US3204106A - Storage-type electroluminescent image amplifier - Google Patents

Description

Aug-31,1965 J. MURR, JR., m1,. 3,204,106

STORAGE-TYPE ELECTROLUMINESCENT IMAGE AMPLIFIER Filed Dec. 28, 1960 #Tram/EY United States Patent O 3,204,106 STORAGE-TYPE ELECTRULUMINESCENT lMAGE AMPLIFIER .lohn Murr, Jr., Allentown, and Harvey 0. Hook, Princeton, NJ., assignors to Radio Corporation of America, a corporation of Delaware Filed Dec. 2S, 1960, Ser. No. 7 8,892 7 Ciaiins. (Cl. Z50-213) This invention relates to electroluminescent devices. In particular, this invention relates to a storage light ampliiier type of electroluminescent devices, wherein an 1nput image is visibly reproduced, in an intensified or arr1- pliiied manner, and in which the input image is stored for any selected period of time.

There are several known types of storage light ampl.- fiers which generally include a layer of electroluminescent material and a layer of photoconductive material in optical feedback relationship. When a potential is applied across .the series combination of the electroluminescent and photoconductive materials, most of the potential drop occurs across the photoconductor in the absence of an input signal. When the photoconductor is excited by an input image, the radiation receiving portions of the photoconductor are in a low resistance state and the potential drop occurs across the electroluminescent layer so that the electroluminescent material produces `an amplified outpu-t image. Usually, part of the output light from the electroluminescent layer is fed back to the photoconductor to store the image.

In devices of the type briefly described above, either the electroluminescent material or the photoconductive material must be divided into elemental units to prevent spreading of the image. The reason for this is that light feedback from Ithe electroluminescent layer is not completely perpendicular to the `surface of the electroluminescent layer. Thus, adjacent areas of photoconductor are excited by the feedback light, and, in turn, 'adjacent areas of the electroluminescent material produce light. If spreading Occurs, and the image is stored for a long period of time, the image is completely replaced by an area of light.

The previous storage light amplifiers have required tedious operation, such as machining, to produce cell barriers to prevent cross-feeding of light from one elemental unit to the other which triggers on the adjacent cells.

It is therefore an object of this invention to provide an improved storage light amplifier.

It is a further object of this invention to provide a novel storage light amplifier characterized by its simplicity of construction and its adaptability for economical production.

These and other objects are accomplished in accordance with this invention by providing a storage light amplifier wherein a first electroluminescent means Iis provided for producing the light output of `the device and a second electroluminescent means is provided for providing the feedback light to the photoconductive means to provide storage of the image. The second electroluminescent means is optically separated from the first electr luminescent means, by an opaque layer, so that the output side of the device does not provide any light feedback and is not directly sensitive to input light. The second or feedback electroluminescent means is separated into elemental units isolated by the photoconductive means so that spreading of the image does not occur.

The invention will be more clearly understood by reference to the accompanying single sheet of drawings, wherein:

FIG. l is an enlarged fragmentary sectional view of a preferred embodiment of this invention; and,

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FIG. 2 is an enlarged fragmentary sectional View of another embodiment of this invention Referring now to FIG. l, there is shown an electroluminescent type 1light amplifier 10 comprising a transparent support member 12. The transparent support member 12 is shown as being a rectangular fiat member but may be of any desired shape or configuration. The

transparent support member 12 may be made of any light transparent material such as glass or plastic. Positioned on one surface of the transparent support member 12 is a light transparent electrically conducting layer or coating 14. The layer 14 may be any conventional transparent conductor such as vapor deposited tin oxide, tin chloride or thin layers of evaporated gold.

Positioned on the transparent conductive layer 14 is a layer 16 of a mixture of electroluminescent phosphor yand a suitable binder. The electroluminescent phosphor 16 may be any of the known electroluminescent materials such as manganese activated zinc sulfide and may be provided in the powdered form when mixed with a suitable binder such as nitro-cellulose. The electroluminescent layer 16 is for the purpose of providing an output image from the device 10. The electroluminescent layer 16 may, for example, be a continuous coating approximately 0.0015 inch thick. The electroluminescent layer 16 may be applied by spraying, blading or other known techniques. Positioned on the electroluminescent layer 16 is a light opaque layer 18 which may be approximately 0.0005 inch thick and which may be made of a material such as black paint or carbon black in an epoxy resin binder. The light opaque material 18 may be applied by spraying. The light opaque layer 18 is non-conducting in a lateral tliizrection, i.e. parallel to the plane of the support member Positioned on the light opaque layer 13 is a discontinuous layer comprising a large plurality or multiplicity if separate particles of electroluminescent phos-` phor 20 The particles 20 may also be made of any suitable known electroluminescent phosphor material, one example of which is manganese activated zinc sulfide.V Since the electroluminescent particles 20 are spaced apart, the average thickness of the array of particles is less than the thickness of one particle. Thus, isolated electroluminescent particles are provided. The isolated electroluminescent particles 20 may be provided by spraying a very dilute electroluminescent paint or dusting a very thin, discontinuous layer of powdered elecrtoluminescent material on a surface made tacky by a partly dry lacquer film. When the electroluminescent particles 20 are applied by spraying, a coating of manganese activated zinc sulfide in a suitable binder, that is just barely visible to the naked eye, has been found to be suitable.

Positioned over all of the separate electroluminescent particles 20 is a photoconductor 22. The photoconduc-` tor 22 is grooved so that a thick photoconductor may be used for its electrical properties and so that light may penetrate to the depths of the photoconductor to excite the photoconductor, as is known. The photoconductor may be approximately 0.008 inch thick, for example, and the grooves may be cut to about 0.004 inch from the top of the electroluminescent particles 20. The photoconductor may be made of any known photoconductive material, one example of which is powdered, copper activated, cadmium sulfide.

Due to the thickness of the photoconductor 22, the light produced by one electroluminescent particle 20 is not suicient to excite adjacent areas of the photoconductor to an extent sufficient to cause image spreading. In other words, the body of the photoconductor Z2 effectively functions as a light opaque shield around each of the isolated electroluminescent particles Z0.

Positioned on the alternate ridges is a different one of a group of electrodes 24. Positioned on the intermediate ridges is a different one of a group of electrodes 26. All of the electrodes 24 are connected to a terminal 38 while all of the electrodes 26 are connected to a terminal 36.

During operation of the device 10, an alternating cur-` rent source, not shown, is connected between the terminal 34 on the transparent conductor 14 and a center tap of a direct current source (not shown). The opposite sides of the direct current source are connected through a reversible switch to terminals .'56 and 3S. By means of the reversible switch, and the direct current source, a potential of opposite polarity rnay be sequentially applied to all the electrodes 26 and all electrodes 24. The direct current source is for the purpose of providing a bias voltage to the photoconductor 22 so that current -flow is only in one direction. `By reversing the polarity of the direct current source, the stored image is quickly erased, for example, in 50 milliseconds or less. The direct current source may be of a magnitude of approximately 300 volts D.C. The alternating current source is of sufficient magnitude to cause the electroluminescent layer 16 and the electroluminescent dots 20 to produce light, when the resistance of the adjacent photoconductor is below a predetermined level. The amount of light produced 4by the electroluminescent dots 20 must be high enough to maintain the photoconductor 22 in its low resistance state for image storage. A source of approximately 180 volts at approximately 1,000 cycles per second has been found to be suicient.

During operation of the device 10, an image to be reproduced is directed into the photoconductor 22. The image may be a visible image, X-rays or other invisible radiations, with the photoconductor being selected for its high sensitivity to the desired wavelengths.

In the areas of the photoconductor which are struck by the input image, the resistance of the photoconductor is decreased so that the electroluminescent particles 20 are excited, and the electroluminescent layer 16 is also excited, since a suicient portion of the potential from the alternating current source is now applied across the electroluminescent layer and the electroluminescent particles 20.

The light produced by the electroluminescent particles 20 is of sufficient magnitude, or brightness, to feed back to the photoconductor 22, in the elemental areas thereof, so that the photoconductor in the illuminated elemental areas is continuously energized and remains at a low resistance state. As long as the input image is stored, the output electroluminescent layer 16 will produce a visible amplified output image corresponding to the original input image.

Due to the presence of the opaque layerv18, light from the electroluminescent layer 16 is prevented from feeding back to, and therefore exciting, the photoconductor 22. Due to the fact that the electroluminescent particles 20 are spaced apart and isolated from each other, the light from these particles does not excite adjacent photoconductive areas so that the image is not diffused or spread. The reason for this is that the body of the photoconductor 22 functions as a light shield between adjacent electroluminescent dots 20 and image spreading7 is eliminated. If the feedback electroluminescent means were continuous, adjacent areas of photoconductor would be excited, which in turn would excite adjacent areas of electroluminescent areas so that the image would spread and eventually become a continuous light source.

In addition to the conventional storage of an image previously described, other interesting effects have been noted in panels of the type shown in FIG. l. For example, after the photoconductor has been exposed to an image, by reversing the polarity of the electric field on the interdigited electrodes 24 and 26, and at the same time by exposing the input photoconductor to a uniform light source (not shown), the negative of the original stored image is produced on the output electroluminescent phosphor 16. The reason for this is that the photoconductor is less sensitive, in the previously exposed areas than it is in the previously un-exposed areas. This negative image may also be stored indefinitely once it has been established by the described procedure.

Another interesting effect is that, by over exposure of the panel to the uniform light source only the border or outline of the original image is retained. It is believed that this effect occurs because of a fatigue effect in the center exposed portions of the photoconductor, when the sensitivity of the photoconductor is reduced below the level at which storage is possible. By means of this phenomena, only the outline of the original stored image is produced on the electroluminescent layer 16. This outline may also be stored indefinitely.

Halftone pictures may be displayed on the storage device 10 by operating at an alternating current voltage level that is slightly below the level at which full image storage is obtained. Under these conditions, the picture fades to black, although the halftone picture takes several minutes until it decays to where it is no longer visible.

Referring now to FIG. 2, there is shown another embodiment of this invention. This embodiment differs from that shown in FIG. l only in that the feedback electroluminescent means is divided into predetermined spaced apart areas. The predetermined spaced apart areas of electroluminescent phosphor 40 may be provided on the opaque layer 18 by holding a metal mesh screen (not shown) in Contact with the opaque layer 13, while the electroluminescent areas 40 are sprayed through the mesh. Then, when the mesh screen is removed, an array of separate, isolated electroluminescent areas 40 is provided. The operation and materials of the embodiment shown in FIG. 2 may be similar to that described in connection with FIG. l.

What is claimed is:

1. An image amplifier comprising a conductive layer, an electroluminescent layer, an opaque layer, a discontinuous electroluminescent layer, and a continuous photoconductive layer, said layers being substantially coextensive and being in electrical contact with each other in the order named, and at least one electrode on said photoconductive layer.

2. An image amplifier as in claim 1 wherein said discontinuous electroluminescent layer is in light exchange relationship with said photoconductor and comprises a plurality of separately isolated elemental areas of electroluminescent phosphor.

3. An image amplier as in claim 1, wherein said discontinuous electroluminescent layer comprises a discontinuous coating of spaced-apart electroluminescent particles.

4. An electroluminescent light amplifier comprising a transparent support member, a light transparent electrical conductor on said support member, a continuous layer of electroluminescent phosphor on said transparent conductor for providing light output from said light amplier, a continuous layer of light opaque material on said electroluminescent phosphor, a plurality of electroluminescent particles spaced apart on said layer of light opaque material, and a photoconductor isolating each of said plurality of electroluminescent particles from any other.

5. An electroluminescent light amplifier as in claim 4 wherein said photoconductor includes a plurality of grooves extending substantially parallel to the plane of said support member, and means for connecting the opposite polarities of a direct current source to said photoconductor.

6. An electroluminescent light amplifier comprising a light transparent support member, a light transparent electrically conductive coating on said support member, a continuous layer of electroluminescent phosphor material on said transparent conductive coating, a continuous layer of light opaque material on said layer of electroluminescent phosphor, a discontinuous layer of spacedapart electroluminescent particles on said layer of light opaque material, and a photoconductor on said array ont electroluminescent particles and on the light opaque layer therebetween.

7. An electroluminescent light amplifier as in claim 6 wherein said photoconductor includes a plurality 0f grooves, the alternate ridges of said photoconductor having a first set of conductors thereon, the intermediate ridges of said photoconductor having a second set of conductors thereon, said sets being adapted to have the alternate polarities of a direct current source applied thereto.

References Cited by the Examiner UNITED STATES PATENTS 2,914,678 ll/59 Kazan Z50-213 2,928,006 3/60 Kruse 250-213 2,957,991 10/60 Kazan 250-213 2,997,596 8/61 Vize 250-213 X 2,999,165 9/61 Lieb 250-213 2,999,941 9/61 Klasen et al. Z50-213 3,002,102 9/61 Palmer Z50-213 FREDERICK M. STRADER, Primary Examiner.

ARCHIE R. BORCHELT, RICHARD M. WOOD,

RALPH G. NILSON, Examiners,

Claims (1)

1. AN IMAGE AMPLIFIER COMPRISING A CONDUCTIVE LAYER AN ELECTROLUMINESCENT LAYER, AN OPAQUE LAYER, A DISCONTINUOUS ELECTROLUMINESCENT LAYER, AND A CONTINUOUS PHOTOCONDUCTIVE LAYER, SAID LAYERS BEING SUBSTANTIALLY COEXTENSIVE AND BEING IN ELECTRICAL CONTACT WITH EACH OTHER IN THE ORDER NAMED, AND AT LEAST ONE ELECTRODE ON SAID PHOTOCONDUCTIVE LAYER.

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