GB2519233A - Scanning method and apparatus for electromagnetic detection - Google Patents

Abstract

THz detection equipment having a scanning mechanism designed to scan a field of view (FoV) 100 in relation to THz detectors is combined with an optical camera 175 so that the THz field of view can be steered in accordance with the output of the camera. Steering can be provided automatically using an algorithm or manually for example using a joystick. The apparatus comprises optical camera 175 with field of view 170, THz scanner 195 with movable mirror 105 for steering the received THz signal to detectors 115, and control means 125. The THz image 134 can be moved around an image 133 from the camera. Also disclosed is a scanning mechanism for scanning the THz FoV, comprising a rotatable mirror and support structures and drive mechanisms.

Description

SCANNING METHOD AND APPARATUS FOR ELECTROMAGNETIC

DETECTION

The present invention relates to a scanning method and apparatus for electromagnetic detection using the electromagnetic spectrum at wavelengths in the centimetre to sub-miflirnetre range.

Embodiments of the invention have particular relevance in the terahertz (THz) region of the electromagnetic spectrum, for instance for sensing or imaging. The Tl-Iz region has particular benefit for many applications, offering high resolution in small systems.

"Terahertz" in this context means the electromagnetic spectrum at wavelengths in the millimetre to sub-millimetre range.

THz radiation has been found a useful tool for detection because sonic materials are transparent to it which are opaque through the visible spectrum. This allows these materials to be "seen through" using THz radiation where they could not using visible optical radiation. Further, objects themselves emit Tl-Iz radiation, including the human body. Because clothing is generally transparent to THz radiation but many objects are not, including for example weaponry, an application has been the detection of objects otherwise concealed about the person.

It is possible to use either active or passive THz detection. In active detection, a THz source can be used to illuminate an object or field of view. hi passive detection, there is no illuminating source, the THz detection regime just receiving THz radiation from an object or fidd of view. Passive systems have an advantage in some applications, such as in relation to humans, because there can be no inference that use of the system can cause physical harm. However, (he signal to noise ratio is relatively low with passive detection, so (hat the speed with which an image can be built, for example, (ends to be limited in comparison with active systems.

In imaging or sensing, it is generally necessary to scan the field of view to deliver radiation onto one or more detectors. hi many scenarios, there can be limited timc in which to carry out ascan, so scan speed is important, hut sufficient data needs to he collected to achieve a reasonable signal to noise ratio to make the data usable. Normally if better sensitivity (Signal to Noise Ratio, SNR) is required then the number of detector elements is increased allowing more of a scene to be observed for longer. At millimeter wave and THz frequencies this is not always possible as there is a fundamental limit as to the number of detector elements that can be mated to a given set of optics due to the longer wavelength and restricted system envelope volume. In addition, the cost of current detector elements is prohibitive and the hackend electronics can in practice restrict the detector layout to a linear array. This is particularly true for sniafl compact systems.

It is known to scan a field of view using a scanning minor or reflector to direct radiation from the field of view to a hank of THz detectors.Such an arrangement is disclosed for example iii International patent apphcation No. W02006013379 in the name Council for the Central Laboratory of the Research Councils. There can be problems however with detector sensitivity, particularly where there is low thermal contrast between an object to be observed and its surroundings. Improvements can be obtained by increasing the number of detectors or slowing the scan speed but the physical size of detectors and the need for short scan times set limits in this respect.

According to a first aspect of embodiments of the invention, there is provided Tl-Iz detection apparatus comprising one or more TI-lz detectors and a scanning mechanism for scanning a THz field of view in relation to the THz detector(s). thc THz dctcction apparatus further comprising: i) a detection system for locating objects of interest. Two examples of such a detection system are: the use o1a human interface via a touch-screen displaying an image of the world in front of the THz imager; the use of Automatic Target Detection (ATD) software processing the THz data itself; and ii) a steering mechanism connectable for receiving a control signal based at least parfly on the detection system output and sending a steering signal for usc in steering the scanning mechanism to scan the THz field of view, whereby the scanning ol the THz field of view is dependent at least partly upon the optical detection output.

"Field of view" in this context is intended to mean the area of a scene being viewed, or sensed, either by the detection system. or by the one or more THL detectors. In the following, the terms "field of view" and "field of regard" are used. With respect to the THz detectors, these are intended to mean: * "Field of view": the area of a scene that is currently being imaged or sensed by the THz detectors due to movement of the scanning mirror(s). For exampk, in imaging to give successive frames in relation to a scene, each frame will show the

THz field of view

* "Field of regard": the largest area of the scene that could he accessible to the THz detectors by steering the scanning mirror(s) without moving the detectors With respect to the detection system. the field of view refers to the angular range within which the detection system is capable of detecting objects of interest.

Using embodiments of the invention means that a scene of interest can be investigated using the detection system for example to identify a likely target, then this information uscd to stccr a THz scanning mechanism with respect to the target. Usc of the one or more THz detectors can then be optimiscd in an efficicnt manner. A known form of scanning mechanism comprises a mirror (or minors) that receives THz radiation from the field of view and dehvers it to the THi detector(s) usually via a collection device. Scanning is achieved by moving the nurror and steering is achieved by controlling that movement.

The detection system and THz detection outputs might both be converted to graphical data to support screen-based representations of the optical and THz fields of view. These screen-based representations preferably accurately relate the positions of the fields of view.

For example, the THz field of view might be shown as a frame or pointer overlaid on the optical field of view. By steering the scanning mechanism, using the visual indicator offered by the screen, the Tl-lz fidd of view can he moved in relation to the detection system output. This will feed back as relative movement of the screen-based representations. The steering mechanism might then he provided as software that receives control signals from one or more control devices, for example another software process. a joystick, keyboard andlor touch screen, converts them as required and delivers a steering signal to the scanning mechanism. The steering signal might for example include data to adjust line speed and offset.

Optionally, the THz field of view is smaller in area than the detection system field of view.

An advantage arises because the data for a small field of view can he collected more sthwly than for a large field of view. Embodiments of the invention can significantly improve the signal to noise ratio of THz detection by controlling the scanning of a THz field of vicw to be smaller in area than the detection system field of view but maintaining the likely target in that reduced field of view. Optionally, the number of THi detectors can he kept relatively low while maintaining an acceptable signal to noise ratio, minimising system size, cost and complexity.

lii other embodiments of the invention, the THL field of view may be of the same order of area, or even larger than the detection system field of view, and may have different relative dimensions from the detection system field of view. For example, the detection output of the detection system might indicate a likely target that extends out of the detection system field of view. In this ease, the Tl-Iz field of view might beneficially overlap the detection system fidd of view hut not fail whofly within it, being directed to cover the position of the wholc likely target.

Further, the scanning mechanism may he steered to provide a scanning pattern in which more inlormation is obtained from a specific area, such as the area of a likely target, than from the rest of a THz field of view. For example. in a scanning raster, the lines might be significantly closer together when scanning the area of the likely target than when scanning the rest of a field of view. Alternatively very few, perhaps just one or two singular THz readings, might he taken from elsewhere in the TI-IZ field of view for comparison purposes rather than for imaging, such as from the sky or a known background.

Preferably. the scanning mechanism can he used to provide a THz held of view according to any of the above options.

Preferably the scanning mechanism is capable of adaptive scanning so that the steering mechanism can steer it to scan in more than one scanning pattern, dependent on the control signal. For example. as well as covering different fields of view, the scanning mechanism might he capable of scanning with adjustable line pitch and/or speed. To achieve this, the control signal(s) supplied to the steering mechanism and delivered as a steering signal to the scanning mcchanism might include data to adjust line spccd and offset.

The scanning mechanism may he based on one or more movable mirrors which relied incoming THz radiation onto one or more THz detectors. In its most basic mode of operation. the image is recovered by scanning (moving) the mirror(s). so that the detector(s) effectively follow a scanning raster relative to the THz field of view. The minor(s) might move so as to produce a scanning raster comprising spaced scan lines. To produce the scanning raster, the mirror moves differently in different directions. For example, the minor might reciprocate about a first axis to create scan lines coupled with a stepped rotation about a second axis to set the scan line spacing.

However, the scan mirror may he configurable to adopt any random scanning regime such as those required for future image processing algorithm-based enhancements such as compressive sensing and compressive imaging.

According to a second aspect of embodiments of the invention, there is provided a scanning mechanism for scanning a THz field of view using at least one movable minor to receive THz radiation from the field of view and deliver it to one or more THz detectors, the scanning mechanism comprising: i) a mirror structure mounted for rotation about a first axis; ii) an intermediate support structure for carrying the mirror structure, the intermediate support structure being mounted for rotation about a second axis; iii) a base support structure for carrying the intermediate support structure; iv) a minor drive mechanism mounted in part on the intermediate support structure and in part on the minor structure, for driving the mirror structure to rotate about the first axis; and v) an intermediate drive mechanism mounted in part on the base support structure and in part on the intermediate support structure for driving the intermediate support structure to rotate about the second axis, whereby the mirror structure can he driven to rotate about either one or both of the first and second axes.

The minor structure might comprise a mirror or a minor support structure having fixings configured to attach a minor.

The two drive mechanisms may be independently operable and comprise for example electromagnetic drive mechanisms. Such a drive mechanism has been found too heavy for use in scanning in known optical arrangements and might be selected for driving a structure about one axis only, where the weight of at least part of the drive can he carried by a base structure. However, it has been found possible in embodiments of the invention for THz scanning to accommodate two electromagnetic drive mechanisms, for scanning in two separate directions, and still achieve high speed and reliability. Using solenoidal electromagnetic drive mechanisms which comprise a coil/core combination, it is possible to provide contactless bcarings whilc maintaining high torque. Becausc the angular range of the mirror only has to be small, say of the order of ±100. it has been found possible to mount the coil/core combination so that the electromagnetic relationship doesn't change over the whole range of scanning. That is, using a curved construction that follows the movement of the nurror structure, the overlap of the coillcore combination can be kept complete, therefore providing maximum torque, throughout the whole angular range. Drive mechanisms according to embodiments of the invention have been found to operate fast and reliaffly in the TI-Iz context, for example being capable of driving a mirror having a surface area as small as 100 cm2. However, the weight of the drive mechanism becomes less significant in relation to larger minors and these also give a larger FOV. Hence embodiments of the invention might comprise a mirror having a reflective surface area of at least 150 cm2. 200 cm2 or greater.

One or both drive mechanisms may comprise a coil suspended about a drive core. Such an arrangement can benefit from continuous adjustability in terms of scanning behaviour.

reliability, silent operation and lack of wear. The coil and drive core can be mounted respectively on one each of the minor structure and the intermediate support structure so as to drive the mirror structure in relation to the intermediate support structure, and again on one each of the intermediate support structure and the base support structure so as to drive thc intermediate support structurc in relation to thc basc support structurc. To achicvc a balanced arrangement, there might be two or more drive mechanisms, placed symmetrically about either or both of the relevant axes, to drive the mirror structure and the intermediate support structure. To minimise weight in the minor structure, the coil of the or each drive mechanism for driving it is preferably mounted on the mirror structure since the coil can have less mass than the core.

Again for the purposes of reliability and low wear, the manner of mounting the mirror structure for rotation about the first axis and/or the intermediate support structure for rotation about the second axis is preferably without rubbing surfaces. A suitable known form of bearing is the flexure bearing in which spring elements fixed at their respective ends to inner and outer shoes, or to an outer housing and an inner shoe, allow relative rotation bctwccn thc shoes and/or housing.

In at least some embodiments of the invention, an array of feedhorns which extends diagonally across the scanning raster might he used. This diagonal layout means that neighbouring feedhorns follow individual scan lines which are more closely spaced than the spacing of the feedhorns themselves in the array. Thus in at least some embodiments of the invention, in which the scanning mechanism comprises a mirror steerable to produce a scanning raster relative to the Tl-lz field of view, the Tl-lz detection apparatus may comprise an array of THz detectors arranged diagonally with respect to at least some lines of the scanning raster.

S

To avoid errors introduced because the position of the THz field of view is not accurately known, the THz detection apparatus may further comprise one or more position detectors for detecting the position of a part of the scanning mechanism that moves in use. An appropriate form of position detector has been found to be capacitive. hi a simple form, one or more dielectric plates are arranged to move without contact between conductive plates. Variations in the capacitance between the conductive plates give a reading of the position of the moving part. Because the capacitive plates are rigiWy attached to the scan mirror and the mechanism is wear free, such an arrangement is accurate and reliahk over time and can be accurately and reliably characterised over a wide temperature range. Thus in embodiments of the invention, at least one dielectric plate and conductive plates might he mounted respectively on the mirror structure and on the intermediate support structure to detect the rotational position ol the mirror about the Iirst axis, and (optionally) again respectively on the intermediate support structure and the base structure to detect the rotational position of the intermediate support structure about the second axis. The plates will usually comprise flat, parallel plates but there may be other more appropriate arrangements.

Where the motor drive and position sensor are contactless in the sense of having air hearings rather than any moving surface that contacts another surface, and the mirror itself is light, a mirror can he scanned at high speed while maintaining accurate positional information and high reliability over timc. For example, such a mirror can be driven to meet the following requirements for scanning a FOV containing people: * ±10 degree mechanical steering capability * acceleration of 10,000 rads/s2 * angular accuracy to be a small fraction of the TI-lz beam width to allow over sampling * the power requirement to be low for a small portable system It is to he understood that any feature described in relation to any one aspect or to any one embodiment of the invention may be used alone, or in combination with other features described, in relation to the same or one or more other aspects or embodiments of the invention if appropriate.

An optically steered. Tl-lz scanning detection system will now be described as an embodiment of the present invention, by way of example only. with reference to the accompanying figures in which: Figure 1 shows schematically equipment for use in the detection system: Figurc 2 shows schcmatically a scanning rastcr across a THz field of view, provided by an embodiment of the invention; Figure 3 shows schematically a scanning raster across a smaller area of the THz field of view, providing enhanced reso'ution compared with the raster of Figure 2; Figure 4 shows a series of scanning rasters providing different levels of resolution; Figure 5 shows schematically a rear view of a mirror and scanning mechanism for use in achieving the scanning rasters of Figure 4; Figure 6 shows schematically a front view of a scanning mechanism for scanning the minor of Figure 5. with the minor itself removed; Figure 7 shows a schematic plan view of a drive mechanism and a capacitive position detector for the mirror shown in Figure 5. in two different scanning positions of the mirror about a vertical axis: Figurc 8 shows a quartcr vicw from abovc of a dctail of the drive mechanism shown in Figure?; Figure 9 shows a schematic side view of a drive mechanism and a capacitive position detector for the minor shown in Figure. in two different scanning positions of the minor about a horizontal axis; Figure 10 shows views of a tiexure bearing for supporting the minor shown in Figure 5; and Figure II shows a schematic plan view of the capacitive position detector shown in Figure 5. in more detail.

The figures are not drawn to scale.

Referring to Figure 1, an optical camera 175 is provided for observing an optical field of view (FOV) 170. THz scanning equipment 195, delivering THz radiation from a THz FOV 100 to a bank of THz detectors 115, is provided alongside the optical camera 175. The optical camera 175 can be directed in conventional manner to observe the optical FOV 170 determined by the direction, aperture and focal length of the camera 175. The optical camera 175 is controlled by user inputs at a user interface 130. The user inputs are interpreted appropriately by an optical camera control 184 and sent via a connection 180 to the camera 175. Such arrangements are known for example in surveillance equipment.

The THz detectors 115 can be scanned to observe the THz FOV by the scanning mechanism 15.5 comprising at kast one mirror 105 that reflects incoming terahertz radiation via a coflection device 110 to the detectors 115. The manner of scanning the THz FOV 100. and indeed the size and position of the THz FOY 100. is determined by the scanning mechanism 195 which in turn is controlled by a steering signal via a connection from a steering mechanism 155 in a central control box 125. The THz FOV 100 can thus he selected and adjusted with respect to the optical FOV 170 as required.

lii the embodiments of the invention described herein, the detectors 115 are receiving incoming radiation and generating a detection signal over a connection 120 to a detector array signal processor 165 in the central control box 125 hut in an active TI-lz radiation system. it would be possible that the detectors 115 are used in the other direction, to send THz radiation via the scanning mechanism 195 to illuminate the THz FOV.

Both the optical camera 175 and the THz detectors 115 are used to produce respective image data in a known manner. The two sets of image data are processed by optical and THz image processors 185, 165 to support visual indicators 133, 134 of the respective fields of view 170, 100 which can he viewed in relation to each other on user interface equipment 130. In order to steer the T1-lz FOV 100 in relation to the optical FOV 170, and so provide correlated THz and optical visual indicators 133, 134, it is necessary to know the relationship between the two FOVs. lii imaging, this can be done visually, by

II

observing the content of the optical and THi images 133, 134 on screen. However, embodiments of the invention can equally be used for sensing THz radiation for non-imaging applications. It may therefore be necessary to correlate the positions of the optical and THz equipment automatically, for example by calibration prior to use in the field. A THz FOV 100 can then be adjusted in relation to an optical FOV 170 by making known changes. The detection position of T1-Iz equipment. or the FOV 100 of the THz detectors 115. might be shown as a pointer, frame or image 134 overlaid on the optical image 133 on screen. a'lowing direct visual correlation of the positions of the two sets of equipment.

Once the relationship between the THz and optical EOVs is known, either by calibration or visually, the screen-based display can be used to steer the THz FOV 100 for example to image or sense an area of the optical FOV 170 whose content appears likely to be of interest.

In practice, the optical camera 175 may be physically very small, to the extent that it can be positioned within the THz FOV 100. This is shown in Figure 1 as an inset 190, the optical camera 175 being mounted on the boresight of the THz detectors 115 with regard to the maximum available THz FOV. The relationship between the two FOVs 100, 170 in usc is then known and pre-calibration isn't necessary.

AU of the electronics that control the optical camera 175, process the optical imagery, steer the scanning mechanism 195, coflect the Tl-lz detector output signal and reconstruct the THz imagery is contained within the central control box 125. Tn addition, to allow an operator to manually steer the THz FOV 100 the output signal from a control mechanism such as a joystick controller 131 or touch screen 132 displaying the optical image 133 overlaid with the THi FOV 134 can he used as a preference with lie user interlace equipment 130.

To support the related optical and TI-k imagery 133, 134 of a screen-based display, the central control box 125 also provides a mirror position data receiver 160 which receives data from capacitive minor position detectors (further described below). It thus provides signal processing equipment 185. 165, 160 configured to receive and process the optical detection output, the THz detection output and minor position data iii supporting the screen-based display The central control box 125 includes the steering mechanism 155. or milTor drive control 155. which provides an interface between a control mechanism, which generates control signals for changing the THz FOV 100, and the scanning mechanism 195. The control mechanism might be manual, such as the joystick controller 131 or touch screen 132.

Alternatively, the control mechanism might he a software process, for example implementing an algorithm for automatic interpretation of feature(s) detected in the optical FOV 170 so as to change the THz FOV 100 appropriately. The steering mechanism 155 receives the control signals, which might include for example values representing x-y co-ordinates, and interprets them to generate steering signals for controlling the scanning behaviour of the scanning mechanism 195.

The central control box 125 is in the digital domain while the scanning mechanism 195 requires analogue input control signals for scanning and has an analogue positional readout. There is therefore a digital to analogue convertor (DAC) 140 and an analogue to digital convertor ADC) 145 at the interface to the scanning mechanism 195 to allow digital steering signals in an incoming direction 135 from the steering mechanism 155 and to provide digital mirror position data in an outgoing direction 150 for use by the position data receiver 160.

RefelTing to Figures 2 and 3, the general principle of changes in FOV can be seen. In Figure 2, the scanning mechanism 195 is controlled to scan a large FOV 100 which covers most of a person and the areas to either side of the person. Indeed, the FOV 100 may cover the whole field of regard 215 of the THz detectors 115. In Figure 3. the scanning mechanism 195 is being steered to scan a much smaller FOV 100, taking in just the torso and right arm.

In Figure 2 most of the scene that is being scanned contains information that is not relative to the subject and hence there is a high level of wasted scanning time and hence the noise in die image is higher or alternatively the scan speed must he slowed down requiring the subject to remain stationary. In Figure 3. the THz FOY 100 is considerably smaller than the field of regard 215. Only the area of the subject that is likely to conceal an object is observed, making the most efficient use of scan time and hence reducing noise.

Alternatively, if the subject is moving through the scene the optical imagery is analysed using a tracking algorithm to move the THz FOV maintaining the signal to noise as if the subject were stationary. Also in Figure 3. the scan angle that is required is much smaller than that used in Figure 2 and so the frame repetition rate can he dramatically increased without an increase in noise.

Referring to Figures 4a to 4d, different scanning rasters 405, 410, 415, 420 are broadly shown. In these, it can he seen that the THz detectors 115 are provided as an array of elements such as leedhorns 400 opening ink) a lace of a block of material. Although the feedhorns 400 remain stationary. the mirror 105 of the scanning mechanism 195 moves such that the THz detectors 115 effectively follow scan lines relative to the FOV 100.

Figures 4a to 4d show this relative movement. The array of feedhorns 400 extends in a direction diagonally across the scanning rasters. It will he understood that this diagonal layout means that neighbouring feedhorns 400 follow individual scan lines which show a different spacing "s"across the scanning rasters than the spacing "d" of the feedhorns 400 themsdves in a row of the array would dictate. In the arrangements shown in Figures 4a to 4d. the individual scan Unes of the eight detectors 400 in a row of the array will he more closely spaced for example than thc spacing "d" of thc eight fccdhoms 400 in that row when perpendicular to the scan line.

The diagonal layout with respect to the scanning direction is shown in Figure 4 in conjunction with rectilinear scanning rasters but other detector layouts and/or rasters could be used, including for example curved and/or non-uniform layouts and/or rasters. Figure 4a shows a raster that might he used for the larger FOV 100 of Figure 2. In this raster, the scan lines are offset by exactly one channel pitch relative to the detectors 115. Image data is therefore gathered just once per frame for each position in the FOV. Figures 4b to 4d show rasters that might be used for the smaller FOV 100 of Figure 3 because the scan lines are offset by less than one channel pitch relative to the detectors 115. Image data is therefore gathered more than once per frame for each position in the FOV. Although it takes longer to scan a fixed area of the FOV, the whole FOV is smaller which means the frame refresh rate might be the same or faster.

Another factor is the line speed. Where scan lines are offset by less than one channel pitch, this gives higher resolution. An option is to increase the line speed, for example from six lines per second to thirty lines per second, to capture a subject's movement more smoothly.

However, in some circumstances it will not he possible to increase the line speed beyond a limiting value, for instancc bccausc an object to bc obscrvcd is close to body tcmpcraturc or the scene background is approaching the temperature of the person. h these cases, the sensitivity or thermal contrast achieved by the camera might he insufficient to support increased line speed.

By being free to move in either axis the scan mirror provides the ability to select any scan pattern that may prove further enhancement in future. For instance, new techniques such as compressive sensing and compressive imaging require known random scanning regimes to achieve significant image processing enhancements.

Adaptive scanning according to embodiments of the invention provides the ability to change the size and shape of the FOV in real time so that it can he adjusted according to rcquircmcnts sct by thc task at hand. A scanning protocol can be devised and adjusted to suit particular circumstances. For example, the FOV might be chosen to be tall and narrow to image an individual person or short and wide to image a crowd seen at a distance. The detector element's EOV 100 can he positioned over the region of the field of regard that is of most interest for the majority of the time. The remainder of the field of regard can be scanned as and when required, to refresh and detect changes to the background that might be new areas of interest. By scanning a smaller FOV, the signal to noise ratio (SNR) can he increased because the data capture rate can he slowed down without a corresponding drop in frame refresh rate. In practice. it may be that data is captured at full speed and successive images are then averaged, provided the positional feedback accuracy of the information provided by the position data receiver 160 is acceptable. The size of the FOV can be adapted along with its position in the field of regard 215. Another use for adaptive scanning could be to track the movement of a person as they move laterally through a scene (field of regard) without the need to move the detectors 115.

The theoretical improvement of the SNR resulting from the use of adaptive scanning is given in the formula below: I Area SNRIThP = 4Areab where: SNR1mp is the improvement factor in signal to noise ratio or thermal sensitivity (NETD. AT).

Area2 is the area of the first (larger) scanned region; and Areab is the area of the second (smaller) scanned region.

For example. an adaptive scan designed to image a hand gun around a subject's waistline might take up 1/16111 thc arca of one dcsigned to imagc a full person. In an ideal world the improvement would be four or the AT would drop from 1K to 0.25K with no loss of frame refresh rate. However, in practice there will be systematic noise which may not reduce according to the above formulae and if not carelufly optimised may have a significant effect.

Referring to Figure 5, a fast 2D steerable scanning mirror 105 is shown which can be used to position the detector elements' FOV 100. The mirror 105 has two axes of rotation 530H, 530V, one horizontal and one vertical. Movement of the mirror about the horizontal axis 530H is slight and incremental from onc scan linc to the next but movement about the vertical axis 530V produces the scan lines and has to be fast. Particularly for the more demanding movement about the vertical axis 530V, what is required is a very fast, agile scanning mirror with as wide an angular movement as possible while maintaining low angular moment of inertia for the minor and its drive motor. Because THz equipment operates at such a long wavelength compared to the optical industry, it is possible to use a relatively light mirror and design it to minimise the rotationa' inertia. A target specification for imaging people nilght have the following main features: ±10 degree mechanical steering capability. This results in an optical scan angie of ±20 degrees in the main axis which, in the embodiment shown in Figure 5, is in the horizontal plane. In principle this will allow a person to be tracked laterally across the field ol regard without the need for the camera to be moved.

* acceleration of 10,000 rads/s2. This allows the mirror to be steered from one extreme position to the other of a reasonable FOV at 60 lines per second * angular accuracy to be a small fraction of the THz beam width to allow over sampling * the power requirement to be low for a small portable system The rotational inertia of a symmetrical object about its centre of mass is givcn by:-1W x 1= xS 4 where: M = the object's mass L = the object's total length, measured through the centre of mass and perpendicular to the axis of rotation S = die shape lactor for that object, always less than or equal to 1.

From this equation we can see that in order to minimise the required rotational inertia and hence the power, the mass of the mirror is key. The rotational inertia is proportional to the square of the size of the mirror. If we double the aperture size then the power required to move the mirror increases by a factor of four. This is an important factor to considcr for system design. For example the power requirement for a small handheld system can be optimized by minimizing the diameter of the mirror. For longer range systems the angular movement required is likdy to he less and the power available higher. Either way this must be considered carefully if a particular speed and/or angular range is required by the application. The moment of inertia is the limiting factor when it comes to acceleration and power requirement. Air resistance is a secondary effect. Mass of the mirror needs to he minimised while retaining stiffness. Fortunately, the surface accuracy required for operation at THz frequencies, such as 2500Hz, is greatly eased compared to optical wavelengths.

Still referring to Figure 5, the mirror 105 is a thin, super-ellipse alununium minor consisting of a thin (0.5mm measured) sheet of aluminium 500 with reinforcing ribs 505 glued on the hack. Its size is determined by the size of the focusing elements used in the optical design (that is, the design of the elements delivering Tl-lz radiation incoming from the FOV 100 to the detectors) but should be sufficiently large that it reflects sufficient proportion of the incoming signal for the application in hand. Where signal to noise is limited then the minor should he sized to reflect at least 95% of the incoming signal. In THz surveillance imaging applications for example, the reflecting surface of the mirror might be roughly. or greater than. 100 cm2, 225 cm2 or 300 cm2. It is mounted as part of a fast scanning mechanism (FSM) 195, driven by two pairs of solenoidal motors 510, 535 (described below in relation to Figures 6 to 8) and mounted on flexural bearings (described hdow in relation to Figure 10). The measured weight of the driven scan head with the mirror but without cables was 1.61kg. A weight less than 2kg was prefened for the application in this case which was to be scanning people in a crowd. The scan angle of the mirror on the X-axis was measured at +1-10°. A very rigid lightweight mirror could be realized out of a stiffer material such as carhon fibre sheet to try to push any vibrational resonances higher in frequency and out of the range of use. A further reduction in incrtia could be realized with the use of lighter stiffer materials such as carbon fibre composites. A reflective front surface of the mirror can he achieved through either gluing or evaporating a thin film of highly conductive metal such as copper or aluminium.

As further described below, the minor 105 is driven as part of a mirror structure 615 (not visible in Figure 5). carried in turn by an intermediate support structure 620 and a base support structure 630. The intermediate support structure 620 and the base support structure 630 each carry pnnted circuit boards 515, 525 providing components for conditioning and generating the capacitive position detector output signal. The base support structure carries the electronic circuits that carries and conditions the analogue input 140 and the analogue output signal 145 that provides the positional readout signals from the capacitive transducers. The scanning mechanism 195 therefore has an analogue inteitace 520 to which the scan mechanism control board 157 that provides the motor driver waveforms and the corresponding positional readout signals, can be connected. The mechanism driver waveforms used to move ad position the scan mechanism are generated digitally by an FPGA processor on a control board housed within the imager control box 125. Two ADCs are used to provide the analogue waveform signals required by the motor driver housed with the scan mechanism control board 157, one for each axis. In addition, two DACs are incorporated into the imager control box 125 to convert the analogue signals provided by the positional readouts on the scan mechanism control board 157, again one for each axis and convert them to digital signals that can he used to generate the images.

Later derivatives will include the circuitry for the scan mechanism controller board into the system control board.

Refening to Figures 7 and 9. the scanning of the minor 500 in orthogonal directions is achieved by mounting the mirror 500 itself for rotational movement about the vertical axis 530V. The motors 510, 535 for driving the movement about the vertical axis 530V are mounted in part on the minor structure 615. which carries the minor 500 and rotates with it about the vertical axis 530V, and in part on an intermediary support structure 620 which is mounted for rotation about the horizontal axis 5301-I. The vertical axis 530V is mounted in fixed relation to the intermediary support structure 620. The motors 510, 535 for driving the movement about the horizontal axis 530H are mounted in part on the intermediary support structure 620 and in part on a base support structure 630. Thus the base support structure 630 takes the thrust generated in scanning both the minor structure 615 and the intermediary support structure 620 about the horizontal axis 530H and the intermediary support structure 620 takes the thrust generated in scanning the minor slructure 615 about the vertical axis 530V.

It will be understood that the terms "vertical" and "horizontal" in relation to the axes 530 are not intended to be absolute. Indeed the "vertical" axis 530V will only occasionally be vertical in the absolute sense because of the rotation of the intermediary support block 620 about the horizontal axis 530H. The terms are only used to distinguish the orthogonal axes in the context of the figures of this specification.

Referring to Figure 6, the mounting and (hive arrangement of the scanning mechanism 195 can be seen in more structural detail. The nurror structure 615 is mounted for rotation about the vertical axis 530V. above the intermediary support structure 620. The latter has a pair of kgs 540 which extend downwards as shown, each fixed at the lower end to a solenoid coil 535 which travels along a curved drive core 510 which is mounted on the base support structure 630, thus providing a solenoidal motor for driving rotation of the inteimediary support structure 620, and therefore also the minor structure 615. about the horizontal axis 530H. The mirror structure 615 extends sideways as shown, fixed at each end to a solenoid coil (visilie in Figure 5) which travels along a further curved drive core 510 (the ends of which only are visible in Figure 6) mounted on the intermediary support structure 620, these together forming solenoidal motor assemblies 600 for driving the minor structure 615 to rotate about the vertical axis 530V.

Because the scanner mechanism 195 needs to be continuously adjusted in teims of its speed, range of movement and path of movement it is extremely important that it is inherenfly reliable. This not only relates to its ability to accurately follow the programmed movement defined by the optical camera or operator hut also that the positional feedback accurately reports back the actual position and path achieved. There is thercfore a requirement that there must be minimal possibility of wear or play in the mechanical drive motors or supporting hearings. In this design. as far as the motors are concerned this has been achieved by using a known form of solenoidal electromagnetic motor. It uses an actuator 535 that does not conic into mechanical contact with any other part being essentially supported in a hovering position around the drive core 510 with the air gap maintained over the full range of travel.

Referring to Figures 7 and 8, each solenoidal motor has an actuator 535 which has the shape of a hollow cylinder. The actuator 535 is carried by a drive core 510 which is curved to follow the direction 800 of the path of the actuator 535 in use. The drive core 510 is long enough that the actuator 535 is at all times carried within the length of the drive core 510, this maintaining the force excited by the drive core 510 on the actuator 535 at a maximum and predictable value throughout the whole scanning movement of the mirror 500.

Steering signals that might be received by a solenoidally driven scanning mechanism 195, from the steering mechanism control box 157 and via the Al 140, will he in the form of currents delivered to the respective actuators 535.

Referring to Figure 10, as far as the bearings are concerned, rotational movement about the vertical and horizontal axes 530V. 530H is allowed via the use of fiexure bearings 1000 such as those manufactured by C-Hex (www.c-flex.com). Each hearing 1000 has a shoe 1015 suspended inside a housing 1010 by orthogonal flexurcs 1005. Figure lOa shows a bearing 1000 in three quarter view and Figure lOb shows end views of the bearing 1000.

Provided the loading and travel of the flexures 1005 are restrained to those within their design parameters, such hearings 1000 provide wear free and infinite life with zero backlash.

Referring to Figures 5. 6. 9 and II, to produce an accurately positioned Tl-lz screen image on the user interface equipment 130, the position of the mirror 500 has to he accuratdy known, including the rotational positions about the axes 530C, 530V. hi known configurations commonly used, a position sensor is fixed to a stationary chassis on which the mirror 500 is mounted and then connected to the moving minor mounting via a flexible couphng. This coupling can he subject to backlash and also deformation resuhing in an apparent mirror position or pointing error. In embodiments of the invention, to read this position accurately in spite of the speed of scan involved, the minor 500 is provided at the rear with a position detector comprising capacitive sensors 605. 900, 705, 1010 which provide mirror position information for interpretation by a position data receiver 160 in the central control box 125. It is extremely important that each positional feedback sensor 605, 900. 705. 1010 provides an accurate output of the actual minor position and the mechanism hy which this is achieved must also he wear free. Referring to Figures 5 and 11, to achieve this in the horizontal plane, the back of the mirror 500 is rigidly attached to a vane 705 which sits between two capacitive pads 1010 etched into the printed circuit boards 515 fixed to the intermediary support block 620. As the vane 705 moves between the pads 1010 the capacitance between them is varied according to the overlapping area between the vane and the pads. Through the use of synchronous demodulation an analogue output signal is produced that is proportional to the vanes position. The mechanically air-spaced sensing capacitor necessarily has a thw capacitance (in the pico-farad range). In order to amplify the resulting small signal, without amplifying stray electrical signals such as power frequency fields (5OHz/6OHz), it is useful to employ a synchronous rectifier running at a relatively high frequency (say 100kHz). The sources shown as +phi and -phi are nominally anti-phase square waves. The tiny signal picked up on the sensor p'ate is first amplified and then synchronously rectified. The AC coupled square wave signal is therefore given a positive gain on the high part of the master clock and a negative gain on the low part of the master clock. Effectively the AC signal has been rectified. If well aligned, the synchronous rectifier will produce a positive DC signal when the sensor plate is mostly exposed to the +phi plate and a negative DC signal when exposed to the -phi plate. Therefore as the moving vane (705) covers varying amounts of the -i-phi and -phi plates, the output of the synchronous rectifier smoothly transitions from a negative DC level to a positive DC level.

Imperfections in the synchronous rectifier mean that its output has small spikes on the clock edges. A multi-pole filter is needed to minimise these spikes. The sensor will he highly linear provided the vane is never allowed to fully cover or uncover either the +phi or -phi plates.

This output signal from the capacitive transducers is sent via the analogue output 145 to the scan mechanism control box to be conditioned and sent over a connection 150 to the central control box 125. A non-contacting wear free configuration is again adopted so that there can be no wear or backlash formed between the scanning mirror 500 and the positional sensor 705, 1010. As the vane 705 and scanning mirror 500 arc effectively one piece of material the position of the vane 705 relates exactly the corresponding position of the minor 500 within the constraints of the materials stiffness. Referring to Figures 6 and 9, the arrangement is repeated in the vertical plane using a vertical vane 605 and capacitive pads 900 etched into the printed circuit boards 525 fixed to the base support structure 630.

With the use of flexure bearings 1000 as shown in Figure 10 rather than roller bearings and the use of capacitive positional sensors 605. 900, 705. 1010 and solenoidal motors 510, 535 that have mechanically wear free maintained air gaps, the scanning mechanism is not only wear free but also effectively silent in operation which is an important factor for many applications.

Claims (19)

1. CLAIMS1. Detection apparatus comprising one or more THz detectors and a scanning mechanism for scanning a THz field of view in relation to the THz. detector(s), the detection apparatus further comprising: i) an optical camera for viewing an optical field of view to provide an optical detection output; and ii) a steering mechanism configured to receive a control signal based at least partly on the optical detection output and to send a steering signal to the scanning mechanism to scan thc THz field of view in accordance with the control signal to provide a THz detection output in relation to the THz field of view, whereby the THz detection output is dependent at least partly upon the optical detection output.
2. 2. Detection apparatus according to claim 1, further comprising a control mechanism for generating the control signal, wherein the scanning mechanism comprises at least one mirror and at least one position detector to detect the position of the mirror and generate mirror position information for use in generating the control signal.
3. 3. Detection apparatus according to claim 2 wherein the control mechanism comprises a software process or a user input device, such as a joystick, keyboard, touch-sensitive screen or mousc. and the steering mechanism is configured to receive the control signal and to convert it to a steering signal compatible with the scanning mechanism.
4. 4. Detection apparatus according to either one ol claims 2 or 3. lurther comprising signal processing equipment configured to receive and process the optical detection output.the THt detection output and the mirror position information to provide a screen-based display showing the positional relationship of the optical and THz fields of view.
5. 5. Detection apparatus according to claim 4, wherein the optical detection output supports a screen-based display of the content of the optical field of view, enabling the control mechanism to contro' the positional relationship ol the optical and THz fields of view based on that content.
6. 6. Detection apparatus according to any preceding claim, wherein the optical camera is mounted at the boresight of the THL field of regard.
7. 7. Detection apparatus according to any preceding claim, configured to scan the THz field of view in a scanning raster, the apparatus comprising an array of at least two neighbouring T1-lz detectors, wherein the direction of spacing between the neighbouring dctectors is diagonal with respect to thc direction of at Icast a portion of thc scanning i-aster, such that the neighbouring detectors follow individual scan lines which are more closely spaced than the spacing of the neighbouring detectors in the array.
8. 8. A scanning mechanism for scanning a THz field of view using at least one movable mirror to receive THz radiation from the field of view and deliver it to one or more THz detectors, the scanning mechanism comprising: i) a mirror structure mounted for rotation about a first axis; ii) an intermediate support structure for carrying the mirror structure, the intermediate support structure being mounted for rotation about a second axis; iii) a base support structure for carrying the intermediate support structure: iv) an electromagnetic mirror drive mechanism mounted in part on the intermediate support structure and in part on the minor structure, for driving the mirror structure to rotate about the first axis; and v) an electromagnetic intermediate drive mechanism mounted in part on the base support structure and in part on the intermediate support structure for driving the intermediate support structure to rotate about the second axis.whereby the minor structure can be driven to rotate about either one or both of the first and second axes.
9. 9. A scanning mechanism according to claim 8 wherein the mirror drive mechanism comprises a solenoidal drive mechanism using a contactiess coillcore combination configured to reciprocate in scanning the THz field of view.
10. 10. A scanning mechanism according to either one of claims 8 or 9 wherein the mirror drive mechanism comprises a core and a coil magnetically suspended in use for reciprocation along the core, configured such that the overlap of the core and the coil, in the axial direction of the coil, remains complete throughout the thU range of the reciprocation.
11. 11. A scanning mechanism according to either one of claims 9 or 10 wherein the core of the mirror drive mechanism is curved to follow the path of the reciprocation.
12. 12. A scanning mechanism according to any one of claims 9 to 11. wherein the mirror drive mechanism comprises a core mounted on the intermediate support structure and a coil mounted on the mirror structure.
13. 13. A scanning mechanism according to any one of claims 8 to 12 wherein the mhTor structure comprises a mirror having a reflective surface area of at least. 100 cm2, 150 cm2 or 200cm2.
14. 14. A scanning mechanism according to any one of claims 8 to 13 wherein both drive mechanisms comprise a coil magnetically suspended about a drive core.
15. 15. A scanning mechanism according to any one of claims S to 14, wherein the mirror structure is mounted for rotation about at least the first axis by means of a flexure bearing.
16. 16. A scanning mechanism according to any one of claims 8 to 15, further comprising at kast one contactless position detector to detect the position of the mirror and generate a mirror position signal.
17. 17. A scanning mechanism according to any nile of claims 8 to 16 wherein the mirror structure comprises an aluminium reflector with reinforcing ribs.
18. 18. Detection apparatus according to any one of claims 1 to 7, comprising a scanning mechanism according to any one of claims 8 to 17.
19. 19. A method of scanning a THz field of view in relation to one or more THz detector(s), the method comprising: i) using an optical camera to view an optical fidd of view to provide an optical detection output; ii) generating a control signal based at least partly on the optical detection output; and iii) using the contro' signa' to steer the one or more THz detector(s) to scan the THzfield of view to provide a THz detection output;whereby the THz detection output is dependent at least partly upon the optical detection output.