Imaging apparatus and method

• US 20070188370A1
• Filed: 03/29/2005
• Published: 08/16/2007
• Est. Priority Date: 03/26/2004
• Status: Active Grant

• Current Assignee: Qinetiq Limited (QinetiQ Group PLC)
• Original Assignee: Quinetiq Limited
• Inventors: Coward, Peter, Salmon, Neil
• Granted Patent: US 7,982,656 B2
• Time in Patent Office: Days
• US Class Current: 342/22
• CPC Class Codes: G01S 13/887 for detection of concealed objects, e.g. contraband or weapons
• CPC Class Codes: G01S 13/89 for mapping or imaging
• CPC Class Codes: G01S 7/411 Identification of targets based on measurements of radar reflectivity G01S7/415 takes precedence
• CPC Class Codes: G01V 8/005 operating with millimetre waves, e.g. measuring the black losey radiation

Abstract

A millimetre wave imaging apparatus and method capable of identifying the presence of relatively thin dielectric materials is disclosed. The method involves taking a plurality of millimetre wave images of a scene at different frequencies and analysing the images at different frequencies to look for frequency dependent effects in the scene. Relatively thin dielectric materials can lead to interference effects which can be detected. In one embodiment the imaging apparatus comprises a millimetre wave imager (2) connected to a processor (16) and at least one variable frequency illumination source (22a . . . d, 24a . . . d). The or each illumination source sweeps the illumination frequency across a reasonably wide bandwidth and the imager captures radiation returned from the scene at a number of different illuminating frequencies.

Claims

1. A millimetre wave imaging apparatus comprising at least one detector arranged to image a scene and a frequency selector providing a plurality of different predominant frequencies of radiation for detection wherein the detector output is recorded for each different predominant frequency and wherein the imaging apparatus further comprises an indicator which, in use, indicates the presence of any frequency dependent variation in the detector output.

2. A millimetre wave imaging apparatus as claimed in claim 1 further comprising at least one illumination source having a variable output frequency wherein the frequency selector periodically varies the output of the or each illumination source.

3. A millimetre wave imaging apparatus as claimed in claim 2 comprising a plurality of illumination sources, the frequency selector being arranged such that each illumination source emits the same frequency at the same time.

4. A millimetre wave imaging apparatus as claimed in claim 2 wherein the or each illumination source is adapted such that it produces the same intensity at each illumination frequency within the operating band.

5. A millimetre wave imaging apparatus as claimed in claim 2 wherein the or each illumination source has an instantaneous bandwidth of approximately 2 Ghz or less.

6. A millimetre wave imaging apparatus as claimed in claim 2 wherein the or each illumination source has an output frequency which varies by 5 GHz or more or 10 GHz or more.

7. A millimetre wave imaging apparatus as claimed in claim 2 wherein the or each illumination source comprises a variable frequency rf noise generator housed in a hollow, reflective panel having a plurality of apertures therein.

8. A millimetre wave imaging apparatus as claimed in claim 1 further comprising a normalisation means for normalising the detector output at each different frequency.

9. A millimetre wave imaging apparatus as claimed in claim 8 wherein the detector comprises a detector array and the normalisation means applies a frequency dependent correction to the output of each detector element in the imaging detector array.

10. A millimetre wave imaging apparatus as claimed in claim 9 wherein the normalisation means comprises a processor adapted to process the output of the detector to apply the frequency dependent correction.

11. A millimetre wave imaging apparatus as claimed in claim 1 wherein the indicator comprises an image processor for analysing the detector output at different frequencies so as to detect any frequency dependent variations therein.

12. A millimetre wave imaging apparatus as claimed in claim 1 wherein the apparatus comprises a display arranged to display an image derived from the outputs of the detector.

13. A millimetre wave imaging apparatus as claimed in claim 1 wherein the frequency selector comprises at least one filter acting on the detector.

14. A millimetre wave imaging apparatus as claimed in claim 13 wherein the filter is a variable filter and the frequency selector controls the passband of the filter.

15. A millimetre wave imaging apparatus as claimed in claim 13 wherein the frequency selector comprises a plurality of different filter elements arranged in parallel such that a plurality of different frequencies are detected simultaneously.

16. A millimetre wave imaging apparatus comprising at least one illumination source and an imaging detector characterised in that the or each source is adapted to illuminate the scene sequentially with a plurality of different illuminating frequencies at substantially the same intensity and the apparatus is adapted to record the output of the detector at a plurality of different illuminating frequencies.

17. A millimetre wave imaging apparatus comprising at least one illumination source and an imaging detector characterised in that the or each source is adapted to illuminate the scene sequentially with a plurality of different illuminating frequencies and record the output of the detector at a plurality of different illuminating frequencies further comprising a normalising means for normalising the detector output at each different illuminating frequency.

18. A method of screening an object for a concealed item comprising the steps of;

arranging a millimetre wave imaging detector relative to an object to be screened so as to be able to form an image of at least part of the object, recording the output from the detector at a variety of different predominant frequencies and analysing the outputs of the detector for any frequency dependent variation from the object.

19. A method as claimed in claim 18 wherein the method further comprises the step of calibrating the outputs of the detector to account for any frequency dependent response of the imaging apparatus.

20. A method as claimed in claim 18 wherein the method further comprises the step of successively illuminating the object with millimetre wave radiation at a plurality of different frequencies and the output from the detector is recorded at each illuminating frequency.

21. A method as claimed in claim 20 wherein the step of illuminating the object comprises illuminating the object with a succession of different frequencies at the same intensity.

22. A method as claim 18 wherein the step of analysing the outputs of the detector recorded at different frequencies includes the step of displaying an image derived from the outputs to an operator.

23. A method as claimed in claim 18 wherein the step of analysing the outputs of the detector recorded at different frequencies includes the step of automatically processing the outputs to detect any frequency dependent variation and triggering an alarm when a frequency dependent variation is identified.

24. A method as claimed in claim 18 wherein the method includes the step of filtering the detector.

25. A method of screening an object for a concealed item comprising the steps of;

arranging a millimetre wave imaging detector array relative to an object to be screened so as to be able to form an image of at least part of the object, successively illuminating the object with millimetre wave radiation at a plurality of different frequencies and recording the output from the detector array at each frequency and analysing the outputs of the detector array for any frequency dependent variation in the scene.