Patent Publication Number: US-2022214160-A1

Title: Edge Extraction Method and Edge Extraction Device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a national phase entry of PCT Application No. PCT/JP2019/020102, filed on May 21, 2019, which application is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an edge extraction method and an edge extraction apparatus, and relates particularly to an edge extraction method and an edge extraction apparatus that use electromagnetic waves. 
     BACKGROUND 
     Imaging using electromagnetic waves, especially an electromagnetic wave with frequencies around 0.1 to 10 THz called a terahertz wave (THz), has high spatial resolution and permeability, and is thus expected to be applied to foreign-matter inspection and non-destructive inspection in security systems or the like. In non-destructive inspection, it is important to identify the shape of an object or content in an object (hereinafter referred to simply as an “object”). Identification of the shape of an object requires information on an outline (edge) of the object. 
     Conventionally, a method has been proposed that uses information regarding rotation of polarization of a terahertz wave to extract an edge of an object in imaging using a terahertz wave (NPL 1). According to NPL 1, when, in an XYZ orthogonal coordinate system, a propagation direction of a terahertz wave is a Z-direction, a terahertz wave that has only X-polarization with an electric field changing in an X-direction is made incident on an object, a Y-polarized component of a transmitted electromagnetic wave is measured, and an edge of the object is extracted from information regarding energy that rotates from X-polarization to Y-polarization. 
     CITATION LIST 
     Non Patent Literature 
     
         
         NPL 1: Van der Valk, Nick C J, Willemine A M van der Marel, and Paul C M Planken. “Terahertz polarization imaging.” Optics letters 30.20 (2005): 2802-2804. 
       
    
     SUMMARY 
     Technical Problem 
     However, if an edge of an object is parallel to an X-polarization plane or a Y-polarization plane, rotation from X-polarization to Y-polarization or rotation from Y-polarization to X-polarization hardly occurs on that edge. For this reason, it is difficult to extract an edge of a plate-like body parallel to an X-polarization plane or a Y-polarization plane of an incident terahertz wave using the conventional method that only uses information regarding rotation between X-polarization and Y-polarization. 
     An object of embodiments of the present invention is to provide an edge extraction method and an edge extraction apparatus that make it possible to extract an edge of a plate-like body parallel to an X-polarization plane or a Y-polarization plane orthogonal to a propagation direction of an incident electromagnetic wave. 
     Means for Solving the Problem 
     To achieve the above-stated object, an edge extraction method according to embodiments of the present invention includes: a step of emitting, toward an object, an electromagnetic wave polarized only in one direction perpendicular to a propagation direction; a step of receiving a transmitted electromagnetic wave that has been transmitted through the object, using a receiving antenna; a step of calculating an intensity in the propagation direction of the transmitted electromagnetic wave based on an intensity of the transmitted electromagnetic wave received by the receiving antenna; and a step of obtaining a spatial distribution of the intensity in the propagation direction of the transmitted electromagnetic wave. 
     As an edge extraction method according to one embodiment of the present invention, the step of receiving the transmitted electromagnetic wave may include: a first step of receiving the transmitted electromagnetic wave using the receiving antenna that is in a first state where a polarization plane of the receiving antenna is parallel to an axis orthogonal to the propagation direction and a direction of polarization of an electromagnetic wave incident on the object, and forms a sharp angle with the propagation direction; and a second step of receiving the transmitted electromagnetic wave using the receiving antenna that is in a second state where the polarization plane of the receiving antenna is parallel to the axis orthogonal to the propagation direction and the direction of polarization of the electromagnetic wave incident on the object, and is orthogonal to the polarization plane of the receiving antenna in the first state, and the step of calculating the intensity in the propagation direction of the transmitted electromagnetic wave may include an intensity calculation step of calculating the intensity in the propagation direction of the transmitted electromagnetic wave based on a ratio between a first intensity of the transmitted electromagnetic wave received by the receiving antenna in the first state and a second intensity of the transmitted electromagnetic wave received by the receiving antenna in the second state. 
     As an edge extraction method according to another embodiment of the present invention, the step of emitting the electromagnetic wave may include: a step of emitting, toward the object, a first electromagnetic wave polarized only in a first direction perpendicular to the propagation direction; and a step of emitting, toward the object, a second electromagnetic wave polarized only in a second direction orthogonal to the propagation direction and the first direction, the step of receiving the transmitted electromagnetic wave may include: a step of receiving a first transmitted electromagnetic wave that is the first electromagnetic wave that has been transmitted through the object, using the receiving antenna in the first state and the receiving antenna in the second state; and a step of receiving a second transmitted electromagnetic wave that is the second electromagnetic wave that has been transmitted through the object, using the receiving antenna in the first state and the receiving antenna in the second state, and the step of calculating the intensity in the propagation direction of the transmitted electromagnetic wave may include: a first intensity calculation step of calculating the intensity in the propagation direction of the transmitted electromagnetic wave based on a ratio between a first intensity of the first transmitted electromagnetic wave received by the receiving antenna in the first state and a second intensity of the first transmitted electromagnetic wave received by the receiving antenna in the second state; and a second intensity calculation step of calculating the intensity in the propagation direction of the transmitted electromagnetic wave based on a ratio between a first intensity of the second transmitted electromagnetic wave received by the receiving antenna in the first state and a second intensity of the second transmitted electromagnetic wave received by the receiving antenna in the second state. 
     An edge extraction method according to another embodiment of the present invention may further include: a step of relatively rotating, around the object, a transmitting antenna for emitting, toward the object, an electromagnetic wave polarized only in one direction perpendicular to the propagation direction and the receiving antenna for receiving the transmitted electromagnetic wave, and repeating the step of emitting the electromagnetic wave toward the object and the step of receiving the transmitted electromagnetic wave, at a plurality of positions around the object; and a step of constructing an outline of the object based on intensity distributions in the propagation direction of the transmitted electromagnetic wave that are obtained based on intensities of the transmitted electromagnetic wave received at the plurality of positions. 
     An edge extraction apparatus according to an embodiment of the present invention includes: a transmitting antenna for emitting, toward an object, an electromagnetic wave polarized only in one direction perpendicular to a propagation direction; a receiving antenna for receiving a transmitted electromagnetic wave that has been transmitted through the object; a calculation device for calculating an intensity in the propagation direction of the transmitted electromagnetic wave based on an intensity of the transmitted electromagnetic wave received by the receiving antenna; and a processing device for obtaining a spatial distribution of the intensity in the propagation direction of the transmitted electromagnetic wave. 
     As an edge extraction apparatus according to an embodiment of the present invention, the receiving antenna may be configured to selectively enter a first state where a polarization plane of the receiving antenna is parallel to an axis orthogonal to the propagation direction and a direction of polarization of an electromagnetic wave incident on the object, and forms a sharp angle with the propagation direction, and a second state where the polarization plane of the receiving antenna is parallel to the axis orthogonal to the propagation direction and the direction of polarization of the electromagnetic wave incident on the object, and is orthogonal to the polarization plane of the receiving antenna in the first state, and the calculation device may include an intensity calculation unit for calculating the intensity in the propagation direction of the transmitted electromagnetic wave based on a ratio between a first intensity of the transmitted electromagnetic wave received by the receiving antenna in the first state and a second intensity of the transmitted electromagnetic wave received by the receiving antenna in the second state. 
     As an edge extraction apparatus according to another embodiment of the present invention, the receiving antenna may include a first receiving antenna and a second receiving antenna, the first receiving antenna may have a polarization plane that is parallel to an axis orthogonal to the propagation direction and a direction of polarization of an electromagnetic wave incident on the object and forms a sharp angle with the propagation direction, the second receiving antenna may have a polarization plane that is parallel to the axis orthogonal to the propagation direction and the direction of polarization of the electromagnetic wave incident on the object and is orthogonal to the polarization plane of the first receiving antenna, and the calculation device may include an intensity calculation unit for calculating the intensity in the propagation direction of the transmitted electromagnetic wave based on a ratio between a first intensity of the transmitted electromagnetic wave received by the first receiving antenna and a second intensity of the transmitted electromagnetic wave received by the second receiving antenna. 
     An edge extraction apparatus according to another embodiment of the present invention may further include: a rotation mechanism for relatively rotating the transmitting antenna and the receiving antenna around the object; and a three-dimensional shape construction device for constructing a three-dimensional shape of the object based on spatial distributions of the intensity in the propagation direction of the transmitted electromagnetic wave that are obtained based on intensities of the transmitted electromagnetic wave received at a plurality of positions around the object. 
     Effects of the Invention 
     According to embodiments of the present invention, an edge of a body parallel to a polarization plane orthogonal to a propagation direction of an incident electromagnetic wave can be extracted from a spatial distribution of the intensity in the propagation direction of the transmitted electromagnetic wave. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a simulation model of an edge extraction method. 
         FIG. 2A  is a diagram showing that an incident electromagnetic wave is an electromagnetic wave that has only X-polarization in the above simulation. 
         FIG. 2B  is a diagram showing electric field intensity of an X-direction component of a transmitted electromagnetic wave in the case where an incident electromagnetic wave has only X-polarization, in the results of the above simulation. 
         FIG. 2C  is a diagram showing electric field intensity of a Y-direction component of a transmitted electromagnetic wave in the case where an incident electromagnetic wave has only X-polarization, in the results of the above simulation. 
         FIG. 2D  is a diagram showing electric field intensity of a Z-direction component of a transmitted electromagnetic wave in the case where an incident electromagnetic wave has only X-polarization, in the results of the above simulation. 
         FIG. 3A  is a diagram showing that an incident electromagnetic wave is an electromagnetic wave that has only Y-polarization in the above simulation. 
         FIG. 3B  is a diagram showing electric field intensity of an X-direction component of a transmitted electromagnetic wave in the case where an incident electromagnetic wave has only Y-polarization, in the results of the above simulation. 
         FIG. 3C  is a diagram showing electric field intensity of a Y-direction component of a transmitted electromagnetic wave in the case where an incident electromagnetic wave has only Y-polarization, in the results of the above simulation. 
         FIG. 3D  is a diagram showing electric field intensity of a Z-direction component of a transmitted electromagnetic wave in the case where an incident electromagnetic wave has only Y-polarization, in the results of the above simulation. 
         FIG. 4  is a diagram showing a configuration of an edge extraction apparatus according to a first embodiment of the present invention. 
         FIG. 5  is a diagram showing a hardware configuration of an information processing device of the edge extraction apparatus according to the present embodiment. 
         FIG. 6  is a diagram illustrating arrangement of an antenna that constitutes a receiving unit of the edge extraction apparatus according to the present embodiment. 
         FIG. 7A  is a diagram illustrating an edge extraction method according to the present embodiment. 
         FIG. 7B  is a diagram illustrating the edge extraction method according to the present embodiment. 
         FIG. 8  is a diagram showing an example configuration of a processing unit of the edge extraction apparatus according to the present embodiment. 
         FIG. 9  is a flowchart showing a processing procedure performed in the edge extraction apparatus according to the present embodiment. 
         FIG. 10  is a diagram showing a main part of an edge extraction apparatus according to a second embodiment of the present invention. 
         FIG. 11  is a diagram showing a configuration of an edge extraction apparatus according to a third embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the edge extraction method according to an embodiment the present invention, an electromagnetic wave that is polarized only in one direction perpendicular to a propagation direction is made incident on an object, and an edge of the object, specifically an edge parallel to a polarization plane of an incident electromagnetic wave is extracted based on electric field intensity of a component in a propagation direction of a transmitted electromagnetic wave that has been transmitted through the object. First, the edge extraction method according to an embodiment of the present invention and a simulation thereof will be described with reference to  FIGS. 1 to 3D . 
       FIG. 1  is a diagram illustrating a simulation model of the edge extraction method according to the present embodiment. An object  20  that is subjected to edge extraction in this simulation is a uniform plate-like member made of a material with a relative permittivity ε r  of  2 , with lengths of 4 mm, 4 mm, and 0.5 m in X, Y, and Z-directions, respectively, as shown in  FIG. 1 . This object  20  has edges that are parallel to the X and Y-directions, respectively. 
     In this simulation, first, a planar wave that has only X-polarization was made incident on the plate-like object  20  that is parallel to an XY plane, as shown in  FIG. 2A , and X, Y, and Z-direction components of an electric field of a transmitted electromagnetic wave that is transmitted through the object  20  were calculated. Note that, here, the frequency of the electromagnetic wave is 300 GHz, and the incident electromagnetic wave is a planar wave. 
       FIGS. 2B, 2C, and 2D  show distributions of electric field intensity in the X, Y, and Z-directions of the transmitted electromagnetic wave in an XY plane 0.05 mm away from a bottom surface of the object  20  when an electromagnetic wave that has only X-polarization is made incident, as an incident electromagnetic wave, on the object  20 . 
     As shown in  FIGS. 2B and 2C , it can be understood that, even if an electromagnetic wave that has only X-polarization is transmitted through the object  20 , the Y-direction component hardly occurs in the electric field of the transmitted electromagnetic wave, and polarization rotation from the X-direction to the Y-direction hardly occurs. This indicates that, when an edge of the object  20  is parallel to the X-polarization plane of the incident electromagnetic wave, polarization rotation from the X-direction to the Y-direction hardly occurs on the edge. For this reason, the edge of the object  20  cannot be extracted from distributions of electric field intensity in the X-direction and the Y-direction of the transmitted electromagnetic wave with respect to the incident electromagnetic wave that has only X polarization. 
     Meanwhile, the incident electromagnetic wave is diffracted at an edge portion of the object  20 , and the transmitted electromagnetic wave thereof then has a Z-direction component. For example, as shown in  FIG. 2D , it can be understood that the Z-direction component of an electric field of the transmitted electromagnetic wave with respect to the incident electromagnetic wave that has only X-polarization strongly occurs on an edge of the object  20  that is parallel to the Y axis. 
     Next, in the simulation model shown in  FIG. 1 , a planar wave that has only Y-polarization was made incident on the plate-like object  20  parallel to the XY plane, as shown in  FIG. 3A , and X, Y, and Z-direction components of an electric field of a transmitted electromagnetic wave that is transmitted through the object  20  were calculated. Note that, here, the frequency of the electromagnetic wave is also 300 GHz, and the incident electromagnetic wave is a planar wave.  FIGS. 3B, 3C, and 3D  show distributions of electric field intensity in the X, Y, and Z-directions of the transmitted electromagnetic wave when an electromagnetic wave that has only Y-polarization is made incident, as an incident electromagnetic wave, on the object  20 , and show distributions of the electric field intensity in the X, Y, and Z directions of the transmitted electromagnetic wave in an XY plane 0.05 mm away from a bottom surface of the object  20 , similarly to  FIGS. 2B, 2C, and 2D . 
     As shown in  FIGS. 3B and 3C , it can be understood that, even if an electromagnetic wave that has only Y-polarization is transmitted through the object  20 , the X-direction component hardly occurs in the electric field of the transmitted electromagnetic wave, and polarization rotation from the Y-direction to the X-direction hardly occurs. This indicates that, when an edge of the object  20  is parallel to the Y-polarization plane of the incident electromagnetic wave, polarization rotation from the Y-direction to the X-direction hardly occurs on the edge. For this reason, the edge of the object  20  cannot be extracted from distributions of electric field intensity in the X-direction and the Y-direction of the transmitted electromagnetic wave with respect to the incident electromagnetic wave that has only Y-polarization. 
     In contrast, as shown in  FIG. 3D , it can be understood that a Z-direction component of the electric field of the transmitted electromagnetic wave with respect to the incident electromagnetic wave that has only Y-polarization strongly occurs on an edge of the object  20  that is parallel to the X-axis. 
     Based on the above simulation results, edges of an object that are parallel to the X-polarization plane and the Y-polarization plane can be extracted by measuring a spatial distribution of electric field intensity of a Z-direction component of an electric field of a transmitted electromagnetic wave obtained when an X-polarized electromagnetic wave and a Y-polarized electromagnetic wave are applied to a plate-like object parallel to an XY plane. 
     First Embodiment 
     Next, an edge extraction apparatus and an edge extraction method according to an embodiment of the present invention will be described with reference to  FIGS. 4 to 8 . An edge extraction apparatus  1  according to the present embodiment includes a transmitting antenna  11   a , a receiving device  12 , a computing processing device  13 , and a display device  14 , as shown in  FIG. 4 . The transmitting antenna  11   a  is connected to a generator device  11  and emits, toward the object  20 , an electromagnetic wave that is polarized only in one direction perpendicular to a propagation direction (Z-direction). The receiving device  12  includes a receiving antenna  12   a  for receiving a transmitted electromagnetic wave that has been transmitted through the transmitting antenna  11   a  and the object  20 . The computing processing device  13  includes a calculation unit  13   a  for calculating an electric field intensity in the propagation direction of the transmitted electromagnetic wave (Z-direction) based on the electric field intensity of the transmitted electromagnetic wave received by the receiving antenna  12   a , and a processing unit  13   b  for obtaining a spatial distribution of the electric field intensity in the propagation direction of the transmitted electromagnetic wave (Z-direction). The display device  14  visualizes and displays the spatial distribution of the electric field intensity in the propagation direction of the transmitted electromagnetic wave (Z-direction). 
     Here, the generator device  11  is a device for generating a terahertz wave. The generator device  11  may be, for example, a device that emits a laser toward a nonlinear crystal and generates a terahertz wave using an optical rectification effect in the nonlinear crystal, or a device that generates a terahertz wave by generating optical carriers in a semiconductor using an ultra-short pulse laser and modulating a photoconductive current in sub-picoseconds. 
     The transmitting antenna  11   a  is, for example, an antenna that emits, toward the object  20 , a terahertz wave generated in the generator device  11  as an electromagnetic wave that is polarized only in one direction perpendicular to the propagation direction (Z-direction), e.g., the X-direction or the Y-direction. Such a transmitting antenna  11   a  may be, for example, a dipole antenna or the like that is formed on a dielectric substrate. The transmitting antenna  11   a  may also include a lens for making an electromagnetic wave to be incident on the object  20  into a planar wave that has a wavefront perpendicular to the propagation direction. The transmitting antenna  11   a  may also be supported so as to be able to pivot around a Z-axis such that the polarization direction can be switched between the X-direction and the Y-direction. 
     For example, a horn antenna can be employed as the receiving antenna  12   a  for receiving the transmitted electromagnetic wave that has been transmitted through the object  20 . To measure Z-direction components that occur as a result of a first incident electromagnetic wave that has only X-polarization and a second incident electromagnetic wave that has only Y-polarization being diffracted on an edge of the object  20 , in the edge extraction apparatus  1  according to the present embodiment, the receiving antenna  12   a  is supported so as to be able to selectively enter a first state where a polarization plane  12   a - 1  of the receiving antenna  12   a  forms a sharp angle, e.g., +45°, with the propagation direction of the transmitted electromagnetic wave (Z-direction), and a second state where a polarization plane  12   a - 2  is orthogonal to the polarization plane  12   a - 1  of the receiving antenna in the first state, as shown in  FIG. 6 . This is because an electromagnetic wave is a transverse wave (vibration in the X-direction or Y-direction), while the Z-direction is the direction in which the electromagnetic wave travels. 
     For example, if the receiving antenna  12   a  is attached to a housing or the like of the receiving device  12  via a support structure  15  that includes a bent support member  15 - 1  and a pivoting mechanism  15 - 2  that pivots around a Z-axis, the polarization plane of the receiving antenna  12   a  can be arranged so as to form an angle of ±45° with respect to the Z-direction in an XZ plane and a YZ plane. 
     The receiving device  12  outputs the electric field intensity of the transmitted electromagnetic wave received by the receiving antenna  12   a.    
     The computing processing device  13  can be constituted by a computer that includes a computing device  131 , an internal memory  132 , an external storage device  133 , an interface (I/F) circuit  134 , an input/output (I/O) device  135 , and so on, which are connected to each other via a bus  136 , as shown in  FIG. 5 . In this case, the later-described calculation unit  13   a  and processing unit  13   b  are realized as a result of hardware resources that constitute the computer cooperating with programs installed in the internal memory  132  and other storage devices. 
     In the edge extraction apparatus  1  according to the present embodiment, the calculation unit  13   a , which is realized by the computing processing device  13 , functions as a calculation device for calculating the electric field intensity of a component in the propagation direction of the transmitted electromagnetic wave received by the receiving antenna  12   a , that is, the Z-direction, based on the electric field intensity of the transmitted electromagnetic wave. 
     The calculation unit  13   a  performs the following computing processing in order to measure a component in the propagation direction of the transmitted electromagnetic wave received by the receiving antenna  12   a , that is, the Z-direction, based on the electric field intensity of the transmitted electromagnetic wave, that is, a component that is diffracted in the Z-direction from the incident electromagnetic wave that has only X-polarization or Y-polarization. 
     First, consideration is given to the case of measuring a component that is diffracted in the Z-direction when the incident electromagnetic wave that has only X polarization is transmitted through the object  20 . The electric field intensity is denoted as A that is measured in the first state where the polarization plane ( 12   a - 1 ) of the receiving antenna  12   a  is parallel to an axis, that is, the Y-direction, orthogonal to both the propagation direction of the incident electromagnetic wave that is incident on the object  20 , that is, the Z-direction, and the direction of polarization of the incident electromagnetic wave, that is, the X-direction, and forms a sharp angle with the propagation direction (Z-direction), e.g., forms an angle of 45° with the Z-direction, as shown in  FIGS. 6 and 7A . Also, the electric field intensity is denoted as B that is measured in the second direction where the polarization plane ( 12   a - 2 ) of the receiving antenna  12   a  is parallel to the Y-direction and is orthogonal to the polarization plane  12   a - 1  of the receiving antenna  12   a  in the first state (in this state, the polarization plane  12   a - 2  is in a state of being rotated by −45° with respect to the Z-direction), as shown in  FIGS. 6 and 7B . Here, when the angle (diffraction angle) of the transmitted electromagnetic wave diffracted with respect to the incident electromagnetic wave at an edge portion of the object  20  is denoted as θ, the diffraction angle θ can be calculated using equation (1) using the electric field intensity A and the electric field intensity B that are measured by the receiving antenna  12   a  in the first state and the second state, respectively. 
     
       
         
           
             
               
                 
                   θ 
                   = 
                   
                     
                       45 
                       ° 
                     
                     - 
                     
                       arc 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         tan 
                         ⁡ 
                         
                           ( 
                           
                             B 
                             A 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Of the transmitted electromagnetic wave, a component diffracted in the Z-direction, that is, an intensity Ez in the Z-direction can be calculated based on equation (2). 
         Ez =√{square root over ( A   2   +B   2 )} sin θ  (2).
 
     Thus, the intensity in the propagation direction (Z-direction) of the transmitted electromagnetic wave can be calculated based on a ratio between the electric field intensity A (first intensity) of the transmitted electromagnetic wave received by the receiving antenna  12   a  in the first state and the electric field intensity B (second intensity) of the transmitted electromagnetic wave received by the receiving antenna  12   a  in the second state. 
     To perform the above computation, the calculation unit  13   a  of the computing processing device  13  can be constituted by a diffraction angle calculation unit  13   a   1  for calculating the diffraction angle θ based on the electric field intensities A and B that are measured when the receiving antenna  12   a  is in the first state and the second state, respectively, and a Z component calculation unit for calculating the magnitude of the Z component of the electric field of the transmitted electromagnetic wave (electric field intensity Ez in the Z direction) based on the electric field intensities A and B and the diffraction angle θ, as shown in  FIG. 8 . 
     Although the above example is in the case where an electromagnetic wave that has only X polarization is an incident electromagnetic wave, a diffraction component in the Z-direction can also be similarly measured in the case where an electromagnetic wave that has only Y polarization is an incident electromagnetic wave. 
     After the above-described electric field intensity Ez in the Z-direction of the transmitted electromagnetic wave is obtained for a plurality of areas in a space including the object  20 , the processing unit  13   b  obtains a spatial distribution of the intensity Ez in the Z-direction. The results can be displayed on the display device  14 . 
     Next, a procedure for edge extraction executed by the edge extraction apparatus  1  according to the present embodiment will be described with reference to  FIG. 9 . 
     First, the orientation of the transmitting antenna  11   a  in the XY-direction is adjusted, and an electromagnetic wave that has only X-polarization is set as an electromagnetic wave to be applied to the object  20  (S 01 ). 
     Next, the state of the receiving antenna  12   a  is set to the aforementioned first state (S 02 ), and the electric field intensity A of the transmitted electromagnetic wave is measured (S 03 ). Then, the state of the receiving antenna  12   a  is set to the second state (S 04 ), and the electric field intensity B of the transmitted electromagnetic wave is measured (S 05 ). After measuring the electric field intensity of the transmitted electromagnetic wave, the electric field intensity Ez in the Z-direction is calculated based on the electric field intensities A and B of the transmitted electromagnetic wave (S 06 ). The calculated electric field intensity Ez is stored in the storage device in association with position information on the XY plane including the measured object  20 . 
     Next, to perform measurement with an electromagnetic wave that has only Y-polarization (S 07 : YES), the polarization direction is set to Y-polarization (S 01 ), and the above-described processing from S 02  to S 06  is repeated. 
     When the above-described processing is performed for one measurement position and then the measurement is also performed for another measurement position (S 08 : YES), the position is changed and the above-described processing from S 01  to S 07  is repeated. 
     After finishing the measurement using the electromagnetic wave that has only X-polarization and the electromagnetic wave that has only Y-polarization for a plurality of measurement positions in a measurement target area including the object  20  (S 08 : NO), a spatial distribution of the electric field intensity Ez in the Z-direction in the XY plane is created based on the electric field intensity Ez stored in the storage device (S 09 ), and the series of processing ends. Note that the spatial distribution of the electric field intensity Ez in the Z-direction may be displayed on the display device  14 . 
     By creating a spatial distribution of the electric field intensity Ez in the Z-direction as described above, it is possible to extract an edge of an object that is parallel to an X-polarization plane or a Y-polarization plane of an incident electromagnetic wave, which has been conventionally difficult to extract. 
     Second Embodiment 
     Next, an edge extraction apparatus according to the second embodiment of the present invention will be described. 
     The edge extraction apparatus  1  according to the above-described first embodiment performs measurement while changing the orientation of the polarization plane of one receiving antenna  12   a  between the first state and the second state, whereas the edge extraction apparatus according to the second embodiment differs from the first embodiment in that, two receiving antennas  12   a   1  and  12   a   2  are arranged in the first state and the second state as shown in  FIG. 10 , respectively, such that the electric field intensities A and B of a transmitted electromagnetic wave in the respective states can be measured simultaneously. 
     Due to having this configuration, measurement with an electromagnetic wave that has only X-polarization and an electromagnetic wave that has only Y-polarization can be performed at a time using the two receiving antennas  12   a   1  and  12   a   2 , whereas, in the first embodiment, the measurement is performed twice using one receiving antenna  12   a . Thus, more efficient measurement can be performed. 
     Third Embodiment 
     An edge extraction apparatus  1 ′ according to the third embodiment of the present invention further includes a rotation mechanism  16  for relatively rotating the transmitting antenna  11   a  and the receiving antenna  12   a  around the object, as shown in  FIG. 11 . A processing device  13 ′ of the edge extraction apparatus  1 ′ according to the present embodiment includes a three-dimensional shape construction unit  13   c  for constructing a three-dimensional shape of the object  20  based on spatial distributions of the electric field intensities in the propagation direction (Z-direction) of a transmitted electromagnetic wave that are obtained based on electric field intensities of the transmitted electromagnetic wave received at a plurality of positions around the object  20 . 
     The edge extraction apparatus  1 ′ according to the present embodiment relatively rotates, around the object  20 , the transmitting antenna  11   a  that emits an electromagnetic wave that is polarized only in one direction perpendicular to the propagation direction and the receiving antenna  12   a  for receiving a transmitted electromagnetic wave, and constructs an outline of the object  20  based on the spatial distributions of the electric field intensities in the propagation direction (Z-direction) of the transmitted electromagnetic wave that are measured at a plurality of positions around the object  20 . 
     According to the edge extraction apparatus  1 ′ according to the present embodiment, it is possible to extract edges of an object that are parallel to an X-polarization plane or a Y-polarization plane of an incident electromagnetic wave and construct a three-dimensional shape of the object  20 . 
     REFERENCE SIGNS LIST 
     
         
         
           
               1 ,  1 ′ Edge extraction apparatus 
               11  Generator device 
               11   a  Transmitting antenna 
               12  Receiving device 
               12   a ,  12   a   1 ,  12   a   2  Receiving antenna 
               13  Computing processing device 
               13   a  Calculation unit 
               13   b  Processing unit 
               13   c  Three-dimensional shape construction unit 
               14  Display device 
               15  Support structure 
               16  Rotation mechanism