US 3675075 A
A solid-state electroluminescent image display device which produces the output thereof in response to an image of the light, X-ray or other types of energy projected on to said device, said output being able to be controlled by varying a DC bias voltage for said device, and which comprises an electroluminescent layer excited by an AC electric field and controllable by a DC field and an energy-responsive layer whose resistivity varies in response to the incident energy such as a photoconductive layer, said two layers being interposed between a first light-pervious electrode and a second electrode in such a manner that the electroduminescent layer is adjacent to said first electrode, a third or composite electrode place between said two layers, the AC exciting voltage being connected between said first and third electrodes and the DC bias voltage between said second and third electrodes.
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Description (OCR text may contain errors)
United States @atent Kohashi 1 July 4, 1972 54 AN ENERGY RESPONSIVE IMAGE 3,293,441 12/1966 Kazan et al. ..250/213 CONVERSION AND AMPLIFICATION 3,315,080 4/1967 1401135111 ..313/10s x DEVICE c/Rcu/r /000 ELECTRIC C/RCU/T Primary ExaminerRoy Lake Assistant Examiner-Palmer C. Demeo Attorney-Stevens, Davis, Miller & Mosher 57 ABSTRACT A solid-state electroluminescent image display device which produces the output thereof in response to an image of the light, X-ray or other types of energy projected on to said device, said output being able to be controlled by varying a DC bias voltage for said device, and which comprises an electroluminescent layer excited by an AC electric field and controllable by a DC field and an energy-responsive layer whose resistivity varies in response to the incident energy such as a photoconductive layer, said two layers being interposed between a first light-pervious electrode and a second electrode in such a manner that the electroduminescent layer is adjacent to said first electrode, a third or composite electrode place between said two layers, the AC exciting voltage being connected between said first and third electrodes and the DC bias voltage between said second and third electrodes.
9 Claims, 2 Drawing Figures AN ENERGY RESPONSIV E IMAGE CONVERSION AND AMPLIFICATION DEVICE This invention relates to an energy-responsive image conversion and amplification device in which an electroluminescent element is energized to luminescence with an alternating electric field and at the same time, the luminescent output of said element being affected by a DC voltage whose magnitude depends on variation in the impedance of an energy-responsive element in relation to incident energy, thus the electroluminescent output from said element being controlled with relation to the incident energy.
More particularly, this invention relates to an image plate for visualizing and amplifying an image of such energy as light, x-rays or gamma rays or further for storing and displaying such an image, on the ground of the above-described principle which is described in detail in the U.S. Pat. No. 3,525,014 granted to the present applicant on Aug. 18, 1970.
In conventional energy-responsive luminescent devices, control of the luminescence has been achieved, in principle, by controlling the alternating current supplied to the electroluminescent element, said alternating current in turn being controlled according to the AC component of a variation in the impedance of a photoconductive element. In such a device, however, the sensitivity of the photoconductive element under AC energization has been too low to be sufficiently useful. According to this invention, a DC control voltage is superimposed on the AC voltage in an ingenious manner so as to permit an expedient DC control. Thus, a remarkably improved energy-responsive luminescent device which has satisfactory characteristics of image conversion and amplification and which also allows for an image to be memorized therein, is obtained.
This invention will be explained hereunder in connection with an embodiment thereof and referring to the attached drawings in which;
FIG. 1 is a schematically shown sectional view of the embodiment of this invention shown with associated electric connection; and
FIG. 2 is a simplified electric circuit diagram equivalent to the arrangement shown in FIG. I.
Now, referring to FIG. I, a layer of AC-DC EL element 100 (this term will be explained in the next paragraph) is provided on one surface of a first light-pervious electrode 110, and a layer of energy-responsive element, for example, consisting of a photoconductive element 200, whose resistivity varies in connection with the excitation by an incident energy, is positioned in the opposite side of said AC-DC EL element 100 in relation to said first electrode 110 in contact with a second electrode 210 which is pervious to the incident energy. A divided or foraminate composite electrode 310 consisting of electroconductive members 301 coated with dielectric material 302 of a high resistivity is interposed between said first and second electrodes.
In this specification, the term AC-DC EL element defines an electroluminescent (hereafter referred to as EL) element which comprises, for example, EL material dispersed in dielectric medium of resistive type or of accumulatively polarizable type which supports the internal electric field when an external polarizing unidirectional voltage is applied thereto and maintains the residual component of said electric field when said external voltage has been removed therefrom, the waveform of the luminescent output from said element due to the excitation with an AC electric field being controllable by a DC electric field applied thereto.
More tangible features of the construction and the manufacturing method of this device will be explained hereunder. Reference numeral 120 indicates a support plate of transparent and heat-resistive material such as glass, which is coated with said first electrode 110 which consists of a heatresistive and light pervious metal oxide such as tin oxide. To this first electrode 110 applied and fused is a layer of the AC-DC EL material in thickness of the order of to 50 microns, said AC-DC EL material comprising powdered dielectric medium of electro-resistive or accumulatively polarizable type such as glass enamel containing electro-resistive (i.e., slightly conductive) metal oxide such as SnO and further containing Li or Li and Ti, and mixed with powdered EL fluorescent material such as ZnS. Then, an electro-resistive (i.e., slightly conductive) layer of an intermediate element 600 is applied onto said AC-DC EL layer. Said element 600 is made from a mixture of, for example, epoxy resin and powdered electro-resistive material such as pulverized carbon or SnO or a mixture of powdered frit, an inorganic black pigment and powder of electro-resistive material such as SnO said mixture being bonded by heating to become an electroresistive and non-pervious layer. Addition of ferro-electric material such as BaTiO to said mixture will be effective for reducing the AC impedance of the resultant layer. Said intermediate layer 600 prevents the luminescent output of the EL element from being fed back to the photoconductive element. However, in case some degree of feedback is desirable, the intermediate layer 600 is made semi-pervious accordingly. Further said layer 600 can be formed as a composite layer, the
additional elements being interposed between the non-pervious layer and the AC-DC EL element 100. For example, an additional layer is made from a mixture of powder of frit or epoxy resin and powder of a ferro-electric and light-reflective material (most preferably BaTiO or an electro-resistive metal oxide such as SnO said mixture being fused and bonded by heating to become an electro-resistive reflecting layer. Thickness of the intermediate layer 600, single or composite, should be 5 to 10 microns for a non-pervious layer and 10 to 20 microns for a reflective layer so that the resistivity of the layer 600 across the thickness is appropriately low as compared with that of the EL element 100. The intermediate element 600 may be endowed with an extremely non-linear resistivity in order to prevent the increase of DC current. This may be achieved by the use of a non-linear resistive material such as CdSzCl or SiC instead of the resistive metal oxide in the above-described composition. At least one of either the resistive reflecting layer or the non-pervious layer can be eliminated in this case.
On the intermediate element 600, or the AC-DC EL element 100 if the intermediate element 600 is omitted, is disposed a divided or foraminate composite electrode 310 which consists of conductors 301, for example, of tungsten or copper wire of about 10 to 30 microns in diameter coated with a highly dielectric material 302 such as polyester resin or glass in thickness of, for example, 2 to 10 microns. In the present embodiment, the composite electrode 310 is formed in the shape of a grid with juxtaposed lines. A composite electrode 310 of another type may be formed either by reticulating the above-described coated conductors or by coating a metallic network with the above-mentioned dielectric material. The space factor of the conductor 301 in said composite electrode 310 should be such that said electrode 310, in spite of its foraminate formation, produces a substantially similar effect with respect to the AC field as that with a solid plate electrode when an AC operating voltage V is applied between the first electrode and this composite electrode 310. In other words, the composite electrode 310 should be constituted in such a form that the exciting AC power for the AC-DC EL element 100 is not greatly affected by the variation in the resistivity of the photoconductive element 200. The space between two adjacent lines in a grid type composite electrode 310 as shown in FIG. 1 should be preferably less than 400 microns. In a net (lattice) type electrode, it is desirable that the network is finer than 50 mesh. Such spaces in the composite electrodes provide passages for the DC current between the AC-DC EL element 100 and the photoconductive element 200.
In order to ensure the similarity of the composite electrode 310 to a solid plate electrode in the effectiveness for an AC field, an auxiliary resistive layer 700 is provided in the same plane as the composite electrode 310 is disposed, filling at least a part of said spaces in the composite electrode 310. The resistivity of said resistive layer 700 is chosen at an appropriate value so that excessive dispersion of the DC current can be prevented. An experiment showed that an auxiliary resistive layer 700 having a surface resistivity of 10 to 10 ohmscm gave a satisfactory result. Accordingly, it will be understood that if the surface resistivity of said AC-DC EL element 100 and said intermediate element 600 fulfils the abovementioned condition, the auxiliary resistive layer 700 is not required. Thus, the composite electrode 310 can be disposed on the intermediate element 600 or the EL element 100 without the auxiliary resistive layer; or further, said electrode 310 can be partly or entirely sunk in said element 600 or 100. In the latter case, the auxiliary resistive layer 700 can be assumed to be included in at least either one of the non-pervious layer or the reflective layer of said intermediate element 600 or in the AC-DC EL element.
If the intermediate layer 600 and the dielectric coating 302 are made of a vitreous material, the auxiliary layer 700 is made from powder of epoxy resin or frit mixed with a resistive metal oxide such as SnO said mixture being fused and bonded by heating. While, if the intermediate layer 600 and the dielectric coating 302 are made of a binder of low melting point such as epoxy resin, a similar binder mixed with the resistive metal oxide is used. In this case, powder of a highly dielectric material, especially a ferro-electric material such as BaTiO may be added to the mixture to reduce'the AC impedance of the resultant layer. Thickness of the auxiliary layer 700 is set, for example, at about 10 to 50 microns to limit the resistivity across the thickness. Further, the auxiliary layer 700 may be endowed with a significantly non-linear resistivity in order to minimize the DC voltage loss across the thickness and the dispersion of the DC current in the direction of the plane. This feature of the layer 700 may be attained by the use of a non-linear resistive material such as CdSzCl or SiC in lieu of the resistive metal oxide in the above-mentioned composition. Moreover, the auxiliary layer 700 may be endowed with functions of a non-pervious layer and a light-reflective layer by the use of the above-mentioned materials selected for the respective purposes. With the use of this type of auxiliary layer, the constitution of the device of this invention can be simplified.
Next, the energy-responsive element 200, that is a photoconductive layer in this embodiment is formed from a mixture of a binder such as epoxy resin and a photoconductive material such as CdS, CdSe or CdS'Se, said mixture being applied in a layer and bonded by heating. In case the dielectric coating 302, the auxiliary resistive layer 700 and the intermediate element 600 are made of a heat-resistive material such as vitreous material as described previously, the mixture applied on the lamination formed on the support plate 120 may be heated at 600 C. for about minutes, for example, in vacuum or in inactive atmosphere of nitrogen, thereby to become a layer of sintered photoconductive element 200 containing CdS, CdSe or CdS'Se.
The sintered photoconductive element, having substantially linear characteristics of voltage versus photoelectric current, makes possible an operation with considerably high sensitivity in comparison with an unsintered element.
In a lamination in which the auxiliary layer 700 is omitted, the photoconductive element 200 is formed in a layer filling the vacant spaces in the composite electrode 310. If the composite electrode 310 is completely sunken in the EL element 100 or the intermediate element 600, the auxiliary layer 700 is formed over said element 100 or 600. While, if said electrode 310 is not entirely sunken in said element 100 or 600, said layer 700 is formed filling the vacant spaces in said electrode 310.
The photoconductive element 200 decreases its resistivity in response to an incident energy such as the light or X-ray. Dark-resistivity of said element 200 should be higher than ohms-cm, for example. Thickness of said element 200 should be chosen to be 50 to 500 microns so that the dark-resistivity in the direction of thickness of said element 200 is similar to or rather higher than the resistivity of the AC-DC EL element 100 in the direction of thickness.
Then, the second electrode 210 is applied over the photoconductive element 200. The second electrode 210 is formed so as to be pervious of the incident energy including the light, X-ray and other radiative rays, and further to be electroconductive. For example, the second electrode 210 is made of foil of gold or aluminum, or vapor-deposited film of a metal, or silver paint. Further, the second electrode 210 may be made in the form of a grid comprising metallic wires of about 10 to 50 microns in diameter juxtaposed with a space of about to 500 (If the composite electrode 310 has the form of a similar grid, the grid of the second electrode 210 should be positioned in such mannerthat the members of said grid cross those of said electrode 310, or come between the latter members in the horizontal projection.) or in the form of a metallic network. The second electrode 210 of such a foraminate formation can be partly or completely sunken in the photoconductive element 200. Further, a second foraminate electrode has an advantage in that the variation of the resistivity in the direction of the plane can be utilized.
The first electrode is connected to one terminal 401 of an AC voltage source 400, and the conductor 301 of the composite electrode 310 to the other terminal 402 of said source 400 through a feeder bar 303, thus the AC operating voltage V being applied between said electrode 110 and 310.
The second electrode 210 is connected to one terminal 501 of a DC bias voltage source 500, while the other terminal 502 of said voltage source 500 is connected to said terminal 402 of said AC voltage source 400 thereby to supply a bias voltage V,, to the second electrode 210. The DC voltage source 500 is arranged so that the DC bias voltage V is variable and the polarity of said voltage V,, can be changed to give the AC-DC EL element 100 different operating characteristics. Thus, depending on whether the switch 5 is in contact with the terminal p or q (refer to FIG. 1), the electrode in the luminescent output side of the AC-DC EL element 100, that is, the first electrode 110 is biased negatively or positively. In the former case, the device is operative mainly for conversion and amplification of an image of the energy as well as erasing of the undermentioned stored image; and in the latter case, mainly for writing of an image and the luminescent display of the stored image.
FIG. 2 is a simplified equivalent circuit of the arrangement described above in connection with FIG. 1. For simplicity of the explanation, functions of the auxiliary resistive layer 700 and the intermediate element 600 are omitted in FIG. 2. Marking R indicates resistance of the photoconductive element 200 across its thickness, said resistance being variable; C capacitance across the dielectric coating 302 of the composite electrode 310, the conductor 301 being one electrode; and C and R respectively capacitance and resistance across the thickness of the AC-DC- EL element 100.
As is seen from FIG. 2, the resistance R,. decreases in accordance with the intensity of the incident energy L, such as the light or X-ray, and consequently the DC bias voltage V distributed across the AC-DC EL element is increased. As a result of this increase in the bias voltage, the waveform of the AC luminescent output L produced by AC power supplied through the capacitance C,, is controlled so that amplitude of said waveform is significantly reduced in particular half cycles of each one cycle of the AC power.
Therefore, with this embodiment, an image L of the energy such as the light or X-ray incident to the photoconductive element is converted and amplified with a high sensitivity and a high amplifing factor to an image of the luminescent output of negative polarity. It should be noted that in the device of this invention, the DC bias voltage V is applied between the energy-responsive element represented by the variable resistance R and the capacitive element represented by the capacitance C Accordingly, dielectric strength of the capacitance C,, on which a relatively low DC voltage corresponding to the DC bias voltage V is imposed, is not required to be very high. Further, adjustment of the source voltage V does not affect the AC power fed to the AC-DC EL element 100.
The above-described luminescent device having the form of a solid image plate can be used as a pre-amplifier for a television camera equipped with a vidicon or one of other image pickup tubes. The high sensitivity and the high energy amplifying factor of the luminescent device of this invention allow the conversion and amplification of an image of considerably low level or the pickup of an image means for sorting, i.e., selecting and separating the luminescent pulses by means of a lightchopper 1000 and a synchronous motor 1010 for driving said chopper 1000 are provided. In this means, the opening interval of said light-chopper 1000 should be shorter than at most one cycle period of the operating AC voltage V, and preferably a half cycle period or shorter, said interval being adjustable. Frequency of the sorting operation is set at'the same number as that of the operating AC voltage V,,.
As shown in the lower part of FIG. 1, the luminescent output image of controlled waveform radiated from the EL element 100 in response to the incident image L is sorted in regard to particular luminescent pulse stored therein. Further, according to this invention, polarity of the obtained video signal is reversed to display a visible image of the positive polarity relating to the incident image of the energy, on the screen of the television system 300. Such a device utilizing the closed television system is very useful as an X-ray television system for the industrial use or the medical use.
If means for adjusting or controlling the image reproduction characteristics such as contrast is provided in the television system, the under-described means for sorting the luminescent pulses is not necessarily needed.
In order to improve the reproduced image, by means of the light-chopper 1000 provided in the optical system of the television camera 2000, the photoelectric screen of an image tube in the television camera 2000 is excited by the output image of the thus sorted luminescent pulse L In a system wherein a television camera is used, the ratio of the frame frequency of the image tube against the sorting frequency for the luminescent pulses or the frequency of the operating AC voltage V,, should be a round number to avoid the beat or the flicker in the television picture, usually the latter frequency being set at a higher number than the former. In order to maintain such a synchronized relation between these two frequencies, the sweep signal or synchronizing signal of the horizontal or vertical scanning of the television system is used as the input signal E of the AC voltage source 400 and the timing or synchronizing signal for the driving signal E of the synchronous motor 1010. That is, said signals E S and E,, are produced from said timing signal in the respective electric circuits 410 and 1020 so that the ratio of the frequency of said signals E and E against that of said timing signal becomes a round number and further the phasic relation between the formers and the latter is adjustable. With this arrangement, adjustability of the operating characteristics can be attained.
In the aforegoing embodiment of this invention, the composite electrode 310 is positioned between the AC-DC EL element 100 and the energy-responsive element or the photoconductive element 200. In this arrangement, the total thickness of the layers including the intermediate element 600, the auxiliary resistive element 700 and any other additional elements, if desired, may be made the same as thickness of the composite electrode or lesser than that so that the whole of said elements can be put in the vacant space of the composite electrode 310. Such a constitution is advantageous in the point that dispersion of the DC current is prevented.
The composite electrode 310 can be partly embedded into at least either one of the DC-AC EL element 100 or the energy-responsive element 200.
Further, relative thickness of the composite electrode 310 is not limited by thickness of the EL element and the photoconductive element 200, but only have to be lesser than the distance between the first electrode and the second electrode 210. As to the position of the composite electrode 310, it is only re uirecl to be between the first electrode 110 and the second e ectrode 210 without any other limitation.
Therefore, at least either one of the EL element 100 or the photoconductive element 200 can be contained within the vacant space of the composite electrode 310.
In the above-described embodiment of this invention, a photoconductive element is used as the energy-responsive element. However, in a device according to the principle of this invention, the energy-responsive element is only required to vary its resistance in response to an excitation by any type of incident energy. Therefore, an energy-responsive element other than the photoconductive element such as a stress sensitive resistance element or a magneto-resistance element can be utilized, the resistivity thereof being controlled by elastic energy or magnetic energy respectively.
What we claim is:
1. An energy-responsive image conversion and amplification device comprising a first electrode which is planar and light-pervious; an electroluminescent layer provided on said first electrode, which can be excited by an AC electric field and waveform of whose luminescent output is changeable by a unidirectional electric field thereacross; a second electrode provided on the opposite side of said electroluminescent layer in regard to said first electrode; an energy responsive layer which varies the resistivity thereof in response to an energy applied thereto and which is provided between said second electrode and said electroluminescent layer; a gridor netshaped third electrode consisting of conductors coated with dielectric material, said third electrode being positioned between said electroluminescent layer and said energy responsive layer; means for applying an AC voltage between said first and third electrodes to cause said electroluminescent layer to luminesce; and means for applying a DC voltage between said second electrode and said first electrode to change the waveform of the AC excited luminescent output according to the variation in the resistance of said energy responsive layer.
2. A device as defined in claim 1, wherein a resistive intermediate layer is provided between said electroluminescent layer and said energy-responsive layer.
3. A device as defined in claim 2, wherein said intermediate layer contains ferro-electric material.
4, A device as defined in claim 2, wherein said third electrode is disposed adjacent the interface between said intermediate layer and said energy responsive layer.
5. A device as defined in claim 2, wherein said third electrode is disposed across the interface between said energy responsive layer and said intermediate layer.
6. A device as defined in claim 2, wherein said third electrode is embedded within said intermediate layer.
7. A device as defined in claim 1, wherein the thicknesswise resistance of said electroluminescent layer is lower than the maximum value of the thickness-wise resistance of said energy responsive layer.
8. A device as defined in claim 2, wherein the maximum value of the thickness-wise resistance of said energy responsive layer is higher than the total value of the thickness-wise resistances of the remaining layers.
9. A device as defined in claim 1, wherein said means for applying a DC voltage includes means for changing the polarity and magnitude of the voltage.