US2158853A - Image reproduction - Google Patents

Description

May 15, 1939 w. D. cool--IDGE 2,158,853

IMAGE REPRODUCTION Filed Oct. 50. 1937 2 Sheets-Sheet l Inventor: Wlam D .Cooiidga May 16 l939 w. p. cooLlDGE 2,158,853

IMAGE REPRODUCTION Filed Oct. 50, 1937 2 Sheets-Sheet 2 Fig. 5.

Inventor: William D-.COO|d e,

ci AMJ, by Attorney.

Patented May 1 6, 1939 UNITED STATES IMAGE REPRODUCTION William D. Coolidge, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application october so, 1937, seri-a1 No. 171,937

5 Claims.

The present invention relates lto image reproduction, and more particularly to a novel method and means for producing a clear and intense visible image by the action of ultra-visible radiations 5 such as X-radiations.

' In conventional X-radiography it is customary to expose a photographic film or plate to X- radiations transmitted through the object desired to be photographed. In diagnostic work in order l0 to produce a record of satisfactory quality, it is frequently necessary to use a relatively powerful X-ray source and to submit the patient to an ex- Dosure of considerable duration. This circumstance, while unavoidable with present day technique, is objectionable in that it may subject the patient and operator to the vdanger of X-rayburns or other physiologically harmful eifects.

It is an object of the present invention to increase the intensity of the visible image which may be obtained by the action of a given 'X-ray source, thereby to permit the use of less powerful sources in the practice of radiography. 'I'his is accomplished, for example, by the use of means whereby a primary image produced by X-rays -(by which term I intend also to indicate gammarays as generated byradium) may be converted into a secondary image of substantially greater intensity. A particular modeof application of the invention includes producingvk an electron beam which corresponds in section pattern with the primary X-ray image and speeding up the electrons of the beam sufliciently to produce an intensified secondary image on a fluorescent.

screen appropriately positioned vin the path oi the beam. In addition to 'the foregoing this inventionhas as a further object-the provision of means whereby an electron beam havinga given section pattern may be accelerated to high velocity without distortion of the pattern by random field currents and without danger of a disruptive velectrical breakdown of the vaccelerating apparatus. i

A still further object includes an apparatus arrangement and mode of procedure by which a primary image which is of relatively slight intensity' and short duration may be reproduced as a secondary image which is of greater intensity and longer duration.

The features of novelty which I desire to protect herein are pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages there- 0f, may best be understood by reference to the following specication taken in connection with the drawings in which Fig. 1 illustrates diagram- (Cl. Z50-71) matically a particular embodiment and mode of practice of the invention; Fig. 2 shows an improved form of image reproduction device comprising a particular aspect of my invention; Figs. 3 and 4 represent primary andsecondary images 5 illustrating the nature and utility of the invention, and Fig. 5 shows diagrammatically a modication of the invention.

Referring to the diagrammatic representation of Fig. 1 in which certain of the dimensions are 10 exaggerated for the purpose of clarity, I have shown a. particular form of the invention as it may be applied in the 'practice of X-radiography. To the left of the figure there is illustrated an X-ray tube comprising acathode I0 and a target 15 oranode l Iadapted to act as a source of X-rays.

These rays may be so directed as toimpinge upon an object which is desired to be radiographically examined and which in the present case is represented as the body of a human-being I4. f 20 The transmitted rayspwhich are modulated in accordancewith the internal structure vofthe object, are recorded-photographically for future examination and' diagnosis. If -the object under .consideration is relatively thick or dense, as is true 25 in the situation illustrated, a satisfactory photographic image can be produced by the technique of the prior art only by the' use of radiations so intense as to constitute at least a potential source of danger to the patient and operator. In ac- 30 cordance with the presentinvention this diilculty is overcome by placing between the camera and the object an intensifying or amplifying system adapted to reproduce a relatively weak X-ray image in such improved form that it may be eiec- 35 tively photographed and recorded.

An exemplary such system may comprise as a first element a iiuorescent screen I6 adapted to be excited to visible luminescence by X-rays (or gamma-rays) passing through the object ll. 40 Such screens are well known in the art of X- radiation and do not require to be described with particularity herein. The primary X-ray image produced on'the screen lB-is focused by an optical system (illustrated schematically as comprising a 45 pair of convex lenses 9 and 20) on the input element of an image reproduction tube 22 of the general type described by V. K. Zworykin and G. A. Morton in v01. 26 of the Journal of the Optical Society of America, pages 182 to 189 in- 50 clusive. I

The particular tube which I have illustrated comprises an evacuated enclosing envelope having a curvilinear end wall 24 provided internally with a photoelectric surface 25 formed, for ex- 55 ample, in the following manner: An extremely thin conducting layer is produced. by sputtering or otherwise on the glass of the end wall 24 and is thereafter coated with a layer of silver. The silver is oxidized and then covered with a layer of caesium or other photoelectrically active material which may be introduced into the tube by any of the appropriate methods conventionally employed in preparing photoelectric surfaces. The surface so prepared is translucent and acts as a cathode which is capable of releasing electrons in a pattern corresponding to that of a light image projected on thesensitized surface. In this connection it is possible and, in some cases, desirable to combine in a single element the fluorescent surface I5 and the photoelectric surface 25 so that the use of an interposed optical system may be omitted. Particular means by which this result may be obtained are described in an application of Irving Langmuir application, Serial No. 172,509 filed November 30, 1937, and assigned to the same assignee as the present invention.

Arranged at the other end of the tube 22 and in cooperative relation with the photoelectric surface 25, there is provided a fluorescent screen 21 formed on the inner surface of the end wall 28. rl'his surface may be composed of known materials, for example, willemite, and is adapted to be excited to visible luminescence under the action of impinging electrons.

In order to cause the luminescence of the screen 21 to correspond with the electron pattern released from the photoelectric surface 25 means are provided for electron-optically focusing such electrons to produce a corresponding image on the fluorescent surface. As illustrated this means comprises an anode or accelerating electrode 30 and cooperating intermediate electrodes adapted to form with the electrode 30 a suitable lens system. The intermediate electrodes may include, for example, a series of rings 32 closely spaced and electrically connected to one another by connections which include resistance elements (indicated in the drawings as solid members bearing the numerals 33). If desired, these intermediate electrodes may alternatively be incorporated in a single cylinder having a relatively high electrical resistance in the axial direction. With either of these arrangements, byimpressing an adjustable voltage on the order of a few hundred volts between the terminals 34 and 35, it is possible to produce a considerable variation in the electrostatic fleld in the vicinity of the cathode and thereby to vary the focal length of the lens system as a whole. By means of a lead-in conductor 36 the anode 30 may be charged to a potential on the order of several thousand volts above that of' the cathode whereby acceleration of electrons proceeding from the surface 25 will occur in the region between the cathode and anode. I f these electrons are given a suflicient velocity the secondary image produced on the screen 21 will be of substantially greater intensity than the primary image formed on the fluorescent surface I6. Consequently, a camera arranged as indicated at 38 will produce a more satisfactory record than has been possible by the methods of the prior art, or will require less intense X-radiation to produce a record of equivalent quality.

The alternative form of image reproduction tube shown in Fig. 2 makes possible a greater inI crease in image intensity than can be realized in the tube 22 previously described. In this embodiment there is provided a photoelectric surface 4l corresponding in essential particulars to the guasta similar surface 25 described in connection with Fig. 1. As in the previous arrangement the pattern of electrons released from this surfac is accelerated and focused toward a cooperating fluorescent screen 42 disposed at the opposite end of an enclosing envelope 43. In this case, however, the tube is divided into a series of successive lens systems each comprising its own accelerating and focusing means. Thus, adjacent the photoelectric surface 40 there is provided a series of focusing rings 44 and corresponding interposed resistors 45. A suitable potential gradient may be maintained between the various rings by means of a potential impressed between the leadin connections 41 and 48, while acceleration and ultimate focusing of the electron beam may be accomplished by means of a higher potential impressed on an accelerating electrode or anode 49 by means of a lead-in connection 50. By suitable adjustment of these potentials the focal length oi the lens system thus constituted may be made such as to cause an image to be produced just beyond the right hand extremity of the anode 49 where it will constitute a virtual object for a second lens system comprising rings 53 and an accelerating electrode or anode 55. It will be understood that the first of the rings 53 may be electrically connected to the anode 49, through one of a series of resistors 5I, whereas the last ring is maintained at a higher potential by means of the connection 54. The anode 55 may be held at a potential several thousand volts above that of the anode 49 by means of a lead-in conductor 56 properly connected to the potential source.

The second lens system thus constituted may in its turn be so focused that its image presents a virtual object for a third succeeding lens system comprising connections 51 and 58, rings 59 and an accelerating electrode 60 adapted to produce an image coinciding with the plane of the screen 42. It is contemplated that any number of succeeding lens stages may be used whereby the electron beam may be brought to extremely high velocity before impingement on the screen 42. At the same time, due to the use of a plurality of uniformly spaced accelerating electrodes, energized with progressively higher potentials, the potential distribution along the length of the tube may be such as substantially to eliminate the possibility of disruptive electrical breakdown of or objectionable field emission from the various electrode surfaces.

The nature of the effects to be obtained by the reproduction system of my invention are indicated generally in Figs. 3 and 4 which represent respectively a primary X-ray image and a secondary image as produced on the fluorescent screen of Fig. 1. It will be seen that whereas the primary image 64 is of relatively faint outline, the corresponding secondary image 65 is clear and of good definition. It is also possible and my invention contemplates that the lens arrangement employed may be such that not only the intensity but the dimensions of the secondary image exceed those of the primary image. This may be accomplished by utilizing a primary image which occupies only a portion of the surface of the screen 25 and choosing the spacing and potentials of the various electrodes of the lens system in such a way that a magnified image is produced on the screen 21. The mechanism by which this may be accomplished is now well understood in the art of electron optics and need not be set out with particularity herein. It will also be understood by those skilled in the art guasta that in the various electron optical focusing systems described in the foregoing the electrostatic focusing means may be replaced or augmented by magnetic means adapted to produce equivalent effects.

Referring to Fig. 5 ll have illustrated an alternative application for utilizing successive accelerations of an electron beam to obtain intensification of a primary image. As in the arrangernents previously described the amplifying sysv tem involved an X-ray source Til, an object 1l,

a fluorescent screen l2, an optical system comprising lenses 'i3 and. lt and a photoelectric surface 15. Associated with the latter, there is provided an electron-optical lens system comprising focusing rings ll and an accelerating electrode lli.

In this case, however, instead of projecting the electrons released from the surface l5 directly on a fluorescent screen they are caused to be intercepted by an intermediate electrode 'lil interposed in their path and adapted to be positively charged with respect to the cathode surface lf. This electrode 'is provided with a surface 8d adapted to emit secondary electrons in numbers greater than the number of primary electrons impinging on the surface. Such a surface may comprise, for example, a base layer of silver oxide modied by the application thereto of a thin dim of caesium in accordance with Well known practice.

ln the illustrated arrangement the surface all acts as a screen on which electrons proceeding from the surface 'l5 are focused to produce an intermediate image corresponding to the section pattern of the electron beam. The secondary electrons thereby developed form a second electron beam having a section pattern corresponding to the intermediate image and may bediverted toward a fluorescent imaging screen 8l where an intensified secondary image will be produced. 'Ihe diversion means illustrated comprises an accelerating electrode 90 adapted to be maintained at a suitable positive potential with respect to the electrode 19 and a plurality of focusing rings 9i which form in combination with the accelerating electrode an electron optical lens of the type hereinbefore described. These elementsare so related electrically and mechanically that the electron beam pattern originating at the screen surface 80 is reproduced on the screen 8l as a corresponding iiuorescent image of augmented intensity. This image may be photographically recorded by means of a camera 89.

While I have shown particular embodiments of my invention, it will be understood that many modifications may 'be made by those skilled in the art without departing from the invention and I aim, in the appended claims, to cover all such modifications as fall within the true spirit and i scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States isz' i. The method which comprises producing a primary visible image by the action of X-radiations, projecting said image on a photoelectric surface, producing an/ electrostatic field in the region of said surface thereby to create an electron beam having a section pattern corresponding to said image, focusing the beam electronoptically to cause said section pattern to be reproduced on a fluorescent screen placed in cooperative relation to said photoelectric surface and accelerating the beam sumciently to produce on said screen a secondary visible image having an intensity substantially greater than that of said primary image.

2. In the art of X-radiography, the method which comprises producing a primary. visible image by the action of Iii-radiations, projecting said image on a photoelectric surface thereby to produce an electron beam having a section pattern corresponding to said image, focusing the beam electron-optically to cause said section pattern to be reproduced on a fluorescent screen placed in cooperative relation to said photoelectric surface, accelerating the beam sufficiently to produce on said screen a secondary visible image having an intensity substantially greater than that of said primary image and photographically recording said secondary image.

3. in combination, a source of -X-rays, a screen arranged in cooperative relation with said source and capable of being excited to visible. luminescence by X/-rays emitted therefrom, a photoelectrically active surface arranged to receive light from said screen, an observing screen capable of being excited to visible luminescence by electrons and arranged to cooperatewith said active surface, and means for accelerating photoelectrons released from said surface toward said observing screen.

Li. In combination, a source of X-rays, a screen arranged in cooperative relation with said source and capable of being excited to visible luminescence by X-rays emitted therefrom,` a photoelectrically active surface arranged to receive light from said screen, a fluorescent imaging screen arranged to cooperate with said surface, means including electrostatic field-producing elements for focusing electrons released from said surface, and means also including said fieldproducing elements for accelerating such focused electrons toward said imaging screen.

5. In an image reproduction system, a source of X-rays, means including said source for producing a iirst electron beam having a section pattern corresponding to the image to be reproduced, a secondary emission surface arranged to intercept said beam, means including said surface for producing a second electron beam havving a section pattern corresponding to that of the first beam, a fluorescent imaging screen arranged to intercept the second beam, and means for accelerating said second beam toward said screen.

WILLIAM D. COOLlDGE.

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