Heat radiation detection device and presence detection apparatus using same

A device for detecting thermal radiation having a detector and a reflector type concentrator. The detector is arranged to have one or more portions exposed to thermal radiation and one or more portions protected from thermal radiation, wherein the detector delivers detection signals on the basis of a temperature difference between the exposed portions and the protected portions. The detector supports pairs of planar thermocouple elements having a cold junction in contact with the protected portion of the detector and a hot junction in thermal contact with the exposed portion of the detector. The reflector type concentrator concentrates thermal radiation coming from a three-dimensional zone. The detector and the reflector type concentrator are combined to define a zone under surveillance and detect any change in the state of thermal unbalance in the zone under surveillance.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a device for detecting thermal radiation,
 and to presence detection apparatus based on such a device. The invention
 can be used particularly, but not exclusively, for detecting intrusion
 into a security zone under surveillance.
 2. Related Art
 In the field of high-security surveillance, e.g. monitoring premises for
 storing nuclear material, it is essential for monitoring objects that are
 in the zone under surveillance failure and without interruption.
 At present, in order to ensure continuity of knowledge concerning a
 situation that is under surveillance, e.g. a quantity of fissile material,
 and in order to keep a zone of finite dimensions under surveillance, use
 is made of systems based on seals, whether electronic or otherwise (with
 identity and/or integrity), of optical surveillance systems (using
 cameras), and of a range of conventional detectors.
 Systems based on seals are generally insufficient because most of them can
 be used only once, and they are incapable of providing remote
 surveillance.
 With video surveillance systems, the large amount of investment that is
 required, the image and signal processing that is necessary for detecting
 an intrusion, and the possibility of error by a spurious image, lead to
 enormous drawbacks when used in high security surveillance setups.
 In most detection applications, traditional surveillance systems based on
 ultrasound are used for measuring radiation from moving sources. The
 principle on which they are based is difficult to use for remote
 surveillance while still allowing movement to take place in a portion of
 the same premises that is not under surveillance.
 Surveillance by means of ultrasound detectors becomes impossible whenever
 some movement is to be expected around the boundaries of the zone under
 surveillance since such detectors respond specifically to movement and
 molecular vibration.
 Numerous pyroelectrical and semiconductor devices are commonly used for
 detecting thermal radiation emitted by a moving source. Since thermal
 radiation varies with the emissivity of a surface, such detectors suffer
 from the major drawback of having their responsiveness dependant on
 wavelength. It is therefore necessary to select a detector as a function
 of the source to be detected since the detector operates in a narrow
 frequency band. In addition, detectors of those types are themselves
 heated by the radiation they receive, and they always require a cooling
 system.
 It can thus be deduced that for an application seeking to provide high
 security surveillance of a specified zone, the systems mentioned above
 present at least one of the following drawbacks:
 responsiveness is nearly always dependent on wavelength, thereby making it
 necessary to select a specific detector as a function of the source to be
 detected;
 with infrared or ultrasound systems, only movement is detected in the
 detection area. Such systems provide detection information due only to the
 movement of a source of thermal radiation in the zone under surveillance,
 but no information concerning presence;
 an optical lens is generally used for focusing incident rays, thereby
 giving rise to an undesirable filter effect;
 encapsulation is normally necessary in order to eliminate external noise
 resulting from convection currents;
 a cooling device needs to be provided in many cases; and
 with ultrasound systems, it is not possible to focus surveillance on a
 particular zone of finite dimensions without suffering disturbances from
 people or objects moving around the boundaries of said zone.
 Thus, an object of the present invention is to provide a device for
 detecting thermal radiation that does not suffer from the above-mentioned
 drawbacks.
 SUMMARY OF THE INVENTION
 The device of the invention is characterized in that it comprises:
 a detector having one or more portions designed to be exposed to the
 thermal radiation, and one or more portions designed to be protected from
 the thermal radiation, the detector delivering a detection signal on the
 basis of a temperature difference between the exposed portions and the
 protected portions; and
 a reflector type concentrator associated with said detector to concentrate
 thereon the thermal radiation coming from a predetermined
 three-dimensional zone.
 Combining a reflector type of concentrator with a detector of the
 above-specified type makes it possible to define a zone under surveillance
 accurately and to detect any change in the state of thermal unbalance in
 the zone under surveillance. It will be observed that the detector
 responds to a small temperature difference representative of the
 difference between the amount of energy absorbed and transformed into heat
 flux in exposed regions and in non-exposed regions.
 Advantageously, the concentrator is a parabolic reflector.
 In a preferred embodiment, the detector is mounted to move relative to an
 axis of the concentrator in order to make it easy to adapt the detection
 field merely by adjusting position.
 Also, the use of a concentrator advantageously replaces the use of an
 optical lens for focusing the incident thermal radiation, thereby avoiding
 transmission losses or losses due to the capacitive effect of the lens
 material.
 The operating principle and the manufacturing technology used for the
 detector make it possible to avoid conventional cooling problems which
 require cryostats or Peltier effect devices which are expensive and
 consume power.
 Even at very low temperature, the detector is sensitive to any object that
 emits thermal radiation in a very broad band extending from 0.75
 micrometers (.mu.m) to 1,000 .mu.m, i.e. covering the range from the
 visible to microwaves. It makes it possible to provide information
 representative of instantaneous variations in energy unbalance between a
 system and its environment, and can thus operate as a heat flux meter
 under varying conditions.
 In a preferred embodiment, the detector is made up of at least one pair of
 planar thermocouple elements having a cold junction in thermal contact
 with a protected portion of the detector, and at least one hot junction in
 thermal contact with an exposed portion of the detector.
 The exposed portion(s) and the protected portion(s) of the detector may
 constitute a common active surface of the detector, being respectively
 constituted by surface elements that are substantially absorbent and by
 surface elements that are substantially reflective relative to thermal
 radiation.
 In an embodiment having a shape that is particularly advantageous, the
 detector comprises a plurality of pairs of thermocouple elements mounted
 in series and disposed in a plurality of lines interconnected at their
 ends so as to form a meandering path, the cold and hot junctions being
 disposed in respective alternate rows extending across the lines and being
 in thermal contact with a material which presents the sensitive surface in
 the form of reflective strips and absorbent strips arranged in rows of
 alternating phase in alignment respectively with the cold junctions and
 with the hot junctions.
 By way of example, the thermocouple elements are made up of alternating
 elements of copper and of constantan deposited on a substrate of
 electrically insulating material such as the material known under the name
 "KAPTON" which is a trademark registered by Dupont de Nemours, and the
 thermocouple elements are covered in a material that is substantially
 transparent to thermal radiation, which may also be KAPTON, the portions
 of the material covering the cold junction being coated in a layer of
 reflecting material. The reflective portions may be obtained by depositing
 gold on the surface of the material covering the thermocouples.
 Advantageously, the surface of the substrate opposite from its surface
 supporting the thermocouple elements is coated in a layer of metal for
 being put into contact with a mechanical support surface.
 The invention also provides presence detection apparatus comprising at
 least one thermal radiation detector device of the above-specified type,
 and control means receiving the detection signal(s) and issuing a signal
 that indicates presence as a function of the detection signal(s).
 Other advantages and characteristics of the invention appear on reading the
 following description of a preferred embodiment given purely by way of
 example, with reference to the accompanying drawings, in which:

DETAILED DESCRIPTION OF THE INVENTION
 FIG. 1 shows a detection assembly 10 comprising a parabolic reflector 12
 and a thermal radiating detector 14. The detector has an active surface
 held facing the reflector by a support structure 16. The reflector 12 is
 held securely around its circumference by a case 18. The support structure
 comprises a first rod 20 secured to one end of the case and extending
 parallel to the main axis A-A' of the reflector 12 in its flare direction.
 A second rod 22 is slidably mounted on the first rod 20 and at right angles
 relative thereto. The second rod 22 is terminated towards its free end by
 a right-angled portion 22a in alignment with the main axis A-A' of the
 reflector 12. The free end of the portion 22a of said second rod 22 has a
 support surface 24 for the detector 14, thus enabling the detector to be
 positioned at a determined point relative to the reflector, e.g. at its
 focus, by sliding the second rod 22 along the first rod 20.
 The detection assembly is fixed to a base by means of fixing tabs 26
 projecting from the case 18.
 FIGS. 2 and 3 show the thermal radiation detector 14 used in the detection
 assembly in greater detail, but in a form that is simplified, particularly
 with respect to the number of active elements. The detector 14 comprises a
 substrate 30 of flexible material, such as "KAPTON", of thickness lying
 substantially in the range 20 micrometers (.mu.m) to 60 .mu.m. The surface
 30a of the substrate that is designed to face the support of the detector
 24 has a deposit of isothermal material 32, e.g. a metal such as copper,
 of a thickness lying substantially in the range 20 .mu.m to 100 .mu.m.
 The other surface 30b of the substrate 30 has a succession of thermocouple
 elements 34, 36 forming a continuous track 38. The track is configured in
 the form of a plurality of parallel main lines that are about 5 mm long
 and about 0.5 mm wide, interconnected at their ends so as to form a
 continuous path that meanders from one end to the other of the track. The
 track 38 is made up of a deposit of constantan coated in copper on
 successive discontinuous short portions 38a. Each copper/constantan
 interface at the surface of the track 38 creates a planar thermocouple
 junction 40a, 40b.
 The configuration of the main parallel lines of the track 38 is such that
 the thermocouple junctions 40a, 40b are disposed in rows perpendicular to
 the main lines of the track.
 The track 38 is covered in a film 42 having a thickness of 5 .mu.m to 25
 .mu.m and made of a material that is substantially transparent to thermal
 radiation and that is electrically insulating, e.g. KAPTON. The surface
 42a of this film opposite from its surface 42b which is in contact with
 the track 38 (FIG. 3), constitutes the active surface of the detector. Its
 function is to respond to thermal radiation by creating alternating cold
 and hot areas over the successive thermocouple junctions 40a, 40b along
 the track 38.
 To this end, the film 42 is coated, on surface portions over junctions that
 are to define cold junctions 40a, in a deposit of reflective material 44,
 e.g. a layer of gold having thickness lying in the range 0.1 .mu.m to 5
 .mu.m. In this way, the reflecting material 44 comprises a series of
 strips overlying every other row of junctions. The surfaces 46 of the film
 42 that are not coated in reflecting material 44 constitute absorbent
 surface portions situated in complementary manner over the junctions that
 are to define hot junctions 40b. These surface portions 46 may optionally
 be covered in a material that is highly absorbent to thermal radiation,
 such as a black paint. Optionally, for an even more absorbent active
 surface 42a, the film may be made of KAPTON of the "special black body"
 type as sold by Dupont de Nemours.
 When the active surface 42a of the detector 14 receives thermal radiation
 due to the appearance of a heat source, e.g. a person, the portions 46 of
 the film 42 that are not coated in reflecting material 44 are subjected to
 a temperature rise that is greater than that to which the portions coated
 in reflecting material are subjected. This generates a temperature
 difference on the active surface of the detector which is transmitted to
 the underlying thermocouple junction elements 34, 36. As a result, an
 alternating sequence of cold and hot junctions 40a, 40b is obtained along
 the track 38 of junctions. Each pair of adjacent cold and hot junctions
 40a, 40b produces an electromotive force by the Seebeck effect. A
 succession of elementary batteries is thus obtained in series along the
 track 38, thereby delivering a detection signal in the form of a potential
 difference between the two ends of the track 38. Ohmic contacts 48
 soldered to the two ends serve to pick up the detection signal. This
 signal is proportional to the temperature difference created in response
 to the thermal radiation concentrated by the reflector 12 on the active
 surface of the detector 14. It is therefore representative of the thermal
 flux received on the active surface 42a of the detector 14.
 The detector 14 is easily implemented by the person skilled in the art of
 printed circuit technology. The device is extremely flat, its thickness
 being less than 0.2 mm, it is of rectangular or square format, or indeed
 of circular format, and it occupies an area that typically lies in the
 range 0.25 cm.sup.2 to 1 cm.sup.2. It typically comprises 150 to 200
 thermoelectric junctions in series, however fabrication technology makes
 it possible to implement a much larger number of junctions on a common
 substrate.
 A detector of the type described provides typical sensitivity in the range
 1.5 .mu.Vm.sup.2 /W to 2.0 .mu.Vm.sup.2 /W, and presents very low internal
 electrical resistance, of the order of 200 ohms. This characteristic makes
 it possible to amplify the signal by a large factor, in the range 1,000 to
 10,000, or even more in some applications, without being troubled by noise
 from the source.
 In radiation detection mode, the detector has a time constant of about 100
 ms.
 The fabrication technology and the choice of component materials for the
 detector make use possible at ambient temperatures up to 250.degree. C. by
 contact.
 FIG. 4 is a block diagram of an example of a surveillance system
 implemented using the detection assembly 10 of FIG. 1.
 In the example, a plurality of thermal radiation detection assemblies 10
 are installed in premises 50, such as a warehouse. Each assembly has a
 well-defined detection field corresponding to a specific surveillance
 zone. The detection field is matched to the required surveillance zone by
 acting on one or more of the following parameters: the shape of the
 reflector 12, its size, the distance between the detection assembly and
 the base of the surveillance zone, the orientation of the reflector in the
 surveillance zone, and possibly also the position of the sensitive surface
 of the detector 14 relative to its reflector 12. With a parabolic
 reflector of small size, a relatively uniform detection field is obtained
 over a conical volume around the detector. The base of the cone, which
 defines the detection surface, can easily be modulated to obtain circular
 outlines or oval outlines of various dimensions by acting solely on the
 distance and the orientation of the reflector relative to said detection
 surface.
 The output signal from each detection assembly is transmitted to a
 respective preamplification stage 52 of gain that is adjustable over a
 range of about 100 to about 10,000. The preamplified signal may optionally
 be processed by filter, peak limiting, or digitizing units (given overall
 reference 54) depending on the nature of the surveillance.
 After preamplification and optional processing, the signals are applied to
 respective channels 56 of a detector 58 for detecting the rate at which
 the amplitude of the signal varies (dV/dt). The detector is associated
 with a computer unit 60 programmed to identify the waveforms of signals
 that correspond to an intrusion. When such a condition is detected, the
 computer unit 60 sends a detection signal to an alarm station 62,
 optionally together with data indicating the zone concerned and the nature
 of the intrusion.
 FIG. 5 is an example of readout, in the form of a graph, of the signals
 picked up by a FIG. 1 detection assembly installed in a corridor under
 surveillance. The ordinate represents the magnitude of the output signal
 expressed in relative values, and the abscissa represents a time scale.
 At rest, the detector issues a base-line signal having relatively small
 fluctuations. When an intrusion occurs due to the presence of a human
 being, a peak of high amplitude is obtained.
 In the example, the readout shows respective detection peaks for a person
 penetrating into the detection zone and jumping (peak A), a person passing
 in normal manner (peak B), and a person entering on tiptoe (peak C).
 Peaks A to C are plotted for various distances between the person and the
 focus of the detection zone covering the range 2 meters (m) to 7 m, as
 shown in the figure.
 It will be observed that the signal obtained is easily detectable in all of
 the above circumstances, in particular because of the fast response time
 of the detector and the very high sensitivity made possible by combining
 the sensor with a reflective type concentrator. It is possible to envisage
 signal discrimination in the computer unit 60 making it possible to obtain
 information about displacement in space and in time within the zone under
 surveillance, and also concerning the number of people present.
 The detection system provides a detection signal that is caused by a change
 within the detector 14 of the thermal balance between radiation that is
 received and radiation that is retransmitted. The presence of an intruder
 in the detection field constitutes a modification of the temperature map
 of the zone under surveillance, either because extra heat is provided (the
 usual case) or because a heat source is masked, which can happen in an
 industrial site. Given that it is practically impossible to know the
 temperature of each point of the surface that is hidden by an intruder,
 and that the temperature of the intruder must be equal to the mean
 temperature of the surface hidden by the intruder if the thermal balance
 is to remain stable, it is extremely difficult to outwit a surveillance
 system based on devices of the present invention.
 The invention lends itself to numerous variants both concerning the thermal
 radiation detector and concerning the concentrator.
 The concentrator may be complex in shape, designed so that the detection
 field is matched to a perimeter of specific outline. In particular, it is
 possible for this purpose to make use of computer-assisted design tools
 operating on software for performing optical ray tracing.