WO2003083508A1

WO2003083508A1 - Improved millimetre wave imaging system

Info

Publication number: WO2003083508A1
Application number: PCT/GB2003/001349
Authority: WO
Inventors: Gordon Neil Sinclair; Neil Anthony Salmon
Original assignee: Qinetiq Limited
Priority date: 2002-03-28
Filing date: 2003-03-20
Publication date: 2003-10-09
Also published as: GB0207361D0; AU2003215770A1

Abstract

An improved millimetre wave imaging system includes means for detecting radiation from a scene at at least two differing illumination intensities, such that separate images may be produced at each illumination intensity. This allows, under some circumstances, objects to be discriminated from their background in one illumination regime where, in another illumination regime, they are indistinguishable. The system may be arranged to observe the scene in a plurality of frequency bands each having different illumination levels, or alternatively it may be arranged to observe at a single frequency band using a switched power illumination source. The invention may also be used to estimate the composition of materials within the scene. A method relating thereto is also disclosed.

Description

Improved Millimetre Wave Imaging System

This invention relates to millimetre wave imaging systems. More particularly, it relates to methods and apparatus for providing an improved capability for millimetre wave imaging systems to detect objects otherwise hidden from the view of an observer.

It is commonplace nowadays for people and baggage to be searched before being admitted entrance to certain facilities. Airports in particular screen both passengers and baggage before allowing them onto a plane. Sea ports and rail ports also conduct searches of vehicles and people as part of the procedure for passing through customs. Metal detectors are often used to check people for weapons etc. when attempting to gain access to buildings or public events etc.

Millimetre wave imagers provide an ability to "see" through certain types of material, such as clothing, plastics etc that are opaque at visible wavelengths, and so are useful for screening objects and people without having to do a manual search. Patent application WO 9847020 describes a millimetre wave imager that is able to provide a detailed image of the radiation emitted from an object at millimetre wave frequencies, and this is able to distinguish knives guns and other objects hidden under clothing.

Millimetre wave imagers of the type described above have a problem in that discrimination of objects in the scene can be poor if the objects and the background to the objects are at similar radiometric temperatures.

According to the present invention there is provided a millimetre wave imaging system comprising a receiver having at least one receive element sensitive to millimetre wave radiation and imaging means for converting radiation received at the receiver into an image, characterised in that the receiver is arranged to independently detect energy from a scene at at least two different background illumination intensities, such that separate images can be produced of the scene illuminated at each illumination intensity.

The current invention allows the images produced at different levels of illumination to be used to generate new information that aids in the discrimination of objects against their background. The received energy from objects within a scene is proportional to their radiometric temperature. At millimetre wave frequencies the radiometric temperature T of depends on six variables:

T = ε T_m + rT_e + tT_b (equation 1 )

where e, r and t are the emissivity, reflectivity and transmissivity of the object being viewed, and T_m is the physical temperature of the object, T_e is the radiometric temperature of the background radiation (the illuminating source), and T_b is the radiometric temperature of whatever is behind the object, transmitting energy through it. Note that e + r + f = 1. Note also, that for many types of object, particularly metallic objects, t will be zero, which will have the effect of removing the last term from the right hand side of Equation 1.

If an object has the same radiometric temperature as its surroundings, it will not be visible to the imager separately from those surroundings. However, whilst an object may have an overall radiometric temperature the same as that of its surroundings, the individual terms that make up its temperature T, i.e. reflectivity, physical temperature etc. may be different to the terms making up the temperature of the surroundings.

By imaging the scene independently at differing levels of background illumination, the value of T_e in equation 1 will be different for each image. Under such conditions two objects that appear to have the same radiometric temperature for T in one image will generally have different values for Tin another. Thus the two objects will be discriminated. Standard data fusion techniques can be used to detect objects that are so discriminated.

Images of the scene at different background illumination levels may be generated by amplitude modulating an illumination source, and synchronising the modulation with the image capture of the imager, such that images taken at different times have different illumination levels. These different images effectively record the scene at differing values of the background temperature T_e, which has the effect of changing the radiometric temperature of the various objects that make up the scene, by an amount depending on their individual millimetre wave characteristics. This is known herein as a time division system.

Images of the scene at different background illumination levels are preferably generated by creating a plurality of images of a scene, each one comprising of radiation from a different frequency band, where each frequency band has a different background illumination level. This is known herein as a frequency division system. For this system, the energy present in each frequency band should be detected independently of the other bands.

A receiver will have at least one receive element, this receive element comprising amplification means, and detection means. A frequency division system will also comprise splitting means to separate the received signal into a plurality of distinct signals, each of which is applied to a filter that corresponds to one of the frequency bands having a particular illumination level.

In a frequency division system, each receive element preferably has a plurality of detectors, each arranged to detect energy from a single frequency band, whilst substantially rejecting energy from other frequency bands.

In a frequency division system, the receive element preferably employs a common signal path for all frequency bands of interest for as far as possible before the signal is split and applied to separate detectors. This will reduce systematic errors present in the receiver front end, by ensuring that all distortions present in this part of the receiver affect all signals equally.

In a frequency division system, the receive element preferably employs a filter bank after the signal has been split and before the split signals are passed to the detectors. This filter bank should be arranged to correspond with the split in illumination power with frequency.

The invention is applicable to both indoor use and outdoor use. If the invention is used indoors, a millimetre wave illumination source can be used to provide sufficient background illumination levels. In a time division system this illumination source must be capable of being modulated, preferably by means of being pulsed on and off at or around the frame rate of the imager. In a frequency division system the illumination source may provide a high power band limited output, where the band matches one of the frequency bands of the filter means. A solid state noise source is particularly suitable as a basis for this. The output of the noise source may be amplified using standard means to produce the desired illumination levels. If used with a frequency division system the noise source may also be filtered using standard techniques, such at by using narrow band amplifiers, or by using a separate filter section. Advantageously, an illuminator incorporating a solid state noise source is provided with means for removing the spatial coherence of the solid state noise source. The noise source may be distributed, such that it comprises a plurality of separate noise sources in different locations. If used in a time division system, these would be synchronised such that they all pulse together. Ambient millimetre wave energy may be used to provide a lower level of illumination intensity.

If the invention is used outdoors the frequency bands may be chosen such that, with just. ambient illumination, there is an appreciable power difference between them. One possible implementation uses a first band in which there is a naturally occurring absorption of energy due to atmospheric effects, and uses a second band where there is less absorption, allowing the radiometric temperature of the sky to be utilised. Alternatively, a band limited illumination source can be provided as for the indoor case.

Under certain circumstances, the invention provides the ability to estimate the composition of different types of materials. If the material in question has zero transmissivity, then images taken at two different background illumination intensities provide enough information to calculate the reflectivity, emissivity and physical temperature of the material. This information can be used to estimate likely material types.

According to another aspect of the invention there is provided a method of generating images using a millimetre wave imaging system comprising a receiver having at least one receive element sensitive to millimetre wave radiation, and imaging means for converting radiation received at the receiver into an image, characterised in that a plurality of separate images of a scene are recorded, a first image being formed of the scene illuminated at a first intensity, and a second image being formed of the scene illuminated at a second intensity.

According to a further aspect of the invention there is provided an illumination means arranged to co-operate with a millimetre wave imaging system characterised in that the illumination means is adapted to be switchable between a first illumination intensity and a second illumination intensity such that the imager is able to provide successive images illuminated at either the first or second illumination intensity.

The invention will now be described in more detail, by way of example only, with reference to the following Figures, of which:

Figure 1 diagrammatically illustrates a typical indoor operating environment; Figure 2 diagrammatically illustrates an object set against a background typical of what may come before the imager;

Figure 3 diagrammatically illustrates a modulation waveform that may be used with the time division implementation;

Figure 4 illustrates in block diagrammatic form a receiver element that may be used with the frequency division system; and

Figure 5 illustrates a graph of atmospheric transmission in a millimetre wave region of interest particularly for the frequency division implementation.

Illustrated in Figure 1 is a typical operating environment of the current invention, when operated indoors. This shows an imager 1 according to the current invention recording an image of a person 2 in a room 3 that is largely non-transmissive at millimetre wavelengths, due to the building materials used. The radiometric temperature of the person 2 as seen by the imager 1 will depend on the background radiometric temperature, the body temperature of the person 2, and the properties of the person 2 and clothing 4 worn. The background radiometric temperature can be controlled by changing the intensity of millimetre wave radiation 5 which would, according to the present invention, be appropriately band limited or amplitude modulated.

Figure 2 shows an object 6, in this case a knife, that is adjacent to, and in front of, a background 7, which may be, for example, the body of a person carrying the knife 6. If the knife 6 were covered by, for example, a layer of clothing, then it would not be visible to the naked eye, but may be visible to a millimetre wave imager under some circumstances. If the radiometric temperature of the knife 6 were sufficiently different to that of its background 7 then the knife would show up. Conversely therefore, if the contrast between the two objects is too low, because the radiometric temperatures are close, then the imager would not be able to distinguish the objects. This is despite the fact that the objects may be very different (eg, skin and metal in this case), and have very different dielectric properties.

Equation 1 above shows that the radiometric temperature of an object or material is made up of three terms. The only one of those terms that is controllable within the scene is the radiometric temperature of the surroundings, the environmental, or background illumination T_e. If two materials that have different millimetre wave properties have the same radiometric temperature when viewed at one value of T_e, their radiometric temperatures will be different from each other when viewed at a different value of T_e.

For example, assume a metal object 6 is against, and in front of, a background object 7 comprising of skin, where each has the following parameters:

Table 1

Then, using Eq 1 , if the scene is viewed with a background radiometric temperature T_e = 290°K, the radiometric temperatures of the foreground object (T_o) and the background object (T_g) will be as follows (assuming physical temperature of the object is 290°K and of the background is 310°K):

T₀ = (0 x 290) + ( 1 x 290) = 290; and

T_g = (0.5 x 310) + ( 0.5 x 290) = 300.

This is a relatively small difference, and would provide very little contrast in an image to enable the two objects to be distinguished from each other. Taking a second image of the same scene at a different background radiometric temperature (say 500 °K) according to the current invention yields the following:

T₀ = (0 x 290) + ( 1 x 500) = 500; and

T_g = (0.5 x 310) + ( 0.5 x 500) = 405.

This is an appreciable difference which would, in this second image, enable the foreground object to be distinguished from the background object.

It is not enough, however, to simply have an increased level of illumination. It will be seen that the values of e, rand t for particular different objects, may be such that when the objects are viewed at some given background illumination level they have the same radiometric temperature, but when viewed at a different background illumination level they have different radiometric temperatures. The scenes should be viewed at at least two, preferably widely different background illumination levels to give a good chance of contrasting the two objects.

The invention also allows the components of both reflectivity, r, and emissivity e, and the physical temperature T_e of objects to be calculated under limited circumstances. The first of these limitations is the requirement that the object be non transmissive, i.e. =0. Then equation 1 can be simplified to T= rT_e + (1-r)T_m (equation 2) as e + r + t - 1. The second limitation is that the reflectivity r of the object must not vary significantly between the plurality of images taken of the object. Based on these assumptions, and using equation 2, then if two images of an object are taken at two differing values of T_e (T_e1 and T_e2), the measured radiometric temperatures Ti and T₂of the object allow the computation of r, e , and T_m as follows: r= (T-i - T₂) I (T_θι -T_θ2), and

T_m = (T_e1 T₂ - T_e2 Ti) l (T_e1 - T_e2 +T₂ - Ti) e= 1-r If the value of r is known, this can be used get a good idea of what the object is made of. For example, a value of unity suggests that the object is metallic, whilst a value of 0.5 suggests skin, and a value of 0.2 suggests a ceramic material. If further images can be taken at other values of T_e then the problem is overdetermined (i.e. there are more equations than there are unknowns), and the resulting redundancy of calculation can be used to increase the accuracy of the calculated values for r, e and T_m.

One embodiment of the current invention employs an imager which is capable of taking sequence of images of a scene over a period of time. Here, a variable power illumination source is arranged to co-operate with the imager such that the illuminator is switched off during the recording of one image, and switched on during the recording of a subsequent image (the time division system). In this way, a scene is recorded at two different background intensity levels. The time delay between recording images at these different illumination levels should be kept small, so there is minimal change in the scene between successive images. An imager of the type discussed in published application WO 9847020 may be used to implement this embodiment, if it is first modified to synchronise its image recordal with an amplitude modulated illuminator. Required modifications would be obvious to one skilled in the relevant arts, and so will not be discussed further. Figure 3 shows a typical modulation waveform of an illuminator that would be used in such an embodiment, t_f is made substantially equal to the frame generation time of the imager being used, and is synchronised in phase with it.

A solid state noise source provides a suitable illumination source, as it is easily modulated at the rates that would be required - typically up to around 5-10Hz. Modulation techniques employed with such noise sources are known to the normally skilled person, and so will not be described further.

Separate images are recorded at differing background illumination intensity levels, and these images may then be compared to detect any differences between them, using standard data fusion techniques. If any differences are detected they may be highlighted to a user, by means of an audible or visual alert, e.g. by appropriately annotating one of the images using standard computer graphical techniques. Preferably, successive images are produced with a time separation small enough such that the scene does not change significantly between them.

A second embodiment of the invention employs an imager that has been modified such that it has receive elements having detectors that are frequency selective (the frequency division system). Here, a first detector is arranged to receive radiation from a first frequency band where there is a lower level of background illumination, and a second detector is arranged to receive radiation from a second frequency band where there is a higher level of background illumination. Separate images files are produced, which are then analysed as described above. An advantage with this embodiment is that the image does not change with time between images, as both are produced simultaneously.

A receive element having two frequency selective detectors is illustrated in Figure 4. Energy entering the imager via receive optics 8 is focused onto the antenna 9 of a receive element 10. From there, the energy is amplified in Low Noise Amplifiers 11 and fed into a power splitter 12, having two outputs. Each of these outputs feeds a filter 13, 14 which feed respective detectors 15, 16. The signals from each of the detectors are used to generate an image, as is known in the prior art. The filters' 13, 14 cutoff frequencies are arranged to correspond with bands of higher and lower background illumination levels, such that the first detector eg 15 is arranged to detect energy in the lower frequency band, and the second detector, eg 16 is arranged to detect energy in the higher frequency band. Note the common optical and amplification path for both frequency bands. This commonality keeps down the systematic errors within the system. If the frequency division system is to be used outdoors, or in a building that is substantially transparent to millimetre wave energy, then advantage can be taken of the natural split in background illumination intensities that occurs through atmospheric effects. Between 50GHz and 70GHz there are many absorption lines due to oxygen in the atmosphere, which give rise to an average attenuation in this band of around 10dB/km, and so provides an illumination radiometric temperature of around 270°K. In the frequency band 80GHz to 100GHz the atmospheric attenuation is relatively low, which gives rise to an illumination radiometric temperature of around 100°K. Consequently, setting the respective filter responses in the receive element 10 to pass these frequencies will lead to the generation of images at differing radiometric temperatures. To increase the contrast, an illuminator can optionally be provided that is filtered to provide radiation only within the lower frequency band. Figure 5 shows the atmospheric transmission with frequency of these frequency bands. Here, the solid line represents the atmospheric attenuation for clear air (at sea level), the dotted line represents thick fog (50m visibility), and the dashed line represents 25 mm/Hour rain. It can be seen that (at least for clear air) there is a large increase in attenuation between approximately 50GHz to 70 GHz, which gives the increased radiometric temperature seen within that frequency band.

If the system is to be used in a closed environment, then a band limited illumination source can be used to generate a high background illumination intensity in a chosen band. This has the advantage that the choice of bands is more flexible, as there is no reliance on atmospheric effects. Solid state illuminators of the type mentioned above are particularly suited to such generation, if appropriate filter means is included. Such filtering means will be known to those normally skilled in the relevant art.

The skilled person will be aware that other embodiments within the scope of the invention may be envisaged, and thus the invention should not be limited to the embodiments as herein described.

Claims

1. A millimetre wave imaging system comprising a receiver having at least one receive element sensitive to millimetre wave radiation and imaging means for converting radiation received at the receiver into an image, characterised in that the receiver is arranged to independently detect energy from a scene at at least two different background illumination intensities, such that separate images can be produced of the scene illuminated at each illumination intensity.

2. An imaging system as claimed in claim 1 wherein each receive element at the receiver is arranged to detect energy at at least two different frequency bands simultaneously, where, in each frequency band, a different illumination intensity occurs, the imaging means converting energy received at each frequency band into a separate image.

3. An imaging system as claimed in claim 2 wherein each receive element has a plurality of detectors, each detector arranged to detect energy in a different frequency band.

4. An imaging system as claimed in claim 3, wherein filter means is provided before the detectors, the filter means having a different transmission band for each detector.

5. An imaging system as claimed in claim 4, wherein the transmission bands of the filter correspond to the frequency bands having different illumination intensities.

6. An imaging system as claimed in any of claims 1 to 5 wherein each receive element has a signal path that comprises an optical path common to all frequency bands and an electrical path that has elements common to all frequency bands.

7. An imaging system as claimed in claim 6 wherein the common elements of the electrical path comprise amplification means.

8. An imaging system as claimed in any of claims 2 to 7 wherein the receiver is arranged to receive radiation from a scene illuminated such that the difference in background illumination intensity levels between the at least two frequency bands is caused by atmospheric attenuation.

9. An imaging system as claimed in claim 8 wherein a first frequency band is substantially within the range 80GHz to 100 GHz.

10. An imaging system as claimed in claim 8 wherein a second frequency band is substantially within the range 50GHz to 70 GHz.

11. An imaging system as claimed in any of claims 2 to 7 wherein the difference in background energy levels between the at least two frequency bands is produced by providing an illuminator that generates radiation at an intensity that is greater at a first frequency band than at a second frequency band.

12. An imaging system as claimed in claim 11 where the illuminator provides radiation substantially in a single frequency band.

13. An imaging system as claimed in claim 1 wherein the system is arranged such that the separate images are taken at different times, with an adjacent pair of images illuminated at different background illumination intensities.

14. An imaging system as claimed in claim 13 wherein the illumination is provided by an illuminator capable of being amplitude modulated.

15. An imaging system as claimed in claim 13 or claim 14 wherein the timing of the separate images is synchronised with an amplitude modulation control of an illuminator.

16. An imaging system as claimed in any of the above claims wherein the system is arranged to use the separate images recorded at different illumination levels to calculate the millimetre wave reflectivity of objects within the images.

17. An imaging system as claimed in any of the above claims wherein the system is arranged to process the separate images recorded at different illumination levels so as to detect any differences between them, and to provide an alert if a significant difference is found.

18. A. method of generating images using a millimetre wave imaging system comprising a receiver having at least one receive element sensitive to millimetre wave radiation, and imaging means for converting radiation received at the receiver into an image, characterised in that a plurality of separate images of a scene are recorded, a first image being formed of the scene illuminated at a first intensity, and a second image being formed of the scene illuminated at a second intensity.

19. A method as claimed in claim 18 wherein the first image is formed using radiation substantially confined within a first frequency band and the second image is formed using radiation substantially confined within a second frequency band.

20. A method as claimed in claim 19 wherein an illuminator is provided that provides a higher illumination intensity in the first frequency band, and a lower power in the second frequency band..

21. A method as claimed in claim 19 wherein the illumination intensity difference between frequency bands is caused by atmospheric attenuation.

22. A method as claimed in claim 18 wherein the first and second images are recorded at different times.

23. A method as claimed in claim 22 wherein an illuminator of variable intensity is provided that is synchronised with the timing of the images such that adjacent images are recorded at different intensities.

24. A method as claimed in any of claims 18 to 23 wherein the separate images recorded at different illumination levels are processed so as to detect any differences between them, and to provide an alert if a significant difference is found.

25. An illumination means arranged to co-operate with a millimetre wave imaging system characterised in that the illumination means is adapted to be switchable between a first illumination intensity and a second illumination intensity such that the imager is able to provide successive images illuminated at either the first or second illumination intensity.