US2975291A - Electroluminescent storage device - Google Patents

Description

March 14, 1961 E. E, LOEBNER ETAL 2,975,291

ELECTROLUMINESCENT STORAGE DEVICE Filed OCl'f. 17, 1957 e ze sra/afa narran/HUMA@ WMS EBUN E. LDEBNER,

HARVEY GII-Imm 5 DDNALD CDARLINE MM gw ELECTROLUMINESCENT STGRAGE DEVICE 'Egon E. Loebner and Harvey 0. Hook, Princeton, NJ., and Donald E. Darling, Levittown, Pa., assignors to Radio Corporation of America, a corporation of Dela- Ware Filed Oct. 17, 1957, Ser. No. 690,814 v 9 Claims. (Cl. Z50-213) This invention relates to electroluniinescent devices and particularly to solid state devices designed to amplify and store radiation images, such as light images.

Solid state light amplifying and storage devices are known which comprise layers of photoconductive and ielectroluminescent phosphor material sandwiched between two transparent sheet electrodes. When an alternating voltage is applied to the electrodes, the two layers function as a voltage divider. Without illumination or irradiation of the photoconductive layer, the division of voltage is such that by far the greater portion appears across the photoconductive layer while the electroluminescent `layer receives only a small voltage which is insuicient to cause appreciable light emission therefrom. In the presence of an incident image, such as light or X-rays, for instance, the impedance of the photoconductive layer lowers, area by area, in accordance with the radiation intensity Variations in the image so that correspondingly greater voltage appears, area by area, across the electroluminescent layer. These Voltage variations result in variations in light emission from the electroluminescent layer which, for a relatively wide range of incident radiation intensities, correspond to but are greater than the variations in the incoming image. The image is thus intensified.

Such a device can store an image indefinitely if a sufficient amount of the electroluminescent light is made to feed back to the photoconductive layer to keep it excited. Modified structures have also been designed specilically for storage purposes. However, rnost of the prior art structures do not have high storage efiiciency, or lack ne definition, or are difficult to fabricate.

It is therefore a primary object of this invention to provide an improved structure, for use in a storage light amplifier, which is easily fabricated.

Another object is to provide a storage light amplifier of very line definition and high feedback efficiency.

'Ihe above and other objects are achieved, in accordance with this invention, by providing a plate of insulating material formed with laterally spaced transparent portions integrally surrounded by opaque portions. The transparent portions are provided with apertures extending through the plate and containing bodies of photoconductive material. The bodies are respectively connected electrically to electroluminescent phosphor capacitor areas disposed adjacent to one side of the plate. Each transp-arent portion of the plate serves as a light duct for the illumination of the photoconductive body therein both by incident light and by light fed back from the electroluminescent area registered therewith. The opaque portions serve to absorb or reliect suicient feedback 4light to prevent the undesirable triggering of neighboring image elements into storage and thus cause a smearing and spreading of the image. A mosaic of mutually isolated transparent conductive areas is arranged as interlayer electrodes between the transparent portions and the phosphor areas in such fashion as t-o permit the n 2,975,291 ?atented Mar. 14, 1961 electroluminescent light emission to be directed most eiciently through the transparent light ducts and onto the surfaces of the photoconductive bodies.

In the drawings:

Fig. 1 is a fragmentary sectional view of a storage light amplifier according to the invention; and

Fig. 2 is a fragmentary perspective view showing a portion of the device of Fig. 1 enlarged and in greater detail.

Referring to Fig. l, the storage light amplifier panel comprises a num-ber of layers which include a first conductive coating 10, an apertured insulating plate 12, a mosaic of conductive patches 26, an electroliuninescent Vphosphor layer 14, and a second conductive coating 16,

in that order. Bodies 18 of pho-toconductive material are contained in the apertures of openings Zit of the plate 12. The first conductive coating l@ is transparent to the input radiations, and the second conductive coating 16 is transparent to the output radiations.

As shown in greater detail in Fig. 2, the -apertured plate 12, preferably of glass, is made up of a great many transparent volumes or light ducts 22 optically separated from one another by a network of relatively narrow opaque portions of light baflies 24. The plate 12 -is a unitary structure, with -the opaque portions 24 prei erably being formed by darkening the original glass of which the plate 12 is com-posed. This process is described in greater detail later. The apertures 2@ extend through the light ducts 22 from side to side of the plate 12. One side of the plate 12 is covered with the continuous transp-arent conductive coating 10, such as a thin film of tin oxide, gold, or aluminum. The coating 10 extends, a slight distance into theapertures 2% to form a narrow conductive band on the inner wall of each aperture. Alternatively the photoconductor body may extend a slight distance outward and beyond the apertures to providea sufficiently large contact area between the photoconductor and the coating 1i). The other side of the plate 12 is coated with the mosaic of mutually insulated conductive patches 26, which are transparent to the feedback radiation. The patches 26 coat the top surfaces of the light ducts 22 and, like the coating 10, extend into the apertures 2t) to form narrow conductive bands or they overlap somewhat the photoconductive body filling the apertures. In one particular method of laying down this mosaic, a network of grooves 28 is first etched into the glass and the grooves are filled with a substance which serves as a mask. The transparent conductive material is then produced on the masked plate, by spraying or otherwise, after which the masking substance is removed from the plate, leaving only the patches 26 coated on the surface of the plate 12.

The apertures 20 are filled with the bodies or plugs 18 of photoconductive material, such as cadmium sulfide o1' cadmium selenide crystalline powder. The powder may be applied dry or it may be mixed with a suitable dielectric binder such as ethyl cellulose or an epoxy resin. rI'he ends of the plugs 18 make good conductive contact with the conductive coatings 1@ and patches 26.

The layer 14 of electroluminescent phosphor is applied over the patches 26, and the phosphor layer 14 in turn is coated with transp-arent conductive material, suc-h as a thin film of gold, forming the second conductive coating 16. The electroluminescent phosphor may lill in the grooves 2S. Alternatively, the grooves may first be filled in with inert material, such as a resin, which is insulating and preferably opaque, and the electroluminescent phosphor may be subsequently applied. Electroluminescent phosphor materials are well known, and in general a material is chosen which closely matches the spectral response of the photoconductive material of the plugs 13. Photoconductive cadmium sulfide, copper and chlorine activated, has a maximum green to red response, so that a green emitting phosphor, such as copper and aluminum activated zinc sulfide can be used with that material. The electroluminescent material is preferably mixed with a transparent dielectric binder material such as et-hyl cellulose or an epoxy resin.`

ln operation, a source 30 of alternating voltage, such as, for example, 800 volts at 200 cycles per second frequency, is connected to the conductive coatings 1t) and 16 which serve as external electrodes. In the absence of an input image on the light amplifier, most of the electric field is developed across the photoconductive plugs 1S and only a small threshold iield appears across the phosphor layer 14, as is well known. When a light or radiation image is projected on the panel from either side thereof, the incoming light rays irradiate the sides of Ithe photoconductive plugs 18, rendering the surfaces more conductive so that a greater field is built up across the adjacent portion of phosphor layer 14, whereupon the latter electrolurninesces. In the areas of the panel where the incoming light is relatively intense, the plugs 18 become more conducting and there is a greater eld built up across the phosphor areas adjacent thereto, so that the light emission from these phosphor areas is relatively high. In the darker areas, the plugs 18 are less conductive, the electric field across the phosphor areas is less, and the light emission is less. In this way, a mosaic of variable intensity areas of a light image is produced and amplilied element by element.

ri`he transparent conductive patches 26 of the mosaic serve to spread the currents uniformly over the adjacent areas of the phosphor layer 14 covered thereby. Furthermore, because of their location on the light ducts 2?., they tend to direct back into the light ducts some of the electrolluminescent light so as to establish feedback and light storage.

It has been shown that it is possible to adjust operating conditions, i.e., voltage and frequency of the power source, of a single optical positive feedback light intensilier cell so that it may have two stable operating states, one which is light emitting and the other which is essentially non-light-emitting. The ability to store indefinitely either of the two states gives rise to the term storage light amplifier. In an image intensifying panel some light unavoidably seeps through, or cross-feeds, to olf cells. Indefinite image storage is achieved when the intensity of this cross-feed light is kept below a certain trigger threshold of the exposed oil cells. Above this threshold, image storage time is finite, and the speed of spreading is related to the amount of light exposure of the oil-cell photoconductor.

The width of the light ducts 22 is made rather large compared to the widths both of the photoconductive plugs 1S and the light battles 24, so that a substantial amount of light, either input or feedback, or both, can illuminate the photoconductive plugs 18. The light batilles 24 form compartments surrounding each light duct 22 and plug 1S so as to confine the light within each compartment. in this way, cross-lumination between adjacent elements and spreading of the stored light pattern is prevented.

More than one photoconductive plug may be disposed Within the confines of each light baille. For instance, it may be desired to locate a trio of such plugs, each responsive to dillerent color light, eg. red, green and blue, with a single bathe, with each plug being responsive to feedback light from a corresponding one of three different color emitting electroluminescent phosphor elements.

It is pointed out that the terms transparent and opaque as applied to the light ducts 2,2 and baliies 24 are used in a relative sense with reference to the feedback light and photoconductor sensitivity. Since, it is only the feedback light which is involved in storage operation, and the feedback light may be a different color than the input light, the light bailes need only be suticiently opaque to the feedback light and then only to that feedback light to which the photoconductor is resistive to prevent triggering of normally dark photoconductive plugs.

To fabricate the plate 12 several known techniques can be used. Thus, it is well known that certain materialsV like alkali halides are transparent to visible radiation when -in a virgin state, but discolor and become non-transmitting in a number of wavelength ranges when exposed to X-ray or similar radiation. An opaque network is obtained in a plate of alkali halide material, such as potassium chloride or potassium fluoride, by bringing it in contact with a mask consisting of a network of X-ray transparent, i.e. low atomic number, material such as lithium, boron or graphite, filled with X- ray opaque, i.e. high atomic number, material such as lead. The alkali halide plate is then exposed to preferably collimated X-radiation through the X-ray mask. Such collimated radiation can be simulated by scanning the plate with a finite X-ray source, however, keeping satisfactory separation between source and mask and providing a suitable thickness of the mask. Instead of a conventional X-ray source, which has to be scanned, a gamma-ray emitting sheet of radio-active material can be used to expose the pattern through the above described mask.

The apertures can be obtained either by ultra-sonic machining, other physical machining, or by physicochemical means described below. Thus, radiation from a heat lamp is allowed to fall through a focused projection system onto the glass plate, thereby producing a heat image of the desired pattern on the plate. A suitable solvent is applied to the plate, the temperature of the solvent being kept well below that at which rapid dissolving action takes place. However, at the focused heat image areas the temperature rises well above the dissolving threshold. Thus, the differential dissolving rate between the heated and cooled parts of the solvent and the plate will result in the production of the desired apertures. It should be pointed out that the formation of the apertures be preferably done before the formation of the opaque network to avoid bleaching of the opaque network by the application of heat.

In another process which utilizes photographically sensitive glass known as Fotoform glass, the Fotoform glass is exposed to ultra-violet light lirst through a photographic master, with clear areas in the master corresponding to the desired positions of the apertures. These areas are then developed by heating the structure to a suitable temperature, about 450 C. until the aperture areas become opalescent. One side of the glass is then ground and polished to provide a suitable surface for a second exposure, this time to the light baille pattern. To this end, a second photographic master, with clear areas corresponding to the desired opaque areas in the glass, is registered with the previously developed aperture pattern on the glass, and a suitable exposure to ultraviolet light ismade. The aperture pattern is then etched through with hydrofluoric acid, the exposed areas being more soluble in the acid than the clear areas. Next, the bailile pattern is developed by heating the plate to about 550 C. although higher temperatures up to 650 C. may be used. The structure is then ground to size and polished on both sides. The batlles are then etched on one side 0f the plate with hydrofluoric acid to obtain grooves. The grooves and apertures are then lled with a suitable masking material, such as Alundum, and both sides of the structure coated with a transparent conductor. The masking material is then removed, leaving the mosaic of conductive patches.

It is thus apparent by means of the invention, a storage light amplifier of simple construction and having fine definition and'high feedback etlciency is provided.

What is claimed is:

1. A light-responsive structure for use in a storage light amplier comprising a plate of glass of substantial thickness which has integrally formed therein a network of opaque portions dividing said plate into a multiplicity of optically separated transparent portions, each of said transparent portions having an opening therethrough, and at least one solid photoconductive body in each of said openings.

2. A structure comprising a plate of.solid insulating material formed with an array of laterally spaced transparent portions each integrally surrounded by an opaque portion, each of said transparent portions having an opening extending therethrough and a solid photoconductive body in each of said openings.

3. A structure comprising a plate of solid insulating material formed with an array of laterally spaced transparent portions each integrally surrounded by an opaque portion, each of said transparent portions having an opening extending therethrough and a solid photoconductive body in each of said openings, and a layer of conducting material on one side of said plate registered with each of said transparent portions and connected to each of said photoconductive bodies.

4. An electroluminescent device comprising a plate of solid insulating material formed with an array of laterally spaced transparent portions each integrally surrounded by an opaque portion, a layer of electroluminescent phosphor adjacent to one side of said plate and comprising elemental areas -exposed to the other side of said plate through said transparent portions, and solid photoconductive areas supported by said plate so that each photoconductive area is exposed to a corresponding one of said phoshor areas through one of said transparent portions but is optically shielded from all other phosphor areas o by said opaque portions.

5. A storage light amplifier comprising an integral support plate formed of solid insulating material which includes an array of spaced transparent portions surrounded by opaque portions, electroluminescent phosphor areas adjacent to one side of said plate, and solid photoconductive plugs within said transparent portions supported by said plate so that each photoconductive plug is optically coupled to a corresponding one of said phosphor areas through said transparent portions but is optically shielded from all other phosphor areas by said opaque portions, said optically coupled photoconductive plugs and phosphor areas being electrically connected together.

6. A storage light amplifier comprising a plate of solid insulating material which includes an array of apertured transparent portions each integrally surrounded by an opaque portion, electroluminescent phosphor areas adjacent to one side of said plate and exposed to the other side through said transparent portions, and solid photoconductive bodies supported in the apertures of said plate and each having surface portions which are exposed to a corresponding one of said phosphor areas but optically shielded from all other phosphor areas.

7. A storage light amplier comprising a glass plate integrally formed with a network of opaque portions of the glass forming compartments with transparent portions of the glass therein, a solid photoconductive plug within each compartment extending from side to side of said plate and surrounded by one of said transparent portions, a mosaic of mutually insulated transparent conductive patches on one surface of said plate and registered with said compartments and connected respectively to said plugs, a layer of electroluminescent phosphor on said mosaic, conductive means on said phosphor layer, and conductive means on the other side of said glass plate, at least one of said last two conductive means being transparent to light.

8. A storage light amplier as in claim 7, wherein the volume of transparent glass is substantially greater than the volume of opaque glass.

9. A storage light amplifier as in claim 7, wherein the volume of transparent glass is substantially greater than the volume of said photoconductive plugs.

References Cited in the le of this patent UNITED STATES PATENTS Sheldon Aug. 13, 1957 UNITED STATES PATENT OEEICE CERTIFICATE OF CORRECTION Patent No. 2,975,291- March I4, 1961 Egon E. Loeloner et al.

vIt is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

In the grant, line 2, and in the heading to the printed specification, line 4, name 01' the third inventor, for Donald E. Darling", each occurrence, read Donald C.

Signed and sealed this 17th day of October 1961.,

(SEAL) Attest:

ERNEST W. SWTDEE DAVID L. LADD Commissioner of Patents USCOMM-DC Attesting Officer

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