US2929935A - Image amplifier - Google Patents

Description

Unite 1 EMAGE A rLmmn Joseph Lempert, West Elmira, N.Y., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa, a corporation of Pennsylvania Application July 23, 1954, Serial No. 445,271

2 Ciaims. (Ci. 250-213) issued September 19, 1950, for Photosensitive Apparatus.

In the attainment of brightness intensification of the above-mentioned degree without such contraction in fsize of the image on the output screen relative to that on the input screen as to make binocular viewing of the output image with a large exit pupil impracticable, I achieve brightness intensification by.utilizing the amplifying properties inherent in the phenomenon known as photoconductivity.

One object of my invention is accordingly to provide a new and improved form of image intensifier.

Another object is to provide a new and improved form of integral tube producing on an output screen a light image which is a replica of greatly enhanced brightness of an image on its input screen.

Another object is to provide a single integral tube of improved form for producing on its output screen an intensified image, with little reduction of linear dimensions, of an X-ray image of its fluorescent-input screen.

Still another object is to provide, in an integral image intensifier tube, means by which the contrast between shadowgraphs of only slightly different intensity-level may be selectively varied at will whereby the images of certain objects or organs may be brought out clearly while other image-portions are suppressed into background.

Other objects of my invention will be made apparent by reading the following description taken in connection with the attached drawings in which:

Figure l is a schematic view in section of a tube embodying the principles of my invention;

Fig. 2 is a schematic cross-section of a target screen employed in Fig. 1; and

Fig. 3 is a schematic view in section of a modified input screen which may be employed in Fig. 1.

Referring in detail to Fig. 1, a vacuum-tight enclosure 1 which may if desired be of glass, has a cathode comprising a layer 2 of aluminum or other metal having a low absorptive power for X-rays and a second layer 3 of a material such as crystalline cadmium sulphide, CdS, cadmium selenide, CdSe, or cadmium telluride, CdTe, which is temporarily rendered photo-conductive over a limited volume in which an X-ray quantum is absorbed. The free face of layer 3 is coated with a layer 4 of conductive material having a relatively high electrical resistivity, and which also is chemically inert to attack by the layer 3, and a photoelectrically emis- States Patent.

Patented Mar. 222, i959 ation incident upon it from a source 6 outside tube 1.

At the opposite end of the tube 1 is an output screen comprising a layer 11 of an electron phosphorsuch as zinc sulphide which may be coated on the glass end wall of tube 1, and which is coated in turn by a layer 12 of aluminum which may be thin enough to be transparent to electrons accelerated into impact upon it from the photoelectrically-emissive layer 5.

A source of direct current voltage 13 accelerates electrons emitted by layer 5 into impact upon layers 12 and 11 and produces a light image in layer 11 which'corresponds in space distribution with the electron image generated at photoelectrically-emissive layer 5 in a manner about to be described. A second direct current voltage source 14 is connected to impress its voltage across the photoconductive layer 3.

When an X-ray field, which is to be pictured, is incident on the screens 2 and 3, it produces a variation of electrical conductivity from point-to-point over the lat ter which may be thought of as a conductivity image of the X-ray distribution striking layer3. The radiation from source 6 is preferably of a type which produces virtually no conductivity in the photoconductive layer 3; radiation of wavelength above 6500 Angstroins is of that type in relation to cadmium sulphide, for example. It should, however, produce a' tree emission of electrons from the photoelectrically-emissive layer .5, and will do so if the latter is of cesium silver oxide or cesiated antimony.

The space within tube 1 may be provided with means to cause electrons emitted at screen 5 to 'follow paths parallel to the axis of tube 1; thismay be done by establishing a magnetic field parallel to said axis by means of windings 15.

-The radiation from source 6 tends to produce a uniform emission of electrons from the surface of screen 5:, and does so when no X-rays are incident on layers 2 and 3. These electrons are drawn away by-the electric field between screen 5 and the aluminum layer, 12 on the output screen. As long, however, as no X-rays strike the photoconductive layer 3, the high resistance of layer 4 prevents any flow radially along its surface, through the external circuit of batteries 13 and 14, to replenish the emitted electrons in the surface layer of photoemitter 5, and no continued flow of electrons into incidence with output phosphor 11 takes place.

When, however, an X-ray image strikes layers 2 and 3, the conductivity image in the latter permits electrons to flow to the surface molecules in photo-emissive layer 5 which lie directly over spots on the screen where the X-ray intensity is high, and a continuous emission of electrons ensues at such spots. Thus an electron-emission image, duplicating in space-distribution the X-ray image, exists at the surface of photoemissive layer 5. This electron image is accelerated into incidence on output layers 11 and 12 and produces a corresponding light-image there.

It will be noted that by subjecting the photoconductive layer 3 to the voltage gradient impressed between layers 2 and 4 energy from this local source is made available to release electrons from the atoms of the photoconductive material and it is found that this makes available energies of the order of thousands of times that present in an X-ray quantum which strikes the atoms. As a result the number of electrons flowing to the surface atoms of screen 5 at an element of the screen area where the X-ray density is high is of the order of at least thousands of times that which the X-ray energy alone could release. Correspondingly the number of electrons which the radiation from source 6 can release per second at such a surface element is greater in similar degree than screen 11. This gives the further advantage in an image intensifier that. extremely high voltage need not be used at source 13.

' While I have described an X-ray image intensifier to responding m2 and 4 in Fig. 1 One way of forming such a lamina would be to embed fine particles of the photoconductive substance in a thin layer of molten glass illustrate the principles of my invention, fields of other radiation such as ultra-violet, alpha and beta particles can constitute the radiation field to be imaged. In this case it may be desirable to replace the layer 2 with a compound screen in which the incident radiation generates an image field of radiant energy to which the photoconductive layer 3 is sensitive; interposing a layer of conductor transparent to said radiation between it and layer 3. The latter might then comprise any material which is rendered photoconductive by the radiant energy just mentioned. For example, if a layer 16 of zinc sulphide faced with NESA conductive glass 17 be substituted for layer 2, antimony trisulphide may be used forlayer 3. This structure is shown in Fig. 3.

Furthermore, instead of'using the electron phosphor layer 11 to receive the electron image generated at photoemissive layer 5, the latter may be focussed on an electron storage screen such as the glass screen of an image orthicon and used for television techniques and the like. The screen comprising layers 2, 3, 4 and 5 may, in fact, be considered an extremely sensitive and efiicient radiation-responsive cathode.

The above mentioned cathode is useful, not only in image intensifying. devices, but in detectors forv nuclear radiation, in which case windows transmissive ofthe particular radiation would be provided in the envelope. Photomultiplier structures could then be provided to respond to the light from output layer 11.

The resolution of which the photoconductive lamina is capable may be increased by employment of the structure shown in Fig. 2 in which filaments 21 of photoconductivematerial are embedded in a matrix of glass or other insulating material with their ends exposed on the two faces of the screen. This structure prevents conduction .of.the electrons laterally within the lamina 3 while maintaining full conduction between the layers on its faces coror other low-melting insulator, and grinding down its opposed surfaces sutficiently to expose the ends of the photoconductive particles after it had cooled.

While I have described a high resistance layer 4, which may be of carbon particles for example, the arrangement is operative if the-layer 4 and voltage source 14 are omitted; or if the source 14 is omitted and layer 4 left floating in potential. By making the layer 4 of a material which is opaque to the radiation from source 6, the need for having the latter of a wave length which does not afiect the photoconductive layer 3 may be avoided.

I claim as my invention:

1. An input screen for an image'amplifier comprising a thin electrically conductive layer sandwiched between a fluorescent layer responsive to the image to be amplified and aradiation sensitive layer, said electricallyconducscreen, said input screen comprising a light sensitive layer responsive to radiation of an image to be amplified, said layer comprised of a plurality of filaments of photoconductive material embedded in a matrix of insulating material, said light sensitive layer sandwiched between a layer of photoemissive material and an electrically conductive layer.

References Cited in the file of this patent UNITED STATES PATENTS 2,523,132 Mason et al. Sept. 19, 1950 2,572,494 Krieger et al. Oct. 23, 1951 2,650,310 White Aug. 25, 1953 2,683,832 Edwards et al. July 13, 1954 2,699,512 Sheldon Jan. 11, 1955 2,747,131 Sheldon May 22, 1956 2,747,132 Sheldon May 22, 1956

Claims (1)

1. AN INPUT SCREEN FOR AN IMAGE AMPLIFIER COMPRISING A THIN ELECTRICALLY CONDUCTIVE LAYER SANDWICHED BETWEEN A FLUORESCENT LAYER RESPONSIVE TO THE IMAGE TO BE AMPLIFIED AND A RADIATION SENSITIVE LAYER, SAID ELECTRICALLY CONDUCTIVE LAYER SUBSTANTIALLY TRANSPARENT TO RADIATION FROM SAID FLUORESCENT LAYER, SAID RADIATION SENSITIVE LAYER COMPRISED OF A PLURALITY OF FILAMENTS OF PHOTOCONDUCTIVE MATERIAL EMBEDDED IN A MATRIX OF INSULATING MATERIAL AND RESPON-

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