Note: Descriptions are shown in the official language in which they were submitted.
Background of the Invention
1. Field of the Invention
This invention relates gener~lly to image producing
tubes and is concerned more particularly with method and
means for enhancing contrast in output images produced by
said tubes.
2. Discussion of the Prior Art
An image producing tube, such as an image intensifier
tube, for example, generally includes an output screen
assembly disposed within an evac:uated envelope for producing
a visible light image which is viewable externally of the
tube. The output screen assembly may be supported on a
transparent substrate which is disposed adjacent a transparent
faceplate portion of the envelope at the output portion of
the tube. Thus, the image produced by the output screen
assembly is directed through the transparent substrate and
through the transparent output faceplate to be seen externally
of the tube.
It has been found that contrast in the output image
viewed externally of the tube is degraded by light scattered
from the image produced by the output screen assembly. This
scattered light is caused by multiple reflections occurring
between opposing surfaces of the transparent substrate,
between the substrate and the output faceplate, and between
opposing surfaces of the output faceplate. Consequently, in
image producing tubes of the prior art, the transparent sub-
strate through which the visible light image passes generally
is made of commercially available gray glass, which is clear
glass chemically darkened to reduce reflections at its opposing
surfaces~
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Ideally, this commercially availabe gray glass will
be darkened an optimum amount for minimizing the scattering of
light from the image and maximizing transmission of non-scattered
light in the image, as well as having other desired charac~
teristics, such as the proper optical quallty~ absorbence,
index of refraction, thermal expansion properties, etc. However,
in practice, it generally is found that gray glass having
these other desired characteristics has not been darkened the
proper amount, and vice versa. Consequently, the gray glass
used in these tubes of the prior art generally involves a
compromise in the amount of darkening required or in one of
the other characteristics desired for its function in the
image producing tubes.
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Summary of the Invention
Accordingly, this inven~ion provides a means and a
method for overcoming these and other disad~antayes o the
prior art. The invention comprises an imaye producing tube
having means for directing an output visible light image
and radiation-darkened transparent means aligned with said
image directing means for receiving said visible light image
and transmitting it as a contrast enhanced image toward an
output portion of the tube. Thus, the radiation-darkened
transparent means may comprise a ~ray glass substrate
supporting an output screen assembly adjacent said output
portion of the tube. Also, the radiation~darkened trans-
parent means may comprise a gray glass output faceplate
portion of the tube envelope at said output portion of the
tube. ~urthermore, the gray glass faceplate portion of the
tube envelope may serve as the gray glass substrate supporting
the output screen assembly adjacent said output portion of the
tube.
The invention also includes a method comprising the
step of controllably exposing a transparent member to
ionizing radiation having a wavelength equal to or shorter
than thirty-six hundred angstroms for suitably darkening the
transparent member to provide it with desired image transmissive
and contrast enhancement properties. The ionizing radiation
preferably is emitted from an energizable source, such as
an X-ray generator, for example, so that the controlled
darkening of the transparent member may be terminated by
de-energizing the source. Thus, a transparent substrate
member supporting the output screen assembly may be con-
trollably darkened, in accordance with this invention,
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either before or after disposing the output screen assemblyon the substrate member. Also, an output faceplate portion
of the image producing tube envelope may be controllably
darkened, in accordance with this invention, during assembly,
or during processing, or after sealing off the tube from a
vacuum system, such as before or after final testing, for
examples. When the darkening of the output faceplate portion
of the tube envelope is carried out after final testing,
contrast enhancement characteristics of the output faceplate
portion may be optimized without disturbing other
characteristics of the completed tube.
The means and method of this invention permits a manufac~
turer of image producing tubes to select a commercially
available glass having the proper characteristics for use in
image producing tubes and controllably darkening the glass to
have desired image transmissive and contrast enhancement proper-
ties without adversely affecting the other characteristics of the
glass. The process involved in the radiation darkening of the
glass is believed to be a creation of ~f" centers, that is the
promotion of elections to meta-stable states within the glass
lattice structure, by the ionizing radiation. Thus, the ionizing
radiation should have an energy which is greater than the energy
required to excite the electrons in the glass lattice structure
to these meta-stable states. The resulting darkened glass has
been found to be very stable and will not bleach over long
periods of time at room temperatures~ It has been found that
the color induced in glass generally used in the fabrication
of image producing tubes will bleach at three hundred degrees
Centigrade in eight hours, thereby providing a convenient method
for reclaiming gray glass that has been darkened excessively.
According to a first broad aspect of the present
invention, there is provided an image producing assembl~ com-
prising: means including a layer of fluorescent ma-terial ~o~
producing a visible light image and directing said image
along a predetermined path; and controllably radiation dar~ened
transmissive means disposed in said path for receiving said
visible light image and transmitting a correspondiny contrast
enhanced image.
According to a second broad aspect of the present
invention~ there is provided an image producing tube compris-
ing: output means for producing a visible light image and
directing said image along a predetermined path; and envelope
means for enclosing said output means, the envelope means
including a controllably radiation darkened transmissive
means disposed in said path for receiving said image and
transmitting a corresponding contrast enhanced image.
According to a third broad aspect of the present
invention, there is provided an image producing assembly
comprising: means for producing a visible light image; and
controllably radiation darkened image transmissive means dis-
posed adjacent said image producing means for -transmitting a
contrast enhanced image of said visible light image.
In the manufacture of the visible image producing
tube, the invention also provides a method of darkening a
light transmissive output member comprising: exposing said
light transmissive output member of said visible image produ-
cing tube a source of radiation having sufficient energy to
cause darkening of said output member; and terminating said
exposure of the output member to the radiation when sufficient
darkening of the output member has occurred.
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Brief Description of the Drawin~
For a better understAnding of this invention, reference
i~ made in the following detailed description to the
drawings wherein:
FIG. 1 is a schematic view of apparatus embodying this
invention;
FIG. lA is an enlarged fragmentary view, in axial section,
of the output end portion of the image producing tube shown
in Fig. 1.
FIG. 2 is a schematic view of the output faceplate end
of the imag~ producing tube shown in Fig. 1 as viewed along the
line 2-2 and looking in the direction o~ arrows;
FIGS. 3A-3C are schematic views of an alternative
embodiment of this invention.
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Des_ription of the Preferred Embodiments
Referring to the drawiny wherein like characters of
reference designate like parts, there is shown in Fig. 1
an image intensifier type of image produciny tube 10 haviny
an evacuated tubular envelope 12. Envelope 12 may terminate
at one end in a flanged end portion of a metallic collar 14
which serves as the cathode terminal of tube 10. The opposing
end portion of metallic collar 14 is circumferentially
sealed to the remainder of envelope 12 which may be made of
another suitable material, such as glass, for example.
Closing the cathode end of envelope 12 is a circular
input faceplate 16 which is peripherally sealed to the 1anged
end portion of collar 14 and is made of radiation transparent
material, such as glass, for example. The inner and outer sur-
faces of input faceplate 16 may be provided with any desired
configurations, such as plano-concave or convexo-concave,
for examples. Disposed adjacent the inner surface of input
faceplate 16 is an aligned input screen assembly 18 which
may be supported directly on the inner surface of faceplate
16, thereby providing it with a conformingly shaped con-
figuration. Alternatively, the input screen assembly 18 may
be supported in close-spaced relationship with the inner
,'~ surface of faceplate 16 as shown in U. S. ~atent
granted to Caraher et al on April 3, 1979 and assigned to
the assignee of this invention.
The input screen assembly 18 comprises a radiation
transparent layer 20 of light reflective material, such as
aluminum, for example, which may serve as the substrate when
the input screen assembly 18 is supported in close-spaced
relationship with the inner surface of input faceplate 16.
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~ayer 20 is preferably made of electrically conductive
material ~o maintain the input screen assembly 18 at
cathode potential during operation o~ tube 10~ Accordingly,
the layer 20 is electrically connected throuyh the metallic
collar 14 and a connecting conductor 22 to a source (not
shown) of cathode potential.
The input screen assembly 18 also includes a scintillator
layer 24 of radiation sensitive, fluorescent material, such
as cesium iodide, for example, overlayiny the radiation
transparent layer 20. Furthermore, input screen assembly 18
includes a photocathode layer 26 of photoemissive material,
such as cesium antimonide~ for example, which may be super-
imposed on the scintillator layer 24~ In some instances, the
input screen assembly may include a transparent barrier
layer (not shown) disposed ~etween the scintillator layer 24
and the photocathode layer 26, and made of mutually compatible
material for preventing adverse reactions between the
respective materials of layers 24 and 26.
In operation, a radiational image of an external object
passes through the transparent input faceplate 16 and the
transparent layer 20 to inpinge on the scintill~tor layer 24.
As a result, photons in the incident image penetrate into
aligned discrete regions of the material in the scintillator
layer 24 and cause these regions to fluoresce locally in
accordance with the intensity of penetrating photons.
Accordingly, the scintillator layer 24 produces a corresponding
dim light image which is reflected by the material of
layer 20 in the direction of photocathode 26~ Con-
sequently, this light image passes through the interposed
barrier layer, if present, and impinges on the adjacent surface
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of layer 26. Thus, photons in the visible light image pene-
trate into aligned discre-te regions of the photoemissive
material in layer 26 and cause the emission of elec~rons 1n
accordance with the intensity of penetrating photons. I'here-
fore, there is emitted from the inner surface of photocathode
layer 26 an equivalent electron image which may be intensity
amplified, such as by electrostatic acceleration and mini-
fication, for example.
An opposing end poxtion of tubular envelope 12 ter-
minates in an output faceplate 30 which is generally circular
and is peripherally sealed to an adjacent reduced diameter end
portion of the envelope. The output faceplate 30 is made of
transparent material, such as glass, for example, and serves
as a substrate for an output screen assembly 32, as shown
more clearly in FIG. lA, disposed directly on its inner sur-
face. Output screen assembly 32 includes an output phosphor
layer 34 overlaying the inner surface of faceplate 32, and an
electron pervious layer 36 overlaying the inner surface of
phosphor layer 34. The phosphor layer 34 comprises an elec-
tron sensitive fluorescent material, such as silver activatedzinc cadmium sulfide, for example; and the electron pervious
layer 36 comprises a light reflective material, such as alum-
inum, for example. The material of electron pervious layer 36,
preferably, is electrically conductive so that it also may
serve as the anode electrode for maintaining the output screen
assembly 32 at an anode potential. Accordingly, the electron
pervious layer 36 may be electrically connected through a res-
pective anode terminal (not shown) in envelope 12 and a connec-
ting conductor 38 to a source (not shown) of anode potential.
The anode potential, thus applied to output screen assembly 32,
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generally is very highly positive with respect to the
cathode potential of input screen assembly 18 for the
purpose of electrostatically accelerating the electron
image emitted from the inner surEace o photocathode layer
26 toward the output screen assembly 32.
Disposed be~ween the input screen assembly 18 and out-
put screen assembly 32 along the length of envelope 12 is a
coaxially aligned series of focussing electrodes 40, 42, 44
and 46 which are insulatingly spaced from one another. Each
of the electrodes 40, 42, 44 and 46 is electrically connected
through respective terminals (not shown) in the envelope 12
and through respective connecting conductors 48, 50, 52 and
~4 to respective sources (not shown) of focussing potential.
Thus, the electrodes 40, 42, 44 and 46 are maintained at
respective focussing potentials with respect to the cathode
potential of input screen assembly 18 for focussing the
electron image emi.tted rom the inner surface of photocathode
layer 26. As a result, the electrostatically accelerated
electron image converges toward a crossover region, such as
at the entrance of electrode 40, for example, and after
passing through the crossover region diverges as an inverted
image.
This diverging inverted image passes through the electron
pervious layer 36 of output screen assem~ly 32 and impinges
on the underlying phosphor layer 34. Consequentlyr electrons
in the diverging inverted image penetrate into aligned
discrete regions of the electron sensitive material in layer
34 and cause these regions to fluoresce locally in accor-
dance with the intensity of the penetrating electrons.
Accordingly, the phosphor layer 34 produces a corresponding
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~isible light image which, due to electrostatic amplification
of the electron image, is much brighter and inverted with
respect to the visible light image produced by the scintillator
layer 24 o~ input screen assembly 18. This brighter visible
light image produced by phosphor layer 34 is reflected by the
ma~erial of layer 36 toward the output faceplate 30 where it
is transmitted out of tube 10 for viewing by an observer aligned
with the output faceplate 30.
However, when the output visible light imaye is trans-
mitted through the output faceplate 30, light may be scattered
from the image due to multiple reflections occurring between
the inner and outer surfaces of faceplate 30, and thereby
degrading contrast in the output viewable image. Accordingly~
this invention provides means and method for selecting clear
glass having otherwise desired optical characteristics for
output faceplate 30 and then darkening it an optimal amount to
provide it with specified "gray" glass characteristics for
selectively absorbing the scattered light and transmitting thP
unscattered light in the image. In accordance with the method
of this invention, the output faceplate 30 of image producing
tube 10 is controllably exposed to an X-ray beam 56 of ionizing
radiation which is incident on the ou er surface of faceplate
30 and has a wavelength shorter than thirty-six hundred
angstroms. The X-ray beam 56, preferably, emanates from an
energizable source, such as X-ray generator 58, for example,
which may be de-energlzed to terminate the resulting darkening
of the output faceplate 30. Generator 58 comprises an X-ray
shielded housing 60 including an X-ray tube 62 having an envelope
64 wherein an electron emitting cathode 66 is disposed for
beaming electrons onto a focal spot area 68 of an anode target
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70. The housing 60 is provided with a conventional pair of
horn-type electrical connectors, 72 and 74, respectively,
whereby an external controllable current source 76 is electri-
cally connected to the cathode 66 through respective conductors
78 and 79, and an external controllable hlgh voltage source 80
is electrically connected between the cathode 66 and the anode
~arget 70 through respective electrical conductors 82 and 83.
Thus, the current source 76 is energized to heat the
cathode 66 in X-ray tube 62 to an electron emitting
temperatùre, and the high voltage source 80 is energized to
maintain the anode target 70 at a high positive potential with
respect to the cathode 66. As a result, electrons are beamed
from the cathode 66 onto the focal spot area 68 of anode
target 70 to generate the divergent X-ray beam 56 which
radiates from the focal spot area 68. The X-ray beam 56
passes through a port 84 in housing 60 and through a collimator
device 85 mounted over the port 84O Accordingly, the
collimator device 86 is spaced a suitabl~ distance from the
output faceplate 30 of image producing tube 10 and is
adjusted to provide the emerging X-ray beam 56 with a
suitable cross-sectional size for irradiating the output
faceplate 30.
The image producing tube 10 is maintained at
room temperature while the output faceplate 30 thereof is
exposed to the irradiating effects of X-ray beam 56 for a pre-
determined time in~erval, such as fifteen minutes, for example,
whereby the output faceplate 30 is darkened sufficiently to
provide it with suitable "gray" glass characteristics. It
is believed that this radiation darkening is caused by the
ionizing effects of X-ray beam 56 creating -fl centers where
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electrons are promoted to meta-stable states within the glass
lattice structure of the output faceplate 30. Thus, the
ionizing radiation used for darkening glass commonly used
for output faceplates of image producing tubes should have
a wavelength at least as short as ultraviolet light, that is
equal to or ~ than thirty-six hundred angstroms, to
provide it with sufficient energy for exciting the electrons
to the desired meta-stable states in the lattice structure
of ylass commonly used for output faceplates in image pro-
ducing tubes. The stability of the electrons in these meta-
stable states within the lattice structure of glass commonly
used for image producing tube output faceplates provides a
darkening discoloration which will not bleach at room temperature
over a considerable length of time, such as on the order of
years, for example. When the output faceplate is provided
with optimal "gray" glass characteristics, the glass darkening
process may be terminated by de-energizing the high voltage
source 80 to stop further beaming of electrons from the cathode
66 onto the focal spot area 68 of anode target 70, thus
cutting-off generation of X-rays in beam 560
It has been found that most of the radiation induced
discoloration in the darkened output faceplate 30 may be
bleached out by maintaining the faceplate 30 at a suitable
temperature for a predetermined interval of time depending on
the type of material in the darkened output faceplate 30. For
example, radiation-darkened glass commonly used for output
faceplates of image producing tubes may be bleached by main-
taining it at about three hundred degrees Centigrade for eight
hours. Accordingly, this invention also provides a method for
relaiming output faceplates which have been excessively
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darkened, such as by over-exposure to the irradiating effects
of X-ray beam 56, for example. After bleaching an over-exposed
faceplate and allowing it to cool to room temperature, the
bleached faceplate may be darkened, as desired, by exposing
it to the irradiating effects of X-ray bearn 56 in accordance
with this invention.
Thus, in the practice of this invention, the darkening
of the output faceplate 30 is controlled to provide it with
optimal gray glass characteristics for selectively absorbing
light scattered from the image produced by output screen
assembly 32 and pe~mitting transmission of unscattered light
in the image~ Consequently, as shown in Fig. 2, the
resulting gray glass, output faceplate 30 produces a contrast
enhanced image 88 for external viewing at the output end of
image produ~ing tube 10. Accordingly, this invention pro-
vides method and means for selecting a clear glass faceplate
having otherwise desirable characteristics, such as proper
optical quality, index of refraction, and thermal expansion
properties, for examples, and contro~lably darkening it to
provide the specified gray glass characteristics required for
producing the gray glass output faceplate 30. Although, this
invention has been illustrated with the tube 10 in a completely
fabricated condition, it is apparent that the invention may be
practiced on the tube at any stage of assembly, processing, or
testingO Thereore, the output faceplate 30 may be darkened
in accordance with this invention prior to assembly in the
envelope 12, prior to sealing-off from a vacuum exhaust system,
or during final testing, for examples.
As shown in Fig. 3A, ~his invention also may be used
.` 30 for controllably darkening a glass plate substrate ~ to
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provide it with desired gray glass characteristics by exposing
it to ionizing radiation from an energizable source, such as
X-ray beam 56 emanating from generator 58, for example. As
shown in Fig. 3B, after the darkening operation, there may
be disposed on the substrate 90 an output screen assembly 32A
comprising a phosphor layer 34A which is similar to phosphor
layer 34 and disposed ad~acent the substrate 90, and an elec-
tron pervious layer 36A which is similar to layer 36 and is
superimposed on the phosphor layer 34A. As shown in Fig. 3C,
the substrate 90 may be attached to an exit end portion of an
electrode 40A which is disposed within an envelope 12A of an
image producing tube lOA similar to the tube 10. Accordingly,
the outpuc screen assembly 32A may be maintained at anode
potential by having the layer 36A electrically connected
through the electrode 40A and a connecting conductor 48A to
a source (not shown) of anode potential. Also, the output
screen assembly 32A is supported by the substrate 90 in close-
spaced relationship with an aligned output faceplate 30A of
tube lOA. ~he output faceplate 30A is similar to the output
faceplate 30 of tube 10 and may be radiation dar~ened by con-
trollably exposing i-t to the X-ray beam 56 emanating from
energizable source 58, as shown in Fig. 1 and Fig. lA.
In similar image producing tube~ of the prior art,
the output faceplate 30A usually is made of a clear glass and
the substrate 90 may be made of a commercially available
"gray" glass generally comprising a clear glass which has been
chemically darkened. Ideally~ the commercially available
"gray" glass will have the optimal amount of darkening for
selectively absorbing scattered light while transmitting
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unscattered light in the output image, as well as having
other specified characteristics, such as optical qualiky,
index of refraction, and -thermal expansion properties, for
examples. However, in practice, it is generally found that
the commercially available "gray" glass having approximakely
khe desired amounk of darkening does not have one or more
of the okher specified characteristics. On the okher hand, a
commercially available "gray" glass having khe other specified
characteristics usually does nok have, even approximakely,
the required amount of darkening. Consequently, a compromise
generally has to be made in the amount of darkening or in one
or more of the okher specified characteriskics in order ko
obkain "gray" glass for fabricating these image producing
tubes of the prior art.
Therefore, this invention provides a means and a
method for fabricating an image producing kube by selecting
a clear glass having okherwise desirable characteriskics, such
as optical qualiky, index of refraction, and thermal expansion
properties, for examples, and radiation darkening ik an
optimal amounk. Thus, in accordance wikh khis invenkion, the
substrate 90 comprises a clear glass which is controllably
darkened by exposure to the ionizing effects of X-ray beam 56
to provide it with optimal gray glass characteristics. Con-
sequently, khe substrate 90 seleckively absorbs khe visible
lighk scatkered from the image produced by output screen assem-
bl~ 32A, due to multiple reflections of the light occurring at
opposing surfaces of the subskrake, and permiks kransmission
of unscakkered lighk in khe image. As a resulk, khe substrate
90 kransmiks a contrask enhanced image ko khe outpuk face-
plate 30A which also has been controllably dar]cened by
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exposure to the ionizing effects of X-r.ay beam 56 for pro-
viding it with optimal gray glass characteristics. Therefore,
the output faceplate 30A selectively absorbs light scattere~
from the contrasted enhanced image transmitted by substrate
90, and permits the transmission of unscattered light in
the resulting image. Accordingly, the output aceplate 30A
transmits a further conkrast enhanced image for external
viewing at the output end portion of tube lOA.
Although the substrate 90 has been described as
radiation-dar~ened before deposition thereon of the output
screen assembly 32A, it may e~ually well have been radiation-
darkened, in accordance with this in~ention, after deposition
of the output screen assembly 32A, such as shown in Fig. 1 and
Fig. lA, for example. Also, it will be apparent th~t the
output faceplate, such as 30 shown in Fig. 1 or 30A shown in
Fig. 3C, for example, may be radiation-darkened in accordance
with this invention at any time during assembly, processing,
or testing of the image producing tube. Thus, the output
faceplate may be radiation darkened before assembly into the
envelope of the tube or before sealing-off the tube envelope
from the vacuum system, or during final test, for examples.
Alternati.vely, the output faceplate may be radiation-darkened
after final testing of the image producing tube is completed,
when other specified characteristics of this have been found
acceptable, and contrast enhancement of the output image may
be adjusted in accordance with this invention without adver-
sely affecting the other specified characteristics of the
tube.
Furthermore, although this invention has been illus-
trated herein using the ionizing effects of an X-ray
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beam emanating from an X-ray generator, it also may be
practiced with the use of other types of ionizing radiation,
such as ultraviolet radiation emanating from a mercury dis-
charge lamp, for example. Also, although this invention has
béen illustrated herein with an image inverting type o image
intensifier tube, it also is useful for enhancing contrast
in the output ima.ges produced by proximity focusing types of
image intensifier tubes or other types of image producing
tubes, such as display tubes of the cathode ray type, for
example.
From the foregoing, it will be apparent that all of the
objectives of this invention have been achieved by the
structures and methods described herein. It also will be
apparent, however, that various changes may be made by
those skilled in the art without departing from the spirit
of the 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 rather
than in a limiting sense.
Cs. 32349
JTM:jp
