US3648052A

US3648052A - Solid-state image-converting device

Info

Publication number: US3648052A
Application number: US793048*A
Authority: US
Inventors: Tadao Nakamura; Shigeaki Nakamura; Tadao Kohashi
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 1969-01-22
Filing date: 1969-01-22
Publication date: 1972-03-07
Anticipated expiration: 1989-03-07

Abstract

A solid-state image-converting device comprising an energysensitive layer and a luminescent element whose luminescence is controlled in response to the variation of the impedance of said energy-sensitive layer, in which said energy-sensitive layer is a composite layer consisting of a layer for sensing the incident energy and a photoconductive layer responding to the light fed back from said luminescent element, thereby to improve the sensitivity of the device greatly.

Description

lJnited States Patent Nakamura et a1.

[151 3,648,052 51 Mar. 7, 1 .972

[54] SOLID-STATE IMAGE-CONVERTING DEVICE [72] Inventors: Tadao Nakamura; Shig'eaki Nakamura, both of Kawasaki-shi; Tadao Kohaslii,

Yokohama, all of Japan 73 Assigneei Matsushita Electric Industrial Co. us,

Osaka, Japan [22] Filed: Jan. 22, 1969 [21] Appl. No.2 793,048

[52] US. Cl. ..250/213 R, 313/108 A [51] lnt.Cl. ..H0lj 31/50 [58 FieldofSearch ..3l3/l08 R, 108 A, 65 R, 65 A;

[56] References Cited UNITED STATES PATENTS 3,031,579 4/1962 Hook et al. ..250/213 Koury ..250/2l8 Sharek... .....250/213 3,101,408 8/1963 Taylor .250/213 X 3,204,106 8/1965. Murr, Jr. et al.. .....250/213 3,247,389 4/1966 Kazan ..250/2l3 3,264,479 8/1966 Peek, Jr ..250/2l3 Primary Examiner-James W. Lawrence Assistant Examiner-D. C. Nelms Attorney-Stevens, Davis, Miller & Mosher [57] ABSTRACT A solid-state image-converting device comprising an energysensitive layer and a luminescent element whose luminescence is controlled in response to the variation of the impedance of said energy-sensitive layer, in which said energy-sensitive layer is a composite layer consisting of a layer for sensing the incident energy and a photoconductive layer responding to the light fed back from said luminescent element, thereby to improve the sensitivity of the device greatly.

6 Claims, 4 Drawing Figures Patented March 7, 1972 3,648,052

2 Sheets-Sheet 1 F I6. I PRIOR ART Iuvsurons T0900 lvmrnnu. n0

.IIHGE/Hfl fimnum 77 000 lronmm Hrrmmsy;

SOLID-STATE IMAGE-CONVERTING DEVICE This invention relates to a solid-state image-converting device for controlling the luminescence from an electroluminescent layer in relation to variation in electric impedance of an energy-sensitive layer caused by light, radiation or other incident energy, particularly to such a device in which the electroluminescence is electrically controlled in relation to variation in impedance of a photoconductive layer which is provided between said energy-sensitive layer and said electroluminescent layer and to which the light from said electroluminescent layer is internally fed back.

Objects and features of this invention will be clarified by the following description which is given with reference to the accompanying drawings, in which;

FIG. 1 is a sectional view of a conventional solid-state image-converting device;

FIGS. 2 and 3 are sectional views respectively of different embodiments of this invention; and

FIG. 4 is a block diagram of another embodiment of this invention.

As shown in H6. 1, the conventional solid-state image-converting device comprises a lamination consisting of an electrode l pervious to light, radiation or other kinds of incident image L, of energy, an energy-sensitive layer 2, an electroluminescent layer 4 which provides the output luminescent image L an opaque layer 3 provided between said energysensitive layer 2 and said electroluminescent layer 4 for preventing luminescent feedback from said electroluminescent layer 4, another electrode 5 pervious to light, and a light-pervious support plate 6 such as a glass plate, a power source being connected between said electrodes 1' and 5 for applying the operating voltage V.

With the above-described structure, the incident image L, to the energy-sensitive layer 2 reduces the electric impedance or resistance of said layer 2 and varies the electric current accordingly. Thus, the electroluminescent layer 4 is excited in response to the incident image L to produce corresponding luminescent output image L In the conventional device as described above, if the energy-sensitive material which the energy-sensitive layer 2 is made of does not have sufficiently high sensitivity or variation of the impedance or resistance relative to the incident energy, variation in the output luminescence from the layer 4 in response to the incident energy will be too small to effect satisfactory image conversion, not to mention the operation with high sensitivity.

Further, if the dark resistivity of the energy-sensitive materia1 is not sufficiently high, an electric current strong enough to cause electroluminescence flows through the electroluminescent layer 4 even where there is no incident energy, thereby lowering the contrast in the output luminescent image L on the layer 4, far from presenting a satisfactory image.

On the other hand,if the dielectric strength of the energysensitive layer 2 is not sufficiently high, an operating voltage high enough to cause a sufficiently large current to flow through the electroluminescent layer 4 will not be able to be applied, thus making an operation with high luminescent output and high sensitivity impossible.

For the above-mentioned reasons, it is an essential requirement for the conventional solid-state image-converting device that the energy-sensitive material has high sensitivity to incident energy, high impedance or resistance when irradiated by no incident energy, and highdielectric strength.

Generally, however, the above requirements are not necessarily fulfilled by the materials commonly used in various conventional energy detectors. That is, a material which satisfies one or two of the above requirements does not fulfill the remaining requirements. Therefore, it is impossible to obtain a satisfactory image-converting device with the above-described conventional constitution.

An object of this invention is to provide a novel solid-state image-converting device which solves the above-mentioned difficulty.

According to this invention, the energy-sensitive layer consists of two layers, one being an energy-sensitive layer which functions as an energy sensor and the other being a photoconductive layer which has high dark resistance and high dielectric strength. And, the opaque layer in the conventional device is replaced with a semitransparent layer so as to transmit the light fed back from the electroluminescent layer. Further, the semitransparent layer may be removed or replaced with a transparent layer, if the materials of the photoconductive layer and the electroluminescent layer'are selected so that the sensitive range of the former is appropriately related to the luminescent range of the latter. With such a constitution, moreover, the device may have image storage function effected by full feedback of the luminescent light.

l-lereunder, embodiments of this invention will be described, which are intended to receive X-rays as incident energy.

The device shown in FIG. 2 comprises a support plate 6 of transparent glass coated with a light-pervious electrode 5, for example, of tin oxide, an electroluminescent layer 4 of the order of 50 microns in thickness which consists of powder of electroluminescent fluorescent ZnS spread over said electrode 5 with the aid of a binder such as epoxy resin, a light feedback control layer 31 of the order of 10 to 20vmicrons in thickness applied on said electroluminescent layer 4, which is a mixture of powder of BaTiO or the like and a binder similar to the abovementioned one, and a composite photoconductive layer 20 stacked on said feedback control layer 31, said composite layer 20 consisting of a first photoconductive layer 21 provided with a gapped electrode 10 thereon and a second photoconductive layer 22, a power source 7 being connected between saidgapped electrode 10 and said light-pervious electrode 5 to apply the operating voltage The incident image L is an X-ray image in this case, and the first photoconductive layer 21 serves as energy-sensitive layer.

The first photoconductive layer 21 which serves as an X-ray sensor is made of a thin film of a material such as CdHgTe, CdSe, PbS or PbSe or a mixture of powder of such material and binding material such as epoxy resin, the thickness of the layer being of the order of 50 to microns. The above materials have been chosen because of high sensitivity and quick response to an incident image. On this first photoconductive layer 21 are provided an electrode in the form of a parallel grid which is made of fine metal wires, for example, tungsten wires of about 10 to 50 microns in diameter disposed with a pitch of approximately 200 to 600 microns, or in the form of a net of 30 to l50 mesh woven with similar metal wires, or in other appropriate forms.

The second photoconductive layer 22 is made of a material which has high dark resistivity and high dielectric strength so as to insure high luminescent output and the operation with high sensitivity, and which appropriately responds to the light fed back from the electroluminescent layer. When it is required to display faithfully, a halftone in response to incident rays, the effective amount of the luminescent feedback must be limited by lessening overlap of the spectral response curves of the photoconductivity of the second photoconductive layer 22 and the luminescent energy from the electroluminescent layer 4 in order to suppress the photoelectric bistable characteristics due to the luminescent feedback from the layer 4 to the layer 22. in this embodiment, the second photoconductive layer 22 consists of photoconductive powder of CdS-CdSe (solid solution of CdS and CdSe) type mixed with a binder such as epoxy resin and applied in a layer of 200 to 400 microns in thickness which is thicker than the layer 21. In this case, the electroluminescent layer 4 is made of a material of green-luminescent ZnS type which gives an intense luminescent output but is not very much responded to by the above-mentioned photoconductive material, in order to control the internal luminescent feedback effect and the spectral response characteristics.

The power source 7 which supplies the operating voltage V is connected between the

electrodes

5 and 10. The incident rays l. to the first photoconductive layer 21 changes the impedance thereof, thereby causing the electroluminescent layer 4 to luminesce. The second photoconductive layer 22 which has high dark resistance and high dielectric strength, decreases its electric impedance on receipt of the light fed back from the electroluminescent layer 4, and the electric current from the power source 7 increases accordingly. As a result, the electroluminescent layer 4 is further excited and an amplified and converted image L is obtained as output. As the increased dielectric strength between both electrodes due to the introduction of the layer 22 allows application of an extremely high voltage, a very luminous output image L, can be obtained. That is, when the first photoconductive layer 21 which has high sensitivity and quick response to the incident energy receives an incident image that is an X-ray image L the lateral resistance R, across the gaps of the gapped electrode 10 effectively decreases with quick response and high sensitivity. Accordingly, a current flows under the operating voltage V from the power source 7 to cause the electroluminescent layer 4 to luminesce, which in turn excites the second photoconductive layer 22 through the semitransparent layer 31. The excited second layer 22 decreases its resistance and thereby further excites the electroluminescent layer 4. Thus, an amplified and converted output image L is obtained from the layer 4.

The luminescent feedback control layer 31 which is essentially made of powder of ferroelectric material such as BaTiO and whose perviousness to light can be predetermined, controls the amount of the light fed back from the layer 4 to the layer 22 as required, and concurrently, increases the intensity of the output light with its good reflecting ability. Further, the high dielectric constant of said material of the layer 31 serves for preventing a substantial drop of the AC voltage, and the high dielectric strength of the layer 31 is very effective to prevent a dielectric breakdown of the layers between the

electrodes

5 and 10.

FIG. 3 shows another embodiment of this invention, which comprises a transparent support plate 6 coated with a lightpervious electrode 5, a first electroluminescent layer 42 provided on said electrode 5 for producing the output light, an opaque layer 31 provided on the layer 42 for intercepting the light coming from said layer 42, a second electroluminescent layer 41 provided on the opaque layer 31 for effecting luminescent feedback, and a composite photoconductive layer 20 provided on said layer 41, said composite layer consisting of a first photoconductive layer 21 provided with a gapped electrode thereon and serving as a sensor to the incident rays and a second photoconductive layer 22, and a power source 7 for supplying the operating voltage V being connected between the gapped electrode 10 and the light-pervious electrode 5.

1f the incident image is an X-ray image for example, the first photoconductive layer 21 which is to serve as a sensor is made of a photoconductive material which quickly responds to the incident X-ray image, and a gapped electrode, for example, in the form of a parallel grid of thin metal wires is provided on the layer 21.

The second photoconductive layer 22 is formed of a photoconductive material which is sensitive to the light fed back from the electroluminescent layer 41 and has high dark resistivity and high dielectric strength as mentioned previously. Further, the material for the second photoconductive layer 22 is selected so as to have a sensitive range of wavelength substantially overlapping the operating range of the electroluminescent material constituting the layer 41. Thereby, the storage operation with the layer 22 for the incident image is made possible by utilizing the photoelectric bistable characteristics due to the internal luminescent feedback. In this case, an orange electro-luminescent material of ZnCdS type is used for the layer 41, and a photoconductive material of CdS type is used for the layer 22. A power source 7 for supplying the operating voltage V is connected between the

electrodes

5 and 10. With this constitution, if the device receives the incident energy, that is, X-ray image L,, the first photoconductive layer 21 which has high sensitivity and quick response characteristics effectively decreases its lateral resistance R, across the gaps of the gapped electrode 10 with the fast response speed and the high sensitivity. The decrease of the resistance R, causes an electric current flowing under the operating voltage V of the power source 7, which makes the electroluminescent layers 41 and 42 to luminesce. The light from the layer 41 excites the second photoconductive layer 22 to decrease the electric resistance thereof, which in turn causes further electric current to flow through the electroluminescent layer 42, thereby causing the layer 42 to luminesce further intensely. In this connection, portions of the layer 41 which are affected by relatively weak incident energy luminesce with accordingly low intensity, feeding back relatively little light to the layer 22. Therefore, if the incident image is removed, said portions restore the initial dark state. However, portions which are influenced by relatively intense incident energy may feed back such a large amount of light to the layer 22 that the feedback and amplification system comprising the layers 41 and 22 is caused to oscillate, and the luminescent output from the corresponding portions of the electroluminescent layer 42 maintains the intense and stable luminescent state. In this state, even if the incident energy is removed, the device cannot restore the initial dark state any more. That is, the device can be used for memory display by setting the incident image L, at a bistable luminescent level.

That is, according to this invention, even the slightest variation in the luminescence of the electroluminescent layer due to variation of resistance or impedance of the sensor such as the photoconductive layer, will be amplified and displayed through an internal feedback and amplification system consisting of an electroluminescent element and a photoconductive element. Therefore, energy-sensitive materials such as magnetoresistance materials or piezoresistance materials, which have not been able to be used in image-converting devices because of their low resistivity, low sensitivity and low dielectric strength, can now be used to form the layer 21, thereby conversion of magnetic energy, elastic energy or other energy rays being made possible. This possibility is attained by introducing a photoconductive element which has high resistivity, high dielectric strength and high sensitivity to visible light, into the internal feedback and amplification system.

In the above embodiments, a gapped electrode has been employed in order to utilize the variation of the resistance or impedance of the sensor or layer 21 mainly in the lateral direction due to incident energy. However, a planar electrode having no gap but being pervious to incident energy can be employed to utilize the resistance change of the layer 21 in the direction of thickness. Further, the input of the device is not limited to an energy image. According to this invention, an incident energy signal L, can be converted to a luminescent output L being controlled in either monostable or bistable manner, with the arrangement shown in FIG. 4, which comprises a sensor element 21 such as a piezoresistance element or a magnetoresistance element, a photoconductive element 22 and a luminescent element 4 such as a electroluminescent element, said elements being electrically connected with a power source 7 and the photoconductive element 22 being disposed so as to receive light fed back from the luminescent element 4.

What we claim is:

1. A solid-state image-converting device comprising a luminescent element whose luminescent intensity is controlled according to the intensity of an electric field applied thereto, and a composite element including a photoconductive element which responds to light fed back from said luminescent element and an energy-sensitive element which varies the impedance thereof in response to incident energy, the variation in the luminescent intensity of said luminescent element due to the variation in the impedance of said energy-sensitive element related to the incident energy being amplified by a luminescent feedback and amplification system consisting of said luminescent element and said photoconductive element.

2. A solid-state image-converting device as defined in claim 1, wherein said energy-sensitive element is a photoconductive layer.

3. A solid-state image-converting device as defined in claim 1, wherein there is further included an intermediate layer provided between said luminescent element and said composite element, which reflects a part of the light from said luminescent element toward said luminescent element and feeds back another part of the light toward said composite element.

4. A solid-state image-converting device as defined in claim 1, wherein said photoconductive element and said luminescent element are chosen so that the sensitive range in wavelength of said photoconductive element markedly overlaps the luminescent spectrum of said luminescent element, in order to attain a bistable state of luminescence.

5. A solid-state image-converting device as defined in claim 2, wherein said photoconductive layer of said composite element is formed so as to have high sensitivity and fast response time to incident light and another photoconductive element is a photoconductive layer which has high dark resistance and high dielectric strength.

6. A solid-state image-converting device as defined in claim 1, wherein said luminescent element consists of a first electroluminescent layer for producing the luminescent output and a second electroluminescent layer for feeding back light to said composite element, with a light-intercepting layer interposed between said first and'second electroluminescent layers.

p' @1 3 I I UNITED STATES PATENT OFFICE 4 CERTIFICATE GE CORRECTIGN Patent No. 3,648,052 Dated March 7, 1972 Inventor(s) 'TADAO NAKAMURA et all It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the title page add the following:

[30] Foreign Application Priority, Data Jan. 26, 1968 Japan 4731/68 Signed and sealed this 31st day of October 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSGHALK Attesting Officer Commissioner of Patents

Claims

6. A solid-state image-converting device as defined in claim 1, wherein said luminescent element consists of a first electroluminescent layer for producing the luminescent output and a second electroluminescent layer for feeding back light to said composite element, with a light-intercepting layer interposed between said first and second electroluminescent layers.