Light amplifying device

2,905,830

Sept. 22, 1959

B. KAZAN

LIGHT AMPLIFYING DEVICE

Filed Dec. 7. 1955

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2,905,330

Patented Sept. 22, 1SS9

United States Patent Office

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partially in section, of an electroluminescent panel in accordance with this invention;

Figure 3 is an enlarged fragmentary sectional view of an embodiment of an electroluminescent panel in acg cordance with this invention; and

Fig. 4 is a fragmentary plan view of the structure of Fig. 2.

Referring now to Figure 1 there is shown a schematic representation of an elemental unit of an electrolumi10 nescent image intensifying and/or storage device in accordance with this invention. The elemental unit includes a pair of ferro-electric elements 1® and 12 that are connected in series with a pair of electroluminescent elements 14 and 16. The ferro-electric elements are di15 electric elements that change their reactance in response to the voltage across the elements. The electroluminescent elements are elements that produce light in response to a voltage applied across the electroluminescent element. The series circuit of the ferro-electric elements 1® and 12 and the electroluminescent elements 14 and 16 are connected across the secondary 18 of a transformer 20. The primary 22 of the transformer 20 is connected to a source 21 of alternating current. The secondary coil 18 of transformer 20 has a grounded 25 center tap 24. Connected between the ferro-electric elements 1® and 12 is one side of a photoconductive element 26. The photoconductive element is an element that has a high impedance in the dark, with the impedance thereof being decreased by light striking the photocon30 ductive element. The change in impedance is largely due to a change in resistance in response to light. The other side of photoconductive element 26 may be connected to one side of a source 2§ of direct current, or to ground by means of switch 36. Also connected between the 35 ferro-electric elements 10 and 12 is one side of a resistor 34. The other side of resistor 34 may be connected to ground or to the positive side of another source 36 of direct current by means of a switch 32. Shunting each of the electroluminescent elements 14 and 16 is a leakage resistor 38 and 40 respectively.

With the photoconductor 26 in the dark, with the switch 30 connected to the positive side of the direct current source 28, and also with the switch 32 connected to ground, the circuit of an elemental unit, as shown in 45 Figure 1, operates as follows: The alternating current source applied to the primary of transformer 22 establishes a varying electric field across each ferro-electric element 1® and 12 and its associated electroluminescent element 14 and 16 respectively. The magnitude of the 50 alternating current source is selected so that the ferroelectric elements operate in their low impedance range. The electroluminescent elements are selected to have a high impedance, as compared to the ferro-electric elements, so that most of the alternating current voltage is 55 thus applied across the electroluminescent elements which causes the elements to luminesce. When in the dark, the impedance of the photoconductor 26 is high as compared to the impedance of resistance 34 and therefore the direct current potential at point 42 is substan60 tially zero.

When light is focused on the photoconductive element 26, the resistance of the element 26 is lowered and there is an increased direct current potential applied at point 42, i.e. between the ferro-electric elements 10 and 12. It 65 should be noted that this direct current potential may be either positive or negative with respect to ground. This increased direct current potential causes a direct current charge to flow into both of the ferro-electric elements 10 and 12. Substantially no direct current voltage 70 is built up across the electroluminescent elements 14 and 16 because of the leakage resistors 38 and 40 respectively. This direct current voltage, superimposed upon the alter2,905,830

LIGHT AMPLIFYING DEVICE

Benjamin Kazan, Princeton, NJ., assignor to Radio Corporation of America, a corporation of Delaware

Application December 7,1955, Serial No. 551,542

Claims. (Cl. 250—213)

This invention relates to light amplifiers and particularly to light amplifiers having improved sensitivity. Also, this invention relates to devices that amplify light and devices that will also store the light in half tone quantities.

This invention contemplates the provision of devices 20 for use in promoting the production and storage of light by a luminescent body under the controlling influence of a variable reactance device, with the reactance being varied by input signals. It is known in the electronic arts that a luminescent body can be made to produce light by the application of an electric field across the luminescent body. This phenomenon is known as electroluminescence. The theory of electroluminescence is not well understood. However, it seems to be agreed that electroluminescence results from a redistribution of electrons in the crystal structure of the electroluminescent material and the consequent emission of radiation from such material.

There are devices presently known which may be used to produce light images in response to an energizing force of one kind or another. Light storage devices are also well known in the art. However, the known devices are complicated; some of them have a relatively low sensitivity; others have a comparatively low response speed; while others have a relatively low light output or 40 picture resolution. Some of the storage devices that are known are not capable of storage of half tone shades, i.e. the storage of intermediate shades between black and white.

Accordingly, an object of this invention is to provide an improved light amplifying device.

It is another object of this invention to provide a novel light amplifying device of increased sensitivity, speed of response, or light output.

It is a further object of this invention to provide an improved light amplifying and picture storage device that is capable of reproducing half tone shades.

In general, the purposes and objects of this invention are accomplished by the provision of a novel electroluminescent panel including new and improved elemental units. Each of the novel elemental units comprises a photoconductive element, a ferro-electric element and an electroluminescent element. A variable field is applied across the electroluminescent element by varying the impedance of the ferro-electric element in response to current through the photoconductive element. The current through the photoconductive element is varied in response to input signals.

The invention will be more clearly understood by reference to the following specification when read in conjunction with the accompanying single sheet of drawings wherein;

Figure 1 is a schematic representation of an elemental unit of an image intensifying device in accordance with this invention;

Figure 2 is qn enlarged fragmentary perspective view,

2,905,880 nating current voltage in the series circuit connected across the secondary 18, changes the impedance of the ferro-electric elements 10 and 12 so that the ferro-electric elements now have a high impedance as compared to the electroluminescent elements 14 and 16. Due to the increased reactance of the ferro-electric elements, there is an increase in potential drop across the ferro-electric elements 10 and 12. The increase in potential drop across ferro-electric elements 10 and 12 decreases the potential drop across electroluminescent elements 14 and 16 at least to the point where the electroluminescent elements 14 and 16 produce less light. When the potential drop across the ferro-electric elements 10 and 12 is high enough, the luminescence from the electroluminescent elements will be cut off completely. This condition, in a total panel, is that of forming a dark image on a light background. When light on the photoconductor 26, which caused the flow of charge producing the increase in direct current potential at point 42, is removed, the photocOnductor 26 returns to its original high impedance condition, and the charge is trapped for a period of‘time. Hence, the change in light from the electroluminescent element is stored.

If switch 30 is connected to ground and switch 32 is connected to the source of direct current potential 36, the operation will be in reverse. In other words, when no light is focused on the photoconductive element -26, direct current voltage will be applied at point 42 and there wil! be no light produced by the electroluminescent elements 14 and 16 since the majority of the potential drop across the secondary of transformer 20 will occur across the ferro-electric elements 10 and 12. In this situation most of the potential drop occurs across the ferro-electric element 10' or 12 because of the direct current potential bias produced by source 36. When light is focused on the photoconductor 26, there is a low resistance for the conduction of the direct current, from source 36, to ground, and thus the bias voltage is removed. With the bias voltage most of the potential drop occurs across the electroluminescent element 14 and 16 and therefore, these elements produce light. This condition, in a complete panel, is that of forming a light image on a dark background.

Referring now to Figure 2 there is shown an enlarged fragmentary perspective view of an electroluminescent panel comprising a plurality of the elemental units shown in Figure 1. The panel 46 comprises a transparent support member or glass plate 48 that supports on one surface a plurality of transparent conductive strips 50. Covering the transparent conductive strips 50 is a layer of electroluminescent material 52 which in turn is covered by a layer of ferro-electric material '54. Spaced apart on the layer of ferro-electric ‘material 54 is a plurality of conducting squares 56. Extending from the center portion-of one row of squares '56 to the center of an adjacent row of conducting squares is a triangular shaped strip of photoconductive material 58. In between each strip of photoconductive material 58 and extending onto the conducting squares 56 is a triangular shaped strip of resistive material 60. Each of the strips of photoconductive material 58 and each of the strips of resistive material 60 has a strip 61 of transparent conductive material for purposes of electrical connection.

Each of the resistive strips 60 is connected to ground, while each of the photoconductive strips 58 is connected to the positive side of the source 62 of direct current potential. The alternate conductive strips 50 are connected to one side of the secondary of a transformer 20' while the intermediate of the conductive strips 50 are connected to the other side of the secondary of transformer 20'.

It should be understood that the thicknesses of the layers in panel '46 are shown, greatly exaggerated for simplicity Of illustration while in actual practice the layers are relatively thin. The panel'46 may- be constructed of the following materials and processes. The transparent support member 48 may be of a material siich as Pyrex glass and may be approximately one quarter of an inch in thickness. The transparent conductive strips 50, which may be about 10 mils wide and spaced 5 mils apart, may be of a material such as tin oxide or tin 5 chloride and may be deposited by any known technique through a suitable mask to provide the separate strips. The transparent conductive strips 61 may also be of a material such as tin chloride or tin oxide deposited through a suitable mask. The electroluminescent layer 10 52, which may be approximately 1 mil in thickness, may » be any of the known electroluminescent phosphors such as copper activated zinc sulphide phosphor and may be deposited by any of the known techniques such as settling ’ or silk screening. The ferro-electric layer 54, which may ' be approximately 5 mils in thickness, may be of a material such as Rochelle salt, barium titanate, barium strontium titanate or the like and may comprise a sintered layer or may be grown as a single flat .crystal. The photoconductive strips 58 may be of a material such .a cadmium 20 sulphide or cadmium selenide and .may -be _:in .the powdered or solid form. As is known, a solid layer of photoconductive materials may be deposited by evaporation; while the powdered form of .photoconductive material may be held in a plastic binder, such as polystyrene, 25 and machined or molded into the triangular shape strips shown. The resistive strips -60 .may be .formed in any well known manner and may be of a material such as carbon particles in a binder. The resistive-strips 60 should have a resistivity of approximately that of the photocon30 ductive strips 58, when in the dark, or-preferably lower.

For example, the resistivity of the strips 60 may be approximately one tenth of that of the _;photoconductor in the dark. The conducting squares .56, which may be about 10 mils square and-spaced apart about.5 mils, may 35 be of any material such as ..gold, or. silver. -The conducting squares 56 may be deposited .by evaporating materials through a suitable mask.

No specific provision , is made in panel 46 for leakage resistance, similar to resistor :38 and 40 of Figure .1, 40 around the .electroluminescent! layer 52. .The: reason for this is that the layer 52 will.have:a-certain amount-of leakage resistance. : If this resistance is : too high, the electroluminescent: phosphor may be mixed with a.conducting powder such.as fine carbon particles or .the like.

Those portions of theriayers.-52iand.-54 /between, a.conductive strip. 50α and conductive square. 56α, for .example, constitute an electroluminescent element and a.ferroelectric element connected electrically in series. -Similarly, those portions of .theTayersc52_:and:.-54 between _so an adjacent conductive ’strip :506 .and the conductive square-56α constitute : another electroluminescent .‘.element and another ferro-electric .element-connected .electrically in series.

The .operation :of the-panel'46 is: substantially -:the same as that, of a/plurality-ofi the .elemental, .units .shown ’^Jj in Figure 1. Thus, with mo.light. on.the -photoconductor, and as was described-.in connection'with Figure 1, the transformer 20' applies an alternating voltage from a conducting strip 50 through the electroluminescent layer θθ 52 through the ferro-electric layer 54 to a. conducting square 56, back through the ferro-electric layer. 54 and the electroluminescent layer 52 to .an adjacent, conduct- j, ing strip'50. This alternating voltage produces an alternating current from a strip 50 through the layers to a y- conducting square'56 back through the layers to the ° adjacent transparent conductive strip‘50. Thus, current is conducted transversely, i.e. in^-the direction indicated by dotted lines 57, -through the panel ’46. -Since· the impedance of the ferro-electric layer‘54 is low as eompared to the electroluminescent layer - 52, most of the voltage drop is across the electroluminescent -layer 52 and this layer produces light. It should be understood that the various layers of materials in-panel 46 are shown as being greatly enlarged for purposes of-’illustration, 75 and in actual fact these/layers are· relativelyrihin. Due

2,905,830 cent material 70. Covering the layer of electroluminescent material 70 is a layer of ferro-electric material 72. Spaced apart on the layer of ferro-electric material 72 is a plurality of conducting squares 74. Covering the 5 conducting squares 74, and the areas of ferro-electric layer 72 therebetween, is a layer of photoconductive material 76. Covering the layer of photoconductive material 76 is a transparent conducting layer 78. The materials and method of construction, for panel 64 may 10 be similar to those previously described in connection with the panel shown in Figure 2.

It should be noted that the panel 64 includes a continuous photoconductive layer rather than the strips of photoconductor and resistive material as described in 15 connection with panel 46. This arrangement is useful for the storage of pictures. For example, with the alternating current voltage applied across adjacent strips 68 by source 21', and with the panel 64 in the dark, conducting squares 74 assume an average potential that is 20 substantially equal to ground potential. When no image is incident upon panel 64 light is produced by the electroluminescent layer 70 since most of the voltage drop from source 21' is across the electroluminescent layer as was explained above. When an image is directed onto 25 panel 64, and with a direct current voltage applied to the transparent conductor 78, the light produces a low resistance through portions of the photoconductor 76. Because of this low resistance through the photoconductor, the potential of the conducting squares in these areas is varied from its original ground potential. This potential, with respect to ground, on the conducting squares 74, produces a biasing voltage pattern on the ferro-electric element 72. This biasing voltage changes the impedance of the ferro-electric element, as was explained above, and thus the light from these areas will be decreased by an amount depending upon the biasing potential. Since the biasing potential depends upon the decrease of resistance of the photoconductor, the decrease in light corresponds to the input image. The charge pattern is stored once the light is removed from a particular spot on the panel, because the photoconductor returns to its original high impedance condition and the charge is trapped for a period of time. This picture can be erased by connecting the transparent conductor 78 to ground and flooding the photoconductor with light.

The device of Figure 3 can also be used to amplify moving input pictures by applying a square voltage wave to the transparent electrode 78. If this voltage varies from zero to a positive potential, or from zero to a negative potential, illumination of the photoconductor with an image causes periodic charging and discharging of the elements in an amount determined by the intensity of the light falling on corresponding photoconductive elements. On the average therefore, the illuminated photoconductive elements will cause an average charge or potential, to exist on the corresponding ferro-electric element; with the level of the charge being proportional to the intensity of the input light. When desired, a stroboscopic light may be flashed on the panel in synchronization with the zero portion of the square wave to insure erasure of the previous image.

The devices in accordance with this invention amplify and/or store light images, in half tone shades, by the combining of the properties of electroluminescent ele65 ments, ferro-electric elements and photoconductors.