Abstract:
Methods and apparatuses of scene illumination for millimeter wave sensing are presented. One embodiment features illuminating a subject with millimeter wave radiation produced by at least one fluorescent light, generating an image with a passive sensor using the millimeter wave radiation reflected from the subject, and analyzing the image to detect representations corresponding to concealed objects associated with the subject. Another embodiment features at least one fluorescent light which illuminates a subject with millimeter wave radiation, and passive millimeter wave sensor which receives the millimeter wave radiation effected from the subject, and generates an image which is analyzed to detect image representations corresponding to concealed objects. Another embodiment features at least one florescent light behind an optically opaque medium which is transparent to millimeter wave radiation, and illuminating a subject with millimeter wave radiation produced by the fluorescent light.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Embodiments of the present invention generally relate to millimeter wave (MMW) imaging, and more specifically, illuminating a scene with diffuse radiation to improve the contrast produced by a passive MMW imaging sensor. 
         [0002]    Electromagnetic energy in the millimeter wave region possess wavelengths which advantageously can be used to form images using relatively small imaging sensors, while being able to penetrate classes of materials which can commonly be used to conceal or camouflage weapons, containers, various other forms of contraband. In other applications, MMW imaging sensors may be used to detect people and/or animals by penetrating through walls. MMW imaging can offer real-time remote detection of hazards of metallic and/or non-metallic materials which can be concealed in a variety of ways, including being cloaked by one or more layers of clothing. 
         [0003]    Either passive or active MMW imaging sensors may be used for concealed contraband detection; however, passive sensors can have the advantage of simplicity, reduced size, and reduced power consumption. Additionally, because passive MMW imaging sensors do not directly provide electromagnetic radiation for scene illumination, the passive sensors are not associated with detrimental physical side effects, or any perceived notions thereof, associated with the irradiation of subjects under surveillance. 
         [0004]    Both passive and active MMW imaging sensors typically rely on contrast to generate useful images. The environmental conditions under which a passive MMW image created can be is one of the principal contributors in providing sufficient contrast. When imaging subjects outdoors, the clear sky can provide an excellent background for MMW imaging because it appears “cold” in the MMW wavelengths, that is, the sky provides little energy in the MMW region and therefore registers with a low temperature. The cold clear sky background can be exploited for detecting concealed objects. 
         [0005]    An example of concealed object detection occurring outdoors on a clear day is as follows. A metal object such as a gun may be visibly concealed on a person under clothing, however, the surface of the gun which is not in contact with the person will “reflect” MMW energy corresponding to the cold sky, because clothing can be transparent to MMW waves. Moreover, the person will tend to have higher than ambient MMW temperature (as will be for fully described below), which can also be readily apparent to a MMW sensor given the transparency of the person&#39;s clothes. The difference in temperature between the person and the concealed gun provides sufficient contrast so the gun can be detected underneath the person&#39;s clothing. 
         [0006]    However, when the above passive MMW imaging scenario occurs indoors, or in some other reduced contrast scenario such as outdoors on a rainy day there may not be sufficient contrast to reliably detect the concealed gun. While energy associated with MMW wavelengths can pass through walls depending upon the material from which they are made and wall&#39;s thickness, MMW contrast can be greatly attenuated indoors. Accordingly, the cold sky may not be available to provide sufficient contrast to detect the concealed gun or any other object behind the person&#39;s clothes. Like other forms of electromagnetic waves, MMW radiation can be polarized. MMW radiation sources and sensors can be configured to utilize polarization to distinguish between various types of materials. For example, objects made from metals will reflect polarized MMW radiation differently than objects made from plastics. These variations in reflected energy can be exploited with subsequent image processing techniques to provide additional information to aid in the classification of concealed objects. 
         [0007]    In the past, various approaches have been tried to improve the ability of passive MMW sensors to make useful images in low contrast scenarios, but have been met with limited success. These approaches include providing MMW illumination sources such as oscillators, noise sources, and point sources, sometimes coupled with diffusers, all operating in the MMW region. These techniques can be problematic because they tend to saturate the MMW sensor, as it may have limited dynamic range. Moreover, images generated utilizing these MMW sources can suffer from glint and scintillation effects which can reduce the ability to exploit the MMW image to detect concealed objects. Other techniques have included providing a controlled environment, such as thermally controlled panels (e.g., a wall chilled using circulated water) to act as a cool radiometric background for improving contrast. Moreover, these approaches can increase complexity and cost as they may involve careful geometric control, precise signal timing, and/or critical alignment of scene being imaged. 
         [0008]    What is therefore needed is a common, unobtrusive source that can provide diffuse MMW illumination in a simple and cost effective manner. 
       SUMMARY OF THE INVENTION 
       [0009]    Accordingly, embodiments of the present invention are directed to providing illumination for passive MMW imaging sensors which may substantially overcome and/or obviate challenges associated with conventional techniques. 
         [0010]    One embodiment directed to detecting concealed objects is presented. The embodiment features illuminating a subject with millimeter wave radiation produced by at least one fluorescent light, generating an image with a passive sensor using the millimeter wave radiation reflected from the subject, and analyzing the image to detect representations corresponding to concealed objects associated with the subject. 
         [0011]    Another embodiment is directed to a device for detecting concealed objects. This embodiment features at least one fluorescent light which illuminates a subject with millimeter wave radiation, and a passive millimeter wave sensor which receives the millimeter wave radiation effected from the subject, and generates an image which is analyzed to detect image representations corresponding to concealed objects. 
         [0012]    Yet another embodiment is directed to a method for generating images using millimeter wave energy. This embodiment features placing at least one radiation source in proximity to a scene, energizing the radiation source so that it fluoresces in the millimeter wave frequency region, illuminating the scene with the fluoresced millimeter wave radiation, and receiving the millimeter wave radiation reflected from the scene. 
         [0013]    Another embodiment is directed to a method for providing illumination to detect concealed objects. This method features placing at least one florescent light behind an optically opaque medium which is transparent to millimeter wave radiation, and illuminating a subject with millimeter wave radiation produced by the fluorescent light. 
         [0014]    Additional features of embodiments of the invention will be set forth in the description which follows, and in part will be apparent from the description, or can be learned by practice of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0015]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention. 
           [0016]      FIG. 1A  depicts an exemplary MMW illumination arrangement and sensor consistent with an embodiment of the invention; 
           [0017]      FIG. 1B  depicts an exemplary image for the embodiment shown in  FIG. 1  when the illumination source is off; 
           [0018]      FIG. 2  shows an exemplary system having a lateral illumination arrangement consistent with another embodiment of the invention; 
           [0019]      FIG. 3  depicts a sensor utilizing modulated MMW radiation consistent with another embodiment of the invention; and 
           [0020]      FIG. 4  depicts a sensor utilizing MMW radiation having polarimetric properties consistent with another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0021]    The following detailed description of the embodiments of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims and equivalents thereof. 
         [0022]      FIG. 1A  depicts a passive MMW sensor utilizing diffuse illumination consistent with an embodiment of the invention. One or more fluorescent lights  120  can placed in proximity to a subject  110  within a scene being surveilled. Fluorescent lights  120  irradiate scene and subject  110  therein with MMW energy. Subject  110  can be carrying a concealed object  140  hidden by one or more layers of clothing. The surveillance can be performed by a MMW sensor  100 , which produces an image based upon MMW radiation  130  reflected from the scene, which can be provided by fluorescent lights  120 . 
         [0023]    Typically, MMW sensor  100  can be a passive sensor, meaning the sensor itself does not supply any MMW energy to illuminate the scene being imaged. However, the invention does not preclude other types of MMW sensors being used, and embodiments could be directed to a MMW sensor which can have the capability of providing MMW energy to illuminate the subject. 
         [0024]    As used herein, frequencies in the MMW region can span between several Mega-Hertz up to Tera-Hertz. MMW sensor  100  can operate at center frequencies of approximately 94 GHz with a bandwidth of approximately 1 to 10 GHz. Any type of passive MMW sensor known in the art may be used, such as, for example, MMW sensors produced by Brijot Imaging Systems. 
         [0025]    MMW sensor  100  may be used to detect any type of concealed object under the proper contrast conditions. Concealed objects which can be detected can be made from, in part or in whole, materials which include metals, both ferrous and non-ferrous, and dielectric materials, such as, for example, plastics, polycarbonates such as Lexan, etc. MMW sensor  100  can detect obvious contraband such as, for example, weapons, plastic explosives, knives, containers with drugs, etc. MMW sensor can also detect concealed objects such as cell phones and other portable electronic devices such as, for example, personal digital assistants, cameras, digital storage devices, and/or audio recorders. Such capability can be especially useful in applications including portal protection, TSA airport screening, urban gang control, schools, prison safety, and border control. 
         [0026]    Image  150 A provided by MMW sensor can include an array of pixels representative of the subject being imaged. Each pixel value represents a magnitude corresponding to the amount of MMW energy in a small area represented by each pixel. Differences in pixel magnitudes in the image can be proportional to contrast. It is the contrast which allows one to interpret the image and derive information from it, and can therefore be desirable to produce images having high contrast. Image  150 A was produced using the MMW radiation  130  from fluorescent lights  120  to illuminate subject  110 . Pixels representing subject  110 , shown as subject representation  152 A, can be darker because subject  110  is primarily emissive, and does not reflect MMW radiation  130  provided by fluorescent lights  120 . Concealed object  140 , which, for example, could be a gun made from metal, can be primarily reflective, and the pixels corresponding to object representation  154 A have higher values and appear brighter in the image against the relatively darker background of subject representation  152 A. The diffuse illumination of subject  110  by MMW radiation  140  permits the high contrast between subject representation  152 A and object representation  154 A. While image  150 A shows pixels having a larger magnitude as lighter in color, and pixels having lower magnitudes as darker, one of ordinary skill in the art would appreciate that this mapping could be inverted. Moreover, image  150 A can be digitally analyzed and/or enhanced to improve detection and/or identification of concealed object  154 A by techniques known to one of ordinary skill in the art. 
         [0027]      FIG. 1B  depicts an image  150 B representative of subject  110  produced without the diffuse illumination provided by MMW radiation  130 , which corresponds to fluorescent lights  120  being turned off. Subject  110  is emissive, and naturally provides MMW energy, so pixels corresponding to subject representation  152 B have higher relative magnitudes that the pixels corresponding to object representation  154 B. In this case, concealed object  140  can reflect the colder temperature corresponding to the surrounding environment. Because subject  110  is being imaged indoors, concealed object  140  is not as cold as it would be if the subject were being imaged outdoors, and as a result object representation  154 B is not very dark. Because the relative MMW radiometric temperature difference between subject  110  and concealed object  140  are not as great as shown in image  150 A, the relative contrast between subject representation  152 B and object representation  154 B is reduced. The reduction in constant depicted in image  150 B can be less effective for detecting concealed objects and can therefore be less desirable. 
         [0028]    Further referring to  FIG. 1A , fluorescent lights  120  can be ordinary fluorescent light tubes commonly found in homes and office buildings. In addition to providing visible light, ordinary fluorescent light tubes can emit enough MMW energy so the scene appears illuminated with 40 to 60 degrees Celsius above typical ambient conditions. In the process of producing visible light, the fluorescent tubes also fluoresce MMW radiation, including frequencies from 35 to 94 GHz. Ordinary fluorescent tubes can provide broad and diffuse illumination with sufficient MMW radiation to provide images with sufficient contrast for performing concealed object detection. To further diffuse the MMW illumination, diffusers which scatter MMW energy could be placed in front of fluorescent lights  120 . Material which could be used for MMW diffusion could be plastic such as, for example, Rexolite, Lexan™, polycarbonate or any other material known in the art. Ordinary fluorescent tubes are commonly used and therefore typically have the advantage of being inexpensive and easy to obtain. Moreover, such tubes are well established, and thus do not present any safety issues or other regulatory concerns which may be present when using other radiation sources. Because ordinary fluorescent tubes have been in long use in both commercial and domestic settings, they are not associated with any perceived health risks and therefore are socially acceptable in the eyes of the public. 
         [0029]    Other embodiments of the invention may utilize radiation sources which can be specifically designed and optimized to fluoresce in the MMW frequency regions. For example, fluorescent tubes may manufactured without an interior phosphor coating to limit fluorescence to energy in the visible wavelengths, which may improve efficiency in the MMW frequencies, improve the ability to conceal the radiation sources, and reduce the cost of manufacturing fluorescent MMW radiation sources. 
         [0030]    Placement of fluorescent lights  120  can be arranged to provide uniform and diffuse illumination of MMW radiation. Uniform MMW illumination may be accomplished using an arrangement consistent with standards commonly associated with office lighting designed to provide uniform optical illumination. The spacing of fluorescent lights  120  can be a function of the distance from subject  110  and the intensity of the MMW illumination desired. As shown in  FIG. 1A , fluorescent lights  120  may be visibly exposed and placed above the subject to be surveilled. One design approach for improving MMW illumination uniformity would be to place lights above a subject and within a boundary formed by a cone of revolution  142 , defined by a 45 degree angle from a vertical reference line, relative to the subject. Other embodiments of the invention having alternative fluorescent light placement are described in more detail below. 
         [0031]      FIG. 2  depicts another fluorescent light bulb placement consistent with another embodiment of the invention. In this embodiment, subject  110  can be observed surreptitiously, as MMW sensor  100  and fluorescent lights  203   a  and  230   b  can be placed behind a screen  220  made from an optically opaque material transparent to MMW energy. MMW sensor  100  can be placed to produce an image most likely to accurately detect concealed objects. In practical systems, MMW sensor  100  can be placed, for example, between 5 and 30 feet from subject  110 . Fluorescent lights  230   b  can be placed alongside subject  110  inside structure  210   b . Structure  210   b  can cover fluorescent lights  230   b  with screen  232  which is optically opaque and MMW energy transparent, so fluorescent lights  230   b  cannot be seen by subject  110 . Screen  232  may be made from material similar to screen  220 . An additional structure  210   a  housing fluorescent lights  230   a  can also be placed along an opposite side of subject  110  to provide additional MMW radiation. Additionally, fluorescent light tubes  230   a  and  230   b  may be aligned differently to illuminate subject  110  with MMW radiation having different polarizations. For example, fluorescent light tubes  230   b  may be placed substantially in vertical alignment with subject  110 ; and fluorescent light tubes  230   a  may be placed substantially horizontally with respect to subject  110 . Embodiments of the invention which exploit the polarization properties of the MMW radiation are presented in more detail below. 
         [0032]    Other embodiments of the invention could include placing fluorescent lights in portals, under floors, behind sheet rock covering walls, or in any place which may be optically opaque but transparent to MMW radiation. 
         [0033]    It has been observed that the MMW radiation produced by fluorescent lights can be modulated based upon the modulation of the energizing voltage. For example, modulation of MMW radiation produced by fluorescent lights can be substantially sinusoidal having a frequency of 60 Hz. This modulation can be used to alter the MMW illumination of subject  110  to produce images having differing characteristics, which can be analyzed and combined to improve the detection of concealed objects. One of ordinary skill in the art would appreciate that frequencies other than 60 Hz, and modulations other than sinusoidal, may be used with other embodiments of the invention. If the modulation frequency chosen is low enough to cause perceptible flickering, the fluorescent lights can be placed behind a screen to mitigate the flickering. 
         [0034]      FIG. 3  depicts a sensor utilizing modulated MMW radiation consistent with another embodiment of the invention. This embodiment exploits the sinusoidal modulation of fluorescent lights  360  associated with the 60 Hz line voltage to form an image of subject  110  when fluorescent lights  360  are in an “on” state, and form an image when fluorescent lights  360  are in an “off” state. The two images may be processed to yield better concealed object detection results. 
         [0035]    Fluorescent lights  360  can be positioned overhead as shown, or placed in some alternative manner proximate to subject  110  to provide diffuse modulated MMW radiation for illumination. Synchronization unit  350  can determine the modulation of fluorescent lights  360  through coupling  370 , and then provide signals to MMW sensor  100  to synchronize components thereof for proper operation with modulated fluorescent lights  360 . MMW sensor  100  further includes MMW imaging sensor  310 , which collects an image of the subject  110 . MMW sensor provides images to an ON integrator  320  and an OFF integrator  330 . The images are directed to each integrator through a switch which can be controlled by synchronization unit  350 . Each integrator  320  and  330  sums data received by MMW imaging sensor  310  over a period of time, such as, for example, 700 microsecond to 10 milliseconds, which can improve the image&#39;s signal to noise ratio. 
         [0036]    An “on” image  380 A can show reflective objects as being bright, as it reflects the MMW radiation provided by fluorescent lights  360 , and the subject as being dark. An “off” image  380 B can show the reflective object as being dark, as it is reflecting cooler radiometric temperatures associated with the environment, and the representation of the subject as being brighter due to the subject&#39;s emissive nature. 
         [0037]    The “on” image and the “off” image are passed from each respective integrator into a formatter  340 . Formatter  340  can provide additional header data, such as whether the image is an “on” image or an “off” image, and can further transform the data into a format suitable for processing. A processor  345  receives the images from formatter  340  and can perform operations to analyze and/or combine the images to improve the probability of detecting a concealed target. After the images have been processed, each image, or a combined image, can be shown on a display  347 . 
         [0038]    Processor  345  can perform, for example, difference processing between the two images which can identify the type of material from which the concealed object is made. Any other processing known to one of ordinary skill in the art may be performed on the two images, either separately or in combination with each other. 
         [0039]    Synchronization unit  350  can operate by monitoring the power line supply fluorescent lights  360  if detector and lights are on same voltage phase. In this case, coupling  370  can be performed using common power lines. Alternatively, a light sensing device, such as, for example, a photodiode, can be the coupling  370 , which can provide a signal which synchronization unit  350  may used to synchronize MMW sensor  100  with fluorescent lights  360 . 
         [0040]    It also has been observed that the MMW radiation produced by fluorescent lights tubes has energy which can be highly polarized down length of tube. When plastic objects are illuminated with polarized MMW energy, the reflections received can vary depending on the orientation of the plastic object. This phenomena can be attributed to the interaction of the MMW electric fields and the dielectric properties of the plastic. However, reflections received from metal objects typically do not vary with orientation. By using polarized MMW radiation sources and sensors, the differing phenomenology between metal and plastic materials can be used to identify and differentiate concealed objects made from metal and plastic. 
         [0041]      FIG. 4  depicts a sensor utilizing MMW radiation having polarimetric properties consistent with another embodiment of the invention. At least one pair of light tubes  405 ,  407  can be positioned in proximity to subject  110 . Fluorescent light tubes  405  and  407  can be set up perpendicular to each so, for example, fluorescent light tubes  405  provide vertical polarization, while fluorescent light tubes  407  provide horizontal polarization. The horizontal and vertical orientations of fluorescent light tubes  405 ,  407  would be set up relative to MMW sensor  100 . As stated before, the fluorescent light tubes may be set above the subject  110  as shown in  FIG. 4 , or can be set in other proximate positions. Additionally, the fluorescent light tubes  405 ,  407  may be positions so that one set is positions vertically to subject  110 , and one set is positioned horizontally to subject  110 . 
         [0042]    In order to separately form images for each polarization, fluorescent light tubes  405 ,  407  can be separately controlled to interleave the illumination corresponding to the H and V polarizations. This could be accomplished by providing separate control signals to fluorescent light tubes  405 ,  407 . In a similar manner as described above in the previous embodiment, fluorescent light tubes  405 ,  407  can be modulated to take advantage of the dual polarization and create MMW images in both the “on” and “off” fluorescent light tube state for each polarization. For example, modulation of MMW radiation produced by fluorescent lights  405 ,  407  can be substantially sinusoidal having a frequency of associated with the power line frequency, e.g. 60 Hz. While the control signals could have the same frequency, they should be 90 degrees out of phase with each other to alternate the polarizations. The control signal could be voltage providing the power to illuminate the fluorescent light tubes. One of ordinary skill in the art would appreciate that frequencies other than 60 Hz, and modulations other than sinusoidal, may be used with other embodiments of the invention. 
         [0043]    This modulation can be used to alter the MMW illumination of subject  110  for both horizontal and vertical polarizations to produce four images, each having differing characteristics. These images can be subsequently analyzed and combined to improve the detection of concealed objects. 
         [0044]    Synchronization unit  450  can orchestrate the modulation of fluorescent lights  405 ,  407  by providing separate H and V control signals. As mentioned above, the signals could be sinusoidal or take on any other form known to one of ordinary skill in the art. Alternatively, synchronization unit  450  could utilize two photo diodes, one sensitive to H polarization, and the other sensitive to V polarization, to monitor which set of fluorescent light tubes  405 ,  407  is irradiating subject  110 , and provide this information to the components of MMW sensor  100  for synchronization. 
         [0045]    MMW sensor  100  further includes a polarization sensitive MMW imaging sensor  410 , which collects an image of subject  110  for both H and V polarizations. MMW sensor can receive a polarization command from synchronization unit  450  to command what polarization the MMW sensor is to receive. Based upon the polarization command and a signal indicating whether the fluorescent light tubes where “on” or “off”, the image is sent to the appropriate integrator via a switch which can be controlled by synchronization unit  450 . The embodiment can include four separate integrators: an ON integrator for H polarization  415 , an OFF integrator for H polarization  420 , an ON integrator for V polarization  425 , and an OFF integrator for V polarization  430 . Each integrator  415 - 430  sums data received by MMW imaging sensor  410  over a period of time, such as, for example, 700 micro-second to 10 milli-seconds, which can improve the image&#39;s signal to noise ratio. 
         [0046]    Both H and V polarization&#39;s “on” and “off” images are passed from each respective integrator into a formatter  440 . Formatter  440  can provide additional header data, such as whether the image is an “on” image or an “off” image, the image&#39;s H or V polarization designation, and can further transform the data into a format suitable for processing. A processor  445  receives the images from formatter  440  and can perform operations to analyze and/or combine the images to improve the probability of detecting a concealed target. After the images have been processed, each image, or a combined image, can be shown on a display  347 . 
         [0047]    Processor  445  can perform, various processing between the four (Hon, Hoff, Von, Voff) images which can identify the type of material from which the concealed object is made. For example, sums and differences between the H and V polarizations can be performed to produce an image which is analogous to a Stokes vector. Any other forms of image and/or signal processing may be performed on the four images, either separately or in other combinations, which are known to one of ordinary skill in the art. 
         [0048]    Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.