U.S. patent number 3,710,181 [Application Number 05/074,334] was granted by the patent office on 1973-01-09 for solid-state image intensifier.
This patent grant is currently assigned to Matsushita Electric Industrial Company, Ltd.. Invention is credited to Tadao Kohashi, Yasuhiko Machida, Norio Suzuki, Kazunobu Tanaka.
United States Patent |
3,710,181 |
Tanaka , et al. |
January 9, 1973 |
SOLID-STATE IMAGE INTENSIFIER
Abstract
A solid-state image intensifier comprising, essentially, an
electroluminescent layer containing 45 to 70 percent by volume of
electroluminescent phosphor mixed with a binding material and a
photoconductive layer in juxtaposition or close association with
the electroluminescent layer. This solid-state image intensifier is
adapted for use as an amplifier of radiant energy or as a converter
of invisible radiation into visible radiation and is designed to be
energized by AC and DC fields. Due to its pecific composition, the
electroluminescent layer has a nonlinear resistance which functions
to keep a DC voltage applied across the photoconductive layer at a
substantially constant value, thereby increasing the
photoconductive sensitivity of the photoconductive layer. The
increase in the DC voltage as applied to the image intensifier
causes the characteristic curve to shift to the low input energy
side, enabling efficient operation of the image intensifier in a
low input energy range.
Inventors: |
Tanaka; Kazunobu (Kadoma,
JA), Machida; Yasuhiko (Kadoma, JA),
Suzuki; Norio (Kadoma, JA), Kohashi; Tadao
(Kadoma, JA) |
Assignee: |
Matsushita Electric Industrial
Company, Ltd. (Osaka, JA)
|
Family
ID: |
22119005 |
Appl.
No.: |
05/074,334 |
Filed: |
September 22, 1970 |
Foreign Application Priority Data
|
|
|
|
|
Sep 22, 1969 [JA] |
|
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44/77175 |
|
Current U.S.
Class: |
315/175; 313/507;
313/525; 315/169.1; 315/169.3 |
Current CPC
Class: |
H05B
33/12 (20130101) |
Current International
Class: |
H05B
33/12 (20060101); H01j 039/00 (); H05b
037/00 () |
Field of
Search: |
;315/169,169TV,175
;313/94,18A |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lake; Roy
Assistant Examiner: Dahl; Lawrence J.
Claims
What is claimed is:
1. A solid-state image intensifier comprising: a photoconductive
layer the AC impedance of which is varied in response to radiant
energy; said photoconductive layer including a photoconductive
material selected from the group consisting of sulfides, selenides
and telurides of cadmium, lead and zinc; an electroluminescent
layer deposited upon and in contact over an extended area with said
photoconductive layer for emitting light in response to an electric
field applied thereto; said electroluminescent layer consisting of
45 to 70 percent by volume of zinc sulfide particles mixed with a
binding material and having an inherent nonlinear resistance which
functions to keep a DC voltage applied across said photoconductive
layer at a substantially constant value; a conductive layer
disposed on said photoconductive layer and permeable to the input
radiation, said input radiation being one of the form of visible
light, X-rays, infrared rays or ultraviolet rays; a transparent
conducting layer disposed on said electroluminescent layer; and DC
and AC voltage sources connected in series between said conductive
and conducting layers.
2. A solid-state image intensifier according to claim 1, in which
said zinc sulfide is activated with copper and aluminum.
3. A solid-state image intensifier according to claim 1, in which
said binding material selected from the group consisting of a
vitreous material such as boro-silicate glass enamel and a plastic
such as epoxy resin.
4. A solid-state image intensifier according to claim 1, in which
continuous layers of a resistive and light-reflective material and
a resistive light-opaque material are interposed between said
photoconductive layer and said electroluminescent layer.
5. A solid-state image intensifier according to claim 4, in which
said light-reflective layer comprises particles of ferroelectric
material such as BaTiO.sub.3 mixed with a resistive plastic.
6. A solid-state image intensifier according to claim 4, in which
said light-reflective layer comprises particles of ferroelectric
material such as BaTiO.sub.3 mixed with a resistive material such
as TiO.sub.2 and a plastic such as epoxy resin.
7. A solid-state image intensifier according to claim 4, in which
said light opaque layer comprises a powdered resistive material
such as CdS:Cl.
8. A solid-state image intensifier according to claim 1, in which
the connections of said DC voltage source are selected that the
positive pole thereof is connected to said electroluminescent layer
side and the negative pole thereof is connected to said
photoconductive layer side.
9. A solid-state image intensifer according to claim 1, in which a
unidirectional field provided by said DC voltage source is varied
to shift the characteristic curve to low input energy range without
any appreciable change in contrast ratio and gamma value.
Description
This invention relates to a solid-state image intensifier including
an electroluminescent layer and a photoconductive layer as elements
thereof. More particularly, the invention relates to such image
intensifers which are adapted for use as amplifiers of radiant
energy or as converters of invisible radiation into visible
radiation.
In the known solid-state image intensifiers comprising an
electroluminescent phosphor and a photoconductor, the
electroluminescence is controlled by an AC field in response to
changes in the impedance of the photoconductor caused by radiant
energy excitation. However, due to its geometric structure the
photoconductor has a predominantly capacitive AC impedance in the
dark or in a low input energy state, resulting in a low
photoconductive sensitivity. This imposes a limitation on the
operating range of the image intensifier to high input energy
range.
It is, therefore, an object of this invention to provide a new and
improved solid-state image intensifier having a wide operational
range including the low input energy range.
It is another object of this invention to provide a solid-state
image intensifier which includes an electroluminescent layer having
a nonlinear resistance and which is operated by AC and DC
voltages.
It is a further object of this invention to provide a solid-state
image intensifer in which a unidirectional voltage applied thereto
is varied to shift the characteristic curve to the low input energy
range without appreciable changes in contrast ratio and gamma
value.
It is yet a further object of this invention to provide a
solid-state image intensifier having an increased output luminosity
and an improved resolution and which is easy to manufacture.
These and other objects of this invention will be apparent from the
following description when taken in conjunction with the
accompanying drawings, in which:
FIG. 1 is a schematic sectional view of a solid-state image
intensifer according to this invention;
FIG. 2 is an equivalent circuit of the image intensifier shown in
FIG. 1;
FIG. 3 is a plot of output luminosity against intensity of input
radiant energy for the solid-state image intensifier of FIG. 1;
FIG. 4 is a plot of output luminosity against volume percentage of
electroluminescent phosphor in an electroluminescent layer; and
FIG. 5 is a voltage-current characteristic of the
electroluminescent layer.
Referring now to the drawings and in particular to FIG. 1, there is
illustrated a fragmentary sectional view of a solid-state image
intensifier or light amplifier constructed in accordance with this
invention. In the figure, reference numeral 11 designates a
transparent support member or glass plate by which is coextensively
supported a transparent conductive film 12. The transparent
conductive film 12 is used as a first electrode and may comprise
metal oxides such as tin oxide. A layer of electroluminescent
material 13 is formed on the transparent conductive film 12. The
electroluminescent material may be constituted of particles of
electroluminescent phosphor embedded in a dielectric material and
having the property of emitting light under the influence of an AC
electric field. It may comprise, for example, zinc sulphide
activated with copper and aluminum and mixed with a suitable
plastic such as epoxy resin. Alternatively, an electroluminescent
layer 13 comprised of phosphor mixed with a vitreous enamel such as
boro-silicate glass enamel may be used. The electroluminescent
layer 13 is approximately fifty microns thick and has a nonlinear
resistive impedance. It has been found that 45 to 70 percent by
volume of the electroluminescent phosphor provides a proper
non-linear resistance. The nonlinear resistance of the
electroluminescent phosphor itself is utilized to accomplish this
invention. If the volume percentage is below 45 percent, the
particles of electroluminescent phosphor are insulated from each
other by means of their surrounding binding material. With the
percentage about 70 percent, the electroluminescent layer 13 is
porous, having so low a mechanical strength that it is impossible
to machine the layer. Further, the layer 13 represents a saturation
in its luminosity with reduced light output.
In our experiments, the boro-silicate glass enamel is, for example,
a compound containing, by weight percentages, SiO.sub.2 14.5 to
44.1%, B.sub.2 O.sub.3 23.7 to 28.7%, ZnO 2.2 to 23.5%, BaO up to
14.6%, Na.sub.2 O 10.9 to 15.4%, K.sub.2 O up to 4.2%, TiO.sub.2 up
to 9.0%, Al.sub.2 O.sub.3 up to 2.7%, and CaO, MgO, Fe.sub.2
O.sub.3 and PbO up to 1.2%, and having a softening point of
45.degree. to 515.degree.C and a volumetric thermal expansion
coefficient of 260 .times. 10.sup.-.sup.7 to 340 .times.
10.sup.-.sup.7 /.degree.C. The boro-silicate glass enamel described
above yields especially good results. In this case, a heat
resisting substrate, for example, such as soda glass plate which
has a higher softening point than that of the boro-silicate glass
enamel and substantially same volumetric thermal expansion
coefficient thereof is used as the transparent support member. The
soda glass has a softening point of 690.degree.C and a volumetric
thermal expansion coefficient of 310 .times. 10.sup.-.sup.7. The
electroluminescent phosphor powder is mixed with the boro-silicate
glass enamel powder by the volume percentage mentioned above and
with an organic solvent such as alcohol. The mixture is applied in
proper thickness on the surface of the transparent conducting
layer, which is heated at a certain temperature. The temperature is
higher than the softening point of the glass enamel material but
lower than that of the transparent support member. For example, the
temperature may range from 500.degree. to 670.degree.C. Thus, the
electroluminescent layer is formed on the transparent conducting
layer.
Deposited upon and in contact over an extended area with the
electroluminescent layer 13 is a resistive light-reflecting layer
14 comprising particles of light-reflecting ferroelectric material
such as BaTiO.sub.3 mixed with a resistive plastic. Instead of the
resistive plastic, a suitable plastic such as epoxy resin mixed
with a resistive material such as TiO.sub.2 may be used. This layer
14 is approximately 10 microns thick.
Deposited upon and in contact over an extended area with the
resistive light-reflecting layer 14 is a resistive light opaque
layer 15 which may comprise a powdered resistive material such as
CdS:Cl. This layer 15 is approximately 10 microns thick. One
function of these resistive layers 14 and 15 is to prevent the
nonlinearly resistive electroluminescent layer 13 from being
damaged as a result of dielectric breakdown by a DC voltage as
applied thereto. Correction of the resistance of the
electroluminescent layer 13 as well as impedance matching of the DC
circuit is provided by the resistive layers 14 and 15. It should be
noted in this connection that since the ferroelectric material such
as BaTiO.sub.3 provides an increased mean dielectric constant for
the resistive light-reflecting layer 14, the electroluminescent
layer 13 is effectively energized by alternating voltages with
reduced AC voltage loss. Further, since the ferroelectric material
has a high specific resistance, the resistance of the
light-reflecting layer 14 can be easily controlled by adjusting the
amount of the material mixed with the binder. On the other hand,
since the light-opaque layer 15 comprises a powdered resistive
material, it is very easy to control the resistance of the layer by
adjusting the amount of the resistive material mixed. The
resistances of the two intermediate layers 14 and 15 may be linear
or nonlinear. It is preferable that, when a DC voltage applied
across the solid-state image intensifier, the resistances of the
intermediate layers are at least smaller than the dark resistance
of the photoconductive layer.
Deposited upon and in contact over an extended area with the
resistive light opaque layer 15 is a photoconducting layer 16
having a thickness ranging from approximately 200 to 500 microns.
The material of this photoconducting layer 16 may, for example,
comprise cadmium sulfide, cadmium selenide or cadmium
sulfo-selenide activated by copper, silver, chlorine, aluminum or
gallium. More generally the photoconductive layer 16 may, for
example, comprises the sulfides, selenides, or telurides, of
cadmium, lead, or zinc, or may be any other known photoconductor
mixed with a suitable plastic binder. The photoconductive layer 16
has its impedance varied under the influence of radiations, such as
light, X-rays, infrared rays or ultraviolet rays.
Finally, upon the photoconducting layer 16, there is deposited in
contact over an extended area therewith a second conducting
electrode 17 which may be a conducting layer of metal oxides such
as tin oxide (SnO.sub.2) or a film formed by evaporating a metal
such as aluminum on the photoconductive layer 16. Alternatively, an
electrode consisting of a plurality of wires arranged in uniformly
spaced parallel relationship, or an apertured or grid-like
electrode may be used. The second conducting electrode 17 is
permeable to an input energy signal I in the form of visible light,
X-rays, infrared rays or ultraviolet rays. Electrical contact is
made with both of the conducting electrodes 12 and 17 so that lead
wires 18 and 19 may be brought out from these electrodes 12 and 17,
respectively. Connected between these lead wires 18 and 19 are a
source 20 of alternating current in series with a source 21 of
direct current. The voltage of the DC voltage source 21 may be
varied as desired. The connections of the DC voltage source 21 is
so selected that the positive pole thereof is connected to the
first conducting electrode 12 through the source 20 and the
negative pole thereof is connected to the second conducting
electrode 17.
Turning now to FIG. 2, there is shown an equivalent circuit of the
solid-state image intensifier shown in FIG. 1. The solid-state
image intensifier is capable of reproducing a positive output image
O which is a replica of the image incident thereon. In the figure,
reference character R.sub.p designates a resistance of the
photoconductive layer 16. The resistance R.sub.p varies in response
to radiant energy excitation. In parallel with resistance R.sub.p
is a capacitance C.sub.p of the photoconductive layer 16. An arrow
is used to indicate the input radiant energy I falling upon the
photoconductive layer 16. A parallel combination of a resistance
R.sub.e and a capacitance C.sub.e, which correspond to the
resistance and capacitance of the electroluminescent layer 13,
respectively, is connected to the parallel combination of the
resistance R.sub.p and the capacitance C.sub.p. The resistance
R.sub.e is nonlinearly variable as a function of DC voltage applied
across the electroluminescent layer 13. The range over which the
resistance R.sub.e varies is appropriately larger than the range
over which the resistance R.sub.p varies in response to radiant
energy excitation. Another arrow is used to indicate the reproduced
image output O.
As shown, a unidirectional voltage or DC voltage of varying
magnitudes and an AC voltage are connected in series between the
serially connected two parallel combinations of resistors and
capacitors. When the DC supply voltage is adjusted to be zero, the
DC voltage V.sub.e is zero, thus increasing the nonlinear
resistance R.sub.e to an extremely large value. Consequently, the
AC impedance of the electroluminescent layer 13 is predominantly
capacitive such as a conventional photoconductive layer. The DC
voltage V.sub.p across the photoconductive layer is also zero,
resulting in the image intensifier having substantially no
sensitivity in the range of low input energy such as conventional
image intensifiers of AC operation.
When the magnitude of the DC voltage is increased, a DC voltage
V.sub.p is applied across the photoconductive layer 16, the DC
voltage V.sub.p being determined by the ratio of R.sub.p to
R.sub.e. Under such conditions, when the input energy I is
increased from low to high energy level, the resistance R.sub.p
decreases, with the resultant decrease in the voltage V.sub.p. This
invites an increase in the voltage V.sub.e, which, however, causes
a reduction in the nonlinear resistance R.sub.e. Thus, there occurs
substantially no change in the ratio of R.sub.p to R.sub.e. It is
to be understood that the voltage V.sub.p as applied across the
photoconductive layer is kept at a substantially constant value by
the action of the voltage-controlled nonlinear resistance
R.sub.e.
It is well known in the art that the photoconductive sensitivity of
a photoconductive powder layer when operated by an AC voltage
increases with the increase of a DC voltage superimposed thereon.
As mentioned above, since the photoconductive layer 16 of the
present solid-state image intensifer has a relatively large DC
voltage V.sub.p to be superimposed on an AC voltage applied across
the photoconductive layer, the photoconductive sensitivity is
greatly improved as compared to that measured when V.sub.p = 0.
This results in improvement of the overall sensitivity of the
solid-state image intensifier against an input energy signal.
According to this solid-state image intensifier, therefore, it is
possible to obtain a sufficient luminosity with a relatively low
input energy signal applied. With the DC supply voltage V
increased, the characteristic curve of the image intensifier is
continuously shifted to a lower input energy range without
appreciable changes in contrast ratio and gamma value.
FIG. 3 is a plot of output luminosity of an input energy signal, in
which various curves represent different values of the DC supply
voltage V. The magnitude of the DC supply voltage V was varied from
0 to 400 volts, with the AC supply voltage V kept at 300 volts at a
frequency of 1 KHz. A radiation of X-rays was employed as the input
energy signal I. As will be seen from FIG. 3, the increase in the
DC supply voltage V causes the characteristic curve to Shift to
lower input energy side, without producing appreciable changes in
contrast ratio and gamma value.
FIG. 4 is a plot of output luminosity against volume percentage of
electroluminescent phosphor particles in the electroluminescent
layer 13 when energized by a constant AC field. In the plot, the
output luminosity of a conventional electroluminescent layer
comprising 20 percent of electroluminescent phosphor and 20% of
SnO.sub.2 is indicated as at 22 by comparison. The SnO.sub.2 has a
good reflectivity to the luminous spectra of the electroluminescent
layer and is employed as a resistive powder mixed with the
electroluminescent phosphor. As will be apparent from the
inspection of FIG. 4, according to this invention, the maximum
output luminosity can be obtained with 70 percent of
electroluminescent phosphor contained in the layer. In this
instance, the electroluminescent phosphor is used itself as a
non-linear resistive powder. In the conventional electroluminescent
layer, it is necessary to have a resistive powder such as SnO.sub.2
mixed with the electroluminescent phosphor in order to provide
resistance to the electroluminescent layer. This leads to a
reduction in the percentage of the electroluminescent phosphor in
the mixture and consequently to a decrease in the output
luminosity. Further, in manufacturing the layer there is a
likelihood of particles of the electroluminescent phosphor being
dispersed nonuniformly in the layer. However, since, as described
above, the electroluminescent layer according to this invention
contains substantially no additional resistive powder as a material
imparting resistance to the electroluminescent layer, it has an
increased output luminosity and is easier to manufacture. Moreover,
the present solid-state image intensifier has an improved
resolution because of its sandwiched structure.
FIG. 5 illustrates a nonlinear voltage-current characteristic of
the electroluminescent layer containing 50 percent of
electroluminescent phosphor. With the volume percentage of the
electroluminescent phosphor varied from 45 to 70 percent, the
electroluminescent layer varies its resistance over a range of two
orders.
As has been described above, the invention provides a new and
improved solid-state image intensifier having a wide range of
operation, an increased output luminosity and an improved
resolution and which is easy to manufacture.
* * * * *