GB2416850A

GB2416850A - Method and device for security searches

Info

Publication number: GB2416850A
Application number: GB0514817A
Authority: GB
Inventors: David John Daniels
Original assignee: ERA Patents Ltd
Current assignee: ERA Patents Ltd
Priority date: 2004-07-20
Filing date: 2005-07-19
Publication date: 2006-02-08
Anticipated expiration: 2025-07-19
Also published as: GB2416850B; GB0416202D0; GB0514817D0

Abstract

A device (1) for detecting an object (8) concealed on a subject (7), comprises a differential radiometer including at least two spaced microwave receivers (4) for detecting microwave energy radiated from the subject and from the object, and processing means (2) for processing signals output by the receivers and for outputting an audio and/or visual differential output signal. A microwave transmitter (3) may be used in an active mode to irradicate the subject, and lens (6) focusses radiation onto the receivers. Further pairs of receivers may be included on the device. Ultrasonic sensors (5) may be included for mapping the surface being scanned, for determining the relative orientation of the device and for determining the extent of curvature of the scanned region. A metal detector may also be included.

Description

24 1 6850

METHOD AND DEVICE FOR SECURITY SEARCHES

FIELD OF THE INVENTION

This application relates to a method and a device for detecting an object concealed on a subject, particularly a human or animal subject. This application especially relates to detection using a radiometer, although additional sensors such as a metal detector may also be incorporated in the device.

BACKGROUND TO THE INVENTION

Detection of concealed objects, for example beneath clothing, on humans or animals either in the open or within buildings is becoming of increasing importance.

Objects such as metallic or non-metallic weapons, explosives or narcotics must be quickly identified during security checks at facilities such as airports, railway stations, sea ports and high profile locations where security is of paramount importance.

Detection is preferably carried out using a hand-held device operated at a short range from the subject, but may also be carried out using a fixed detector installed in a building, for example a portal incorporating multiple detectors.

Radiometers for performing such detection generally comprise a receiver or multiple receivers for detecting microwave energy radiated from an object. Such radiation is caused by either the object's own equivalent internal microwave temperature, or by reflected microwave energy from sources of radiation such the sky, when the device is to be used as a passive system in the open, or fluorescent lighting, when the device is to be used as a passive system indoors.

There is a problem in that some known radiometers have a poor ability to accurately detect concealed objects and, as a result, such devices have a high false alarm rate, indicating a concealed object to be present when there is none. This may be due to a number of factors. The device may be unable to discriminate between a background signal and a signal radiated from an object under investigation. Also, the radiated signal may not be incident the device. This may be due to Incorrect orientation of the device on behalf of a user, or the signal may be being reflected from a highly curved subject surface.

There is therefore a need In the art for an improved method and device for detecting an object concealed on a subject for use in security searches. The present invention has been devised to overcome, or at least ameliorate, at least some of the above problems. It is therefore an object of the present invention to provide a radiometer device, which is able to more accurately detect concealed objects. It is another object of the present invention to provide a radiometer device which is easy to use.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, a device for detecting an object concealed on a subject comprises a differential radiometer including at least two spaced microwave receivers for detecting microwave energy radiated from the subject and from the object, and processing means for processing signals output by the receivers and for outputting an audio and/or visual differential output signal.

Any difference between the microwave energy radiated by one surface, such as human skin, and the microwave energy radiated by another surface, such as a weapon, is perceptible by the output signal which indicates both the presence and location of a concealed object.

The receivers preferably each comprise a receive antenna and an amplifier, and output a DC voltage signal. The device preferably includes a first pair of receivers which may be identical. A plane of operation of the device should lie normal to the direction of detection of the first pair of receivers. A first operational scan direction of the device lies parallel to a spacing axis of the first pair of receivers.

The receivers may be a direct detection receivers or be down converted using a local oscillator, mixer and intermediate frequency amplifier. These detected voltage signals may be further amplified and processed by a processor which may provide a sum and a difference output signal. The sum and difference output signals may be multiplied together to provide a drive voltage. The drive voltage may be used to provide a device output signal in the form of an audio or video output.

Preferably, the drive voltage may be input to a voltage to frequency converter for converting the drive voltage to a drive frequency. The drive frequency is preferably used to provide an audio output in a 100Hz to 5kHz range. By providing a pair of receivers, and hence a difference output, the device may reduce any background noise signals and enhance object detection, thus Improving the accuracy of the device and reducing the possibility of false alarms. By multiplying the sum and difference signals output by the processor the device may provide enhanced object detection, thus improving the accuracy of the device and reducing the possibility false alarms.

Further preferably, the device includes a second pair of receivers located adjacent one another. The second pair of receivers may be identical to one another and/or identical to the first pair of receivers. Each receiver of the second pair of receivers is orientated to receive a reflected electromagnetic signal from approximately the same direction as one another. The second pair of receivers are preferably located orthogonally to the first pair of receivers. A second scan direction may be defined as a direction perpendicular to the first scan direction in the plane of operation of the device.

By providing two pairs of receivers the device may operate in two orthogonal scan directions in the plane of operation. The plane of operation should be approximately coplanar with a surface of interest, i. e. the surface of a portion of a subject under investigation. To ensure that the device is operated at a correct orientation, the device may be provided with a optical boresight to track the plane tangential to the surface of interest. Signals output by the second pair of receivers may be amplified, processed and output in a similar manner as the signals output by the first pair of receivers.

The surface of interest may be either convex or concave in shape. Where the surface is convex, the device may provide unwanted readings due to the necessary displacement between adjacent receivers of the first and/or second pairs of receivers.

Where the surface is concave, the surface may act as a focus and one of the receivers may experience a large increase in received microwave energy.

Accordingly, it is preferable that the plane of operation of the device is approximately coplanar with a tangent to the surface of the portion of the subject under investigation as far as is possible. To assist this, the device may further be provided with at least two ultrasonic sensors. The ultrasonic sensors may provide information regarding the range and contours of the portion of the subject. The ultrasonic sensors may output a signal to indicate a suitable operating region for the device both in terms of a range of the device from the subject surface, and In terms of a lateral position.

The microwave energy radiated from the subject and object may emanate from sources of radiation such as the sky, when the device is to be used as a passive system in the open, or fluorescent lighting, when the device Is to be used as a passive system indoors.

Alternatively, the microwave energy radiated from the subject and object may emanate from a transmitter included in the device, when the device Is to be used as an active system. Where a transmitter is provided in the device, the transmitter Is preferably located between the receivers of the first pair of receivers and further preferably is centrally located between the receivers of both the first and second pairs of receivers. The transmitter may radiate at a total power of less than or equal to 1 microwatt (,aW) to minimise radiological damage to the subject. Where a transmitter is not provided, the receiver amplifiers may operate at an increased gain to provide sufficient detection sensitivity. This increased gain may be approximately 50dB.

The device may operate at a range of frequencies suitable for the type of object to be detected. Frequencies below 15GHz are unsuitable for transmission by the transmitter as these may penetrate beneath the skin of the subject under investigation and give rise to unwanted radiated power from internal structures such as bone and tissue. Suitable ranges of operation are between 18-26GHz, 28-36GHz, 70-80GHz and 90-100GHz.

The device is preferably a hand-held device which may be operated at a short range from the subject at a distance of approximately 5cm to approximately 30cm.

Most preferably the device is operated at a distance of approximately 1 Ocm from the subject. The device may be used to detect one or more of a metallic weapon, a non-metallic weapon, an explosive or a narcotic. The device preferably is able to detect objects concealed beneath clothing worn by the subject.

Since the device may be operated at short range, a zone from which each receiver receives microwave energy radiated is small. Preferably, each receiver receives the microwave energy from an approximately circular zone having a diameter of approximately 4cm, most preferably approximately 1 cm. In one embodiment where the first pair of receivers and a centrally located transmitter are provided, the first pair of receivers receive reflected signals from two small approximately circular zones of sensitivity of the subject, the zone of sensitivity of the transmitter being approximately elliptical and encompassing the two small circular zones of the receivers. Where the second pair of receivers is also provided, the receivers receive microwave energy from four small approximately circular zones of sensitivity of the subject, the zone of sensitivity of the transmitter being approximately circular and encompassing the four small circular zones of the receivers.

A lens arrangement may be arranged to maintain a zone, from which the receivers receives a signal from the subject and/or object, at a suitable diameter over a range of distances from the subject at which the device may be expected to operate.

The depth of focus of the sensitive zone may be optimised and the system may be guided to operate within the depth of focus by use of ultrasonic sensors. The effect of curved surfaces of interest may be significantly reduced by focusing the receive and, if appropriate, the transmit antenna pattern to create zones of sensitivity that are small enough to substantially render the curved surface equivalent to a plane surface.

According to a second aspect of the invention, a method of detecting an object concealed on a subject comprises the steps of detecting microwave energy radiated from the subject and from the object using at least two spaced microwave receivers, and processing signals output by the receivers to output an audio and/or visual differential output signal.

The method may further comprise the steps of multiplying a sum and a difference voltage signal generated from voltage signals output by a pair of said receivers to produce a drive voltage, converting said drive voltage into a drive frequency, and outputting a noise based on said drive frequency.

DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic diagram of a radiometer device in accordance with the present invention; Figures 2a and 2b are schematic diagrams of two footprints of sensitivity according to single and dual scan modes of operation of the radiometer device of Figure 1; Figure 3 is a schematic diagram of the processor circuit; Figure 4 is a schematic diagram of the circuit of Figure 3 further including a spectral analyser; Figure 5 is a perspective view of a physical embodiment of the device of Figure 1 having a single scan mode of operation; Figure 6 is a schematic diagram of the lens arrangement of the device of Figure 1; Figure 7 Is an example of an output from the spectral analyser according to the embodiment of Figure 4; and, Figure 8 is a schematic diagram of a system including the device in accordance with the present invention and a metal detector.

DETAILED DESCRIPTION

An embodiment of the present invention will now be described with reference to the accompanying drawings. Turning first to Figure 1 there is shown a schematic diagram of a radiometer device 1. The device 1 includes a processor 2 electrically connected to a transmitter 3 and a pair of receive antennas 4, one receive antenna 4 being located diametrically opposite the other receive antenna 4 of the pair with respect to the centrally located transmitter antenna 3. Also electrically connected to the processor 2 is a pair of ultrasonic sensors 5. The pair of receive antennas 4 has a lens arrangement 6.

The radiometer device may be used in an active mode whereby the transmitter antenna 3 transmits an electromagnetic wave at a total power of less than 1 microwatt (1.uW). The radiometer device is suitable for use in the detection of metallic or non-metallic weapons, explosives or narcotics concealed below clothing on humans or animals. The device should be used at a short range of 5-30cm from a region of a subject under investigation. The device should be moved by a user in a scanning motion over the surface of the region of the subject under investigation. The total microwave power radiated by the transmitter antenna 3 is reflected, at least partially, by the surface of the subject and by any object located between the subject and the radiometer device. The total microwave power radiated by the subject is likely to be different in structure to the total microwave power radiated by the object.

The receive antennas 4 are electrically connected to the processor 2 such that the device 1 operates as a differential radiometer device. Each receive antenna of the pair output a signal, which is amplified by amplifiers to produce a DC voltage output.

These voltages are further amplified and processed by the processor 2 to provide a sum and difference output. As the radiometer device is processed in a scanwise manner over the surface of the subject, where an object has been concealed beneath clothing worn by the subject, one receive antenna of the pair of antennas located approximately in line with the direction of scan will observe the total microwave power radiated by the subject 7, and the other receive antenna 4 of the pair will receive the total microwave power radiated by the object 8. Accordingly, a difference output, output by the processor 2, calculated from the voltage difference between the two receive antennas 4 of the pair of receive antennas will detect the difference between the two total microwave power incident on the receive antennas, irrespective of background radiation. This difference output may then be multiplied with a sum output, being the sum of the two outputs from the pair of receive antennas, to amplify the difference output. This product voltage provides a drive voltage for a voltage to frequency converter in the processor 2 which produces an audio output in a 100 Hz to KHz range to be output through either a speaker or to a set of headphones. A schematic circuit diagram is shown in Figure 3.

Where the radiometer device 1 is moved in a scanwise manner over a subject 7 having a planar surface of a uniform material, the total microwave power radiated should be approximately the same for both receive antennas 4. As a result, the drive voltage for the voltage to frequency converter is low and hence the audio output is a low frequency output. Where the device 1 is moved in a scanwise manner over a portion of a subject 7 where a metallic object 8 is positioned between the radiometer device and the subject 7, as the device 1 is moved to a position where one of the receive antennas receives the total microwave power radiated from the subject 7 and the other receive antenna 4 receives the total microwave power radiated by the subject 7, the drive voltage output by the processor 2 is much larger and hence the audio output is a high frequency output. For example, where the object 8 is a concealed knife the metallic surface of the knife will reflect the electromagnetic wave transmitted by the transmitter antenna 3 appreciably better than the skin of a person.

Where two pairs of receive antennas 4 are provided disposed at orthogonal axes, two orthogonal scan directions may be simultaneously detected so that the radiometer device may be moved in two orthogonal scan directions. The device 1 is able to detect best when moved in one of the two scan directions corresponding to the orthogonal axes of orientation of the receive antennas 4 but may be moved in a plurality of scanwise directions. Although the device 1 described herein has up to two pairs of receive antennas 4, three or four or more pairs of receive antennas 4 may also be provided, one antenna of each pair provided diametrically opposite the other antenna of each pair with respect to the central transmitter antenna 3.

The device can operate best when the receive antennas 4 are orientated normal to a plane of the surface being scanned. Where the receive antennas are not orientated normal to the plane of the surface being searched the electromagnetic wave emitted by the transmitter may be reflected by the subject 7 or object 8, where present, such that the reflected waves are not detected by some of the receive antennas.

Accordingly, a situation may arise whereby one of the receive antennas in each pair of receive antennas 4 detects a reflected electromagnetic wave that the other receive antenna 4 of the pair receives little or no reflected wave. This situation may indicate the presence of a concealed object where there is none. Alternatively, highly concave or convex portions of the surface of the subject 7 under investigation may provide focus points or specular dissipation points giving rise to anomalous readings and increasing the possibility of false alarms.

The device 1 according to this embodiment includes a pair of ultrasonic sensors for mapping the surface of interest on the subject. The function of this mapping is two-fold. Firstly, the mapping provides information as to the relative orientation of the device 1 to the region of the surface in question. As has been described above the device 1 operates most effectively where the orientation of the receive antennas 4 are normal to the plane of the surface under investigation. Secondly, the mapping provides information as to the extent of curvature within the region of the surface under investigation. As has also been described previously the device 1 operates most effectively where the zone of sensitivity on the subject 7 from which each receive antenna 4 receives a reflected wave should be approximately circular and have a diameter preferably less than 4 cm and ideally 1 cm.

To provide a zone of sensitivity for each received antennas 4 of such constant shape and size a lens arrangement 6 is provided for pair of receive antennas 4. The lens arrangements 6 may be fixed or movable to alter the focus spots. The lens arrangement 6 comprises a lens holder 9, a lens 10 and a screen 11 as shown in Figure 6. The centrally located transmit antenna 3 transmits an essentially unfocussed spot. This illuminates via lens 10 the plane of the subject 7. The conical screen 11 provides absorption of any unwanted reflected energy. As can be seen, the zones of sensitivity are small and focused so that the device 1 operates over small discrete areas thus improving the sensitivity of the device and minimising the possibility of false alarms. The depth of focus of the sensitive zone is optimised and the system may be guided to operate within the depth of focus by use of ultrasonic sensors. The lens arrangements 6 may alternatively be electrically coupled to the processor 2 which is driven by signals from the ultrasonic sensors 5. The lens may therefore be adjusted to accommodate for highly concave or convex regions of the subject 7 to maintain approximately constant spot sizes. Any number of ultrasonic sensors 5 may be provided to increase the accuracy of the mapping of the surface.

The device 1 is operable over a range of frequencies. Suitable frequency ranges are between 18 to 26 GHz, 28-36 GHz, 70-80 GHz and 90-100 Ghz.

Frequencies below 15 Ghz are unsuitable because of their penetration beneath the skin of the subject 7.

A physical representation of the embodiment described above is shown in Figure 5. The transmit and receive antennas 3,4,4 are clearly shown. Although the embodiment described above has been described with reference to a hand-held device 1 for use in a short range of 5-30 cm, the device 1 according to the present invention is equally suitable for mounting in a portal detection which may be permanently installed in a building. The circuit diagram of a further embodiment of the invention is shown in Figure 4. This embodiment Is similar to the single or dual scan embodiments described above and further includes a spectral analyser 12 for classifying the material of the object 7 when detected. The processor circuit of Figure 3 further has a mixer and a local oscillator whose frequency is swept over the range of interest. This provides an intermediate frequency output, which is rectified and then analysed to detect periodic patterns, which are related to the physical characteristics of the object 8. These characteristics could be one or more of a thickness or relative dielectric constant. Typical spectral characteristics of an explosive object 8 concealed on a human subject 7 is given in Figure 7 together with a sample spectra.

In a yet further embodiment, a system 13 Includes the device in accordance with the embodiment described above and a metal detector 14. A metal detector coil is provided around a periphery of the lens holder 9 of the device 1. The metal detector coil 15 provides the additional capability of detecting and distinguishing metal on the subject 7. The metal detector coil 15 can be used to detect substantial masses of metal in proximity to the coil and the system 13 can provide improved identification of the type of material by means of suitable processing of the signals from the radiometer device 1 and the metal detector 14. A schematic diagram of the system 13 is shown in Figure 8.

Claims

1. A device for detecting an object concealed on a subject, comprising a differential radiometer including at least two spaced microwave receivers for detecting microwave energy radiated from the subject and from the object, and processing means for processing signals output by the receivers and for outputting an audio and/or visual differential output signal.

2. A device according to claim 1, wherein each receiver comprises a receive antenna and an amplifier.

3. A device according to claim 1 or claim 2, wherein the receivers output a DC voltage signal.

4. A device according to any one of the preceding claims, wherein a pair of said receivers are provided.

5. A device according to claim 4 when dependent on claim 3, wherein the processing means comprises means for processing the DC voltage signals output by said pair of said receivers to provide a sum and a difference voltage and for outputting said sum and difference voltage outputs.

6. A device according to claim 5, wherein the processing means further comprises means for multiplying the sum and the difference outputs to provide a drive voltage and for outputting said drive voltage.

7. A device according to claim 6, wherein the processing means further comprises a voltage to frequency convertor for converting the drive voltage to a drive frequency as the audio and/or visual differential output signal.

8. A device according to any one of claims 4 to 7, wherein said pair of receivers are disposed spaced along an axis, such that, in use, both said receivers are approximately normal to and facing a plane tangential to a surface of interest of the subject.

9. A device according to claim 8, wherein the receivers in said pair are identical.

10. A device according to claim 8 or 9, wherein said spacing axis is disposed in a plane such that, in use, said plane is substantially parallel to the plane tangential to the surface of interest as the device and subject are moved scanwise relative to one another.

11. A device according to any one of claims 8 to 10, further comprising a boresight used for ensuring correct positioning of the device relative to the plane tangential to the surface of interest.

12. A device according to any one of claims 8 to 11, further comprising at least two ultrasonic sensors used for ensuring correct positioning of the device relative to the plane tangential to the surface of interest.

13. A device according to any one of claims 8 to 12, wherein four said receivers are provided, arranged in pairs, said second pair of receivers being spaced along an axis substantially orthogonal to said spacing axis of said first pair of receivers such that, in use, both said receivers of said second pair are approximately normal to and facing a plane tangential to a surface of interest of the subject.

14. A device according to claim 13, wherein the receivers in each of said pairs are identical.

15. A device according to claim 13 or 14 when dependent on claim 10, wherein said spacing axes are disposed in the same plane.

16. A device according to any one of claims 13 to 15, wherein said processing means outputs an audio and/or visual differential output signal for each of said pairs of receivers.

17. A device according to any one of the preceding claims, wherein each receiver has an associated zone of sensitivity, said zones being substantially circular.

18. A device according to claim 17, wherein a diameter of each said zone is less than approximately 4cm.

19. A device according to claim 18, wherein said zone diameter is approximately 1cm.

20. A device according to any one of the preceding claims, further comprising a radiation transmitter.

21. A device according to claim 20, wherein said transmitter radiates at a power of less than approximately 1,uW.

22. A device according to claim 20 or 21, wherein said transmitter is disposed centrally between a pair of said receivers.

23. A device according to any one of claims 20 to 22, wherein said transmitter has an associated zone of sensitivity.

24. A device according to claim 23, when dependent on claim 17, wherein said transmitter zone of sensitivity is shaped to encompass substantially all of each said receiver zones of sensitivity.

25. A device according to claim 24, wherein said transmitter zone of sensitivity is substantially elliptical or circular in shape.

26. A device according to any one of the preceding claims, wherein said device is adapted for use at a range of approximately 5 to 30cm.

27. A device according to claim 26, wherein said device range is approximately 1 Ocm.

28. A device according to any one of the preceding claims, wherein said audio differential output signal, where provided, is approximately in the range 100Hz to 5kHz.

29. A device according to claim 28, wherein said audio differential output signal is modulated to provide range information.

30. A device according to any one of the preceding claims, operable at frequencies above approximately 15GHz.

31. A device according to claim 30, operable at f requencies between approximately 18 to 26GHz, 28 to 36GHz, 70 to 80GHz or 90 to 100GHz.

32. A device according to any one of the preceding claims, further comprising focusing means disposed between the receivers and the subject.

33. A device according to claim 32, wherein the focusing means comprises a lens arrangement.

34. A device according to claim 32 or 33, wherein the focusing means focuses the microwave energy radiated from the subject and object towards the receivers.

35. A device according to any one of claims 32 to 34, wherein a conical housing is disposed between the receivers and the focusing means.

36. A device according to any one of the preceding claims, further comprising means for classifying material properties of the object.

37. A device according to claim 36, wherein said classifying means includes a spectrum analyser.

38. A device according to claim 36 or 37, further comprising a mixer and a local oscillator electrically connected to, or forming part of, the processing means, the frequency of the local oscillator being, in use, swept over a suitable range, for providing a frequency output relating to material properties of the object, the device further comprising a rectifier for rectifying said frequency output.

39. A device according to claim 38 when dependent on claim 37, wherein said frequency output is fed to the spectrum analyser for detecting periodic patters relating to physical characteristics of the object.

40. A device according to any one of the preceding claims, further comprising a metal detector.

41. A device according to any one of the preceding claims, adapted for detecting metallic or non-metallic weapons, explosives or narcotics objects concealed beneath clothing on a human or animal subject.

42. A method of detecting an object concealed on a subject, the method comprising the steps of detecting microwave energy radiated from the subject and from the object using at least two spaced microwave receivers, and processing signals output by the receivers to output an audio and/or visual differential output signal.

43. A method according to claim 42, further comprising the steps of multiplying a sum and a difference voltage signal generated from voltage signals output by a pair of said receivers to produce a drive voltage, converting said drive voltage into a drive frequency, and outputting a noise based on said drive frequency.