US2569872A - Electron discharge tube - Google Patents

Description

Oct. 2, 1951 J. w. SKEHAN ETAL 9,87

ELECTRON DISCHARGE TUBE Filed Dec. 24, 1949 3 Sheets-Sheet 1 ATTORNEY-5 1951 J. w. SKEHAN ETAL 2,569,872

ELECTRON DISCHARGE TUBE Filed Dec. 24, 1949 3 Sheets-Sheet 2 INVENT R 9 ATTORNEYS 51 J. w. SKEHAN m 2,569 872 ELECTRON DISCHARGE TUBE Filed Dec. 24, 1949 3 Sheets-Sheet 3 INVENT f 1 -4 w. 3216 sy iutl' S Bau (CAM; 31416; W

ATTORNEYS Patented Oct. 2, 1951 ELECTRON DISCHARGE TUBE Joseph W. Skehan, Stamford, and Thomas H.

Rogers, New

Canaan, Machlett Laboratories, Incorporated, Spring- Conn., assignors to dale, Conn., a corporation of Connecticut Application December 24, 1949, Serial No. 134,926

19 Claims. (01.313-57) This invention relates to electron discharge devices of the type disclosed in Horsley Patent No. 2,482,275, issued September 20, 1949, in which electrons emitted by an annular filament are focused to cause them to pass through the area defined by the filament and to form a beam impinging upon'a target surface, and radiation produced by the target passes through the area within the filament and through a window in the tube envelope. More particularly, the invention is concerned with a novel electron discharge device involving the use of the Horsley invention, in which the electron beam is caused to scan the target surface and means are provided for converting into a useful beam the radiation produced at the target by impingement of the electron beam. The new discharge device may be employed advantageously for X-ray production or as a cathode ray tube in projection television.

A form of the new device producing X-rays may be used for radiological purposes and, with especial benefit, in fluoroscopic examinations. In such examinations as now carried on, the X- ray tube employed produces X-rays in the form of a cone of considerable size and the rays-pass simultaneously through all parts of the object under examination and fall upon a fluoroscopic screen. The fluoroscopic image appearing on the screen then results from differential absorption of the X-rays by the different parts of the object. With all parts of the object thus. simultaneously irradiated, the image on the screen lacks brightness and definition to such an extent that fluoroscopy is not satisfactory for many purposes. The lack of brightness cannot be overcome by merely increasing the intensity of the X-ray beam, because the patient's tolerance to X-ray exposure would too quickly be exceeded. The proposal has been made, in order to overcome these objections, that the X-ray beam employed in fiuoroscopy be of small size and be caused to scan the object, so that the different parts of the object will be traversed consecutively by the rays, and the X-radiation, after transmission through the object, will fall upon a block of fluorescent material, causing it give off visible and ultra-violet light modulated in intensity in accordance with the differences in absorption of the X-ray beam by difierent parts of the object.

The light would then be focused and allowed to fall upon the light-sensitive element of a photomultiplier tube, and the resultant signal would be amplified by a suitable vacuum tube amplifier and fed into a television receiving system, whose sweep-circuit would be synchronized with the scanning sequence of the X-ray'source, so that the X-ray image would be presented on a cath-. ode-ray tube screen and its brightness could be controlled independently of the intensity of the X-ray beam.

It appears that the various components necessary for the successful performance of the proposed system are available in reasonably satisfactory form, with the exception of an X-ray tube capable of producing a scanning beamof X-rays of suitably small cross-section area, adequate intensity, and suitable pattern of scanning. The present invention, accordingly, is directed to the, provision of a novel X-ray tube producing a beam which scans the object under examination and, in the new tube, the scanning effect is obtained by causing the electron beam impinging upon a target surface to scan that surface. The tube is provided with a cathode having a central opening coaxial with the target and a filament of circular form and substantial length, which encircles and is concentricwith the opening. The tube includes an X-ray transparent window coaxial with the target and the cathode opening, and focusing means, which cause the electrons emitted by the cathode to converge along curved trajectories and combine into a beam directed toward the target. The beamis acted on by deflecting means, which cause the beam to scan the target surface. X- rays emitted by the target then pass through the opening in the cathode and through the window, and a ray-proof element having a small opening in the common axis of the cathode and the target and serving as an image forming means is disposed in the path of the X-rays. Accordingly, as the electron beam scans the target, the X-ray beam passing through the small opening in the ray-proof element scans the object in the reverse sense. All parts of the object within the field of the X-ray ,beam are thus traversed consecutively by the X-rays.

A form of the new discharge device useful in projection television overcomes certain objec- 1 tions to present systems, as follows. The most satisfactory projection television systems now in use employ a Schmidt optical system, which comprises a spherical concave mirror external to and aligned with; the fluorescent screen of the tube and anaspheric corrector plate at the center of curvature of the mirror. In addition to being expensive because of the added cost of the optical elements, such a system occupies much space, since the mirror is usually at least twice the di-: ameter of the screen and the aspheric plate en:

circles the neck of the tube and has approximately the same diameter as the mirror. Thus, the complete unit has overall dimensions much greater than and varying with the size of the tube used.

In the application of the discharge device of the invention for television projection purposes, the target surface of the device is provided with a. suitable phosphor coating and the envelope of the device is provided with a window transparent to visible light and aligned axially with the target. Beyond the window and coaxial therewith is mounted a lens system for concentrating and focusing the light from the target to form an image upon a viewing screen remote from the device. Since the useful light does not pass through the target surface of the new device, the target can be made of metal and provided, if necessary, with cooling means. The electron beam used may, accordingly, be of higher intensity without doing damage to the target surface and the increased light thus obtained compensates for inefficiency of the optical features.

For a better understanding of the invention, reference may be made to the accompanying drawing, in which Fig. 1 is a view in longitudinal section through one form of X-ray tube embodying the invention; and

Fig. 2 is a plan view of part of the tube of Fig. 1;

Fig. 3 is a longitudinal section of a tube embodying the invention and suitable for use in projection television;

Fig. 4is a fragmentary longitudinal sectional view through a tube of the invention provided with alternative modulating means; and

Fig. 5 is a plan view of part of the tube shown in Fig. 4.

The tube illustrated in Fig. 1 comprises an evacuated envelope in having an enlarged section ll formed in part by a glass wall l2, on one open end of which is sealed a tubular metal element l3 terminating at its outer end in a radial flange l3a. A target anode assembly extends into the enlarged section II of the envelope through the open end of element l3, and the assembly includes a heavy block of metal l4, shown as having a concave target surface coaxial with the envelope and facing inward. The targetsurface may be formed of a sheet 15 of'tungsten embedded in the surface of block II, which may be of copper, or the target surface may be formed by a thin gold plating on the face of the block. The block l4 may be provided with embedded cooling coils I6, through which a coolant supplied through tubes l1, l8 may be circulated. The block I4 is mounted in a tubular metal member is having a radial flange Isa at its free end, the flange lying in contact with flange l3a and being secured thereto as by brazing, indicated at 20. If desired, a flat ring may be secured to the upper surface of flange IS, the ring having an inner diameter somewhat less than that of the ,tubular member l9, or the ring may be replaced by a disc 2| having openings, through which the tubes l1, l3 extend. At its inner end, the block l4 carries a projecting hollow shield 22 having a rolled edge 220.

A tubular metal member 23 is sealed in the end of the glass section l2 opposite to that in which the anode structure is mounted, and a rayproof plate 24 having a central opening is ,secured to the end of member 23. A hollow neck 25 of tapering diameter and made of ceramic or other dielectric material is secured in place by of the flange.

metaiized ceramic hard soldering processes with its large end fixed in the opening in plate 24. Within the enlarged section H of the tube, the neck has a metallic extension 25a with an outwardly rolled edge, the extension having a radial flange at its small end secured to plate 24. The small end of neck 25 is seated upon and secured to the end wall 26 of the end section 21 of the envelope by means of a. flanged collar 26a.

The wall 26 is formed with a central opening defined by an internal flange 28, which is coaxial with the neck and the target and contains a beryllium diaphragm 28a having a central opening. Flange 28, neck 25, and extension 2511 are coated with a continuous thin high resistance conductive coating such as that known commercially as Aquadag. End section 21 includes a hollow glass section 29 sealed at one end to a cylindrical flange at the outer periphery of wall 26 and, at its other end, to one end of a hollow metal member 30 having a. radial flange 3| at its other end. A hollow metal member 32 lies beyond member 30 and at one end has a radial flange 33 lying in contact with flange 3| and secured thereto by brazing. The other end of member 32 is seated against and secured to a plate 34 having a tubular extension 35 of substantially less diameter than members 30 and 32 and extending through those members to terminate close to end wall 26. The extension has a central opening provided with an internal flange 35a and a window 36 of X-ray 'transparent material, preferably beryllium, is mounted in the opening against the inner face A ray-proof lead diaphragm 31 having a central pinhole 31a is mounted in the opening against the outer face of flange 35a and a filter 38 lies against the outer face of the diaphragm 31. The filter and diaphragm are held in place by an externally threaded annular nut 39 screwed into the opening.

The portion of the extension 35 inwardly from window 36 forms a cathode generally designated 40 and, at its inner end, the cathode is provided with a flange 4| of L-section, which, with the wall of the extension, forms a circumferential channel/encircling flange 28 and spaced laterally therefrom. A filament 42 of generally circular form is mounted in the channel on supports 43' and a lead 44 is connected to one end of the filament and extends outwardly through an insulating bushing 45 through the cathode wall. The lead is connected to a conductor 46, which lies between extension 35 and members 3| and 32 and is connected to a lead 41 sealed through-an insulating bushing 48 mounted-in an opening in the wall of member 32. Lead 41 is connected to a source 49 of filament current.

A focusing coil 50 in'casing 5| of paramagnetic material encircles the hollow neck 25 of the envelope near its small end. The coil is supplied with energy from a source 52. Deflecting coils 53 enclosed in insulation and of the type used in television receiver tubes, encircle the neck 25 between the focusing coil and plate 24. The deflecting coils are energized by means of conventional sweep circuits (not shown).

The means for supplying energy to the tube are diagrammatically illustrated as including sources 54 and 55 connected in series. One side of source 54 is grounded and the other side is connected to source 55 and to the end wall 26 of the end section 21 of the envelope end to the hollow neck 25. Theother side of the larger source 55 is connected to the anode assembly. Plate I4 is rounded.

aseaava In the operation of the X-ray tube, electrons emitted by filament I! are focused by the walls of the channel, in which the filament lies, and their initial travel out of channel is in a direction away from the anode. The inner surface of the wall 58 of the cathodeand window I. deflne a concavity, the surface of which is at the same potential as the filament. and the wall and window act as focusing means, which cause the electrons to :travel along convergent curved trajectories. so that the electrons pass through the opening in diaphragm Ila and through hoilow flange 20. The flange, the ,diaphragm, and focusing coil 50 act to form theelectrons into a beam, which travels along the common axis of neck and the target toward the latter. Neck 25 cannot be made ofunetal because eddy currents induced by the scanning coil would cause overheating. Because it is desirable to maintain uniform potential along this portion, however, a thin high resistance conductive coating is applied to the inner surface of the neck. By making the coating thin and of high resistance material. eddy currents in the coating are negligible. The energization of the deflecting coils 53 by the sweep circuits causes the electron beam to scan the target surface of the anode and. because of the ray-proof disc 31 and plate 24, the only X- rays which can issue from the tube in the general direction of the axis are those passing through the pin hole 31a in the disc. Accordingly, as the electron beam scans the target surface. the X- ray beam issuing from the pin hole has a scanning movement opposite to that of the electron beam. The parts of the object to be examined lying within the fleld of the X-ray beam are, accordingly, consecutively irradiated in accordance with the scanning sequence determined in conventional manner by the design of the sweep circuits.

The modified form of the electron device of the invention shown in Fig. 3 is similar to that shown in Fig. 1, except for the target surface, the window, and the provision of optical elements outside the window and of means for modulating the intensity of the electron beam.

The Fig. 3 tube comprises an evacuated envelope having an enlarged section I i' of the same construction as section II of the Fig. 1 tube. A target anode assembly extends into section it and the assembly includes a block H of metal having a concave target surface coaxial with the envelope and facing inward. The target surface has a phosphor coating forming a screen II, which fluoresces when electrons impinge thereon, and the block may be provided with embedded cooling coils l6 for circulation of a coolant supplied through tubes ll, II. The block carries a hollow shield 22' similar to shield 32.

The inner end of the section II of the tube is attached to an opaque plate 24' having a central opening and a hollow neck 25 of ceramic or other dielectric material and of tapering diameter extends through the opening with its large end mounted in place in the opening. The neck has a metallic extension 25a mounted by means of a flange on plate 24 within section Ii and the small end of the neck is seated upon and secured to the end wall 26 of the end section 21' of the envelope by means of a flanged collar 28a.

The wall 26' is formed with a central opening deflned by an internal flange 28' coaxial with the neck and the target, and flange II, the inner surface of neck 2!. and the neck extension Ila are coated with a continuous thin high resistance through the cathode wall.

conductive coating of a material, such as that known commercially as Aquadag." The end section 21' of the envelope is similarin construction to end section 21 of the tube of Fig. 1 and has a tubular extension I! similar to extension I! but internally threaded throughout substantially its entire length. At its inner end, extension 35' has a central opening defined by an internaL flange 35a, and a glass window ll is mounted to close the opening and is sealed vacuum-tight to a washer II of the metal known commercially as Kovar" or some other suitable metal. the washer being in turn brazed to the flange inwardly from the window.

A pairs of lens mounts are threaded into extension 35' and the inner mount includes a ring I! having a central opening defined by an externally threaded flange. A lens element 59 is mounted to close the opening through ring it and is held in place against the flange by a cap overlying the element and threaded on the outside of the flange. The second lens mount comprises an externally threaded tubular member 8| carrying a lens element 62 at its outer end, which is held in place by a cap 83 overlying the element and threaded on the end of member if.

The portion of extension 35 inwardly from window 3i forms a cathode generally designated 40' and, at its inner end, the cathode is provided with a flange ll of L-section which, with the wall of the extension, forms a circumferential channel encircling flange 28' and spaced laterally therefrom. A filament 42' of generally cir cular form is mounted within the channel on leads 43', N, which extend outwardly through insulating bushings 45 mounted in openings The leads are connected to conductors 46', which lie within the end section 21 between the extension 35 and the outer wall of the section. Conductors 46" are connected to leads 1' sealed through insulating bushings 48' in openings in the wall of section 21'. The leads 4'! arejconnected, respectively, to the positive and negative terminals of a source of fllament current.

A focusing coil 50' in a casing of paramagnetic material encircles the neck 15' near its small end and is supplied with energy from a source 52'. Deflecting coils 53' enclosed in insulation and of the type used in television receiver tubes encircle neck 25' between the focusing coil and plate ll. The deflecting coils are energized by means of conventional sweep circuits (not shown) The tube of Fig. 3 is supplied with energy in the same manner as the tube of Fig. 1, the end plate 34', which is integral with extension I". being grounded and potential being maintained on plate 26' and the anode assembly. The tube is provided with means acting on the electron beam to modulate the intensity thereof and, in the construction illustrated, the modulating means comprises a transformer 64, the secondary of which is connected between the negative fllaxnent lead 41 and a suitable part of the assembly, such as the plate 34', which has a good electrical connection with the cathode channel ll The operation of the tube of Fig. 3 is generally similar to that of the tube of Fig. 1, except that. when the electron beam strikes the fluorescent screen i5, visible light instead of X-rays are produced. The light passes from the screen through neck I! and window 36' and is then condensed by the lens system consisting of elements 5! and I. The image produced upon the screen is pro- Jected upon an external viewing screen.

asses-m "-Iii the construction illustrated, the concave surface of the cathode '40 in front of the filament channelacts as a focusing means for the electrons emitted by the filament. and, when no signal is impressed on the primary of the transformer 64, the cathode and such focusing means are at the same potential as the filament. When a'signal is impressed on the transformer priem'ary,-the potential of the cathode focusing means is changed relative to that of the filament, with the result that moreor less electrons are de-' flected to pass through the opening in flange 28'. the intensity of the light produced at the screen is proportional to the density of the electron stream impinging on the screen, the amount of light produced at any point on the screen may bejcontrolled' by impression of suitable signalson the primary f the modulation means. i

' It" is to be understood that the modulation means "described above is merely by way of example, and any other means, which produces a'variation in the potential gradient at the electron-emitting surface of the cathode in accordance"withthe modulating signal, may be em ployed'. Another example of such modulating means is illustrated in Figs. 4 and 5-and it involves the use of agrid 65 made of a pair of circular concentric wires 86, 61 connected by radial tie wires 8'. The outer circular wire 61 is provided with radial pins 69 and the grid is mounted in the cathode do" with pins 61 embedded in msulating supports 10 inserted in bores in the wall of the cathode. One of the pins 69 is connected by a conductor II to a lead 12 connected to one side of the secondary of a transformer 13,-the

other side of the secondary being connected to' the grounded plate 34". The grid is so positioned that the electrons emitted by the filament 42" pass through the grid between the wires thereof and, in accordance with the modulation signal impressed thereon, the grid permits a varying number of electrons to pass through it, sothat the intensity of the electron beam is correspondingly varied. 1 I In both forms of the new discharge device, the flange 2!, 28' serves as the first anode and performs the dual functions of focusing and accelerating the electrons. The continuous conductive coating on the inner surfaces of the flange 212i, the neck 25, and the neck extension fie, 25a keeps the entire region between enlarged section II, If and end section 21, 21' at constant potential. so that the electron beam is not distorted." The target is maintained at a higher positive potential than flange 28, 28' in order to cause further acceleration of the electrons and produce greater effects. For some purposes, however, the electrons may be sufliciently acceleratedby the first anode alone and, in that case, the target can be maintained at the'same potential as the first anode.

The electron beam may be focused either by an electric field or by a magnetic field, and the,

We claim:'

1. An electron tube, which comprises an evacu ated envelope having a window for escape of radiant energy, an anode having a target surface within the envelope coaxial with andspaced from the window, cathode means within the en-- velope adjacent the window, the cathode means having a central opening coaxial with the target surfaceand window and a filament encircling and concentric with the opening, means for fo cusin'g electrons emitted by the filament to cause them'to' pass through said central opening and form a beam traveling toward the target surface, means outside the tube and acting on the beam between the cathode and the target surface-to cause the beam to scan the target surface. and image forming means in the path of the radiant energy traveling from the target surface through the cathode opening for converting said energy into a usefui beam traveling away from the -en-- velope and having a lateral movementsimilar to the scanning movement of the electron beam but opposite in sense. v

2. Anelectron tube, which comprises an" evacuated envelope having a window for escape of radiant energy, an anode having a target surface within the envelope coaxial with and spaced from the window, cathode means within the envelope adjacent the window, the cathode means having a central opening coaxial with the target surface and window and a filament encircling and con-' centric with the opening, said cathode means also including. means for focusing" electrons emitted by the filament to cause'them'to pass through said central opening and form a beam traveling toward the target. surface, means for deflecting the beam to causeitto scan the target surface,. and image forming means .in the path of radiant energy traveling from the target sur-, face through the cathode opening for converting said energy into a useful beam traveling away from the envelope and having a lateral movement similar to thescanning movementof the electron beam but opposite in sense.

3. An electron tube, which comprises an evacuated envelope having a windowfor escape of radiant energy, an anode having a target surface within the envelope coaxial with and spaced from the window, cathode means within the. envelope adjacent the window, the cathode means having a central openin'g coaxial with the target surface and window and a filament encircling and concentric with the opening, means both inside and outside the tube for focusing electrons emitted by the filament to cause them to pass through said central opening and form a beam travelingito ward the target surface, means for deflecting the beam to cause it to scan the target surface, and image formingzmeans in thepath of radiant energy'traveling from the target surface'through the cathode opening" for converting said energy into a useful beam traveling away from the envelope and having a lateral'movement similar the scanning movement of the electron beam but opposite 'in sense. V

4 An electron tube, which comprises an evacuated envelope having a window for escape of radiant energy. an anode having a target surface within the envelope coaxial with and spaced from the window. cathode means within the envelope, adjacent the. window. the cathode means havin a. central opening coaxial, with thetarget surface and.window and. amamentencimlmg andv concentric with the opening, means for focusing elecing said energy into a useful beam traveling away from the envelope and having a lateral movement similar to the scanning movement of the electron beam but opposite in sense.

5. An electron tube, which comprises an evacuated envelope having a window for escape, of radiant energy, a target anode having a target surface within the envelope coaxial with and spaced from the window, the cathode means having a central opening coaxial with the target surface and window and a filament encircling and concentric with the opening, means, including a hollow anode, for focusing electrons emitted by the filament to cause them to pass through said central opening and form a beam traveling toward the target surface, and means outside the envelope and acting on the beam between the hollow anode and the target surface to cause the beam to scan the target surface.

6. An electron tube, which comprises an evacuated envelope having an end section provided with a wall formed at least in part by a window transparent to radiant energy, an enlarged section, and a neck connecting the end and enlarged sections and diminishing in diameter, toward the end section, an anode in the enlarged section having a target surface producing radiant energy when struck by an electron beam, the target surface being coaxial with the neck and window, a cathode in the end section having an opening coaxial with the neck and a filament encircling and concentric with the opening, means for focusing electrons emitted by the filament to cause them to pass through said central opening and form a beam traveling toward and impinging upon the target surface, and means for deflecting the beam to cause it to scan the target surface.

'7. An electron tube, which comprises an evacuated envelope having an end section provided with a wall formed at least in part by a window transparent to radiant energy, an enlarged section, and a neck connecting the end and enlarged sections and diminishing in diameter toward the end section, an anode in the enlarged section having a target surface producing radiant energy when struck by an electron beam, the target surface being coaxial with the neck and window, a cathode in the end section having an opening coaxial with the neck and a filament encircling and concentric with the opening, means for focusing electrons emitted by the filament to cause them to pass through said central opening and form a beam traveling toward and impinging upon the target surface, and means encircling the neck and operable to deflect the beam to cause it to scan the target surface.

8. An electron tube, which comprises an evacuated envelope having a window for escape of radiant energy, an anode having a target surface within the envelope coaxial with and spaced from the window, cathode means within the envelope adjacent the window, the cathode means having a central opening coaxial with the target surface i0 and window and a filament encircling and concentric with the opening, means for focusing electrons emitted by the filament to cause them to pass through said central opening and form a beam traveling toward the target surface, means for deflecting the beam to cause it to scan the target surface, and image forming means in the path of radiant energy traveling from the target surface through the cathode opening for converting said energy into auseful beam traveling away from the envelope and having a lateral movement similar to the scanning movement of the electron beam but opposite in sense.

9. An X-ray tube, which comprises an envelope having awindow transparent to X-rays, an anode having a target surface within the envelope coaxial with and spaced from the window, cathode means within the envelope adjacent the window, the cathode means having a central opening coaxial with the target surface and window and a filament concentric with the opening, means for and form a beam traveling toward the target surface, means for deflecting the beam to cause it to scan the target surface, and a ray-proof shield in the path of the X-rays leaving the target surface and having a small opening on the common axis of the target surface and the cathode opening.

10. An X-ray tube, which comprises an envelope having a window transparent to X-rays, an anode having a target surface within the envelope coaxial with and spaced from the window, cathode means within the envelope adjacent the window. the cathode means having a central opening coaxial with the target surface and window and a filament concentric with the opening means for focusing electrons emitted by the filament to cause them to converge along curved trajectories and pass through said central opening and form a beam impinging upon the target surface, means for deflecting the beam to cause it to scan the target surface, and a rayproof shield outside the window and having a small opening on the common axis of the target surface and the cathode opening.

11. An X-ray tube, which comprises an envelope having a window transparent to X-rays, an anode having a target surface within the envelope coaxial with and spaced from the window, cathode means within the envelope adjacent the window, 'the cathode means having a central opening coaxial with the target surface and window and a filament concentric with the opening, means, including the window, for focusing electrons emitted by the filament to cause them to converge along curved trajectories and pass through said central opening and form a beam impinging upon the target surface, means for deflecting the beam to cause it to scan the target surface, and a ray-proof shield outside the window and having a small opening on the common axis of the target surface and the cathode opening.

12. An X-ray tube, which comprises an evacuated envelope having an end section provided with a wall formed at least in part by a metallic window transparent to X-rays, an enlarged section, and a neck connecting the end and enlarged sections and diminishing in diameter toward the end section, an anode in the enlarged section having a target surface producing X-rays when struck by an electron beam, the target surface being coaxialwith the neck and window, a cathode in the end section having an opening coaxial with the neck and a filament encircling and concentric with the opening, means for maintaining potential upon said wall of the end section to "cause electrons emitted by the filament to converge and pass through the cathode opening and opening, said element having a small opening on the common axis of the target surface and neck.

13'. A cathode ray tube, ,which comprises an evacuated envelope having a transparent window, an anode having a fluorescen target surface within the envelope coaxial with a d spaced from the window, cathode means within'the envelope adjacent the window, the cathode means having a central opening coaxial with the target surface and window and a filament encircling and concentric with the opening, means for focusing electrons emitted by the filament to cause them to ,pass through said central opening and form a beam impinging upon the target surface, the light produced by the target surface traveling through said central opening and window, means for deflecting the electron beam to cause it to scan the target surface, means for varying the intensity of the electron beam, and image forming optical means in the path of the light beam issuing through the window.

14. A cathode ray tube, which comprises an evacuated envelope having a transparent window, an anode having a fluorescent target surface within the envelope coaxial with and spaced from the window, cathode means within the envelope adjacent the window, the cathode means having a central opening coaxial with the target surface and window and a filament encircling and concentric with the opening, means both inside and outside the envelope for focusing electrons emitted by the filament to cause them to pass through said central opening and form a beam impinging upon the target surface, the light produced by the targetv surface traveling through said central opening and window, means for varying the intensity of the electron beam, means for deflecting the electron beam to cause it to scan the target surface, and image forming opticentric with the opening, means both inside and outside the envelope for focusing electrons emitted by the filament, the internal focusing means causing the electrons to converge along curved paths and pass through the cathode opening and the external focusing means acting on the electrons thereafter to form them into a beam impinging upon the target surface, the light produced by the target surface traveling through the cathode opening and window, means for varying the intensity of the electron beam, means for deflecting the electron beam to cause it to scan the target surface, and image forming optical means in the path of the light beam issuing through the window.

16. A cathode ray tube, which comprises an v evacuated envelope having an end section with a metallic wall having an opening closed by a transparent window, an enlarged section, and a neck connecting the end and enlarged sections and diminishing in diameter toward the end section, an anode having a fluorescent target surface within the enlarged section and facing the large end of the neck, the target surface being coaxial with theneck, a cathode in the end section having an opening coaxial with the neck and a filament surrounding and concentric with the opening, the cathode also including focusing means for directing electrons emitted by the filament away from the anode. means for maintaining potential upon said wall of the end section to cause the electrons to converge and pass through the cathode opening and enter the neck, focusing means acting on the electrons within the neck and forming them into a beam impinging upon the target surface, deflecting means acting on the electron beam and causing it to scan the target surface, the light produced by the target surface traveling through the neck, cathode opening, and window, and image forming optical means in the path of the light beam issuing through the window.

17. A cathode-ray tube, which comprises an evacuated envelope having an end section with a metallic wall having an opening closed by a transparent window, an enlarged section, and a neck connecting the end and enlarged sections and diminishing in diameter toward the end section, an anode having a fluorescent target surface within the enlarged section and facing the large .end of the neck, the target surface being coaxial with the neck, a cathode in the end section having an opening coaxial with the neck and a filament surrounding and concentric with the opening, the cathode also including focusing means for directing electrons emitted by the filament away from the anode,.means for maintaining potential upon said wall of the end section to cause the electrons to converge and pass through the cathode opening and enter the neck, focusing means acting on the electrons within the neck and forming them into a beam impinging upon the target surface, means for varying the potential maintained upon said wall to vary the number of electrons entering the neck and thereby vary the intensity of the electron beam, deflecting means acting on the electron beam and causing it to scan the target surface, the light produced by the target surface traveling through the neck, cathode opening, and window, and image forming optical means in the path of the light beam issuing through the window.

18. An electron tube, which comprises an evacuated envelope having a window for escape of radiant energy, an anode having a target surfacewithin the envelope coaxial with and spaced from the window, cathode means within the envelope adjacent the window, the cathode means having a central opening coaxial with the target surface and window and a filament encircling and concentric with the opening, the cathode means including walls acting to cause electrons emitted by the filament to converge along curved trajectorles and pass through said central opening, a hollow anode acting on the electrons as they approach said central opening to form them into a beam traveling toward the target surface, means acting on the beam between the hollow anode and the target surface and causing the beam to scan the target surface, and image forming means in the path of radiant energy traveling from the target surface through the cathode opening for converting said energy into a useful beam traveling away from the envelope and havina a lateral movement similar to the scanning movement of the electron beam but opposite in sense.

19. In an electron tube having an evacuated envelope, the combination of an anode having a target surface within the envelope, cathode means within the envelope having a central opening coaxial with the target surface and a filament encircling and concentric with the opening, the cathode means including means for causing electrons emitted by the filament to con- JOSEPH w. SKEHAN. moms H. ROGERS.

REFERENCES CITED I The following references .are'of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,685,928 Morrison Oct. 2, 1928 1,946,288 Kearsley Feb. 6, 1934 2,467,462 Brown. Jr. Apr. 19, 1949

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