(19)
`
`(12)
`
`Europaisches Patentamt
`European Patent Oflice
`Otfice européen des brevets
`
`(11)
`
`EP 0 809123 A2
`
`EUROPEAN PATENT APPLICATION
`
`(43) Date of publication:
`26.11.1997 Bulletin 1997/48
`
`(21) Application number: 97108189.8
`
`(22) Date of filing: 21.05.1997
`
`(84) Designated Contracting States:
`DE FR GB IT
`
`(30) Priority: 22.05.1996 US 651243
`
`(71) Applicant: OMRON CORPORA11ON
`Kyoto (JP)
`
`(72) Inventors:
`- Huguenin, Richard G.
`South Deerfield, Massachusetts 01373 (US)
`- Moore, Ellen
`South Deerfield, Massachusetts 01373 (US)
`
`(54)
`
`Millimeter wave imaging system
`
`A millimeter wave detection and image genera-
`(57)
`tion system utilizing folded optics for reduced size.
`Means (212) for scanning a received image over a radi-
`ation detection arrays to improve resolution, is provided.
`
`1
`mi
`
`in:
`
`(51) 1m.c1.6; G01V 3/12, H01Q 19/06,
`G01 R 29/08
`
`- Kolodzinski, Robert
`Northampton, Massachusetts 01060 (US)
`- Kapitzky, John E.
`Florence, Massachusetts 01060 (US)
`
`(74) Representative:
`Kahler, Kurt, Dipl.-lng.
`Patentanwalte
`Kahler, Kick, Fiener et col.,
`Vorderer Anger 268
`86899 Landsberg/Lech (DE)
`
`Autofocusing (44, 164, 240) of objects in the field of
`view of the millimeter wave detection and image gener-
`ation system is also provided.
`
`’
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`1/25
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`DOJ EX. 1027
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`

`
`EPO 809123 A2
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`Description
`
`FIELD OF THE INVENTION
`
`The present invention relates to an electromagnetic
`wave imaging system, in particular to a millimeter wave
`imaging system. More particularly the present invention
`relates to a compact camera for detecting millimeter
`wavelength radiation and generating images therefrom.
`
`BACKGROUND OF THE INVENTION
`
`Plastic weapons and explosives concealed under
`clothing present unique challenges to conventional con-
`traband detection technology Non-metallic, non-mag-
`netic objects cannot be detected by conventional
`systems that use low frequency magnetic fields. Com-
`mon techniques that can reveal these types of contra-
`band involve x-rays or other ionizing radiation. These
`techniques are, or are perceived to be, hazardous and
`thus, their use has been limited.
`It is known, however, that all objects naturally emit
`and reflect a broad spectrum of electromagnetic radia-
`tion. The level of radiation emitted or reflected by an
`object is determined by a number of factors such as the
`material and surface properties of the object and by its
`temperature. It is also known that the human body is an
`especially good emitter of millimeter waves, i.e., electro-
`magnetic radiation characterized by wavelengths in the
`range of 1 to 10 millimeters with corresponding frequen-
`cies of 300 GHz to 30 GHz. By contrast, metal objects
`are very poor emitters and excellent reflectors of millim-
`eter waves. Dielectric objects such as plastics, ceram-
`ics, plastic explosives, powdered drugs, etc., have
`emission properties that are between those of
`the
`human body and metals. Most clothing and many build-
`ing materials are virtually transparent
`to millimeter
`waves.
`
`Since all objects either reflect or emit millimeter
`waves, two different techniques have evolved to exploit
`one or the other of these properties. Passive imaging
`utilizes a camera with high sensitivity to detect the natu-
`ral millimeter wave emissions from objects or people,
`and requires a sensitive receiver to distinguish small dif-
`ferences in emissions. The emitted radiation is proc-
`essed by receivers in the camera, which convert the
`millimeter wave signal down to video. The strength of
`the video signal is roughly proportional to the power
`level
`in the emitted radiation. Different video signal
`strength is encoded to be displayed as pixels ranging
`from black through gray to white on a visual display.
`A hidden plastic weapon in a person's clothing
`stands out as a dark (bright) silhouette against a bright
`(dark) body. Whether the warm body is dark or bright
`depends on the data acquisition program. Passive
`imaging is preferred when (1) irradiating living subjects
`is unacceptable and (2) when the user does not want to
`cause transmissions from the imaging system which
`could be detected by a monitor. The imaging system
`
`described in this disclosure is a passive imaging cam-
`era.
`
`Active imaging takes advantage of reflection of mil-
`limeter waves from objects or people. In this method the
`user transmits signal from a millimeter wave generator
`to the subject under investigation. The millimeter waves
`are reflected back to the camera, where they are con-
`verted down to video signal and processed to pixels of
`varying intensity or color on a display. The transmitted
`signal which illuminates the subject may be attached to
`the camera or it may be set up separately. Active imag-
`ers may use the same high sensitivity camera as is
`used in passive imaging.
`The foregoing properties have been exploited in
`systems utilizing millimeter waves for contraband detec-
`tion. Examples of such systems are described in the fol-
`lowing U.S. Patents.
`U.S. Patent No. 5,227,800 discloses an active
`imaging millimeter wave contraband detection system.
`In that system, millimeter wave generators are provided
`for illuminating objects in the field of view of a millimeter
`wave camera. Millimeter waves reflected off objects in
`the field of view are received by the camera module and
`processed in a focal plane receiver array. Signal
`strength measurements by each element in the array
`are used in forming an image of objects in the field of
`view.
`
`U.S. patent No. 5,047,783 discloses a millimeter
`wave passive imaging system having improved image
`resolution. A rotating refractive wedge is provided to
`redirect the signal energy incident on the focal plane
`array In redirecting the signal energy the area sampled
`by each element of the focal plane array is expanded.
`Image resolution is thereby improved by processing the
`signals in this expanded area.
`While the foregoing systems represent significant
`advances over other prior art imaging techniques, such
`systems are often not well suited for use in a compact
`unit. Thus, the image resolution techniques of the prior
`systems require additional lenses and motors and thus
`result in increased size and complexity.
`
`SUMMARY OF THE INVENTION
`
`The present invention satisfies to a great extent the
`foregoing need for a compact millimeter imaging system
`by providing a method of forming an image based on
`incoming millimeter wavelength radiation wherein a sig-
`nal received at a first surface of a transreflector plate is
`filtered by the transreflector plate so that only those sig-
`nals having a preselected polarization are passed; the
`polarized signals are then reflected and the polarization
`sense is rotated ninety degrees by a load switching twist
`reflector;
`the reflected rotated polarized signals are
`reflected from a second surface of the transreflector
`
`plate and then received and detected at a radiation
`detector assembly. The signals received and detected
`at the radiation detector assembly are used to generate
`an image. Resolution of the image is improved by scan-
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`ning the load switching twist reflector to redirect the
`reflected,
`rotated polarized signals over an array of
`detectors. Piezoelectric transducers may be used to
`move the load switching twist reflector to produce the
`scanning.
`In another aspect of the im/ention, the distance
`between the transreflector plate and the load switching
`twist reflector is adjusted to focus the reflected rotated
`polarized signals on the radiation detector assembly.
`This adjustment can be performed using an ultrasonic
`range sensor to measure the distance to the subject
`under investigation.
`In its apparatus aspects, an imaging system is pro-
`vided having a millimeter wavelength bandpass filter for
`passing signals of a predetermined frequency in the
`range of 30 gigahertz (GHz) to 300 GHz. A transreflec-
`tor plate having a polarized filtering first surface is pro-
`vided for filtering the bandpass filtered signal to pass
`signals of predetermined polarization. A layered load
`switching twist reflector having a reflecting layer and a
`polarization rotating layer reflects and rotates the polar-
`ized signals filtered by the transreflector. The reflected
`and rotated signals are then reflected from a reflective
`second surface of the transreflector and received and
`
`detected by a radiation detector assembly. Piezoelectric
`transducers are mounted to the load switching twist
`reflector and operated to improve resolution of the
`image by moving the twist reflector to redirect and thus
`scan a received image over the radiation detector
`assembly. Other embodiments may use voice coils or
`linear motors in place of the piezoelectric transducers.
`Adjustment means may also be provided in the
`foregoing system for adjusting the distance between the
`transreflector and the load switching twist reflector to
`focus the signals received at the radiation detector
`assembly.
`One object of the present invention is to provide a
`compact contraband detecting imaging system for gen-
`erating millimeter wave images for detecting nonmetal-
`lic, non-magnetic objects in addition to metallic objects,
`and to provide a mechanism for redirecting an image
`received at an array of radiation detectors to improve
`image resolution.
`It is another object of the invention to provide a mil-
`limeter wave camera as described in the preceding par-
`agraph with folded optics for reducing the size of the
`camera.
`
`It is another object of the invention to provide a mil-
`limeter wave imaging system which has a mechanism
`for automatically focusing the system.
`These and other objects, advantages and features
`of the invention will become more readily apparent, and
`the nature of the invention may be more clearly under-
`stood, by reference to the following detailed description
`of the invention, the appended claims, and to the sev-
`eral drawings attached herein.
`
`10
`
`15
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a block diagram representation of the mil-
`limeter wave camera of a preferred embodiment of the
`present invention.
`FIG. 2 is a block diagram representation of a con-
`sole unit in accordance with a preferred embodiment of
`the present invention.
`FIG. 3 is a partial cutaway perspective view of a mil-
`limeter wave camera of a preferred embodiment of the
`present invention.
`FIG. 4 is a plan view of a portion of the focal plane
`array of the millimeter wave camera of. FIG. 3.
`FIG. 5 is a block diagram of the focal plane array
`signal processing circuitry.
`FIG. 6 is a side elevation view of the millimeter
`wave camera of FIG. 3.
`
`FIG. 7 is an exploded view of the load switching
`twist reflector of the millimeter wave camera of FIG. 3.
`
`FIG. 8 is an illustration of a diode array of the load
`switching twist reflector of FIG. 7.
`FIG. 9 is a rear elevation view of the millimeter wave
`camera of FIG. 3.
`FIG. 10 is a cross-sectional view of the millimeter
`
`wave camera taken along the lines x-x of FIG. 6.
`FIG. 11 is a perspective view of the local oscillator
`of the millimeter wave camera of FIG. 10.
`
`FIG. 12 is a partial cutaway perspective view of the
`CCD camera housing assembly of the millimeter wave
`camera of FIG. 10.
`FIG. 13 is a functional flow chart of the software
`
`operation of the radiometric imaging processor of FIG.
`2.
`
`FIG. 14 is a functional flow chart of the software
`
`operation of the super resolution processor of FIG. 2.
`
`DETAILED DESCRIPTION OF A PREFERRED
`EMBODIMENT OF THE PRESENT INVENTION
`
`Referring now to the Figures wherein like reference
`numerals indicate like elements, FIG. 3 shows a millim-
`eter wave camera 20 in accordance with a preferred
`embodiment the present invention. The millimeter wave
`camera 20 of FIG. 3 is designed to receive millimeter
`wavelength radiation from objects in the field of view of
`the camera and generate images therefrom.
`Shown in FIGS.
`1 and 2 are block diagram repre-
`sentations of the optical, electrical, autofocus and cool-
`ant systems of a millimeter wave camera system in
`accordance with a preferred embodiment of the present
`invention. The millimeter wave camera system com-
`prises the millimeter wave camera 20, FIG. 1, and a
`console unit 206, FIG. 2, for supplying power and con-
`trol signals to the millimeter wave camera 20 and for
`outputting images generated by the system.
`Addressing first the optical system, in FIG. 1, radia-
`tion 22 from the field of view is received first at a macro
`
`In the present embodiment of the invention,
`lens 208.
`the macro lens is an add-on lens which is provided to
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`extend the focal range of the millimeter wave camera
`20. The radiation from the field of view is transmitted
`
`next to the passive optics 210 of the millimeter wave
`camera 20, either directly or via the macro lens 208.
`The passive optics 210 includes the bandpass filter 24,
`lens 28 and transreflector plate 34, the operation of
`which will be described below in connection with FIG. 3.
`
`When millimeter wave images are to be generated,
`the signals 36 (FIG. 1) passed through the passive
`optics 210 are transmitted then to the switchable twist
`reflector 108 (FIG. 7) of the load switching twist reflector
`38 (FIG. 1). The switchable twist reflector 108 is biased,
`via the voltage source 144, to reflect the signals 36. The
`reflected signals are then transmitted to the focal plane
`array 30 for processing by the array antennas. The pie-
`zoelectric transducer assemblies 212 are connected to
`
`the load switching twist reflector 38 to improve resolu-
`tion of the images received at the focal plane array, dur-
`ing
`generation
`of millimeter wave
`images,
`in
`accordance with the method described below in con-
`nection with FIG. 7. The switchable twist reflector 108
`
`includes a diode array 133 (FIG. 8) which is biased
`under control of signals transmitted from the console
`206 (FIG. 2) via the STFI driver 214.
`The signals 36 reflected from the switchable twist
`reflector 108 and received at the focal plane array 30
`include the radiation signals from the field of view as
`well as a locally oscillated signal of approximately 47
`GHz generated by the local oscillator 166. The local
`oscillator signal is combined with the radiation signals
`from the field of view in the focal plane array 30. As
`described below in connection with FIG. 12, the local
`oscillator signal serves to down convert the radiation
`signals 22 received from the field of view to approxi-
`mately 2.5 GHz. Power is supplied to the local oscillator
`166 by the local oscillator bias supply 216.
`The signals as processed in the focal plane array
`30 are transmitted next
`to a video multiplexer 218
`wherein the signals are packaged for transmission and
`processing at the console 206 (FIG. 2). These signals
`are transmitted to the console through the millimeter
`wave camera input output ports 220 (FIG. 1). At the
`console 206, as shown in FIG. 2, the packaged signals
`are stored in the console host computer 222, wherein
`they can be operated upon by a radiometric imaging
`processor 224.
`The radiometric imaging processor is responsible
`for performing five operations, including: 1) controlling
`the data acquisition hardware (i.e., A/D converters, load
`switching twist reflector 38 (FIG. 1), noise generator 174
`and piezoelectric transducers 212) in response to com-
`mands from the host computer, 2) sequencing and digi-
`tizing data during the load, noise and sixteen scanned
`positions‘ portion of a data frame, 3) calibrating the data
`of the sixteen scanned positions via information col-
`lected during the load and noise portions of the data
`frame, 4) arranging all of the data into a coherent image,
`and 5) placing the final image in a buffer, not shown, for
`image processing and transmission for display.
`
`Referring now to FIG. 3, the structure and operation
`the millimeter wave camera components are
`of
`described herein with reference to radiation signals,
`generally designated by reference character 22,
`received from the field of view. The radiation signals 22,
`emitted by an object in the field of view, are incident first
`upon a bandpass filter 24 of the millimeter wave camera
`20. The bandpass filter 24 permits millimeter wave-
`length signals 26 of a desired frequency range to pass
`while blocking signals outside the desired frequency
`range.
`In the present embodiment, the bandpass filter
`24 is selected to pass signals in the frequency range of
`91.5 GHz to 96.5 GHz.
`
`The signals 26 that pass through the bandpass filter
`24 are then focused by a lens 28 for transmission to a
`focal plane array 30. The lens 28 of the present embod-
`iment of the invention is a biconvex hyperbolic type lens
`selected for focusing millimeter wavelength radiation in
`the desired wavelength range.
`Prior to being received at the focal plane array 30,
`however, the focused signals 32 are transmitted to a
`transreflector plate 34. The transreflector plate 34 is
`polarized to permit only those signals 36 having a
`desired polarization (i.e. vertical, horizontal or circular)
`to pass through. The millimeter wavelength signals that
`do not have the desired polarization are reflected by the
`transreflector plate 34.
`In the camera of the present
`embodiment the transreflector plate 34 is selected to
`pass vertically polarized signals.
`The polarized signals 36, passed by the transreflec-
`tor plate 34, are incident upon and reflected from a
`switchable twist reflector 108 (FIG. 7) of a load switch-
`ing twist reflector 38.
`In addition to reflecting the polar-
`ized signals, the switchable twist reflector 108 shifts the
`polarization of the reflected signals by ninety degrees,
`resulting in rotated polarized signals 40.
`The rotated polarized signals 40 are transmitted
`back to the transreflector plate 34 and, since they are
`horizontally polarized, are reflected from the transreflec-
`tor plate 34. The reflected, rotated polarized signals 42
`are then transmitted to the focal plane array 30 where
`the millimeter wave signals are detected.
`The focal plane array 30 of the millimeter wave
`camera of FIG. 3 is preferably a 16 x 16 element array.
`These 256 elements are scanned in a 4 x 4 raster scan
`
`(see FIG. 13) to produce 4096 pixels. The 4096 pixels
`are processed to produce an image having 1024 pixels.
`The resolution (see FIG. 14) of the processed image is
`3mm x 3mm at a distance of 0.5m, and 19mm x 19mm
`at a distance of 3.0m.
`
`The focal plane array 30 is constructed from a layer
`dielectric such as Duroid”" (Rogers Corporation) with
`metallization on both sides. The antennas 64 and cir-
`
`cuits are made by etching away the metal in the appro-
`priate pattern. FIG. 4 shows the RF portion of the focal
`plane array 30, which is in two separate layers. The lin-
`early tapered slot antennas 64 are outlined in dashed
`lines because they are on the back side of the circuit.
`The cross hatching denotes metallization on the back
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`sides, while the areas without cross hatching are dielec-
`tric. The top side of the antenna portion has no metalli-
`zation. The antenna array in this circuit has slotline
`architecture. Such antennas 64 are referred to in the art
`
`as endfire traveling wave slot antennas" as originally
`described by P.J. Gibson, "The Vivaldi Aerial," Proc. of
`the European Microwave Conf., Brighton, UK (1979),
`pp. 101-105.
`The rest of the RF circuit uses microstrip architec-
`ture. As shown in FIG. 4, the circuit elements are metal
`on a dielectric background, with a ground plane under
`the whole portion. The electric field is coupled from the
`antenna to the mixer 66 via a transition 68 and a trans-
`
`mission Iine 70. D.C. bias is applied to the mixer by the
`bias pads and lines 72. The intermediate frequency (IF)
`portion of the circuit extends to the left of the bias pads
`and contains, as shown in greater detail in FIG. 5, the IF
`amplifiers 76, detectors 78, and video amplifiers 80. The
`differential video output comes out of the millimeter
`camera output 220 (FIG. 1), and is applied through the
`console host 222 (FIG. 2) to the video output device 228
`(FIG. 2). The FIG. 4 circuit board is a standard printed
`circuit board which also contains circuit elements and
`
`I.C.'s needed to form the focal plane array processing
`circuit depicted in FIG. 5. The back side of the IF portion
`of the dielectric circuit is completely metallized to form a
`ground plane. The IF and microstrip RF portions of the
`focal plane array 30 are mounted to a metal tray 74 for
`stiffness. The metal trays 74 are stacked in precise
`spacing to form focal plane array 30, as shown in FIG.
`3, so that the correct horizontal and vertical distance
`between antennas 64 is maintained. The antennas 64
`
`extend out beyond the metal trays 74 (FIG. 4).
`The focal plane array 30 which may be used in the
`system of the present invention is of the type described
`in U.S. Patent No. 5,227,800, issued July 13, 1993 and
`U.S. Patent No. 5,202,692, issued April 13, 1993 the
`disclosures of which are incorporated herein by refer-
`ence.
`
`The passive millimeter wave camera of this inven-
`tion must detect weak signals. To enhance weak sig-
`nals, filter out unwanted noise, and get high dynamic
`range,
`the focal plane array has a superheterodyne
`receiver at each of
`the antennas 64. This type of
`receiver is described by Tiuri, M.E., "Radio Telescope
`Receivers", Ch. 7, Radio Astronomy, by Kraus, J.D., at
`236-293 (McGraw-Hill 1966).
`The superheterodyne receiver has a subharmonic
`mixer and local oscillator 166 (see FIG. 11). The band-
`width of the incoming millimeter wave signal spans 91.5
`GHz to 96.5 GHz. The local oscillator signal of approxi-
`mately 47 GHz is multiplied up to approximately 94
`GHz, and is mixed with the upper and lower sidebands
`of approximately 2.5 GHz bandwidth.
`In other words,
`this camera has a double sideband receiver with an IF
`
`of approximately 2.5 GHz. The local oscillator signal is
`launched from a feed 170 and is detected by the focal
`plan array 30 (FIGS. 3, 4) without reflecting from the
`load switching twist reflector 38 (FIG. 3) or the transre-
`
`flector plate 34.
`The use of local oscillators in this manner, in millim-
`eter wave cameras,
`is well know in the art and is
`described in a number of U.S. Patents including U.S.
`Patent No. 5,227,800, issued July 13, 1993 and U.S.
`Patent No. 4,910,528, issued March 20, 1990, the dis-
`closures of which are incorporated herein by reference.
`As shown in FIG. 11,
`in the present embodiment
`the local oscillator consists of two signal generators
`178, 180. Each signal generator produces a signal of
`approximately 47 GHz. The power of each signal gener-
`ator 178, 180 is combined in power combiner 182 to
`produce a single approximately 47 GHz signal having
`the combined power of each of the signal generators
`178, 180. The phase of each of the signal generators
`178, 180 is locked by the process of injection locking
`whereby leakage between the two signal generator
`sources is used to lock the phase.
`The combined signal of approximately 47 GHz is
`passed through an isolator 184 to suppress unwanted
`reflections. The signal is then provided via a wave guide
`168 to a horn antenna 170 for transmission to the focal
`
`plane array 30. Electrical leads 186, 188, 190, 192 are
`provided for driving signal generators 178, 180. Further
`a cooling block 194 is provided for dissipating heat gen-
`erated by the signal generators 178, 180. Water, or
`other suitable fluid,
`is provided into the cooling block
`194 through input hose fitting 196. The fluid passes
`through a channel, not shown, in the cooling block 194
`before passing out of the hose fitting 198.
`Another aspect of the superheterodyne receiver of
`the focal plane array 30 is the process of load compari-
`son, or Dicke switching, also described by Tiuri (Id.).
`The process of load comparison enables the effects of
`gain fluctuations to be compensated. The noise source
`174 (FIG. 12) is at 94 GHz and is transmitted directly
`into the focal plane array antennas 64 (FIG. 4) without
`reflecting off the switchable twist reflector 108 (FIG. 7)
`of the load switching twist reflector 38 (FIG. 3) or off the
`transreflector 34 (FIG. 3). This occurs during that part of
`the duty cycle when the switchable twist reflector 108 is
`transmitting. The noise signal is received and detected
`by the focal plane array antennas 64 (FIG. 4). This noise
`signal is of a known strength and is used as a standard
`to measure the strength, or noise temperature, of all
`other incoming millimeter wave signals from the subject.
`During that part of the duty cycle when the noise is
`being detected, the switchable twist reflector 108 (FIG.
`7) transmits the millimeter wave signal from the scene to
`the silicon carbide layer 128 (FIG. 7), where it dissi-
`pates.
`In operation, the noise generator 174 shown in FIG.
`12 is used to normalize the gain between the antennas
`64 of the focal plane array 30, FIG 3. The normalization
`process is accomplished by first operating the load
`switching twist reflector 38, FIG. 3, to transmit the sig-
`nals 36 received from the field of view. Vlfith the signals
`36 from the field of view transmitted by the load switch-
`ing twist reflector 38, no radiation from the field of view
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`will be received at focal plane array 30. The noise gen-
`erator 174 then transmits a signal, of a known level and
`having the same character as the signals being meas-
`ured from the field of view, toward the focal plane array
`30 via the horn antenna 176. When the signal having
`the known signal level is received at each of the anten-
`nas 64 of the focal plane array 30 the gain between
`channels can then be normalized. This process is com-
`monly referred to as flat fielding. A metallized quartz
`disk 204 is provided to prevent the passage of RF fre-
`quencies which would otherwise interfere with the cam-
`era operation.
`In the preferred embodiment of
`the
`present invention, the quartz disk is gold plated.
`The load switching twist reflector 38, as shown in
`FIG. 7, is composed of three subcomponents: an alumi-
`num plate 106, a switchable twist reflector 108 and a
`quasioptical load 107. The aluminum plate 106 serves
`as a mounting substrate for the switchable twist reflector
`108 and quasioptical load 107, and as a base for con-
`necting the bearing assemblies. The switchable twist
`reflector 108 consists of three sandwiched layers includ-
`ing a quasioptical monolithic tile array layer 132, a per-
`forated plate filter 134, and a quarter wave plate 110.
`The quarter wave plate 110 operates to change the
`polarization state of the polarized signals 36 received
`from the transreflector plate 34. The switchable twist
`reflector 108 can be biased to reflect received signals or
`to transmit received signals. As will be described below,
`through the use of the switchable twist reflector 108, the
`radiation received at the focal plane array 30 can be
`switched between the load, i.e., radiation received from
`the field of view, and a known signal generated for use
`in adjusting the gain between antennas 64 of the focal
`plane array 30. The quasioptical load 107 comprises a
`silicon carbide layer 128 and a fused silica layer 130.
`The silicon carbide layer 128 is mounted to the alumi-
`num plate 106.
`The millimeter wave radiation is transformed from
`
`linear vertical polarization to circular polarization by the
`quarter wave plate 110. It is then incident on the perfo-
`rated plate high pass filter 134, which reflects all fre-
`quencies below the cutoff frequency, about 80 GHz.
`After reflection by the perforated plate filter 134, the
`beam is incident on the quasioptical monolithic tile array
`layer 132. FIG. 8 shows a diode array 133 that is on the
`surface of the monolithic tile array layer 132 which faces
`the lens 28 (FIG. 3). The diode array 133 employed is
`similar to that described in U.S. Patent No. 5,170,169
`and in the article "Ouasi-Optical Millimeter Wave Hybrid
`and Monolithic PIN diode Switches", K. D. Stephan, P.H.
`Spooner and PF Goldsmith,
`IEEE Trans. Microwave
`Theory Tech., vol. 41, pp. 1791-1798, Oct. 1993.
`As shown in FIG. 8, the building blocks of the diode
`array 133 are 12.7 x 12.7 mm tiles comprising a GaAs
`substrate 135, a passive mesh on one side, and an
`active mesh on the other side. The mesh is a two-
`
`dimensional array of thin metal strips arranged in paral-
`lel and perpendicular fashion. The active mesh has a
`PIN diode 142 located between the nodes 138 of the
`
`mesh in both directions. The passive mesh has the
`identical metallization pattern as the active mesh, but
`has solid metallization in place of the PIN diodes 142.
`The active and passive meshes are fabricated onto the
`GaAs substrate 134. A mesh by itself is inductive, and
`performs as a high pass filter. Two passive meshes in
`series form a quasioptical bandpass filter.
`The switching function of the diode array 133 is
`controlled by the PIN diodes 142. These devices have a
`high impedance when they are unbiased or reversed
`biased, and a very low impedance when fonivard biased.
`The monolithic tile array 132 transmits the millimeter
`wave signal when the diode array 133 is forward biased
`and reflects the signal when the diode array 133 is
`reverse or zero biased. The details of how the transmis-
`sion and reflection works are described in U.S. Patent
`No. 5,170,169.
`The diodes 142 are biased by the voltage source
`144 which connects to the bias leads 146 on the circuit
`
`board 136 surrounding the array of tiles. In the ON state
`the signals transmitted through the monolithic tile array
`layer 132 (FIG. D are received at the fused silica layer
`130. The first surface of the fused silica layer 130 oper-
`ates as a mounting surface for the printed circuit board
`136 (FIG. 8) which provides the bias to the monolithic
`tile array 132. The fused silica layer 130 also serves as
`an antireflection layer to prevent the reflection of signals
`from the silicon carbide layer 128.
`The signals transmitted through the monolithic tile
`array layer 132 are then received at the silicon carbide
`layer 128 which operates as a load material to absorb
`the transmitted signals. In addition, silicon carbide is a
`rigid material and thus the silicon carbide layer 128 is
`provided to allow the switchable twist reflector 108 to be
`driven by the piezoelectric transducer to high resonant
`frequencies which could not be achieved with the alumi-
`num plate 106. The silicon carbide layer 128 also serves
`as thermoccnductor to dissipate heat generated by the
`diodes in the monolithic tile array layer 132.
`To facilitate the management of power require-
`ments, the diode array layer 132 of the present embodi-
`ment is biased in quadrants. As shown in FIG. 9, four
`diode array drivers 118, 120, 122, 124, are provided for
`biasing the four quadrants of the diode array layer 132.
`A switchable twist reflector which may also be used in
`the millimeter wave camera of the present invention is of
`the type described in U.S. Patent No. 5,170,169 issued
`December 8, 1992, the disclosure of which is incorpo-
`rated herein by reference.
`As shown in FIG. 9, encasing the focal plane array
`30 of the millimeter wave camera 20 is a cooling block
`126. The cooling block 126 is mounted to the focal plane
`array mounting frame 48. When the focal plane array
`mounting frame 48 is shifted the focal plane array 30
`(FIG. 3) is thereby also shifted. As heat is generated by
`the focal plane array 30, the fluid passing through the
`cooling block 126 absorbs and carries away the heat. A
`heat exchanger, not shown, is provided in the fluid con-
`duit for dissipating the heat absorbed by the fluid. A
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`EP0809123A2
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`is also provided for circulating the
`pump, not shown,
`fluid through the cooling block 126.
`is pro-
`In operation, water, or other suitable fluid,
`vided into the cooling block 126 through the inlet hose
`fitting 150. The fluid then passes through an internal
`channel, not shown, at one side of the cooling block 126
`and then out of an outlet hose fitting 154. The fluid is
`transmitted, via a hose 156, to an inlet hose fitting 158
`at the opposite side of the cooling block 126. The fluid
`then passes through a second internal channel, not
`shown, within the cooling block 126 and then out of the
`outlet channel 152.
`
`Also shown in FIG. 9 is a drive gear 160 secured to
`the threaded rod 52. A second drive gear 162, secured
`to the focusing motor 50 (FIG. 3), drives the drive gear
`160. A motor controller 164 provides drive signals to the
`focusing motor 50 to drive the threaded rod 52 to move
`the focal plane array mounting frame 48 forwardly or
`rearwardly. The drive signals provided to the focusing
`motor are generated by the motor cont