US2919352A - Electroluminescent device - Google Patents

Description

Dec. 29, 1959 B. KAzAN 2,919,352

ELECTROLUMINESCENT DEVICE Filed Sept. 27, 1957 2 Sheets-Sheet l (Ezfc'rfaizMfn/r 42,12

INVENTOR. BEM/AMW KAzA/v Arroz/vir A Dec. 29, 1959 B, KAZAN 2,919,352

ELECTROLUMINESCENT DEVICE v Filed Sept. 27, 1957 2 Sheets-Sheet 2 INVENToR. /vuA/V//v /fAzA/V ELECRLUMNESCENT DEVICE Beniamin Kazan, Princeton, NJ., assignor te Radio Corporation of America, a corporation oi' Delaware Application September 27, 1957, Serial No. 686,702

11 Claims. (Cl. 2519-413) This invention relates to electroluminescent devices, and in particular to such devices designed to store light signals or images so that they may be viewed for an extended time and erased at will.

The present invention incorporates electroluminescent cells and photoconductive elements in a unique physical and electrical relation designed to produce a light amplifying and storage system having a higher degree of liexibility in adapting to diierent desired storage conditions than devices previously known.

An object of this invention is to provide electroluminescent cells and photoconductive elements arranged in such optical feedback relationship as to provide a high degree of optical feedback efficiency.

Another object is the provision of a light amplifying and storage device having a high overall gain, thereby permitting it to respond to very weak light signals and to. store the same.

Another object is the provision of a light storage device wherein the storage time, or persistence of the signal information viewed, is controlled.

A feature ofV the invention is the structural simplicity of the device which attains said high feedback efficiency.

Another feature is the provision in a light storage dis; play panel of a simple means to erase the stored information periodically by momentarily interrupting optical feedback paths between electroluminescent cells and photoconductive elements.

In the drawings:

Fig. l is a fragmentary sectional View, partly schematic, of two pairs of electroluminescent cells and photoconductive elements arranged in opticalv feedbackA relation in accordance with the invention;

Fig. 2 isa plan view of one side of a light storage display panel incorporating an array of pairs of electrolurninescent cells'and photoconductive elements similar to those of Fig. 1;

Fig. 3 is a sectional view taken along lines 3 3V of Fig.'y 2, showing both sides of the panel;V

Fig. 4 is a diagrammatic view showing a light storage display panel with means for periodically erasing the stored information; and

Fig. 5 is a sectional view of the panel of Fig. 4.

In Fig. 1, which shows a single picture element, there is provided a transparent insulating support, or glass plate 1li. On one side ofA the plate 14)' is arranged a rst electroluminescent cell 12 which includes a transparent conductive coating 14v on the plate 10, an electroluminescent phosphor layer 16 and another transparent conductive coating l in that order. On the same side of the plate but displaced laterally from the electroluminescent cell 12 is a lirst photoconductive element 20 constituted by two spaced apart conductors 22 and 24 bridged by a small volume 26 of photoconductive material. The electroluminescent cell 12 and photoconductive element 2i) are connected in series with a iirst alternating current voltage source 28, as by conductive connections between one side of the source 28 and one side (coating 14) of atent 0 2,919,352 Patented Dec. 29, 17959 the electroluminescent cell 1 2, between the other side (coating 18) of the electroluminescent cell 12 and one side (conductor 22) of the photoconductive element 20, and between the other side (conductor 24') of the photoconductive element 20 and the other side of the voltage source 28. l

The transparent conductive coatings 14 and 18 may be thin films of evaporated metal, such as gold. Alternatively, the coating 14 on the glass plate 10 may be formed by spraying a solution of stannic chloride, methanol, and

water onto a heated glass plate 10. The conductors 22 and 24 may be similarly formed. Alternatively, they may be formed by evaporating a metal, such as gold, or by spraying silver paint through a Suitable mask, or by silk screening ysilver paste in the proper pattern. These conductors 22 and 24 need not be transparent.

The electroluminescent phosphor layer 16 may comprise particles of any well known electrolumnescent phosphor embedded in a transparent dielectric matrix. Some examples of electroluminescent phosphors are zinc sullide activated by copper and Zinc selenide activated by manganese. Suitable dielectrics are ethyl cellulose, polystyrene, or epoxy resins.

The photoconductive material 26' may consist of any of the well known materials having a variable impedance characteristic in response to radiant energy. Examples of photoconductive materials are cadmium sulde and cadmium `selenide which may be produced by vapor deposition, as sintered material, or 'as crystals in powder form, and mixed with a `dielectric binder of the same kind as described above.

The voltage source 28 may be alternating currentof several hundred to severalthousancl cycles per second frequency and may be SOO-.10,00 volts, depending on the On the opposite side of the plate 10- aredisposeda sec-` ond electroluminescent cell 12 and the second photoconductive element 2W identical tothe first cell. 12 and element 20, and connected-in series with aY second alternating current voltage source 28. conductive element 20l is directly oppcsitev the first' electroluminescent: cell- 1-2, and ,the` second electroluminescent cell 12T is. directly opposite the first photoconductive element 20. The elements and cells are so. arranged'in order to providev a continuous or closed feedback loop between the two series circuits, as will now be described.

Each of the two series circuits, when considered` separately, constitutes an elemental light amplifier. For instance, with no light incident on the lirst photoconductive element 20, the impedance of that element is substantially greater than that of the electroluminescentcell 12.

With suitable photoconductors the impedance ratio is about 10 to 1' or greater for the example given previously. Consequently, the small fraction of the supply voltage appearing across the electroluminescent cell is not of sutlicient magnitude to cause visible light emission therefrom. When light strikes the photoconductive element, its impedance-,drops inv accordancewith the light intensity so that a correspondingly greater fraction of the voltage develops across the electroluminescent cell and light is emitted from thecell, Since a small change in intensity of incident light falling on the photoconductive element can` produce a large change in the voltage developed across the electroluminescent cell and a corre-r The; second photospondingly large change in the electroluminescent light, light amplification is achieved.

To achieve maximum light gain, the highest supply voltage is applied which will not quite develop suficient voltage across the electroluminescent cell to produce visible electroluminescence therefrom in the absence of light on the photoconductive element. Thegain may be reduced by lowering the supply voltage.

Since the first electroluminescent cell i2 is optically coupled to the second photoconductive element 20', some of the amplified light emitted by the electroluminescent cell 12 impinges on the second photoconductive element 20. This initiates a similar light amplifying action in the second circuit to produce light emission from the second electroluminescent cell 12. Since the second electroluminescent cell 12 is optically coupled to the first photoconductive element 20, some ofthe light emitted by the second electroluminescent cell 12' impinges on the rst photoconductive element 20 to produce a regenerative action, by reason of which both electroluminescent elements may, under proper circumstances, continue to emit light after removal of the input light on the first photoconductive element 20. The holding or storage action occurs if the light emitted by the second electroluminescent cell 12 and reaching the first photoconductive element 20 is at least as great as the incident light which first excited it. Because the initial incident light undergoes two stages of amplication, this condition is readily attained.

One feature of the invention is that the light feedback process may be controlled to satisfy three conditions.

One of these conditions is light amplification without storage. The second condition is light amplification with retention of half tone light information for a limited time. The third condition is one of light amplification with indefinite storage and without half-tones. Each of these conditions may be attained simply by altering the gain of the second light amplifying circuit to control the degree of feedback light reaching the first photoconductive element 20. More specifically, the gain may be altered by varying the magnitude of the second supply voltage 28.

For the first condition, the second supply voltage 28 is set at zero. This renders the second light amplifying circuit inoperative and no feedback light may reach the first photoconductive element 20. With the first supply voltage energized, the first light amplifying circuit operates in the normal manner to produce amplified light emission from the first electroluminescent cell 12 when incident light falls on the first photoconductive element 20. The output light may be viewed from either side of the plate since both electrode coatings of the first electroluminescent cell 12 are transparent. Light, dark, and half-tone information is reproducible. When the incident light is removed, theoutput electroluminescent light is extinguished at a rate determined by the decay time of photoconductive element because of the absence of feedback light to maintain the rst photoconductive element 20 energized.

For the second condition, that of light amplification with extended retention of light information for a longer time, use is made of small amounts of positive feedback, i.e. the second supply voltage 28 is turned von so that the second light amplifying circuit becomes operative, feeding back to the first photoconductive element 20 a low level light below the level of the input light signal. For this purpose the gain of the second amplifier, which can be adjusted by adjusting its supply voltage 28 should be less than unity so that its output is low relative to the output ofv first amplier. Also, the product of the gains of the two individual amplifying circuits is less than unity to prevent unstable half-tone operation. Under these conditions, the overall speed of response will be less than the speed due to the rst photoconductive element 20 alone. In this type of operation, the individual gains of the two elemental amplifiers can be further changed if desired by varying the applied voltages, provided that the feedback light from the second electroluminescent element 12' is held below a critical level to prevent instability.

In each of the situations where the light decay is controlled by controlling relative gains of the two circuits, it is assumed that both phtoconductive elements 20 and 20' are of the same kind of material and hence react similarly to dierent levels of light. Additional control of decay can be realized by selecting different photoconductive materials for the individual amplifiers and controlling their respective gains. Cadmium selenide, for instance, is a relatively fast photoconductor, whereas cadmium sulfide is relatively slow.

ln the third condition, namely that of light ampliiication with indefinite storage, both circuits may be operated at high voltage and high gain such that a high level of feedback light reaches the first photoconductive element 20. This mode of operation, which is referred to herein as bi-stable operation, is useful for storing on or off light information. In addition, the large overall gain of the two amplifying circuits, which operate in cascade to provide a gain much greater than unity, and the high degree of light feedback permits the device to store weak short pulses of light.

In bi-stable storage operation, if the input light level is above a critical level, the feedback light returning to the first photoconductor will exceed a threshold and the regenerative action will automatically drive the first electroluminescent element to a saturation level or on level. If the input light level is below the critical level, the regenerative action will not take place because of the lower gain of the amplifiers at low light levels, and the first electroluminescent element 12 will remain in the olf condition after the input light is removed.

In another mode of storage operation, the two light amplifying circuits may be operated with voltages at different frequencies. For example, assume that the second voltage source 28' has substantially higher frequency than the first voltage source 28 (10,000 cycles per second for the second voltage and 600 cycles per second for the first voltage). Since the impedance of an electroluminescent cell is primarily a capacitive reactance, it is apparent that at the high frequency specified the impedance ratio between the second photoconductive element 20 and the second electroluminescent cell 12' will be much higher than 10 to 1 as specified previously at 500 to 1000 cycles per second. With low voltage operation of the second circuit, therefore, it is apparent that a relatively high level of light is required to excite the second photoconductive element 20 before its impedance can be reduced to the point where the second electroluminescent cell 12' will have at least a threshold voltage developed across it. Since the amount of light reaching the second photoconductive element 20' is determined by the amount of light incident on the first photoconductive element 20, the feedback loop can be made to operate only if the incident light on the first photoconductive element 20 exceeds a specified level. This mode of operation would be advantageous for displays where it is desired to blank out spurious low light level signals.

For displaying and storing light images over an eX- tended area, a projection screen is made of two arrays 29 and 29 of cells and elements similar to those of Fig. l. Such a screen lis shown in Figs. 2 and 3. For the first array 29, one side of a glass plate 30 is coated with a plurality of transparent conductive strips 32, spaced apart and running parallel. These strips 32 are coated in turn with strips 34 of electroluminescent phosphor. The phosphor strips 34 are coated with small transparent conductive elements 36, spaced apart from each other to form rows and columns of these elements over the one side of the plate 30. The phosphor strips 34 overlap` fao'rega se 'one edge of the transparent conductive strips 32, and the transparentconductive elements 36 overlap the corresponding edge of the phosphor strips 34. A plurality of elongated conductors 38 are disposed on the glass plate each parallel to the strips 32. and 34. Each conductor 38 is spaced slightly from the overlapping edges of the conductive elements 36. The spaces between the elements 36 and conductors 38 are filled with photoconductive material 40 to form a plurality of photoconductive elements 42. The conductors 38 and conductive strips 32 are connected to opposite sides of a voltage source 44. In such an array, each photoconductive element 42 is in series with an electrolumi'nescent cell 46 which is formed by a conductive element 36 and the registered portions of the phosphor and conductive strips 34 and 32 respectively.

A similar array 29' of photoconductive elements 42' and electrolumine'scent cells 46 is disposed on the opposite side of the plate with these elements 42 and cells 46 connected to a second voltage source 44 and located opposite the cells 46 and elements 42 respectively of the one side to provide optical coupling between the elemental light amplifiers on the two sides of the plate 30 and thus to provide a great number oflight feedback loops in the manner previously described. An image projected on one side of the plate 30 may be amplified and viewed as a stored image on the same side or on the opposite side of the plate 30.

To minimize cross-lumination between adjacent elemental light amplifiers, which may result in undesired spreading of the light image pattern, each elemental light feedback loop may be optically isolated from the neighboring loops by means of a network 47 of light opaque cells. The network 37 may be formed in the glass plate 30, as shown in Figs. 2 and 3, and may comprise darkened glass portions which are formed in the glass plate by exposing the glass to ultra violet light or X-ray radiation through a suitable mask. Alternatively, the glass plate may be etched or machined to form grooves extending a substantial depth in the plate, and the grooves then lled with an opaque substance. If the network 47 is not used, the groups of elements and cells should be spaced apart a sui-licient distance from adjacent groups to prevent undesired cross-lumination.

In some applications, for instance radar, it may be desired to erase previously stored portions of a light image before storing new image information. For this purpose, instead of one plate supporting the two arrays 29 and 29 of cells and elements, two spaced apart plates 50 and 52 may be used with each plate supporting one array, as shown in Figs. 4 and 5. The two plates 50 and 52, which may be circular, are joined at their outermost extremities to an annular spacer 53. For simplicity, only the outline of one of the arrays 29 is shown in Fig. 4. The arrangement of cells and elements in each array is the same as that shown in Figs. 2 and 3. In addition, a strip 54 of light opaque material is pivotally supported between the two plates 50 and 52 and arrays 29 and 29 so that it is free to rotate about an axis coinciding with the centers of both plates. The plane of the strip 54 is parallel to the planes of both plates 50 and 52 and is made long enough to reach the outer most extremities of the arrays. It is seen that at any one position of the opaque strip 54, the feedback loop between the two arrays 29 and 2.9 is interrupted to produce a dark area in the image coinciding with the opaque strip 54. All other areas remain unaffected. If the strip 54 is rotated, the entire stored image will be erased. Furthermore, the rotation of the opaque strip 54 may be synchronized with the rotation of a radar antenna to sequentially erase radial lines of the stored image so that new information can be written on the erased screen.

The opaque strip S4 may be driven at the center of the plates 50, 52 by a motor 56, as shown. Alternatively, the strip 54 may comprise an opaque sector in a thin lStrip 54.

in the embodiments described above, only a few elemental units .have been shown in the gures, for convenience. However, it will be understood that an actual panel will contain a great many more of these units in order to obtain good resolution.

The invention thus provides means embodied in relatively simple structure by which light images may vbe vamplified and stored eficiently with varying degrees of storage time, which images, may, in addition, be erased at will.

What is claimed is:

l. An electroluminescent device comprising a first electrical series circuit including a first electrolurninescent cell and a first photoconductive element said cell and said element disposed in a rst plane, and a second electrical series circuit including a second electroluminescent ell and a second photoconductive element said second cell and said second element disposed in a second plane, said second plane spaced from said first plane, with said tirst electroluminescent element optically coupled to said second photoconductive element and said second electroluminescent element optically coupled to said first photoconductive element so that a continuous optical feedback loop is formed.

2. The invention as iu claim 1, wherein said photoconductive elements have different response time charac teristics.

3. An electroluminescent device comprising a iirst electrical series circuit including a first electroluminescent cell, a first photoconductive element and a first alternating current voltage source said cell and said element disposed in a first plane; a second electrical series circuit including a second electroluminescent cell, a second photoconductive element and a second alternating current Voltage source said second cell and said second element disposed in a second plane, said second plane spaced from said iirst plane, with said first electroluminescent cell optically coupled to said second photoconductive element and said second electrolurninescent cell optically coupled to said rst photoconductive element so that a continuous optical feedback loop is formed.

4. The invention as in claim 3, wherein the frequency of said second alternating current voltage source is substantially different from the frequency of said iirst alternating current voltage source.

5. The invention as in claim 3, wherein said voltage sources are so related that the product of the gains of said two circuits is less than unity.

6. The invention as in claim 3, wherein said voltage sources are so related that the product of the gains of said two circuits is substantially greater than unity.

7. An electroluminescent device comprising a transparent insulating plate, a iirst electroluminescent cell and a rst photoconductive element arranged side by side on one side of said plate and electrically connected in series, and a second electrolurninescent cell and a second photoconductive element arranged side by side on the side of said plate opposite fro-m said one side and electrically connected in series, with said rst electroluminescent cell oppositesaid second photoconductive element and said first photoconductive element opposite said second electroluminescent cell.

8. An electroluminescent display panel comprising a transparent insulating plate and an array of elemental units on each of two sides of said plate, each unit comprising an electroluminescent cell and a photoconductive element arranged side by side and electrically connected in series, each unit being located opposite a respective unit on the other side of said plate with the photocon- 7 ductive element of each unit opposite the electroluminescent cell of the opposite unit.

9. An electroluminescent storage device comprising two transparent insulating plates disposed face to face and spaced apart, each plate supporting on a surface thereof an array of elemental units, each unit including an electroluminescent cell and a photoeonductive element arranged side by side and electrically connected in series, each unit on one plate being located opposite a respective unit on the other plate, with the photoconductive element of each unit opposite the electroluminescent cell of the opposite unit to produce a light feedback loop between the two arrays, and movable opaque means intermediate said two plates for interrupting said feedback loop in selected areas.

10. The invention as in claim 9, wherein said arrays are disposed on the outside of said plates.

11. The invention as in claim 10, wherein said movable opaque means comprises an opaque strip of extended length which is rotatable about the center of said device and in a plane parallel to said plates.

References Cited in the file of this patent Marshall et al.: Quarterly Report #3, Fellowship in Computer #347, Mellon Institute of Industrial Research, Iuly 17, 1952.

#2 Quarterly Report #3, Second Series Fellowship on Computer, Mellon Institute of Industrial Research, June 30, 1954.

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