Title:
SOLID-STATE IMAGE CONVERTER HAVING COMPOSITE ENERGY SENSING ELEMENT
United States Patent 3710127
Abstract:
A solid-state image converting device wherein the luminescence of a luminescent element is controlled by the variation in the impedance of an energy sensitive element with incident energy. The energy sensitive element is a composite element comprising a highly sensitive and highly responsive element and a high resistance and high breakdown voltage element, whereby a highly sensitive and highly responsive device to which a sufficient operating voltage can be applied is provided.


Inventors:
Kohashi, Tadao (Yokohama, JA)
Nakamura, Tadao (Kawasaki, JA)
Nakamura, Shigeaki (Kawasaki, JA)
Application Number:
05/135417
Publication Date:
01/09/1973
Filing Date:
04/19/1971
Assignee:
Matsushita Electric Industrial Co., Ltd. (Osaka, JA)
Primary Class:
Other Classes:
250/330, 250/484.2
International Classes:
H05B33/12; (IPC1-7): H01J31/50
Field of Search:
250/213R,83.3HP,213A
View Patent Images:
US Patent References:
Primary Examiner:
Borchelt, Archie R.
Assistant Examiner:
Grigsby T. N.
Parent Case Data:


CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of co-pending U. S. Pat. application Ser. No. 793,137 filed Jan. 22, 1969 and abandoned upon the filing of the present application.
Claims:
What is claimed is

1. A solid state image converting device comprising a pair of electrodes, a composite energy sensitive element and an electrically luminescent element, both of said elements being disposed adjacent to each other and interposed closely between said pair of electrodes, wherein one of said pair of electrodes which rests on said composite energy sensitive element is a foraminous electrode and said composite energy sensitive element is composed of a first energy sensitive element adjacent said foraminous electrode and a second energy sensitive element adjacent said electrically luminescent element, the response to said first element to incident energy being more rapid than that of said second element, and the dielectric strength and the dark impedance of said second element being higher than those of said first element.

2. A solid state image converting device according to claim 1, wherein said foraminous electrode comprises a parallel-grid electrode made of fine metal wire.

3. A solid state image converting device according to claim 1, wherein said first energy sensitive element is a photoconductive element including at least one of CdSe and CdHgTe and said second energy sensitive element is a photoconductive element including CdS or CdS-CdSe.

Description:
The present invention relates to a solid-state image converting device wherein the luminescence of a luminescent element such as an electroluminescent layer is controlled by the variation in resistance or impedance of an energy sensitive element such as a photoconductive layer with incident energy such as visible light, radiation or the like.

A conventional solid state image converting device comprises laminated layers of an incident energy image transmissive electrode, a photoconductive layer, an opaque layer for preventing optical feedback, an electroluminescent layer for producing output light images, and a light transmissive supporting plate such as a glass plate coated with a light transmissive electrode. An operating voltage is applied across the two electrodes by a power source. When an incident image falls upon the photoconductive layer, the resistance of the layer decreases. In response to the decrease in resistance of the photoconductive layer the electroluminescent layer is excited to produce an output light image. In a device operating on such principle, unless the dark resistance of the photoconductive layer is high, a current flows through the electroluminescent layer to make the layer considerably luminous even when no input image is incident upon the photoconductive layer because the impedance of the photoconductive layer is low. Thus, the contrast of an output light image corresponding to an input image is degraded preventing the obtaining of a good quality output image. Moreover, unless the breakdown voltage of the photoconductive layer is high, the operating voltage of the device cannot be made high. Consequently, a high output, high sensitive operation of the device is impossible because a high current cannot be allowed to flow through the electroluminescent layer due to the low operating voltage.

In view of the abovementioned consideration, as a photoconductive material composing the photoconductive layer in a conventional solid state image converting device, a very high dark resistance and very high breakdown voltage photoconductive material is required. To meet such requirement, materials containing cadmium sulphide have been employed as photoconductive material. On the other hand, a device operative with a very low X-ray dose and yet having a suitably rapid response characteristic is required for a medical device amplifying and converting a radiation image such as X-ray image into a visible light image.

Conventional photoconductive materials containing cadmium sulphide are not necessarily high in their sensitivity to X-rays, and moreover have the vital disadvantage that their response time is very long. These disadvantages of low sensitivity and long response time can be overcome by employing photoconductive materials having a band gap narrower than that of the photoconductive materials containing CdS, such as photoconductive materials containing CdSe or CdHgTe instead of those containing CdS. However, such photoconductive materials are low in their dark resistance and breakdown voltage. Consequently, such photoconductive materials cannot be employed in solid state image converting devices of the conventional structure.

Therefore, an object of the present invention is to provide a novel solid state image converting device which eliminates the abovementioned difficulties.

In the present invention the abovementioned difficulties are overcome by composing the photoconductive layer of a composite layer comprising a photoconductive layer having a high dark resistance and breakdown voltage and another photoconductive layer having a dark resistance and breakdown voltage lower than those of the former layer.

According to the present invention there is provided a solid state image converting device comprising a composite energy sensitive element composed of a first energy sensitive element having a high sensitivity to an input energy and a second energy sensitive element having a high dark impedance and a high breakdown voltage, an electrically luminescent element for emitting light in response to the variation in impedance of said composite energy sensitive element corresponding to the intensity of said input energy, and two electrodes for applying an electric field to said composite energy sensitive element and said electrically luminescent element.

The present invention will become more apparent from the following detailed description of the invention made with reference to the accompanying drawings, in which:

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

FIG. 2 is a schematic cross-sectional view of a solid state image converting device embodying the invention.

In FIG. 1, a conventional solid image converting device comprises an electrode 1 pervious to an image L1 of an input energy such as light rays, radiations, or the like, a photoconductive layer 2, an opaque layer 3, an electroluminescent layer 4 for producing an output light image L2, and a transparent supporter plate 6 coated with a transparent electrode 5. A power supply 7 applies an operating voltage V between the electrodes 1 and 5.

An embodiment of the solid state image converting device of the invention shown in FIG. 2 comprises a transparent glass plate 6 coated with a transparent electrode 5 made of, for example, stannic oxide, an electroluminescent layer 4 about 50 microns thick on the transparent electrode 5, the electroluminescent layer being made of ZnS phosphor powder bonded by a binder such as epoxy resin, an opaque layer 3 about 5 microns thick provided on the electroluminescent layer 4, the opaque layer 3 being made by mixing opaque powder such as carbon black in a binder similar to that used in the electroluminescent layer 4, and a composite photoconductive layer 20 disposed on the opaque layer 3, the composite photoconductive layer 20 being a lamination consisting of a first photoconductive layer 21 provided with a porous electrode 10 and a second photoconductive layer 22. A power supply 7 applies an operating voltage V across the porous electrode 10 and the transparent electrode 5. As an input image L1, an X-ray image is employed.

The first photoconductive layer 21 is a layer of a thickness of about 50 to 80 microns made of powder of a photoconductive material having low dark resistance and breakdown voltage but having a rapid response characteristic and capable of producing a large photocurrent, such as CdSe, CdHgTe, etc., bonded together by a binder such as epoxy resin. The layer 21 is provided with a foraminous electrode such as a grid electrode formed of, for example, tungsten filaments having a diameter of about 10 to 50 microns arranged at intervals of about 200 to 600 microns, or a net electrode of about 30 to 150 mesh formed by weaving metal filaments. The second photoconductive layer 22 is sensitive to the X-ray image L1 to a certain degree and has breakdown voltage and dark resistance higher than those of the first photoconductive layer 21. The second photoconductive layer 22 is made of powder of a material containing, for example, CdS or CdS-CdSe (solid solution of CdS and CdSe) with a binder such as epoxy resin, and is thicker than the layer 21, for example, about 200 to 400 microns thick. The thickness of the layers 21 and 22 can be selected depending on the dark resistance, breakdown voltage, and rate of variation in the resistance or impedance against input energy or sensitivity of each photoconductive material employed.

The operating voltage V is supplied by the power source 7 between the electrodes 5 and 10. By the provision of the second photoconductive layer 22 of high dark resistance and breakdown voltage a very high operating voltage can be applied to the electrodes in a dark and suitably weak output light L2 state compared with the case of no provision of such second photoconductive layer 22.

When input X-rays L1 are incident upon the first photoconductive layer 21, the lateral resistance Rl of the interstice portions of the layer 21 exposed to the X-rays L1 decreases effectively due to a rapid response and a high sensitivity. The decrease in the lateral resistance Rl increases the effective area of the foraminous electrode 10, and hence the space factor to the transparent plane electrode 5. With this increase in space factor the displacement current between the electrodes 10 and 5 increases. Depending on an increase in the displacement current the light output L2 of the electroluminescent layer 4 varies. As the input energy L1 is increased further, the foraminous electrode 10 becomes effectively a continuous electrode and, at the same time, the intensity of the X-rays having passed through the first photoconductive layer 21 becomes very large. Consequently, also the second photoconductive layer 22 becomes effectively sensitive to X-rays in spite of its low sensitivity. Due to the excitation of the second photoconductive layer 22 by the transmitted X-rays the resistance Rt of the layer 22 in the direction of its thickness decreases. Since the decrease in the resistance Rt is substantially equivalent to the decrease in distance between the electrodes 10 and 5, the displacement current between the electrodes 10 and 5 increases further to produce more intense output light L2.

Generally, the response time of a photoconductive material becomes shorter as the degree of its excitation caused by an input energy increases. Since the variation in the impedance of the second photoconductive layer 22 contributing to the luminescence of the electroluminescent layer 4 is effective when the input energy L1 is high, a sufficiently rapid response region becomes substantially the operating region even if a layer having a long response time at a low intensity of X-rays L1 is used as the second photoconductive layer 22. Thus, at low levels of the incident energy L1 the first photoconductive layer 21 operates, and at high levels of the input energy L1 the second photoconductive layer 22 operates. As a result the device effectively has a rapid response, a high breakdown voltage and a high dark resistance which enables the application of a high operating voltage, thus providing a very bright image with high sensitivity, high contrast, and wide brightness range.

In this embodiment the first photoconductive layer 21 is predominantly employed in its variation in the lateral resistance Rl. For this purpose the foraminous electrode 10 can be provided in such a manner that a portion thereof is exposed above the surface of the first photoconductive layer 21 and the remainder is buried therein as shown in FIG. 2, the electrode 10 is completely buried in the layer 21, is disposed between the first and second photoconductive layers 21 and 22, or is disposed on the outer surface of the first photoconductive layer 21.

In the above case the variation in the resistance of the layer 21 in the direction of thickness can additionally be utilized. Instead of utilizing the variation in the lateral resistance of the layer 21 mainly, the variation in the resistance in the direction of thickness can mainly be utilized by appropriately increasing the thickness of the layer 21 and by providing an input energy transmissive continuous electrode such as, for example, an evaporated thin metal film electrode or transparent conductive film electrode on the surface of the layer.

In the above-described embodiment, although X-rays are utilized as an input energy L1, infrared rays can also be used as the input energy L1. In this case, by employing an infrared photoconductive material, which has a low dark resistance and a low breakdown voltage and which has not been possible to be employed in solid state image converting devices, such as PbS, PbSe, CdHgTe, etc. as the first energy sensitive layer 21 and by employing CdSe which has a certain degree of sensitivity to infrared light as the second energy sensitive layer 22, an infrared light image can be converted into a visible light image.

In the above description photoconductive materials are utilized as materials of the layers 21 and 22. However, since it is sufficient for the layers 21 and 22 to have a variation in resistance or impedance in response to an input energy, piezoelectric materials, magnetoresistive materials, etc. can also be utilized as materials of the layers 21 and 22. In these cases, elastic energy, electromagnetic energy, etc. can be used as the input energy L1. Also, although an electroluminescent material was utilized as material of the luminescent layer 4 in the above embodiment, solid laser materials or other luminescent materials can be used as material of the luminescent layer 4 because the layer 4 has only to be electrically controlled in its luminescence.