Illumination Strategies to Achieve Effective Indoor Millimetre Wave Imaging for Personnel Screening Applications

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1 Illumination Strategies to Achieve Effective Indoor Millimetre Wave Imaging for Personnel Screening Applications Rory Doyle, Brendan Lyons, Alan Lettington*, Tony McEnroe, John Walshe, John McNaboe, Peter Curtin Farran Technology Ltd, Ballincollig, Co. Cork, Ireland Tel: ; Fax: ; rdoyle@farran.com *The University of Reading, Whiteknights, Reading RG6 6AF, UK ABSTRACT The ability of millimetre-waves (mm-wave) to penetrate obscurants be they clothing, fog etc. enables unique imaging applications in areas such as security screening of personnel and landing aids for aircraft. When used in an outdoor application, the natural thermal contrast provided by cold sky reflections off of objects allow for direct imaging of a scene. Imaging at mm-wave frequencies in an indoor situation requires that a thermal contrast be generated in order to illuminate and detect objects of interest. In the case of a portal screening application the illumination needs to be provided over the imaged area in a uniform, omni-directional manner and at a sufficient level of contrast to achieve the desired signal to noise ratio at the sensor. The primary options are to generate this contrast by using active noise sources or to develop a passive thermally induced source of mm-wave energy. This paper describes the approaches taken to developing and implementing an indoor imaging configuration for a mmwave camera that is to be used in people screening applications. The camera uses a patented mechanical scanning method to directly generate a raster frame image of portal dimensions. Imaging has been conducted at a range of frequencies with the main focus being on 94GHz operation. Experiences with both active and passive illumination schemes are described with conclusions on the merits or otherwise of each. The results of imaging trials demonstrate the potential for using mm-wave imaging in an indoor situation and example images illustrate the capability of the camera and the illumination methods when used for personnel screening. 1. INTRODUCTION Millimetre-wave imaging takes advantage of the penetrating property of radiation at this wavelength and can, for example, identify threat objects concealed on or about the person. The ability to detect multiple threat materials concealed on a person provides a powerful tool for security checkpoint situations where rapid screening of people is required. Monolithic microwave integrated circuits (MMICs) operating up to 100GHz and beyond have greatly expanded the potential for creating high performance and commercially viable millimeter-wave (mm-wave) imaging systems. The available MMIC technology can now provide small form factor and high performance imaging sensors. At the same time, the cost per sensor is still prohibitively expensive when a fully populated imaging array is considered. Employing a scanning system to view the scene and relay pixel information onto a single or small number of sensors can greatly reduce the cost of a millimeter wave imager while at the same time provide sufficient performance for security channel operation. An imaging system based on a scene scanning approach has been built and demonstrated. It allows for either passive or active operation within a compact form factor that makes it suitable for installation in security screening applications. One of the target uses of this scanner is portal screening of personnel for high-resolution imaging of concealed threat objects. Other applications lie in the area of longer distance surveillance type monitoring of checkpoints and crowds. The design parameters of this imager were to provide rapidly refreshed, raster imagery with high efficiency across all Passive Millimeter-Wave Imaging Technology VIII, edited by Roger Appleby, David A. Wikner, Proceedings of SPIE Vol (SPIE, Bellingham, WA, 2005) X/05/$15 doi: /

2 millimetre wave frequencies. The system design included adjustable focus to image close-in for portal screening applications or at long distances for use in surveillance situations. An overriding requirement was to produce a system that could be manufactured at a low cost where it could be economically justified to use the imager in general public area security screening situations. Plane Mirror Receivers Focussing Mirror Fold Mirror (a) (b) Figure 1: The core of the optical mechanism shows the rotating plane and focussing mirrors while a fold mirror is used to maintain a compact design (a). The raytrace diagram (b) illustrates the focussing extremes of the linear array of receivers with a representation of a person in the scene. 2. IMAGER DESIGN AND DEVELOPMENT The imager design is based on a patented scanning system [1]. Figure 1(a) shows the basic components of the scanning system that consist of rotating and folding mirror optics. The rotation of the plane and concave mirrors, combined with their relative inclination, focus a spot on the target onto the receiver. As the mirrors rotate this scene spot travels in a vertical line creating a line scan in the scene. A third mirror is used to fold the optical path to keep the overall design compact. A linear array of receivers captures parallel lines on the scene to produce a full frame on each rotation of the concave/plane mirrors. The main features and benefits associated with this design are summarised as follows Scanning method - Rotating mirrors focus a line-scan of the scene onto the millimetre-wave receivers each receiver creates a vertical line of the image a sub-array of receivers is used to create a full frame on each rotation of the mirrors. Optical configuration - The scanning camera uses only reflective optics in directing and focusing the incident radiation. This ensures that almost all of the signal reaches the receiver maximising the system efficiency and signal to noise ratio for the processing electronics. The fact that there are no frequency dependent optical 102 Proc. of SPIE Vol. 5789

3 elements in the signal path means that the camera is independent of the frequency being detected and can be used interchangeably over the millimetre-wave and into the terahertz spectrum. High refresh rates - The line-scan is generated by rotating two circular mirrors at constant speed each going in one direction only. This configuration allows for a high rate of revolution of the mirrors and consequently a fast refresh rate in the image. An imager design based on a receiver array of 16 elements will produce images at a 10Hz refresh rate. Raster scan - The rotation of the mirrors generates a line-scan of the scene. The linear array of receivers is used to generate the vertical image lines and directly produces a raster frame of the scene. The uniformity of the raster output allows for in-line image processing to be performed on the data. Active and passive operation - Because there is no manipulation of signal in the optical path the camera can be configured as either a passive receive only unit or as an active transceiver system. Low Cost - The low receiver count and simple mechanics combine to provide a physically compact millimetre-wave camera with high performance and low cost of manufacture. The typically configured system has parameters as listed below. These are nominal values and can be modified at the design stage of the imager implementation. System aperture - Concave focusing mirror 600mm aperture Field of view - ±10 in the vertical (mechanically variable) in the horizontal (programmable). The standard portal field of view is captured in an image of 32 horizontal lines and 200 horizontal rows. Angular resolution at 94GHz Temperature resolution - <1 K Front End Direct detect, 94GHz centre frequency, 10GHz bandwidth, <5dB noise figure, >45dB gain. Refresh rate - 10Hz for 16 channels Dimensions Camera: L=800mm x W=1460mm x H=1800mm. Illumination chamber: Circular outline of 1.6m diameter and 2.2m height Figure 2: Final imager configuration including illumination source after industrial design is complete. The raytrace diagram in figure 1(b) shows the optical design of the scanning system and figure 2 shows the final imager format. Proc. of SPIE Vol

4 3. OBJECTIVES FOR PEOPLE SCREENING APPLICATIONS To implement an effective means of personnel screening there are a number of objectives to be achieved. Primary among these are the successful detection of potential threat materials such as metallics, ceramics, plastics and dielectrics. The resolution requirements for detecting these materials can vary in that a small metallic object of a few cm 2 could represent a threat in the form of a blade whereas an explosive sheet would be need to be considerably larger in order to constitute a stand alone threat object. For many applications the requirement to scan people will have a target of completing the process in a fixed minimum time. The ideal will be a walk through scenario where a free-flow of people can be maintained through a control area. The practical limit at present will be a stop/scan/walk-on procedure where the subject turns in front of the imaging device. This will require a minimum of a real time imaging output from the scanner in order to facilitate good detection and rapid throughput. A still image output could find application where low throughput can be accepted once an image could be produced in the order of seconds. In the target market of high throughput people screening, a user friendly and easily interpreted image display would greatly improve the effectiveness of the identification of concealed threat objects. Millimetre-wave images are generally a grayscale representation of the emitted and reflected energy from the scene. An operator examining such imagery would be greatly assisted by a software tool that could identify and extract potential threat objects and for instance superimpose suspect items on a video image of the person being screened. 4. ILLUMINATION FOR MM-WAVE IMAGING Imaging at millimeter-wave frequencies uses receivers that distinguish millimeter-wave energy variations in the scene. To obtain a useful image a contrast must be present to illuminate the items of interest in the observed area. The image consists of the variations in emissions and reflections of the millimeter wave energy that are detected by the sensor. For people screening, the main source of image contrast comes from the reflected energy coming off of the subject and any concealed items that the person is carrying. The body having a large water content is approximately 50% reflective whereas metals and ceramics will have a higher reflectivity and absorbing materials such as leathers and plastics will have a lower reflectivity. Millimetre Wave Imager Illumination Source Wide angle illumination pattern onto person Person being imaged Illumination Source Figure 3: An illustration of an illumination configuration used to provide wide angle and uniform contrast to a person standing the scene. For practical mm-wave imaging a thermal contrast of several tens of Kelvin is needed given that the resolution of receivers is generally in the order of one degree. In order to distinguish features and objects in the scene a minimum number of distinct levels in the grayscale image are needed and for effective threat detection this resolution should approach or exceed 100 resolvable levels. To create this contrast in the scene sources that are readily available are (i) naturally occurring thermal contrast e.g. cold sky, (ii) thermally generated blackbody contrast and (iii) actively derived radiation from amplifiers and noise sources. 104 Proc. of SPIE Vol. 5789

5 The objective in illuminating a subject for millimeter-wave imaging is to create an even distribution that originates uniformly from a wide angle source. The illumination also needs to be sufficiently wideband to cover the span of the receiver bandwidth. The contrast achieved from cold sky in outdoor imaging is a good example of a wide angle (hemispherical) source providing a high contrast (~200 K) to the scene. Replicating this type of illumination for indoor imaging is a significant requirement for successful millimeter-wave imaging. Figure 3 shows a potential illumination scheme for indoor millimeter wave imaging. A number of difficulties can arise in creating a high performance indoor illumination source. These relate to issues such as materials properties used in thermal blackbody illumination and image artifact introduced from active sources. A number of documented problems arise when active sources are used, in particular the directionality of the source that results in selective illumination of the subject and image degradation due to glint and speckle [2]. 5. HUMAN BODY MODEL One of the main application areas for millimeter-wave imaging is in people screening for concealed threat object detection. Imaging the human body in the millimeter wave spectrum presents a particular set of characteristics in terms of response and image quality. At wavelengths of millimeter scale, the body can be regarded as a flat semi-reflective surface. Skin has a large water content and has roughly equal absorption and reflection for incident millimeter-wave energy. The body also has its own emissivity but compared to typical contrast differentials from illumination sources this emission can be ignored. Against this semireflective response presented by the body, objects with high reflectivities such as metals and ceramics and absorbing materials such as plastics can be seen as light or dark contrast against a midrange background. (a) (b) Figure 4: Outdoor imaging has the benefit of a readymade source of illumination. However, the relative strength of overhead illumination tends to create an over emphasis of the upper body in the image (a). By using a side reflector and a means of reducing the direct overhead illumination an even and useful outdoor image can be produced (b). 6. OUTDOOR IMAGING A key advantage of outdoor imaging is the readily available contrast provided by the sky background. Nominally at liquid Nitrogen temperatures, the sky can contribute up to a 200 K contrast compared to the surrounding ambient. This differential can mean very good signal to noise levels in the image and results is good quality imagery without the need for assisted illumination. Apart from the fact that it requires access to uncovered outdoor imaging area, there are however a number of disadvantages to working outdoors. A significant disadvantage is the variability of contrast as this is dependent on the level of cloud cover on a dull day the contrast can be reduced by up to an order of magnitude. Sky illumination is nominally hemispherical in origin. However, atmospheric attenuation of the source as the horizon is Proc. of SPIE Vol

6 approached means that the profile of the illumination is biased to a distribution that is dominated by a stronger overhead level. Figure 4 illustrates this situation and how it can be overcome albeit with the addition of redirecting reflectors. While outdoor situations can provide a good test environment for portal style millimeter-wave imaging they do not offer a stable environment for people screening. Applications that could benefit from the use of cold sky contrast are the use of millimeter waves in airborne visibility assistance where runways and landing positions could be identified through poor weather conditions using millimeter-wave imaging. Figure 5 demonstrates the principle of this approach. Metal reflecting sky Reflection on tarmac Figure 5: An example of outdoor imaging where the cold sky produces the scene contrast. Note the reflector in the distance showing sky temperature (bright) and its subsequent reflection off of the car park tarmac. 7. INDOOR IMAGING To image in an indoor situation, a level of contrast must be generated in the scene. The choices available are a number of active sources such as oscillators, amplifiers etc. or passive sources emitting blackbody radiation by virtue of a temperature differential. A number of methods have been proposed and described in the literature including active arrays based on wideband oscillators [2] and tuned cavities [3]. A major drawback of using these active methods is the potential for image artifact in the form of speckle, glint and ringing. Trials on active sources were carried out using noise sources and amplifier components. Very high levels of scene contrast were achieved with these sources estimated at several hundred degrees. The main objective in this work was to create an even distribution over the portal area. Scattering of the source achieved this to some extent but overall, the expected problems due to the directionality and intensity of the beam dominated the results. Based on the initial findings, the use of active sources was not continued until an effective means of scattering/diffusion of the source could be developed. The use of thermally generated blackbody radiation has the benefit of producing a uniform wideband spectrum over the area of the source. To get a thermal contrast compared to the scene, the source can be either heated or cooled. Cooled sources have the advantage of achieving contrast compared to the body once any reduction below ambient is achieved. Heating on the other hand has to overcome body temperature differentials and move beyond this before contributing to scene contrast. Despite this disadvantage the ease of producing a high differential by heating compared to maintaining a cooled contrast means that a heated illumination scheme is generally more practical. 106 Proc. of SPIE Vol. 5789

7 In order to achieve the uniform illumination coverage over the person, large area panels were constructed to provide indoor contrast in the scene. Areas comparable to the portal scene were used to generate the radiation source and these were angled onto the person to minimise front to back distance from the person to the illumination panel. A common feature of millimeter wave scanners is the need to leave an opening in the illumination source for the scanning system to see the subject figure 6. This opening results in a gap in the illumination pattern that can cause shadowing on the person. In the imaging system described here, this gap can be filled by using the scanning optics to direct illumination into the scene. Imager Source Imager Figure 6: Illumination schemes for indoor imaging a means of providing broadcast blackbody illumination onto the subject is shown on the left hand diagram while the scanning optics are used in the right hand illustration to provide illumination onto the person in the scene. To generate the desired illuminating contrast, an appropriate microwave emitting material must be heated (or cooled). Properties required of the emitter material are a low reflectivity and a long term stability at temperature extremes. In terms of the overall system, the illumination panels should have a low power dissipation and cost of manufacture. After experimentation with a range of commercial and custom created materials, a configuration has been developed that provide the necessary contrast while still achieving the overall system goals. (a) (b) Figure 7: Millimetre-wave images taken indoors with illumination provided by means of thermally generated blackbody sources. A person with a concealed knife is shown in (a) while the presence of an explosive simulant being held to the chest is visible in (b). Proc. of SPIE Vol

8 In the course of the development of the illumination panels, material and construction methods were built that provided several hundred degrees of contrast. A final specification has been chosen that provides a stable contrast level in the range of 100 K. This has been the basis of the indoor imaging trials conducted using the imager to date. Examples of indoor images taken with the indoor illumination set up are shown in figure 7. These demonstrate the ability of the imaging system to detect concealed threat objects. In the first image Figure 7(a) a knife is clearly identified on the person. The body is seen as a halftone of the gray level providing adequate contrast to the complete reflection of the knife blade. A significant target of millimeter-wave imaging is to be able to detect thin sheet dielectric materials. The threat represented by this material type is principally sheet explosives strapped to the body. The relative transparency of this type of material makes it very difficult to detect. Using only the radiometric detection capability it is possible to identify such materials as shown in figure 7(b). Here a thin sample of explosive simulant (approx 1cm thickness) is being held against the body. This sample can be imaged but there is a possibility of false detection or non-detection because of the low contrast response from these targets. A more conclusive approach to identifying thin sheet materials is being investigated where the optical response of thin sheet layers is being detected. Using the potential for identifying interference effects from a thin sheet material on the body a characteristic response can be identified and used to detect thin sheet dielectric materials. An example of the approach is shown in figure 8 where the optical response of different views of thin sheet dielectrics are compared. As can be seen in this example it is possible to positively identify the presence of a dielectric sheet of only a few millimeters thickness. Thin dielectric sheets on a background with properties similar to a human body Figure 8: Thin sheet dielectric can be positively identified by isolating the interference response of the material. In this example the same scene produces different responses in the rightmost sheet when viewed under differing conditions. REFERENCES 1. A. H. Lettington, D. Dunn, N. E. Alexander, A. Wabby, B. N. Lyons, R. Doyle, J. Walshe, M. Attia, I. Blankson, Design and development of a high performance passive mm-wave imager for aeronautical applications Proc. SPIE Vol 5410, pp , Orlando, April J. Stiens, G. Poesen, G. Koers, R. Vounckx, Noise sources in active imaging systems in the millimeter wave domain, Proc NEFERTITI Workshop on Millimetre Wave Photonic Devices and Technologies for Wireless and Imaging Applications, Brussels, January R. Appleby, R. N. Anderton, S. Price, G. N. Sinclair, P. R. Coward, Whole Body 35GHz security scanner, Proc SPIE Vol 5410, pp , Orlando Proc. of SPIE Vol. 5789