U.S. patent number 3,777,201 [Application Number 05/314,105] was granted by the patent office on 1973-12-04 for light amplifier tube having an ion and low energy electron trapping means.
This patent grant is currently assigned to Litton Systems, Inc.. Invention is credited to Bernard Caesar Einstein.
United States Patent |
3,777,201 |
Einstein |
December 4, 1973 |
LIGHT AMPLIFIER TUBE HAVING AN ION AND LOW ENERGY ELECTRON TRAPPING
MEANS
Abstract
A light amplification device such as an image intensifier or low
level sensing device is disclosed which includes a photocathode
spaced from an aluminized target electrode and a microchannel plate
intermediate said cathode and target. A thin non-self-supporting
substantially optically transparent layer of material of a
substance and thickness so as to be essentially transparent to high
energy electrons, on the order of 100 to 1,000 electron volts, and
light is situated atop the front end of the microchannel plate,
covering the passages therein, in order to trap ions, which
otherwise would travel to the photocathode, neutral gas ions, and
to absorb scattered low energy electrons generated by secondary
emission at the rim portion of the individual tubes in said
microchannel plate, which would otherwise travel into the
microchannel plate passages and to pass any light which passes
through the photocathode and transmit any light which penetrates
through the photocathode. The microchannel plate is spaced by a
predetermined first distance from the photocathode, with its
covered end facing the photocathode, and is spaced by a second
distance, larger than the first distance, from the aluminized
target electrode.
Inventors: |
Einstein; Bernard Caesar
(Redwood Estates, CA) |
Assignee: |
Litton Systems, Inc. (San
Carlos, CA)
|
Family
ID: |
23218576 |
Appl.
No.: |
05/314,105 |
Filed: |
December 11, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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237343 |
Mar 23, 1972 |
|
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124107 |
Mar 15, 1971 |
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Current U.S.
Class: |
313/526;
250/214VT; 313/103CM; 313/103R; 313/528 |
Current CPC
Class: |
H01J
31/507 (20130101); H01J 43/246 (20130101) |
Current International
Class: |
H01J
31/50 (20060101); H01J 31/08 (20060101); H01j
031/26 (); H01j 031/50 (); H01j 043/00 () |
Field of
Search: |
;313/65R,68R,94,96,105,103 ;250/213 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lawrence; James W.
Assistant Examiner: Grigsby; T. N.
Parent Case Text
This application is a division of application, Ser. No. 237,343,
filed Mar. 23, 1972.
This application is a continuation-in-part of my earlier filed
application for patent, Ser. No. 124,107, filed Mar. 15, 1971.
Claims
What is claimed is:
1. An image intensifier which includes:
a phosphor display screen of a predetermined area;
a metal backing layer covering the rear surface of said phosphor
display screen;
a photocathode for emitting electrons in response to incident
light;
a microchannel plate type electron multiplying element located
between said display screen and said photocathode, having a front
input side facing said photocathode and a rear output side facing
the back surface of said display screen, said electron multiplying
element being spaced from said photocathode by a first
predetermined distance within the range of 0.005-inches to
0.020-inches and being spaced from said display screen by a second
predetermined distance greater than said first predetermined
distance in the range of 0.030-inches to 0.050-inches, and a thin
non-self-supporting substantially optically transparent layer of
material coupled to and covering the front end of said microchannel
plate, including the passages therein.
2. The invention as defined in claim 1 wherein said
non-self-supporting layer is of a thickness in the range of 50 A to
400 A.
3. The invention as defined in claim 2 wherein said material
possesses a secondary emission characteristic of at least 3.0
measured at 400 volts.
4. The invention as defined in claim 2 wherein said material
possesses a specific gravity in the range of 1.0 to 4.0.
5. The invention as defined in claim 2 wherein said material
comprises the metal aluminum.
6. The invention as defined in claim 2 wherein said material
consists of a member selected from the group consisting of
aluminum, boron, beryllium, boron carbide, silicon oxide, silicon
dioxide, magnesium oxide, magnesium fluoride, and aluminum oxide.
Description
This invention relates to light amplification devices and, more
particularly, to image intensifiers and low light level sensing
devices.
BACKGROUND OF THE INVENTION
Light amplification devices and, particularly, image intensifiers
and low light level sensing devices find application for
surveillance in all situations where limited lighting is available
and, particularly, in those applications where the only available
light is moonlight or starlight, lighting conditions which to the
ordinary person with the unassisted eye are usually considered
almost complete darkness. Under these conditions a prime
application of such apparatus is to detect intruders in civilian
area surveillance or to detect and locate enemy personnel in
battlefield surveillance. The apparatus can be designed to permit
the information to be acted upon locally or it can be designed to
electronically relay the detected images by means of conventional
and well-known television apparatus to a remote point.
In one conventional apparatus for those applications light
amplification devices are incorporated within and made an integral
part of conventional television image transmitting camera tubes,
such as the vidicon or image orthocon. In such a camera tube the
light amplification device serves as the "front end" which first
detects and amplifies an image and places the amplified image upon
an output target electrode. The target electrode, in turn, is
serially scanned in the camera portion and the image is converted
into a series of serial electrical signals which may be transmitted
to a remote point by cable or radio and detected and displayed upon
the cathode ray tube of a television receiver.
Another conventional apparatus for the use heretofore mentioned
incorporates the light amplification device in combination with
optical binoculars or "goggles" for direct viewing so that guard or
field personnel can observe areas under their personnel
surveillance under conditions of almost complete darkness and act
upon that information.
In still other equipments light amplifying devices can be
incorporated within cathode ray tubes to enhance the intensity of a
reproduced image to be displayed and reference may be made to prior
art sources for specific examples.
Basically, the operative elements of one common type of light
amplification device are found within an evacuated (in-vacuum)
housing or tube with an optically transparent front window. The
images received at the window are directed to a photocathode, a
material which emits electrons in proportion to the intensity of
light incident thereon. The light image or pattern is projected
upon a predetermined area of the photocathode surface and a
corresponding pattern of charges or electron currents are generated
in the photocathode. The electrons of this mosaic of electron
intensities are directed under the influence of a supplied
electrostatic field to an electron multiplying element. The
electron multiplying element increases the number of traveling
electrons and at its output provides a corresponding mosaic or
image of electron currents. Under the influence of a second
provided electrostatic field these electrons are accelerated either
directly or through an electron lens focusing system toward a
target, suitably phosphor, which, because of the electron
multiplication, provides a dislayed or otherwise usable image of a
greater light intensity than the image received at tube window.
In one type of light amplifier device the electron multiplying
element is a transmission-emission dynode. The
transmission-emission dynode is of a high secondary emission
material. Thus those electrons traveling from the photocathode and
incident upon the dynode cause the re-emission of a larger number
of secondary electrons.
A second conventional electron multiplying element uses as an
alternative to the dynode a microchannel plate. The microchannel
plate comprises a bundle of very small cylindrical tubes packed
together parallel and forming essentially a thick layer of material
with a very large number of passages or openings through the layer.
The inner passage walls are coated with a high secondary emission
material and an electric static field is placed thereacross. The
end surfaces of the plate are coated with a layer of electrically
conductive material which serve as electrodes. This coating does
not cover the openings. Electrons traveling from a particular
location on the photocathode are directed by the electrostatic
field to and enter a correspondingly located passage in the
microchannel plate, and is incident upon the passage walls. Since
the walls of each passage are coated or formed with a material
having a characteristic high coefficient of secondary emission the
incident electron knocks out, re-emits, from the wall surface at
least two more electrons and they, in turn, travel in a general
direction toward the end of the tube. By design, the length and
diameter of the passage is such that these electrons, in turn,
again strike the passage walls at subsequent locations and increase
further the number of electrons traveling toward the end of the
passage. The increased number of electrons, hence "amplified"
electron intensity, exits from the individual passage in the
microchannel plate. Under the influence of another supplied
electrostatic field the exiting electrons are accelerated toward a
corresponding location on the target electrode, typically a
phosphor screen. By similar action at all other locations on the
photocathode and microchannel plate, a visual image or mosaic
representative of the original image received by the amplifier tube
and first focused on the photocathode is displayed upon the target
electrode.
By way of example of light amplifier devices and the possible
arrangements of the basic photocathode-electron multiplier-target
electrode and the additions and variations thereto and thereof the
following U.S. Pat. Nos. can be considered: 3,497,759; 3,480,782;
3,478,213; 3,346,752; 3,345,534; 3,440,470; 3,387,137; 3,513,345;
3,528,101; and 2,903,596.
Because of its potentially greater amplification and performance,
light amplifier devices using a microchannel plate as the electron
multiplying element are the present day choice in "second
generation" light amplifier tube structures. The device, however,
has heretofore had several significant limitations.
One limitation in the microchannel plate type light amplifier is
contrast resolution. As previously described, the microchannel
plate element consists of a bundle of small tubes forming passages.
Each of these tubes has a rim or edge surrounding the passage
opening. Even with the multitude of passages through the plate the
area taken up by the tube edges is on the order of fifty percent.
Hence, electrons traveling toward the microchannel plate are
incident sometimes upon the tube edges. Such electrons are bounced
back or collide with other electrons at the plate surface and knock
out from the surface one or more such electrons. These electrons
first move in a direction against the now opposed electrostatic
field where they are decelerated and then are turned around under
the influence of such electric field and then accelerated into the
tube passages in the microchannel plate. In passing into the
microchannel plate these electrons act like any other electron in
normal operation as previously described. Unfortunately, there is
an uncertainty as to into which one or more of the tube passages
all or any one of such electrons will proceed. In being knocked out
from an edge surface of one tube at a random velocity and direction
the electron when turned around under the influence of the electric
field may go through that particular tube or any one of the closely
packed adjacent tubes.
The reproduction quality of photographic image reproduction
requires that a point of light received from one location be
reproduced or "imaged" at a second location as a point. If, for
some reason due to defects in the optical system or otherwise, the
light emitted from a point is "scattered" and reproduced at several
closely adjacent points essentially a "blur" is obtained. The limit
of resolution can be determined in one manner by locating another
point of light closely adjacent the first light point and moving
the two together. The reproduced light images should be
distinguishable and it is apparent that if the two light sources
show up at the image location as one single blur that the distance
between light sources at which this occurs is a limit to the
resolution capability of the imaging system.
The electrons emitted from particular spacial locations on the
photocathode surface in the light amplifiers may be considered
analogous to the previously described light points. Thus, if the
electrons generated at the microchannel plate surface by electron
collisions can flow into one or more adjacent passages somewhat
randomly the output image of the intensifier essentially appears
blurred. This means that the point source of electrons is not 100
percent accurately reproduced at the image location and that, with
respect to the transfer function defined by the MCP resolution,
degradation in performance results.
This characteristic is specified by those skilled in the art in
terms of the number of "raster" or "TV" lines per unit of raster
height which can be placed upon the display screen and be
discernible. As the number of lines per unit height are increased
the more densely packed together these lines become until the limit
of resolution is reached: the point at which time these adjacent
lines merge together or blur and it is impossible to determine the
edge of any one line and the beginning of any space between lines.
As a result of the problem previously discussed present light
amplifier devices have typically maximum resolution capabilities on
the order of 400 TV lines per unit of raster height with the raster
height, in turn, being specified as four-tenths of 1 inch.
A further limitation is inherent. As is apparent to those familiar
with television type pickup and display apparatus, there are
instances, however infrequent, where an electron current instead of
colliding with the wall and causing the emission of other electrons
will, instead, cause the desorption of an atom as a positively
charged ion, an atomic particle that is of a mass several hundred
times larger than an electron. In the light amplifier device the
original electrons are directed to the microchannel plate under the
influence of a large electric field the latter of which by the
convention adopted points in a direction from the photocathode to
the microchannel plate. This same electric field, however, is
oriented such as to accelerate any such large mass positive ions
for travel in a direction toward the photocathode. Thus the
positive ions, when generated, travel to and collide with the
photocathode with deleterious effect. The structure and method of
fabrication of microchannel plates is such that those positive ions
derived from the microchannel plate structure may be, for example,
water ions, H.sub.2 O.sup.+ or Cesium ions, Cs.sup.+. In colliding
with the photocathode consisting of a different chemical substance,
such as S-20, the ion can combine with the photocathode material to
form compounds that do not possess photocathode properties. And in
colliding the kinetic energy released by the ion erodes the
photocathode mechanically. Both due to the release of large kinetic
energy at the photocathode and the chemical changes caused to the
photocathode, the photocathode deteriorates resulting in a serious
loss of sensitivity in the amplifier tube, evidenced by a faded
picture with diminishing brightness and diminishing contrast. As a
result, the normal operating life of the presently available
microchannel type intensifier tubes in which images of acceptable
quality are provided may be on the order of 50 to 100 hours.
The ion bombardment problem thus described is not significant in
many of those light amplifying devices which use the
transmission-emission dynode electron multiplier structure. This is
because the dynode acts as a trap for any ions generated due to
electron incidence upon other elements in the tube and the
proximity of the dynode to the photocathode precludes large ion
velocities. However, because of the other advantages of the
microchannel plate, primarily substantially higher gain, the light
amplifying devices using the microchannel plate are superior and
are preferred.
Ion bombardment problems are not unique to light amplifiers and are
recognized in the prior art with various means heretofore devised
or proposed to minimize or eliminate the problem. One common
example is provided in television cathode ray tubes in which a
magnetic field diverts the traveling positive ion to the side of
the tube envelope. Absent this diversion the positive ion would
proceed instead to the phosphor faceplate and gradually erode a
spot in the middle of the screen. Another example of a type of ion
trap appears in a direct view light amplifier as disclosed in U.S.
Pat. No. 3,350,594, issued Oct. 31, 1967, to Davis, illustrating a
light amplifier device of the "first generation" type which does
not include a microchannel plate or equivalent electron multiplying
element. The approach therein suggested is to coat the backside of
the phosphor display screen with a porous coating of aluminum atop
the normal aluminum layer conventionally applied to the back of the
screen for other purposes, the object being to capture any positive
ions produced at the phosphor screen within the porous layer so
that they cannot travel back toward the photocathode. This
structure appears to be necessitated because the light amplifying
device there shown simply does not incorporate any electron
multiplier or multiplying elements such as a dynode or microchannel
plate, which elements would necessarily provide a large obstacle to
the travel of the positive ions from the phosphor screen back to
the photocathode which, if included, minimizes this problem.
It also appears known to merely place a thick metal layer on and at
the front end of a microchannel plate merely to stop or trap
positive ions. In the specification of U.S. Pat. No. 3,603,832 a
low light level amplifier tube structure is disclosed in which it
is proposed to remove the conventional thick light opaque aluminum
backing, the light and ion trap, located on the back of the
phosphor target electrode or display screen, as variously termed, a
combination referred to as an "aluminized" screen located spaced in
back of the rear end of the microchannel plate, and to place that
thick metal layer instead as an alternative at the front end of the
microchannel plate atop an insulator layer. In so doing, patent
U.S. Pat. No. 3,603,832 notes that the metal backing as applied to
the phosphor display screen in prior devices requires a very high
voltage, suitably on the order of 5,000 volts, to give those
electrons exiting the rear end of the microchannel plate sufficient
kinetic energy and momentum to pass through the thick metal layer
backing to the phosphors of the display screen and the application
of such a large voltage requires a large physical spacing between
the display screen and the microchannel plate to avoid destructive
voltage arc-overs. Further according to that patent, the large
physical spacing between the aluminized display screen and the
microchannel plate reduced the light output intensity from the
phosphor display screen and caused other undesired optical effects.
As a compromise the patent hence proposed to remove the thick
aluminum layer from the phosphor display screen, which permits a
lower accelerating voltage between the microchannel plate and the
phosphor display screen, from 5,000 volts down to 1,500 volts, by
example, and position the display screen more closely to the exit
end of the microchannel plate. Hence the light intensity output
from the electron bombarded phosphors in the display screen is the
same as or better than in preceding devices. Because some ion trap
is necessary the cited patent suggests locating the thick metal
layer at the front end of the microchannel plate and this required
additional modifications to the prior devices. Namely as taught in
the cited patent the physical distance between the front end of the
microchannel plate and the photocathode is increased and the
accelerating voltage between the photocathode and the front end of
the microchannel plate is also increased, from a low voltage of
perhaps 1,000 volts to a high voltage of 5,000 volts, in order to
sufficiently accelerate electrons to pass through the
"repositioned" thick metal layer, just as in the case of the prior
art device where the metal layer was located on the back of the
phosphor display screen. In so doing, U.S. Pat. No. 3,603,832
effects a compromise or selection in location of a tube element,
the thick metal layer, rather than proposing an entirely new
device. There is no disclosure that a metal layer can be placed on
both the front end of the microchannel plate and the back of the
phosphor display screen or that the distance between photocathode
and microchannel plate should remain small and the accelerating
voltage therebetween should remain low, so that the low energy
secondary electrons created on the input end surface of the
microchannel plate can be reabsorbed by the trap layer and increase
resolution, and the number of elastically scattered secondary
electrons, which do have the higher energy and can penetrate into
randomly located adjacent microchannel plate holes, is kept to a
minimum; or that low energy electrons can be absorbed by a metal
layer in combination therewith to provide substantially increased
contrast resolution and noise reduction and whereas by increasing
the voltage between the photocathode and microchannel plate more
higher energy secondary electrons are being generated which would
not be absorbed by the metal layer and which would therefore
decrease contrast resolution.
A further limitation is inherent in the nature of the photocathode
itself. While characterized as an opaque element, considered
quantitatively it is approximately 90 percent opaque and can
actually transmit as much as 10 percent of the incident light.
Should light pass through the photocathode and be incident upon the
metal layer located at the front end of the microchannel plate,
that light can be reflected from the metal layer back to the
photocathode and circulates between the metal plate and the
photocathode to generate improperly positioned electrons by further
photocathode emission. And thus a point source of light becomes
displayed as an enlarged point on the phosphor screen, a phenomenon
characterized as "blooming."
OBJECTS OF THE INVENTION
Accordingly, it is an object of the invention to provide an
improved light amplifier device.
It is an additional object of the invention to provide a light
amplifying device having improved contrast resolution
capabilities.
It is a still additional object of the invention to provide a new
light amplifier device having improved life and resolution
capabilities and avoids blooming.
It is a still further object of the invention to minimize or
eliminate entirely in a light amplifier tube positive ion
bombardment of the photocathode and eliminate low energy electrons
without reducing the amplifier gain.
And it is still another object of my invention to increase the
operational life of and the quality of performance during that life
of a light amplifier tube.
SUMMARY OF THE INVENTION
In accordance with the foregoing objects of the invention, the
invention comprises in a light amplification device a photocathode
spaced from a target electrode, suitably a phosphor screen backed
by a thick metal layer, and a microchannel plate type of electron
multiplier between the cathode and screen, with the plate spaced
more near to the cathode than to the screen. A very thin optically
transparent layer of a material having a low atomic mass is
situated atop and covers the front end of the microchannel plate
which confronts the photocathode. The layer is of a thickness in
the range of 50 to 400 A and, in one example, comprises the metal
aluminum. The front of the microchannel plate is positioned close
to the photocathode and a voltage in the range of 200 to 1,000
volts is applied therebetween to establish an electric field
gradient, within the range of 1 .times. 10.sup.4 volts/cm to 4
.times. 10.sup.4 volts/cm therebetween. A substantially larger
voltage in the range of 3,000 to 8,000 volts is applied between the
rear end of the microchannel plate and the metal backed phosphor
screen to establish therebetween an electric field gradient,
suitably in the range of 3 .times. 10.sup.4 to 6 .times. 10.sup.4
volts/cm. In accordance with the invention, the covering metal
layer absorbs or dissipates scattered low energy electrons
generated by secondary emission at the front surface of the
microchannel plate and thereby prevents such electrons from
entering the plate passages. Additionally, the covering layer acts
as a barrier to positive ions traveling from the microchannel plate
in the direction of the photocathode and to any existent neutral
gas atoms. And any light which passes through the substantially
opaque photocathode is permitted to pass through the metal layer
rather than allowed to create multiple reflections in the space
between the photocathode and metal layer.
The foregoing and other objects and advantages of my invention
together with modifications, substitutions and equivalents thereof
and other variations and additional advantages thereto become more
readily apparent from consideration of the following detailed
description together with the figures of the drawing in which:
DESCRIPTION OF DRAWINGS
FIG. 1 illustrates symbolically a light amplifier device which
embodies the principles of my invention;
FIG. 2 illustrates pictorially a small section, A, of the
microchannel plate and metal layer used in connection with the
illustrated embodiment of FIG. 1; and
FIG. 3 illustrates schematically the stpes of manufacturing a
microchannel plate and layer combination in accordance with a novel
method.
DETAILED DESCRIPTION OF INVENTION
Inasmuch as all elements and details of light amplifiers, other
than the improved element, are conventional and known to those
skilled in the art, FIG. 1 illustrates the basic elements of a
direct view type light amplifier device, which incorporates the
invention, in symbolic form. The light amplifier includes an
envelope or housing 1 represented by the dashed lines, suitably
glass or ceramic materials, and the inside of the housing is in
vacuum. The housing includes an optically transparent faceplate 2,
represented by dashed lines, with which to permit entry of an
optical image and, in the direct view tube, an optically
transparent rear window 4 through which to view the displayed
"amplified" image.
The conventional electron or light optical system 3 is indicated by
the dashed lines. This element as is well known can simply be a
transparent space with which to allow the light image to pass, or a
complicated, though conventional, structure for converting the
received light image into a source of electrons representative of
that image, i.e., an electron image and then into an intermediate
display. Additionally, a conventional optical lens focusing system,
not illustrated, may be located in a conventional manner in front
of faceplate 2.
A photocathode 5 is situated at the front end of the tube to
receive the light image upon its surface. Photocathode 5 is a
circular disk having a predetermined area constructed and supported
within the tube envelope in a well-known manner. Suitable
photocathode materials are cesium and antimony and the preferred
material is sodium potassium--cesium antimony combination,
commercially sold under the designation S-20.
A target or display electrode 13 is located at the rear of the
envelope 1. By target electrode I refer generically to the last
electron receiving electrode in the light amplifier tube, whether
it be a direct display type or storage type of tube. As is
conventional in a display type of light amplifier the target
electrode 13, usually referred to as the "screen" in a direct view
tube, is usually of a circular disk-shaped geometry and consists of
a coating of an electro-luminescent material, such as P-20
phosphor. Suitably the phosphor target is formed as a coating on
the tube window 4. In turn the phosphor is conventionally backed or
coated with a thick electron permeable layer of aluminum 16,
suitably 500 to 1,000 A in thickness, to enhance its electrical
conductivity and function as an electrode and to function as an ion
trap and a light trap.
Spaced from and located in between photocathode 5 and target 13,
and supported by conventional means, not illustrated, is a
microchannel plate 7. The microchannel plate is a conventional type
of electron multiplier and is cylindrical in geometry. Microchannel
plate 7 consists typically of a plurality of glass tubes of small
diameter closely packed together and fused into a unit. In the
conventional case in excess of 100,000 individual tubes are
incorporated within and make up this plate. Each of the tubes
contains a passage 9 therethrough opening on both the front and
back faces or sides of the plate. Typically, the diameter of this
passage is on the order of 2 mils and the openings approximate
fifty percent of the total face area. By conventional techniques
the outer edges or rims of these tubes on both front and back sides
are coated with a conductive metal, lead or lead oxide, not
illustrated, to form electrically conductive end surfaces for the
microchannel plate and so as to place all the tube ends on the
front and on the back sides, respectively, electrically in common.
The inside walls 11 of the glass tubes in microchannel plate 7 are
coated with a highly resistive electrically conductive material and
a high secondary emission coating. Usually this comprises a lead
and lead oxide coating conventionally produced by hydrogen
reduction of lead oxide glass. While the internal coating is
considered electrically conductive it is highly resistive and on
the order of 100 megaohms, and thus is not an electrical short
circuit.
The embodiment of FIG. 1 may be modified to include an electron
focusing arrangement in the space between the microchannel plate
and target 13. Such elements are conventional and can be included
by choice without departing from the invention.
Suitable conductor means symbolically illustrated in FIG. 1, and
labeled 8, 10, 12, and 14, provide electrically conductive paths
from the photocathode 5, front end of microchannel plate 7, back
end of microchannel plate 7, and target 13, respectively, to
corresponding terminals on the exterior of tube envelope 1.
A thin film or layer 15 of metal, suitably aluminum, is attached or
coupled to the front end of microchannel plate 7 and covers the
entire front surface and all the open ends of passages 9. The film
layer is of a thickness, d3, preferably in the range of 50 to 400
A, or 75 A by way of specific example, and is of a material which
has a low atomic mass or low specific gravity of 1.0 to 4.0.
Suitably the layer 15 comprises aluminum which in this dimension is
"substantially" transparent to electrons having energies in excess
of several hundred electron volts while opaque, absorptive, or
dissipative, however effected, of electrons having energies of less
than 20 electron volts and is transparent substantially to light.
In addition the aluminum layer forms a positive barrier to
relatively large mass large volume positive ions and neutral gas
atoms. One obvious addition to be noted at this point is to provide
in addition a very thin aluminum oxide skin on the aluminum layer
on the underside abutting the front face of the plate. The front of
the layered microchannel plate is spaced by a distance, d1, as
close as is practical to the rear of photocathode; within the range
of 0.005-inches to 0.020-inches, and 0.012-inches by way of
specific example. In one example the length of the microchannel
plate with the layer is approximately 0.025-inches in length front
to back. And the distance between the rear end of the microchannel
plate and the metal backed phospher display screen 13, d2, is
within the range of 0.030-inches to 0.050-inches preferably and is
0.038-inches in this specific example.
For operation of the light amplifier suitable sources of electrical
energy or "bias" supplies are provided and symbolically illustrated
as batteries in FIG. 1. A battery 17 has its positive polarity
output connected to lead 10 and its negative polarity output
connected to lead 8 to place the battery voltage between the
photocathode and the microchannel plate. Typically the voltage of
this supply is on the order of 400 to 1,000 volts, particularly 600
volts, in order to establish an electrostatic field of a
predetermined intensity gradient between the front face of the
microchannel plate and photocathode 5, which field is represented
symbolically in FIG. 1 by the arrow labeled E1. Such a gradient E1
is equal to V1/d1. The gradient E1 is preferably on the order of 32
.times. 10.sup.3 volts per centimeter within the range of 15
.times. 10.sup.3 volts/cm to 40 .times. 10.sup.3 volts/cm. A second
bias source voltage is represented by battery 19. Battery 19 has
its positive polarity terminal connected to lead 12 and its
negative polarity terminal connected to lead 10 of the light
amplifier. Battery 19 may be of a voltage on the order of 300 to
1,000 volts, typically 800 volts. This places the battery voltage
between the front and rear ends of the microchannel plate and
establishes an electric field of predetermined intensity between
the front and back ends of microchannel plate 7 in a direction
toward the back of plate 7. This electric field is represented by
the arrow and labeled E2 in FIG. 1 and is typically 12.6 .times.
10.sup.3 volts per centimeter. A third bias source voltage is
represented by battery 21 in FIG. 1. Battery 21 has its positive
polarity terminal connected to lead 14 and its negative polarity
terminal connected to lead 12 of the light amplifier. This places
the voltage of battery 21 between the rear of the microchannel
plate and the aluminized phosphor screen 13. Battery 21 provides
voltages on the order of 3,000 to 8,000 volts, typically 5,000
volts, which establishes a predetermined electric field between and
in a direction from the back end of microchannel plate 7 to
aluminized phosphor display screen 13. This field is represented in
FIG. 1 by the arrow and symbol E3 and is preferably within the
range of 3 .times. 10.sup.4 to 7 .times. 10.sup.4 volts/cm. In one
specific example the gradient E3 is 5 .times. 10.sup.4
volts/cm.
An exploded view of a segment of the front end of microchannel
plate cut out by the dashed lines in FIG. 1, which are labeled A,
is presented in FIG. 2 to assist in the explanation of the
operation and effects of invention. Thus FIG. 2 illustrates the
microchannel plate 7, several of the individual tubes 9 which form
the microchannel plate, the walls 11 of tubes 9, and the layer 15
situated atop microchannel plate 7.
As previously described, aluminum layer 15 is of a thickness of 75
A. Layers of this minute thickness are not self-supporting and
would crumble and fall apart if an attempt were made to form such a
layer, lift it, and place it upon the microchannel plate.
Accordingly special techniques are necessary to satisfactorily
couple the aluminum layer to one surface of plate 15.
One alternative is to form the layer in place on the microchannel
plate by a conventional phosphor filming technique. Such a
technique requires the immersion in water during processing of the
plate but does not require the film 15 to be self-supporting.
A second approach is to take a thick self-supporting film of
aluminum oxide produced by conventional means such as anodization
and then evaporate aluminum onto the front side of the aluminum
oxide film. This layer can then be placed atop the microchannel
plate. For adhesion the film and plate may be fused together by
simply passing electrical current between the film and the plate.
In this the aluminum oxide remains and serves to give increased
transmission-secondary emission multiplication.
I prefer, however, to fabricate the aluminum film by the novel
method illustrated in connection with FIG. 3. In this method a thin
self-supporting film of lacquer (nitrocellulose) is first formed by
conventional techniques. This step is represented in FIG. 3 as
block 22. The lacquer film is then taken and placed atop the
microchannel plate as represented by block 23. Preferably prior to
placing the lacquer film in place I connect a vacuum pump to the
back side of the microchannel plate in order to produce a suction
at the front surface as is represented by the dashed lines of block
24. When the lacquer film is placed atop the microchannel plate the
vacuum assists holding this film in place. Because the lacquer
material is very thin and electrostatically charged the lacquer
film adheres to the microchannel plate immediately and the vacuum
pump is removed. The microchannel plate with lacquer layer is then
placed in a conventional bell jar for aluminum deposition.
By conventional means such as measurement of quartz crystal
oscillation frequency as a function of aluminum deposition the
desired thickness of aluminum is then evaporated atop the lacquer
film as is represented by block 25. In this way the thin aluminum
coating which is not self-supporting is maintained as a layer by
the thin self-supporting lacquer layer. Subsequently, the entire
assembly is then placed in an oven where the assembly is baked in
an air atmosphere at a temperature of about 325.degree. Centigrade
for approximately one to two hours as is represented by block 26.
It is noted that this and other processing may permit the aluminum
to oxidize on its surfaces, forming aluminum oxide, an electrical
insulator. While unconfirmed, as hereinafter becomes apparent the
existence of the oxide does not detract from and possibly enhances
the operation of the invention.
The lacquer vaporizes during baking and disappears while the
aluminum layer sinks down into place atop the microchannel plate.
Normal electrostatic forces assist in retaining the aluminum in
place. The aluminum is in electrical contact with the electrode
coating on the front surface of the microchannel plate.
In operation an image is received at the front faceplate 2 of the
light amplifier 1 illustrated in FIG. 1, and this image is directed
upon the surface of photocathode 5. Photocathode materials produce
electron currents in proportion to the magnitude of the incident
light. Thus photocathode 5 at its output back surface produces an
electron image or, as otherwise stated, an image of electron
currents. Electric field E1, where E1 = V1/d1, accelerates all the
electrons toward the electron multiplying structure, namely,
microchannel plate 7 and aluminum layer 15. These electrons are
accelerated through the voltage V1. Upon reaching aluminum layer 15
the electrons have been accelerated up to an energy level of 400 to
1,000 electron volts corresponding to the voltage applied between
photocathode 5 and microchannel plate 7.
As previously discussed the thickness and substance of aluminum
layer 15 is such as to make the layer "effectively" transparent to
electrons of such high energy levels in that such electrons either
pass through the aluminum layer 15 or knock out a corresponding
electron and the electron travels forward into a correspondingly
located one of the tube passages 9.
Otherwise stated, the electrons which are generated due to electron
collisions with the front surface of the microchannel plate are low
energy level electrons typically on the order of 3 to 5 electron
volts, a rather small energy level in contrast to the approximately
600 electron volt energy of the incident electron traveling from
the photocathode. The low energy electron travels into the aluminum
layer and therein loses what little energy it has due to
interaction and collision with the multitude atoms and electrons in
the aluminum layer and is therefore unable to pass through the
length of aluminum layer into one of the passage openings. By
contrast, any conductive material plated or otherwise formed on the
front surface of the microchannel plate which, for example, is the
electrode element previously described, which is found on the plate
in the form provided by the manufacturer, does not cover the entire
open ends of the passages and does not present any barrier to
scattered electrons. Further, because the plate 7 is located close
to the photocathode, permitting use of voltages on the order of 600
volts, higher accelerating voltages which create scattered
electrons of higher energy levels that might pass through layer 15
are avoided.
For clarity of explanation the path of one electron, e1, serves to
illustrate the conventional operation of the microchannel plate 7.
Electron e1 is derived from photocathode 5 due to the incidence of
light, .lambda.i, at the indicated location on the photocathode and
is accelerated toward the electron multiplier. The electron goes
through "transparent" metal layer 15 and then into a passage 9
where it comes under the influence of electric field, E2,
established by source 19. Because of its random transverse travel
the electron collides with the passage walls. The passage walls are
coated with high secondary emission material having a secondary
emission coefficient of at least 2, or, in other words, greater
than one at average impact velocity. Thus the electron knocks out
at least two additional electrons and these, in turn, accelerate
and travel, due to electric field E2, in general toward the back of
the passage. As illustrated in the example in FIG. 1 these two
electrons having random velocity vector angles, in turn, strike the
walls of the passage at a subsequent location and, in turn, knock
out four electrons. By suitable choice of the length and diameter
of the tubes this process of increasing the quantity of electrons
continues. And a large number of electrons exit from the back side
of microchannel plate 7. Hence, the initial electron e1 which
entered the front of the plate is amplified or "multiplied" many,
many times. Upon exit from the rear of the passage 9 in
microchannel plate the electrons enter high level electric field,
E3. The electric field accelerates the electrons to a large energy
level, through 5,000 volts typically, and they travel and pass
through metal backing 16 and strike the surface of the phosphor
screen or target 13, as variously termed, at a corresponding
predetermined location. With conventional electron focusing systems
between the plate and target this location can be varied, but in
the embodiment illustrated, it is a direct corresponding location.
As is conventional, the phosphor emits light, .lambda.o, in
proportion to the amount of electron bombardment and thus the
initial low level of light responsible for the generation of the
single electron from photocathode 5 is amplified to a much higher
light level or brightness at the phosphor screen 13, much higher
than the light which would have been produced by the single
original electron.
While the foregoing theory of operation has been discussed in
connection with a single electron, the operation occurs
concurrently with all of the incident light in the image or mosaic
placed over the entire surface of the photocathode and with all the
passages in the microchannel plate so that the electron image
formed at photocathode 5 appears as a light image at the display
target electrode 13.
Prior to my invention by the addition of aluminum layer 15 to the
front of the microchannel plate 7 it was possible for an electron
such as illustrated by e2 in FIG. 1, to enter a passage in the
microchannel plate and knock out a positive ion, "+." Due to the
nature of construction of the microchannel plate tube this would
most likely be a cesium ion or an oxygen ion or a water ion.
Electric field E1 which accelerates negatively charged electrons
toward plate 7, instead accelerates positively charged ions toward
the photocathode. In striking the photocathode these particular
ions would combine with the photocathode material to form a
different compound, a compound which would not possess
photocathodic properties and lowered or destroyed, eventually, the
effectiveness of the photocathode.
Secondly, the mass of an ion is, of course, hundreds of times
greater than an electron and when accelerated through the electric
field possesses relatively large amounts of kinetic energy. Upon
striking the photocathode the ion releases this energy and erodes
the photocathode material and reduces its photocathodic
properties.
As a result of the foregoing effects, the light amplifiers lost
sensitivity, presented a faded picture with diminishing brightness
and diminishing contrast typically after an operating life of no
more than 50 to 100 hours.
Instead, in the construction of the invention aluminum layer 15
absorbs or acts as a barrier to and prevents these large mass ions
from reaching photocathode 5, and in this way acts as an "ion
trap." In addition, any ions or neutral gas molecules originating
from any other tube components behind the microchannel plate cannot
pass through layer 15.
Image amplifiers under life test have presently been in operation
for in excess of 1,000 hours in contrast to the 50 to 100 hours
provided with prior art tubes and this is accomplished without
reduction in the gain overall of the light amplifier.
Theoretically, the ultimate increase in the operating life of the
light amplifier expected from this improved construction and which
will be demonstrated in the future is expected to increase by a
factor of 100 to 1,000 times over that previously available.
Further consideration of the invention is better illustrated and
understood in connection with FIG. 2 which shows a cutaway section
A from FIG. 1. In normal operation of the light amplifier,
electrons such as e3, of 400 electron volts, pass through aluminum
layer 15 and enter one of the tubes 9 in microplate 7.
With the modification to the aluminum layer suggested in which a
skin of high secondary emission material, suitably aluminum oxide,
is applied to the underside of the aluminum layer, electron e3 as
represented in FIG. 2 passes through the aluminum and strikes the
high secondary emission material. Inasmuch as such material has a
secondary emission coefficient preferably greater than 2, two
electrons, e31 and e32, are shown emerging from the back surface of
layer 15 and traveling into the passage. In this the layer
functions in addition as a transmission-emission dynode. However
even without the suggested coating, it is possible in many
instances for such electrons, such as e3, to knock out some
secondary electrons.
Assuming momentarily the deletion from FIG. 2 of aluminum layer 15
the prior art microchannel plate light amplifier devices and an
attendant disadvantage of same as well as a prime and unexpected
feature of the invention can be better understood. Electrons such
as e4, as represented in FIG. 2, traveling from the photocathode
are many times incident upon an edge surface bordering the
passages, 9. This is not uncommon. As previously noted,
approximately 50 percent of the apparent surface area of the
microchannel plate open to the passage while the other 50 percent
represents actual material. This represents the manufacturer's
compromise between the desire of as many passages as possible in a
given space versus the mechanical requirements of rigidity for the
multichannel plate element.
In those instances the electron knocks out from the surface one or
more additional electrons, which are represented by way of example
as e41 and e42. These electrons depart the surface and travel at an
angle with respect to the surface of the microchannel plate but
with a component of velocity in the direction of the photocathode.
These electrons are low energy level "scattered" electrons and,
typically, possess energies on the level of 3 to 5 electron volts.
The electric field E1 previously described in connection with FIG.
1 decelerates and turns these electrons around and they travel into
passages in the microchannel plate. In the passages 9 these
electrons act and are "multiplied" in substantially the same manner
as any other electron as previously described in connection with
the electron multiplication properties of the microchannel
plate.
Unfortunately, because these electrons are "scattered" in any
direction and of varying low energy levels there is an uncertainty
as to which one of the passages in microchannel plate into which
they will travel. In FIG. 2, electron e41 is shown traveling into
one passage while electron e42 is shown traveling into an adjacent
passage. As is also apparent, there are additional passages above
and below the illustrated passages in the 3-dimensional body. It is
also possible for the energy level of the scattered electrons to be
such that it can travel, instead, to the next adjacent tube into
which the previously discussed electron e3 traveled.
Inasmuch as each of the electrons represents light of a received
image, it is apparent that light intended to be positionally
located on the phosphor display screen at a position corresponding
to the juncture of the adjacent passages where e4 is incident the
light is, instead, displaced a predetermined position and presented
at one location, that through which electron e41 emerges, or, in
addition or alternatively, at a second location, the one to which
e42 will travel, as well as many other passages above and below and
around those illustrated to which the scattered electrons can
travel. Thus it is possible to obtain instead of a sharp point
location on the phosphor screen a rather "blurred" representation;
the contrast or resolution is thus not as great as possible. This
factor is referred to in the design and specification of cathode
ray tubes and light amplifying tubes as a "limiting resolution
factor" and this limiting resolution factor is specified in the
number of television lines (scanning lines) per unit of raster
height. The limit of resolution is specified as the number of lines
on the screen for a given height which can be viewed before the
lines merge and blur and become indiscernible. Typically, on prior
art microchannel plate light amplifying tubes the upper threshold
of resolution was 400 TV lines per unit of raster height, the
raster height being typically four-tenths of 1 inch. The capability
of resolution of a phosphor screen itself is limited by phosphor
spot size and conventionally the display screen 13 of FIG. 1 is
inherently capable of a resolution greater than 1,000 lines per
unit of raster height.
Considering now the incorporation of the aluminum layer 15 in the
light amplifier. As previously described in connection with the
operation in FIG. 1 the aluminum screen acted as a barrier to
positive ions, which ions were many hundreds of times larger in
both mass and volume than an electron. In addition, quite
unexpectedly, the aluminum layer also absorbed or captured the
electrons low energy level scattered electrons such as those
generated by secondary emission from the edge surface of the
microchannel plate. Thus electrons such as e41 and e42 are now
absorbed or captured within layer 15 covering passage 9 and they
cannot travel into the microchannel plate. The accelerating voltage
within the range of 400 to 1,000 volts, as previously noted, is low
and at most generates a minimum of higher energy secondary
"scattered" electrons that might pass through layer 15 to diminish
contrast resolution as a higher accelerating voltage could do.
Because the existence of only these low energy level scattered
secondary electrons was a substantial factor limiting the
resolution of the light amplifier, their elimination without the
generation of those of increased energy permits an increased
resolution capability for the light amplifier. In point of fact, in
a tube constructed in accordance with the teachings of this
invention, a 30 percent increase was obtained in the limiting
resolution over a corresponding tube constructed without the
aluminum layer 15. As contrasted with an upper threshold of
resolution of 400 TV lines per unit of raster height obtained with
prior art tubes the structure of the invention makes it possible to
distinguish 600 lines per unit of raster height.
Should any light pass through photocathode 5 and be incident upon
the metal layer 15, it will pass through the transparent layer and
through the microchannel plate without interfering in the operation
of the image intensifier tube. This avoids the problem of
reflecting light from layer 15 back to photocathode 5 and
generation of electrons in a different position possible with an
optically opaque thick metal layer and which could cause
"blooming," is avoided.
In the specific example of my invention the layer 15 which forms
the ion and electron trap is a metal, aluminum. However other
materials of a low density, suitably a specific gravity below 4,
can be used as an alternative. Boron and beryllium are examples.
Although the preceding examples are metals, I have also found that
nonmetals, normally electrical insulators, are equally suitable. By
way of example, some such nonmetals which can be used to form the
layer 15 include boron carbide, aluminum oxide, silicon oxide,
boron nitride, silicon dioxide, magnesium oxide and magnesium
fluoride. Whatever alternative material is used, the layer 15 is
formed in place on top of the microchannel plate by the same
processes described previously for the specific example of
aluminum. Thus the foregoing or any equivalent element compound or
composition of matter may be used which can undergo processing by
the preferred process. Basically, the material should not be water
soluble, should not oxidize at temperatures less than 300.degree.
Centigrade, and do not reduce in a hydrogen atmosphere at
temperatures on the order of 435.degree. Centigrade.
Moreover, the structure of the invention requires the materials to
be of a low density and hence they must have a specific gravity
less than 4.0 and, preferably, the specific gravity of the material
is in the range of 1.0 to 4.0.
As a further consideration and refinement to the invention, it is
desirable that the materials used as the ion and electron trapping
layer, such as layer 15, possess a high secondary emission
coefficient suitably greater than 3.0 at 400 volts. This insures
greater output for electron multiplication processes and renders
the resulting tube of higher quality and less susceptible to
"noise." This characteristic is exhibited by most of the specific
examples given.
As previously described, the front edge surface of the microchannel
plate 7 as obtained from the manufacturer is electrically
conductive. Hence if layer 15 is an electrical conductor it will be
in direct contact with the front edge of the microchannel plate,
and, accordingly, the voltage V1 which is applied to the
microchannel plate and the photocathode 5 creates a voltage drop
across a distance d1 slightly shorter by the distance d3, the
thickness of the layer, and, accordingly, a slightly greater
voltage gradient E1, which is equal to V1/d1. However inasmuch as
the thickness of the layer is insignificant in relation to the
distance d1 between the photocathode 5 and microchannel plate, for
all practical purposes the effect of the thickness of the
microchannel plate on the established electric fields may be
disregarded. Thus, where the covering layer 15 is of a nonmetal
which is not electrically conductive but electrically dielectric,
the voltage extends across the space between photocathode 5 and
front edge surface of microchannel plate 7. However as in the
preceding case, the thickness of the layer is so small relative to
the distance between the photocathode and microchannel plate that
its effect upon the voltage applied therebetween or the voltage
gradient E1 thereacross may be disregarded. Hence, with either type
of construction I may refer to the potential difference or voltage
between the photocathode and the front end of the modified
microchannel plate as that between the photocathode and the front
edge of the microchannel plate, disregarding the existence of the
thin layer 15, and likewise the electric field gradient E1
established between the photocathode and microchannel plate is the
same for all practical purposes as that gradient established by
disregarding layer 15. And thus it is understood where I describe a
voltage or field between the microchannel plate and photocathode
that such voltage or gradient may actually appear across an
insignificantly foreshortened space in the case where layer 15 is
an electrically conductive metal and is intended to cover such
structure. Likewise, essentially the same is true of the distance
between the rear of the microchannel plate and the target
electrode. Hence where I refer to a voltage V3 between the two
elements or a field E3, (V3/d2), between those two elements it is
understood such language is intended to cover the distance between
the microchannel plate and the metal backing layer on the target
electrode.
The foregoing detailed description and illustration of the
preferred embodiment of my invention are presented solely for
purposes of explanation and not by way of limitation. As is
apparent to those skilled in the art many modifications,
substitutions and equivalents to the foregoing details can be made
without departing from the spirit and scope of my invention. It is
therefore understood that the inventions are to be broadly
construed limited only by the breadth and scope of the appended
claims.
* * * * *