US2645721A

US2645721A - Image intensification apparatus

Info

Publication number: US2645721A
Application number: US271881A
Authority: US
Inventors: Ferd E Williams
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1952-02-16
Filing date: 1952-02-16
Publication date: 1953-07-14
Anticipated expiration: 1970-07-14
Also published as: GB726333A; DE930467C

Description

.lilly 14, 1953 F. E. WILLIAMS IMAGE INTENSIFICATION APPARATUS Filed Feb. 16, 1952 w Inventar- `Ferd E. Williams,

His Attor-he'g.

Patented July 14, 1953 ETD STS MAGE vINTENSIFICATION APPARATUS Ferd E. Williams, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York 16 .Cla-ims.

This invention relates to image intensification apparatus and, in particular, to tubes wherein X-ray or gamma ray images are converted into intensified light images by electron optical means'.

One of the principal uses of X-rays or gamma rays is found in the study of the internal structure of objects. In such study, the object to be examined is irradiated with X-rays and the transmitted rays are permitted to impinge upon a iiuorescent screen or X-ray sensitive phosphor screen whereby a visible light image is formed. The chief diiculty encountered in this employment of X-rays is that of obtaining a visible light image having sunicient intensity and definition to permit adequate observation of the internal structure of the object under examination.

In order to intensify `the visible light images produced by X-rays transmitted through an object, it has been proposed heretofore that the f light emitted by the fluorescent or X-ray sensitive phosphor be directed to impinge upon a photosensitive surface whereby an electron image corresponding to the light image is emitted from the photosensitive surface. The electron image is then focused by electron optical means on an electron-responsive phosphor screen to produce an intensified light image thereon. This latter image then becomes the final image which is utilized for observation purposes.

The above-described prior art apparatus, while it has produced many satisfactory results, has been found to have an inherent limitation resulting from the low quantum efciency of known photosensitive materials. Even with the mos-t efcient photosensitive materials, e. g., those of the caesium-antimony type, several light quanta from the X-ray responsive phosphor are required to Produce one emitted electron .from the photo.- sensitive material. Thus, it is seen that, .although the brightness of the final light image produced in the rabove-described apparatus may be increased to a desired level by increasing the acceleration of the electrons emitted from the -photosensitive material, the image definition is restricted by the low quantum eiciency of the photosensitive material.

It is, accordingly, a principal object of the presen-t invention to provide image intensification apparatus capable of producing exceptionally high intensiiication.

Another object of the present invention is to provide image intensication apparatus `capable of producing an exceptionally Well-defined iinal visible image.

A further object of the present invention is to provide image intensication apparatus in which no chemically active photosensitive materials are required.

It is a still further object of the present invention to provide image intensification apparatus in which the number of electrons utilized to form the final intensified light image is independent of the number of photons of vvisible light emitted from the X-ray responsive phosphor.

It is yet another object of this invention to provide image intensification apparatus in which image -de-magniiication is avoided.

According to a principal aspect of the invention, there is provided an image intensification tube which comprises an image conversion means spaced from an electron gun which directs a defocused electron beam toward the image conversion means. facing the electron gun, the image conversion means includes a photoconductor such as amorphous selenium or `cadmium sulphide, a thin transparent conductive element, and an X-ray responsive phosphor such as lzinc: sulphide activated with silver. Positioned ltraversing the defocused electron ybeam between the electron gun and the `image conversion means is a fine mesh which is coated on the side facing the image conversion means with an electron-responsive phosphor such as zinc cadmium sulphide. Electrons in the defocused electron beam pass through the voids in the mesh and impinge upon the photoconductor to impart a uniform charge to the surface thereof. X-rays transmitted from an object under observation are directed to strike the X-ray responsive phosphor whereby a visible light image corresponding to the X-ray image is emitted. The light from this image irradia-tes portions of the photoconductor, and such irradiated portions are discharged through the transparent conductor element. Thus, a charge pattern reversely corresponding to the X-ray image is produced on the surface of the photoconductor facing the electron gun. vElectrons arriving thereafter from the electron gun are repelled from the charged portions of the photoconductor and their ,directions of travel reversed. The reversely traveling electrons impinge upon the electron-responsive phosphor to produce a desired intensied final light image having high definition and brightness.

The features ofthe invention desired t0 be protected herein are pointed out with particularity in the `appended claims. vThe invention itself, together with further objects and advantages In progressive order from the side' thereof, may best be understood by reference to the following description, taken in connection with the accompanying drawings, in which Fig. 1 is a schematic sectionalized View of one embodiment of the invention; Fig. 2 is a magnified fragmentary section representation of the image conversion means of Fig. l; Fig. 3 is a greatly enlarged perspective view of a fragment of the mesh of Fig. l; and Fig. 4 is a schematic sectionalized view of another embodiment of the invention.

Referring particularly now to Fig. 1, there is shown in schematic fashion image intensification apparatus comprising an evacuable tube I having an envelope 2 within which. are disposed an image conversion means 3, an electron permeable conductive mesh 4 and an electron gun 5. Image conversion means 3 may be conveniently supported adjacent the end of envelope 2 by a mechanical support (not shown), or attached to the inner surface of tube envelope 2 as shown by actual preparation on the inner surface of envelope 2 or by any suitable means such as an adhesive. Image conversion means t is adapted to receive X-rays which have been transmitted through an object under examination and to form on its innermost exposed surface a charge pattern or image which reversely conforms to the image created by the incident X-radiation.

The components of image conversion means 3 are `more clearly illustrated in the magnified fragmentary view of Fig. 2. The component shown contiguous with the inner surface of tube envelope 2 is an X-ray responsive phosphor or fluorescent material 6 which is capable of emitting light photons in response to X-radiation incident thereupon. Thus when X-ray responsive phosphor 6 is irradiated through tube envelope 2 with a de- -sired pattern of X-rays, it will uoresce and form a light image corresponding to the X-ray pattern. X-ray responsive phosphors suitable for such application include zinc sulphide activated with silver, zinc cadmium sulphide Vactivated with silver, calcium tungstate and magnesium tungstate. As illustrated, the X-ray responsive phosphor 6 should be prepared so as to be of quite large particle size, e. g. about microns diameter, and should be deposited with suitable binders such as dilute potassium silicate in a relatively thick layer, e. g. about 100 milligrams per square centimeter in thickness. To avoid feedback excitation, it is preferable that the X-ray responsive phosphor have a predetermined emission spectrum as will be more fully explained hereinafter. Suitable methods of preparing X-ray sensitive phosphors are described in the text entitled .Luminescence of Solids, by H. W. Leverenz, published by John Wiley and Sons in 1950. In this connection, specific attention is directed to chapter 3.

Adjacent the X-ray responsive phosphor 6 is a thin transparent conductive layer or element 'I' which has a thickness Preferably of the order about 0.01 millimeter to 1 millimeter depending upon its area and the desired resolution of the image conversion means. A suitable transparent conductor of this nature is a conductive glass manufactured by the Pittsburgh Plate Glass Company under the trade-name N esa. The transparent element may also consist of ordinary glass, e. g., borosilicate glass, which has been treated in a well known manner with stannic chloride to form a conductive surface of stannic oxide. Upon the inner surface of transparent conductive element 'I is deposited a photoconductor 8 which has the indicated by. dotted lines 22.

property of being rendered conductive through portions of its cross-section which have been irradiated with visible light. Thus, when the interior surface 9 of photoconductor 8 is sprayed with electrical charge carrying particles such as electrons, the charge upon any portion of the surface 9 may be dissipated by irradiating with visible light a portion of surface I which is opposite the selected portionrof surface 9. The charges so dissipated are then conducted away through transparent conductive layer I. Photoconductor B is preferably deposited in a layer of from about 1 to 10 microns in thickness and may consist of such materials as cadmium sulfide, zinc sulfide, and amorphous or non-crystallized selenium. Suitable methods for preparing and depositing photoconductor 8 are described in the text entitled Photoelectricity, published by John YWiley and Sons in 1949 by V. K. Zworykin and E. G. Ramberg. Particular attention is directed to chapter 10 wherein appropriate descriptive material may be found.

Electrons for collection upon the surface 9 of photoconductor 8 may be generated by electron gun 5 which comprises a thermionic cathode, shown in the form of a filament II, an apertured focusingcup I2 and a cylindrical anode I3, all of which are supported in the manner illustrated by conductive leads introduced into envelope 2 through a suitable seal I3. Filament II may be energized by a source of direct current conventionally represented by a battery I4, and focusing cup I2 may be maintained at a positive potential of about 10 volts with respect to lament II by means of a battery I5. Anode I3, which includes a washer I6 for the purpose of providing an aperture I'I, may be maintained at a positive potential with respect' to focusing cup I2 and filament II by means of a source of direct voltage conventionally represented by battery I8, one side of which is connected to ground as indicated. The voltage of source I8 should be in the neighborhood of a few hundred volts.

In order that desired image intensification may be obtained according to the invention, the electrons emanating from electron gun 5 must be accelerated to a relatively high velocity. This is accomplished by depositing a conductive film I9 upon a portion of the inner periphery of tube envelope 2 and maintaining this film at a high potential with respect to electron gun 5. High voltage may be imparted to conductive nlm I9 through a ring-sealed conductive washer 29 which makes contact with conductive nlm I9 as illustrated. rThe high voltage, which may be of the order of 10 kilovolts, may be supplied to washer 2&1 through a conductor 2| connected to a suitable source of direct voltage which is here designated B+. Conductive film I9 may consist of graphite or silver which has been applied in well known ways. In addition to serving as a second anode for accelerating the electrons from gun 5 to a high velocity, conductive film I9 provides an electron lens effect to assure uniform crosssectional current density in the electron beam Thus, conductive film I9 facilitates the production of a uniform cross-sectional current density beam in which the electron trajectories are approximately parallel to. an optical axis 23 as is illustrated by the portions of dotted lines 22 extending to the right of washer 20.

To form a nal intensified visible image, an electron-responsive phosphor 24 is deposited upon theside of' mesh d opposite electron gun 5 as illustrated in the greatly enlarged perspective View of Fig. 3. Mesh l!` is supported about its periphery by washer and may be attached thereto by any suitable means such as silver solder. As stated hereinbefore, mesh 4 should be quite fine and may consist of a nickel mesh having 400 lines per inch with 50% voids. Such a mesh may be obtained from the Buckbee-Mears Company of St. Paul, Minnesota. The electronresponsive phosphor 24 may be deposited upon mesh 4 by any of the well known methods of spraying, vliquid settling, or flotation. A description of such methodsmay be found in the abovementioned text entitled Luminescence of Solids. Phosphor 24 may also be deposited by chemical deposition as described in an article by F. J. Studer, D. A. Cusano,.and A. H. Young, appearing in the Journal of the Optical Society of America, Vol. 41, p. 559 (1951). Phosphor 24 preferably consists of zinc cadmium sulphide, or a mixture of zinc sulphide and zinc cadmium sulphide both of which have been activated with silver. Other suitable materials known to those familiar with this art may be employed With eicacy. p

In the operation of the apparatus of the invention, the accelerated electrons generated by electron gun 5 form a crossover as indicated at and provide therebeyond defocused electron beam 22. The electrons are further accelerated and constrained into approximately parallel trajectories with respect to optical axis 23 by the potential applied to conductive nlm Iii whereby an electron beam having substantially uniform cross-sectional current density is produced. A substantial proportion of the accelerated electrons pass through the voids in mesh li and proceed to impinge upon the surface 9 of photoconductor 8. In a matter of milliseconds or less, however, surface 9 accumulatesenough charge so that electrons arriving at a later time are repelled and caused to return and impinge upon electron-responsive phosphor 24. To assurersufcient collection of charge by surface 9 of photoconductor 8 and also to provide a path for the dissipation of charges from photoconductor 8, a circuit comprising a resistor 25 and a source of direct Voltage 21 may be connected to the peripheries of photoconductor 8 and conductive layer l through a lead 28 suitably sealed into tube envelope v2. It is pertinent to observe here that the polarity of battery 21 need not always be positive with respect to focusing cup I2. In practice, it has been found preferable to adjust the polarity of battery 21 to produce the best nal light image upon phosphor 24, and the voltage required for this purpose has been found to Vary between about 1*- 25 volts.

After the surface 9 of photoconductor 8 has been charged uniformly by the collection of electrons from beam 22, the irradiation of X-ray responsive phosphor 6 with an X-ray pattern produces a corresponding light image from which photoconductor 8 is irradiated to dissipate charge from corresponding portions of surface 9 through conductive layer i. X-ray excited phosphor 5 continues to be incident upon portions of photoconductor 8, electrons in beam 22 which arrive at these portions will be collected and dissipated through conductive film 1. Electrons from beam 22 which. arrive at non-irradiated portions of photoconductor 8 will continue to be reversed in direction by the charge remaining upon such non-irradiated portions and caused to impinge upon electron- As long as light from theY responsive phosphorV 24. In this manner a reverse light image of the object under examination is formed upon electron-responsive phosphor 24. This final intensified light image is viewed through a .suitable optical system 29 which may include a. condensing lens 28 and a viewing screen 29 positioned exteriorly of envelope 2.

It will not be understood that the final intensiiied image formed upon electron-responsive phosphor 24 has very high definition. Since the number of electrons generated by electron gun 5 may be increased to any desired amount, an electron beam of very high current density may be produced. When the current density of the electron beam is increased, the number of electrons striking electron-responsive phosphor` 24 to form the final visible image is correspondingly increased and hence the definition of the final image may be made very high. Furthermore, it is apparent that the apparatus of Fig. 1 does not utilize any chemically active materials; hence the difficulties` of y'evacuating tube. I are greatly reduced.

To avoid the possibility that light from the electron-responsive phosphor 24 may irradiate to some extent` the photoconductor and result in a deleterious feedback effect, the phosphors and4 the photoconductor should be selected to have particular emission spectra and a photosensitivity spectrum, respectively. More particularly, the emission spectrum of electron-responsive phosphor 24 shouldy be selected, in conjunction with the photosensitivity spectrum of photoconductor 8, such that light from electron-responsive phosphor 24 does not modify the charge upon photoconductor 8. Similarly, the emission spectrum of the X-ray vresponsive phosphor should be selected, in conjunction with the photosensitivity spectrum of photoconductor 8, such that light from X-ray responsive phosphor 6 does modify the charge upon photoconductor 8. These purposes may be effectuated by the use of proper materials. A suitable example is the following:

(a) For the X-ray responsive phosphor, hexagonal zinc sulphide activated with silver having a peak emission at 4400 angstroms; for the photoconductor, amorphous red selenium having high sensitivity at 4400 angstroms and negligible sensitivity at 5600 angstroms; for the electronresponsive phosphor, hexagonal zinc cadmium sulphide activated with silver and having a peak emission at about 6000 angstroms. The hexagonal zinc sulphide should be -prepared to have a relatively large particle size, e. g. about 20 microns diameter, and should be deposited about 100 milligrams per sq. centimeter thick. The amorphous red selenium should be a few microns thick and is prepared by evaporation on a surface at room temperature. It is never heated, otherwise crystallization to another form results. 'Ihe particle size of the hexagonal zinc cadmium sulphide should be smaller than that of the X-ray l luminescent cell 3l attached mechanically or with an adhesive to the exterior of envelopey 2 as illustrated, or supported separate from but.

near the envelope. vPhotosensitivey material has the property of emitting electrons when irradiated with visible light while electroluminescent cell 3| rhas the property of emitting visible light when energized by an alternating current source 33. In this manner an electron beam 32 having a desired highly uniform current density is produced and accelerated by the potential upon conductive coating I9.

Photosenstitive layer or screen 30 is -preferably composed of antimony on which a caesium layer has been deposited by ashing in a vacuum and which, after such deposit, has been heated to a temperature between 150 C. and 210 C. to form a caesium antimony compound (CSSb; probably a mixture of CSzSb and CSaSb). Other suitable photosensitive surfaces may be produced by flashing caesium on arsenic or bismuth or any mixture of arsenic, antimony or bismuth; heating is also satisfactory. Surfaces :produced by ashing other metals of the alkali group, particularly rubidium, or mixtures of these metals on deposits of arsenic, antimony or bismuth or mixtures of these may also beemployed with elicacy.

,Electroluminescent cell 3l is a light-emitting device made after the manner. of a iiat plate capacitor, except that one of the plates is made of a transparent conducting material such as tin oxide and the space between the plates is occupied by a thin layer of Adielectric material in which a phosphor suchas zinc sulphide is suspended. When such a device is energized by alternating current, the phosphor emits light which is visible through the transparent conducting layer. The intensity of the light increases as the voltage and frequency of the alternating current increase. A suitable cell of this nature is disclosed in the United States patent application of Jerome S. Prener, assigned to the assignee of the present invention, led September 8, 1951, and having Serial No. 245,696.

In both the embodiments illustrated, a magnetic field generated byv a solenoidal winding disposed about the exterior of tube envelope 2 may be employed to secure desired axial focusing of the electron beam. Also, it should be denoted that for any level of incident X-ray intensity which, of course, determines the conductance of the photoconductor, the current density of the electron beam should be adjusted for maximum contrast in the final intensied lightv image.

It is to .be particularly observed that the term mesh is intended herein to include all equivalent forms of apertured elements capable of performing the lfunction set forth f in connection therewith, e. g. apertured plates, screens, sieves,

embodiments as may be within the true spirit and scope of the foregoing description. ,y

What I claim as new and desire-to secure b Letters Patent of the United `States is:

1. An image intensification tube comprising means for generating a defocused electron beamv directed along an optical axis; an image conversion means spaced from said beam generating means and traversing said optical axis including an ,X-ray responsive phosphorior receivingiXm ray images and forming corresponding light images, and a photoconductor adjacent said X- ray responsive phosphor between said electron beam generating means and said X-ray responsive phosphor for receiving electrons from said beam generating means and transforming the light images formed by said X-ray responsive phosphor into reversely corresponding charge images; yan apertured electron-permeable element positioned traversing said optical axis between said beam generating means and said photoconductor and having an electron-responsive phosphor deposited on the side thereof opposite said beam generating means whereby electrons in said electron beam may pass through said apertured electron-permeable element without exciting said electron-responsive phosphor, the charge images onsaid photoconductor being effective to reverse the direction of travel of electrons passing through said apertured electronpermeable element and approaching the portions of the charge images whereby the reversed electrons impinge upon said electron-responsive phosphor to form intensified light images reversely corresponding to the X-ray images formed on said X-ray responsive phosphor.

2. An image intensification tube comprising means for generating a defocused electron Abeam directed along an optical axis; an image conversion means spaced from said beam generating means and traversing said optical axis including an X-ray responsive phosphor for receiving X- ray images and forming light images corresponding to the X-ray images, a thin transparent conductive element adjacent said X-ray responsive phosphor between said beam generating means and said X-ray responsive phosphor, and

a photoconductor contiguous with the surface of said transparent conductive element remote from said X-ray responsive phosphor for receiving and storing electrons from said electron beam, the light images upon said X-ray responsive phosphor being effective to irradiate portions of said photoconductor and discharge electrons therefrom through said transparent conductive element to produce reverse charge images on said photoconductor; a mesh vpositioned to traverse said electron beam between said beam generating means and said image conversion means; an electron-responsive phosphor deposited on the side of said mesh remote from said beam generating means whereby electrons in said electron beam may pass from said beam generating means through the interstices of` said mesh without exciting said electron-responsiveY phosphor, the charge images on said photoconductor Ibeing effective to reverse the direction of travel of electrons passing through said mesh and appreaching the portions of the charge images whereby the reversed electrons impinge upon said electron-responsive phosphor to form intensified light images reversely corresponding to the X-ray images formed on said X-ray responsive phosphor.

3. An image intensification tube as in claim 2 in which said beam generating means comprises an electron gun including a thermionically emissive cathode land an accelerating anode.

4. An image intensification tube as in claim 2 in which said beam generating means comprises an electro-luminescent cell for generating visible light and a photoemissive layer adjacent said cell for emitting electrons along said optical axis in response to the visible light emanating from said cell.

5. An image intensification tube comprising means for generating a defocused electron beam directed along an optical axis; an image conversion means spaced from said beam generating means and traversing said optical axisincluding an X-ray responsive phosphor for receiving X-ray images and'forming light images corresponding to the .EZ-ray images, a thin transparent conductive element adjacent said X-ray responsive phosphor between said beam generating means and said X-ray responsive phosphor, and a photoconductor contiguous with the surface ofsaid transparent conductive element remote from-said X-ray responsive phosphor' for receiving and storing electrons from said electron beam, the light images upon said X-ray responsive phosphor being eiective to irradiate portions of' said photoconductor and discharge electrons therefrom through said transparent conductive element to produce reverse charge images on said photoconductor; a mesh positioned to traverse said electron beam between said beam generating means and said ima-ge conversion means; an electronresponsive phosphor deposited on the side of said mesh remote from said beam generating means whereby electrons in said electron beam may pass from said beam generating means through the interstices of said mesh Without'exciting said electron-responsive phosphor, the charge images on said photoconductorbeing eiective to reverse the direction of travel of electrons passing through said mesh and approachingl the portions of the charge images whereby the reversed electrons impinge upon said electron-responsive phosphor to form intensied light images reverselycorresponding to the X-ray images formed on said X-ray responsive phosphor, and means for observing said intensi'ed light images.

'6. An image intensification tube comprising means for generating a defocused electron beam directed along an optical axis; an image conversion means spaced from said beam generating means and traversing said optical axis including an X-ray responsive phosphor for receiving X- ray images and forming light images corresponding to the X-ray images, a thin transparent conductive element adjacent said X-ray responsive phosphor between said beam generating means and said X-ray responsive phosphor, and a photoconductor contiguous with the surface of said transparent conductive element remote from said X-ray responsive phosphor for receiving and storing electrons from said electron beam, t-he light images upon said X-ray responsive phosphor being effective to irradiate portions of said photoconductor and discharge electrons therefrom through said transparent conductive element to produce reverse charge images on said photoconductor; a mesh positioned to traverse said electron beam between said beam generating means and said image conversion means; electron-optical means adjacent said mesh for causing the electrons in said beam to follow paths approximately parallel to said optical axis; an electron-responsive phosphor deposited on the side of said mesh remote from said beam generating means whereby electrons in said electron beam may pass from said beam generating means through the interstices of said mesh without exciting said electron-responsive phosphor, the charge images on said photoconductor being effective to reverse the direction of travel of electrons passing through said mesh and approaching the portions of the charge images whereby the reversed electrons impinge upon said electronresponsive phosphor to form intensied light iii images' reversely corresponding tothe X-ray images formed on said X-ray responsive phosphor.

'7. An image intensification tube as in claim 4 in which said electron-optical means comprises a conductive coating along a portion or" the interior surface of said tube.

8. An image intensification tube as in claim 4 in which said electron-optical means comprises a conductive coating along a portion of the interior surface of said tube on the side of said mesh adjacent said beam generating means, said conductive coating being connected to said mesh.

9. Image intens ation apparatus comprising means for generating a disperse electron beam directed along an optical axis; an image conversion means spaced from said beam generating means and traversing said optical axis including an X-ray responsive phosphor for receiving -ray images and forming light images corresponding to the E-ray images, a thin transparent conductive element adjacent said X-ray responsive phosphor between said beam generating'means and said X-ray responsive phosphor, and a photoconductorcontiguous with the surface of said transparent conductive element remote from said X-ray responsive phosphorfor receiving and storing electrons froml said electron beam, the light images upon said X-ray responsive phosphor being effective to irradiate portions of said photoconductor and discharge electrons vtherefrom through said transparent conductive element to produce reverse charge imageson said photoconductor; alcircuit connected to transparent conductive element for dissipating the discharged electrons from said photoconductor; an electron-permeable mesh positioned to traverse said electron beam between said beam generating means and said image conversion means; an electron-responsive phosphor deposited on the side of said mesh remote from said beam generating means whereby electrons in said electron beam may pass from said beam generating means through the interstices of said mesh without exciting said electron-responsive phosphor, the charge images on said photoconductor being effective to reverse the direction of travel of electrons passing through said mesh and approaching the portions of the charge images whereby thereversed electrons impinge upon said electron-responsive phosphor to form intensied light images reversely corresponding to the X-ray images formed on said X-ray responsive phosphor; and means for viewing said intensified light images.

10. Image intensication apparatus comprising an evacuable envelope, means for generating a disperse electron beam within said envelope directed along an optical axis; an image conversion means within said envelope spaced from said beam generating means and traversing said optical axis including an X-ray responsive phosphor for receiving X-ray images and forming light images corresponding to the X-ray images, a thin transparent conductive element adjacent said X-ray responsive phosphor between said beam generating means and said X-ray responsive phosphor, and a photoconductor contiguous with the surface of said transparent conductive element remote from said X-ray responsive phosphor for receiving and storing electrons from said electron beam, the light images upon said X-ray responsive phosphor being effective to irradiate portions of said phctoconductor and discharge electrons therefrom through said transparent conductive element to produce reverse charge images on said photoconductor; a circuit connected through said envelope to said transparent conductive element for dissipating the discharged electrons from said photoconductor; an electron-permeable mesh positioned to traverse said electron beam between said beam generating means and said image conversion means; electron-optical means adjacent said mesh for causing the electrons in said beam to follow paths approximately parallel to said optical axis; an electron-responsive phosphor deposited on the side of said mesh remote from said beam generating means whereby electrons in said electron beam may pass from said beam generating means through the interstices of said mesh without eX- citing said electron-responsive phosphor, the charge images on said photoconductor being effective to reverse the direction of travel of electrons passing through said mesh and approaching Vthe portions of the charge images whereby the reversed electrons impinge upon said electron-responsive phosphor to form intensied light images reversely corresponding to the X-ray images formed on said X-ray responsive phosphor; and means for viewing said intensied light images from the exterior of said envelope.

11. Image intensication apparatus as in claim 10 in which said beam generating means comprises an electron gun including a thermionically emissive cathode and an accelerating anode.

12. Image intensication apparatus as in claim 10 in which said beam generating means comprises an electro-luminescent cell for generating visible light and a photo-emssive layer adjacent said cell for emitting electrons along said optical axis in response to the visible light emanating from said cell.

13. Image intensification apparatus as in claim 10 in which said electron-optical means comprises a conductive coating along a portion of the interior surface of said envelope.

14. Image intensication apparatus as in claim 10 in which said electron-optical means comprises a conductive coating along a portion of the interior surface of said envelope on the side of said mesh adjacent said beam generating means, said conductive coating being connected to said mesh and being maintained at a relatively high potential with respect to said beam generating means.

15. Image intensication apparatus as in claim 14 in which said circuit is connected through said envelope to both said transparent conductive element and said photoconductor.

16. Image intensication apparatus as in claim 14 in which said circuit is connected through said envelope to both said transparent conductive element and said photoconductor and includes a source of direct voltage for maintaining a difference of potential between the potential of origination of said electrons and the potential of said photoconductor. l

FERD E. WILLIAMS.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,203,347 Batchelor June 4, 1940 2,277,246 McGee et al Mar. 24, 1942 2,523,132 Mason et al. Sept. 19, 19150 2,577,038 Rose Dec. 4, 195,1