US20020044276A1 - Millimeter wave scanning imaging system - Google Patents
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- US20020044276A1 US20020044276A1 US09/851,672 US85167201A US2002044276A1 US 20020044276 A1 US20020044276 A1 US 20020044276A1 US 85167201 A US85167201 A US 85167201A US 2002044276 A1 US2002044276 A1 US 2002044276A1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
- G01S13/88—Radar or analogous systems specially adapted for specific applications
- G01S13/887—Radar or analogous systems specially adapted for specific applications for detection of concealed objects, e.g. contraband or weapons
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B15/00—Measuring arrangements characterised by the use of electromagnetic waves or particle radiation, e.g. by the use of microwaves, X-rays, gamma rays or electrons
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V8/00—Prospecting or detecting by optical means
- G01V8/005—Prospecting or detecting by optical means operating with millimetre waves, e.g. measuring the black losey radiation
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
- H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
- H01L27/144—Devices controlled by radiation
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- H—ELECTRICITY
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- H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
- H01L27/144—Devices controlled by radiation
- H01L27/146—Imager structures
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- H01L27/1462—Coatings
- H01L27/14623—Optical shielding
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
- H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
- H01L27/144—Devices controlled by radiation
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- H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
- H01L27/144—Devices controlled by radiation
- H01L27/146—Imager structures
- H01L27/14643—Photodiode arrays; MOS imagers
- H01L27/14649—Infrared imagers
Definitions
- the present invention relates to millimeter wave devices, and more particularly, imaging devices that utilize scanned millimeter waves.
- 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.
- millimeter wave devices are 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.
- 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.
- 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.
- 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.
- 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.
- MEMs microelectromechanical
- the MEMs scanner is a biaxial scanner having a central reflector coated with a conductor.
- 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.
- 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.
- 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.
- 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.
- FIG. 1 is a block diagram of a scanning imager according to one embodiment of the invention.
- 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.
- FIG. 3 is a top plan view of a biaxial MEMs reflector for reflecting millimeter wave energy.
- FIG. 4 is a diagram of a two mirror scanning assembly, including a horizontal reflector and a vertical reflector.
- FIG. 5 is diagram of a millimeter wave scanner using a polygon-type reflector.
- FIG. 6 is diagram of a two mirror scanning assembly, including a mechanically resonant scanner.
- FIG. 7 is block diagram of a millimeter wave detector within the scanning imager of FIG. 1.
- FIG. 8 is a diagrammatic representation of differences between a sinusoidal scan and a linear scan.
- FIG. 9 is a diagrammatic representation of relative timing of scanned data versus uniformly spaced data.
- FIG. 10 is system block diagram of a millimeter wave imaging system, including timing synchronized to the scanning reflector.
- FIG. 11A is a top plan view of a multi-element scanning reflector.
- FIG. 11B is a side cross-sectional view of a multi-element scanning reflector of FIG. 11A.
- FIG. 12 is a diagrammatic representation of a millimeter wave system viewing objects behind an optically opaque barrier.
- FIG. 13 is a diagrammatic representation of a human portable millimeter wave imaging system.
- 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.
- the scanning assembly 44 redirects millimeter wave energy from a series of locations in the external environment 42 toward a millimeter wave detector 46 .
- 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.
- the scanning assembly 44 provides a sense signal to the electronic controller 48 that indicates the orientation of the scanning assembly 44 .
- 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.
- 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 .
- 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 .
- 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 .
- the central reflector 52 pivots periodically in a sinusoidal pattern, as shown in FIG. 2.
- FIG. 2 One skilled in the art will recognize that for non-resonant systems, other scanning patterns may be used.
- 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.
- the substantially raster pattern is often preferred because of its compatibility with typical signal processing techniques.
- 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.
- 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.
- the scanning assembly 44 of FIG. 3 is a microelectromechanical (MEMs) device formed from a silicon substrate.
- MEMs microelectromechanical
- 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.
- the central reflector 56 in this embodiment is able to pivot quickly through a periodic pattern.
- magnetic fields from a separate source 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 .
- 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 .
- 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.
- 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.
- the voltage on the plates is varied at the resonant frequency of the central reflector 56 .
- the scanning assembly 44 may be formed from separate horizontal and vertical scanners 200 , 202 .
- the separate scanners are mesomechanical devices, although MEMs devices could also be used.
- the horizontal scanner 200 is a resonant device with a high Q (>100)
- the scanner 200 can operate with a relatively low drive power.
- 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.
- 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.
- the vertical reflector 204 will pivot by about 10-20 degrees to produce a 20-40 degree field of view.
- 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.
- 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 .
- the horizontal scanner 200 scans at a substantially higher rate than the vertical scanner 202 .
- the horizontal reflector 214 has a width D of 2 inches (50.8 mm) and the millimeter wave energy is at 1.2 THz.
- the number of pixels can be approximately:
- the motor 220 can drive the reflector 214 at speeds on the order of 1,000-100,000 rpm.
- the frame rate with a 10% allowance for frame transition would be:
- 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.
- the resolvable pixels will be 106.37.
- a 100 by 100 pixel image would give a frame rate of about 18 frames per second.
- 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.
- 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.
- 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.
- 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.
- the vertical scanner 202 is structured very similarly to the resonant 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 .
- the vertical scanner 202 is typically not resonant.
- the vertical scanner 202 directs millimeter wave energy toward the horizontal scanner with vertical deflection at about 30-100 Hz.
- 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 .
- 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.
- the diode 170 and converter 174 are super cooled by a conventional super cooler 176 .
- Such super coolers are commercially available devices.
- 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.
- 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 .
- VCLO voltage controlled local oscillator
- 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 .
- 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 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.
- 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 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 .
- 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.
- the sampling pulse arrives at time t 1B , rather that time t 1A , as would be the case for a linear scan rate.
- FIG. 9 shows graphically the determination of clock timing for a 35-pixel line.
- a typical line may include hundreds or even thousands of pixels.
- 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.
- the actual locations of evenly spaced pixels correspond to nonlinearly spaced counts.
- 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.
- the third pixel is to be stored in a memory location corresponding to count 200 and arrives at count 720 .
- 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.
- the system of FIG. 10 (described below) imposes a latency on the output of data, in a similar fashion to synchronous memory devices.
- 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 .
- 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.
- 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 .
- the sampling frequency is an integral multiple of the scanning frequency f SCAN .
- 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.
- the pattern memory 102 Responsive to the clock from the phase locked loop 100 , the pattern memory 102 outputs data to an edge detector 104 .
- 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.
- the pattern memory 102 outputs data with few transitions during this portion of the scan.
- 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”
- 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.
- 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.
- 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 .
- 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.
- the system is not limited coherent radiation, diffractive effects of the multiple reflector elements will be minimized.
- 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.
- 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 .
- 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 .
- the scanning imager 46 may use non-raster scanning, such as vector scanning to image an area.
- 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.
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Abstract
Description
- The present invention relates to millimeter wave devices, and more particularly, imaging devices that utilize scanned millimeter waves.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- FIG. 1 is a block diagram of a scanning imager according to one embodiment of the invention.
- 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. - FIG. 3 is a top plan view of a biaxial MEMs reflector for reflecting millimeter wave energy.
- FIG. 4 is a diagram of a two mirror scanning assembly, including a horizontal reflector and a vertical reflector.
- FIG. 5 is diagram of a millimeter wave scanner using a polygon-type reflector.
- FIG. 6 is diagram of a two mirror scanning assembly, including a mechanically resonant scanner.
- FIG. 7 is block diagram of a millimeter wave detector within the scanning imager of FIG. 1.
- FIG. 8 is a diagrammatic representation of differences between a sinusoidal scan and a linear scan.
- FIG. 9 is a diagrammatic representation of relative timing of scanned data versus uniformly spaced data.
- FIG. 10 is system block diagram of a millimeter wave imaging system, including timing synchronized to the scanning reflector.
- FIG. 11A is a top plan view of a multi-element scanning reflector.
- FIG. 11B is a side cross-sectional view of a multi-element scanning reflector of FIG. 11A.
- FIG. 12 is a diagrammatic representation of a millimeter wave system viewing objects behind an optically opaque barrier.
- FIG. 13 is a diagrammatic representation of a human portable millimeter wave imaging system.
- As shown in the simplified block diagram of FIG. 1, a
scanning imager 40 is aligned to anexternal environment 42. Theimager 40 includes ascanning assembly 44 that acts as the principal scanning component. As will be described in greater detail below with reference to FIG. 2, thescanning assembly 44 redirects millimeter wave energy from a series of locations in theexternal environment 42 toward amillimeter wave detector 46. - 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. Anelectronic controller 48 receives the electrical signal and produces data indicative of the millimeter wave energy level. - At the same time, the
scanning assembly 44 provides a sense signal to theelectronic controller 48 that indicates the orientation of thescanning assembly 44. Responsive to the sense signal, theelectronic controller 48 stores the produced data in amemory device 49 in locations corresponding to the orientation of thescanning assembly 44. Thecontroller 48 thus builds a memory map indicative of the millimeter wave energy versus scan angle. - A first
dielectric lens 50, positioned between the scanningassembly 44 and theelectronic controller 48 improves the sensitivity of theimager 40 by gathering and focusing the millimeter wave energy from thescanning assembly 44 onto thedetector 46. A seconddielectric lens 52 is positioned between the scanningassembly 44 and theexternal environment 42. Thesecond lens 52 gathers and focuses millimeter wave energy from theexternal environment 42 onto thescanning assembly 44. Additionally, the relative positions of the first andsecond lenses detector 46 and atarget object 54 in theexternal environment 42. - As will now be described with reference to the simplified block diagram of FIG. 2, the
scanning assembly 44 includes acentral reflector 56 that pivots about twoorthogonal axes central reflector 56 is conductively coated such that it reflects millimeter waves toward thedetector 46. Because thecentral reflector 56 is substantially planar, the energy reflected toward thedetector 48 comes from asmall region 62 of animage field 64. - As the
central reflector 56 pivots about thefirst axis 58, thesmall region 62 moves in afirst direction 66 in theimage field 64. The field of view of thedetector 46 thus sweeps through a line in theimage field 64. - Where the
scanning assembly 44 is a mechanically resonant system, thecentral 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. - At the same time that the
central reflector 56 sweeps in the first direction, thecentral reflector 56 also pivots about thesecond axis 60 at a rate that is substantially lower than the scan rate about thefirst 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, thesmall region 62 scans theimage 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. - Because the
small region 62 traverses substantially theentire image field 64 during the raster sweep, thedetector 46 receives energy sequentially from theentire image field 64. For each location of thesmall region 62, thedetector 46 outputs an electrical signal corresponding to the millimeter wave energy coming from the location. Theelectronic controller 48 receives the electrical signal and identifies image data that represents the received energy from theimage field 64. As described above, theelectronic controller 48 can correlate the image data to the location in the image field, because thescanning assembly 44 supplies the sense signal indicative of the scan angle. Theimager 40 can thus generate an entire map of thetarget object 54 from the image data. - FIG. 3 shows one embodiment of the
scanning assembly 44 where thecentral reflector 56 is mounted to a pivotingring 66 by a pair oftorsion arms 68. The pivotingring 66 is mounted in turn to asubstrate 70 by a pair ofsecondary torsion arms 72. Each of thetorsion arms central reflector 56 and pivotingring 66 to pivot about respective orthogonal axes. - In a preferred embodiment, the
scanning assembly 44 of FIG. 3 is a microelectromechanical (MEMs) device formed from a silicon substrate. To make thecentral reflector 56 reflective to millimeter waves, thecentral reflector 56 includes aconductive coating 67, such as an aluminum or gold film. Redirection of millimeter waves from fixed reflectors is known in the art. - Unlike typical fixed reflectors, the
central reflector 56 in this embodiment is able to pivot quickly through a periodic pattern. To pivot thecentral reflector 56 about a first axis, magnetic fields from a separate source (not shown) interact with currents flowing through conductive traces on the pivotingring 66, thereby sweeping the pivotingring 66 about the first axis. Since the pivotingring 66 carries thecentral reflector 56, the motion of the pivotingring 66 produces corresponding motion of thecentral reflector 56. - The
central reflector 56 can pivot about a second axis relative to the pivotingring 66. A pair ofconductive plates central reflector 56. In operation, a driving voltage is applied alternatingly to the firstconductive plate 81 and then thesecond plate 83. The voltage difference between the drivenplate central reflector 56 produces a torque that causes thecentral reflector 56 to pivot about the second axis. - The
central reflector 56 andtorsion arms 68 are dimensioned so that thecentral 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 thecentral reflector 56 andtorsion 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 thecentral reflector 56. - 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 andtorsion arms central reflector 56 will typically be substantially larger for millimeter waves than a MEMs mirror in an optical device. - As shown in FIG. 4, the
scanning assembly 44 may be formed from separate horizontal andvertical scanners horizontal scanner 200 is a resonant device with a high Q (>100), thescanner 200 can operate with a relatively low drive power. Typically, the resonant frequency of thescanner 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. - The
vertical scanner 202 is formed from avertical reflector 204 mounted to ashaft 206 driven by amotor 208. Themotor 208 is a commercially available device that rotates thevertical reflector 204 linearly from one extreme to another about a first axis. Typically, thevertical reflector 204 will pivot by about 10-20 degrees to produce a 20-40 degree field of view. Depending upon the application, thevertical 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. - The
horizontal scanner 200 includes ahorizontal reflector 214 mounted to ashaft 218 of amotor 220. Thehorizontal reflector 214 is positioned in the field of view of thedetector 210 as deflected by thevertical reflector 204. Themotor 220 spins thehorizontal reflector 214 about a second axis orthogonal to the first to provide a horizontal component to the scanning pattern. The field of view of thedetector 210 thus covers a two-dimensional image field 222. - The
horizontal scanner 200 scans at a substantially higher rate than thevertical scanner 202. For example, in one embodiment, thehorizontal 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.
- In a vacuum environment, the
motor 220 can drive thereflector 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.
- 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. - 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.
- As shown in FIG. 5 the
horizontal scanner 200 can use apolygonal reflector 224 to increase the frame rate. Thepolygonal 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 thereflector 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. - 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.
- 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 theresonant scanner 250 is a movingmirror 252 mounted to aspring plate 254. The dimensions of themirror 252 andspring plate 254 and the material properties of thespring plate 254 are selected so that themirror 252 andspring 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. - A ferromagnetic material mounted with the
mirror 252 is driven by a pair ofelectromagnetic coils spring plate 254 and body of themirror 252. Driveelectronics 268 provide electrical signals to activate thecoils coils mirror 252. If the frequency and phase of the electric signals are properly synchronized with the movement of themirror 252, themirror 252 oscillates at its resonant frequency with little power consumption. - In this embodiment, the
vertical scanner 202 is structured very similarly to the resonanthorizontal scanner 200. Like thehorizontal scanner 200, thevertical scanner 202 includes amirror 262 driven by a pair ofcoils drive electronics 268. However, because the rate of oscillation is much lower for vertical scanning, thevertical scanner 202 is typically not resonant. As described above, thevertical scanner 202 directs millimeter wave energy toward the horizontal scanner with vertical deflection at about 30-100 Hz. Advantageously, the lower frequency allows themirror 262 to be significantly larger than themirror 214, thereby reducing constraints on the positioning of thevertical scanner 202. - 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.
- As shown in FIG. 7, the
detector 46 includes a verysensitive detector diode 170 fed by acollector 172. Thedetector 46 drives a high gain amplifier and downconverter 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, thediode 170 andconverter 174 are super cooled by a conventionalsuper cooler 176. Such super coolers are commercially available devices. - 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.
- 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. - An A/
D converter 180 receives the output from theconverter 174 and produces digital data in response. Acontroller 182 receives the digital data and also receives a sense signal indicating the scan position. In response, thecontroller 182 generates a data map that is stored in amemory 184. Additionally, thecontroller 182 can output the data map through aRF transmitter 186 or on adisplay 188. For most applications, thecontroller 182 includes a pre-processor that processes the data from thedetector 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 thememory 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. - 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 t1A the sinusoidal scan will reflect energy from position P1 to thedetector 46. A linear scan assumption would place the corresponding data in thememory 184 at a location corresponding to position P2. - 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 P1, the sampling pulse arrives at time t1B, rather that time t1A, as would be the case for a linear scan rate. - 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.
- 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 count200 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. - 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. - Turning to FIG. 10, timing is referenced to the
scanning assembly 44. As noted above, thescanning assembly 44 includes a high-Q resonant scanner as the principal scanning component. As a high-Q resonant system, thescanning assembly 44 scans at its resonant frequency fSCAN. The resonant frequency depends upon the specific geometry, materials, and other characteristics of the scanner. Additionally, the scanning frequency fSCAN may vary in response to changing environmental conditions, including temperature and pressure. - As the
scanning assembly 44 scans thetarget object 54, the scanning assembly also outputs the sense signal to a phase lockedloop 100. The phase locked loop locks to the scanning frequency fSCAN and outputs a sampling signal at a sampling frequency that is synchronized to the scanning frequency fSCAN. Typically, the sampling frequency is an integral multiple of the scanning frequency fSCAN. - 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 theimager 40. Thepattern memory 102 is programmed with data that correspond to the adjusted counts corresponding to the proper memory location. - Responsive to the clock from the phase locked
loop 100, thepattern memory 102 outputs data to anedge detector 104. In response to transitions of the data, theedge detector 104 provides pulses that form the sampling clock. Near the edges of the scan, thescanning 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. - Accordingly, the
pattern memory 102 outputs data with few transitions during this portion of the scan. For example, corresponding locations in thepattern 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” - 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 acorrection 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. - 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. - FIGS. 11A and 11B show an alternative embodiment of a
millimeter wave scanner 120 that includes an array ofreflectors 122 that pivot about parallel axes. Each of thescanners 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 thereflectors 122 synchronously so that all of thereflectors 122 pivot together. Theoverall scanner 120 appears to operate very similarly to a conventional louvered window shade with individual components pivoting synchronously, although theindividual reflectors 122 typically pivot periodically. Because thereflectors 122 pivot synchronously, they form an effective reflector that is substantially larger than anyindividual reflector 122. However, because the dimension of eachindividual reflector 122 transverse to the axis of rotation is substantially smaller than that of the overall effective reflector, theindividual 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. - As shown in FIG. 12, the
scanning imager 40 may be used to identify information aboutobjects opaque barrier 134, such as a concrete wall. Where thedetector 46 is sufficiently sensitive, thedetector 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 separatemillimeter wave source 136. Theseparate source 136 may be within the viewed environment or may be placed opposite anotheropaque barrier 138. - As shown in FIG. 13, the
scanning imager 40 of FIG. 1 can be assembled with theMEMs scanner 44 of FIG. 3, aminiature super-cooler 150 and abattery 152 to produce a human portable system. Since thescanning imager 40 can operate with ambient millimeter wave energy or with a separate millimeter wave source, it is not necessary for anoperator 154 to transport a millimeter wave source. Theportable imager 40 is particularly useful for circumstances where human portability is advantageous. For example, as shown in FIG. 10, theimager 40 may be used by police or other operatives to view a hostage or otherhostile environment 158 from behind awall 160. - 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 theimager 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.
Claims (33)
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