Patent Publication Number: US-2022224845-A1

Title: Imaging device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of PCT International Application No. PCT/JP2020/024787 filed on Jun. 24, 2020, designating the United States of America, which is based on and claims priority of Japanese Patent Application No. 2019-185627 filed on Oct. 9, 2019. The entire disclosures of the above-identified applications, including the specifications, drawings and claims are incorporated herein by reference in their entirety. 
    
    
     FIELD 
     The present invention relates to imaging devices. 
     BACKGROUND 
     Conventionally, an imaging device which images an object that is hidden under people&#39;s clothes or the like and cannot be visually identified directly is known (see PTL 1, for example). 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: U.S. Pat. No. 8,835,849 
       
    
     SUMMARY 
     Technical Problem 
     Conventionally, an imaging device that (i) includes: a point light source that emits electromagnetic waves that transmit through people&#39;s clothes or the like to a person; and a detector that receives reflected waves of the electromagnetic waves emitted from the point light source, and (ii) images an object hidden under the person&#39;s clothes or the like is known. In the use of such a conventional imaging device, electromagnetic waves that transmit through people&#39;s clothes or the like are specularly reflected by a human body, a metal, or the like. The conventional imaging device can therefore image only an area, of a human body, a metal, or the like that specularly reflects the electromagnetic waves emitted from the point light source, which is defined by an angle at which the reflected waves are incident on the detector. Accordingly, with the conventional imaging device, it is difficult to image, with high accuracy, the shape of a human body, a metal, or the like that is hidden under people&#39;s clothes or the like and cannot be visually identified directly. 
     In view of this, the present invention provides an imaging device capable of imaging more accurately, than the conventional imaging device, the shape of an object that is hidden under people&#39;s clothes or the like and cannot be visually identified directly. 
     Solution to Problem 
     An imaging device according to an aspect of the present disclosure includes: a first light source including a first emission surface from which a sub-terahertz wave is emitted to a measurement target; and a first detector including a first image sensor that detects an intensity of a reflected wave generated by the measurement target reflecting the sub-terahertz wave emitted from the first emission surface. Note that the term “sub-terahertz wave” refers to an electromagnetic wave having a frequency that ranges from 0.08 THz to 1 THz, inclusive. 
     Advantageous Effects 
     With the imaging device according to an aspect of the present disclosure, it is possible to image more accurately, than a conventional imaging device, the shape of an object that is hidden under people&#39;s clothes or the like and cannot be visually identified directly. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other advantages and features will become apparent from the following description thereof taken in conjunction with the accompanying Drawings, by way of non-limiting examples of embodiments disclosed herein. 
         FIG. 1  is a diagram illustrating the relationship between attenuation and frequency when electromagnetic waves transmit through a matter. 
         FIG. 2  is a diagram illustrating the relationship between the frequency of electromagnetic waves and resolution. 
         FIG. 3  is a block diagram illustrating a configuration of an imaging device according to Embodiment 1. 
         FIG. 4  is a schematic diagram illustrating a configuration of a light source according to Embodiment 1. 
         FIG. 5  is a schematic diagram illustrating a cross section showing how a detector according to Embodiment 1 receives reflected waves. 
         FIG. 6  is a schematic diagram illustrating a cross section showing how a detector according to a first comparative example receives reflected waves. 
         FIG. 7  is a schematic diagram illustrating how the imaging device according to Embodiment 1 is installed. 
         FIG. 8  is a block diagram illustrating a configuration of an imaging device according to Embodiment 2. 
         FIG. 9  is a schematic diagram illustrating a cross section showing how a detector according to Embodiment 2 receives reflected waves. 
         FIG. 10  is a schematic diagram illustrating a cross section showing how a detector according to a second comparative example receives reflected waves. 
         FIG. 11  is a schematic diagram illustrating a cross section showing how the imaging device according to Embodiment 2 is installed. 
         FIG. 12  is a flowchart illustrating an image analysis process. 
         FIG. 13  is a schematic diagram illustrating part of an outer appearance of an imaging device according to a variation. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     (How the Inventors Conceived an Aspect of the Present Disclosure) 
     The inventors have conducted a dedicated study on an imaging device which can perform imaging that enables detection of a hazardous object (e.g., a knife or the like) hidden, for instance, under people&#39;s clothes or the like, or in a bag. 
     Hereinafter, the study conducted by the inventors will be described. 
     The inventors have studied on the relationship between attenuation and frequency when electromagnetic waves transmit through a matter used as a material for clothes, bags, or the like. 
       FIG. 1  is a diagram illustrating the relationship between attenuation and frequency when electromagnetic waves transmit through a matter. 
     As illustrated in  FIG. 1 , electromagnetic waves having a frequency of at most 1 THz can transmit through many of matters used as materials for clothes, bags, or the like. 
     As a result of the study, the inventors have obtained the knowledge that it is appropriate to utilize electromagnetic waves having a frequency of at most 1 THz in order to detect a hazardous object hidden, for instance, under people&#39;s clothes or in a bag. 
     The inventors have also studied on a frequency that achieves resolution with which the shape of a hazardous object can be imaged. 
     The relationship between the frequency (wavelength) of electromagnetic waves and resolution is expressed by Equation 1 known as Abbe&#39;s formula. 
     
       
         
           
             
               
                 
                   δ 
                   = 
                   
                     
                       λ 
                       
                         2 
                         ⁢ 
                         N 
                         ⁢ 
                         A 
                       
                     
                     = 
                     
                       λ 
                       
                         2 
                         ⁢ 
                         n 
                         ⁢ 
                         sin 
                         ⁢ 
                         θ 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     In Equation 1, δ denotes resolution, λ denotes the wavelength of electromagnetic waves, NA denotes the number of apertures of a lens, n denotes the refractive index of a medium between an object and the lens, and θ denotes a maximum angle with respect to the optical axis of a light beam incident on the lens from the object. When approximation is performed assuming d&gt;&gt;D where D denotes the entrance pupil diameter of an imaging lens and d denotes the distance from the entrance pupil position of the imaging lens to the object, Equation 2 is yielded. 
     
       
         
           
             
               
                 
                   δ 
                   = 
                   
                     
                       λ 
                       n 
                     
                     * 
                     
                       
                         
                           
                             D 
                             2 
                           
                           + 
                           
                             d 
                             2 
                           
                         
                       
                       D 
                     
                     ⁢ 
                     
                       ∼ 
                     
                     ⁢ 
                     
                       
                         λ 
                         ⁢ 
                         d 
                       
                       
                         n 
                         ⁢ 
                         D 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
             
           
         
       
     
       FIG. 2  is a diagram illustrating a graph into which Equation 2 is transformed under the conditions of D=0.5 m and d=2.5 m where n=1 assuming an air environment. 
     As illustrated in  FIG. 2 , it is possible to image the shape of a hazardous object such as a knife by utilizing electromagnetic waves having the frequency of at least 80 GHz (0.08 THz). 
     As a result of the study, the inventors have obtained knowledge that it is appropriate to utilize electromagnetic waves having the frequency of at least 0.08 THz in order to image the shape of a hazardous object such as a knife. 
     In other words, through these studies, the inventors have obtained knowledge that it is appropriate to utilize sub-terahertz waves that are electromagnetic waves having a frequency that ranges from 0.08 THz to 1 THz, inclusive, in order to perform imaging that enables the detection of a hazardous object hidden, for instance, under people&#39;s clothes or the like, or in a bag. 
     It is known that sub-terahertz waves do not affect human bodies. Therefore, utilizing sub-terahertz waves as electromagnetic waves to be emitted to a human body does not cause any problems in terms of safety. 
     On the other hand, sub-terahertz waves specularly reflect off a human body, a metal, or the like. Therefore, in imaging utilizing electromagnetic waves (sub-terahertz waves in this case) emitted from a point light source, it is difficult to image, with high accuracy, the shape of a human body, or a metallic hazardous object such as a knife, as has conventionally been the case. In order to solve this problem, the inventors have studied on the shape of a light source that emits sub-terahertz waves. As a result, the inventors have obtained the knowledge that if a light source that emits sub-terahertz waves is an area light source, it is possible to irradiate an object to be imaged with sub-terahertz waves from various angles, thereby enabling more accurate imaging, than that performed by a conventional imaging device, of the shape of an object such as a human body, a metal, or the like which specularly reflects sub-terahertz waves. 
     Based on all of the knowledge described above, the inventors have arrived at the following imaging device. 
     An imaging device according to an aspect of the present disclosure includes: a first light source including a first emission surface from which a sub-terahertz wave is emitted to a measurement target; and a first detector including a first image sensor that detects an intensity of a reflected wave generated by the measurement target reflecting the sub-terahertz wave emitted from the first emission surface. 
     The imaging device performs imaging utilizing sub-terahertz waves emitted from the first light source which is an area light source. 
     With the imaging device, it is possible to image more accurately, than a conventional imaging device, the shape of an object such as a human body, a metal, or the like which is hidden under people&#39;s clothes or the like and cannot be visually identified directly. 
     The first light source may include: at least one point light source that emits a sub-terahertz wave; and an optical element that generates, from the sub-terahertz wave emitted from the at least one point light source, a sub-terahertz wave to be emitted from the first emission surface. 
     The optical element may include a reflector that diffusely reflects the sub-terahertz wave emitted from the at least one point light source, to generate a sub-terahertz wave to be emitted from the first emission surface. 
     The optical element may include a diffuser that diffusely transmits the sub-terahertz wave emitted from the at least one point light source, to generate a sub-terahertz wave to be emitted from the first emission surface. 
     The first emission surface may be a curved surface. 
     The curved surface may include part of an inner surface of a spheroid. 
     The spheroid may be a sphere. 
     The first emission surface may include part of an inner surface of a spheroid. The first light source may include: a point light source that emits a sub-terahertz wave; and an optical element that generates, from the sub-terahertz wave emitted from the point light source, a sub-terahertz wave to be emitted from the first emission surface. The first light source may be disposed at one of two focal points of the spheroid. 
     The imaging device may further include: a second light source including a second emission surface from which a sub-terahertz wave is emitted to the measurement target; and a second detector including a second image sensor that detects an intensity of a reflected wave generated by the measurement target reflecting the sub-terahertz wave emitted from the second emission surface. 
     The first image sensor may output a first image that is based on the intensity of the sub-terahertz wave detected. The second image sensor may output a second image that is based on the intensity of the sub-terahertz wave detected. The imaging device may further include: an image processing unit which blends the first image and the second image in a lighten only mode to generate a blended image, and outputs the blended image generated. 
     The image processing unit may determine whether at least one of the first image or the second image includes an object having a predetermined characteristic. When it is determined that the at least one of the first image or the second image includes an object having the predetermined characteristic, the image processing unit may output a predetermined first detection signal. When it is determined that the at least one of the first image or the second image does not include an object having the predetermined characteristic, the image processing unit may generate the blended image and further determine whether the blended image includes an object having the predetermined characteristic. When it is determined that the blended image includes an object having the predetermined characteristic, the image processing unit may output a predetermined second detection signal. 
     The sub-terahertz wave may be an electromagnetic wave having a frequency that ranges from 0.08 THz to 1 THz, inclusive. 
     Hereinafter, specific examples of an imaging device according to an aspect of the present disclosure will be described with reference to the drawings. Each of embodiments described herein illustrates a specific example of the present disclosure. Note that the numerical values, shapes, elements, the arrangement and connection of elements, steps (processes), an order of the steps, etc. described in the following embodiments are mere examples, and do not intend to limit the present disclosure. Moreover, the figures are schematic diagrams and are not necessarily accurate illustrations. Hereinafter, the term “planar surface” refers not only to a surface that is accurately planar, but also to a surface that is substantially planar. In addition, the term “spheroid” refers not only to a surface that is accurately a spheroid, but also to a surface that is substantially a spheroid. 
     It should be noted that general or specific aspects of the present disclosure may be implemented using a system, method, integrated circuit, computer program, computer-readable recording medium such as a CD-ROM, or any combination of systems, methods, integrated circuits, computer programs, or computer-readable recoding media. 
     Embodiment 1 
     The following describes an imaging device that emits sub-terahertz waves to a person, receives reflected waves generated by the person reflecting the sub-terahertz waves, and detects the intensity of the received reflected waves, to image a hazardous object such as a knife hidden by that person under his/her clothes or the like. 
       FIG. 3  is a block diagram illustrating a configuration of imaging device  10  according to Embodiment 1. 
     As illustrated in  FIG. 3 , imaging device  10  includes light source  20 , detector  30 , and image processing unit  40 . 
     Light source  20  emits sub-terahertz waves to a measurement target (person  100  in this case). 
       FIG. 4  is a schematic diagram illustrating a configuration of light source  20 . 
     As illustrated in  FIG. 4 , light source  20  includes point light source  21  and optical element  23 . 
     Point light source  21  emits sub-terahertz waves radially in all directions in the vicinity of point light source  21 . 
     Optical element  23  has emission surface  22  and generates, from the sub-terahertz waves emitted from point light source  21 , sub-terahertz waves to be emitted from emission surface  22 . Emission surface  22  here is a planar surface. Therefore, optical element  23  functions as an area light source that emits sub-terahertz waves from emission surface  22  which is a planar surface. The emission surface from which sub-terahertz waves are emitted is a planar surface for the sake of explanation, but may be the inner surface of a spheroid to be described later, the inner surface of part of a sphere, or any curved surface. 
       FIG. 5  is a schematic diagram illustrating a cross section showing how light source  20  functions as an area light source that emits sub-terahertz waves and how detector  30  to be described later receives reflected waves generated by a measurement target. 
     As illustrated in  FIG. 5 , optical element  23  includes diffuser  24  and has a principal surface on its front side as emission surface  22 . 
     Diffuser  24  diffusely transmits the sub-terahertz waves emitted from point light source  21 , to generate sub-terahertz waves to be emitted from emission surface  22 . Diffuser  24  is a flat plate that is parallel to emission surface  22  when viewed from a macro perspective. When viewed from a micro perspective, on the other hand, tiny bumps are formed on the entire surface of diffuser  24  so that sub-terahertz waves diffuse when transmitting through diffuser  24 . 
     In light source  20 , point light source  21  is disposed at the rear side of optical element  23 , as illustrated in  FIG. 5 . The sub-terahertz waves emitted from point light source  21  enter optical element  23  from the principal surface on the rear side of optical element  23  to reach diffuser  24 . The sub-terahertz waves that have reached diffuser  24  diffusely transmit through diffuser  24 . The sub-terahertz waves that have diffusely transmitted through diffuser  24  then propagate to emission surface  22  and are emitted outward from emission surface  22 . 
     Referring back to  FIG. 3 , the description of imaging device  10  continues. 
     Detector  30  includes image sensor  31 . 
     Image sensor  31  transforms an image generated by sub-terahertz waves emitted from a subject into an electric signal that is in accordance with the intensity of the sub-terahertz waves. Image sensor  31  then generates an image that is based on the electric signal into which the sub-terahertz waves are transformed. Hereinafter, generating, by image sensor  31 , an image including an image of a subject is also referred to as “imaging”. When sub-terahertz waves emitted from light source  20  are reflected by a measurement target (person  100  in this case) which is a subject and the reflected waves reach image sensor  31 , image sensor  31  receives the reflected waves. 
     As described above, sub-terahertz waves specularly reflect off a human body, a metal, or the like. Therefore, image sensor  31  receives reflected waves from an area, of the body of person  100  and the knife hidden by person  100 , which is defined by an angle at which reflected waves resulting from the specular reflection are incident on image sensor  31 . Image sensor  31  then detects the intensity of the reflected waves received. 
     As described above, light source  20  functions as an area light source that emits sub-terahertz waves from emission surface  22 . Therefore, light source  20  can irradiate person  100  with sub-terahertz waves from various angles. Image sensor  31  can thus receive reflected waves from area  101  that is a relatively wide range of the surfaces of person  100 , i.e., the body of person  100 , and a knife hidden by person  100 . Accordingly, imaging device  10  is capable of imaging area  101  that is a relatively wide range of the surfaces of the body of person  100  and the knife hidden by person  100 . 
       FIG. 6  is a schematic diagram illustrating a cross section showing how detector  30  receives reflected waves in an imaging device according to a first comparative example, from which optical element  23  has been removed, that is, an imaging device configured to emit sub-terahertz waves emitted outward from point light source  21 , as-is. 
     With the imaging device according to the first comparative example, image sensor  31  can receive only reflected waves from area  101   a , of the surfaces of the body of person  100  and the knife hidden by person  100 , which is defined by an angle at which reflected waves resulting from specular reflection are incident on image sensor  31 , as illustrated in  FIG. 6 . Accordingly, the imaging device according to the first comparative example can image only area  101   a  that is a relatively narrow area of the surfaces of the body of person  100  and the knife hidden by person  100 . 
     Thus, imaging device  10  according to Embodiment 1 is capable of more accurately imaging the shapes of the body of person  100  and the knife hidden by person  100 , compared to the imaging device according to the first comparative example. 
     Referring back to  FIG. 3 , the description of imaging device  10  continues. Detector  30  outputs an image generated by image sensor  31  to image processing unit  40 . 
     Upon receiving the image from detector  30 , image processing unit  40  outputs the received image to an external device and also performs image processing on the received image and outputs the result of the image processing to the external device. 
     The image processing performed by image processing unit  40  may be, for example, a process of determining whether an image outputted from detector  30  includes an object having predetermined characteristics (e.g., an object having the characteristics of a knife) and outputting a predetermined detection signal (e.g., an alarm indicating that an object having the characteristics of a knife is imaged) when it is determined that the image includes an object having the predetermined characteristics. Image processing unit  40  may include, for example, a processor and memory, and perform the process by the processor executing a program stored in the memory. 
     Imaging device  10  having the above configuration is installed, for example, in a pathway at an airport, in the vicinity of an exit of a station, or the like. 
       FIG. 7  is a schematic diagram illustrating how imaging device  10  is installed in a pathway at an airport. 
     Imaging device  10  may be installed in such a manner, for example, that optical element  23  and detector  30  are embedded inside a wall along pathway  200  that is in a crank shape at an airport, as illustrated in  FIG. 7 . 
     In  FIG. 7 , sub-terahertz waves emitted from emission surface  22  inside the wall transmit through the wall and person  100  is irradiated with the sub-terahertz waves. Then, reflected waves generated by person  100  reflecting the sub-terahertz waves transmit through the wall again to be incident on detector  30 . This enables imaging device  10  to image a hazardous object such as a knife hidden under the clothes or the like of person  100  walking along pathway  200 . 
     Embodiment 2 
     The following describes an imaging device according to Embodiment 2 which has a configuration obtained by partly modifying the configuration of imaging device  10  according to Embodiment 1. Hereinafter, the imaging device according to Embodiment 2 will be described mainly focusing on the difference between the imaging device according to Embodiment 2 and imaging device  10 . 
       FIG. 8  is a block diagram illustrating a configuration of imaging device  10   a  according to Embodiment 2. 
     As illustrated in  FIG. 8 , imaging device  10   a  includes first light source  20   a  and second light source  20   b  as modified from light source  20  of imaging device  10  according to Embodiment 1, first detector  30   a  and second detector  30   b  as modified from detector  30  of imaging device  10 , and image processing unit  40   a  as modified from image processing unit  40  of imaging device  10 . 
     First light source  20   a  emits sub-terahertz waves to a measurement target (person  100  in this case). 
       FIG. 9  is a schematic diagram illustrating a configuration of first light source  20   a  as well as a cross section showing how first light source  20   a  functions as an area light source and how first detector  30   a  to be described later receives reflected waves. 
     As illustrated in  FIG. 9 , first light source  20   a  includes point light source  21   a  and optical element  23   a.    
     Point light source  21   a  emits sub-terahertz waves radially in all directions in the vicinity of point light source  21   a.    
     Optical element  23   a  has first emission surface  22   a  and generates, from the sub-terahertz waves emitted from point light source  21   a , sub-terahertz waves to be emitted from first emission surface  22   a . First emission surface  22   a  here is the inner surface of a spheroid. Therefore, optical element  23   a  functions as an area light source that emits sub-terahertz waves from first emission surface  22   a  which is the inner surface of a spheroid. 
     As illustrated in  FIG. 9 , optical element  23   a  includes reflector  24   a  and has first emission surface  22   a  as the inner curved surface of reflector  24   a.    
     Reflector  24   a  diffusely reflects sub-terahertz waves emitted from point light source  21   a  to generate sub-terahertz waves to be emitted from first emission surface  22   a . When viewed from a macro perspective, reflector  24   a  has the same or larger size but is similar in shape compared to first emission surface  22   a , and two focal points of reflector  24  match two focal points of first emission surface  22   a . When viewed from a micro perspective, on the other hand, tiny bumps are formed on the entire surface of the reflection surface of reflector  24   a  so that the reflected sub-terahertz waves diffuse. 
     In first light source  20   a , point light source  21   a  is disposed at one focal point  301   a  of the two focal points of first emission surface  22   a , as illustrated in  FIG. 9 . The sub-terahertz waves emitted from point light source  21   a  enter optical element  23   a  from first emission surface  22   a  and reaches reflector  24   a . The sub-terahertz waves that have reached reflector  24   a  are diffusely reflected by reflector  24   a . The sub-terahertz waves that have diffusely reflected by reflector  24   a  are then transmitted to first emission surface  22   a  and emitted outward from first emission surface  22   a . For example, person  100  in the vicinity of other focal point  302   a  of the two focal points of first emission surface  22   a  is irradiated with the sub-terahertz waves emitted outward. 
     Second light source  20   b  in  FIG. 8  has the same function as that of first light source  20   a , and has a shape that is in a mirrored relationship with the shape of first light source  20   a . Therefore, second light source  20   b  can be explained by replacing point light source  21   a  with point light source  21   b , optical element  23   a  with optical element  23   b , reflector  24   a  with reflector  24   b , one focal point  301   a  with one focal point  301   b , and other focal point  302   a  with other focal point  302   b  in the description of first light source  20   a , while keeping the description on the shape of second light source  20   b  unchanged. 
     Referring back to  FIG. 8 , the description of imaging device  10   a  continues. 
     First detector  30   a  includes first image sensor  31   a . First detector  30   a  is the same as detector  30  according to Embodiment 1. In other words, first image sensor  31   a  is the same as image sensor  31  according to Embodiment 1. 
     Second detector  30   b  is the same as first detector  30   a . Therefore, second detector  30   b  can be explained by replacing first image sensor  31   a  with second image sensor  31   b  in the description of first detector  30   a.    
     As described above, first light source  20   a  functions as an area light source that emits sub-terahertz waves from first emission surface  22   a . First light source  20   a  is therefore capable of irradiating person  100  in the vicinity of other focal point  302   a  with sub-terahertz waves from various angles. First image sensor  31   a  can thus receive reflected waves from area  102  that is a relatively wide range of the surfaces of person  100 , that is, the body of person  100  and a knife hidden by person  100 . Accordingly, imaging device  10   a  is capable of imaging area  102  that is a relatively wide range of the surfaces of the body of person  100  and the knife hidden by person  100 . 
       FIG. 10  is a schematic diagram illustrating a cross section showing how first detector  30   a  receives reflected waves in an imaging device according to a second comparative example which has a configuration in which reflector  24   a  of imaging device  10   a  is modified to reflector  24   aa . When viewed from a macro perspective, reflector  24   aa  here has the same shape as reflector  24   a , but when viewed from a micro perspective, the entire surface of the reflection surface of reflector  24   aa  is smoothly formed so that reflected sub-terahertz waves specularly reflect off the reflection surface. Therefore, sub-terahertz waves emitted from one focal point  301   a  and reflected by reflector  24   aa  all travel toward other focal point  302   a  no matter which portion of reflector  24   aa  the reflected sub-terahertz waves have been reflected. Therefore, in the imaging device according to the second comparative example, image sensor  31   a  can receive only reflected waves from area  102   a , which is located at other focal point  302   a , of the surfaces of the body of person  100  in the vicinity of other focal point  302   a  and the knife hidden under the clothes of person  100 , as illustrated in  FIG. 10 . Accordingly, the imaging device according to the second comparative example can image only area  102   a  that is a relatively narrow area of the surfaces of the body of person  100  and the knife hidden by person  100 . 
     Thus, imaging device  10   a  according to Embodiment 2 is capable of more accurately imaging the shapes of the body of person  100  and the knife hidden by person  100 , as compared to the imaging device according to the second comparative example. 
     Referring back to  FIG. 8 , the description of imaging device  10   a  continues. 
     First detector  30   a  and second detector  30   b  respectively output a first image and a second image respectively generated by first image sensor  31   a  and second image sensor  31   b  to image processing unit  40   a.    
     Upon receiving the first image and the second image from first detector  30   a  and second detector  30   b , respectively, image processing unit  40   a  outputs the received first image and second image to an external device, and also performs image processing on the received first image and second image and outputs the result of the image processing to the external device. 
     The image processing performed by image processing unit  40   a  may be, for example, determining whether the first image and the second image respectively outputted from first detector  30   a  and second detector  30   b  each include an object having predetermined characteristics (e.g., an object having the characteristics of a knife), and outputting a predetermined detection signal (e.g., an alarm indicating that an object having the characteristics of a knife is imaged) when it is determined that at least one of the first image or the second image includes an object having predetermined characteristics. The image processing performed by image processing unit  40   a  may also include a process of: blending the first image and the second image in a lighten only mode to generate a blended image in the case where it is determined that at least one of the first image or the second image includes an object having the predetermined characteristics; determining whether the blended image includes an object having the predetermined characteristics; and in the case where it is determined that the blended image includes an object having the predetermined characteristics, outputting a predetermined detection signal. Image processing unit  40   a  may include, for example, a processor and memory, and perform the process by the processor executing a program stored in the memory. 
     Imaging device  10   a  having the above configuration is installed, for example, in a pathway at an airport or in the vicinity of an exit of a station. 
       FIG. 11  is a schematic diagram illustrating a cross section showing how imaging device  10   a  is installed in a pathway in the vicinity of an exit of a station. 
     Imaging device  10   a  may be installed in such a manner, for example, that optical element  23   a  and optical element  23   b  are embedded inside walls along pathway  400  in the vicinity of an exit of a station, as illustrated in  FIG. 11 . More specifically, imaging device  10   a  may be installed in such a manner, for example, that optical element  23   a  is embedded inside wall  401   a  that is one of the side walls along pathway  400  and optical element  23   b  is embedded inside wall  401   b  that is the other of the side walls along pathway  400 . Thus, imaging device  10   a  may be installed in such a manner that first light source  20   a  and second light source  20   b  are provided on opposite sides of pathway  400  and first detector  30   a  and second detector  30   b  are also provided on opposite sides of pathway  400 . Accordingly, first detector  30   a  detects, using first image sensor  31   a , the intensities of reflected waves generated by a measurement target (person  100  in this case) positioned in pathway  400  reflecting sub-terahertz waves emitted from first emission surface  22   a  and sub-terahertz waves emitted from second emission surface  22   b , whereas second detector  30   b  detects, using second image sensor  31   b , the intensities of reflected waves generated by a measurement target (person  100  in this case) positioned in pathway  400  reflecting sub-terahertz waves emitted from first emission surface  22   a  and sub-terahertz waves emitted from second emission surface  22   b . In this case, it is desirable that optical element  23   a  and optical element  23   b  be installed so that other focal point  302   a  of optical element  23   a  substantially coincides with other focal point  302   b  of optical element  23   b  on the center line of pathway  400 . By thus placing optical elements  23   a  and  23   b , it is possible to irradiate an area in the vicinity of other focal point  302   a  or other focal point  302   b  (hereinafter referred to as “focal area”) with sub-terahertz waves emitted from point light source  21   a  and sub-terahertz waves emitted from point light source  21   b  from various angles. Therefore, first image sensor  31   a  and second image sensor  31   b  respectively included in first detector  30   a  and second detector  30   b  can receive reflected waves from area  103  that is a relatively wide range of the surface of person  100  walking in the focal area, that is, the body of person  100  walking in a focal area and the surface of the knife hidden by person  100 , as illustrated in  FIG. 11 . Accordingly, imaging device  10   a  is capable of imaging area  103  that is a relatively wide range of the surfaces of the body of person  100  and the knife hidden by person  100 . Moreover, first image sensor  31   a  and second image sensor  31   b  respectively included in first detector  30   a  and second detector  30   b  receive reflected waves from mutually different angles from area  103 . Accordingly, imaging device  10   a  is capable of imaging the body of person  100  and the knife hidden by person  100 , which are the same subject, from mutually different angles. 
     Hereinafter, an operation performed by imaging device  10   a  having the above configuration will be described. 
     As one example, imaging device  10   a  performs an image analysis process. The image analysis process is a process in which imaging device  10   a  images a first image and a second image and outputs, based on the imaged first image and second image, a detection signal which is an alarm indicating that an object having the characteristics of a knife is being imaged. 
       FIG. 12  is a flowchart illustrating the image analysis process performed by imaging device  10   a.    
     The image analysis process is started when a person enters a focal area. 
     When a person enters a focal area, imaging device  10   a  detects that the person has entered the focal area. Imaging device  10   a  may detect that a person has entered the focal area, for example, by receiving a signal indicating that the person has entered the focal area from an external sensor that detects a person entering the focal area. 
     When it is detected that the person has entered the focal area, first light source  20   a  and second light source  20   b  emit sub-terahertz waves at the same timing in synchronization with each other (step S 100 ). Moreover, first detector  30   a  and second detector  30   b  image the person having entered the focal area at the timing when first light source  20   a  and second light source  20   b  emit the sub-terahertz waves in synchronization with each other (step S 110 ). First detector  30   a  and second detector  30   b  then respectively output a first image and a second image to image processing unit  40   a.    
     When the first image and the second image are output, image processing unit  40   a  receives the first image and the second image that have been output and outputs the received first image and second image to an external device (step S 120 ). Image processing unit  40   a  then determines whether at least one of the first image or the second image includes an object having the characteristics of a knife which are predetermined characteristics (step S 130 ). 
     In the process of step S 130 , in the case of not determining that at least one of the first image or the second image includes an object having the predetermined characteristics (step S 130 : No), image processing unit  40   a  blends the first image and the second image in a lighten only mode to generate a blended image, and outputs the generated blended image (step S 140 ). Image processing unit  40   a  then determines whether the generated blended image includes an object having the characteristics of a knife which are the predetermined characteristics (step S 150 ). 
     In the case of determining that at least one of the first image or the second image includes an object having the predetermined characteristics (step S 130 : Yes) in the process of step S 130  and also in the case of determining that the blended image includes an object having the predetermined characteristics (step S 150 : Yes) in the process of step S 150 , image processing unit  40   a  outputs, to an external device, a detection signal which is an alarm indicating that an object having the characteristics of a knife is being imaged (step S 160 ). 
     Imaging device  10   a  ends the image analysis process when the process of step S 160  ends or in the case of not determining that the blended image includes an object including the predetermined characteristics (step S 150 : No) in the process of step S 150 . 
     OTHER EMBODIMENTS 
     As described above, the imaging device according to an aspect of the present disclosure has been described based on Embodiment 1 or Embodiment 2, but the present disclosure is not limited to these embodiments. Various modifications to the embodiments which may be conceived by those skilled in the art, as well as embodiments resulting from arbitrary combinations of elements from different embodiments may be included within the scope of one or more aspects of the present disclosure so long as they do not depart from the essence of the present disclosure. 
     (1) Embodiment 2 has described that imaging device  10   a  includes: first light source  20   a  including optical element  23   a  having first emission surface  22   a  which is the inner surface of a spheroid; and second light source  20   b  including optical element  23   b  having second emission surface  22   b  which is the inner surface of a spheroid. In contrast, an imaging device according to a variation that is another example of an aspect of the present disclosure may include: instead of optical element  23   a , a first light source according to the variation which includes first emission surface  22   a  whose shape has been changed from the inner surface of a spheroid to the inner surface of part of a sphere; and instead of optical element  23   b , a second light source according to the variation which includes second emission surface  22   b  whose shape has been changed from the inner surface of a spheroid to the inner surface of part of a sphere, as modified from imaging device  10   a  according to Embodiment 2. 
       FIG. 13  is a schematic diagram illustrating part of an outer appearance of the imaging device according to the variation. 
     In imaging device  10   b  according to the variation, first light source  20   c  according to the variation includes point light source  21   a  and optical element  23   c  having first emission surface  22   c  which is the inner surface of part of a sphere, as illustrated in  FIG. 13 . Point light source  21   a  is disposed in the vicinity of the center of the sphere. Second light source  20   d  according to the variation includes point light source  21   b  and optical element  23   d  having second emission surface  22   d  which is the inner surface of part of the sphere. Point light source  21   b  is disposed in the vicinity of the center of the sphere. 
     (2) Embodiment 1 has described that optical element  23  includes diffuser  24 . In contrast, optical element  23  may be diffuser  24  per se, as another example. In this case, the surface of diffuser  24  is emission surface  22 . 
     (3) Embodiment 2 has described that optical element  23   a  and optical element  23   b  include reflector  24   a  and reflector  24   b , respectively. In contrast, optical element  23   a  and optical element  23   b  may be reflector  24   a  per se and reflector  24   b  per se, respectively. In this case, the reflection surface of reflector  24   a  and the reflection surface of reflector  24   b  are first emission surface  22   a  and second emission surface  22   b , respectively. 
     (4) Embodiment 1 has described that light source  20  includes one point light source  21 . However, the number of point light sources included in light source  20  does not need to be limited to one and may be plural. In this case, optical element  23  generates, from sub-terahertz waves emitted from a plurality of point light sources, sub-terahertz waves to be emitted from emission surface  22 . 
     (5) Embodiment 2 has described that first light source  20   a  and second light source  20   b  respectively include one point light source  21   a  and one point light source  21   b . However, the number of point light sources included in first light source  20   a  or second light source  20   b  does not need to be limited to one and may be plural. In this case, optical element  23   a  and optical element  23   b  generate, from sub-terahertz waves emitted from a plurality of point light sources, sub-terahertz waves to be emitted from first emission surface  22   a  and sub-terahertz waves to be emitted from second emission surface  22   b , respectively. 
     (6) An aspect of the present disclosure may be not only the imaging device according to Embodiment 1 or Embodiment 2, but also an imaging method implementing steps performed by characteristic components included in the imaging device. In addition, an aspect of the present disclosure may be a program causing a computer to execute each of the characteristic steps included in the imaging method. Moreover, an aspect of the present disclosure may be a non-transitory computer-readable recording medium having such a program recorded thereon. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure can be widely used for imaging devices that image objects.