Pyroelectric infrared detector

This invention concerns a pyroelectric infrared detector, comprising a substrate and polymeric pyroelectric elements mounted to both surfaces of the substrate, in which the electrode plates on one of the polymeric pyroelectric element are connected electrically to the electrode plates of opposite polarities on the other of the polymeric pyroelectric elements respectively.

BACKGROUND OF THE INVENTION 
This invention concerns a pyroelectric infrared detector and, more 
specifically, an infrared detector using polymeric pyroelectric elements. 
It is well known that certain types of polymer such as polyvinylidene 
fluoride, polyvinyl fluoride and the like show both pyroelectric and 
piezoelectric properties when subjected to poling procession and, due to 
their easy processability, wide variety of application uses are 
considered. 
Infrared detectors constituted with these polymers as pyroelectric elements 
are, however, defective in that electric signals produced from 
piezoelectric effect due to mechanical actions such as vibration or 
bending are much greater than electric signals produced from pyroelectric 
effect due to incident infrared rays and S/N ratio as the infrared 
detector is very poor as compared with detectors using ferroelectric 
ceramics as pyroelectric elements. In order to overcome such defects, a 
detector using a pair of elements overlapped and connected to each other 
at their electrodes of same polarities so that the piezoelectricity caused 
by the bending of the elements may be offset are disclosed, for example, 
in Japanese Patent Laying Open No. 99869/1977, but the piezoelectricity 
resulted from the strain in the elements along the thickness can not be 
offset and the S/N ratio for the electric signals obtained from the 
infrared detector is not yet satisfactory. 
An object of this invention is to provide a pyroelectric infrared detector 
in which signal to noise ratio in the electric signals can be improved 
significantly. 
Another object of this invention is to provide a pyroelectric infrared 
detector having a highly excellent detection sensitivity for infrared 
irradiation. 
A further object of this invention is to provide a pyroelectric infrared 
detector which can reduce electric signals resulted from piezoelectric 
effect due to mechanical actions applied thereto and process a 
satisfactory S/N ratio even placed under the conditions where many 
vibrations are present. 
A further object of this invention is to provide a pyroelectric infrared 
detector which can offset electric signals resulted from pyroelectric 
effect due to fluctuations in the atmospheric temperature and electric 
signals resulted from piezoelectric effect due to compression strains 
respectively. 
A still further object of this invention is to provide a pyroelectric 
infrared detector which has a good processability and can be formed in a 
small size. 
In accordance with this invention, a pyroelectric infrared detector is 
provided which comprises a substrate and polymeric pyroelectric elements 
each having electrode plates on both surfaces thereof and mounted to both 
surfaces of the substrate and in which electrode plates on one of the 
pyroelectric elements are connected electrically to the electrode plates 
of opposite polarities on the other of the pyroelectric elements, the 
thickness of the substrate is greater than the thickness of the 
pyroelectric elements and infrared rays are adapted to irradiate on one of 
the pyroelectric elements. 
The substrate having a thickness greater than that of the pyroelectric 
element is, desirably, formed so that the product of the modulus of 
elasticity and the thickness is great. In preferred embodiments, the 
substrate can be formed so that its product of the modulus of elasticity 
and the thickness is greater than that of the pyroelectric element by a 
factor of 5, preferably, 10 and, more preferably, 20. Materials usable 
herein for the substrate can include, for example, metal, glass, ceramics, 
plastics or rubber, and the substrate is preferably prepared from 
materials having modulus of elasticity greater than that of the polymeric 
pyroelectric element. 
The polymeric pyroelectric element having electrode plates provided on both 
surfaces thereof can be formed by orienting and polarizing a polymer film 
or membrane made of a homopolymer such as polyvinylidene fluoride or 
polyvinyl fluoride, a copolymer comprising vinylidene fluoride or vinyl 
fluoride as a main component, or a polymer blend comprising either of the 
above homopolymer or copolymer as a main component. In a preferred 
embodiment, the film- or membrane-like pyroelectric element is preferably 
formed to a thickness of 1 to 100 .mu.m and more preferably 2 to 50 .mu.m. 
Materials for the electrode plates provided to both surfaces to the 
polymeric pyroelectric element usable herein include gold, silver, 
nickel-chromium alloy, aluminum or carbon and the electrode plates are 
formed through vapor deposition of these materials on the pyroelectric 
element or through bonding of a film or membrane made of these materials 
on the pyroelectric element. In a preferred embodiment, the electrode 
plates are formed to a thickness approximately equal to or less than the 
thickness of the pyroelectric element. In a further preferred embodiment, 
the electrode plates are preferably formed to a thickness of 10 A to 2000 
A, and more preferably 50 A to 1000 A so that the heat generated by 
infrared irradiation is preferably conducted to the pyroelectric element. 
In another preferred embodiment, the electrode plate used as the incident 
surface of the infrared radiation can be prepared from materials 
transmittable to infrared rays, for example, a transparent material so 
that the irradiated infrared rays can transmit it and directly reach the 
pyroelectric element. 
In the electrode plates thus formed, the electrode plates on one of the 
polymeric pyroelectric elements are connected electrically to the 
electrode plates of opposite polarities on the other of the pyroelectric 
elements, for example, by way of lead wires. The electrode plate of the 
opposite polarity means herein, for example, an electrode plate on which 
electric charges such as positive or negative charges different from those 
on the other electrode plate are induced due to pyroelectric when infrared 
rays are irradiated on both of the pyroelectric elements to result in 
temperature increase in the same direction in both of the pyroelectric 
elements and thus produce pyroelectricity in each of the pyroelectric 
elements. 
In a preferred embodiment of this invention, polymeric pyroelectric 
elements are disposed so that the electrode plates of each of the 
polymeric pyroelectric elements contacting to the surfaces of the 
substrate show the polarities opposite to each other. By disposing each of 
the pyroelectric elements in such a manner, the electrode plates on the 
sides contacting to the surfaces of the substrate can be set in common to 
each of the pyroelectric elements, which, as the result, enables to use a 
substrate prepared by vapor depositing or bonding a conductive membrane or 
layer as the electrode plate over the entire outer surface or a substrate 
made of a conductive material also serving as the electrode plate. This 
can save lead wires for connection. In another preferred embodiment, each 
of the elements is mounted to the substrate so that the electrode plates 
on each of the pyroelectric elements contacting to the surfaces of the 
substrate are on the same polarity. 
An infrared detector is constituted by adapting to irradiate infrared rays 
on one of the pyroelectric elements mounted to both surfaces of the 
substrate. The incident infrared rays may be focussed by way of a lens or 
specific spectrum components therein may be removed through color filters. 
Moreover, infrared rays may be irradiated merely through a transparent 
glass plate or irradiated directly.

PREFERRED EMBODIMENT 
In FIG. 1 and FIG. 2, a disc-like substrate 1 made of ceramics is mounted 
on its one surface 2 with a pyroelectric element 5 having thin film 
electrode plates 3 and 4. The disc-like pyroelectric element 5 is formed, 
for example, by orienting and polarizing a film made of a polymeric 
material such as polyvinylidene fluoride thereby applying it pyroelectric 
property. The pyroelectric element 5 is mounted to the substrate 1 
generally by applying adhesives between the electrode plate 4 and the 
substrate 1 to secure the electrode plate 4 to the substrate 1 where 
electrode plates 3 and 4 are previously formed on both surfaces of the 
pyroelectric element 5 through vapor deposition of gold or silver, or by 
applying adhesives between the pyroelectric element 5 and the electrode 
plate 4 to secure the pyroelectric element 5 to the electrode plate 4 
where the electrode plate 4 is previously formed on the surface 2 of the 
substrate 1 through vapor deposition of gold or silver. 
The incident infrared radiation is generally rendered intermittent by 
chopper means because the pyroelectric element produces pyroelectricity 
corresponding to the differentiation value with respect to the changes in 
temperature. 
The thickness D of the substrate 1 is formed greater than the thickness d 
of the element 5, and the product of the modulus of elasticity and the 
thickness D of the substrate 1 may be set greater than the product of the 
modulus of elasticity and the thickness d of the element 5 by a factor of 
5, 10 or 20. The pyroelectric element 9 having electrode plates 7 and 8 is 
mounted to the outer surface 6 of the substrate 1. While the pyroelectric 
element 9 and the electrode plates 7 and 8 are formed respectively in the 
same manner as the pyroelectric element 5 and the electrode plates 3 and 
4, the pyroelectric element 9 is mounted to the substrate 1 so that the 
element 9 is polarized in the direction shown by an arrow B where the 
element 5 is polarized in the direction of an arrow A. With the 
pyroelectric elements 5 and 9 mounted respectively on both surfaces of the 
substrate 1 in such a manner, the electrode plate 3 on one element 5 and 
the electrode plate 8 on the other element 9 are connected by way of a 
lead wire 10, while the electrode plate 4 on the element 5 and the 
electrode plate 7 on the other element 9 are connected by way of a lead 
wire 11. That is, the electrode plate 3 is connected to the electrode 
plate 8 of the opposite polarity and the electrode plate 4 is connected to 
the electrode plate 7 of the opposite polarity respectively by way of the 
lead wires 10 and 11. With the substrate 1 and the pyroelectric elements 5 
and 9 thus assembled, an infrared detector 13 is formed by adapting to 
irradiate infrared rays 12, for example, on the side of the pyroelectric 
element 5. 
Since the pyroelectric elements 5 and 9 are supported on the substrate 1, 
vibrations or sonic waves are applied, for example, to the detector 13 as 
described above, bending of the pyroelectric elements 5 and 9 is 
suppressed, thus, the value of the piezoelectricity caused by the bending 
is negligible and no substantial electric charges are induced to the 
electrode plates 3 and 4, as well as 7 and 8. 
If compression waves in air such as sonic waves are applied to the detector 
13, compression strains are produced equally along the thickness of each 
of the pyroelectric elements 5 and 9, by which positive electric charges 
are induced to the electrode plates 3 and 7 and negative electric charges 
are induced to the electrode plates 4 and 8 respectively, for example, as 
shown in FIG. 3. Since the electrode plates 3 and 8 are connected by way 
of the lead wire 10 and the electrode plates 4 and 7 are connected by way 
of the lead wire 11 respectively in the detector 13, the electric charges 
induced to these electrode plates are offset to each other. 
While on the other hand, if electric charges are induced to each of the 
electrode plates resulted from the pyroelectric effect due to fluctuations 
in the atmospheric temperature as shown in FIG. 3, these electric charges 
are also offset in the same manner. Consequently, no substantial electric 
noise signals resulted from the piezoelectricity due to bending or 
compression strain or the pyroelectricity due to fluctuations in the 
atmospheric temperature are produced at all between terminals 14 and 15 
connected respectively to the lead wires 10 and 11. 
While on the other hand, upon irradiation of the infrared rays 12, since 
only the pyroelectric element 5 is heated, positive and negative electric 
charges resulted from pyroelectricity are induced respectively to the 
electrode plates 3 and 4 as shown in FIG. 4. Consequently, electric 
signals due to infrared radiation can be obtained between the terminals 14 
and 15. 
More specifically, since the infrared radiation and the heat conduction to 
the pyroelectric element 9 are substantially interrupted by the substrate 
1, no substantial charges are induced to the electrode plates 7 and 8 in 
the irradiation of the infrared rays 12. Consequently, electric charges 
induced in the electrode plates 3 and 4 are not offset to each other but 
produce as they are as electric signals between the terminals 14 and 15. 
As foregoings, in the detector 13, since the noise signals can 
substantially be lowered and, on the other hand, electric signals due to 
infrared radiation to be detected can be issued as they are, S/N ratio in 
the required electric signals can be improved significantly. 
While the electrode plates 4 and 7 are disposed separately from each other 
in the foregoing embodiment, an electrode layer 51 may be formed entirely 
on the outer surface of the substrate 1 and used as the opposing electrode 
plate for both of the pyroelectric elements 5 and 9, for example, as shown 
in FIG. 5. Provision of the electrode plate in such a manner can save the 
lead wire and the wiring work for the lead wire to thereby improve the 
processability. 
In this embodiment, the electrode layer 51 is not necessarily formed for 
the entire outer surface of the substrate 1 but it may be formed in the 
same area as that of the pyroelectric elements 5 and 9 at the portion 
facing to the pyroelectric elements 5 and 9 while formed as a strip at 
other portions. Electrical connection of the electrode layers on the side 
of the pyroelectric element 5 and on the side of the pyroelectric element 
9 by way of the strip can improve the processability, as well as 
preferably reduce the heat conduction from the electrode layer on the 
pyroelectric element 5 to the electrode layer on the pyroelectric element 
9, by which the changes in the temperature can be produced only on the 
side of the pyroelectric element 5 upon infrared irradiation. 
As shown in FIG. 6 and FIG. 7, the detector 13 can be used in combination 
with an impedance converter 61 in a canned casing. The impedance converter 
61 shown in FIG. 6 and FIG. 7 comprises an N type field effect transistor 
62, a resistor 64 connected at one end to the gate 63 of the transistor 
62, and a resistor 66 connected at one end to the source 65 of the 
transistor 62, in which each of the other ends of the resistors 64 and 66 
are connected. The converter 61 and the detector 13 constituted as above 
are combined to the detector 13 by connecting the terminal 14 to the gate 
63 and the terminal 15 to joined ends of the resistors 64 and 66 (wiring 
connection is not shown in FIG. 6). The detector 13 mounted in the casing 
67 is supported by way of a support 68 on the stem 69 and the converter 61 
is also secured on the stem 69. The stem 69 is provided with terminals 70, 
71 and 72, the terminal 70 being connected to the drain 73 of the 
transistor 62, the terminal 71 being connected to the source 65 of the 
transistor 62 and the terminal 72 being connected to joined ends of the 
resistors 64 and 66 respectively. The top of the canned casing 67 opposing 
to the pyroelectric element 5 is formed with a window 74, which is 
provided with a material 75 composed of silicon, germanium or the like 
that can transmit infrared rays. 
By mounting the detector 13 and the impedance converter 61 formed as a 
source follower integratedly in the canned case 67 and combining them, 
electric signals caused by infrared radiation more excellent in the S/N 
ratio can be issued from the terminal 71. In the use of the device shown 
in FIG. 6, a DC power source is connected between the terminals 70 and 72. 
The casing 67 is preferably formed not only to inhibit incidence of 
infrared rays to the pyroelectric element 5 except from the window 74 but 
also to cut off any vibration with directional qualities i.e. sound wave. 
However, there may be provided with some small apertures on the casing as 
long as the said effects substantially are shown to a considerable extent. 
Further, when the external atmosphere is isolated completely, it is 
preferable to seal inert gas such as argon and nitrogen so as to prevent 
from oxidizing the electrodes and the like.