adequate charge removal. Such a tin oxide layer also satisfies a third requirement, i.e., it is transparent to electrons emerging from channels 26.

FIGS. 3 and 4 illustrate modifications of the light generating portion of the image storage device of the present invention. In these embodiments, elements similar to those of FIG. 1 are indicated by primed reference numerals. As shown in FIG. 3, a transparent conductive layer 63 may be disposed between faceplate 14' and cathodoluminescent layer 61. When such an arrangement is employed, the thickness of the cathodoluminescent layer should be just sufficient to absorb electrons from multichannel plate 24. If layer 61 is too thick, an electron-repelling charge can accumulate on that side thereof which faces microchannel plate 24.

In FIG. 4, a plate 65 of cathodoluminescent glass is utilized as the faceplate, and no conductive film is required on the surface thereof. Instead, means such as a conductive path 67 could be disposed around the circumference of plate 65. A glass composition suitable for use as plate 65 is set forth in Example 57 of Table IV of U.S. Pat. No. 3,654,172. This cathodoluminescent glass is sufficiently conductive under electron bombardment so that no additional conductive electrode is needed to remove charge buildup resulting from the relatively low energy electrons that bombard plate 65.

Referring again to FIG. 1, an electrical field E.sub.2 will exist between the output surface of the microchannel plate and conductive film 62 since voltage source 58 is connected across these elements. Electrons 34" and 36" are focused by field E.sub.2 and are accelerated toward and penetrate thin transparent conductive film 62 and thereby bombard cathodoluminescent layer 60 which generates light represented by undulated arrows 68 and 68'. Since film 62 is transparent, some of the light represented by arrows 68' propagates through film 62 and through channels 26 after which it may impinge upon photocathode 18 or photoemissive layer 22. In order to provide sufficient optical feedback to maintain image storage properties, the present invention requires both a substantially transparent conductive film 62 and good registration between all components, this latter requirement being satisfied by utilizing miminum spacing between the microchannel plate and both the photocathode and the cathodoluminescent material.

Electrons leaving channels 26 could be focused by a magnetic field represented by arrow B.sub.2 so that they travel in a path substantially perpendicular to the output surface of the microchannel plate and to the conductive film 62. However, if electrical field E.sub.2 is at least 10 volts/0.001 inch there will normally be no need for magnetic focusing between these two members.

The embodiment illustrated in FIG. 1 can be utilized as a computer terminal or a storage device for conventional television or other optical images. When used as a computer terminal, shutter 28 would be unnecessary since the storage device could be associated with a cathode ray tube, for example, which could provide an image from a single scanned frame. When used in conjunction with a conventional television tube, movable element 32 of shutter 28 would be maintained to open position B for a time sufficient to permit an entire image to be focused upon the photocathodes, and thereafter, the movable element 32 would be moved to position A to prevent additional exposure of the photocathodes by the changing image.

When the system of FIG. 1 is operated in the "write" mode, optical image 16 is focused onto photocathodes 18 and 22 causing electrons 34 and 36 to be emitted in response thereto. These electrons are focused and accelerated by field E.sub.1 toward multichannel plate 24 which, by a secondary emission process, boosts the number of input electrons by a gain factor G. These electrons 34" and 36" which are provided by multichannel plate 24 are focused and accelerated onto phosphor screen 60 which converts them into an optical image 68 that is visible to an observer looking toward faceplate 14.

A portion 68' of the light emitted by phosphor layer 60 passes back through transparent conductive film 62, and it is this feedback light which makes possible the "storage function" of the device which function is illustrated in the simplified diagram of FIG. 5 which illustrates only those elements of FIG. 1 that are necessary for a description of the storage mode. The storage function is based upon the fact that electron excitation of the phosphor screen produces light that is not directional. Whereas some of the excited photons represented by undulated arrow 68 form an output image, others of the excited photons are redirected back through multichannel plate 24 as indicated by undulated arrows 68'. Some of these feedback photons impinge upon photocathode 18 or photoemissive layer 22 and excite photoelectrons represented by arrow e.sup.-. These photoelectrons are in turn amplified by multichannel plate 24 to provide numerous output electrons Ge.sup.- which again bombard the phosphor screen 60 to generate photons, some of which are used in the feedback process to maintain the storage function.

To completely erase all of the stored information, the voltage across the multichannel plate is dropped to the point where essentially zero electron gain results. This could be accomplished by opening switch 56 of FIG. 1 or by changing the voltage of one or both of sources 54 and 58. To selectively erase, the selected active channels 26 are driven into saturation by focusing a spot of light onto that portion of the photocathodes which is involved in the feedback process for the information to be erased. The light level must be high enough to drive that area into saturation. No additional information can be written into the erased area since this method of erasure provides a solid area of light rather than the absence of light. This form of selective erasure is required since individual channels cannot presently be selectively biased to cutoff.

Whereas the embodiment of FIG. 1 contains photocathode 18 and photoemissive layer 22, a memory storage panel requires the use of only one of these photoemissive layers. It is noted that an embodiment having only photoemissive layer 22 would benefit from a transparent conductive layer 20 closely spaced from the photoemissive layer for the purpose of accelerating and focusing photoelectrons into the channels of the microchannel plate.

The embodiment of FIG. 1 could also function as an electronic blackboard by directing a beam of light from a source such as pencil beam source 70 onto faceplate 14. This input light beam which is represented by undulated arrow 72, is preferably highly directive and must be of sufficient intensity to excite photoelectrons from the photocathode after passing through cathodoluminescent layer 60, conductive film 62 and channels 26. Best results are obtained when this writing light beam is collimated and when the direction thereof is parallel to the longitudinal axes of channels 26. This mode of operation requires that either a transparent phosphor or a cathodoluminescent glass such as that illustrated in FIG. 2 be utilized to permit adequate light transmission. As illustrated in the diagram of FIG. 6, that portion of the input light which reaches photocathodes 18 and 22 generates photoelectrons e.sup.-.

Whereas the conductive layer 20 of FIG. 1 was described as being a transparent conductive layer that was transparent to photo image 14, an electronic blackboard of the type wherein the input light beam is directed toward the cathodoluminescent layer would have no need for a transparent conductive layer such as layer 20, and improved operation is achieved if a reflective layer such as metallic layer 76 of FIG. 4 is disposed adjacent to the photocathode to reflect feedback light and input light. Since layer 76 reflects light, the substrate on which it is deposited could be opaque. A layer of a metal such as aluminum, gold or silver about 1,000 A in thickness will provide the necessary reflective properties. Input light beam 72 is illustrated in FIG. 6 as passing through cathodoluminescent layer 60, microchannel plate 26 and photocathode 18. This light then reflects from metallic layer 76 and again passes through photocathode 18 before impinging upon photoemissive layer 22. Thus, the presence of reflecting surface 76 improves the efficiency of the device. The aforementioned reflective shutter surface 33 of FIG. 1 would function in a manner similar to reflective layer 76. Thus, if the device of FIG. 1 were to be used as an electronic blackboard, movable shutter element 32 could be maintained in closed position A.

The diagram of FIG. 7 illustrates that means other than an optically activated photocathode may be employed to provide an electron image. In this embodiment, means 80 such as an electron gun, digitally scanned electron generating means, or the like, is disposed within vacuum tight container 10 which is illustrated by dashed line. Electrons provided by means 80 are multiplied by the microchannel plate 26 and the output electrons represented by arrow Ge.sup.- bombard cathodoluminescent layer 60 to provide output light represented by undulated arrow 82. Feedback light represented by undulated arrow 84 strikes photoemissive layer 22 which releases photoelectrons, thereby providing the storage function.

Claims

I claim:

1. An image storage device comprising a

multichanneled plate having a plurality of parallel channels, the inner surfaces of which contain a resistive, secondary-electron emissive layer, the openings in the ends of said channels lying on first and second surfaces which constitute input and output ends, respectively, of said multichannel plate,

at least one photoemissive layer at the input end of said multichannel plate, and

means disposed at the output end of said multichannel plate for generating in response to electrons provided by said multichannel plate an optical image comprising two components, one of said components providing an output image and the other of said components providing feedback light that is transmitted through the channels of said multichannel plate to said photoemissive layer, said feedback light generating a sufficient amount of photoelectroms to maintain an image on said photoemissive layer.

2. An image storage device in accordance with claim 1 wherein said means for generating an optical image comprises a plate of cathodoluminescent glass and means disposed at the periphery of said plate for conducting electrical charge therefrom.

3. An image storage device in accordance with claim 1 wherein said means for generating an optical image comprises cathodoluminescent means disposed at the output end of said multichannel plate and means disposed on said cathodoluminescent means for preventing charge buildup on said cathodoluminescent means and for transmitting one of said optical image components.

4. An image storage device in accordance with claim 3 wherein said means for preventing charge buildup is disposed on that portion of said cathodoluminescent means facing said multichannel plate.

5. An image storage device in accordance with claim 3 wherein said means for preventing charge buildup is disposed on that portion of said cathodoluminescent means remote from said multichannel plate.

6. An image storage device in accordance with claim 3 wherein said cathodoluminescent means comprises a continuous layer of phosphor.

7. An image storage device in accordance with claim 3 wherein said cathodoluminescent means comprises a plate of cathodoluminescent glass.

8. An image storage means in accordance with claim 3 wherein said at least one photoemissive layer comprises a substrate, a conductive film disposed upon the surface of said substrate and a planar photocathode disposed upon the surface of said conductive film.

9. An image storage device in accordance with claim 8 wherein said substrate and said conductive film are transparent.

10. An image storage device in accordance with claim 9 further comprising a shutter disposed adjacent to that side of said substrate opposite said microchannel plate.

11. An image storage device in accordance with claim 10 wherein said shutter has a reflective surface facing said substrate.

12. An image storage device in accordance with claim 8 wherein said conductive film is reflective to light generated by said cathodoluminescent means.

13. An image storage device in accordance with claim 12 wherein said cathodoluminescent means comprises a layer of transparent phosphor.

14. An image storage device in accordance with claim 12 wherein said cathodoluminescent means comprises a plate of cathodoluminescent glass.

15. An image storage device in accordance with claim 12 further comprising first and second conductive layers on said input and output ends, respectively, of said multichannel plate, said at least one photoemissive layer further comprising a layer of photoemissive material disposed upon said first conductive layer and extending into said channels.

16. An image storage device in accordance with claim 15 further comprising means for directing a beam of light onto said cathodoluminescent means.

17. An image storage device in accordance with claim 1 further comprising first and second conductive layers on said input and output ends, respectively, of said multichannel plate, said at least one photoemissive layer comprising a layer of photoemissive material disposed upon said first conductive layer.

18. An image storage device in accordance with claim 17 wherein said at least one photoemissive layer further comprises a substrate, a conductive film disposed upon the surface of said substrate and a planar photocathode disposed upon the surface of said conductive film.

19. An image storage device in accordance with claim 1 further comprising means for directing a beam of light onto said cathodoluminescent means.