US3293441A - Image intensifier with ferroelectric layer and balanced impedances - Google Patents

Description

Dec. 20,1966 B.KAzAN ETAL 3,293,441

IMAGE INTENSIFIER WITH FERROELEGTRIC LAYER AND BALANCED IMPEDANoEs v Filed May 12, 1965 Low/LEVEL RAmATmN PATVERN Zi?, I'

l oco/vac'r/j/f /ff/ekoafcr/e/c v afar/ML M/A/fscf/vr Cpc' www WAGE JOHN 5. W//VSLOW INVENTORIS ATTORNEY l United States Patent Mice 3,293,441 IMAGE INTENSIFER WITH FERROELECTREC LAYER AND BALANCED MPEDANCES Benjamin Kazan, 2062 Tigertail Road, Los Angeles, Calif.

90049, and .lohn S. Winslow, 4380 Canyon Crest Road,

Altadena, Calif. 90065 Filed May 12, 1965, Ser. No. 456,901 2 Claims. (Cl. Z50- 213) The present invention relates in general to 4light amplifiers, and more particularly relates to a ferroelectric image intensifier device.

This application is a continuation-in-part of the original application tiled May 6, 1963, and given Serial No. 278,287, now abandoned.

Because of its ability to intensify and store images as well as convert them from one wavelength to another, combined with its structural simplicity and compactness, the two-layer solid-state light amplier stimulated considerable interest when first demonstrated a few years ago. Such amplifiers were developed to the point where they exhibited radiant energy gains of several hundred, using visible input light, and were capable of producing output images of a quality exceeding that of commercial 50G-line TV pictures. When operated with X-ray input images, these intensiers produced an output brightness 100 times greater than a conventional uoroscope screen, at the same time greatly increasing the image contrast. However, devices of this type have a number of limitations, namely:

(l) Slow speed of response, which interferes with the reviewing of moving objects;

(2) Limited resolution because of the photoconductor thickness, which is necessary to withstand the operating voltages;

(3) Relatively high input radiation threshold;

(4) Inherently high contrast ratio or gamma, thereby limiting its usefulness in some applications; and

(5) Restriction to the use of photoconductors with extremely high sensitivity and low dark current.

The above limitations result both because of the operating principles of such a two-layer device and the inherent properties of the materials utilized therein. To be more specic, an example of such a two-layer device is disclosed in the patent to William O. Reed entitled Photosensitive Radiant-Energy Transducers, issued November 26, 1963, and having Patent No. 3,112,404. As taught in the patent, Reed uses a laminar structure that includes photoconductive and electroluminescent layers sandwiched between a pair of transparent electrodes, an A.C. voltage source being connected across the electrodes. However, in this kind of two-layer device, the A.C. impedance of the photoconductive layer must be considerably higher than the A.C. impedance of the electroluminescent layer if the photoconductive layer is to have any control action at all. In other words, unless the capacitance of the photoconductive layer is very much smaller than that of the electroluminescent layer, the phosphor or electroluminescent material would be emitting light irrespective of the excitation of the photoconductor, an obviously undesirable characteristic.

This can be explained by schematically representing each of the two layers with the parallel combination of a capacitor and resistor, the two combinations being connected in series between an A.C. voltage source.

Since the resistance of the unilluminated photoconductive 3,293,441 Patented Dec. 20, 1966 layer is extremely high, it will be obvious to anyone skilled in this art that the capacitance of the layer must be very low as compared to the capacitance of the electroluminescent layer, that is to say, the A.C. impedance of the photoconductive layer must be considerably higher than that of the electroluminescent layer, or else a major portion of the applied voltage would appear across the electroluminescent layer. This means that unless the proper conditions exist, a significant amount of electrical current will ow through the electroluminescent layer, even though the photoconductive layer is unilluminated which, in turn, would cause the phosphor to emit light even in the absence of any radiant-energy input.

On the other hand, with the necessary relatively high A.C. impedance present in the photoconductive layer, the incidence of light thereon causes its resistance to drop to a very low value, which, from a practical point of view, shorts or by-passes the A.C. impedance. As a result, a major portion of the applied A.C. voltage is thereby developed across the electroluminescent layer and, in response thereto, the iiow of current through this layer materially increases so that there is a sharp increase in the amount of light emitted by it.

It is thus seen .that if the Reed device is to function at all, or at least with any elfectiveness, the photoconductive layer must be made to have a very much smal-ler capacitance than the adjacent electroluminescent layer. This is done by making the photoconductive layer thicker than it might otherwise be.

To a large degree, the limitations enumerated above were overcome by a new solid-state image intensifier approach as taught in the patents to Benjamin Kazan entitled Light Amplifying Device, Patent No. 2,905,830, issued September 22, 1959, and the patent to Richard K. Orthuber entitled Solid-State Radiation Amplilier, Patent No. 3,054,900, issued September 18, 1962. In this approach, three layers are employed consisting, respectively, of a photoconductor, a ferroelectric material, and an electroluminescent phosphor. By means of the added ferroelectric layer, the over-all operation compared to a two-layer photoconductiveelectroluminescent amplier is improved in many respects. Specifically, the ferroelectric amplifier approach allows the fabrication of image intensifiers with the following advantages:

(1) Increase of radiant-energy gain;

(2) Increase of speed of response;

(3) Increase of output brightness;

Decreaseof input radiation threshold;

Control of input threshold;

Use of D.C. operated photoconductive layers; Control of output image polarity;

Control of contrast or gamma; and

Use of photoconductors with increased dark currents.

However, notwithstanding the improvements mentioned, previous designs of ferroelectric image intensiiers, as may be seen from the above-said patents to Kazan and Orthuber, have all required complex electroding with one or more of the layers being broken into elements, segments or lines. This complex electroding and breaking up of the layers is made necessary by the fact that the ferroelectric material, such as barium titanate or triglycine sulfate, has such a high resistivity as compared to the resistivity of the photoconductive and electroluminescent materials that if they were all placed together in a simple three-layer arrangement of the kind suggested by the Reed patent, then there would not be any control action by the photoconductive layer, that is to say, when light irnpinged on the photoconductor material, there would still not be any significant change in the voltage or electric eld,

across the ferroelectric and electroluminescent layers. In other words, the change in the electric elds across .the ferroelectric and electroluminescent layers would be negligible, with the result that the current ow through these layers would likewise change very little. l

Hence, a three-layer laminar arrangement as suggested, for example, by the combinati-on of the Grthuber and Reed patents, would not work because they would be very little fluctuation of the light emitted by the electroluminescent layer in response to the fluctuations of light incident upon the photoconductive layer. In other words, in order to produce the desired amplifying action, the D.C. voltage across the ferroelectric layer must change significantly in response to the variations of conductivity or resistance of the photooonductive layer if the A.C. current through the fer-roelectric and electroluminescent layers is to be modulated to 1a signicant degree. This cannot be achieved simply by combining the materials of Orthuber with the structural arrangement of Reed. Consequently, to overcome this problem, it has been necessary in the prior a-rt to segmentize at least the photoconductive layerV and to employ complex electroding as a result thereof in order'to obtain the desired amplifying action.

The present invention permits the construction of an intensifier using only continuous layers of materials without requiring an array of accurately-spaced electrodes over the image area. As a result, the basic construction of such an image intensifier device is very greatly simpliiied, higher resolution is obtainable, and improved -uniformity can be expected. The present invention accomplishes these results through the alteration of the extremely high resistivity characteristics of the ferr-oelectric, to suchan extent that the resstances of the photoconductive, ferroelectric and electroluminescent layers are of the same order of magnitude, that is to say, comparable to each other in value, and this is done by doping the ferroelectric material, the term doping being a term that is Well understood in the art. A further benefit derived from the present invention lies in the fact that the capacitive impedance of the phot-oconductive layer need not be as great as in the Reed case, preferably smaller in value than the capacitive impedance of the electroluminescent layer, with the result that a much thinner photoconductive layer is used here than in the prior art.

It is, therefore, an object of the present invention to provide an improved three-layer solid-state light-amplifying device.

It is another object of the present invention to provide a three-layer solid-state image-intensifier device in which only continuous layers are used.

It isa furthe-r object of the present invention to provide a three-layer solid-state image-intensifier device whose construction is very greatly simplified.

The novel features which are believed to be characteristie of the invention both as to its organization and method 'of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawing in which an embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawing is for the purpose of illustration and description only and is not intended as a deiinition of the limits of the invention.

FIGURE l is a cross-sectional view of an embodiment constructed in accordance with the present invention;

FIGURE 2 is a schematic lrepresentation or equivalent circuit of an elemental picture element of an image intensifying device according to the present invention; and

FIGURE 3 is a cross-sectional view illustrating a modification of the FIGURE 1 embodiment.

Considering now the drawing, reference is made in particular to FIG. 1 wherein an image intensifier in accordance with the present invention is shown to include a sandwich arrangement of a photoconductive layer 10, a ferroelectric layer 11 and an elect-roluminescent layer 12, the ferroelectric layer being sandwiched between the photoconductive and electroluminescent layers. Photoconductive layer 10 may be 1 to 2 mils thick, or less, and, although photoconductive materials are well known, by way of example, it may be made of a material such as cadmium sulphide or cadmium selenide and may -be either in the powdered `or solid form. As for ferroelectric layer 11, it may be between 5 to 10 mils thick, may be made of a material such as triglycine sulfate, Rochelle salt, barium tit-anate, barium strontium titanate, or the like, land may be a sintered layer or may be grown as a single at crystal. Finally, electroluminescent layer 12, which may be approximately 1 to 2 mils in thickness, may be any of the known electroluminescent phosphors, such as copperactivated zinc sulphide phosphor, and may be deposited by kany of the known techniques such as settling or silk screening.

The free surfaces of photoconductive and electroluminescent layers 10 and 12, that is to say, the top and bottom surfaces of the above-described sandwich panel, are each covered with a transparent conducting layer, the one -overlying the phot-oconductive layer being designated 13 and the other one, namely, the one covering the electroluminescent layer, being designated 14. Transparent conducting l- ayers 13 and 14 are lm thin and may be of a material such as tin oxide or tin chloride and may be deposited by any known technique. These layers may also be made with thin iilms of metal, such as gold, evaporated onto their support surfaces -by the process of vacuum deposition, the layers being thin enough to permit light to pass through them without diiculty.

Electrically connected between layers 13 and 14 are direct-current and alternating-current voltage sources connected in series, the D.C. source being designated 15 and the A.C. source being designated 16. As an example of the voltages that may be involved, D.C. source 15 is preferably volts and A.C. source 16 is preferably about 75 volts R.M.S. As shown in the figure, voltage source 15 is connected so that its negative terminal is connected to layer 13. As for voltage source 16, the frequency of the signal produced by this source is in the audio range, preferably about 5,000 cycles per second. Finally, also included in the FIG. 1 embodiment is a transparent supp-ort member or glass plate 17 that supports the entire layer assembly on Ione of its surfaces. More specifically, as is shown in the figure, layers 10-14 rest on plate 17 so that conducting layer 14 is sandwiched in between layer 12 and plate 17. Transparent support members 17 may be made of a material such as Pyrex glass and may be approximately 1A of an inch in thickness.

The underlying principle of the image intensifier device shown in FIG. l is presented in FIG. 2 by means of a schematic circuit which represents an elemental picture element in the intensier. In other words, the schematic circuit in FIG. 2 sets forth the electrical circuit equivalents of an elemental portion or segment of layers 10, 11 and 12. Thus, the equivalent of photoconductive layer 10` is the parallel combination of variable resistor Rpc and capacitor Cpe, the equivalent of ferroelectric layer 11 is the parallel combination of xed resistor Rf@ and variable capacitor Cie, and the equivalent of electroluminescent layer 12 is the parallel combination of fixed resistor Rel and fixed capacit-or Cel. These three parallel combinations, designated like the layers they represent, namely, 10, 11 and 12, are connected in series between voltage sources 15 and 16 previously described.

In considering the operation orf this circuit, it will be assumed that the values of resistance are suiciently high so that the alternating-current path is to be considered as bei-ng through the capacitors. On the other hand,

the D.-C. conductivity of the capacitors is assumed to be negligible, with the result that the direct-current path is to be considered :as being through the resistors. Stated otherwise, resistors Rpc, Rie and Re, may be considered as forming a voltage divider with capacitors Cpe, Cfe and Cel respectively bridged across them, the sum of the three resistances having a much yhigher value than the sum of the three capacitive limpe-dances. It should also be mentioned or reiterated at this point that the resistances of the three resistors are comparable to one another in value, that is to say, of the same order of magnitude, and that this is achieved not only by appropriately choosing the thicknesses of the layers to provide the necessary -operating conditions, but also by doping t-he -ferroelectric material. The fact that erroelectric materials can be prepared with conductivity -is indicated in the article entitled Ferroelectricity in SbSI by E. Fatuzzo, et al., published on page 2036 .in Physical Review, vol. 127, No. 6, September l5, 1962. Similar information =is available in the German article entitled Aufbau and IEigenschaften von PTC-Widerstanden published on pages 63 and 64 o-f the periodical Elektronische Rundschau, vol. 17, No. 2, 1963.

Accordingly, in operation, the effect of, radiation on the photoconduc-tive element is to reduce its resistance, that is, the resistance of resistor Rpc, thereby reducing the voltage drop across this last-mentioned resistor and correspondingly increasing the voltage drops across resistors Rfe and Rel. This increase Iin the D.-C. voltage across the lferroelectric element increases its A.C. impedance, that is to say, the impedance of capacitor Cfe in the equivalent circuit, so that the A.C. current fiow through the photoconductive, .-ferr-oelectric land electroluminescent eleme-nts is correspondingly reduced. Consequently, there is a drop -in the output light from the electroluminescent element. In other words, for the reasons give-n, the -iiow o-f A.C. current through the phosphor is therefore reduced and the light output lowered. This is brought about by the fact that the ferroelectric element has a square-.shaped hysteresis loop. As a result, it acts, essentially, as an extremely non-linear A.C. impedance, it blocking action toward the iiow of A.-C. current rising rapidly with `an -increase of field across it. Because of this, an lincrease o-.f light on the photoconductive element, causing an increase in iield -across the ferroelectric element, causes a sharp drop .in the light emitted from the electroluminescent element.

Similarly a reduction in input radiation will, for the reasons given, ultimately produce an increase in output light.

Hence, in etiect, a small change in input radiation on the photoconductor produces a large change in the output light.

As we previously mentioned, the equivalent circuit in FIG. 2 is taken for an elemental picture element in the FIG. l embodiment and, as w-ill be recognized, any such embodiment would necessarily include .a great many of these elemental picture elements. However, the principles underlying lone such picture element are equally applicable to all picture elements. Moreover, since layers -14 are so thin, electrical current iiows straight down through them and does not, to any practical extent, iiow -or .spread sideways. Accordingly, it may be said that each elemental picture element is isolated from or acts independently from every other picture element, which means that the underlying principles of a single picture element are lalso valid [for the embodiment as a whole. Thus, a light image passing through nlm 13 and irnpinging upon photoconductive layer 10 will result in an intensi-fied output ima-ge visible thro-ugh Iglass plate 17 and, because of the elimination of any electrode structure or groovi-ng technique, the resolution is greatly improved, being limited only by the laye-r thickness.

A modification of the embodiment presented in FIG. l

S is shown in FIG. 3 wherein like or similar parts or elements are .similarly designated. Thus, in FIG. 3, the photoconductive, ferroelectric and eleotroluminescent layers are respectively designated 10i, 11 and `12. Likewise, the transparent conducting layers are designated 13 and 14, the D.-C. and A.C. voltage sources are respectively designated 15 and 1-6, and the Iglass plate is designated 17. With but one exception, the elements recited are mounted or -arranged as they were before and, also, are made of the same materials and dimensions. Hence, to avoid being redundant, nothing iurther need be said with respect to these elements. As to the exception mentioned, which constitutes the modiiication, Va thin resistive layer that is opaque to light is interposed between ferroelectric layer 11 and electrolmninescent layer l2. This opaque resistive layer, designated .18, is preferably only a fraction of a mil in thickness and may be a thin sheet of plastic, such as an epoxy resin, containing lampblack. Again, layer .1-8 may be a tine mosaic of conducti-ng elements, such as Igold, that are isolated -rom each other. Such a mosaic could be deposited on either the rferroelectric layer or the electroluminescent layer by evaporatin-g the mosaic material, such as the gold previously mentioned, through a very fine lgr-id.

The yfunction of layer 18 lis to prevent or minimize light feedback, that -is to say, lits function is to eut down or reduce the amount lof light returning to the input rfrom the electroluminescent layer. Layer 18 may also serve to reflect light toward Vglass faceplate 17.

Although a couple of. particular arrange-ments of the invention have been yillustrated above by way of example, it is not intended that `the invention -be limited thereto. Thus, by way of example, in the arrangement of FIG. 3, opaque resistive layer 18 may, with equally good effeet, be -deposited between photoconductive layer 10 and Ierroelectric layer 11 rather than between the rerroelectric layer and electroluminescent layer 12. Aigain, ias an example, if yferroelectric layer 11 is transparent to output light, then the positions of the electroluminescent and -ferroelectric layers may be reversed, that is to say, the abovesaid sandwich may be c-omposed of photoconductive, eleetroluminescent and ferroelectric layers in the order named. Accordingly, the invention should be considered to include any and all modiii-cations, alterations or equivalent arrangements .falling within the scope of the annexed claims.

Having thus described the invention, what is claimed is:

1. Light amplifying apparatus comprising: continuous layers of a photoconductive material, a ferroelectric material and an electroluminescent mate-rial in a sandwich arrangement with the photoconductive layer being an outside layer and in which the layers are in contact with each other throughout their `face-to-face surfaces, the parameters of said layers being such that the sum of their direct-current impedances is considerably ,greater than the sum of their alternating-current impedances, said ferroelectr-ic layer being doped to reduce its directcurrent impedance to .a magnitude that is respectively of the same order as the direct-current impedances of the photoconductive and electroluminescent layers; and transparent conducting layers on the top and bottom surfaces, respectively, of said sandwich arrangement.

2. Image intensifier apparatus compris-ing: continuous layers of a photocondructive material, .a ferroelectric material and an electr-olluminescent material in a sandwich arrangement in which the rerroelectric layer is positioned between the photoconductive and electroluminescent layers and i-n which said layers are in contact With each other throughout their iace-to-face surfaces, said ferroelectric layer being doped to reduce its resistance to a magnitude that is similar in value to those of the photoconductive and electroluminescent layers; continuous transparent conducting layers on the top and bottom surfaces, respectively, orf said sandwich arrange- 7 ment; and drect-mlrrent and alternating-current voltage 2,989,636 sources connected in series between said transparent con- 3,041,490 ducting layers. 3,054,900 3,112,404

References Cited by the Examiner UNITED STATES PATENTS 2,905,813() 9/1959 Kazan Z50-213 8 Lieb Z50-213 X Rajdhman et al. Z50-213 X Ofrt-huber Z50-213 Reed Z50-213 RALPH G. NILSON, Prmmy Examiner.

M. A. L-EAVITT, Assistant Examiner.

Claims (1)

1. A LIGHT AMPLIFYING APPARATUS COMPRISING: CONTINUOUS LAYERS OF A PHOTOCONDUCTIVE MATERIAL, A FERROELECTRIC MATERIAL AND AN ELECTROLUMINESCENT MATERIAL IN A SANDWICH ARRANGEMENT WITH THE PHOTOCONDUCTIVE LAYER BEING AN OUTSIDE LAYER AND IN WHICH THE LAYERS ARE IN CONTACT WITH EACH OTHER THROUGHOUT THEIR FACE-TO-FACE SURFACES, THE PARAMETERS OF SAID LAYERS BEING SUCH THAT THE SUM OF THEIR DIRECT-CURRENT IMPEDANCES IS CONSIDERABLY GREATER THAN THE SUM OF THEIR ALTERNTING-CURRENT IMPEDANCES, SAID FERROLECTRIC LAYER BEING DOPED TO REDUCE ITS DIRECTCURRENT IMPEDANCE TO A MAGNITUDE THAT IS RESPECTIVELY OF THE SAME ORDER AS THE DIRECT-CURRENT IMPEDANCES OF THE PHOTOCONDUCTIVE AND ELECTROLUMINESCENT LAYERS; AND TRANSPARENT CONDUCTING LAYERS ON THE TOP AND BOTTOM SURFACES, RESPECTIVELY, OF SAID SANDWICH ARRANGEMENT.

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