Image intensifier with ferroelectric layer and balanced impedances

Dec. 20, 1966 b. Kazan etal 3,293,441

IMAGE INTENSIFIER WITH FERROELECTRIC LAYER AND BALANCED IMPEDANCES

Filed May 12, I960

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INTENSIFIED VISIBLE OUTPUT IMAGE

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Benjamin Kazan

John &amp;. wjrsiow

INVENTORS

BY

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Attorney

United States Patent Office

3,293,441

Patented Dec. 20, 1966

3,293,441

IMAGE INTENSIFIER WITH FERROELECTRIC LAYER AND BALANCED IMPEDANCES

Benjamin Kazan, 2062 Tigertail Road, Los Angeles, Calif.

90049, and John S. Winslow, 4380 Canyon Crest Road, 5

Altadena, Calif. 90065

Filed May 12,1965, Ser. No. 456,901

Claims. (Cl. 250—213)

The present invention relates in general to light amplifiers, and more particularly relates to a ferroelectric image intensifier device.

This application is a continuation-in-part of the original application filed 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 amplifier 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 500-line TV pictures. When operated with X-ray input images, these intensifiers produced an output brightness 100 times greater than a conventional fluoroscope screen, at the same time greatly increasing the image contrast. However, devices of this type have a number of limitations, namely:

(1) 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 specific, 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 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 flow 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 flow 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 effectiveness, the photoconductive layer must be made to have a very much smaller 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 Amplifier,” 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 photoconductive-electroluminescent amplifier is improved in many respects. Specifically, the ferroelectric amplifier approach allows the fabrication of image intensifiers with the following advantages:

Increase of radiant-energy gain;

Increase of speed of response;

Increase of output brightness;

Decrease of 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 intensifiers, 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 (1) (2) (3) . (4) (5) , (6) (7) (8) (9)

3,293,441 patent, then there would not be any control action by the photoconductive layer, that is to say, when light impinged on the photoconductor material, there would still not be any significant change in the voltage or electric field across the ferroelectric and electroluminescent layers. In other words, the change in the electric fields across the ferroelectric and electroluminescent layers would be negligible, with the result that the current flow through these layers would likewise change very little.

Hence, a three-layer laminar arrangement as suggested, for example, by the combination of the Orthuber 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 photoconductive layer if the A.-C. current through the ferroelectric and electroluminescent layers is to be modulated to a significant 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 art to segmentize at least the photoconductive layer 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 simplified, 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 ferroelectric, to such an extent that the resistances 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 photoconductive 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 is a further 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 characteristic 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 definition of the limits of the invention.

FIGURE 1 is a cross-sectional view of an embodiment constructed in accordance with the present invention;

FIGURE 2 is a schematic representation 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 electroluminescent 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 titanate, barium strontium titanate, or the like, and may be a sintered layer or may be grown as a single flat 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 any 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 photoconductive layer being designated 13 and the other one, namely, the one covering the electroluminescent layer, being designated 14. Transparent conducting layers 13 and 14 are film 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 films 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 difficulty.

Electrically connected between layers 13 and 14 are direct-current and alternating-current voltage sources connected in series, the D.-C. source being designated IS and the A.-C. source being designated 16. As an example of the voltages that may be involved, D.-C. source 15 is preferably 100 volts and A.-C. source 16 is preferably about 75 volts R.M.S. As shown in the figure, voltage source IS 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 support member or glass plate 17 that supports the entire layer assembly on one 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 % of an inch in thickness.

The underlying principle of the image intensifier device shown in FIG. 1 is presented in FIG. 2 by means of a schematic circuit which represents an elemental picture element in the intensifier. 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 R_po and capacitor C_pc, the equivalent of ferroelectric layer 11 is the parallel combination of fixed resistor R_ie and variable capacitor C_fe, and the equivalent of electroluminescent layer 12 is the parallel combination of fixed resistor R_ei and fixed capacitor C_ei· 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 of this circuit, it will be assumed that the values of resistance are sufficiently high so that the altemating-ourrent path is to be considered as being through the capacitors. On the other hand,

3,293,441 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 R_pc, Rf_e and R_ej may be considered as forming a voltage divider with capacitors C_pe, C_Ieand C_ei respectively bridged across them, the sum of the three resistances having a much higher value than the sum of the three capacitive impedances. 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 the ferroelectric material. The fact that ferroelectric 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 15, 1962. Similar information is -available in the German article entitled “Aufbau and Eigenschaften von PTC-Widerstanden” published on pages 63 and 64 of the periodical Elektronische Rundschau, vol. 17, No. 2, 1963.

Accordingly, in operation, the effect of radiation on the photoconductive element is to reduce its resistance, that is, the resistance of resistor R_pe, thereby reducing the voltage drop across this last-mentioned resistor and correspondingly increasing the voltage drops across resistors R_fe and R_ei. This increase in the D.-C. voltage across the ferroelectric element increases its A.-C. impedance, that is to say, the impedance of capacitor C_fein the equivalent circuit, so that the A.-C. current flow through the photoconductive, ferroelectric 'and electroluminescent elements is correspondingly reduced. Consequently, there is a « electroluminescent element. In other words, for the reasons given, the flow of 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 flow of A.-C. current rising rapidly with an increase of field across it. Because of this, an increase of light on the photoconductive element, causing an increase in field 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 light.

Hence, in effect, 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. 1 embodiment and, as will be recognized, any such embodiment would necessarily include a great many of these elemental picture elements. However, the principles

S is shown in FIG. 3 wherein like dr similar parts or elements are similarly designated. Thus, in FIG.. 3, the photoconductive, ferroelectric -and electroluminescent layers are respectively designated 10, 11 and 12.. Likewise, the transparent conducting layers are designated 13 and 14, the D.-C. and A.-C. volt-age sources are respectively designated 15 and 16, and the glass 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 further need be said with respect to these elements. As to the exception mentioned, which -constitutes the modification, a thin resistive layer that is opaque to light is interposed between ferroelectric layer 11 and electroluminescent layer 12. 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 .18 may be -a fine mosaic of conducting elements, such as gold, that are isolated -from each -other. Such a mosaic could be deposited on either the ferroelectric layer or the electroluminescent layer by evaporating the mosaic material, such as the gold previously mentioned, through a very fine grid.

The -function of layer 18 is to prevent -or minimize light feedback, that is to say, -its -function is to cut down or reduce the amount -of light returning to the input from the electroluminescent layer. Layer 18 may also serve to reflect light toward glass faceplate 17.

Although a -couple of. particular arrangements of the invention have been illustrated 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 efdrop -in the -output light from the ³&amp; deposited between photoconductive layer 10 and ' ' -ferroelectric layer 11 rather than between the ferroelectric layer -and electroluminescent layer 12. Again, as an example, if ferroelectric layer 11 -is transparent to output ,_n light, then the positions of the electroluminescent and ferroelectric layers may be reversed, that is to say, the abovesaid sandwich may be composed of photoconductive, electroluminescent and ferroelectric layers in the order named. Accordingly, the invention should be considered to include any -and all modifications, alterations or equivalent arrangements falling within the scope of the annexed claims. . .