US3479515A - Thermally coupled image amplifier using internal feedback - Google Patents

Description

Nov. 18, 1969 B. B. SNN/m 3,479,515

THERMALLY COUPLED IMAGE AMPLIFIER USING INTERNAL FEEDBACK F'iled March 25, 1965 IN VEN TOR. 5in/.www 5ml/ar United States Patent O U.S. Cl. Z50-213 9 Claims ABSTRACT OF THE DISCLOSURE A method and device in which a photoconductor is used to convert an electromagnetic-radiation image (e.g., in the visible, or near-visible spectrum) to patterns of electric current, the heat generated by the current pattern thereafter being used to quench or stimulate the fluoresence of an externally-excited phosphor to reform the original image in fluorescent light. Increased contrast is obtained in the iiuorescent image by means of an internally positioned electroluminescent material which respods to the electric current to feed-back additional luminescence to the photoconductor, providing a further increase in current and, thereby providing an intensification of the image-quenching (or -intensifying) heat patterns.

This invention relates to improvements in image arnpliers and more particularly to image amplifiers employing thermosensitive phosphors the stimulated light emission of which is either quenched or intensied by the application of heat.

Many systems are known in the art for producing image amplification and conversion. One such system utilizes a sandwich comprising, sequentially, a transparent electrode layer, a photoconductive layer, an opaque electrically conducting layer, an electroluminescent layer and a second transparent electrode layer, said electrodes being connected to a source of electrical potential. It should be noted here that it is a characteristic of photoconductive materials that their electrical resistance decreases with exposure to electromagnetic radiation thus permitting increased current iiow, the magnitude of said current being directly proportional to the intensity of the incident radiation. It should also be noted that it is a characteristic of electroluminescent materials that their luminescence increases with increases in the density of electrical current flowing through them. It follows, therefore, that when a radiation image impinges upon the surface of the photoconductive layer of the image amplifier being described, with a voltage applied across the transparent electrodes, current liows through the irradiated portions of the photoconductive layer an-d through the electroluminescent layer to produce a replica of the original light image in the electroluminescent layer. That portion of the current which is produced by the incident light is commonly referred to as photocurrent.

Another system of image amplification known in the art utilizes the heat produced by the ow of electrical current through a photoconductive layer and an excited thermosensitive phosphor layer to modulate the light emission or fluorescence of the excited thermosensitive phosphor layer. The light emission of the phosphor layer may increase or decrease with increases in temperature depending upon the composition of the particular phosphor employed in the layer. This system of image amplication employs a sandwich comprising, sequentially, a transparent electrode layer, a photoconductive layer, a thermosensitive phosphor layer and a second transparent electrode layer, said electrode layers ybeing connected to a source of electrical potential and the phosphor layer being excited by an externally applied source of stimulating 3,479,515 Patented Nov. 18, 1969 ICC radiation, e.g., an ultraviolet lamp. When an image of electromagnetic radiation impinges upon the surface of the photoconductive layer of such a system current ows through the irradiated portions of the photoconductive layer and through the phosphor layer to produce a heat pattern in the phosphor layer of a magnitude and distribution `dependent upon the intensity and distribution of the incident radiation and the product of the electrical resistance and the square of the current iiowing through the photoconductor and phosphor layers. This heat pattern produces a light pattern in the phosphor layer which may be either a positive or a negative replica of the original light image depending upon whether the uoresence of the phosphor layer is quenched or stimulated by the local increases in temperature produced by the heat pattern.

An examination of the image amplifiers described above, one employing an electroluminescent layer in combinatioh with a photocon-ductor layer, and the other ernploying a thermosensitive phosphor in combination with a photoconductor layer, reveals that an improved image amplifier might be obtained by combining the principles of both image amplifiers into a single amplier, that is, by utilizing the current to stimulate the electroluminescent layer and the heat generated by such current flow to modulate the uorescence of the excited thermosensitive phosphor layer, thereby producing both an electroluminescent and a phosphorescent replica of the original incident light image. If these principles are combined and the individual layers arranged satisfactorily, the electroluminescent image generate-d by the photocurrent will .stimulate the photoconductive layer to generate additional photocurrent which will produce more electroluminescence, etc., so that as a consequence of this optical feedback a small amount of incident radiation will produce a relatively large amount of photocurrent and therefore a relatively large amount of heat to modulate the uorescence of the excited thermosensitive phosphor layer.

Such an improved image amplilier is contemplated by the present invention which is a preferred species of the invention of co-pending application Ser. No. 442,696 of Nelson R. Nail. According to the latter invention, current is passed through and heats an imagewise radiated photoconductor which in turn imagewise heats a thin heat conductive electrode which conducts the heat to a thermosensitive phosphor coated on the outside of the thin electrode, The phosphor is preferably one which iiuoresces under actinic radiation and whose fluoroescence is thermally quenched, 'but it may be one which is preexcited by actinic radiation, non-fluorescent, and whose phosphorescence is stimulated by heat. According to the present invention, an electroluminescent layer is included between the photoconduetor and the heat conductive electrode. The imagewise current flow excites the electroluminescent layer, but the emitted light is not seen as in the electroluminescent system described hereinabove. Instead, the light from the electroluminescent layer is fed back to further illuminate the photoconductor thus producing greater current liow. Thus, it acts as an intensifying screen excited by the current rather than by the original radiation. One might expect the electroluminescent feedback to spread the light and reduce resolution in the image. One might also expect the extra layer to contribute to the spread of the heat in the two layers between the electrodes, which in turn would be added to the spread of the heat in the electrode itself all of which would reduce resolution of the image in the outside thermosensitive phosphor layer. One surprising result is that the `deterioration of resolution is not great and the increase in contrast is well worth any such loss that in incurred.

When the thin electrode is made transparent and the thermosensitive layer is a pre-excited thermosti'mulated one, the electroluminescence adds to and reinforces the thermoluminescence. However, stimulated phosphorescence is generally less contrasty than quenched fiuorescence, and transparent electrodes have higher electrical resistance which cuts down on current flow and hence on the contrast of the heat image. Thus the opaque electrode embodiments are much preferred. In the preferred arrangements, therefore, the electroluminescent image and the heat image are positive relative to the original radiin contrast is well worth any such loss that is incurred. image is negative.

It is thus an object of this invention to provide an improved image amplifier which is highly efficient and which produces substantial image amplification.

It is a further object of this invention to provide an improved image amplification system which gives negative or positive images with respect to the applied image.

It is yet another object of this invention to provide an improved image amplifier which utilizes electroluminescent image feedback to produce greater image amplification.

It is still another object of this invention to provide an improved image amplifier which can operate at very high output intensity levels.

These and other objects will be evident from the following description and the accompanying drawing which is a schematic view of the layer arrangement of a .preferred embodiment of the image amplifier of the present invention.

Referring to the accompanying drawing layer 11 comprises a transparent electrode, layer 12 comprises a photoconductive layer, layer 13 comprises an electroluminescent layer, layer 14 comprises an opaque layer having a high degree of heat and electrical conductivity and layer 15 is an excited thermosensitive phosphor layer which may optionally be covered with a protective transparent layer. For optimum efficiency of the system it is imperative that the sandwich comprising the layers 11 through 15 be made to have as low a thermal capacity as is consistent with the optimum efficiency of the individual layers.

In the operation of this embodiment of the image amplifier of the present invention an alternating electrical potential is applied between electrodes 11 and 14 and the phosphor layer 15 is excited by an external source of actinic radiation. When an image comprising a pattern of electromagnetic radiation is cast upon the photoconductive layer 12, locally decreasing its electrical resistance, current flows through the irradiated portion of the layer generating a heat pattern in the layer in accordance with the energy equation where P is the power dissipated, I is electrical current and R is the electrical resistance f layer 12. The heat pattern generated in the exposed areas of the photoconductive layer is then transmitted through the thin opaque conductive layer 14, which has little lateral thermal conductivity due to the temperature gradients of the system, to the excited thermosensitive phosphor layer to either quench its fluorescence or intensify it depending upon the characteristics of the phosphor employed. Thus, either a negative or a positive amplifier replica of the original image may be produced in accordance with the type of fheremosensitive phosphor employed; a phosphor the fluorescence of which is quenched on heating affording a negative replica of the original image and a phosphor the uorescence of which is stimulated by heat affording a positive replica of the original image.

In a preferred embodiment of the present invention, a layer of photoconductive copper-doped cadmium sulfide in an epoxy resin was coated at a thickness of about 0.007 inch on an electrode comprising stannic oxide strips of about 0.001 inch thickness, 0.010 inch width, and with a center-to-center distance of about 0.025 inch on glass.

The cadmium sulfide photoconductive layer was overcoated with an 0.001 inch layer of an electroluminescent material such as manganese-doped zinc sulfide in a silicone alkyd resin. A` silver paste was then applied in a thin layer over the electroluminescent layer. A thermosensitive phosphor coating comprising about 49% of zinc sulfide, 49% of cadmium sulfide, 2% of sodium chloride, 400 parts per ,million of silver and 2 parts per million of nickel in a non-fluorescent binder was then applied over the silver paste. The phosphor layer of the resulting sandwich was then irradiated with ultraviolet light t0 produce a uniform glow of fluorescent light which was reduced slightly in intensity on the application of volts AC at a frequency of from 50 cycles to l0 kilocycles across the silver paste and stannic oxide glass electrodes (due to the small amount of dark current flowing in the photoconductive layer). An image of white light was then applied to the photoconductive cadmium sulfide layer through the stannic oxide glass electrode to produce in the thermosensitive phosphor layer an image which was negative with respect to the applied image, the fluorescence of the phosphor layer being quenched in the areas corresponding to the illuminated areas of the photoconductive layer. Since the brightness of the fluorescent image in the thermosensitive phosphor layer was significantly greater than the brightness of the image applied to the photoconductive layer, image amplification was accomplished.

While the preferred embodiment described above produced a negative replica of the applied image in the thermosensitive phosphor layer, it is equally possible t0 produce a positive replica of the applied image by using a thermosensitive phosphor in the image amplifier which stores excitation energy and subsequently releases it in the form of visible light when the temperature of the phosphor is increased.

As was mentioned above, it is imperative in the construction of the image amplifier that the sandwich comprising layers 11 through 15 as designated on the accompanying drawing be made to have as low a thermal capacity as is consistent with the optimum efiiciency of the individual layers if maximum sensitivity and efficiency are to be obtained. For example, the low thermal capacity transparent electrode layer could comprise a thin transparent conductive coating of evaporated metal on a transparent support having a low thermal capacity such as a thin film of polyethylene terephthalate. The photoconductive layer might be a thin layer of any suitable photoconductive material such as cadmium sulfide, zinc oxide, amorphous selenium, lead sulfide, antimony sulfide, lead selenide, arsenic selenide, etc. By proper choice of the photoconductive material employed, the image amplifier may be made responsive to electromagnetic radiations other than visible radiation, e.g., long X-rays, ultraviolet rays, near infrared rays, etc. It is equally obvious that with certain wavelengths of radiation not visible to the eye which produce heat on absorption in various materials, the photoconductive layer may be replaced by a layer which absorbs such radiation.

It is to be noted that as used in the specification and claims, the term amplify and its derivatives such as amplifying, amplification and amplifier are deemed to embrace not only amplification of a particular type of electromagnetic radiation, but also the conversion of an image of one type of electromagnetic radiation to an image of another type of electromagnetic radiation. For example, on the one hand, according to the present invention, an incident image of visible radiation may he amplified to produce an image of visible radiation of greater intensity or, on the other hand, an incident infrared image, ultraviolet image, etc., may be converted to an image of visible radiation, etc.

The thermosensitive phosphors useful in the practice of this invention may be selected from proprietary products such as those sold by the United States Radium Company or they may be prepared by methods known in the art. It is obvious that, by the .proper selection of the thermosensitive phosphors, the color of the amplified image, the brightness of the image, and the heat input energy requirements may all be varied to suit a particular application of the image amplifier.

Although specific embodiments of this invention have been described and illustrated above, it is obvious to those skilled in the art that other embodiments are possible which are within the scope of the present invention as described above and in the appended claims.

What is claimed is:

1. A method of amplifying electromagnetic radiation energy comprising the steps of casing an image of such radiation energy upon means for converting incident electromagnetic radiation energy into electrical conductivity, impressing an electrical potential across said electromagnetic radiation energy converting means in order to produce an electrical current pattern within such radiation converting means corresponding to the image of electromagnetic radiation cast thereon, causing said electrical current pattern to pass through means for converting electrical energy into electromagnetic energy such that said electromagnetic energy produced by said electrical current is cast back upon said electromagnetic energy converting means, said electrical current pattern also producing a corresponding thermal image in said electromagnetic radiation converting means, and utilizing said thermal image to produce a light image in means for converting a thermal energy pattern into a light energy pattern, said thermal energy pattern converting means having no electrical current passing therethrough.

2. An electromagnetic radiation energy amplification device comprising means for converting electromagnetic radiation energy into electrical conductivity, means for converting electrical energy into electromagnetic radiation energy, means for impressing an electrical potential across said electromagnetic radiation energy converting means and said electrical energy converting means to cause an electrical current to pass through said respective means, the passage of current through said electrical energy converting means causing the production of thermal energy therein, and means, in thermal contact with said electrical energy converting means, for converting said thermal energy into light energy, said thermal energy means having no electrical current passing therethrough.

3. An electromagnetic radiation energy amplication device comprising a transparent electrode layer, a photoconductive layer, an electroluminescent layer, an opaque heat conductive electrode layer, a thermosensitive phosphor layer in thermal contact with said opaque electrode layer, a source of electrical potential connected across said electrode layers and an external source of actinic radiation to excite said thermosensitive phosphor layer, said phosphor layer being positioned relative to said opaque electrode layer so that current flowing between said electrode layers does not flow through said thermosensitive phosphor layer.

4. The electromagnetic radiation energy amplification device of claim 3 wherein said thermosensitive phosphor layer comprises a phosphor the fiuorescence of which is quenched by heat.

5. The electromagnetic radiation energy amplification device of claim 3 wherein said thermosensitive phosphor layer comprises a phosphor which stores energy which is released as light energy on heating.

6. An electromagnetic radiation energy amplification device comprising a sandwich including at least five layers positioned relative to each other in the following positions:

a transparent electrode layer as a first layer,

a photoconductive layer as a second layer,

an electroluminescent layer as a third layer,

an opaque heat-conductive electrode layer as a fourth layer, and a thermosensitive phosphor layer as a fifth layer, whereby, when an electrical potential is impressed across said electrode layers and the thermosensitive phosphor layer is excited by an external source of actinic radiation, an image of electromagnetic radiation cast upon said photoconductive layer through said transparent electrode layer produces an electrical current pattern, which passes between said electrodes but not through said phosphor layer, the density of current at any point being a function of the intensity of the radiation cast upon said photoconductive layer at that point,

said current pattern generates light in said electroluminescent layer, said light being cast back upon said photoconductive layer in accordance with said image of electromagnetic radiation to intensify said current pattern,

said current pattern also produces a thermal image corresponding to said current pattern and said radiation image, said thermal image being transferred to said thermosensitive phosphor layer to cause a light energy pattern to be emitted by said thermosensitive phosphor layer, the intensity of said emitted light being substantially greater than the intensity of the electromagnetic radiation image cast upon said photoconductive layer.

7. The electromagnetic radiation energy amplification device of claim 6 wherein said thermosensitive phosphor layer comprises a phosphor the fluorescence of which is quenched by heat.

8. The electromagnetic radiation energy amplification device of claim 6 wherein said thermosensitive phosphor layer comprises a phosphor which stores energy which is released as light energy on heating.

9. The electromagnetic radiation energy amplification device of claim 6 wherein said thermosensitive phosphor layer comprises a mixture of about 49% of zinc sulfide, 49% of cadmium sulfide, 2% of sodium chloride, 400 parts/million of silver, 2 parts/million of nickel and a non-fluorescent binder.

UzS. Cl. X.R.

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