US3303373A

US3303373A - Target assembly comprising insulating target, field and collector meshes

Info

Publication number: US3303373A
Application number: US340279A
Authority: US
Inventors: Hemmo R Alting-Mees
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1964-01-27
Filing date: 1964-01-27
Publication date: 1967-02-07
Anticipated expiration: 1984-02-07

Description

1967 H. R. ALTlNG-MEES 3,303,373

TARGET ASSEMBLY COMPRISING INSULATING TARGET. FIELD AND COLLECTOR MESHES Filed Jan. 27, 1964 2 Sheets-Sheet l Fig.l.

3,303,?! 73 FIEI'JD 1967 H. R. ALTlNG-MEES TARGET ASSEMBLY COMPRISING INSULATING TARGET,

AND COLLECTOR MESHES 2 SheetsSheet 2 Filed Jan. 27, 1964 United States Patent 3,303,373 TARGET ASSEMBLY COMPRISING INSULATING TAR GET, FIELD AND CQLLECTOR MESHES Hemmo R. Alting-Mees, Horseheads, N.Y., assignor to Westinghouse Electric Corporation, East Pittsburgh,

Pa., a corporation of Pennsylvania Filed Jan. 27, 1964, Ser. No. 340,279 12 Claims. (Cl. 313-65) This invention relates to pickup tubes and more particularly to an improved electrode assembly for these devices.

One particular application of this invention is within an image orthicon type pickup tube. More specifically, this specific embodiment is directed to the type of image orthicon tube in which a field terminating mesh is provided between the target electrode and the reading electron beam source. An image orthicon type of tube consists of a photocathode onto which the input radiation or light from an input scene is directed and which in turn emits electrons in response to the input radiation. The photo electrons from the photocathode are in turn accelerated and focused onto a thin dielectric target of a material such as glass. It is also normal practice to provide a fine mesh screen adjacent the target and on the side thereof facing the photocathode. This mesh may be referred to as a collector electrode. The energy of the electrons directed onto the dielectric storage target are sufficient to cause secondary emission from the bombarded dielectric surface and thereby establish a charge pattern on the opposite side of the dielectric target corresponding to the visible light image that is directed onto the photocathode. This charge pattern or image established on the opposite side of the target with respect to the photocathode is read out by an electron beam referred to as a reading beam in a manner well known in the art to derive an electrical signal representative of the visible information directed onto the pickup tube.

In order to insure that the reading electron beam scanning the target to read the charge information out will approach the target normal over the entire surface, there may be provided an electrostatic field terminating mesh. This field terminating mesh is normally mounted substantially parallel to and adjacent the target .on the reading electron gun side thereof and is connected to a fixed potential. The field terminating mesh is primarily for the purpose of correcting the electrostatic field in the neighborhood of the target and increasing the strength of the decelerating field in front of the target. This mesh reduces distortion due to deflection of the beam by the electrostatic field from the charge pattern stored on the target. A more specific discussion of the field within the scan or read section of the image orthicon is described in US. Patents 2,545,382 and 2,579,351.

There are several arrangements known in the art for positioning the target electrode and the collector mesh between the reading gun and the photocathode, such as described in copending application, Serial No. 265,425 filed March 15, 1963, now US. Patent 3,287,585, by R. R. Randels and assigned to the same assignee as this invention. In addition, there are also several structures known in the art for mounting the field terminating mesh which is the mesh positioned adjacent the target and between the target and the electron scanning gun. Due to the necessary processing procedure of an image orthicon, it is necessary to insert the target, the collector mesh and the field termminating mesh after the associated lens system is mounted within the image orthicon tube. One prior art structure supports the field terminating mesh within the lens structure of the image section by providing an additional lens electrode to support the field terminating mesh. This 3,303,373 Patented Feb. 7, 1967 electrode is an addition to the normal electrode provided for supporting the target and the collector mesh facing the photocathode. Another prior method of supporting the field terminating mesh in a position adjacent to the target member is to mount the field terminating mesh on the electron gun structure of the image orthicon.

These mounting techniques are subject to several objections, for example, in the first structure the snap-in field mesh assembly required must have a larger diameter than the target support since the target must pass through the field mesh support member. This larger electrode support for the field mesh makes it diificult to mount the field terminating mesh. In the insertion of the assembly through the small portion of the image orthicon envelope, it is difiicult to prevent scratching the coating on the small portion of the image orthicon envelope resulting in dirt in the tube and possible damage to the tube. It is also diflicult to provide precision spacing between the collector and the mesh field mesh and to provide a ruggedized image orthicon tube.

The second prior structure requires a complicated jigging arrangement and is inherently dirty and again accurate spacing is very difiicult to maintain between the field terminating mesh and the target assembly. This is particularly so in that the spacing is primarily determined by the sealing of the electron gun structure to the envelope at the base.

Another important consideration in the mounting of the field terminating mesh is to provide the desired electrostatic field configuration pattern between the field terminating mesh and the target to insure that the electron beam does approach the surface of the target normal thereto over the whole usable area.

It is accordingly an object of this invention to provide an improved electrode assembly for an image pickup tube.

It is another object to provide an improved electrode support arrangement for supporting the field terminating mesh within an image orthicon pickup tube.

It is another object to provide an improved electrode support for the field terminating mesh to provide the desired electrostatic field between the field terminating mesh and the target electrode.

In accordance with my invention, I provide an electrode assembly in which the field terminating mesh is mounted and secured to the support ring of the target and the associated collector mesh positioned between the target and the photocathode and is thereby mounted within the normal electrode or target support cup' of the image orthicon. In addition, I provide a deflection shield member or ring about said field terminating mesh to provide the necessary field configuration within the space between the field terminating mesh and the target.

Further objects and advantages of the invention will become apparent as the following description proceeds. The features of novelty which characterize the invention will be pointed out in particularity in claims annexed to and forming a part of the description.

For a better understanding of the invention, reference may be had to the accompanying drawings, in which:

FIGURE 1 is a cross sectional view of an image orthicon pickup tube embodying the teachings of my invention;

FIG. 2 is an enlarged partial view, partly in section, of the electrode arrangement illustrated in FIGURE 1 and incorporating the teachings of my invention;

FIG. 3 is a perspective view of the electrode arrangement in FIGURE 2 partly broken away to illustrate the teachings of my invention;

FIG. 4 is a diagrammatic view illustrating the field pattern established by my invention within the area between the field terminating mesh and the target member.

Referring now to FIGURE 1, there is illustrated an image orthicon pickup tube comprising an envelope of a suitable material such as glass. The envelope 10 includes an enlarged or image section 12 at one end thereof and a tubular neck portion 14 or scanning section forming the remaining portion of the envelope. The tubular neck portion 14 is of smaller diameter than the enlarged image section 12. Positioned within the tubular neck portion 14 of the envelope is a conventional image orthicon electron structure 16 for providing an electron beam of low velocity which is focused and accelerated onto a target member 40 within the image section. The electron gun 16 consists of a heater 18, a cathode 19, a control grid 20 and agrid 21 which also serves thefunction of the first dynode of the electron multiplier. An additional electrode 22 is also provided coaxial with the electron gun 16 with an aperture 23 provided therein. An anode 24 which is illustrated as a conductive wall coating is provided on the inside surface of the tubular portion 14 and extends from the electron gun 16 to the enlarged tubular envelope portion 12.

Positioned within the image section 12 and adjacent the target 40 is a field shaping electrode 30. In addition, a field terminating mesh 50 is provided between the electron gun 16 and the target 40. The field shaping electrode 30 reduces the velocity of the electron beam as it approaches the target 40 and shapes the field adjacent the target 40. The field provides the desired field pattern between the terminating mesh 50 and the target 40 to provide a field that will normalize the electrons approaching the target 40.

An electromagnetic coil (not shown) surrounds the tubular envelope 10 and provides a focusing field for the electron beam from the gun 16 to provide normalized approach of the electrons to the surface of the glass target 40. A neck yoke (not shown) is also provided to provide means of scanning or deflecting the electron beam generated by the electron gun 16 to cause the electron beam to scan a raster on the surface of the target 40. The raster scanned on the target 40 does not cover the entire area of the target and is rectangular in shape. The above mentioned deflection neck yoke provides by means of suitable circuits connected thereto the necessary scanning magnetic fields for the electron beam.

Suitable potentials are applied to the electrodes within the tubular neck portion 14 by means of terminals (not shown) provided through the base portion of the neck.

The enlarged portion 12 of the envelope 10 is closed by a light transmissive face plate portion 32. A photocathode surface 34 is provided on the inner surface of the face plate 32. The input scene is optically focused upon the photocathode surface 34 and causes the emission of photoelectrons from the photocathode 34 in proportion to the amount of light striking the elemental areas of the photocathode 34. The photoelectron emission from the photocathode surface 34 is focused onto the target 40 by means of a magnetic field provided by coil (not shown) around the envelope portion 12 and the accelerating field provided by electrodes 36 and 38.v The photoelectrons are accelerated to such an extent that they strike the surface of the target 40 with suflicient energy to be above the first crossover potential of the target and create a greater number of secondary electrons than bombarding primary electrons. A collector or mesh screen 42 is closely spaced from the surface of the target 40 and is maintained during operation at a potential near the cathode potential of the electron gun 16.

The low velocity electron beam of the electron gun 16 scanning the surface of the target 40 will bring the potential of the target 40 to approximately that of the cathode of the gun 16. At this potential, the scanning electron beam will not land on the surface of the target '40 but will be electrostatically reflected back along the axis of the tube 10. The secondary emission caused by the bombardment of the target by the photoelectrons from the photocathode 34 will cause the surface of the target 40 facing the photocathode 34 to charge in a positive direction to provide a positive charge pattern corresponding to the light input directed onto the photocathode 34. Due to the extreme thinness of the target 40, positive charges on the image side of the target 40 will set up a corresponding positive potential pattern on the reading or the electron gun side of the target 40. The target 40 is of a thin insulating material such as glass. As the electron beam of the gun 16 scans the surface of the target 40, electrons from the beam will be drawn to the positive areas of the target surface and will be deposited and will neutralize the positive potential pattern on the target 40. This will tend to charge the surface of the target '40 to cathode potential again. The remainder of the electrons in the electron beam which are not required to return the surface of the target 40 to cathode potential will be reflected back towards the electron gun 16. The negative charges deposited on the gun side of the target 40' will unite by conduction through the glass target 40 with the positive charges on the photocathode side of the target 40. The return electron beam from the target 40 will be modulated by the removal of electrons from the beam and deposited upon the surfaces of the glass target to neutralize the positive charge pattern formed thereon. The modulated return beam is returned to the electron gun 16 and will strike the .first dynode 21 of a multiplier section surrounding the electron gun 16. The electron multiplier section includes a plurality of dynodes 25. The modulated return is thus amplified by the multiplier sections and collected on a collector electrode 26 which in turn is connected to an appropriate output circuit to provide the video signal. The above description and operation is substantially that of a conventional image orthicon and the applicant has described his invention incorporated in such a device in order to adequately explain and set forth the invention.

Referring now to FIGS. 2 and 3 for a more detailed description of the electrode system in and around the target electrode 40, the target support cup or electrodes 38 as well as

electrodes

30 and 36 are cylindrical electrodes longitudinally spaced and in a coaxial relationship. These electrodes are normally secured to each other by ceramic rods 31 spaced about the circumference of the electrodes and secured to the electrodes by straps. The

electrodes

30, 36 and 38 are supported within the inner section by a pin member 33 projecting through the wall portion 41 which are in turn secured to the ceramic rods 31. The pins 33 passing through the wall 41 are also normally utilized to supply voltage to the electrodes within the image section.

The target support cup 38 is a cylindrical member and includes an inturned flange 39 with a cylindrical sleeve 43 integrally secured to the inturned flange 39. It is within this cylindrical portion 43 that the field mesh 50, the target 40 and the collector mesh 42 are mounted and secured within the image orthicon tube. The assembly including these three elements and supporting structure is illustrated in FIG. 3. The structure illustrated in FIG. 3 is the assembly that is assembled exterior of the envelope and then inserted into the image orthicon and secured within the support cup electrode 38 as illustrated in FIG. 2.

The collector electrode 42 includes an annular support ring 45. The target member 40' includes an annular support ring 46. The collector electrode 42 includes a mesh 47 of a suitable material such as copper. The mesh 47 has a thickness of about 0.5 mil and about 600,000 apertures per square inch. The support ring 45 may be of any suitable material such as an alloy of steel, iron and aluminum. The target 40 includes the support ring 46 and a thin film 49 of a suitable insulating material such as glass.

t-rode 42 are positioned and held together so that the glass membrane 49 and the mesh 47 are spaced at a distance of about 1 to 100 mils. The target 40 and the collector electrode 42 are held together by :an outer clamping ring or member 52 and an inner retainer ring or member 54 with a resilient spacer member 56 also positioned between the members 52 v and 54. The target clamping ring 52 consists of an annular member having an inturned flange and of such dimensions as to permit the support rings 45 and 46 to be positioned therein as illustrated in FIG. 2. The target retaining ring 54 is also an annular member with an inturned flange 58 and a. cylindrical integral flared portion 60. The target retaining ring 54 is of such dimensions as to encompass the support rings 46 and 45 and the spacer ring 56. A plurality of clamping lugs 62 are provided about the periphery of the clamping rings 52 and are bent over to lock the retainer ring 54, to form the assembly illustrated in FIG. 2. It should be appreciated that the above target assembly of the target 40 and the collector electrode 42 are assembled exterior of the tube. The field terminating mesh electrode 50 is also assembled and secured to this target assembly exterior of the tube.

The field terminating electrode 50 includes a support ring 70. The support ring 70 may be of a suitable nonmagnetic material such as an alloy of steel, iron or aluminum. The ring 70 also includes an inturned flange 72 to which a mesh 74 is secured. A weld ring '76 is secured over the mesh 74 as illustrated in FIG. 2. The mesh 74 may be of a suitable material such as copper having a thickness of about 0.5 mil and having =about'250,000 apertures per square inch. The inturned flange 72 on the support ring 7 0 is provided on the edge facing the photocathode. The mesh 74 is positioned with respect to the storage membrane 49 so as to be substantially parallel thereto and at a distance of about 100 to 500 mils.

The field terminating electrode 50 is secured to the target assembly 40 by means of four connecting members 81. The connecting members 81 consist of a ceramic pillar 82 with connecting tabs 80 secured to each end of the pillars 82. The connecting tab 80 at one end of the pillar 82 is brazed to the support ring 70 and the tab 80 at the other end of the pillar is brazed or spot welded to the outer clamping ring 52 of the target. In the specific embodiment illustrated, these connecting members 81 are positioned about the periphery of the

rings

70 and 52 and are equally spaced. Two of these members 81 lie in a plane substantially parallel to the horizontal scanning by the electron beam and the other two members 81 lying in a plane substantially perpendicular to the plane of the first two members. By this arrangement the connecting members 81 are removed from the scan master as far as possible. In this manner, distortion caused by non uniform electric field at pillars is kept at a minimum. In addition, two contact members 86 of resilient material are spot welded to the support ring 70 and provide electrical contact to the wall coating 24 on the inner surface of the tubular portion 14 of the envelope.

A deflection electrode 90 is also secured to the support ring 70. The deflection electrode 90 consists of an annular ring 91 to provide a cylindrical portion of substantially the same diameter as the support ring 70. The length of this deflection electrode is about 100 mils as illustrated in FIGS. 2 and 3. It is necessary to remove portions of this deflection electrode 90 as illustrated in FIG. 2 to provide access to the support ring 70 of the connecting members 81. The complete assembly as illustrated in FIG. 3 is assembled on the outside of the tube envelope and then by proper jigging members is inserted through the tubular portion 14 of envelope and secured into the support cup 38 as illustrated in FIG. 2. The flared portion 60 on the target retainer ring 54 is forced into the cylindrical portion 42 of the support cup 38 and spring tabs 88 lock and secure the entire assembly within the support cup electrode 38. It should be pointed out that it is possible to eliminate the field shaping electrode 30 and mount it in a similar manner as the field terminating electrode 50 and thus permit more versatility in design. This would permit one to vary the size, shape and position of the electrode 30 or the position of the field mesh electrode 50 with respect to the electrode 30.

The field deflection electrode was found necessary in order to achieve linearity in the image orthicon field mesh tube. It was found that in a structure without the field deflection shield 90, the voltage required on the electrode 30 to correct for distortion was of a higher value than available in most conventional power supplies. A normal field mesh tube has a voltage of about 75 volts on the electrode 30 and it was found in the system described herein without the electrode 90 that it was necessary to go to at least volts which is near the maximum voltage available in most image orthicon camera setups. It was found that without the electrode 90, the electrostatic lines near the field mesh 50 were curved away from center whereas they should be parallel to the field terminating mesh. It was found by providing the deflection electrode 90 that parallel field lines could be obtained in a tube with a target to field mesh spacing of 400 mils with the target support 38 at zero potential, the electrode 30 at 90 volts and the Wall coating at volts. This of course meant that the field mesh electrode 50 as well as the deflection electrode 90 were operating at about 160 volts.

It was also found that with different heights of the electrode 90, the voltage required on electrode 30 was essentially a linear function of the height of the shield 91. It is also thought that the spacing betweenthe field mesh electrode 50 and the target 40 affects the Voltage on electrode 30. In FIG. 4, there is illustrated the field configuration with the voltages indicated above. It was found that with'this design that linearity was provided across the entire raster portion of the image orthicon. By providing the deflection electrode 96 and permitting variation in height of 91 thereof, the potential required for the electrode 30 may be determined. It is entirely feasible that with adequate deflection electrode height 'it would be possible to entire-1y dispense with the field shaping electr0de 30.

While there have been shown and described what are present and considered to be preferred embodiments of the invention, modifications thereto will readily occur to those skilled in the art. For example, an additional grid electrode could be mounted between the collector grid 42 and the photocathode 34 in the same manner as the electrode 50 is mounted to form a unitary structure. It is not desiredthere'fore that the invention be limited to the specific arrangements shown and described and it is intended to cover in the appended claims all such modifications that fall within the true spirit and scope of the invention.

' I claim as my invention:

1. A pickup tube comprising an envelope and including therein an electron beam source for forming and directing a beam of electrons along an axial path, a target electrode assembly mounted to intercept said electron beam, a photocathode sensitive to an input radiation positioned on the opposite side of said target electrode assembly with respect to said electron beam source, an annular electrode support member for said target electrode assembly, said target electrode assembly including a target of insulating material, a collector electrode positioned on the side of said target facing said photocathode and a field terminating electrode positioned on the side of said target facing said electron source, said target including a film of insulating material and an annular supporting means, said field terminating electrode including a mesh and an annular supporting means, said field terminating electrode mechanically secured to said supporting means for said insulating target and insulated therefrom, said field terminating electrode also including an annular field deflection electrode mechanically and electrically connected to said field terminating supporting means and extending in a direction toward said target member, a wall coating provided on the inner surface of said envelope and electrical contacting members secured to said field terminating supporting means and in contact with said wall coating to permit operation of said wall electrode and said field terminating mesh at the same potential.

2. A pickup tube comprising an envelope and having therein an electron beam source for forming and directing a beam of electrons along an axial path, a photocathode sensitive to an input radiation positioned at the opposite end of said envelope with respect to said electron beam source, annular electrode members positioned between said photocathode and said electron source, a target electrode assembly of smaller diameter than said annular electrode members positioned between said photocathode and said electron source including an insulating target, a collector electrode positioned on the side of said target facing said photocathode and a field terminating electrode positioned on the side of said target facing said electron source, said target including an insulating film and an annular support ring, said collector electrode including a mesh and an annular support ring, an annular clamping means for retaining said target and said collector electrode and securing said target and said collector electrode to an inner portion of one of said annular electrodes, said field terminating electrode including a mesh and an annular support ring and means for mounting said field terminating electrode to said clamping means.

3. A pickup tube comprising an envelope having a first tubular portion of a first diameter and second tubular portion of a diameter greater than said first portion, an electron beam source positioned in 'said first envelope portion, for forming and directing a beam of electrons along an axial path, a photocathode sensitive to an input radiation positioned within said second envelope portion, a plurality of annular electrodes positioned within said second envelope portion and between said photocathode and said electron source, a unitary target electrode assembly of smaller diameter than said annular electrode members and supported within one of said annular electrode members, said target electrode assembly including an insulating target, acollector mesh electrode positioned on the side of said target facing said photocathode and a field terminating mesh electrode positioned on the side of said target facing said electron source.

4. A pickup tube comprising an envelope having a first tubular portion of a first diameter and second tubular portion of a diameter greater than said first portion, an electron beam source positioned in said first envelope portion, for forming and directing a beam of electrons along an axial path, a photocathode sensitive to an input radiation positioned within said second envelope portion, a plurality of annular electrodes positioned within said second envelope portion and between said photocathode and said electron beam source, a unitary target electrode assembly supported within one of said annular electrode members, said target electrode assembly including an insulating target, a collector electrode positioned on the side of said target facing said photocathode and a field terminating electrode positioned on the side of said target facing said electron source, said field terminating electrode including an electrical conductive mesh lying in plane transverse to said axial path and an annular member for supporting said mesh and including a substantially tubular portion of less diameter than said annular electrode members extending from said plane toward said target.

5. A pickup tube comprising an envelope and including therein an electron beam source for forming and directing a beam of electrons along an axial path, a target electrode assembly mounted to intercept said electron beam, a photocathode sensitive to an input radiation positioned on the opposite side of said target electrode as-' sembly with respect to said electron beam source, an annular electrode support member for said target electrode assembly, said target electrode assembly including a target of insulating material, said target including a film of insulating material and an annular supporting means, a field terminating electrode including a mesh and an annular supporting means, said field terminating electrode mechanically secured to said annular support means for said insulating target by means of spaced insulating members, said field terminating electrode also including an annular field deflection electrode mechanically and electrically connected to said field terminating supporting means and extending in a direction toward said target member, a wall coating provided on the inner surface of said envelope and flexible electrical contacting member secured to said field terminating supporting means and in contact with said wall coating to permit operation of said wall electrode and said field terminating mesh at the same potential.

6. A pickup tube comprising an envelope and having therein an electron beam source for forming and directing a beam of electrons along an axial path, photocathode sensitive to an input radiation positioned at the opposite end of said envelope with respect to said electron beam source, annular electrode members positioned between said photocathode and said electron source, a target electrode assembly of less diameter than said annular electrode members positioned between said photocathode and said electron source including a target, a collector electrode positioned on the side of said target facing said, photocathode and a field terminating electrode positioned on the side of said target facing said electron source, said target including an insulating film and an annular support ring, said collector electrode including an electrical conductive mesh and an annular support ring, an annular clamping means for holding said target and said collector electrode and securing said target and said collector electrode to an inwardly extending flange on one of said annular electrodes, said field terminating electrode including a mesh and an annular support mean-s, said field terminating electrode mechanically secured to said annular clamping means by four equally spaced insulating members.

7. A pickup tube comprising an envelope having a first tubular portion of a first diameter and second tubular portion of a diameter greater than said first portion, an electron beam source positioned in said first envelope portion for forming and directing a beam of electrons along an axial path, a photocathode sensitive to an input radiation positioned within said second envelope portion, a plurality of annular electrodes positioned within said second envelope portion and between said photocathode and said electron source, a unitary target electrode assembly of lesser diameter than said annular electrodes supported within one of said annular electrode, said target electrode assembly including an insulating target, a collector grid electrode positioned between said target and said photocathode and a field terminating grid electrode positioned between said target and said electron source.

8. A pickup tube comprising an envelope having a first tubular portion of a first diameter and second tubular portion of a diameter greater than said first portion, an electron beam source positioned in said first envelope portion, for forming and directing a beam of electrons along an axial path, a photocathode sensitive to an input radiation positioned within said second envelope portion, a plurality of annular electrodes positioned within said second envelope portion and between said photocathode and said electron beam source, a unitary target electrode assembly supported within one of said annular electrode members, said target electrode assembly including a target layer of insulating material, a collector electrode positioned on the side of said target facing said photocathode and 1 field terminating electrode positioned on the side of said target facing said electron source, said field terminating electrode including an electrical conductive mesh lying in plane transverse to said axial path and an annular electrically conductive member in electrical contact with said mesh including a substantially tubular portion of less diameter than said annular electrode members extending from said plane toward said target.

9. A pickup tube comprising an envelope and including therein an electron beam source for forming and directing a beam of electrons along an axial path, a target electrode assembly mounted to intercept said electron beam, means for deflecting said electron beam to scan a rectangular raster on said target, sensitive to an input radiation positioned on the opposite side of said target electrode assembly with respect to said electron beam source, an annular electrode support member for said target electrode assembly, said target electrode assembly including a planar target member of insulating material, a planar collector electrode positioned between said target and a field terminating electrode positioned between said target and said electron source and parallel to said target, said target including a film of insulating material and an annular supporting means secured within said an nular electrode support member, said field terminating electrode including an electrical conductive mesh parallel to said target and an annular supporting means, said field terminating electrode mechanically secured to said supporting means for said target by four rods of insulating material perpendicular to said target, said rods equally spaced about the periphery of said target and outside said raster, said rods positioned substantially midway between the corners of said rectangular raster, said field terminating electrode also including an annular field deflection electrode mechanically and electrically connected to said field terminating supporting means and extending toward said target member.

10. An image tube comprising an envelope having a first tubular portion of a first diameter and second tubular portion of a diameter greater than said first portion, an electron beam source for forming and directing a beam of electrons along an axial path positioned in said first envelope portion, a photocathode sensitive to an input radiation positioned within said second envelope portion, an annular electrode positioned within said second envelope portion and between said photocathode and said electron source, said annular electrode having an outer diameter greater than the diameter of said first envelope portion, a unitary target electrode assembly having a diameter less than the diameter of said annular electrode and supported within said annular electrode, said target electrode assembly including an insulating target, a collector grid electrode positioned on the side of said target facing said photocathode and a field terminating grid electrode positioned on the side of said target facing said lar portion of a diameter greater than said first portion, an electron beam source for forming and directing a beam of electrons positioned in said first envelope portion, an input radiation sensitive electrode positioned within said second envelope portion for generating electrons in response to input radiation, an annular electrode member positioned within said second envelope portion and between said input electrode and said electron source, a unitary target electrode assembly of lesser diameter than said first tubular envelope portion and said annular electrode member and secured within the inner portion of said annular electrode member, said target electrode assembly including an insulating target, a collector electrode positioned between said target and said input electrode and a field terminating electrode positioned between said target and said electron source, said field terminating electrode including an electrical conductive mesh lying in plane transverse to the longitudinal axis of said envelope and an annular member for supporting said mesh and including a substantially tubular portion in electrical contact with said mesh and of less diameter than said first tubular envelope portion extending from said plane toward said target.

12. An image tube comprising an envelope having a first tubular portion of a first diameter and a second tubular portion of a diameter greater than said first portion, an electron beam source positioned in said first portion for forming and directing a beam of electrons along an axial path, a photocathode sensitive to an input radiation positioned within said second portion, an annular electrode positioned within said second portion and between said photocathode and said electron source, said annular electrode having an outer diameter greater than the diameter of said first portion, a unitary target electrode assembly having a diameter less than the diameter of said first envelope portion and said annular electrode member, said target electrode assembly including an insulating target, a first grid electrode and a second grid electrode, said target including a layer of insulating material with supporting means, said second grid electrode including an annular supporting member mechanically secured to said target supporting means for said target by rods of insulating material perpendicular to said target, said rods equally spaced about the periphery of said target.

References Cited by the Examiner UNITED STATES PATENTS 2,743,150 4/1956 Rudy 313 2,914,690 11/1959 Sommer 313-65 3,137,803 6/1964 Ney et al. 31367 3,202,854 8/1965 Ochs 313-65 3,225,237 12/1965 Cope 313-65 JAMES W. LAWRENCE, Primary Examiner.

R. SEGAL, Assistant Examiner.