CA1152789A

CA1152789A - Infrared condensing lenses

Info

Publication number: CA1152789A
Application number: CA000365575A
Authority: CA
Inventors: Naohiro Murayama; Masafumi Yoshida
Original assignee: Kureha Corp
Current assignee: Kureha Corp
Priority date: 1979-11-30
Filing date: 1980-11-26
Publication date: 1983-08-30
Also published as: FR2470980A1; GB2065916A; DE3045203C2; DE3045203A1

Abstract

ABSTRACT OF THE DISCLOSURE
The infrared rays condensing lens for use in an infrared ray sensor or the like contains a Fresnel convex lens composed of a synthetic resin capable of transmitting infrared rays and a membrane or film for preventing the penetration of visible rays disposed on at least one of the surfaces thereof. The membrane or film is formed by spattering or vacuum evaporating an inorganic material capable of substantially interrupting visible rays and transmitting at least a portion of rays in the infrared range. The infrared rays condensing lens is transmissive of a large amount of the infrared ray transmission, and has a short focal length and a small transmittance of visible rays. It has advantages that it can be manufactured by simple manufacturing steps using cheap materials.

Description

llS2789 i BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention relates to an infrared condensiilg lens (here~after referred ~o also as an infrared rays condensing lens).

2. Brief Description of Prior Art It is known of infrared rays sensing elements composed of materials givinglsome electrical response upon the radiation of infrared rays, such materials being as a pyroelectric material producing electric charges by the radia-tion of infrared rays and a photoconductive material changingits electroconductivity by the radiation of infrared rays.
The infrared rays sensing elements have been used in a combination with infrared ray condensers for a variety of infrared ray sensors which have been employed as an infrared ray detecting part in apparatuses such as, for example, a fire alarm for detecting a fire; an invader detecting device for detecting the infrared rays radiating from the human body or other living bodies; a transfer detecting device for detecting a transferring object such as human beings, vehicles or the like passing through a passage or the like on which infrared rays are being radiated; an infrared ray image pickup unit for producing electrical signals correspond-ing to optical images of infrared rays; an infrared ray communication unit for communicating apart with infrared rays and so on.
As an infrared ray condenser, there has been used a con~ex lens in a u~ual form or a concave mirror. Where it is used merely for the detection of infrared rays, the infra-red condenser is designed in a manner adaptable to receive infrared rays from their source at an area as wide as possible i and condense them on an infrared rays sensing element, whereby the sensitivity of the infrared ray sensor is rendered high.
Where it is employed for picking up infrared ray images, the infrared ray condenser can provide a function to focus an outside infrared ray image into t~e image on the sensing surface of its infrared ray sensing element. In either case, the surface of the condenser receiving infrared rays is preferably as big as possible within the scope acceptable from the design of an apparatus or device.
Both the pyroelectric and photoconductive materials constituting the infrared rays sensing elements are also highly sensitive to visible rays so that it is desired to remove noises resulting from the visible rays with the infrared ray condenser in an infrared ray sensor particularly where accurate information with respect to the infrared rays to be sensed is required.
Whexe prior art infrared ray condensers are composed of convex lenses or spherical lenses, D
materials capable of transmitting a wide range of infrared rays and interrupting visible rays have heretofore been used, ¦~
such material being as crystalline materials such as silicone, germanium, sapphire or the like. When such an extremely hard and neve~theless brittle material is used to gi-~e a convex lens or a spherical lens, however, many skilled laborious ~70rk is required to cut the single crystal plate of the crystalline material into a convex shape and then grind and polish it elaborately, thereby causing the manufacturing and material costs to rîse. Accordingly, the use of such crystalline material for usual infrared ray condensers other than high-quality ones such as the infrared ray image ~icku~ unit is not

- 3 --115~789 very appropriate.
On the other hand, where the infrared ray condensers have been of a condensing mirror or a convex mirror, it is required to provide an infrared rays sensing element at the side of the mirror receiving the infrared rays so that the shadow of the infrared rays sensing element falling on the mirror surface becomes the dead angle.
Therefore r where the condensing mirror is used, for example, the infrared ray image pickup unit presents problems that the image are broken or get blurred so that it is inappropriate to apply the condensing mirror to such a unit. And, where the condensing mirror is applied to an apparatus or device which is used merely for the detection of infrared rays, it causes a defect that an amount of the infrared rays to be sensed is reduced. Furthermore, where the condensing point of the condensing mirror is adjacent to the surface of the mirror receiving the infrared rays, it is necessary to mount the infrared rays sensing element adjacent to the infrared rays receiving surface so that it causes disadvan-tages that there are many limits in manufacturing and design-ing on procedures for mounting the sensing element and wiring.
In order to improve the defects and disad-vantages presented in the convex lens and the concave mirror as stated hereinabove, it has now been found that a convex lens composed of a synthetic resin which is very easy to be processed to form a desired shape can be used for an infrared ray condenser in an infrared ray sensor. It has been found, howeverr that as the transmittance of infrared rays of the synethetic resin is unexpectedly poor and the synthetic resin having a little bit greater thickness does not permeate or transmit most of the effective infrared rays, it is found difficult to provide a practically applicable infrared ray condenser with a high condensing efficiency from a convex lens prepared as from such a synthetic resin.
Thus, wavelengths of predominant infrared rays radiating from the human body that is the object for detection by means of an invader detecting device are ~rom 8 to 13 microns and average about lo microns. The wavelength of the infrared rays from carbon dioxide flame that is the main object for detection by means of a fire alarm is about

4.3 microns. Currently known synthetic resins, however, absorb well infrared rays having wavelengths ranging from several microns to several tens microns. Among those synthe-tic resins, a polyethylene which is relatively small in absorbing the infrared rays in the above wavelength range q can transmit only from 20 to 30 percent of the infrared rays having the wavelength of 10 microns when the polyethy-lene lens is 1 mm thick, although the polyethylene lens having a thickness of 0.1 r~m can transmit from 85 to 90 percent of the 10 microns infrared rays.
Where a convex lens is prepared from a mate-rial having a poor transmittance of infrared rays, the size of the lens having a thin thickness as stated here~nabove and a desired shape is rendered so larger while keeping its - 25 similar figures that its thickness is f~rther increased, whereby the convex lens cannot allow most of the effective infrared rays at its central portion to transmit therethrough.
Furthermore, the enlargement of the size of the lens while keeping its similar figures lengthens its focal length so that, in order to control an increase in the focal '

- 5 -... , . .. _ , .

length, the enlargement of the curvatuxe of the lens causes a further increase in the thickness thereof at its center portion.
For example, when a convex lens for condens-ing infrared rays, having the focal length of 50 mm, capable of transmitting 50% or more of the infrared rays having a wavelength of 10 microns at the maximal thickness portion thereof is prepared by a polyethylene, the thickness of the lens should be at largest 600 microns when calculated from the transmittance of infrared rays of the polyethylene so that its maximum size is restricted to about 11 mm. Such a lens is too small in a condensing area and its transmittance of infrared rays is poor. When such a lens having the focal length of 300 microns is preparedj its-transmittance of- -infrared rays is increased to about 70%, but the size ofthe lens is reduced to 7.7 mm so that the condensing area is half as much as a lens having the thickness of 600 microns.
As stated already hereinabove, infrared ray condensers for use in an infrared ray sensor is necessarily designed so as not to transmit visible rays. Since a synthetic resin such as a polyethylene has a transmittance of visible rays larger than silicon, germanium or sapphire, this all the more prevents the synthetic resinous convex lens from being empLoyed as an infrared ray condenser.

.. . . .

lS2789 S~MMARY OF THE INVENTION
Therefore, an object of the present invention is to provide an infrared condensing lens having a large aperture and a great transmittance of infrared rays, thus a large amount of transmission of the infrared rays.
; Another object of the present invention is to provide an infrared condensing lens having a short focal length in comparison with a large amount of the transmission of the infrared rays.
A further object of the present invention is to provide an infrared condensing lens having a small transmittance o~ visible rays in comparison with a favorable transmittance of infrared rays.
A still further object of the present invention is to provide an infrared rays condensing lens which can be manufactured by means of simple manufacturing steps with a cheap material.
Accordingly there is provided an infrared condensing lens comprising a Fresnel convex lens made of a synthetic resin capable of transmitting infrared rays toward the thic~ness of said lens and a membrane or film for preventing the penetration or transmission of visible rays, said membrane or film disposed on at least one of the surfaces perpendicular to the thickness direction of said Fresnel convex lens, being made of an inorganic material capable of substantially interrupting visible rays and providing a transmittance of at least a portion of rays in the infrared range.
~arious other objects, advantages and features of the present invention will become readily apparent from the - 30 ensuing detailed description, and the novel features will be paxticularly pointed out in the appended claims.

-7~

BRIEF DESCRIPTION OF THE DRAWINGS
.
FIG. 1 is a schematic representation illustrating an example of the infrared ray sensors using an infrared rays condensing lens according to the present invention.
FIG. 2 is a partially notched perspecti~e view illustrating one embodiment of the structure of the infrared ray sensor of FIG. 1.

-7a-1~152~789 FIG. 3 is a cross-sectional view illustrating the infrared rays condensing lens shown in FIGS. 1 and 2.
FIG. 4 is a schematic representation illus-trating another embodiment using the infrared rays condensing lens according to the present invention.
FIG. 5 is a schematic representation in ' perspective illustrating an example of an infrared ray image pickup unit to which the infrared rays condensing lens according to the present invention is adaptable.
FIG. 6 is an exploded perspective view illustrating infrared rays sensing elements and electrodes ~ in the infrared ray image pickup unit shown in FIG. 5.
s DESCRIPTION OF THE PREFERRED EMBODIMENTS
,. ..
The infrared ray sensorsas shown in FI~S. 1 to 3 will be described in detail hereinafter.
Referring to FIG. 1 showing the schematic representation illustrating the infrared ray sensor, an infra-s red rays sensing element 10 is composed of a pyroelectric membrane 12 obtainable from a pyroelectric polymer such as, for example, a fluorine compound, e.g., polyvinylidene fluo-ride, polyvinyl fluoride, a vinylidene fluoride-trifluoroethy-lene copolymer or the like, a pyroelectric inorganic material such as, for example, a titan compound, e.g., lead titanate, barium titanate, lead titanzirconate or the like or other pyroelectric materials.
The pyroelectric membrane 12 is provided at its both sides with electrodes 14a and 14b. The electrode 14b ? at the side receiving the infrared rays may be composed of a thin layer prepared from a material capable of absorbing -- . , 1:1527~39 infrared rays well, such as gold black or the like. The electrode 14a at the side opposite to the side receiving infrared rays may be composed of a material reflectingthe infrared rays such as aluminum or the like. Both of the electrodes 14a and 14b are preferably designed so as to allow their surfaces to introduce the infrared rays in an effective manner to the pyroelectric membrane 12. As long as the purpose is fulfilled, any variation and modification may be made in a manner, for example, such that the electrode 14b at the side receiving the infrared rays may be a t~ans-parent electrode, while the electrode 14a at the side opposite to the infrared rays receiving side is composed of an infra-red rays absorbing material as stated hereinabove or such that the electrode 14b may be a transparent electrode, while a pigment capable of absorbing infrared rays may be mixed in pyroelectric membrane 1~ or coated thereon.
The electrodes 14a and 14b may be substan-tially the same in shape and form as the pyroelectric membrane 12 where the infrared ray sensor is employed merely for the purpose of detecting a fire or detecting an invader. However, where the infrared ray sensor is employed for the infrared ray image pickup unit or for detecting the position of a fire for an automatic fire-extinguishing unit, at least one of the elec-trodes may be preferably a structure comprised of a plurality of divided strips or pieces. Accordingly, the shape, size and number of the electrodes may vary in an appropriate manner accordlng to the purpose of use, design and the like of the infrared ray sensors. When the electrodesl4a and/or 14b are divided into plural strips or pieces, impedance converting circuits 16 and detecting circuits 18 may be ' _ g _ ..... _ ..... . _ '` :

provided according to the number of the divided electrodes.
; One of the electrode 14a of the infrared ~, rays sensing element 10 is grounded and the other of the , electrode 14b is connected to the impedance converting circuit 16 comprised of a field effect transistor (FET).
The output side of the impedance converting circuit 16 is in turn connected to the detecting circuit 18 for detecting electric signals (electric current or voltage) from which a desired output can be obtained. The structure as stated hereinabove is generally known so that a detailed descrip-, tion thereon is omitted herein.
, The condensing of infrared rays on the infrared rays sensing element 10 may be conducted by means of the infrared rays condensing lens 20 according to the ~,~ 15 present invention which is disposed in front of the infrared " rays sensing element. Turning now to FIG. 2, there is shown an embodiment illustrating the structure of an infrared ray sensor 24 in which the infrared rays sensing element 10 and the infrared rays condensing lens 20 are assembled in , 20 a cylindrical housing 22.
s As shown in FIG. 2, the infrared rays ' condensing lens 20 is provided at the front open side of the cylindrical case 22. At the middle portion of the cylin-drical housing 22 is provided with a printed circuit board 26 ; 25 in an arrangement substantially parallel to the lens 20.
At the front side of the printed circuit board 26 facing the ,, lens 20, there is attached the infrared rays sensing element , 10, and at the rear or back side thereof opposite to the above front side, there are attached electrical parts 28 ,,.

, 30 constituting necessary circuits. A signal line or code 30 ,., 1~5*789 is drawn to the outside from the rear or back end portion of the cylindrical housing 22 opposite to the front end portion where the lens 20 is mounted. The signal line or code is connected at the other end to a signal presenting unit 32 for indicating signals to the outside such as an alarm, a display device or a voice producing device or a computer or the like.
Referring to FIG. 3, there is shown a detail of the infrared rays condensing lens according to the present invention. The lens 20 is composed of a Fresnel convex lens ~, 10 34 and a membrane or film 36 for preventing the penetration or transmission o~ visible rays, said membrane or film being pro-vided on the flat surface of the lens 34. The lens 34 may ~e comprised of a synthetic resin having a favorable transmittance of infrared rays inciuding a thermoplastic resin such as, for example, olefinic resins, e.g., polyethylene, polypropylene or ethylene-propylene copolymer; fluorine-containing resins, e.g., polyvinyl fluoride, polyvinylidene fluoride or polytetra-fluoroethylene; or an acetylenic resin, e.g., polyacetylene.
The membrane or film 36 for preventing the penetration of visible rays may be comprised of an inorganic material capable of substantially interrupting the visible rays and penetrating or transmitting at least a portion of rays in the infrared range, said inorganic material being, for example, germanium, silicon, an indium compound, e.g., indium antimonide, indium phosphide or indium arsenide or a gallium compound, e.g., gallium antimonide or gallium arsenide. The inorganic material may be allowed to adhere to the flat surface of the lens 34 indirectly or directly ~y spattering, vacuum deposition or other appropriate techniques. The membrane or film 36 is not necessarily provided on the flat surface of the lens and may : ` ~
~s~9 ~

be disposed on a surface opposite to the flat surace thereof or on both of the surfaces thereof.
Generally, a Fresnel lens can ~e greatly reduced in weight by dividing the continuous lens surface into a succession of concentric rings having a cross section as indicated in FIG. 3, the rings being assembled in correct relationship on the flat surface thereof. The surface of the ring is not necessarily a surface (for example, a portion of a sphere) whose cross section in the radial direc-~0 tion has a curved line like the continuous lens surface.
The ring surface may be in a form of the circumferential surface of a truncated cone. In other words, it may be a surface whose cross section has a straight line (or a tangent line) represented by differentiating the above curved line. Accordingly~ the ring may be a small prism in the form of a ring having the surface as described hereinabove. The Fresnel convex lens is a Fresnel lens of the type having the continuous lens surface which corresponds to a surface of a convex lens.
As a Fresnel convex lens is designed by removing portions as much as possible where the rays are penet-rating or transmitting straight through, as compared with usual convex lenses or spherical lenses, it may be rendered considerably thinner than usual convex lenses having the identical focal length and a ratio of the central portion to the circumferential porion thereof in thickness may be rendered small. Accordingly, for example, where a Fresnel convex lens having a lens diameter of several tens mm is prepared by, for example, a polyethylene, the thickness of the Fresnel convex lens may be to the extent of several tens - ~2 -~, microns to several hundreds microns and the Fresnel lens can provide a good transmittance of infrared rays and allow a small difference in the infrared ray transmittance between the central portion and the circumferential portion thereof.
Referring to FIG. 3, the thickness of the , Fresnel convex lens 34 may be determined according to the thickness of a flat plate or a base plate excluding nearly triangular cross-sectional portions 34a, the lens size, the focal length, the number of the concentric rings or the like.
If the number of the concentric rings would be increased, it is theoretically possible to provide a Fresnel convex lens having any thin triangular cross-sectional portions 34a.
However, if the number of the concentric rings would be too many, there may be a case where the annular concentric grooves 34c between each adjacent rings formed on the surface thereof may act as a lattice. Accordingly, as it is difficult to assemble too many concentric rings on the lens surface in order to make the triangular cross-sectional portions 34a extremely thin, the thickness of the Fresnel convex lens may range gene-rally ~bout 10 microns or more, preferably from about 10 microns to 1,000 microns, and more preferably from about 50 microns to 500 microns. A Fresnel lens composed of a synthetic resin having a thickness thicker than 1,000 microns is not practical because of poor transmittance resulting from the absorption of infrare~ rays by the synthetic resin used. For example, where a polyethylene which is said to be least in the absorp-tion of infrared rays, it can transmit or penetrate only 20 to 30% of the infrared rays when its thickness is 1,000 microns, although it can penetrate ~5 to 90~ of the rays when its thick-ness is 100 microns. Although the infrared ray absorption ~ 13 -115Z7~39 of Fresnel convex lenses 34 may vary greatly with the type and kind of synthetic resins to be used as a material therefor, it is preferred that the Fresnel convex lenses have 30% or more, preferably 50~ or more, of the transmittance of infrared rays having a wavelength of at least one of 4.3 microns and lO microns.
The Fresnel convex lens 34 may be designed so as to have, for example, a focal length of 40 mm, a diameter of about 50 mm, a pitch width (a distance between the grooves 34c) of about 0.2 mm (200 microns) and a thickness of about 0.4 mm (400 microns). In this case, the number of the concen-tric rings may be about 250. The thickness of the lens~may be such that the thickness of the flat portion or base portion 34b is about 0.2 mm,and the both thickest or outermost triangular cross-sectional portions 34a may be about 0.2 m~ (200 microns) thick.
The membrane or film 36 for preventing the penetration or transmission of visible rays may be preferably 0.02 micron or thicker, although a membrane or film having i a thlckness of 0.01 micron may be optionally used because it can interrupt a considerable amount of visible rays. The upper limit on the thickness of the membrane or film 36 is not particularly necessary; however, the thicker the membrane or film is, the more the differential thickness due to irregularity in thicknesses may also be. Accordingly, the reflective index of the infrared rays in the membrane or film 36 composed of germanium, silicon or the like is so irregular that the condensing performance or effect of the lens is impaired.
It is accordingly disadvantageous to thicken the membrane or film 36 to an unnecessary level and its thickness may be s ` - 14 -`~
~1527~1g ~, preferably about 1 micron or thinner.
For example, another membrane or film for the prevention of reflection may be provided on the membrane or film 36 for preventing the penetration of visible rays.
S The reflection preventing membrane or film may be composed of, for example, silicon monoxide, cerium dioxide, zinc sulfide or the like and disposed on the surface of the mem-brane or film 36 for preventing the penetration of the visible rays by means of vacuum de~osition or other suitable J
techniques. As the reflection preventing membrane or film is required to be provided at the incidence side of the Fresnel g convex lens 34, it is at the side of the flat surface of the lens when the rays are falling upon the flat surface thereof and vice versa. Accordingly, where the membrane or film 36 is at a side opposite to the incidence side of the Fresnel convex lens 34, the membrane or film for the prevention of reflection is provided at the side opposite to the membrane or film 36. As shown in FIG. 3, however, the reflection preventing membrane or film may be provided on the surface of the membrane or film 36 for preventing the penetration or transmission of visible'rays, whereby both of the membranes or films having uniform thicknesses may be easily formed.
The thickness of the reflection preventing membrane or film may be, in the visible range, as conventionally , 25 presented, as follows:

, d =

~ ' , ! - 15 - ~

j;.
, ~152'789 were: d = thickness~
A = wavelength of infrared rays whose incidence should be prevented, and n = reflective index, The thickness of the reflection preventing film or membrane may be~ for example, about 0,1 micron.
In the infrared ray sensors as shown in FIGS. ~ to 3, infrared rays 40 is radiated from the side of the visible rays preventing membrane or film 36 on the infrared rays condensing lens 20. The infrared rays 40 is condensed by means of the Fresnel convex lens 34 after the penetration or transmission of the membrane or film 36 and then introduced into the pyroelectric membrane 12 of the infrared rays sensing element 10. The infrared rays which fell upon the sensing element 10 cause the production of electric charges due to polarization and produce a potential between the electrodes 14a and 14b on the sensing element 10~ This potentia,l or electric signals are fed through the impedance converting circuit 16 to the detecting unit 18 from which the detecting signals are dispatched. The detecting signals are then fed to the signal presenting unit 32, thereby giving a desired presentation or producing a necessary represen-', ~ tation.
, 25 In place of the pyroelectric light sensing element as employed in the infrared ray sensors shown in FIGS. 1 to 3, inclusive, an infrared photoconductive material comprising an infrared semiconductor such as an infrared photo diode, an infrared photo transistor, an infrared photo , r llS2789 thyr1stor or the like may be employed as an infrared rays sensing element.
FIG. 4 illustrates an example of an infrared ray sensor that uses as an infrared rays sensing element an infrared photoconductive material such as, for example, lead sulfide (PbS), indium antimonide (InSb), cadmium sulfide (CdS) or the like. In FIG. 4, the parts in common with the infrared sensors presented in FIGS. 1 to 3 are indicated by the same numerals. Accordingly, a description thereon is omitted from the description which follows.
In the infrared ray sensor i~lustrated in FIG. 4, too, the same infrared rays condensing lens 20 as used in the infrared ray sensors illustrated in FIGS. 1 to 3 is employed. A pair of the electrodes 14a and 14b are provided at the both sides thereof and connected in series to a bias power source 52 and an ampere méter 54 and, when necessary, to the signal presenting unit 32. In this case, where the infrared ray sensor is used merely for the detec-tion of a ~ire or for the detection of an invader, the elec-trodes 14a and 14b may be in each case a solid electrode which maybe composed of one piece and which may have a margin portion for the protection of a short circuit at the peripheral pcrtion thereof when needed.
Accordin~ly, when infrared rays 40 are radiated upon the infrared ~hotoconductive material 50, the electric current flowin~ through the ampere meter 54 ¦ ~s increased so that the rneasurement of the current can detect the amount of the infrared rays.

~152789 ! In FIGS. 5 and 6 there is shown an example of the infrared ray image pickup unit in which the same infrared d rays condensing lens as used in the infrared ray sensors illustrated in FIGS. 1 to 3 is mounted at the front side of an infrared rays sensing element 60, although an i llustration thereon is omitted in the drawings for brevity. In this example, the parts in common with the infrared ray sensor illustrated in FIG. 4 are indicated by the same numericals, and a description thereon is omitted for avoiding duplicate d explanation.
As shown in FIGS~ 5 and 6, there are provided d at either of the both sides of the sensing element 60 a number of strip electrodes 62a, 62b, ..., and 62n which are arranged in a relationship parallel to each other and also at the other side thereof a number of strip electrodes 64a, 64b, ..., and 64n which are in turn arranged parallel to each other. In FIGS, 5 and 6, the strip electrodes 62a, ~i 62b, ..., and 62n are arranged in a relationship substantially orthogonally intersecting the other group of the strip elec-trodes 64a, 64b, ... , and 64n. However, this orthogonally intersecting arrangement between the two groups of the strip electrodes is not required and one of the groups of the .
strip electrodes may be arranged in an intersecting relation-ship with the other group of the strip electrodes obliguely at an appropriate angle to each other.
The strip electrodes 62a, 62b, ..., and 62n are commonly connected to each other through ON-OFF switches 66a, 66b, ..., and 66n, respectively. Similarly, the strip electrodes 64a, 64b, ..., and 64n are commonly connected to each other through ON-OFF switches 66a, 66b, .. , and 66n, ~-. ,;

!

,. . . ...

1152 7~39 - i ii respectively. One of the common connections is connected in series through the bias power source 52 and the ampere meter 54 to the other common connection and, when necessary, to the signal presenting unit 32.
Thus, when the ON-OFF switches 66a, 66b, and 66n are turned on and off in turn at . constant time intervals and the other group of the ON-OFF switches 68a, 68b, ..., 68n is turned on and off in turn at intervals of a period of time for which the ON-OFF switches 66a, 10 66b, , and 66n aré being turned on and off in turn in the whole round, image PiCkUp signals corresponding to the shape, distribution and the like of an infrared ray image in front of the infrared condensing lens are produced.
Accordingly, with a picture receiving device or unit as the signal presenting unit, the respective infrared ray F
image can be reproduced.
Either of the groups of the strip electrodes may be replaced by a printed electrode as in the case of FIG. 4. In this case, there may be provided a slit plate t 20 having a slit extendingin directions perpendicularly or obliquely intersecting the lengthwise directions of the . rest of the strip electrodes. As the slit plate is moved .
or transferred in a straight direction at a ~,~
; constant speed, an infrared ray i.mage can be produced in such a mode as in FIGS. 5 and 6.
Where an optical chopper capable of intro-ducing infrared rays intermittantly into the infrared rays sensing element is disposed in front of the sensing element ~, ,.

:IL1527B9 `
as in the example illustrated in F~GS. 5 and 6 as well as in the examples illustrated in FIGS. 1 to 4, intermittant infrared ray detecting signals can be produced.
While the present invention is illustrated 1 5 with specific embodiments, it will be recognized by those -¦ skilled in the art that any variation thereon and modifica-tion therefrom may be made therein without departing from the scope of the present inventive concepts of the present invention as defined by the following claims.
' '~

~1, , .-~ .
- 20 - :1

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An infrared condensing lens comprising: a Fresnel convex lens made of a synthetic resin capable of transmitting infrared rays toward the thickness of said lens and a membrane or film for preventing the penetration or transmission of visible rays, said membrane or film disposed on at least one of the surfaces perpendicular to the thickness direction of said Fresnel convex lens, being made of an inorganic material capable of substantially interrupting visible rays and providing a transmittance of at least a portion of rays in the infrared range.

2. The infrared condensing lens according to Claim 1, wherein one of said surfaces of the Fresnel convex lens is a flat surface.

3. The infrared condensing lens according to Claim 1, where said Fresnel convex lens has a 30% or more transmittance of at least one of the infrared rays having wavelengths of 4.3 microns and 10 microns.

4. The infrared condensing lens according to Claim 1, wherein said synthetic resin is a thermoplastic resin selected from the group consisting of an olefinic resin, a fluorine-containing resin and an acetylenic resin.

5. The infrared condensing lens according to Claim 4, wherein said synthetic resin is a polyethylene.

6. The infrared condensing lens according to Claim 1, wherein said inorganic material is germanium, silicon, an indium compound or a gallium compound.

7. The infrared condensing lens according to Claim 6, wherein the indium compound is indium phosphide, indium arsenide or indium antimonide.

8. The infrared condensing lens according to Claim 6, wherein the gallium compound is gallium antimonide or gallium arsenide.

9. The infrared condensing lens according to Claim 1, wherein said Fresnel lens has a maximal thickness of from about 10 to 1,000 microns.

10. The infrared condensing lens according to Claim 1, wherein said membrane or film for preventing the penetration or transmission of visible rays is provided at the surface thereof with a membrane or film for preventing the reflection of rays.

11. The infrared condensing lens according to Claim 10, wherein said membrane or film for preventing the reflection of rays is comprised of silicon monoxide, cerium dioxide or zinc sulfide.

12. The infrared condensing lens according to Claim 1, wherein said lens is constructed for use in condensing infrared rays in an infrared ray sensor and the infrared ray sensing element in said infrared ray sensor is disposed close to a point where the rays through the lens are condensed.