WO2008078112A1

WO2008078112A1 - Environmental conditioning apparatus, a chamber for use thereof and a related detection method and apparatus

Info

Publication number: WO2008078112A1
Application number: PCT/GB2007/005021
Authority: WO
Inventors: Christopher Mark Mann; Leslie Green; Claes Bergstedt; David Haskett; Gregory Finn
Original assignee: Thruvision Limited
Priority date: 2006-12-23
Filing date: 2007-12-24
Publication date: 2008-07-03
Also published as: WO2008078112A8

Abstract

Thermal surfaces are used to condition the environment of a subject in black body radiation imaging apparatus. The surfaces improve the image contrast in detecting concealed objects and can be built into the walls of passages and cubicles for scanning human subjects. Thermal surfaces may be warmed and/or cooled according to conditions. In one embodiment for use in imaging a body carrying a concealed object, cooled surfaces are used to immerse the body and object in a reduced level of black body radiation in the field of view of an imager, and a warmed surface is used behind the body to match the effective temperature of the body. The body and warmed surface merge to appear as a blank area, both avoiding the imaging of any anatomical detail and making the object easier to detect. An alarm or alert can be generated when an object is detected and a second, conventional camera can be used to provide a conventional visual view of the subject area to an operator which can be overlaid with a black body view of a concealed object at any time that such an object is detected.

Description

ENVIRONMENTAL CONDITIONING APPARATUS, A CHAMBER FOR USE THEREOF AND A RELATED DETECTION METHOD AND APPARATUS

The present invention relates to environmental conditioning apparatus, a chamber for use thereof and a related detection method and apparatus, for use in detecting objects using the electromagnetic spectrum at wavelengths in the centimetre to sub-millimetre range.

Embodiments of the invention are relevant from the microwave to the terahertz region of the electromagnetic spectrum. However, the terahertz region has particular benefit for many applications in offering high resolution in small systems and specific embodiments of the invention are described below which operate in the terahertz region. "Terahertz" in this context means the electromagnetic spectrum at wavelengths in the millimetre to sub-millimetre range.

This range is referred to herein generally as the terahertz spectrum. Terahertz radiation has been found a useful tool for imaging and other purposes because some materials are transparent to it which are opaque through the visible spectrum. This allows these materials to be "seen through" using terahertz radiation where they could not using visible optical radiation. For example, terahertz wavelengths have been used in imaging the earth' s surface through the atmosphere and for improving visibility in bad weather (for example for flying or driving) . Some materials can be distinguished under terahertz radiation because of their distinctive transmissivity or reflectivity and this has been used for example in detecting food or chemical components. Further, objects themselves can emit terahertz radiation, including the human body. This has been used for example in medicine for detecting skin cancer. Because clothing is generally transparent to terahertz radiation but weaponry is not, another application has been the detection of weaponry otherwise concealed about the person. Cameras for imaging an object by use of the terahertz ("THz") spectrum are known. For example, an arrangement is described in International Patent Application WO 2004038854 in the name Agence Spatiale Europeenne. In this arrangement, the camera is based on a double bank of horn antennae which each pick up terahertz radiation, in use, which is directed into a mixer channel to extract an intermediate frequency signal using a local oscillator. This known heterodyne technique allows smaller detectors to be used at room temperature in the terahertz range than might otherwise be necessary and so supports finer resolution.

Passive imaging systems are possible, using the black-body radiation emitted from all physical objects. Such imaging systems are used at wavelengths ranging from micro waves, to mm- waves, sub-mm waves, terahertz waves, and continuing through the far-infrared range, several of these designations being relatively synonymous in practice.

The term "operating wavelength" will be used herein to refer to the actual wavelength band over which a particular imaging system is operating.

Passive imaging systems which work by sensing different amplitudes of radiation at the operating wavelength are unable to distinguish between natural thermal ("grey body") emission of an object and the reflected power from the surroundings. This can affect the imaging process.

There is particular interest in imaging human subjects with certain concealed objects hidden under their clothing. Such objects are typically cooler than the body temperature but tend to be physically warmer than the ambient air temperature due to the warming effect caused by contact or close proximity to the body. Such objects might therefore be considered easy to detect by virtue of their different physical temperature. However, the difference in physical temperature is relatively indeterminate due to the variable amount of clothing present and variable proximity to the body. Additionally, the different reflectivity of the body compared to the object may mean that in certain circumstances some objects may assume similar equivalent black body temperatures to the body, greatly reducing or removing contrast in the image.

It is non-optimal in security applications to have a system which shows a degraded performance over a range of conditions . For example, no black-body radiation sensing system could be expected to work in an indoor ambient temperature of 37 ⁰C since the body, object and environment are all at the same temperature, thus giving a black-body temperature of 37 ⁰C for all parts of the image.

It is non-optimal in security applications to have a system which shows a degraded performance over a range of conditions. However, for passive terahertz imaging an effective black body contrast is required. Every environment and measurement requirement relies on a complicated thermo-dynamic and physical situation. For example, at first glance a very non-ideal situation for a blackbody imaging system would be to operate in an indoor ambient temperature of 37 ⁰C where the body, object (at steady state temperature) and environment are all at the same temperature, thus giving a black-body temperature of 37 ⁰C for all parts of the image. However, in reality if the person and object has come in to the room from an environment where it is not 37C (either hotter or cooler) it may take a considerable time for the object to stabilize at 37C. Therefore this situation would in fact be ideal for imaging the object. Alternatively if the person is perspiring then clothing that is in contact with the skin and is therefore damp will actually be cooler due to the evaporation process. The object may act as a barrier to the clothing in front of it from becoming wet and therefore appear to be warmer. Each environment and situation must be considered in detail and all physical processes will affect the level of terahertz black body radiation contrast available to the imager. However, to a first order it is the environment that will set the level of contrast available.

According to a first aspect of the present invention, there is provided environmental conditioning apparatus for use in controlling the immersion of an object in black-body radiation at an operating wavelength, the apparatus comprising a thermally insulated panel having a temperature control element and a low reflectivity surface between the temperature control element and the object, in use of the apparatus, which surface is provided by material having low reflectivity at the operating wavelength.

Preferably, the panel further comprises thermally insulating material between the low reflectivity surface and the object, in use of the apparatus, said thermally insulating material having a low transmission loss at the operating wavelength.

Such a panel will radiate black body radiation to a greater or lesser degree, depending on whether the temperature control element is set to produce a relatively high or relatively low temperature of the panel . It can thus be used to increase or decrease an ambient black body temperature in relation to an immersed object to a degree depending on the proportion taken by the panel of a field of view of the subject.

The term "operating wavelength" may include more than one wavelength at a time, or one or more wavelength ranges.

Optionally, the panel also has a heat spreading layer, between the temperature control element and the low ' reflectivity surface, this improving evenness in the thermal output of the panel . Clearly, it is desirable in any imaging process that the image contrast should be sufficient. It has been realised, in making the present invention, that there are two factors present in an environment which affect image contrast in imaging by black-body radiation, these being the ambient temperature and the equivalent black-body temperature of radiated emissions present. It would be possible to get image enhancement by cooling the ambient temperature from a typical 20 ⁰C room temperature to say 5 ⁰C. This is neither convenient nor comfortable. The second factor however is more amenable to adjustment. Apparatus according to embodiments of the invention ^■ provide a way of controlling the temperature of equivalent black-body emissions in a localised environment which is convenient, comfortable and inexpensive .

The equivalent black-body temperature T_BB of an object is given by

T_BB = T₀ (1 -Po) + T_E . Po where T₀ is the physical object temperature Po is the reflectivity of the object (1 -po) is the emissivity of the object T_E is the equivalent environmental temperature

Hence the black body temperature of a body perceived by imaging apparatus using black-body radiation (often referred to herein as the "effective" temperature of an object, body or surface) is a combination of the body's own temperature, multiplied by its emissivity, together with the radiation it is reflecting from its environment tempered by its reflectivity. Indeed it might be noted that for an object to be seen by a black body radiation imager, it only needs to be different in effective black body temperature from its background. It could in fact be hotter in ⁰C though in practice it is usually cooler. The effect of a panel according to the first aspect of the invention is to change the level of radiation that a body, and any object carried by the body, is reflecting from its environment.

An aim of imaging by use of black-body radiation is to detect an object carried by a body. The object will only be detected if the perceived black-body temperature of the body and object are sufficiently different. To use the example of a human body (37 ⁰C) carrying an object in an ambient temperature of 27 °C, the apparent temperature differential between the body and the object which would give the image contrast in perceived black body radiation would be 10 ⁰C at first sight. However, other factors have to be taken into account. Looking just at the body carrying the object, if the reflectivity of the body at the operating wavelength is only 0.5, then the temperature differential is reduced because the perceived black body temperature of the human body is not 37 ⁰C but only 32 ⁰C. This follows from using the above equation:

T_BB = T₀ (1 -P₀) + T_E . po = 37 (1 - 0.5) + 27 . 0.5 = 32 ⁰C

Further, the temperature of the carried object itself is likely to be raised perhaps a couple of degrees because it is physically close to the human body which is a heat source. If the reflectivity of the object is 0.9, the perceived black body temperature of the object is then: 29 (1 - 0.9) + 27 . 0.9 = 27.2

This gives a temperature differential between the body and the object of only 4.8 ⁰C. If the reflectivity of the object is less than 0.9, the differential will be reduced further.

Using an embodiment of the present invention, however, the value of T_E can be manipulated to improve the differential.

Given an environment consisting of ambient temperature objects and cooled and/or heated illumination apparatus, the equivalent environmental temperature T_E will depend on how much of the subject sees the environmental conditioning apparatus. If half the solid angle field of view from the subject into its environment contains a heating or cooling surface, then the equivalent environmental temperature T_E seen at the subject will be half way between the unmodified ambient temperature and the temperature of the cooled or heated surface.

According to a second aspect of the present invention, there is provided a chamber for use in imaging at least part of a subject by means of black body radiation, the chamber comprising at least one subject location, at least one camera location and at least one illumination surface for illuminating a subject in the subject location with black body radiation at an effective temperature different from the ambient temperature in the chamber in use, wherein the surface is at least partially located to illuminate the subject location within the field of view of a camera at the camera location, in use of the chamber.

A subject location in this context is a location at or on which a subject will be located in use of the chamber for viewing by the camera. It may be for example either a predictable path to be followed by a subject through the chamber, such as the floor of a corridor, or a support for supporting a stationary subject, such as a specified location on a floor. A camera location is a location in which a camera can be placed in use of the chamber to view at least part of the subject location and might conveniently for example, but not necessarily, comprise a mounting for a camera.

"Chamber" as used in this specification is intended to encompass any space, indoor or outdoor, in which imaging can be done with some control over the environment of a subject or subject to be imaged. It is intended to be interpreted broadly so as to include corridors and spaces where a subject or target, whether moving or stationary, will be suitably positioned for imaging and at the same time can be illuminated with black body radiation, whether from a local or remote source and/or by reflection, so as to modify the effective temperature of the subject or target in a predictable or controllable manner.

If "f" denotes the proportion of the solid angle field of view from the subject in any one direction into its environment which is filled by the panel, then:

T_E = Tp f + T_Λ (1 - f) where the "P" and "A" subscripts relate to panel and ambient temperatures respectively. The significance of this equation is perhaps easier to see when it is slightly rearranged:

T_E = T_A + ΔT_PA f where ΔT_PA is the difference between panel temperature and the ambient temperature.

Closer inspection of the above equation shows that a "cold beam" which is small in cross section cannot be used to illuminate a subject to a practical extent. The maximum possible temperature difference ΔT_PΛ when using a panel or surface that has a cooling effect relative to the environment is around 300 ⁰C, the limiting factor being absolute zero. Even then, "£" has to be larger than 0.03 to change T_E by 10 °C. The spatial extent of the irradiating beam in practice thus has to be large relative to the environment of the subject for the effect to be significant. For example, in an arrangement for detecting objects concealed on human bodies in a normal indoor setting, it might be preferred that "f" is at least 0.5.

It is an option to use a hot beam to change T_E, generated by a heated rather than a cooled panel or surface relative to the ambient temperature, but such arrangements tend to suffer from specular reflection from metallic or other reflective objects, giving intense points in the image which do not give a true image shape. In more detail, and taking the example of a subject which comprises a concealed object carried on a human body, as long as the reflectivity of the object is not identical to that of the body, panels or surfaces which are either hotter or colder than the ambient temperature can change the image contrast. Referring to Figure 1, in the case that the reflectivity of the object is greater than that of the body, then the image contrast (indicated by the vertical separation between the lines representing the perceived temperatures of the body and the object) across a range of T_E either side of T_A is greatest at "Cl" as shown where T_E is low, that is, using a cool panel to create ΔT_PA. Referring to Figure 2, conversely in the case that the reflectivity of the object is lower than that of the body, the image contrast is greatest at "C2" as shown where T_E is high, that is, using a warm panel to create ΔT_PA.

According to a third aspect of the present invention, there is provided a chamber for use in imaging at least part of a subject comprising a body carrying a concealed object, the chamber comprising at least one subject location and at least one camera location, wherein the chamber further comprises a temperature- controllable surface and a temperature controller for controlling the temperature of the surface to have a known effective temperature distribution, the surface being positioned at least partially in the field of view of a camera at the camera location.

Embodiments of the invention in its third aspect can offer significant advantage in providing an area of an image in which there is a reference level of effective temperature. However, if the temperature-controllable surface has an effective temperature in use which is at least substantially the same as that of the body as viewed by the camera, then there is the very significant advantage that anatomical detail can be removed where the body is a human body. This represents a real breakthrough in terms of security screening acceptability and a real advantage over competing technologies such as backscatter X-ray and active millimetre wave systems.

"The same" in this context does not necessarily in practice mean identically the same but may only mean sufficiently similar to make a significant reduction in image contrast between the body and the background as viewed by a camera at the camera location.

Preferably an embodiment of the invention comprises a chamber which meets the requirements of the invention in both its second and third aspects. That is, a chamber which provides both at least one illumination surface and a temperature-controllable surface.

According to a fourth aspect of embodiments of the invention, there is provided a thermally adjustable panel for use in providing the temperature-controllable surface of embodiments of the invention in its third aspect, the panel having: i) a temperature control element; ii) a surface at least part of which has a first level of reflectivity with respect to black body radiation; and iii) a shutter having a second level of reflectivity with respect to black body radiation, the shutter being movable relative to the surface so as to control the area of the surface having the first level of reflectivity which is exposed to the camera in use of the apparatus .

The shutter could be provided in several different forms. For example, it could comprise a set of planar strips which can be rotated about their longitudinal axes to control the area of the surface exposed. Alternatively, it could comprise a screen with apertures therein, the size of respective apertures being variable to control said area of the surface exposed. A panel according to embodiments of the invention in its fourth aspect could be used in a chamber which is enclosed, as in a building, or in a chamber which is at least partially open, for example being provided in a street or other outdoor environment.

According to a fifth aspect of the present invention, there is provided detection apparatus for use with a chamber as described above, the detection apparatus comprising: i) a camera having at least one terahertz radiation detector for producing a terahertz radiation image signal in relation to the field of view of the camera; and ii) a signal processor for processing terahertz radiation image signals produced by the camera to detect one or more preselected characteristics thereof; wherein the signal processor is adapted to provide a trigger output, for use in triggering a triggerable device, on detection of at least one of said one or more pre-selected characteristics .

Embodiments of the invention in its fifth aspect support the provision of a security measure such as an alarm, or some change in behaviour of the detection apparatus, perhaps a change in the colour or the resolution of at least part of a display, when a terahertz radiation-based camera detects a significant change in the field of view. "Significant" here may be based for instance on a numerical measure derived by experiment or perhaps on data gained from previous behaviour of the detection apparatus. Where a temperature-controllable surface has been provided, having an effective temperature in use which is at least substantially the same as body temperature as viewed by the camera, then the normal image signal of a terahertz radiation- based camera might provide a screen area which is blank and featureless on a display device. A significant change in this context might then comprise the appearance of a feature not previously present in the blank screen area, this being caused for instance by the arrival in the field of view of a concealed object showing a different effective temperature from that of the temperature-controllable surface.

In some applications, it may be preferred to provide a more conventional image of a field of view, as well as a terahertz radiation-based image signal. In this case, a visual camera might also be provided and the two images displayed in parallel or superimposed. In this respect, there might be provided detection apparatus for use in a chamber as described above, the apparatus comprising: i) a camera having at least one terahertz radiation detector for producing a terahertz radiation image signal in relation to the field of view of the camera; and ii) a visual imaging device for producing a visual image signal in relation to the field of view of the camera, the visual image signal being based on received radiation which is outside the terahertz region of the electromagnetic spectrum.

Preferably, the apparatus further comprises: iii) a signal combiner capable of combining image signals produced by the camera and by the visual imaging device to provide a visual display signal having at least a first portion determined by the terahertz radiation image signal and at least a second portion determined by the visual image signal.

This allows areas of an image based on a terahertz radiation signal to be overlaid by a visual image. This can help an operator to relate an image of a concealed object to a real scene and may also support the suppression of anatomical detail appearing in an image since it is the terahertz camera that might capture it and any critical parts of an image as seen by the terahertz camera can then be "blanked out" by overlaying the visual image. According to a sixth aspect of embodiments of the present invention, there is provided a method of detecting a carried object, which method comprises the steps of: i) illuminating a detection location with black body radiation so as to modify the effective temperature of one or more surfaces in the detection location; and ii) imaging a field of view in the detection location by means of black body radiation to produce a black body image signal.

Advantages of embodiments of the invention discussed above in relation to the first to fifth aspects can also apply to embodiments of the invention in its sixth aspect. In a method according to this sixth aspect, used for detecting an object carried by a body in the detection location where the body has a different effective surface temperature from that of the object, the method may therefore further advantageously comprise the step of providing a background to the body, the background having an effective temperature matched at least substantially to, or at least as high as, that of the body.

Additionally or alternatively, the method may comprise the steps of: iii) imaging the field of view by means of electromagnetic radiation in the visible and/or near-visible spectrum to produce a visual image signal; and iv) generating an overlaid image signal, comprising in part the black body image signal and in part the visual image signal.

It is to be understood that any feature described in relation to any one embodiment or aspect of the invention may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments . Illumination apparatus according to an embodiment of the invention will now be described, by way of example only, with reference to the accompanying figures in which:

Figure 1 shows a graph of perceived black-body temperature difference between a body and an object in the case that the reflectivity of the object is greater than that of the body;

Figure 2 shows a graph of perceived black-body temperature difference between a body and an object in the case that the reflectivity of the object is lower than that of the body; Figure 3 shows a cross section of a layered illumination panel according to an embodiment of the invention;

Figure 4 shows in three quarter view from above a curved illumination panel of the type shown in Figure 3 in use;

Figure 5 shows in plan view three alternative corridor arrangements using flat illumination panels;

Figure 6 shows in vertical cross section a corridor using flat illumination panels, and in particular the effect of floor temperature on an object in the corridor;

Figure 7 shows in vertical cross section a corridor with improved exposure to floor temperature;

Figure 8 shows in plan view two alternative arrangements of a corridor with added restrictions to increase illumination of a subject;

Figure 9 shows in plan view a further alternative arrangement for a corridor;

Figure 10 shows in plan view a further alternative arrangement for a corridor;

Figure 11 shows in plan view a corridor arrangement with optimised background; Figure 12 shows in front elevation a body carrying an object against a background in two cases, an unmodified background and a warm background;

Figure 13 shows imaging results in the case of an object carried on the leg, firstly back or front, and secondly to the side; Figure 14 shows a scanning cubicle using illumination panels according to embodiments of the invention; Figure 15 shows in side elevation the scanning cubicle of Figure

14;

Figure 16 shows in plan view the scanning cubicle of Figure 14 with an alternative cool wall arrangement; Figure 17 shows in vertical, longitudinal cross section a corridor for use in the invention with improved floor construction;

Figure 18 shows in greater detail and exploded view the improved floor construction of Figure 17; Figure 19 shows an element for use in a thermal panel of a corridor, perhaps in combination with the floor construction of

Figures 17 and 18;

Figure 20 shows an alternative imaging and display arrangement for use in embodiments of the invention; Figures 21A through to 23B show components of screen views that might be obtained using the imaging and display arrangement of

Figure 20;

Figures 24A through to 25B show a variation of the scanning cubicle of Figure 14 with a removable warm background,' together with schematic examples of images that might be obtained;

Figures 26A and 26B show an arrangement in which a view of a subject can be obtained through the floor of a corridor, and a schematic example of an image that might be obtained thereby;

Figure 27 shows two further camera arrangements that might be used to obtain images of subjects moving along a corridor; and

Figures 28A, 28B and 29 show alternative forms of background with controllable reflectivity for use for example in the cubicle of Figure 14.

Figures 1 and 2 are discussed above.

Referring to Figure 3, a simple layered illumination panel 300 that produces black-body radiation that can be used to adjust T_E, with low power consumption and without substantially changing the ambient air temperature, has five layers. These are: • a thermal insulator 305 with low loss at the operating wavelength, such as a foamed plastic

• a low reflectivity, radiation absorbent material 310 such as "Eccosorb®" or pure wool carpet • an optional heat spreading plate, 315 typically aluminium

• a heating or cooling temperature control element 320

• a heavy thermal insulator 325

Eccosorb® is a range of microwave absorbing products of Emerson & Cuming Microwave Products Ltd.

The exact material used for the front transparent thermal insulation 305 is a matter of choice and operating wave length. It needs to block the general conduction of heat out of the panel but show low transmission losses at the terahertz wavelengths of interest. An example of a suitable material is low density polystyrene which is a great insulator but pretty much completely transparent to terahertz radiation. The amount of insulation present primarily relates to the power consumed by the panel 300 rather than the functionality. Thicker insulation gives less power consumption but too much thickness will attenuate the radiated emissions too greatly.

The temperature control element 320 may take any suitable form, such as resistive wires or a heated fluid, connected to a controllable energy supply and optionally thermostatically controlled. For most applications, the distribution of temperature should be as evenly delivered as possible, although in some circumstances there may be advantage in having a known, non-even distribution.

There are a great many forms of chamber that might use one or more illumination panel (s) 300 to provide controlled environments which are similar from the point of view of the physics of the operation. The specific form chosen is a question of cost, space and operational convenience. Some examples are given, but these in no way should be considered as limiting the possible range of constructions that can be chosen.

All the schemes can also use mechanically moved absorbing shutters which cover/uncover the panels 300 on demand, thereby changing the illumination rapidly. One such shutter is described below with reference to Figures 24 and 25.

Referring to Figure 4, if a human subject 405 is walking towards or facing an imager 400, it is only the front face of the subject 405 that is visible. It is therefore only the front face of the subject 405 that needs to be "illuminated". (The term "illuminated" is used herein whether the panel 300 is a cooling or a heating panel.)

Suitable forms of imager 400 are known, based on terahertz radiation detection. Such an imager 400 will be based on at least one terahertz radiation detector, or an array of detectors, and electronics sufficient to produce an image signal which can be stored, transmitted over a network and/or used to drive a display such as a liquid crystal display screen.

It is more convenient to consider the subject 405 to be stationary when considering illumination angles. However, it should be borne in mind that in practice the subject 405 will be moving. It is also more convenient to consider the two dimensional plan view rather than looking at solid angles, although it should be understood that the field distribution is three dimensional. In the following, two dimensional plan views are shown but it should be understood that the description relates to the three dimensional counterparts.

Figure 4 shows in plan view a curved panel 300 in front of a human subject 405, viewed by an imager 400. The panel 300 extends from floor to ceiling and thus, within the curved panel

400 and from the direction of the imager, the subject 405 is apparently completely immersed in the panel's "ambient light level". However this is only the case if floor and ceiling are neglected. If the floor and ceiling are taken into account, the subject 405 is only partially immersed. Thus Figure 4 shows an arrangement according to an embodiment of the invention in which a panel 300 can be used to raise or lower T_E, the equivalent environmental temperature in relation to the subject 405, by filling a proportion "f" (more than half in this case) of the solid angle field of view from the subject 405 in the direction towards the imager 400. In the arrangement of Figure 4, if the floor and ceiling of the chamber were also provided by panels 300, "f" would have the value "1".

Figure 4 shows a form of open chamber in which a human subject can be imaged. Referring to Figures 5 and 6, it is more convenient to use flat panels 300 since flat panels are easier to construct. For example, a subject 405 may pass down a chamber provided as a corridor comprising flat panels 300 as side walls 505.

Figure 5 shows three variants of a corridor in schematic plan view and Figure 6 shows a corridor in schematic vertical cross section, thus including the floor 610 and ceiling 605.

In the two dimensional plan models as shown in Figure 5, the maximum angle on the front face of the subject 405 would have to be 180 degrees for total immersion. If the sides 505 of the corridors are provided by panels 300, each side 505 will subtend an angle 510 at the front face of the subject 405 and the immersion angle ^ΛΛIA" is the sum of the angles 510 subtended by the two sides 505 of the corridor.

In an open-ended corridor, the dimensions of the corridor will affect the immersion angle IA. The immersion angle IA is higher in a narrow corridor of a given length compared with a broad corridor of the same length, being given by the formula: immersion angle = 2 x arctan (2 x length/width)

In embodiments of the present invention, sufficient immersion angle is achieved when the length of the corridor is more than 3 times the width, in a corridor with panelled sides but un- panelled floor and ceiling.

If a corridor has restrictions 515, creating perhaps a door sized opening at an end thereof, the above formula is unchanged. It is just that the opening is now effectively the "width". A benefit, however, lies in the human factors issue that a corridor does not seem so cramped. Such an arrangement is shown in the third example in Figure 5.

As mentioned above, the floor and ceiling decrease the degree of immersion of the subject 405 in a corridor if they have no temperature control element. To increase immersion, it is easy to install panels 300 on the ceiling 605 but panelising the floor 610 is less easy due to the requirement for robustness. The transparent insulator 305 on the panel 300 tends to be too soft to carry a human weight. In the cross sectional view of a corridor shown in Figure 6, it is evident that a point 600 low down on a subject 405 "sees" more of the floor temperature than a point 615 further from the floor. An unfortunate side effect is therefore that the lower half of a human subject 405 will appear warmer than the upper half when using cooled panels 300 as the sides 505 of the corridor.

Referring to Figure 7, to reduce the effect of the floor it is possible to provide angled sides 700 to the floor 610 of a corridor but this type of corridor cross section would be unsuitable in some circumstances, for instance for wheelchairs or people using walking sticks or frames.

Referring to Figure 8, a working layout for a corridor for detecting concealed objects on human subjects comprises a straight section with entrance and exit at angles to the straight section. Constrictions 515 at the entrance and exit to the straight section set the "width" of the corridor. Concealed imagers 400 at one end or preferably both ends are directed along the length of the straight section, the side walls of which are provided by cooled panels 300. The ceiling (not shown) would also have thermal panels 300 fitted for optimum performance. A good practical width of the corridor in use is between 1.0 and 1.5 metres.

In Figure 8, it might be noted that there are two imagers 400 pointing directly at each other. One can be used to image the front of a subject 405 at the same time as the other is used to image the back of the subject 405. Having two imagers 400 pointing directly at one another however is not ideal and, in practice, they should be slightly offset. Further, metal panels or objects in the field of view of each imager 400 are unhelpful and to be avoided since they can reflect "stray" light and give ghost images .

In the second embodiment shown in Figure 8, an exit of the corridor is branched.

The restrictions 515 in the corridor increase the illumination of the subject 405 when they are within the cooled panel area, thereby improving the image contrast. The restrictions 515 should leave a gap of no less than 0.8 metres in the corridor.

If only one imager 400 is used, the restrictions 515 should be placed at the end of the corridor closer to the imager 400 and omitted from the far end. The restrictions 515 give an undesirable background to a subject if cooled, this being further discussed below in relation to Figures 11 and 12.

Referring to Figure 9, an alternative form of corridor simply diverts a subject 405 towards and then away from a single imager 400. The corridor provides a side chamber to the main direction of the corridor with a small plastic barrier 900 at waist height, causing the human subject 405 to walk an appropriate path. This allows the use of a single imager 400, optionally on a pan mount, to take both front and rear views of the subject 405. The sides 505 of the corridor/chamber are provided by cooling panels 300, including the wall to either side of the imager 400.

Although not shown separately, each imager or camera 400 is mounted in use by means of a suitable mounting and this might be fixed, adjustable or capable of panning movement, either manual or driven. The mounting may be incorporated in the imager or camera 400 or may be provided separately.

Referring to Figure 10, a further alternative arrangement provides an "L" shaped corridor, again with a single imager 400 or a pair of adjacent imagers 400. Cooling panels 300 provide the walls of the corridor and the imager (s) 400 can take a view down either or both leg(s) of the corridor.

Referring to Figures 11 and 12, in a modification of the arrangement shown in Figure 9, the plastic barrier 900 has a triangular cross section in plan view and the imager 400 sits in a tapered portion of the corridor/chamber. The sides of the tapered portion are provided by cooled panels 300, these acting as both side walls and restrictions. In this embodiment, the tapered portion of the corridor/chamber may include a cooled panel 300 in the floor, entry to the tapered portion being discouraged by a light weight plastic barrier 1110.

It is inadvisable to image a subject 405 against a cool background. Although the contrast between the background and the subject 405 will be increased, the contrast of objects hidden on the subject 405 will actually be decreased. The subject is therefore best imaged against a relatively warm background.

In Figure 11, a background panel 1105 with a controllable surface temperature is shown behind the subject 405 in relation to the imager 400. If this temperature-controllable background panel 1105 is warmed to body temperature, it has the effect of removing the human subject 405 from the image to leave just a concealed object 1210. An image that might be produced by the imager 400 is shown in Figure 12. In a first view with no warm background panel 1105, it can be seen that the outline of the human subject 1200, 1205 appears in the image. This makes a concealed object 1210 difficult to detect. In the second view of Figure 12, where a warm background panel 1105 is used behind the subject 405, only the concealed object 1210 is visible, making the object 1210 significantly easier to detect. Such an arrangement has an additional advantage in that no anatomical detail can be shown if the outline of the human subject 1200, 1205 is not imaged.

A suitable background panel 1105 is made from material of high thermal conductivity such as copper or aluminium sheet and can be provided with resistive heaters. The panel 1105 (or panels) preferably ensures that heat is distributed evenly over the whole surface facing the camera 400. To avoid spurious results, it may be preferred that the panel (s) 1105 has sufficient thermal conduction to reduce the thermal variation across the surface of the panel 1105 facing the camera 400 to less that the minimum δT (that is, the minimum detectable temperature variation) of the imaging system. For example, this may be about 0.2°K. As with the illumination panels 300 described above, a good front surface with low reflectivity can be provided by Eccosorb® or pure wool carpet. It has been found relatively easy to fabricate a background panel 1105 with a temperature variation across it, as viewed by the camera 400, of less than 1°K. This can be checked for example using an infrared camera with a δT of 0.1⁰K. The overall temperature can be controlled by a suitable control device 1125 of known type.

If the outline of the human subject 1200, 1205 is to be eradicated altogether, the actual temperature of the panel (s) 1105 needs to match the human body temperature fairly closely. However, it would be possible to process an image signal from the camera 400 to set a threshold in the data so that a displayed image showed all areas at the human body temperature and above in a single colour. Thus the outline of the human subject 1200, 1205 would disappear from a displayed image against the background panel 1105 as long as the temperature of the background panel 1105 was at or above the human body temperature. Alternatively, it would be possible to set a temperature band in the data obtained with a camera 400 within which band a displayed image shows just a single colour or pattern. If the band is set to include the human body temperature, again the outline of the human subject 1200, 1205 would disappear from a displayed image against the background panel 1105 as long as the temperature of the background panel 1105 was within the temperature band. This arrangement may be found useful where the terahertz camera 400 has a very good δT because in this case some body anatomy can start to appear, particularly where cooled panels 300 are used. Variations in viewing angle combined with the illumination angle of the cooled panels 300 can start to show up body features due to the finite reflectivity of the skin.

It will be understood of course that the temperature band within which no detail is shown would have to exclude the expected temperature of any concealed object it is intended to detect.

Since this can be extremely close to the human body temperature, it may not be practical in many circumstances to use an excluded temperature band. As shown in Figure 11, there may be cooled panels 300 behind the camera 400, illuminating a human (or animal) subject 405. The cooled panels 300 particularly, but potentially also other aspects of the environment of the subject 405, will modify its effective skin temperature. This needs to be taken into account in setting the temperature of the background panel 1105. The effective terahertz temperature of a person's skin is not the same as its physical temperature. Because the skin is partially reflective it appears cooler than the 37.5°C one might expect, say 34°C. When a cooled panel 300 is introduced, the skin will appear colder still. However, the temperature of the background panel 1105 just has to be dropped appropriately.

Another factor to be taken into account is the reflectivity of the background panel (s) 1105, particularly where there are cooled panels 300 behind the camera 400. The background panel (s) 1105 need to produce very little reflection so that the effective temperature is as independent as possible of other features of the environment of the subject 405.

Figure 11 also shows an imaging system for showing images captured by the terahertz camera 400. This comprises signal processing software supporting a display screen 1115 for receiving an image signal from the imager 400. The screen 1115 may be provided for example together with a computing platform supporting the signal processing software to process the received image signal, or data associated with it. It is possible to trigger an alarm 1120, or other equipment, when one or more conditions or pre-selected characteristics are met in relation to the received image signal. This is further discussed below with reference to Figures 20, 21A and 21B.

Referring to Figure 13, it may be necessary to have increased resolution in order to detect a concealed object on a leg. At 6m between an imager 400 and a subject 405, the full torso can be imaged but parts of the leg will only relate to a few, perhaps a single, pixel at this range. This depends in part on the nature of the imager 400 as some imagers have an elongate detecting aperture giving poor resolution in at least one direction due to poor interpolation between pixels. Using such an imager 400 means that items carried on the front or back of a leg might be distinguishable but an item 1210 carried on the side of the leg may not be detectable. In the first pair of images in Figure 13, a field of view showing the lower leg is shown, first as viewed in real time from the position of the imager 400 and second as imaged. An object 1210 is concealed on the back of the leg. It can be seen that the lower temperature of the concealed object 1210 means that the image produced clearly has the lower portion of leg apparently missing. In the second pair of images in Figure 13, the concealed object 1210 is carried on the side of the leg and the image produced shows very little difference between the two legs 1300.

There are several steps that can be taken to improve detection of objects concealed about the legs. The imager 400 can be moved closer to the subject 405, for example to 3m. An imager 400 with better resolution in one direction than another can be rotated so that the resolution is improved in a required direction. A warm panel 1105 can be placed behind the subject 405, as discussed in relation to Figures 11 and 12.

Referring to Figure 14, the imager 400 need only block a small fraction of the forward direction from a subject 405. This means that it is possible to place a cool panel 300 behind the imager 400 to increase the image contrast. The imager 400 can be placed close to the subject 405, for example at 3m as mentioned above, and moved to scan the subject 405, for example on a vertical pole 1400. Further, if the subject is stationary, a cool floor section 1405 can be placed under the subject 405. It is possible to provide a full body scan using this arrangement within 2 seconds, using a known imager 400 marketed as the T 4000 by ThruVision Ltd. It would be possible to move the imager 400 even closer to the subject 405 using modified optics. In a particularly convenient arrangement, floor switches can be provided, so that the subject 405 can enter the cubicle or chamber, trigger frontal scan, turn around and trigger a scan from the rear. This arrangement is shown in side view in Figure 15.

In the embodiment shown in Figures 14 and 15, the mounting for the imager 400 incorporates both a collar 1415 and the pole 1400.

Referring to Figure 16, two sets of cool panels 300 can be provided behind and to either side of the imager 400 and a warm panel 1105 can be provided behind the subject 405.

Referring to Figures 17 and 18, in order to improve "illumination" of the subject 405 by cool panels, it is possible to provide a cool panel 1700 in the ceiling of the corridor and reflectors 1705 in the floor. In this arrangement, cooling black body radiation 1710 that would otherwise be lost is used to illuminate the subject 405 from below and thus improve immersion. The reflectors 1705 can be provided as sloped metal surfaces in the upper face of a layer 1810 of the floor. These might be protected by a layer of polystyrene 1805 and a PTFE/Nylon upper coating 1800.

Referring to Figure 19, it is also possible to use directional elements in a warm or cooling panel 1105, 300. For example, individual heating or cooling element 1900 may each be seated behind a flared cavity 1905 to reduce losses.

In embodiments of the invention, an ability is provided to adjust and manipulate a background in order to enhance the presence of foreign objects on subjects. This provides some benefits as described above and at least some of these benefits can be exploited to provide a better operator-oriented working environment. For example, referring to Figure 11, where an imager 400 is providing an image signal capable of display on a screen 1115, it is possible to process the image signal or the screen display so as to operate some form of alarm 1120 when one or more conditions are met . An operator may not then have to watch as assiduously.

Referring to Figures 20, 21A and 21B, significant advantage can be gained by using an arrangement such as that shown in Figure 11 in which only a concealed object 1210 is imaged by an imager 400 because background temperature equalising is provided in relation to a body carrying the concealed object 1210 by a temperature equalising surface 1105.

In the arrangement of Figure 11, an image is effectively "cleaned" of all image detail except that of a concealed object 1210, if present. This lends itself to the use of an alarm 1120 since the normal view of the imager 400 would be blank. It is only when a concealed object 1210 arrives in the field of view that the image signal would produce anything but a blank screen 1115. This makes it particularly easy to trigger an alarm 1120 whenever a concealed object 1210 is brought into the field of view. It is merely necessary to monitor for example the image signal output by the imager 400, or the screen 1115 itself, to detect a pre-selected characteristic such as a significant change and trigger the alarm 1120. The change could be a level change in the image signal or a transition from a blank, or "clean", screen to a screen showing a feature such as the outline of a detected concealed object against the clean background. Signal processing software for monitoring the image signal or the screen and triggering the alarm 1120 can be run on any appropriate platform, such as a personal computer supporting the screen 1115 and the definition of a pre-selected characteristic can be set as appropriate to the particular circumstances and/or equipment in use. The alarm 1120 can alert an operator (with either on-screen or off-screen information) and/or initiate some other triggerable device, process or procedure. The triggerable device, process or procedure could be embodied as any one or a combination of mechanical, electrical or electromagnetic devices or processes, or the provision of any human sensory targeted signal such as a visual or audible signal.

A scene change associated with the presence of a concealed or foreign object could create a visual alarm signal in a number of ways. Firstly, the image displayed could indicate the presence of a foreign object by changing one or more colours to a colour indicating an alarm condition. In this case, the triggerable device will comprise a colour adjustor for adjusting one or more colours of a display. This can easily be implemented in more than one way. One way would be to configure the system such that the apparently coolest area(s) of the screen 1115 have a neutral or "cool" colour but any area of the screen 1115 indicating a higher temperature has a "hot" colour to provide the alert. This would operate so that when there is no concealed object in the field of view, the screen 1115 would be uniformly neutral. As soon as a concealed object comes into the field of view, the coolest area of the screen 1115 is now the part showing the object. This stays a neutral colour but the whole of the rest of the screen 1115 will now switch to a "hot" colour, giving a relatively dramatic alert condition to an operator. Alternatively, the area of the screen 1115 showing the object could take on the "hot" colour and the rest of the screen 1115 be arranged to remain neutral. In either case, other visual effects could be applied, such as flashing.

In an embodiment producing an image signal supporting a "clean" screen as background to significant features such as a detected concealed object, it is not necessary to show the "cleaned" area of the screen to an operator at all. A visual screen which is blank during non-alert status could be found a distraction to the system operator. Referring to Figures 20, 21A and 21B, an alternative mode of operation is to overlay at least part of the image, particularly a blank area, from the black body radiation- based imager 400 with an image from a conventional type of visual camera 2000 to produce an overlaid image signal 2025. This can be done for instance using a signal processor 2005 as a signal combiner to deliver the overlaid image signal 2025 to a visual display screen 1115. Now the "clean" area of the screen 1115 in terms of the image signal 2015 produced by the black body camera 400 is effectively transparent to an image signal 2020 from the visual camera 2000. The signal processor 2005 can also monitor the image signal 2015 produced by the camera 400. If a concealed or foreign object is present in the field of view of the cameras, the image signal 2015 from the black body camera 400 will show a significant variation which will be detected by the signal processor 2005 which will in turn trigger a change in the display 1115. The change in the display 1115 can be produced by a change in the overlaid image signal 2025 in which the relative portions determined by the image signal 2015 from the black body camera and the image signal 2020 from the visual camera 2000 will change, in this case from one hundred per cent being determined by the image signal 2020 from the visual camera 2000 to the situation where a portion of the signal 2025 relating to the concealed object is instead determined by the image signal 2015 from the black body camera. In this scenario, the equipment supporting the display 1115 constitutes a triggerable device which responds to a trigger output of the signal processor 2005. The display 1115 will now deliver a view of the object which will appear to the operator as overlaid on an otherwise ordinary visual image of the field of view. The object as it appears on-screen can additionally be highlighted in some manner, such as a bright colour or flashing, to attract the attention of an operator. In this manner of operation, the operator will normally be watching a dynamic scene and can concentrate on other security indicators such as human behaviour until such time as a foreign object is detected by the black body imager 400 and the signal processor 2005. It is not necessary that the visual camera 2000 responds only to light in the visible region of the electromagnetic spectrum, or indeed to light in the visible spectrum at all. It could instead or additionally respond to infra-red light for example, and thus provide a night view of a scene. The visual camera

2000 is there to provide a view of the scene which is different from that provided by the black body camera 400 and thus it is only necessary that the visual camera 2000 will detect radiation outside the terahertz region of the electromagnetic spectrum.

Figure 20 shows examples of signal types that might be produced by the two cameras 400, 2000 when a foreign object (not shown) carried by a human subject 405 is present in a scene having a warm panel background (not shown) of the type shown in Figure 11. The image signal 2015 from the black body imager 400 shows a flat signal except for where the object introduces detail. Figure 21B shows how this image signal 2015 might appear on a display, the object here appearing as a box shape 1210. The image signal 2020 from the visual camera 2000 shows detail over the whole field of view. Figure 21A shows (schematically only) how this image signal 2020 might appear on a display, the field of view here showing human forms 2105, 2100 and the small plastic barrier 900 described above in relation to Figure 9. The signal processor 2005 substitutes flat portions of the image signal 2015 from the black body imager 400 with the detail of the corresponding portions of the image signal 2020 from the visual camera 2000 to produce an overlaid image signal 2025 to drive the visual display 2010.

Many other types of on-screen or off-screen triggerable alarm or alert system could be devised and any combination of these alarm/alert systems can be implemented simultaneously. For example, the signal processor 2005 may apply a threshold condition to the image signal 2015 from the black body imager 400 and trigger an alarm whenever the level of the signal 2015 breaches the threshold. If there is no concealed object, the image signal 2015 will stay at a level corresponding to the temperature equalising surface 1105 but a concealed object 1210 can produce a significant drop in the apparent temperature. It is even envisaged that an operator need not be present. In this case for example there may be no visual camera 2020 and the signal from the black body camera 400 may be transmitted to a signal processor at a remote location and not used at all to produce an image related to the field of view but simply used to drive a triggerable device as necessary. In other embodiments of the invention, further security measures can be triggered automatically by a significant change in the field of view. One example is that the imaging system shifts into a higher resolution mode in order to help an operator or object identification system to discern the identity of a foreign object. In this case, the triggerable device comprises a resolution adjustor which might for example change the pixel size in at least a region of the screen which is of particular interest because it shows a portion of the image signal 2015 associated with the drop in apparent temperature. Another example is operation of security devices operated for example by means of a switch or transducer, such as security doors, external audible or visual alarms, automated communication systems or the like.

In another form of thresholding, it is possible to set a size threshold with regard to triggering the alarm or alert system. Particularly where a threshold is being applied to a "cleaned" image signal 2015 from the black body imager 400, where a temperature equalising background panel 1105 is used to remove all but a suspect object from the image signal 2015, the signal processor 2005 can be set to trigger the alarm/alert only where the object has a size above a threshold size. This type of thresholding can be used to differentiate between a small item, such as a mobile telephone for example, versus a large item such as a kilo of contraband. It would also be relatively easy, for example using known forms of pattern-matching software, to look for specific shapes such as guns and circles which might be for example DVDs.

In some scenarios, particularly where a black body camera 400 has very good temperature resolution and one or more cooling panels 300 is used to illuminate a subject 405, it is possible that the image signal 2015 contains data relating to unwanted anatomical detail. In this case it may be preferred that the signal processor 2005 is set to detect a portion of the image signal indicating an effective temperature at or close to that of the temperature equalising surface and to delete image data from that portion of the image signal. In this case, a display unit 1115 might act as a triggerable device as it will be driven by a processed image signal 2015 to show at least one blank area where image data has been deleted. The meaning of "close to" in this context is likely to be decided by factors in any particular set of circumstances, in particular how close the likely effective temperature of any object it is desired to detect will be. It would clearly be undesirable to delete image data in relation to such an object and the range of effective temperature at which image blanking is done will be determined accordingly.

Where two or more cameras 400, 2000 are used and the image signals need to be related, there is a need to co-ordinate the fields of view so that features detected by one camera can be related to an image signal produced by another. There are a number of ways in which this can be done. The cameras can be physically aligned, using whatever positioning equipment each is provided with, or the image signals can be related by use for example of either manual, physical alignment of the images or, more accurately, a feature map matching technique. Image overlay itself can be performed using standard video production/editing techniques. In practice, it may not be necessary to overlay images from more than one camera but simply to present the different images side by side, one being for example a THz image and another being a visual video image. In this and other embodiments, the images may not cover fields of view having the same perimeters. In this arrangement, it is possible to provide a superimposed reference "box" visible in the visual video image that represents at least the approximate field of view for the THz image .

Referring to Figures 22A and 22B, a particularly advantageous option where both a THz image and an optical image of the same subject are available is to use the optical image to determine whether a suspect object detected in the THz image is concealed. This can be done by observing the outline shape 2200 of a suspect object in the THz image (Figure 22A) and comparing it with the optical image (Figure 22B) to determine if the object is concealed under clothing. The outline shape 2200 of the object found in the terahertz image can be used to interrogate the optical image to look for a similar shaped object 2205 in the same relative position. That is, the signal processor 2005 can be set to look for an object outline 2205 in the optical image which:

• has the same shape as the outline shape 2200 of the object found in the terahertz image

• is present in the same position

• is present at the same time

This can be done whether the THz image is a "clean" image, using a background panel 1105 at body temperature to eradicate the human form 405, 1205 from the image, or otherwise. If a matching outline 2205 does appear in the optical image (Figure 22B) , it can be concluded that the object is not concealed and is likely to be a hand bag, suitcase, laptop bag or the like. In this case, it is an option then not to set off an alarm but to track the object and person subsequently, using optical object recognition. Referring to Figures 23A and 23B, the situation is shown where an object outline 2200 is found in the THz image (Figure 23A) but no similar object is found in the optical image (Figure 23B) . In this situation, it can be concluded that it is likely the object is concealed beneath an optically opaque material such as clothes and hence may constitute a concealed object. Detection of an apparently concealed object in this way can be used to set off a "concealed object" alarm.

From a security and functionality perspective, both instances are useful as it is possible to make a significant, even dramatic, reduction in the false alarm rate for concealed objects (since many people carry bags which would otherwise falsely appear as target concealed objects) and also identifies every bag in a passing stream of people. Once the outline shape and colour of the bag is subsequently extracted, or "learnt", from the optical image information, the bag ands its carrier can be tracked through an integrated security system using other means such as optical recognition software. This means that the bags and their carriers can be followed through a crowd to ensure that they pass through the correct screening procedure such as X-ray inspection.

Referring to Figures 24A, 24B, 25A and 25B, as described above with reference to Figures 11, 12 and 21, a temperature equalising panel 1105 behind a subject body 405 can make a significant difference to the image obtained from the terahertz radiation camera 400. If a concealed object is cooler than the subject 405, then it may be seen alone as an object 1210 against the warm background panel 1105 as shown in Figure 12. However, if the object has a similar reflectivity to skin and is at body temperature, then there will be nothing in the image captured by the terahertz radiation camera 400, as shown in Figure 24B. If the temperature equalising panel 1105 were not behind the subject body 405 in relation to the camera 400, and the background to the subject 405 were at a different temperature from the body, then unless it is transparent the object may at least be "seen" by the terahertz radiation camera 400 as a distortion of the outline of the subject body 405. Such an image is shown in Figure 25B where an outline 1200 of the subject body 405 is seen, which outline 1200 has a detectable distortion 1210 on the leg, created by a view of the object.

In order to obtain views with and without a temperature equalising panel 1105 behind the subject body 405, using the same camera 400 and viewing chamber, it is possible to install a shutter wall 2400 of strips that can be rotated through 90° in the manner of a Venetian blind. This is shown in Figures 24A and 25A. In Figure 24A, the shutter wall 2400 is open to expose the temperature equalising panel 1105 behind the subject body 405 to the camera 400. In Figure 25A, the shutter wall 2400 is closed, covering up the temperature equalising panel 1105 and presenting a surface at a different effective temperature from the body 405.

The strips of the shutter wall 2400 could be made from reflective material such as metal and positioned in front of the temperature equalising panel 1105. In the arrangement shown in Figures 24 and 25, illumination panels 300 are provided behind and to the sides of the camera 400 with respect to the subject body 405. The shutter wall 2400 when closed, as shown in Figure 25A, will reflect radiation from the illumination panels 300 back to the camera 400, giving a potentially clear outline of the body 405 and any bulges caused by objects strapped to the body 405.

Referring to Figures 28A and 28B, with or without the illumination panels 300 the situation can be different. There may be many situations where the physical temperature around a subject body 405 is complex, unknown or can change and where all sorts of surfaces of different emissivities and physical temperatures might be illuminating the subject body 405. In this case, it can be desirable to have a thermally adjustable panel, referred to below as a backdrop 2800, whose black body temperature, as seen by the camera 400, can be at least roughly tuned to match that of the subject body 405. It can be difficult to change the temperature of a temperature equalising panel 1105, as described above, very quickly and/or evenly. In a variation, a backdrop 2800 may comprise an absorbing panel 2820 (high emissivity) which is heated to a temperature at least slightly above body temperature, for instance to 38.5°. In front of it in relation to a camera 400 (not shown) , a shutter comprising two highly reflective panels 2805, 2810 with an array of apertures 2815 in each are mounting in sliding relationship so that the apertures of respective panels 2805, 2810 can be brought in and out of register with each other. In this way, the surface area presented to the camera can be tuned using the shutter 2805, 2810 from highly reflective, where the apertures 2815 are entirely out of register and the camera sees only the surfaces of the highly reflective panels 2805, 2810, through to significantly absorbing, where the apertures are entirely in register with each other and the camera sees a high proportion of the absorbing panel 2820. Figures 28A and 28B show in particular an arrangement where the apertures are approximately half overlapping.

For example, if the absorbing panel 2820 has an emissivity 0.95 or greater, the highly reflective panels 2805, 2810 might each have an emissivity of not more than 0.01 or as low as 0.001. A suitable material for the highly reflective panels 2805, 2810 might be a metal such as brushed aluminium or similarly finished, conductive surface. The overall effect of this configuration is to provide an adjustable reflectivity that can be tuned to be similar or the same as that of the skin in whichever frequency the camera is operating.

In an arrangement as shown in Figures 28A and 28B, the shape of the apertures 2815 may be varied, for example to rectangular or tapered. The circular shape shown in Figures 28A and 28B tends to limit the maximum amount of the absorbing panel 2820 which can be shown by bringing the apertures into register and another shape might be preferred. The apertures don't have to be the same size or shape in the two respective reflective panels 2805, 2810. It may however be preferred to keep the size of the apertures when in maximum register to below that at which they can be resolved by the camera 400. This might be for example to a maximum diameter or long side of less than 0.1 of an equivalent measurement of the camera's beam footprint. Further, small apertures are less likely to have a frequency-dependent effect.

One of the reflective panels 2805 might in practice be mounted on the absorbing panel 2820, the other reflective panel 2810 being mounted for sliding movement in front of it with respect to a camera 400.

Referring to Figure 29, in a variation, just one highly reflective panel 2810 with an array of apertures 2815 in it might be mounted in sliding relationship with the absorbing panel 2820, the surface of the panel 2820 having a non-uniform distribution of reflectivity. For example, an array of fibrous areas 2900 alternating with highly reflective areas might be present on the face of the panel 2820. The single reflective panel 2810 can now be moved to expose or cover the areas of low reflectivity 2900.

In a further variation, the shutter might be provided as a set of planar strips which can be rotated about their longitudinal axes to control the area of the surface of the absorbing panel 2820 exposed to the view of the camera. A shutter, or "shutter wall" 2400, of this type is described above in relation to Figures 24A and 25A. In yet a further alternative, the shutter might comprise a sheet of material with apertures each having individual stopping devices for stopping down the aperture, in the manner of a conventional photographic camera shutter to control light coming in to the camera .

An advantage of using a temperature-controllable background panel 1105, or a backdrop 2800, is that its temperature and reflectivity can be known. This means that the presence of a background panel 1105 or backdrop 2800, or part of a background panel 1105 or backdrop 2800, which is "in shot" in use of a terahertz camera 400 can be used to calibrate an associated imaging system. By having a surface which has been set to a fixed temperature in view of the camera 400, the imaging system can be calibrated for changing terahertz lighting conditions as might appear outdoors or in a non temperature-controlled room during the day. Thus the background panel (s) 1105 or backdrop 2800 may be useful even if not set to equalise body temperature in order to eradicate the human form from images obtained.

Further, the use of a background panel 1105 or backdrop 2800 "in shot" introduces an ability to get the absolute temperature of a subject object 405, which may be a person. For example this might be useful to detect a person whose body temperature indicates they might have an illness such as SARS or they are at a heightened stress level which could be an indicator of mal- intent .

Referring to Figures 26 and 27, it is possible to view a subject 405 from additional angles so as to image a concealed object that would not normally appear to a camera 400 with a substantially horizontal field of view. Thus in Figure 26A, a camera 400 is embedded in a transparent floor 610, or under a platform that the person walks over, and views the subject 405 against a heated panel 1105 in the ceiling. In this arrangement, the imager 400 may just scan from side to side to give information in a transverse direction, the movement of the person 405 as they walk providing the perpendicular information in the image. A warm wall 1105 is positioned over the person 405 at ceiling level to clean any anatomical detail out of the image (see Figure 26B) which will show a concealed object 1210 and nothing else, as long as the object's effective temperature is different from that of the person 405. As shown in Figure 27, two alternative camera views would be from above at an oblique angle, to see the front of a person pushing a trolley in front of them, and from behind. To image from above, the positions of the imager 400 and the warm wall 1105 are switched with regard to the arrangement shown in Figure 26A.

In the above description, reference is made to a subject being a body carrying an object. Although, as described, the subject may be a human carrying an object concealed in clothing, embodiments of the invention might well be found suitable for use with subjects of another type. For example an animal carrying an object would be substantially equivalent as a subject but there may be other forms of subject for which the invention would be useful. The preferred characteristics are that the "body" and the object should be at different black body temperatures, for instance because they are at different absolute temperatures and/or have different reflectivities, and it is not essential that the "body" should be living.

Claims

CIiAIMS

1. Environmental conditioning apparatus for use in controlling the immersion of an object in black-body radiation at an operating wavelength, the apparatus comprising a thermally insulated panel having a temperature control element and a low reflectivity surface between the temperature control element and the object, in use of the apparatus, which surface is provided by material having low reflectivity at the operating wavelength.

2. Environmental conditioning apparatus according to Claim 1, the panel further comprising thermally insulating material between the low reflectivity surface and the object, in use of the apparatus, said thermally insulating material having a low transmission loss at the operating wavelength.

3. Environmental conditioning apparatus according to either one of Claims 1 or 2, the panel further comprising a heat spreading layer, between the temperature control element and the low reflectivity surface, for improving evenness in the thermal output of the panel.

4. A chamber for use in imaging at least part of a subject by means of black body radiation, the chamber comprising at least one subject location, at least one camera location and at least one illumination surface for illuminating a subject at the subject location with black body radiation at an effective temperature different from the ambient temperature in the chamber in use, wherein the surface is at least partially located to illuminate the subject location within the field of view of a camera at the camera location, in use of the chamber.

5. A chamber according to Claim 4 wherein the at least one illumination surface is adapted to illuminate the subject with black body radiation at an effective temperature lower than the ambient temperature .

6. A chamber according to Claim 4 wherein the at least one illumination surface is adapted to illuminate the subject with black body radiation at an effective temperature higher than the ambient temperature .

7. A chamber according to any one of Claims 4, 5 or 6 wherein the illumination surface provides at least part of a surface of the chamber.

8. A chamber according to any one of Claims 4 to 7 wherein the illumination surface is curved.

9. A chamber according to any one of Claims 4 to 8 wherein the illumination surface fills at least half of a 180° solid angle field of view from the subject location towards the camera location.

10. A chamber according to any one of Claims 4 to 9 wherein the chamber comprises a corridor having sides, a floor and a ceiling and the illumination surface provides at least part of the sides and/or ceiling of the corridor.

11. A chamber according to any one of Claims 4 to 9 wherein the chamber comprises a cubicle having sides, a floor and a ceiling and the illumination surface provides at least part of the sides and/or ceiling of the cubicle.

12. A chamber according to any one of Claims 4 to 11 wherein the chamber has a floor provided with reflecting elements for reflecting illumination from one or more illumination surfaces towards the subject.

13. A chamber according to any one of Claims 4 to 12 wherein at least one illumination surface comprises one or more temperature controlling elements and at least one directional element for directing illumination produced by the temperature controlling element (s) .

14. A chamber according to any one of Claims 4 to 13 wherein the camera location comprises a mounting for a camera.

15. A chamber according to any one of Claims 4 to 14, wherein the chamber further comprises a temperature-controllable surface for providing a background to a subject comprising a body carrying an object at the subject location in the field of view of a camera at the camera location, the temperature-controllable surface having a controlled effective temperature in use.

16. A chamber according to Claim 15, further comprising a control device for controlling the effective temperature of the temperature-controllable surface in use.

17. A chamber according to either one of Claims 15 or 16 wherein the effective temperature of the temperature- controllable surface in use is at least substantially the same as that of the body as viewed by the camera.

18. A chamber for use in imaging at least part of a subject comprising a body carrying a concealed object, the chamber comprising at least one subject location and at least one camera location, wherein the chamber further comprises a temperature- controllable surface and a temperature controller for controlling the effective temperature of the surface, the surface being positioned at least partially in the field of view of a camera at the camera location, in use of the chamber.

19. A chamber according to Claim 18 wherein the known effective temperature distribution has a variation of not more than 2⁰K in use, over the area of the temperature-controllable surface in the field of view.

20. A chamber according to either one of Claims 18 or 19, wherein the temperature-controllable surface has an effective temperature in use which is at least substantially the same as that of the body as viewed by the camera.

21. A chamber according to any one of Claims 4 to 20 wherein the body and the object have different reflectivities in use of the chamber for imaging by use of at least one operating wavelength.

22. A chamber according to any one of Claims 4 to 21 wherein at least part of at least one illumination surface is provided by a panel according to any one of Claims 1 to 3.

23. A chamber according to any one of Claims 4 to 22 wherein the camera location comprises a camera having at least one terahertz radiation detector.

24. A thermally adjustable panel for use in providing the temperature-controllable surface of any one of Claims 18 to 23, the panel having: i) a temperature control element; ii) a surface at least part of which has a first level of reflectivity with respect to black body radiation; and iii) a shutter having a second level of reflectivity with respect to black body radiation, the shutter being movable relative to the surface so as to control the area of the surface having the first level of reflectivity which is exposed to the camera in use of the apparatus.

25. A panel according to Claim 24 wherein the shutter comprises a set of planar strips which can be rotated about their longitudinal axes to control said area of the surface exposed.

26. A panel according to Claim 24 wherein the shutter comprises a screen with apertures therein, the size of respective apertures being variable to control said area of the surface exposed.

27. A panel according to Claim 24 wherein the shutter comprises at least two screens with apertures therein, at least one of the screens being movable in use to adjust the degree to which the apertures of the respective screens are in register.

28. A panel according to Claim 24 wherein said surface of the panel has a non-uniform distribution of reflectivity, and wherein the shutter comprises at least one screen with apertures therein, the screen being movable in use to adjust the degree to which the apertures expose a part or parts of the surface having said first level of reflectivity.

29. Detection apparatus for use with a chamber according to any^¬ one of Claims 4 to 23, the detection apparatus comprising: i) a camera having at least one terahertz radiation detector for producing a terahertz radiation image signal in relation to the field of view of the camera; and ii) a signal processor for processing terahertz radiation image signals produced by the camera to detect one or more pre- selected characteristics thereof; wherein the signal processor is adapted to provide a trigger output, for use in triggering a triggerable device, on detection of at least one of said one or more pre-selected characteristics .

30. Detection apparatus according to Claim 29 wherein a preselected characteristic comprises a significant change in a terahertz radiation image signal produced by the camera.

31. Detection apparatus according to Claim 30 wherein said significant change comprises a signal level change from a signal level representing an effective temperature corresponding to body temperature in the field of view.

32. Detection apparatus according to either one of Claims 30 or 31 wherein said significant change comprises a signal level change indicating a discernible object in the field of view.

33. Detection apparatus according to Claim 32 wherein said signal level change is sustained so as to indicate a discernible object in the field of view having a size greater than a threshold size.

34. Detection apparatus according to either one of Claims 32 or 33 wherein said signal change comprises a signal level change from a signal level representing an effective temperature corresponding to body temperature in the field of view to a level indicating the discernible object, the apparatus being for use in detecting carried objects.

35. Detection apparatus according to any one of Claims 29 to 34, further comprising a triggerable device arranged to receive a trigger output of the signal processor.

36. Detection apparatus according to Claim 35 wherein the triggerable device comprises at least one of the group comprising an audible or visual alarm, a switch or a transducer.

37. Detection apparatus according to Claim 35 wherein the triggerable device comprises a colour adjustor for a display showing an image of at least part of the field of view of the camera .

38. Detection apparatus according to Claim 35 wherein the triggerable device comprises a resolution adjustor for a display showing an image of at least part of the field of view of the camera .

39. Detection apparatus according to Claim 37 wherein the colour adjustor is adapted to adjust the colours of the display such that an area of the display indicating the coolest temperature in the field of view is shown in a different colour to one or more area(s) of the display indicating a higher temperature.

40. Detection apparatus according to Claim 39 wherein the area of the display indicating the coolest temperature in the field of view at any one time is shown always in the same colour or colour range.

41. Detection apparatus according to Claim 29, for use in a chamber having a temperature equalising surface for providing a background to a subject in the field of view of the camera, in use, the temperature equalising surface having an effective temperature in use which is at or close to that of at least part^' of the subject as viewed by the camera, wherein a pre-selected characteristic detectable by the signal processor comprises a portion of the image signal indicating an effective temperature at or close to that of the temperature equalising surface; and wherein the trigger output in respect of said characteristic is adapted to delete image data from said portion of the image signal .

42. Detection apparatus for use in a chamber according to any^¬ one of Claims 4 to 23, the apparatus comprising: i) a camera having at least one terahertz radiation detector for producing a terahertz radiation image signal in relation to the field of view of the camera; and ii) a visual imaging device for producing a visual image signal in relation to the field of view of the camera, the visual image signal being based on received radiation which is outside the terahertz region of the electromagnetic spectrum.

43. Detection apparatus according to Claim 42, the apparatus further comprising: iii) a signal processor for comparing at least part of a terahertz radiation image signal produced by the camera with at least part of a visual image signal produced by the visual imaging device in order to detect one or more related features of said signals.

44. Detection apparatus according to Claim 43, wherein the signal processor is adapted to provide a trigger output, for use in triggering a triggerable device, in the event that a terahertz radiation image signal of the camera shows one or more pre-selected characteristics.

45. Detection apparatus according to Claim 44 wherein a preselected characteristic comprises a significant change in detected terahertz radiation within its field of view.

46. Detection apparatus according to Claim 45 wherein the signal processor is adapted to provide any one of at least two different outputs, a first output being provided on detection of one or more of said related features and a second output being provided on detection of said significant change in the absence of any said related feature.

47. Detection apparatus according to any one of Claims 42 to 46, the apparatus further comprising: iii) a signal combiner capable of combining image signals produced by the camera and by the visual imaging device to provide a visual display signal having at least a first portion determined by the terahertz radiation image signal and at least a second portion determined by the visual image signal.

48. Detection apparatus according to Claims 43 and 47 wherein the first and second portions are selected at least partially in accordance with said one or more related features.

49. Detection apparatus according to either one of Claims 47 or 48 for use in a chamber having a temperature equalising surface for providing a background to a subject in the field of view of the camera, in use, the temperature equalising surface having an effective temperature in use which is at or close to that of at least part of the subject as viewed by the camera, wherein the portion (s) of the display signal determined by the visual image signal corresponds to one or more regions of the field of view in which the terahertz radiation image signal indicates a level of radiation at or close to that detected from the temperature equalising surface.

50. A method of detecting a carried object, which method comprises the steps of: i) illuminating a detection location with black body radiation so as to modify the effective temperature of one or more surfaces in the detection location; and ii) imaging a field of view in the detection location by means of black body radiation to produce a black body image signal.

51. A method according to Claim 50, for detecting an object carried by a body in the detection location, the body having a different effective temperature from that of the object, the method further comprising the step of providing a background to the body, the background having an effective temperature at least close to, or higher than, that of the body.

52. A method according to Claim 50, for detecting an object carried by a body in the detection location, the body having a different effective temperature from that of the object, the method further comprising the step of providing a background to the body, the background having an effective temperature matched at least substantially to that of the body.

53. A method according to any one of Claims 50 to 52, further comprising the steps of: iii) imaging the field of view by means of electromagnetic radiation in the visible and/or near-visible spectrum to produce a visual image signal; and iv) generating an overlaid image signal, comprising in part the black body image signal and in part the visual image signal.