US3708673A

US3708673A - Image intensifier tube

Info

Publication number: US3708673A
Application number: US00154243A
Authority: US
Inventors: A Blacker
Original assignee: Machlett Laboratories Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1971-06-10
Filing date: 1971-06-10
Publication date: 1973-01-02
Anticipated expiration: 1990-01-02

Abstract

An image intensifier tube comprising a generally tubular envelope having disposed therein, in axially spaced relationship with one another, a spherically curved photocathode located adjacent an input faceplate at one end of the envelope, a frustoconical anode, an electron image decelerating electrode, a microchannel plate, and an imaging screen which is located adjacent an output faceplate at the other end of the envelope. The microchannel plate is provided with means for ensuring substantially uniform amplification of an electron image emitted by the photocathode and, preferably, also is provided with means for preventing the passage of visible light through the apertures thereof.

Description

United States Patent [191 Blacker, Jr.

[ 1 Jan. 2, 1973 [54] IMAGE INTENSIFIER TUBE Allen Palmer Blacker, Jr., New Milford, Conn.

[75] Inventor:

[73] Assignee: The Machlett Laboratories, Incorporated, Springdale, Conn.

[22] Filed: June 10, 1971 [21] Appl. No.2 154,243

52 U.S.Cl. ..2s0/213v'r,313/95,31s/11 51 Int.Cl ..H0lj31/50 58 Field of Search .250/213 VT; 313/95, 103, 104,

FOREIGN PATENTS OR APPLICATIONS 742,801 9/1966 Canada ..250/2l3 R Primary Examiner-Leland A. Sebastian Attorney-Harold A. Murphy et al.

[57] ABSTRACT An image intensifier tube comprising a generally tubular envelope having disposed therein, in axially spaced relationship with one another, a spherically curved photocathode located adjacent an input faceplate at one end of the envelope, a frustoconical anode, an electron image decelerating electrode, a microchannel plate, and an imaging screen which is located adjacent an output faceplate at the other end of the envelope. The microchannel plate is provided with means for ensuring substantially uniform amplification of an electron image emitted by the photocathode and, preferably, also is provided with means for preventing the passage of visible light through the apertures thereof.

10 Claims, 7 Drawing Figures PATENTEDJAN 2 I975 SHEET 2 BF 2 vw mm tttiltt222:2: Nm

IMAGE INTENSIFIER TUBE BACKGROUND OF THE INVENTION The invention herein described was made in the course of and under a contract, or subcontract thereunder, with the Department of Defense.

This invention relates generally to light amplifier tubes and is concerned more particularly with image intensifier tubes utilized for direct viewing of objects illuminated by visible or invisible radiation.

An image intensifier tube is a device for converting a radiational image of an external object directly into a bright visual image. Generally, an image intensifier tube includes an evacuated tubular envelope having an input screen assembly disposed adjacent a radiation transparent faceplate at one end of the envelope and an imaging screen assembly disposed adjacent an output faceplate at the other end. The input screen assembly usually comprises a layer of photoemissive material which constitutes the photocathode of the tube; and the imaging screen assembly generally comprises a layer of phosphor material which produces the output visual image. Usually, the imaging screen assembly is maintained at a relatively high positive potential with respect to the photocathode for the purpose of establishing a strong electrostatic field therebetween.

In operation, photons of radiant energy emanating from localized areas of an external object pass through the input faceplate of the image intensifier tube and impinge on corresponding localized areas of the photocathode. As a result, the photocathode emits an equivalent electron image which is accelerated by the strong electrostatic field toward the imaging screen assembly. The accelerated electron image, thus amplified, impinges on the phosphor layer of the imaging screen assembly with sufficient kinetic energy to produce a corresponding visual image which may be viewed through the output faceplate of the tube.

One type of prior art, image intensifier tube is provided with an input faceplate having a concave inner surface which supports a conforming photocathode layer. The photocathode is supported in spaced axial relationship with a small diameter end portion of an anode cone, such that a central portion of the photocathode is axially aligned with an aperture disposed in the small diameter end of the cone. The anode cone'extends longitudinally within the tube envelope and has a large diameter, open end which is axially aligned with a transversely disposed, imaging screen assembly having a concave inner surface. Thus, the described type of image intensifier tube has an imaginary axis of symmetry which extends from the central portion of the photocathode, along the axial center line of the anode cone and terminates in a central portion of the imaging screen assembly.

When operating an image intensifier tube of the described type, the anode cone is maintained at a relatively high positive potential with respect to the photocathode. Consequently, a strong electrostatic field having arcuately curved, equipotential surfaces is established between the concave inner surface of the photocathode and the opposing, tapering surface of the anode cone. The intensity vectors associated with this electrostatic field are directed radially from the inner surface of the photocathode to the small diameter end of the anode cone. Thus, an electron image emitted from the concave inner surface of the photocathode has a conforming curvature and is accelerated, by the electrostatic field, toward the small diameterend of the anode cone. Consequently, this curved electron image converges toward a crossover region which is centered about the axis of symmetry and located adjacent the aperture in the small diameter end of the anode cone. After passing through the crossover region and the aperture, the electron image travels longitudinally through the anode cone toward the imaging screen assembly. However, as a result of passing through the crossover region, the electron image traveling through the anode cone is inverted and has a reversed curvature which corresponds to the concave inner surface of the imaging screen assembly. Consequently, when this inverted, electron image impinges on the imaging screen assembly, the resulting visual image is inverted and has uniform resolution characteristics.

In order to increase the brightness of the output visual image, the described tube structure has been modified by locating the imaging screen assembly a greater axial distance away from the anode cone and disposing a microchannel plate in the approximate position formerly occupied by the imaging screen assembly. Generally, the microchannel plate comprises a glass disc having a plurality of through holes extending between opposing planar surfaces of the disc. Thus, electrons in the inverted image emerging from the anode cone enter respectively aligned holes in the microchannel plate and collide with the walls of the holes. Each collision produces a multiplicity of secondary electrons which, in turn, also collide with the walls of the associated holes. As a result, each electron in the incident image is multiplied many thousands of times thereby producing a corresponding image having greater electron density. Consequently, when this denser electron image is accelerated by a suitable electrostatic field and impinges on the axially aligned, imaging screen assembly, the resulting visual image is much brighter than the output image produced by an unmodified image intensifier tube of the described type.

However, the bright visual image produced by the modified image intensifier tube does not have the uniform resolution qualities of an image produced by the unmodified image intensifier tube. Some of the non-uniform resolution characteristics present in the image produced by the modified image intensifier tube are the result of placing a microchannel plate having opposing planar surfaces in the position formerly occupied by an imaging screen assembly having a concave inner surface. Since the inverted image emerging from the anode cone has a reversed curvature which corresponds to the concave inner surface of the imaging screen assembly, only an annular portion of this curved electron image can be in focus when the image is incident on the microchannel plate. As a result, a major portion of the electron image is out of focus when the microchannel plate increases the electron density of the image. Accordingly, a major portion of this denser electron image is out of focus when it impinges on the imaging screen, thereby producing a bright visual image having non-uniform resolution characteristics.

An obvious solution to this problem is to provide a microchannel plate having a concave inner surface similar to the imaging screen assembly. A suitable microchannel plate for this purpose is shown in U.S. Pat. No. 3,407,324 which is granted to M. Rome and issued on Oct. 22, 1968. However, as disclosed in the referenced patent, a microchannel plate having a concave surface will have holes which vary in length from the center of the plate to the periphery thereof. Consequently, the diameters of the respective holes must vary in accordance with the lengths in order to maintain the hole length to hole diameter ratio substantially constant across the diameter of the microchannel plate. A microchannel plate of this type is very difficult to produce and, consequently, is very expensive. A more practical solution to the problem of non-uniform resolution would not only be simple and inexpensive but also would allow some degree of adjustment to be made to meet" varying conditions within respective image intensifier tubes of the modified type.

Furthermore, when a microchannel plate is positioned between the anode cone and the imaging screen, as described, another type of non-uniform resolution, known as black spot, may be manifested in the output visible image. Thus, a specific area of the visible image, usually located off the axis, may appear darker than the'other portions of the visual display. This dark spot seems to be caused by an aligned portion of the microchannel plate failing to produce the required number of secondary electrons for maintaining uniform amplification of the incident electron image. Thus, a solution of the non-uniform resolution problems caused by the planar microchannel plate being positioned between the anode cone and the imaging screen also must include means for correcting the underlying cause of the black spot" problem.

SUMMARY OF THE INVENTION Accordingly, this invention provides an image intensifier tube having a decelerator electrode disposed between the anode cone and the microchannel, in axially aligned, spaced relationship therewith, for the pur- 4 pose of shaping the electrostatic field between the anode cone and the microchannel plate whereby the inverted electron image having a reversed curvature will be provided with a more suitable configuration and velocity for impinging on the microchannel plate.

This invention also provides a microchannel plate having a plurality of through holes extending between opposing surfaces which are coated with respective conductive films such that conductive material is deposited in an asymmetrical manner around the respective end apertures of each through hole. Preferably, the microchannel plate comprises a dielectric disc having opposing planar surfaces between which a plurality of through holes extend at an angle with each of the planar surfaces.

The preferred embodiment of this invention comprises an image intensifier tube having a generally tubular envelope closed at one end by an input faceplate having a concave inner surface which supports a conforming photocathode comprising a layer of photoemissive material. The photocathode is disposed in axial spaced relationship with a small diameter end portion of a hollow anode cone, such that a central portion of the photocathode is axially aligned with an aperture disposed in the small diameter end of the cone.

ported on the inner surface of an output faceplate of the tube and a conductive film of reflective material disposed on the inner surface of the phosphor layer, which conductive film constitutes another accelerating electrode of the tube.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of this invention, reference is made to the drawings wherein:

FIG. 1 is an axial sectional view of an image intensifier tube which embodies the decelerator electrode of this invention;

FIG. 2 is a diagrammatic representation showing a typical electrostatic field established within an image intensifier tube which is similar to the image intensifier tube shown in FIG. 1 but which does not have the decelerator electrode of this invention;

FIG. 3 is a diagrammatic representation showing a typical electrostatic field established within the image intensifier tube shown in FIG. 1;

FIG. 4 is a schematic view of the image intensifier tube shown in FIG. 1;

FIG. 5 is an enlarged fragmentary plan view of the top surface of the microchannel plate shown in FIG. 1;

FIG. 6 is an enlarged fragmentary view in axial section of the microchannel plate shown in FIG. 5, taken along the line 6-6 and looking in the direction of the arrows; and

FIG. 7 is an enlarged fragmentary view in axial section of a microchannel plate having opposing surfaces metallized symmetrically relative to respective end apertures of holes extending through the plate.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring more particularly to the drawings wherein like characters of reference designate like parts throughout the several views, there is shown in FIG. 1 an image intensifier tube 10 having a generally tubular envelope 12 which is closed at one end by a coaxially disposed, input faceplate l4 and a supporting ring 16 of conductive material, suchas kovar, for example. The faceplate 14 generally comprises a'plurality of fiber optic rods hermetically sealed in side by side relationship to form a cylindrical bundle which is provided with a substantially flat surface 18 at one end and a centrally disposed, concave surface 20 at the other end. Deposited on the concave surface 20 is a conforming photocathode 22 comprising a conductive layer of photoemissive material, such as cesium antimony, for example.

The faceplate 14 is hermetically attached by conventional means to the ring 16 such that the concave surface 20 is axially aligned with the opening in ring 16, and the circular edge of photocathode 22is disposed in electrical contact with an annular portion of ring 16 adjacent the inner periphery thereof. An annular portion of ring 16 adjacent its outer periphery is circumferentially attached, as by welding, for example, to a contiguously disposed flange 24 which constitutes the cathode terminal of tube 10.

Flange 24 extends outwardly from one end of an axially disposed cathode sleeve 30 which is made of conductive material, such as copper, for example. Transversely disposed in the wall of cathode sleeve 30 is an exhaust tubulation 28 through which the envelope 12 is evacuated during processing of the tube and which is sealed off, in a conventional manner, after processing is completed. The other end of sleeve 30 is provided with an inwardly extending, radial flange 32 having an inner rolled edge 34 which forms a centrally disposed aper- 'ture 36. The aperture 36 is axially aligned with the opening in ring 16 such that the rolled edge 34 and the inner surface of photocathode 22 form an arcuately curved, equipotential surface when a suitable voltage potential is applied to the cathode terminal 24.

Radial flange 32 is hermetically attached, by conventional means, to one end of an axially disposed, hollow cylinder 40 which is made of dielectric material, such as ceramic, for example. The other end of cylinder 40 is peripherally sealed, by suitable means, to one side of a substantially flat ring 42 which is made of conductive material, such as kovar, for example. The ring 42 constitutes the anode terminal of tube and extends inwardly of envelope 12 to support a coaxially disposed, hollow cone 44 which is made of conductive material, such as stainless steel, for example. The anode cone 44 is provided with a large diameter, open end 46 having an outwardly extending, radial flange 48 which is fixedly attached, as by welding, for example, to an annular portion of ring 42 adjacent the inner periphery thereof. Cone 44 extends longitudinally within envelope 12, in spaced relationship with the inner surface of cylinder 40 and insulatingly through the centrally disposed aperture 36 formed by the rolled edge 34. The small diameter end 50 of the cone 44 is disposed in spaced axial relationship with the inner surface of photocathode 22 and has a centrally disposed aperture 52 therein which is axially aligned with a central portion of photocathode 22.

The other side of ring 42 is circumferentially attached, by conventional means, to one end of axially disposed, hollow cylinder 54 which is made of dielectric material, such as ceramic, for example. The other end of cylinder 54 is peripherally sealed to one side ofa substantially flat ring 56 which is made of conductive material, such as kovar, for example. Ring 56 constitutes a third terminal of tube 10 and extends radially inward of envelope 12 to support a coaxially disposed, frustoconical ring 58 which constitutes the decelerator electrode of this invention. The ring 58 is made of conductive material such as stainless steel, for example, and has a large diameter open side 57 provided with an outwardly extending radial flange 59 which is fixedly attached, as by welding, for example, to an annular portion of ring 56 adjacent the inner periphery thereof. The ring 58 has an opposing small diameter, open side 55 which is disposed in spaced axial relationship with the anode cone 44 such that the tapering wall of ring 58 is aligned with the tapering wall of cone 44 and electrostatically appears to be a continuation thereof.

The large diameter end of ring 58 is disposed in spaced axial relationship with a transversely disposed, microchannel plate 60 having opposing

planar surfaces

62 and 64, respectively, and a plurality of closely spaced, through holes 63 extending therebetween. The

surfaces

62 and 64, respectively, are coated, as by evaporation, for example, with

respective films

66 and 68 of a suitable metal, such as nickel, for example. The metallized surface 62, adjacent the decelerator electrode ring 58, has an outer annular portion circumferentially attached, by suitable means, to one side of a substantially flat ring 70 of conductive material, such as kovar, for example. Ring 70 constitutes a fourth terminal of tube 10 and is circumferentially attached, by conventional means, to one end of an axially disposed, hollow cylinder 72. Cylinder 72 is made of dielectric material, such as ceramic, for example, and is peripherally sealed, by suitable means, at the other end to ring 56.

The metallized surface 64 of microchannel plate 60 has an outer annular portion circumferentially attached, by suitable means, to one side of a substantially flat ring 74 of conductive material, such as kovar, for example. Ring 74 constitutes a fifth terminal of tube 10 and is circumferentially attached, by conventional means, to one end of an axially disposed, hollow cylinder 76. Cylinder 76 is made of dielectric material, such as ceramic, for example, and is peripherally sealed, by suitable means, at the other end, to one side of a substantially flat ring 78 of conductive material, such as kovar, for example. Ring 78 constitutes the sixth terminal of tube 10 and has an inner peripheral surface hermetically sealed, by conventional means, to an outer peripheral surface of a coaxially disposed, substantially flat faceplate 80 which closes the other end of tubular envelope l2.

Faceplate 80 constitutes the output faceplate of tube 10 and may be made of a suitable transparent material, such as glass, for example. Supported on the inner surface of faceplate 80 is an imaging screen assembly 82 comprising a layer 84 of phosphor material, such as zinc cadmium sulfide, for example, which may be deposited directly on the inner surface of the faceplate 80. Disposed directly on the inner surface of the phosphor layer 84 is a conductive film 86 of light reflecting material, such as aluminum, for example. The film 86 extends radially outward beyond the perimeter of phosphor layer 84 and electrically contacts an annular portion of the terminal ring 78. Thus, the film 86 constitutes the imaging screen electrode and is transparent to accelerated electrons but reflects visible light photons emitted by the phosphor layer 84, thus enhancing the output visible image.

FIG. 2 shows an image intensifier tube 10a which has an electrode structure similar to the structure of image intensifier 10, except for the omission of the decelerator electrode ring 58 of this invention. The respective electrodes of tube are considered as being maintained at suitable direct current potentials for the purpose of illustrating typical electrostatic fields established between the electrodes. For example, the photocathode 22a and electrically connected cathode sleeve 30a may be maintained at ground potential, while the anode cone 44a may be maintained at approximately 9 kilovolts and the input side 62a of microchannel plate 60a may be maintained at approximately 4 kilovolts, above ground potential. Thus, there will be established between the concave inner surface of the photocathode 22a and the small diameter end 50a of the anode cone 44a an electrostatic field having conforming equipotential surfaces 100a which are indicative of a radially symmetrical field. Consequently, all portions of a curved electron image emitted from the concave inner surface of the photocathode 22a will be uniformly accelerated toward the small diameter end 50a of the anode cone 44a. As a result, the curved electron image will converge as it travels toward a crossover region located adjacent the aperture 52a in small diameter end 50a of the cone 44a; and, after passing through the crossover region and aperture 52a, it will diverge as an inverted image within the anode cone 44a.

An equipotential surface 102a located adjacent the aperture 52a indicates that the edge portions of the electron image will be accelerated at a faster rate than the central portion, as the image is passing through the aperture 52a. Consequently, the central portion of the image will come to a focus at a greater distance from the plane of the aperture 52a than the edge portions. Thus, the resulting image will be curved in a direction opposite to the curvature of photocathode 22a. This reverse curvature of the electron image will be further enhanced by the decelerating electrostatic field established within the anode cone 44a. As shown by the equipotential surface 104a, the central portion of the electron image will be decelerated at a faster rate than the edge portions of the image. Therefore, the central portion of the image will come to a focus at an even greater distance from the plane of the aperture 52a than the edge portions. Consequently, the resulting image will be curved even more in a direction opposite to the curvature of the photocathode 22a. However, when the electron image emerges from the large diameter, open end of the anode cone 44a, the equipotential surfaces 106a established there indicate that the edge portions of the image will be decelerated at a faster rate than the central portion. Consequently, the edge portions of the electron image will come to a focus at almost the same distance from the plane of aperture 52a as the central portion. Thus, the resulting image will be curved only slightly in the reverse direction.

Subsequently, this almost planar image will enter an accelerating electrostatic field established adjacent the input surface 620 of the microchannel plate 60a. The equipotential surfaces 110a of this field indicate that the edge portions of the electron image will be accelerated at a much faster rate than the central portion. Consequently, if the central portion of the image comes to a focus in a plane substantially coinciding with the plane of the input surface 62a, the edge portions of the image will come to a focus before reaching the surface 620 of microchannel plate 60a. As a result, when the edge portions of the electron image reach the input surface 62a, they will be out of focus and will be amplified by microchannel plate 60a in this out-of-focus condition.

FIG. 3 shows the electrode structure of image intensifier tube which includes the decelerator electrode ring 58 positioned between the large diameter end 46 of anode cone 44 and the input surface 62 of microchannel plate 60. For the purpose of comparing tube 10 with tube 10a, corresponding electrodes of the respective tubes are maintained at substantially equal voltage potentials. A low voltage potential, relative to the respective potentials of the anode cone 44 and the input surface 62 of microchannel plate 60, is applied to the decelerator electrode ring 58. For example, the decelerator electrode ring 58 may be maintained at the same potential as the photocathode 22. Consequently, an electron image emerging from the large diameter, open end 46 of anode cone 44 will enter a decelerating electrostatic field. Thus, as noted in the discussion of image intensifier tube 10a, the edge portions of the electron image will be decelerated at a faster rate than the central portion. As a result, the edge portions of the image will come to a focus at almost the same distance from the plane of aperture 52 as the central portion of the image. Thus, the resulting image will be curved slightly in the reverse direction.

The decelerator electrode ring 58 is disposed transversely between the decelerating field established adjacent the large diameter end 46 of anode cone 44 and the accelerating field established adjacent the input surface 62 of microchannel plate 60. Therefore, the configuration of the decelerator electrode ring 58 and the voltage potential applied thereto may be optimized to shape the equipotential surfaces of the adjacent decelerating and accelerating fields, respectively, as desired. Furthermore, the potential applied to the decelerator electrode ring 58 may be adjusted to comply with variable conditions or structural features within the tube. As a result, the field established within and adjacent to the aperture in ring 58, as indicated by the respective equipotential surfaces 108, will decelerate the edge portions of the electron image relative to the central portion. Consequently, the edge portions of the image will come to a focus in substantially the same plane as the central portion. Thus, the decelerator ring 58 at the potential applied thereto has the effect of eliminating the curvature in the image and provides means for obtaining a substantially planar, final image.

With the decelerator electrode ring 58 maintained at approximately photocathode potential, the resulting electrostatic field established adjacent the input surface 62 of microchannel plate 60 will have equipotential surfaces which are substantially planar. Consequently, the electron image, as modified by the field established within and adjacent to the aperture of ring 58, will be drawn axially toward the input surface 62 of plate 60. As a result, all portions of the electron image will come into focus simultaneously on the input surface 62 of plate 60. Thus, the electron density of the image will be multiplied by microchannel plate 60 while all portions of the image are in focus.

A high positive potential with respect to the potential of the input surface 62, such as 5,000 VDC, for example, is applied to the opposing metallized surface 64 of plate 60 for the purpose of establishing an accelerating field between the opposing

planar surfaces

62 and 64, respectively. Consequently, the electrons in the incident image and the associated secondary electrons generated within the respectively aligned holes 63 will be drawn toward the output surface 64 of plate 60. As a result, a correspondingly denser electron image will emerge from the holes 63 of plate 60 and enter a strong electrostatic field. This field is established by applying a very high positive potential with respect to the potential of the output surface 64, such as 10,000 VDC, for example, to the conductive film 86 of imaging screen assembly 82. Consequently, the denser electron image leaving the output surface 64 of plate 60 will be accelerated at very high velocity toward the imaging screen assembly 82 before appreciable spreading of the image can take place. Thus, the highly accelerated electron image will pass through the conductive film 86 and impinge on the underlying layer 84 of phosphor material. As a result, the material of phosphor layer 84 will emit a corresponding visible light image which may be viewed through the transparent output faceplate 80.

As shown in FIG. 4, photons of visible light may pass through the central portion of photocathode 22 and travel along an axial path, such as 112, for example. Consequently, these photons will pass through the aperture 52 in cone 44 and be reflected by the input surface 62 of plate 60 along an axial path, such as 113, for example. On the other hand, if the holes 63 in plate 60 are perpendicularly disposed with respect to the opposing

planar surfaces

62 and 64, respectively, the photons of visible light may pass through aligned holes 63 and be reflected by the inner surface of conductive film 86 back through aligned holes 63 and along an axial path, such as 113, for example. As a result, reflected light will be incident on the central portion of inner surface of photocathode 22 and will cause an emission of photoelectrons therefrom. These electrons will pass through the aperture 52 of anode cone 44 and will be multiplied by the central portion of microchannel plate 60. As a result, a bright spot will appear on the aligned central portion of the imaging screen 84.

The portion of this bright spot problem which is caused by light reflected from the input surface 62 of microchannel plate 60 may be avoided by providing the input surface 62 with a fine grind finish, such as 15 microns, for example. Thus, visible light incident on the input surface 62 will not be reflected along a path, such as 113, for example, but will be diffused and thereby dispersed within the enclosure of anode cone 44. The portion of the bright spot problem which is caused by light reflected form the conductive film 86 of imaging screen assembly 82 may be avoided by providing the microchannel plate 60 with holes 63 which extend at an angle between the opposing

planar surfaces

62 and 64, respectively, as shown in FIG. 6. Thus, the biased holes 63 in plate 60 and the anode cone 44 will provide an optical baffle which will permit the passage of electrons but prevent the passage of visible light. It has been found that holes 63 biased at an angle of to with respect to the normal, provide a satisfactory baffle and ensure the collisions of incident electrons with the walls of aligned holes 63.

However, it has been found that when a microchannel plate having biased holes extending between opposing planar surfaces is disposed between the anode cone 44 and the input surface 62 of plate 60, as described, a dark spot may appear in the visual output image. This dark spot, usually, is located at some radial distance away from the center of the visual image and seems to be caused by non-uniform amplification of the electron image by the microchannel plate 60. Investigation disclosed that electrons entering holes 63 in the portion of plate 60 aligned with the dark spot will pass through these holes without colliding with the walls thereof. It was determined that these electrons were approaching the entrance apertures of the aligned holes 63 at an angle which would produce respective collisions with the walls of the aligned holes. However, the respective trajectories of the incident electrons were being changed by a micro-lensing effect produced by metal plated symmetrically around the respective entrance apertures of the holes and having a voltage potential applied thereto.

As shown in FIG. 7, when a metal, such as nickel, for example, is deposited on the input surface 62a to form the superimposed metal film 66a, some of the metal also is deposited on the wall surfaces of the respective holes 63a, adjacent the respective entrance apertures thereof. As a result, the entrance apertures of holes 63a may be encircled by respective metal sheaths 650. When a voltage potential, such as 4,000 VDC, for example, is applied to the metallized surface 620 for the purpose of establishing an adjacent electrostatic field, as described, aligned portions 114 of the field will extend symmetrically into the entrance apertures of respective holes 630 due to the voltage potential also being applied to encircling metal sheaths 65a. Consequently, when an electron approaches the entrance aperture ofa hole 63a at an angle close to the axial centerline of the hole, the electrostatic force exerted by the portion of the field extending symmetrically with the entrance aperture may be sufficient to divert the electron into a trajectory extending parallel with the axial centerline of the hole. Thus, the electron will pass through the hole 63a without colliding with the wall thereof and producing secondary electrons. As a result, a dark spot will appear on the aligned portion of the imaging screen 84.

It was found that the black spot" problem could be avoided by depositing the metal on the input surface 62 of plate 60 in a manner which ensured that metal would be deposited asymmetrically relative to the respective entrance apertures of the holes 63, as shown in FIG. 6. Thus, when a voltage is applied to the metal film 66 disposed on surface 62 the resulting electrostatic field will not extend symmetrically into the entrance apertures of the respective holes 63. Since the voltage potential is also applied to the asymmetrically deposited metal adjacent the respective entrance apertures of the holes 63, the associated equipotential surfaces will be distorted asymmetrically also. Consequently, when an electron approaches the entrance aperture of a hole 63, at an angle close to the axial center line of the hole, it will be diverted by the asymmetrical portion of the electrostatic field established therein into a trajectory which terminates in the wall of the hole 63. Thus, the electron will collide with the wall of the hole 63 and produce a copious quantity of secondary electrons. Accordingly, these electrons will impinge on an aligned portion of the imagining screen 84 and produce a portion of the output visual image.

Thus, there has been disclosed herein a novel image intensifier tube having a decelerator electrode ring positioned between the large diameter end of an anode cone and an axially spaced microchannel plate. In the illustrative embodiment, the decelerator electrode ring 58 is provided with a frustoconical configuration which is aligned with the anode cone 44 such that the inner surface of cone 44 and the ring 58 appear electrostatically continuous and distortion of the adjacent field due to abrupt structural changes is avoided. However, the decelerator ring 58 may be provided with any other configuration that would produce equipotential surfaces having a desired shape. For example, the decelerator electrode ring 58 could be a flat ring having an inner rolled edge, such as rolled edge 34, for example, which could be axially aligned with the large diameter, open end of cone 44, as the rolled edge 34 is axially aligned with the photocathode 22, for example. Furthermore, the decelerator electrode ring 58 may have any other voltage potential applied thereto which will shape the adjacent decelerating and accelerating fields, respectively, as desired.

Thus, it will be apparent that the objectives of this invention have been achieved by the structures shown and described herein. However, it also will be apparent that various changes may be made by those skilled in the art without departing from the spirit and scope of this invention as expressed in the appended claims. It is to be understood, therefore, that all matter shown and described herein is to be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A light amplifier tube comprising:

an evacuated envelope having an input faceplate and an output faceplate;

a photocathode disposed within the envelope adjacent the input faceplate;

an imaging screen disposed within the envelope adjacent the output faceplate;

a first electrode disposed between the photocathode and the imaging screen and spaced therefrom;

a second electrode disposed between the first electrode and the imaging screen and spaced from the first electrode;

means for establishing a first electrostatic field having equipotential surfaces disposed between the photocathode and the first electrode and a second electrostatic field having equipotential surfaces disposed between the first electrode and the second electrode; and

adjustable means for modifying the equipotential surfaces of the second electrostatic field.

2. A light amplifier tube as set forth in claim 1 wherein the second electrostatic field comprises an electron decelerating field adjacent the first electrode and an electron accelerating field adjacent the second electrode. 7

3. A light amplifier tube as set forth in claim 2 wherein the adjustable means is positioned in adjoining portions of the decelerating and accelerating fields respectively.

4. A light amplifier tube as set forth in claim 3 wherein the adjustable means comprises an annular electrode having means for applying a voltage potential thereto.

5. A light amplifier tube comprising:

an evacuated envelope having opposing input and output faceplates;

a photocathode disposed within the envelope adjacent the input faceplate and axially aligned therewith;

an imaging screen disposed within the envelope adjacent the output faceplate and axially aligned with the photocathode;

an electrode sleeve disposed longitudinally between the photocathode and the imaging screen and in axially aligned spaced relationship therewith;

a microchannel plate disposed between the electrode sleeve and the imaging screen and in axially aligned spaced relationship therewith, the plate having opposing metallized surfaces and a plurality of through holes extending therebetween;

means for establishing an electrostatic field having equipotential surfaces between the electrode sleeve and the adjacent metallized surface of the microchannel plate; and

adjustable means disposed in said electrostatic field for modifying the equipotential surfaces thereof.

6. A light amplifier tube as set forth in claim 5 wherein the opposing metallized surfaces of the microchannel plate are planar.

7. A light amplifier tube as set forth in claim wherein the plurality of holes extend at a uniform angle with the opposing planar surfaces.

8. A light amplifier tube as set forth in claim 5 wherein the metallized surfaces of the microchannel plate have respective portions adjacent the end apertures of the respective holes metallized asymmetrically with respect to the axial center lines of the respective holes.

9. A light amplifier tube comprising:

an evacuated envelope having opposing input and output faceplates, the input faceplate having a concave inner surface;

a photocathode disposed on the inner surface of the input faceplate in conforming relationship therewith;

an anode cone longitudinally disposed between the photocathode and the imaging screen, the cone having a small diameter apertured end disposed in axially aligned spaced relationship with the central portion of the photocathode and an opposing large diameter open end;

a microchannel'plate disposed between the large diameter open end of the anode cone and the imaging screen and in axially aligned spaced relationship therewith, the plate having opposing planar metallized surfaces and a plurality of through holes extending at a uniform angle therebetween;

means for establishing an electrostatic field having equipotential surfaces between the large diameter end of the anode cone and the adjacent metallized surface of the microchannel plate; and adjustablemeans disposed in the electrostatic field for modifying the electrostatic surfaces thereof. 10. A light amplifier tube as set forth in claim 9 wherein the adjustable means includes an annular electrode disposed between the large diameter, open end of the cone and the adjacent metallized surface of the microchannel plate and also includes means for applying a controlled voltage potential thereto.

Claims

1. A light amplifier tube comprising: an evacuated envelope having an input faceplate and an output faceplate; a photocathode disposed within the envelope adjacent the input faceplate; an imaging screen disposed within the envelope adjacent the output faceplate; a first electrode disposed between the photocathode and the imaging screen and spaced therefrom; a second electrode disposed between the first electrode and the imaging screen and spaced from the first electrode; means for establishing a first electrostatic field having equipotential surfaces disposed between the photocathode and the first electrode and a second electrostatic field having equipotential surfaces disposed between the first electrode and the second electrode; and adjustable means for modifying the equipotential surfaces of the second electrostatic field.

2. A light amplifier tube as set forth in claim 1 wherein the second electrostatic field comprises an electron decelerating field adjacent the first electrode and an electron accelerating field adjacent the second electrode.

5. A light amplifier tube comprising: an evacuated envelope having opposing input and output faceplates; a photocathode disposed within the envelope adjacent the input faceplate and axially aligned therewith; an imaging screen disposed within the envelope adjacent the output faceplate and axially aligned with the photocathode; an electrode sleeve disposed longitudinally between the photocathode and the imaging screen and in axially aligned spaced relationship therewith; a microchannel plate disposed between the electrode sleeve and the imaging screen and in axially aligned spaced relationship therewith, the plate having opposing metallized surfaces and a plurality of through holes extending therebetween; means for establishing an electrostatic field having equipotential surfaces between the electrode sleeve and the adjacent metallized surface of the microchannel plate; and adjustable means disposed in said electrostatic field for modifying the equipotential surfaces thereof.

7. A light amplifier tube as set forth in claim 5 wherein the plurality of holes extend at a uniform angle with the opposing planar surfaces.

9. A light amplifier tube comprising: an evacuated envelope having opposing input and output faceplates, the input faceplate having a concAve inner surface; a photocathode disposed on the inner surface of the input faceplate in conforming relationship therewith; an imaging screen disposed within the envelope adjacent the output faceplate and axially aligned with the photocathode; an anode cone longitudinally disposed between the photocathode and the imaging screen, the cone having a small diameter apertured end disposed in axially aligned spaced relationship with the central portion of the photocathode and an opposing large diameter open end; a microchannel plate disposed between the large diameter open end of the anode cone and the imaging screen and in axially aligned spaced relationship therewith, the plate having opposing planar metallized surfaces and a plurality of through holes extending at a uniform angle therebetween; means for establishing an electrostatic field having equipotential surfaces between the large diameter end of the anode cone and the adjacent metallized surface of the microchannel plate; and adjustable means disposed in the electrostatic field for modifying the electrostatic surfaces thereof.

10. A light amplifier tube as set forth in claim 9 wherein the adjustable means includes an annular electrode disposed between the large diameter, open end of the cone and the adjacent metallized surface of the microchannel plate and also includes means for applying a controlled voltage potential thereto.