US2927234A

US2927234A - Photoconductive image intensifier

Info

Publication number: US2927234A
Application number: US549038A
Authority: US
Inventors: Kazan Benjamin
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1955-11-25
Filing date: 1955-11-25
Publication date: 1960-03-01
Anticipated expiration: 1977-03-01

Description

March l, 1960 B, KAZAN PHo'rocoNDucTIvE IMAGE INTENSIFIER Filed Nv. 25. 1955 INVENTOR. 55mm/HN KHZHN TOR/V57 5m'. au

United States Patent O PHOTOCONDUCTIVE IMAGE INTENSIFIER 4Benjamin Kazan, Princeton, NJ., assignor to 'Radio Corporation of America, a corporation of Delaware Application November 25, 1955, Serial No; 549,038

Claims. (Cl. 313-65) This invention relates to photoconductive image intensiier devices, and particularly to Vdevices that will in- `tensfy -visible images, and will intensify and make visible infrared images or X-ray images.

There are several known types of devices that intensify images by means of photoemissive effects. As is known, currents produced by photoconductive action are rela- ,tively high as compared to currents produced by photoemissive action. Therefore, it is desirable to utilize devices having photoconductors as the photosensitive element in order that the device will respond to weak signals. iln devices prior to this invention that have used .photoconductors as the sensitive element, the information has normally been obtained by scanning the rphotoconductor with an electron beam, and obtaining the majority of the gain in amplifying systems outside of the tube.

It is therefore an object of this invention to provide an improved photoconductive intensiiier device.

It is another object of this invention to provide an improved photoconductive intensiiier device that is ,extremely sensitive and is capable of visibly reproducing weak images of the visible or invisible ranges of the spectrum.

'I'hese and other objects are accomplished in accordance with this invention by providing a novel image intensifying device which includes, within an evacuated envelope, a target electrode comprising a photoconductive layer and a phosphor layer superimposed, one on the other. A stream of electrons is directed onto the phos- .phor at such a low energy that it produces little or no light when no images are directed onto the photocon- ,ducton When an image is focused onto the photoconductive layer, the photoconductor becomes more conductive and a charge is established on the phosphor so that the landing velocity of the stream of electrons is increased, in a pattern corresponding to the image whereby the phosphor emits light in the areas of the focused image. This emitted light is visible and corresponds to the original image. The original image may he visible, infrared or X-ray, and the reproduced image vis of an intensity that is much greater than the original image.

The invention will now be described in detail in conjunction with the accompanying single sheet ofdrawings wherein:

Figure 1 .is a transverse sectional view of a photocon- .dilutive image intensifying device in accordance with this invention;

Figure 2 is :an enlarged fragmentary sectional view of .an embodiment of the target structureshown in Figure l; and,

Eigure 3 is a transverse sectional View ,of an embodiment .of .a photoconductive image intensifying device in accordance with this invention.

Referring now to Figure 1 indetail, there is shown a transverse sectional view of a photoconductive VVimage intensifying device, or tube, in Aaccordance with this invention. vThe tube '10.comprisesan evacuated envelope 11. images are directed ontothe envelope lthrough a wall FFice 15 and are viewed through a Wall '13. Arranged in :an oiaxis relationship to the envelope 11 is a neck portion 17 that encloses an electron gun 19. The electron lgun 19 may be any conventional type ,and includes a cathode 21, a control electrode 20, and one or more accelerating electrodes 22. The gun 19 is of the type that isy adapted to produce a wide angle stream of electrons 23.

Arranged within the envelope Y11, inthe path of `the stream of electrons 23, and adjacent to the image input wall I5, ofthe envelope 11, is a target electrode 2'5. The target electrode 25 includes a transparent ,support yplate '27 which may be made of a material such `as glass. 'When desired, the target 25 may be supported directly 'on the input image wall 15 .of envelope 11 andthe transparent support plateV 27- omitted. VO11 `the. electron gun side of the transparentY support plate 27 there .is providedatransparent conductive coating 34) which may l.be any .well known light transparent, electrically conductive material such as tin chloride or .tin oxide. On the surjface of the transparent conductivercoating 30 is provided a layer, of approximately l() mils thickness, of `photoconductivev material 32, such as cadmium sulphide pow- Ader. .On the exposed surface Vof photoconductive layer '32 there is provided an opaque resistive layer 34 that may be .formed of a material suchas conducting cadmium sul,- phide, or carbon particlesin anplastic binder, and may Vlbe approximately one to two mils thick. On the opaque Vresistive layer 34 there is provided a layer of phosphor material 36 that may be of .a material such aszinc oxide. Supported within the envelope 11and closely adjacent to the target 25, `is la `conducting fine mesh screen '3.8 o f anyV high transparency. The mesh screen 38 is supported over an 'open end o'f a hollow tubular electrode V39,. 'fIhe velectrode 39 .may take the form of a conductive coating applied to the inner surface of envelope 11 and screen A38 may be supported by electrodes extending through 'the walls of the envelope. The photoconductive llayer 32 is a resistive layer that has .a low conductivity inthe dark, and a high conductivity when light is focused thercon.r

Since, during operation, there is a potential drop across .the .photoconductor 32, the photoconductive layer 32 should be Vthick enough to sustain a potential drop 'inthe order of 500 volts withoutV breakdown. Due to Ythis f limitation, it .may bedesirable to .utilize aphotoconductor of the powdered form such as `cadmium selenide and cad'- miumV sulphide, either of which may be made intoa thick layer, as the photoconductive layer 32. .It should be .understood that -photoconductors that are deposited Vb y evaporating material, egg.' antimony trisulphide, may also be utilized, andthese layersmade thick enoughto adequatelysustainthe voltage drop across the photoconductor.

"When it is.desred to visiblyV reproduce `infrared images, a photoconductive material should be .usedfor -the photoconductivelayer 32 that is sensitive in the infrared range of the spectrum. One .example of such a photoconductor is'lead sulphidef- When it isjdesired to visibly reproduce .X-ray images, Ya photoconductor, such as cadmium sulphide, should be utilized whichresponds well to frequencies ,inV this range of the spectrum. i

"The purpose of the opaquelayer 34 in the target 225 .is tol prevent "light 'feedback 'from the phosphor :36"toithe photoconductive layer 32. 'In otherwords, Vit is to prevent vprevent loss o'fepicture resolution. This normally'results linthe opaquelayer "3'4 being relativelythin. The 'apague' resistive layer "34 may 4be -formedof a mosaic, 'oraptub rality of opaque conductive islands such as evaporated aluminum islands (not shown) that are spaced apart so that the lateral conductivity across the target 25 is extremely low, but the conduction through the islands, is relatively high. The opaque layer 34 may also be formed of known conductive materials such as carbon particles in a suitable binding material.

The phosphor layer 36 should be relatively conductive, or be very thin, so that the potential on the exposed surface of the phosphor layer 36 Will be substantially the same as the potential on the gun side of the photoconductive layer 32. In other words, when in the dark, the majority of the voltage drop across an elemental crosssectional unit of the target should occur across the photoconductive layer 32 rather than across the phosphor layer. One example of a highly conductive phosphor is zinc oxide.

During one form of operation, and using the potentials shown in Figure 1 as an example, a potential of 500 volts positive with respect to the grounded cathode Z1 is applied to the transparent electrode 30. With the electron beam turned off, the phosphor surface 36 acquires a potential of 500 volts by leakage to the electrode 30. When the electron beam is turned on, the phosphor surface is driven in a negative direction until the electrons in the beam are equal to the charge leakage through the target, i.e. the leakage from the scanned surface of phosphor 36 to the transparent conductor 30. By adjusting the density of the electron beam, the potential of the surface of the phosphor may be stabilized at a few volts positive, e.g. tive to ten volts, with respect to the cathode 21. At this point a current equal to the primary electron beam current will ow through the series connection of the phosphor layer, the opaque layer, and the photoconductive layer to the electrode 30. Due to the resistance of these layers, a certain voltage drop, of approximately 495 volts, is established across the target in the dark. When the phosphor is at this stabilized potential, the landing velocity of the electrons from gun 19 onto phosphor 36 is very low and no light is produced thereby when the electrons land on the phosphor 36 to replace the charge conducted through the target. Thus, as long as the photoconductive layer is maintained in the dark, its resistance remains high and the primary electron current owing through it from the gun 19 will build up a relatively large Voltage vdrop across it.

When an image is directed onto the photoconductor 32, the resistance of the photoconductor is lowered in the areas struck by light from the image. This decrease in resistance, decreases the voltage drop through the target and thus increases the potential of the phosphor with respect to the cathode 21. The electrons now land at higher velocities in these areas and produce visible images. These images vary in accordance with the input signals.

The device of Fig. 1 may also be constructed using a phosphor which is relatively insulating. If such a phosphor is used, a target as shown in Fig. 2 may be employed. This target comprises a transparent support plate 41 having a transparent conductive coating 43 on one surface thereof. On the transparent conductive coating 43 is a layer of photoconductive material 46 which in turn supports an opaque resistive layer 47. .On the opaque resistive layer 47 is a phosphor layer. 48 that comprises a mosaic of phosphor areas 48 with the resistive layer 47 exposed between the phosphor areas 48. In this arrangement, the phosphor areas 48 assume approximately the potential of the exposed opaque layer 47, which acts as a collector electrode (assuming that more secondary electrons are dislodged from the target than primary electrons land on the target). Since no secondary electrons from the phosphor areas can reach screen 38, because of the suppressing eld, the net current entering the opaque layer 47 is equal to the priymary current, and the operation will be similar to that described for the target of Figure 1. The materials for the target shown in Figure 2 may be similar to those described in connection with Figure l.

Referring now to Figure 3 there is shown a cross sectional view of an embodiment of this invention comprising an evacuated envelope 52 having an electron gun 54 in one end of the envelope for producing an electron beam 56. The electron gun 54 may be any conventional type that is adapted to be scanned over the surface of a target by any continual means such as horizontal and vertical deflection plates 53. The electron beam 56 is directed toward the target 5S that comprises a phosphor layer 60 that is supported in light exchange relationship with the balance of the target 58. The balance of the target 58 includes a transparent support plate 59, a transparent conductive coating 62, a photoconductive layer 64, an opaque resistive layer 66 and a phosphor layer 68. The materials for the layers of the target 58 may be substantially the same as those previously described with the phosphor layer 60 being of a material similar to the phosphor 36 described above. In this embodiment of the invention, the electron beam 56 is video modulated to produce a visible picture on the phosphor 60. This picture is intensified by the photoconductor 64, and the phosphor 68, similar to that described in connection with Figure 1.

In any of the embodiments of this invention, the opaque resistive layer may be omitted, and the device used to store a transverse picture for any desired length of time. The storage occurs because now the photocon'- ductor is in light exchange relationship with respect to the phosphor, which produces a regenerative feedback action that will continuously energize elemental areas of photoconductor and phosphor which have been triggered on.

The device in accordance with this invention may be scanned by an electron beam, rather than sprayed with an electron stream, when it is desired -to increase the instantaneous current density in the device.

Image intensifying devices in accordance with this invention are very sensitive, and utilize the relatively high sensitivity of photoconductive materials. Also, these devices do not have the problems of electron optically focusing imaging currents.

What is claimed is:

l. An image intensifying device, comprising an evacuated envelope, a source of electrons within said envelope, a target electrode in said envelope and in the path of said electrons, said target electrode including4 a transparent conductive electrode, a layer of photoconductive material on said conductive electrode, an opaque resistive layer on said layer of photoconductive material and a layer of phosphor material on said opaque resistive layer, said layer of phosphor material being exposed and on the surface of said target toward said source of electrons. y

2. An `image intensifying device, comprising an evacuated envelope, an electron source Within said envelope, a tar-get electrode in the path of said electrons and including a transparent conductor, a layer of photoconductive material on said transparent conductor, an oapque resistive layer on said photoconductive material, a mosaic of phosphor on said opaque resistive layer, elements of said phosphor mosaic being exposed and spaced apart on the surface of said opaque resistive layer in the path of said electrons.

3. An image intensifying device comprising an evacuated envelope, a target electrode within said envelope and including a transparent support member, a rst layer of phosphor material on one side of said support member, a transparent conductive coating on the other surface of said support member, a layer of photo-conductive material on said transparent conductive coating, an opaque resistive coating on said photoconductive material, and a second layer of phosphor material on said opaque coating, means within said envelope for producing an electron beam, means for scanning said beam over said rst layer of phosphor material, means within said envelope for producing a stream of electrons, and means for directing said stream of electrons onto said second phosphor layer.

4. An image intensifying device, comprising an evacuated envelope, a target within said envelope, said target including a rst phosphor material, a photoconductor and a second phosphor material, means Within said er1-4 velope for producing an electron beam, means for scaning, an opaque resistive coating on said photoconduc-l tive material and a second phosphor material on said resistive coating, au electron lgun Within said envelope for producing an electron beam, means VWithinsaid envelope for scanning said beam over said first phosphor material t at energies that produce light, the resistance of saidk ning said beam over said first phosphor material, a Y

source of electrons within said envelope, means for directing electrons from said source onto said second phosphor material, and said second phosphor material and said photoconductor forming an electrical series path whereby the electrons flowing through said photoconductor originate at said source.

5. An image intensifying device, comprising an evacuated envelope, a target electrode within said envelope and including a transparent support member, a irst layer of phosphor material on one side of said support member, a transparent conductive coating on the other surface of said support member, a` layer of photocon- Y ductive material on said transparent conductive Vcoatl Re. 23,802

' photoconductor being decreased by said light, an electron gun within said envelope for producing a stream of electrons, and kmeans for directing said stream of eleotronsvonto said second phosphor material.

References Cited in the le of this patent K UNITED STATES PATENTS l 1,954 iVV