Abstract:
An imaging system includes a scanning reflector that sweeps through a periodic scan pattern to redirect millimeter wave energy from a target object to a detector. The imaging system may include one or more millimeter wave lenses that gather and focus the millimeter wave energy from the target object onto the reflector or the detector. The detector is super-cooled to increases sensitivity, so that the imaging system does not require an illumination source. For each location on the target object, the detector monitors the intensity of the millimeter wave energy and an electronic controller builds a memory map from the detector data. Because different materials block millimeter wave energy differently, the detector data, and thus the memory map corresponds to the structure of the target object. In one embodiment, the scanning reflector is a resonant scanner. The scanner may be a microelectromechanical (MEMs) or mesomechanical scanner.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to millimeter wave devices, and more particularly, imaging devices that utilize scanned millimeter waves.  
         BACKGROUND  
         [0002]    Millimeter wave devices promise many useful applications, because such devices provide a small solution to many local transmission applications. Moreover, millimeter wave devices may be useful in detecting objects behind optically opaque barriers, much like X-rays. Advantageously, millimeter wave devices can take advantage of many optical techniques, such as focusing lenses and reflectors. This capability provides flexibility in developing small components with unique capabilities.  
           [0003]    One application of millimeter wave devices is for imaging through opaque materials, such as concrete walls and plastic boxes. Conventional millimeter wave imaging devices utilize highly sensitive detectors fed by fixed waveguides. To image a target object, the entire device is moved until the waveguide is aligned to the target object. The waveguide then collects millimeter wave energy emitted or reflected by the target object and directs the millimeter wave energy to the detector.  
           [0004]    Typically, the field view of the waveguide is quite small. Consequently, the portion of the target object that can be image at any one time is quite small. Imaging the entire target object can therefore involve moving the entire device through a series of many orientations. At each location, the millimeter wave energy is sampled and stored. Gradually, an entire data set is built up. From the data set, signal processing can produce an image of the target object.  
           [0005]    One difficulty with this approach is the time required to generate the entire data set. Gathering data in this fashion can be tedious and costly.  
           [0006]    This difficulty becomes particularly problematic where the target object is moving. In such a circumstance, the time lag between data taken for a first orientation and data for a second orientation may be sufficiently large that the target object may move significantly during the time lag. The final data set may represent portions of the image taken for different positions of the target object. Consequently, the data set may represent a highly distorted image of the target object.  
         SUMMARY OF THE INVENTION  
         [0007]    A millimeter wave scanning imager scans an image field to collect millimeter waves from an external environment. A sensitive detector monitors the millimeter wave energy received from the external environment and produces an electrical signal indicative of the energy received. An electronic controller samples the electrical signal to produce image data corresponding to the scanned millimeter wave energy.  
           [0008]    Because the electronic controller concurrently monitors the scan position, the electronic controller can determine the corresponding location in the external environment for producing the image data. Accordingly, the electronic controller can build an image data set representative of the external environment.  
           [0009]    In one embodiment, the imager includes a scanner that scans one or more reflectors through a periodic two-dimensional scan pattern. The scan pattern may be a raster pattern or another type of pattern, such as a vector or spiral pattern.  
           [0010]    One embodiment of the scanner is a microelectromechanical (MEMs) scanner. The MEMs scanner is a biaxial scanner having a central reflector coated with a conductor.  
           [0011]    In another embodiment, the scanner includes two mechanically resonant scanners driven by electromagnetic coils. The central reflectors of the scanners formed from a metal that reflects the millimeter wave energy.  
           [0012]    To improve the sensitivity, the imager also includes dielectric lenses that gather and focus the millimeter wave energy onto the detector. One embodiment also includes additional dielectric lenses with variable positioning to adjust the imaging distance of the imager.  
           [0013]    To improve the detector sensitivity, the imager also includes a super cooler that cools detector to a very low temperature. The very low temperature reduces the detector noise to improve the signal to noise ratio of the imager. Consequently, the imager does not require an illuminating millimeter wave source.  
           [0014]    Because the millimeter wave imager is formed from small components, including a small super cooler, and does not require a separate source, the imager may be small and light enough to be human portable. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0015]    [0015]FIG. 1 is a block diagram of a scanning imager according to one embodiment of the invention.  
         [0016]    [0016]FIG. 2 is a block diagram of a scanning assembly within the imager of FIG. 1, including a central reflector  56  that pivots about two orthogonal axes.  
         [0017]    [0017]FIG. 3 is a top plan view of a biaxial MEMs reflector for reflecting millimeter wave energy.  
         [0018]    [0018]FIG. 4 is a diagram of a two mirror scanning assembly, including a horizontal reflector and a vertical reflector.  
         [0019]    [0019]FIG. 5 is diagram of a millimeter wave scanner using a polygon-type reflector.  
         [0020]    [0020]FIG. 6 is diagram of a two mirror scanning assembly, including a mechanically resonant scanner.  
         [0021]    [0021]FIG. 7 is block diagram of a millimeter wave detector within the scanning imager of FIG. 1.  
         [0022]    [0022]FIG. 8 is a diagrammatic representation of differences between a sinusoidal scan and a linear scan.  
         [0023]    [0023]FIG. 9 is a diagrammatic representation of relative timing of scanned data versus uniformly spaced data.  
         [0024]    [0024]FIG. 10 is system block diagram of a millimeter wave imaging system, including timing synchronized to the scanning reflector.  
         [0025]    [0025]FIG. 11A is a top plan view of a multi-element scanning reflector.  
         [0026]    [0026]FIG. 11B is a side cross-sectional view of a multi-element scanning reflector of FIG. 11A.  
         [0027]    [0027]FIG. 12 is a diagrammatic representation of a millimeter wave system viewing objects behind an optically opaque barrier.  
         [0028]    [0028]FIG. 13 is a diagrammatic representation of a human portable millimeter wave imaging system.  
     
    
     DETAILED DESCRIPTION  
       [0029]    As shown in the simplified block diagram of FIG. 1, a scanning imager  40  is aligned to an external environment  42 . The imager  40  includes a scanning assembly  44  that acts as the principal scanning component. As will be described in greater detail below with reference to FIG. 2, the scanning assembly  44  redirects millimeter wave energy from a series of locations in the external environment  42  toward a millimeter wave detector  46 .  
         [0030]    The millimeter wave detector  46 , which will be described in greater detail below with reference to FIG. 7, responds to the millimeter wave energy by producing an electrical signal. An electronic controller  48  receives the electrical signal and produces data indicative of the millimeter wave energy level.  
         [0031]    At the same time, the scanning assembly  44  provides a sense signal to the electronic controller  48  that indicates the orientation of the scanning assembly  44 . Responsive to the sense signal, the electronic controller  48  stores the produced data in a memory device  49  in locations corresponding to the orientation of the scanning assembly  44 . The controller  48  thus builds a memory map indicative of the millimeter wave energy versus scan angle.  
         [0032]    A first dielectric lens  50 , positioned between the scanning assembly  44  and the electronic controller  48  improves the sensitivity of the imager  40  by gathering and focusing the millimeter wave energy from the scanning assembly  44  onto the detector  46 . A second dielectric lens  52  is positioned between the scanning assembly  44  and the external environment  42 . The second lens  52  gathers and focuses millimeter wave energy from the external environment  42  onto the scanning assembly  44 . Additionally, the relative positions of the first and second lenses  50 ,  52  can be varied to adjust the effective distance between the detector  46  and a target object  54  in the external environment  42 .  
         [0033]    As will now be described with reference to the simplified block diagram of FIG. 2, the scanning assembly  44  includes a central reflector  56  that pivots about two orthogonal axes  58 ,  60 . The central reflector  56  is conductively coated such that it reflects millimeter waves toward the detector  46 . Because the central reflector  56  is substantially planar, the energy reflected toward the detector  48  comes from a small region  62  of an image field  64 .  
         [0034]    As the central reflector  56  pivots about the first axis  58 , the small region  62  moves in a first direction  66  in the image field  64 . The field of view of the detector  46  thus sweeps through a line in the image field  64 .  
         [0035]    Where the scanning assembly  44  is a mechanically resonant system, the central reflector  52  pivots periodically in a sinusoidal pattern, as shown in FIG. 2. One skilled in the art will recognize that for non-resonant systems, other scanning patterns may be used.  
         [0036]    At the same time that the central reflector  56  sweeps in the first direction, the central reflector  56  also pivots about the second axis  60  at a rate that is substantially lower than the scan rate about the first axis  58 . The field of view thus sweeps along a path that has components along the first and second axes, as represented in FIG. 2. Because the scan rate in the first direction is substantially higher than the rate in the second direction, the small region  62  scans the image field  64  in a sinusoidal pattern that approximates a raster pattern. One skilled in the art will recognize that other scan patterns may be used, although the substantially raster pattern is often preferred because of its compatibility with typical signal processing techniques.  
         [0037]    Because the small region  62  traverses substantially the entire image field  64  during the raster sweep, the detector  46  receives energy sequentially from the entire image field  64 . For each location of the small region  62 , the detector  46  outputs an electrical signal corresponding to the millimeter wave energy coming from the location. The electronic controller  48  receives the electrical signal and identifies image data that represents the received energy from the image field  64 . As described above, the electronic controller  48  can correlate the image data to the location in the image field, because the scanning assembly  44  supplies the sense signal indicative of the scan angle. The imager  40  can thus generate an entire map of the target object  54  from the image data.  
         [0038]    [0038]FIG. 3 shows one embodiment of the scanning assembly  44  where the central reflector  56  is mounted to a pivoting ring  66  by a pair of torsion arms  68 . The pivoting ring  66  is mounted in turn to a substrate  70  by a pair of secondary torsion arms  72 . Each of the torsion arms  68 ,  70  twists torsionally to allow the central reflector  56  and pivoting ring  66  to pivot about respective orthogonal axes.  
         [0039]    In a preferred embodiment, the scanning assembly  44  of FIG. 3 is a microelectromechanical (MEMs) device formed from a silicon substrate. To make the central reflector  56  reflective to millimeter waves, the central reflector  56  includes a conductive coating  67 , such as an aluminum or gold film. Redirection of millimeter waves from fixed reflectors is known in the art.  
         [0040]    Unlike typical fixed reflectors, the central reflector  56  in this embodiment is able to pivot quickly through a periodic pattern. To pivot the central reflector  56  about a first axis, magnetic fields from a separate source (not shown) interact with currents flowing through conductive traces on the pivoting ring  66 , thereby sweeping the pivoting ring  66  about the first axis. Since the pivoting ring  66  carries the central reflector  56 , the motion of the pivoting ring  66  produces corresponding motion of the central reflector  56 .  
         [0041]    The central reflector  56  can pivot about a second axis relative to the pivoting ring  66 . A pair of conductive plates  81 ,  83  are positioned on opposite sides of the second axis and aligned to the central reflector  56 . In operation, a driving voltage is applied alternatingly to the first conductive plate  81  and then the second plate  83 . The voltage difference between the driven plate  81  or  83  and the corresponding part of the central reflector  56  produces a torque that causes the central reflector  56  to pivot about the second axis.  
         [0042]    The central reflector  56  and torsion arms  68  are dimensioned so that the central reflector  56  oscillates at a desired resonant frequency. The system has a relatively high Q, so that only a small portion of the energy in the central reflector  56  and torsion arms  68  is lost during a sweep. Consequently, the amount of energy that must be added to cause pivoting is reduced relative to a low Q system. For high efficiency, the voltage on the plates is varied at the resonant frequency of the central reflector  56 .  
         [0043]    The structure and fabrication of such biaxial MEMs devices is described in detail in U.S. Pat. No. 5,629,790 to Neukermans et. al., entitled MICROMACHINED TORSIONAL SCANNER, which is incorporated herein by reference. One skilled in the art will recognize that the central reflector  56  and torsion arms  68 ,  70  will be sized and shaped appropriately for millimeter waves. For example, the central reflector  56  will typically be substantially larger for millimeter waves than a MEMs mirror in an optical device.  
         [0044]    As shown in FIG. 4, the scanning assembly  44  may be formed from separate horizontal and vertical scanners  200 ,  202 . As shown in FIG. 4, the separate scanners are mesomechanical devices, although MEMs devices could also be used. Because the horizontal scanner  200  is a resonant device with a high Q (&gt;100), the scanner  200  can operate with a relatively low drive power. Typically, the resonant frequency of the scanner  200  is greater than 1000 Hz, and may be greater than 10 kHz. As one skilled in the art will recognize from the calculations below, higher frequencies can produce higher resolutions for a given scan angle.  
         [0045]    The vertical scanner  202  is formed from a vertical reflector  204  mounted to a shaft  206  driven by a motor  208 . The motor  208  is a commercially available device that rotates the vertical reflector  204  linearly from one extreme to another about a first axis. Typically, the vertical reflector  204  will pivot by about 10-20 degrees to produce a 20-40 degree field of view. Depending upon the application, the vertical scanner  202  will typically follow a saw-tooth or triangular scan pattern. However, other scan patterns, such as stair-step or sinusoidal patterns may be used in some applications.  
         [0046]    The horizontal scanner  200  includes a horizontal reflector  214  mounted to a shaft  218  of a motor  220 . The horizontal reflector  214  is positioned in the field of view of the detector  210  as deflected by the vertical reflector  204 . The motor  220  spins the horizontal reflector  214  about a second axis orthogonal to the first to provide a horizontal component to the scanning pattern. The field of view of the detector  210  thus covers a two-dimensional image field  222 .  
         [0047]    The horizontal scanner  200  scans at a substantially higher rate than the vertical scanner  202 . For example, in one embodiment, the horizontal reflector  214  has a width D of 2 inches (50.8 mm) and the millimeter wave energy is at 1.2 THz. For an expected 60 degree (1.047 rad) field of view, the number of pixels can be approximately:  
           P=Dθ/λ= 50.8 mm*1.047 rad/0.25 mm=212.75 pixels.  
         [0048]    In a vacuum environment, the motor  220  can drive the reflector  214  at speeds on the order of 1,000-100,000 rpm. For a 200 by 200 pixel image where the reflector spins at 60,000 rpm (1000 Hz), the frame rate with a 10% allowance for frame transition would be:  
           F= 1000 Hz/(200 lines*1.1)=4.54 frames per second.  
         [0049]    If both sides of the reflector  214  are reflective, the effective scan rate will be doubled, providing a frame rate of 9.08 frames per second. This rate is below typical display rates of many systems, but is sufficient for many applications.  
         [0050]    One skilled in the art will recognize that a variety of other designs may desirable for some applications. For example, where the millimeter wave energy is at 600 GHz, the resolvable pixels will be 106.37. In such an imager, a 100 by 100 pixel image would give a frame rate of about 18 frames per second.  
         [0051]    As shown in FIG. 5 the horizontal scanner  200  can use a polygonal reflector  224  to increase the frame rate. The polygonal reflector  224  is an eight sided reflector with each face canted at a respective angle. Consequently, each face provides a respective vertical component to the vector angle of the scan. Because the reflector  224  has eight sides, the imager will produce 8 lines per rotation of the polygon. Following calculations described above, a polygon having 1 inch sides would produce about 34 pixels per line for a 20 degree field of view and a 0.25 mm wavelength.  
         [0052]    In this embodiment, the effective horizontal scan rate is 3,000 Hz at 60,000 rpm. A 100 by 100 pixel display would then have a frame rate of 27 frames per second.  
         [0053]    For higher frame rates and a more stable system, in some applications a resonant mechanical scanner  250  may form the horizontal scanner, as shown in FIG. 6. The principal scanning component of the resonant scanner  250  is a moving mirror  252  mounted to a spring plate  254 . The dimensions of the mirror  252  and spring plate  254  and the material properties of the spring plate  254  are selected so that the mirror  252  and spring plate  254  have a high Q with a natural oscillatory (“resonant”) frequency on the order of 1-20 kHz, where the selected resonant frequency depends upon the application.  
         [0054]    A ferromagnetic material mounted with the mirror  252  is driven by a pair of electromagnetic coils  256 ,  258  to provide motive force to mirror  252 , thereby initiating and sustaining oscillation. The ferromagnetic material is preferably integral to the spring plate  254  and body of the mirror  252 . Drive electronics  268  provide electrical signals to activate the coils  256 ,  258 . Responsive to the electrical signals, the coils  256 ,  258  produce periodic electromagnetic fields that apply force to the ferromagnetic material, thereby causing oscillation of the mirror  252 . If the frequency and phase of the electric signals are properly synchronized with the movement of the mirror  252 , the mirror  252  oscillates at its resonant frequency with little power consumption.  
         [0055]    In this embodiment, the vertical scanner  202  is structured very similarly to the resonant horizontal scanner  200 . Like the horizontal scanner  200 , the vertical scanner  202  includes a mirror  262  driven by a pair of coils  264 ,  266  in response to electrical signals from the drive electronics  268 . However, because the rate of oscillation is much lower for vertical scanning, the vertical scanner  202  is typically not resonant. As described above, the vertical scanner  202  directs millimeter wave energy toward the horizontal scanner with vertical deflection at about 30-100 Hz. Advantageously, the lower frequency allows the mirror  262  to be significantly larger than the mirror  214 , thereby reducing constraints on the positioning of the vertical scanner  202 .  
         [0056]    The details of mechanically resonant scanning are described in greater detail in U.S. Pat. No. 5,557,444 of Melville, et al., entitled MINATURE OPTICAL SCANNER FOR A TWO AXIS SCANNING SYSTEM which is incorporated herein by reference.  
         [0057]    As shown in FIG. 7, the detector  46  includes a very sensitive detector diode  170  fed by a collector  172 . The detector  46  drives a high gain amplifier and down converter  174  that produces an output signal corresponding to modulation of the millimeter wave energy. To improve the overall signal to noise ratio of the system, the diode  170  and converter  174  are super cooled by a conventional super cooler  176 . Such super coolers are commercially available devices.  
         [0058]    To improve receiver selectivity, the receiver may employ a single or double superheterodyne conversion detector. This design methodology allows for tuning of the received millimeter wave energy over a band of useable frequencies.  
         [0059]    Tuning is accomplished by the use of a voltage controlled local oscillator (VCLO)  175 , which mixes with the original impinging millimeter wave energy and produces subsequent first and second stage intermediate frequency if output signals which are then further filtered and amplified into amplifier and down converter  174 . This conversion process results in better overall sensitivity, increased signal to noise ratio performance, and the ability to “see through” or penetrate various types of materials.  
         [0060]    An A/D converter  180  receives the output from the converter  174  and produces digital data in response. A controller  182  receives the digital data and also receives a sense signal indicating the scan position. In response, the controller  182  generates a data map that is stored in a memory  184 . Additionally, the controller  182  can output the data map through a RF transmitter  186  or on a display  188 . For most applications, the controller  182  includes a pre-processor that processes the data from the detector  170  according to conventional video image processing techniques to remove non-linearities and other image artifacts. In many applications, software drivers, dedicated image processors and other signal processing techniques are applied either before or after the data is stored in the memory  184 . The level of processing and location of these additional processing components will be dictated by the particular application. For example where sufficient computing resources are available in a notebook computer, much of the processing may be done on the notebook computer. In other applications, it may be desirable to perform additional processing at a remote location.  
         [0061]    Where the scanning system of FIG. 4 is a resonant or other nonlinear scanning system, equally spaced physical locations on the target object  54  do not correspond to equally spaced sampling times. As shown by broken line in FIG. 8, the timing of data is often premised upon a linear scan rate. That is, for equally spaced subsequent locations in a line, the data arrive at constant intervals. A resonant scanner, however, has a scan rate that varies sinusoidally, as indicated by the solid line. For a start of line beginning at time to, the sinusoidal scan initially lags the linear scan. Thus, at time t 1A  the sinusoidal scan will reflect energy from position P 1  to the detector  46 . A linear scan assumption would place the corresponding data in the memory  184  at a location corresponding to position P 2 .  
         [0062]    To address this non-linearity, the system of FIG. 7 employs unevenly spaced pulses of an adjusted clock to clock data out of the A/D converter  180 , instead of typical equally spaced clock pulses. Generally, for a pixel to be located at position P 1 , the sampling pulse arrives at time t 1B , rather that time t 1A , as would be the case for a linear scan rate.  
         [0063]    [0063]FIG. 9 shows graphically the determination of clock timing for a 35-pixel line. One skilled in the art will recognize that this example is simplified for clarity of presentation. A typical line may include hundreds or even thousands of pixels. As can be seen, the pixels will be spaced undesirably close at the edges of the field of view and undesirably far at the center of the field of view. Consequently, the image will be compressed near the edges of the field of view and expanded near the middle, forming a distorted image.  
         [0064]    As shown by the upper line, pixel location varies nonlinearly for pixel counts equally spaced in time. Accordingly, the actual locations of evenly spaced pixels, shown by the lower line, correspond to nonlinearly spaced counts. For example, the first pixel in the upper and lower lines arrives at the zero count and should be located in the zero count location. The second pixel should be stored in a memory location corresponding to the 100 count; but does not arrive until the 540 count. Similarly, the third pixel is to be stored in a memory location corresponding to count  200  and arrives at count  720 . One skilled in the art will recognize that the figure is merely representative of the actual calculation and timing. For example, some output counts will be higher than their corresponding input counts and some counts will be lower. Of course, a pixel will not actually be stored before its corresponding data arrives.  
         [0065]    To address this condition, the system of FIG. 10 (described below) imposes a latency on the output of data, in a similar fashion to synchronous memory devices. For the example of FIG. 9, a single line latency (3400 count latency) would be ample. With such a latency, the first pixel would be stored at count  3400  and the second would occur at count  3940 .  
         [0066]    Turning to FIG. 10, timing is referenced to the scanning assembly  44 . As noted above, the scanning assembly  44  includes a high-Q resonant scanner as the principal scanning component. As a high-Q resonant system, the scanning assembly  44  scans at its resonant frequency f SCAN . The resonant frequency depends upon the specific geometry, materials, and other characteristics of the scanner. Additionally, the scanning frequency f SCAN  may vary in response to changing environmental conditions, including temperature and pressure.  
         [0067]    As the scanning assembly  44  scans the target object  54 , the scanning assembly also outputs the sense signal to a phase locked loop  100 . The phase locked loop locks to the scanning frequency f SCAN  and outputs a sampling signal at a sampling frequency that is synchronized to the scanning frequency f SCAN . Typically, the sampling frequency is an integral multiple of the scanning frequency f SCAN .  
         [0068]    Rather than using an actual counter and a look up table to identify the adjusted counts, the system of FIG. 10 uses a simplified structure in which the sampling signal drives a pattern memory  102  to produce an adjusted clock that controls timing of operations in the imager  40 . The pattern memory  102  is programmed with data that correspond to the adjusted counts corresponding to the proper memory location.  
         [0069]    Responsive to the clock from the phase locked loop  100 , the pattern memory  102  outputs data to an edge detector  104 . In response to transitions of the data, the edge detector  104  provides pulses that form the sampling clock. Near the edges of the scan, the scanning assembly  44  is moving slowly, due to its sinusoidal motion. Consequently, it is desirable for pulses of the sampling clock to be spaced far apart in time.  
         [0070]    Accordingly, the pattern memory  102  outputs data with few transitions during this portion of the scan. For example, corresponding locations in the pattern memory  102  may contain all “0s” or a long string of “0s” followed by a long string of “1s.” Near the middle of the scan, the scanning assembly moves at its highest rate. It is desirable therefore, to sample data quickly in this region. Consequently, the corresponding locations in the pattern memory contain interleaved “0s” and “1s” 
         [0071]    The output of the edge detector  104  clocks data through the A/D converter  80  to sample the down converted data from the amplifier and down converter  74 . The sampled data forms an address in a correction buffer  106  to produce corrected data. The correction buffer contains gamma corrected data that correct for gain distortion and other nonlinear characteristics of the system.  
         [0072]    The corrected data is then stored in a frame buffer  110  to be output through a register in response to a system clock. The system clock typically comes from reader control circuitry or another clock that is independent of the sampling clock. The data output from the register can then be processed in a conventional fashion to identify information about the target object or to generate a visual display of the target object.  
         [0073]    [0073]FIGS. 11A and 11B show an alternative embodiment of a millimeter wave scanner  120  that includes an array of reflectors  122  that pivot about parallel axes. Each of the scanners  120  may be a MEMs device, a motor driven scanner such as those of FIG. 5, or a resonant scanner, such as those of FIG. 7. The drive electronics  218  (FIG. 1) drive all of the reflectors  122  synchronously so that all of the reflectors  122  pivot together. The overall scanner  120  appears to operate very similarly to a conventional louvered window shade with individual components pivoting synchronously, although the individual reflectors  122  typically pivot periodically. Because the reflectors  122  pivot synchronously, they form an effective reflector that is substantially larger than any individual reflector  122 . However, because the dimension of each individual reflector  122  transverse to the axis of rotation is substantially smaller than that of the overall effective reflector, the individual reflectors  122  can be made resonant at higher rates for given drive currents. One skilled in the art will recognize that, because the system is not limited coherent radiation, diffractive effects of the multiple reflector elements will be minimized.  
         [0074]    As shown in FIG. 12, the scanning imager  40  may be used to identify information about objects  130 ,  132  behind an optically opaque barrier  134 , such as a concrete wall. Where the detector  46  is sufficiently sensitive, the detector  46  can detect millimeter wave energy from ambient sources, such as the sun. In some applications, however, it may be desirable to augment ambient energy with a separate millimeter wave source  136 . The separate source  136  may be within the viewed environment or may be placed opposite another opaque barrier  138 .  
         [0075]    As shown in FIG. 13, the scanning imager  40  of FIG. 1 can be assembled with the MEMs scanner  44  of FIG. 3, a miniature super-cooler  150  and a battery  152  to produce a human portable system. Since the scanning imager  40  can operate with ambient millimeter wave energy or with a separate millimeter wave source, it is not necessary for an operator  154  to transport a millimeter wave source. The portable imager  40  is particularly useful for circumstances where human portability is advantageous. For example, as shown in FIG. 10, the imager  40  may be used by police or other operatives to view a hostage or other hostile environment  158  from behind a wall  160 .  
         [0076]    Although the invention has been described herein by way of exemplary embodiments, various other embodiments may be within the scope of the invention. For example, the scanning imager  46  may use non-raster scanning, such as vector scanning to image an area. Moreover, the range of the imager  46  may be optimized to allow viewing from substantial distances. Such embodiments would allow viewing through low vision environments, such as fog. This capability would be useful for such applications as docking boats or landing aircraft. Accordingly, the invention is not limited, except as by the appended claims.