U.S. patent number 3,708,673 [Application Number 05/154,243] was granted by the patent office on 1973-01-02 for image intensifier tube.
This patent grant is currently assigned to The Machlett Laboratories, Incorporated. Invention is credited to Allen Palmer Blacker, Jr..
United States Patent |
3,708,673 |
Blacker, Jr. |
January 2, 1973 |
IMAGE INTENSIFIER TUBE
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.
Inventors: |
Blacker, Jr.; Allen Palmer (New
Milford, CT) |
Assignee: |
The Machlett Laboratories,
Incorporated (Springdale, CT)
|
Family
ID: |
22550588 |
Appl.
No.: |
05/154,243 |
Filed: |
June 10, 1971 |
Current U.S.
Class: |
250/214VT;
313/528; 313/534; 315/11 |
Current CPC
Class: |
H01J
31/507 (20130101); H01J 31/501 (20130101) |
Current International
Class: |
H01J
31/08 (20060101); H01J 31/50 (20060101); H01j
031/50 () |
Field of
Search: |
;250/213VT
;313/95,103,104,105 ;315/11 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Sebastian; Leland A.
Claims
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.
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 5 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 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.
Description
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 diameter end 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 purpose 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. The anode cone extends axially within the tube
envelope and has a large diameter, open end disposed in axially
aligned, spaced relationship with an annular decelerator electrode.
Axially spaced from the decelerator electrode and aligned therewith
is a microchannel plate having opposing metallized surfaces which
constitute respective accelerating electrodes of the tube. Disposed
in axial spaced relationship with the microchannel plate is an
imaging screen assembly comprising a layer of phosphor material
supported 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 14 and a supporting ring 16 of
conductive material, such as 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
22 is 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 aperture 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 10 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 of a 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 12.
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 10a 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 62a 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 62a 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 10
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 110 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 5.degree. to
10.degree., 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 65a. When a voltage
potential, such as 4,000 VDC, for example, is applied to the
metallized surface 62a 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 63a due to the voltage potential also being
applied to encircling metal sheaths 65a. Consequently, when an
electron approaches the entrance aperture of a 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
115 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.
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