WO2002084239A2

WO2002084239A2 - Pyroelectric infrared detector with parabolic reflector

Info

Publication number: WO2002084239A2
Application number: PCT/GB2002/001659
Authority: WO
Inventors: Ian Weaver
Original assignee: Spectraprobe Limited
Priority date: 2001-04-10
Filing date: 2002-04-10
Publication date: 2002-10-24
Also published as: WO2002084239A3; GB0108952D0

Abstract

A thermal detector (10a,b,c,d) for detecting thermal radiation comprises a first sensor (16) for sensing thermal radiation, located substantially at the focal point of a focusing means such as a parabolic reflector (18) for focusing thermal radiation onto the first sensor. The sensor (16) includes a single pair of electrodes (22,24) between which pyroelectric material (20) is disposed. The detector does not include a chopping mechanism, but is scanned manually over an area of interest to provide an intermittent signal.

Description

A THERMAL DETECTOR

The present invention relates to a thermal detector.

Thermal energy emitted by, for example, a human body, can be detected using a thermal (i.e., infra-red) detector. Generally the type of detector used is a "fixed" detector which comprises an array of detector elements which "look" at different spatial regions.

An example of a fixed detector is disclosed in US Patent No. 4,379,971 (Statitrol). This detector comprises a sensing device which is placed at the focal point of a concave mirror. The sensing device includes a thin strip of pyroelectric material. One side of the pyroelectric material is coated with a continuous layer of metal to form an electrode. The other side of the pyroelectric material is coated with a plurality of electrodes. The outputs of the electrodes are connected to a differential amplifier. As a thermal energy source moves past the view of the stationary detector, a varying output signal from the sensor is generated. The variation of the voltage and polarity of the output signal from the sensor with respect to time determines the relative size and the actual movement of the infra-red source.

One disadvantage with the use of pyroelectric detectors is that, because they respond to a change in temperature rather than absolute temperature, a means of modulating the thermal radiation incident upon the detectors is required. For the case of a detector having a single detector element, modulation is usually carried out via the use of a "chopping" mechanism, such as a tuning fork chopper which comprises paddles which periodically cross, or a simpler chopper such as a disc having cut-away portions which is spun, thereby periodically permitting the passage of thermal radiation.

A disadvantage of a thermal detector which includes a chopping mechanism is that it is more expensive to manufacture than a chopperless detector. In addition, the more moving parts the detector has, the greater the likelihood that such a moving part may fail. However, for the type of fixed detector having a plurality of detector elements which is disclosed above, the action of a thermal radiation source moving across the detector elements acts as a chopping mechanism, and so a separate chopping mechanism is not required. Nevertheless, these types of detectors do have disadvantages. One disadvantage is that more than one detector element is required, and (with the associated electronics) such a detector can be expensive to manufacture. Another disadvantage with fixed detectors is that they do not have a 360 degree field of view.

An aim of the invention is to provide a portable thermal detector. Another aim of the invention is to provide a thermal detector which includes a single detector element and does not require a chopping mechanism. A further aim of the invention is to provide a thermal detector which has less moving parts than other thermal detectors, and is cheap to manufacture. A yet further aim of the invention is to provide a thermal detector which has less moving parts than other thermal detectors, and is therefore more rugged than other thermal detectors.

According to a first aspect of the invention there is provided a thermal detector as claimed in claims 1 to 20.

According to a second aspect of the invention there is provided a method of detecting thermal radiation as claimed in claim 21.

A number of embodiments of the invention will now be described, by way of example only, with reference to the accompanying Figures, in which: -

Figure 1 shows schematically a cross-section of a first thermal detector;

Figure 2 shows schematically a cross-section of a second thermal detector;

Figure 3 shows schematically a cross-section of another thermal detector; and

Figure 4 shows schematically a cross-section of a further thermal detector.

Referring to Figure 1, there is shown a thermal detector (10a) comprising a housing (12) attached to a handle (14), similar to a torch. The detector housing (12) includes a sensor

(16) located at the focus of a concave parabolic reflector (18). The sensor (16) is formed from a layer of pyroelectric material (20) (such as lithium tantalate or vinylidene fluoride trifluoroethylene) sandwiched between a pair of electrodes, i.e., a thin upper electrode (22) held at ground potential, and a lower electrode (24). The lower electrode

(24) is connected to electronics (26) which, in this case, are located in the detector housing (12).

Operation of the detector (10a) will now be described. In use, the entire detector assembly is moved by the user so as to scan (or sweep) the radiation acceptance angle of the detector assembly across an area of interest, in order to locate a thermally emitting body. As the use of a pyroelectric sensor (16) requires the incoming thermal radiation to be intermittent, the sweeping motion acts as the chopping mechanism in a conventional unit. When a thermal body is in the line of sight of the detector, thermal radiation enters the device as shown by the downward arrows A. The radiation is focused by the parabolic reflector (18) onto the upper electrode (22) of the sensor (16). The radiation passes through the upper electrode (22) to the pyroelectric material (20), heating the pyroelectric material. The increase in temperature of the pyroelectric material (20) causes an electric charge to be generated - the size of the electric charge being proportional to the intensity of the radiation. The voltage between the upper (22) and the lower (24) electrodes is measured by the electronics (22). If the size of the signal generated by the sensor (16) is more than a predetermined threshold, the user is alerted that a thermal radiation signal has been detected. This may be by way of an LED (not shown), or an audio signal such as a "beep". The user is therefore provided with a means of narrowing down the area of interest, and eventually locating the thermal body.

A second thermal detector (10b) is shown in Figure 2. This detector again comprises a housing (12) attached to a handle (14), and a sensor (16) situated at the focus of a parabolic reflector (18). However, the detector (10b) also includes a pass-band interference filter (28) centred at 9.5 micrometers (the wavelength of the radiation emitted by a human body at 37°C) with a FWHM (full-width at half maximum) of approximately 5 micrometers. This filter (28) is placed in front of the sensor (16) so that only thermal radiation of the desired wavelength will be detected.

A further embodiment of the invention is shown in Figure 3. This thermal detector (10c) includes a first (16) pyroelectric sensor situated at the focus of the parabolic reflector

(18), and a second pyroelectric sensor (17) situated behind the first sensor (16) so that incoming thermal radiation is not incident upon the second sensor. However, the second sensor (17) can be positioned in any suitable position such that incoming radiation is not incident upon it. The second sensor (17) also comprises a layer of pyroelectric material

(20) sandwiched between upper (22) and lower (24) electrodes. The first (16) and second (17) sensor are connected to the electronics (26). As pyroelectric materials have piezoelectric characteristics, small changes in pressure will affect the accuracy of the signal produced by the sensors (16,17). In order to minimize the effects of pressure changes within the device, the detector (10c) also has vents (40) formed in the detector housing (12) for equalizing the air pressure within the detector.

As incoming thermal radiation is not incident upon the second sensor (17), the second sensor measures the amount of environmental noise present, and generates an electrical signal indicative of this. By environmental noise, it is meant variations in pressure as well as background electrical noise etc. As both sensors (16,17) are subject to the same environmental conditions, the signal from the second sensor (17) can be subtracted from the signal generated by the first sensor (16). In this way, a detector which is less sensitive to environmental noise than the detectors (10a) and (10b) is provided.

A further embodiment of the invention is shown in Figure 4. This detector (lOd) includes first (16) and second (17) pyroelectric sensors located side by side at the focus of the parabolic reflector (18). A first pass-band interference filter (28) is positioned between the incoming radiation and the first sensor (16), and a second pass-band interference filter (30) is positioned between the incoming radiation and the second sensor (17). A third pyroelectric sensor (32) is located behind the first (16) and second (17) pyroelectric sensors. The function of the third pyroelectric sensor (32) is to act as an environmental discriminator, in the same way as the second sensor (17) in the aforedescribed detector (10c). The first filter (28) has a pass-band centred at 9.5 micrometers with a FWHM of 1 micrometer, and the second filter (30) has a pass-band centred at 3.5 micrometers with a FWHM of 1 micrometer.

For thermal radiation generated by a human body at 37 °C, the size of the signal generated by the first sensor (16) will be approximately 25 times the size of the signal by the second sensor (17). The electronics (26) can be configured such that the user is only alerted when the ratio of the first sensor signal to the second sensor signals is, say, between 1:20 and 1:30. Therefore, if a human body is being searched for, and the detector (lOd) is swept past a hot water pipe at 100 °C, the ratio of the size of the first and second sensor signals will be approximately 7. As this ratio is lower than the preselected ratio, then the user will not be alerted. Alternately, the electronics could be configured such that this "false signal" from the hot water pipe is indicated to the user by, for example, a different colour LED, or a different sounding "beep".

Variation may be made to the aforementioned embodiments without departing from the scope of the invention. For example, the focusing means may be circular, elliptical, or any other suitable shape.

Although reference has been made to the detection of human bodies at 37 °C, the detectors could be configured to detect thermal bodies of a higher or a lower temperature.

Claims

1. A thermal detector (10a,b,c,d) for detecting thermal radiation, the detector comprising: a first sensor (16) for sensing thermal radiation, the first sensor located substantially at the focal point of a focusing means (18) for focusing thermal radiation onto the first sensor, characterised in that the first sensor (16) includes a single pair of electrodes (22,24) between which pyroelectric material (20) is disposed, and that the detector does not include a chopping mechanism.

2. A thermal detector (10a,b,c,d) according to claim 1 wherein the focusing means (18) is a parabolic reflector.

3. A thermal detector (10b,d) according to claim 2 including a first thermal radiation filter (28) disposed between the direction of the incoming thermal radiation and the first sensor (16).

4. A thermal detector (10c,d) according to claim 3 further including a second sensor (17).

5. A thermal detector (10c,d) according to claim 4 wherein the second sensor (17) is disposed adjacent the first sensor (16).

6. A thermal detector (10c) according to claim 5 wherein the second sensor (17) is situated such that incoming thermal radiation is not incident thereupon.

7. A thermal detector (lOd) according to claim 5 wherein the second sensor (17) is situated such that incoming thermal radiation is incident thereupon.

8. A thermal detector (lOd) according to claim 7 including a second thermal radiation filter (30) disposed between the direction of the incoming thermal radiation and the second sensor (17).

9. A thermal detector (lOd) according to claim 8 further including a third sensor (32) disposed adjacent the first (16) and the second (17) sensors.

10. A thermal detector (10c,d) according to any of claims 4 to 9 wherein the second (17) sensor comprises a layer of pyroelectric material (20) disposed between a first (22) and a second (24) electrode.

11. A thermal detector (10c,d) according to claim 9 or claim 10 wherein the third (32) sensor comprises a layer of pyroelectric material (20) disposed between a first (22) and a second (24) electrode.

12. A thermal detector (10b,d) according to any of claims 3 to 11 wherein the first thermal radiation filter (28) is a pass-band interference filter.

13. A thermal detector (10b,d) according to any of claims 8 to 12 wherein the second thermal radiation filter (30) is a pass-band interference filter.

14. A thermal detector (10b,d) according to claim 12 wherein the first thermal radiation filter (28) has a pass-band centred at approximately 9.5 micrometers.

15. A thermal detector (lOd) wherein according to claim 13 wherein the second thermal radiation filter (30) has a pass-band centred at approximately 3.5 micrometers.

16. A thermal detector (10a,b,c,d) according to any preceding claim including alerting means which, in use, alerts a user that thermal radiation has been detected.

17. A thermal detector (10a,b,c,d) according to claim 16 wherein the alerting means includes an LED.

18. A thermal detector (10a,b,c,d) according to claim 16 or claim 17 wherein the alerting means includes means for generating an audio signal.

19. A thermal detector (10a,b,c,d) according to any preceding claim, further including a power supply means for powering the device.

20. A thermal detector (10a,b,c,d) according to claim 19, wherein the power supply means is a battery.

21. A method of detecting thermal radiation using the thermal detector (10) claimed in any of claims 1 to 20, the method including the steps of: scanning the thermal detector across an area thought to include a source of thermal radiation, such that an intermittent signal is obtained as the source of thermal radiation is focussed on the sensor, and alerting a user if such a source has been detected.