Patent Publication Number: US-2021181013-A1

Title: Electromagnetic wave detection apparatus and information acquisition system

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Japanese Patent Application No. 2018-94103 filed on May 15, 2018, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to an electromagnetic wave detection apparatus and an information acquisition system. 
     BACKGROUND 
     Devices such as a DMD (Digital Micromirror Device) that include an element for changing a propagation direction of electromagnetic waves incident on each pixel are known. For example, an apparatus that forms a primary image of an object on a surface of a DMD and then forms a secondary image of the primary image formed on the surface of the DMD on a surface of a CCD (Charge-Coupled Device) via a lens is known (see PTL 1 set forth below). 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent No. 3507865 
     SUMMARY 
     An electromagnetic wave detection apparatus according to a first aspect includes: 
     a first propagation unit comprising a plurality of pixels along a reference surface and configured to propagate electromagnetic waves incident on the reference surface in a particular direction at each of the pixels; 
     a second propagation unit including a first surface configured to propagate electromagnetic waves incident from a first direction in a second direction and propagate electromagnetic waves propagated in a third direction in a fourth direction, a second surface configured to separate electromagnetic waves propagated in the second direction and propagate the electromagnetic waves in the third direction and a fifth direction, a third surface configured to emit electromagnetic waves propagated in the fourth direction, a fourth surface configured to emit electromagnetic waves propagated in the fifth direction towards the reference surface and to propagate electromagnetic waves incident again from the reference surface in a sixth direction, a fifth surface configured to propagate electromagnetic waves propagated in the sixth direction in a seventh direction, and a sixth surface configured to emit electromagnetic waves propagated in the seventh direction; 
     a first detector configured to detect electromagnetic waves emitted from the third surface; and 
     a second detector configured to detect electromagnetic waves emitted from the sixth surface. 
     An information acquisition system according to a second aspect includes: 
     an electromagnetic wave detection apparatus that includes
         a first propagation unit comprising a plurality of pixels along a reference surface and configured to propagate electromagnetic waves incident on the reference surface in a particular direction at each of the pixels,   a second propagation unit including a first surface configured to propagate electromagnetic waves incident from a first direction in a second direction and propagate electromagnetic waves propagated in a third direction in a fourth direction, a second surface configured to separate electromagnetic waves propagated in the second direction and propagate the electromagnetic waves in the third direction and a fifth direction, a third surface configured to emit electromagnetic waves propagated in the fourth direction, a fourth surface configured to emit electromagnetic waves propagated in the fifth direction towards the reference surface and to propagate electromagnetic waves incident again from the reference surface in a sixth direction, a fifth surface configured to propagate electromagnetic waves propagated in the sixth direction in a seventh direction, and a sixth surface configured to emit electromagnetic waves propagated in the seventh direction,   a first detector configured to detect electromagnetic waves emitted from the third surface, and   a second detector configured to detect electromagnetic waves emitted from the sixth surface; and       

     a controller configured to acquire information regarding the surroundings of the electromagnetic wave detection apparatus, based on electromagnetic waves detected by the first detector and the second detector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a schematic configuration of an information acquisition system that includes an electromagnetic wave detection apparatus according to a first embodiment; 
         FIG. 2  is a diagram illustrating a schematic configuration of the electromagnetic wave detection apparatus illustrated in  FIG. 1 ; 
         FIG. 3  is a timing chart illustrating radiation timing and detection timing of electromagnetic waves, for explaining the principle of distance measurement performed by a distance measuring sensor made up of a radiation unit, a second detector, and a controller illustrated in  FIG. 1 ; 
         FIG. 4  is a diagram illustrating a schematic configuration of an electromagnetic wave detection apparatus according to a second embodiment; 
         FIG. 5  is a schematic configuration diagram illustrating an example variation of the electromagnetic wave detection apparatus according to the second embodiment; 
         FIG. 6  a diagram illustrating a schematic configuration of an electromagnetic wave detection apparatus according to a third embodiment; 
         FIG. 7  is a diagram illustrating a schematic configuration of an electromagnetic wave detection apparatus according to a fourth embodiment; 
         FIG. 8  is a schematic configuration diagram illustrating an example variation of the electromagnetic wave detection apparatus according to the fourth embodiment; 
         FIG. 9  is a diagram illustrating a schematic configuration of an electromagnetic wave detection apparatus according to a fifth embodiment; 
         FIG. 10  is a diagram illustrating a schematic configuration of an electromagnetic wave detection apparatus according to a sixth embodiment; 
         FIG. 11  is a schematic configuration diagram illustrating an example variation of the electromagnetic wave detection apparatus according to the sixth embodiment; 
         FIG. 12  is a diagram illustrating a schematic configuration of an electromagnetic wave detection apparatus according to a seventh embodiment; 
         FIG. 13  is a diagram illustrating a schematic configuration of an electromagnetic wave detection apparatus according to an eighth embodiment; 
         FIG. 14  is a schematic configuration diagram illustrating an example variation of the electromagnetic wave detection apparatus according to the eighth embodiment; and 
         FIG. 15  is a schematic configuration diagram illustrating an example variation of the electromagnetic wave detection apparatus according to the first embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an embodiment of an electromagnetic wave detection apparatus to which the present disclosure is applied will be described with reference to the drawings. An electromagnetic wave detection apparatus that includes a primary imaging optical system for forming an image of incident electromagnetic waves and a separation surface for separating electromagnetic waves having passed through the primary imaging optical system can separately detect each of the separated electromagnetic waves. Because in such an electromagnetic wave detection apparatus the separation surface needs to be disposed on an image side of the primary imaging optical system, it is necessary to lengthen a back focus of the primary imaging optical system. However, lengthening the back focus may deteriorate imaging characteristics such as imaging performance, brightness, and an angle of view, due to design restrictions of the primary imaging optical system. Thus, it is desired to reduce the length of the back focus. In order to reduce the length of the back focus, it is conceivable to approximate an angle between a main axis of the primary imaging optical system and the separation surface to 90°. However, if the angle approximates 90°, interference may occur between a detector configured to detect electromagnetic waves reflected by the separation surface and the primary imaging optical system, whereby actual manufacturing can become complicated. Accordingly, the electromagnetic wave detection apparatus to which the present disclosure is applied includes a surface that propagates electromagnetic waves having transmitted through the primary imaging optical system towards the separation surface and, further, propagates electromagnetic waves propagating from the separation by the separation surface towards the detector, whereby the length of back focus can be reduced. By reducing the length of the back focus, image forming characteristics such as imaging performance, brightness, and an angle of view of the primary imaging optical system can be secured. 
     An information acquisition system  11  that includes an electromagnetic wave detection apparatus  10  according to a first embodiment of the present disclosure includes the electromagnetic wave detection apparatus  10 , a radiation unit  12 , a scanner  13 , and a controller  14 , as illustrated in  FIG. 1 . 
     In subsequent drawings, a broken line connecting functional blocks indicates a flow of a control signal or communicated information. Communication represented by a broken line may be wired communication or wireless communication. A solid line projecting from each functional block indicates a beam of electromagnetic waves. 
     The electromagnetic wave detection apparatus  10  includes a first imaging unit  15 , a first propagation unit  16 , a second propagation unit  17 , a second imaging unit  18 , a first detector  19 , and a second detector  20 , as illustrated in  FIG. 2 . 
     The first imaging unit  15  includes, for example, at least one of a lens and a mirror. The first imaging unit  15  propagates an image of electromagnetic waves of an object ob serving as a subject incident from a first direction d 1  with respect to the electromagnetic wave detection apparatus  10  towards a first surface s 1  of the second propagation unit  17 . The first imaging unit  15  forms an image of electromagnetic waves of an object ob at a position remote from the first surface s 1 . The first direction d 1  includes, for example, a direction that is parallel to a principal axis of the first imaging unit  15  and directed to the first imaging unit  15  from an object plane and to an image plane from the first imaging unit  15 . 
     The first propagation unit  16  is provided in a path of electromagnetic waves that have been incident on the first surface s 1  of the second propagation unit  17  and emitted from a fourth surface s 4 . Further, the first propagation unit  16  may be provided at or in the vicinity of a primary imaging position of the object ob located remote from the first imaging unit  15  by a predetermined distance. 
     In the first embodiment, the first propagation unit  16  is provided at the primary image formation position. The first propagation unit  16  may have a reference surface ss on which electromagnetic waves is to be incident after having passed through the first imaging unit  15  and the second propagation unit  17 . The reference surface ss is made up of a plurality of pixels px aligned in two dimensions. The reference surface ss is a surface that causes an action such as, for example, reflection and transmission of electromagnetic waves in at least one of a first state and a second state, which will be described below. The first propagation unit  16  may form an image of electromagnetic waves of the object ob formed by the first imaging unit  15  on the reference surface ss. The reference surface ss may be perpendicular to a propagation axis of electromagnetic waves emitted from a fourth surface s 4 . 
     The first propagation unit  16  propagates electromagnetic waves incident on the reference surface ss in a particular direction. The first propagation unit  16  can switch each of the pixels px between a first state of propagating electromagnetic waves in a first direction d 1  as the particular direction and a second state of propagating electromagnetic waves in a second direction d 2  as another particular direction. According to the first embodiment, the first state includes a first reflection state in which electromagnetic waves incident on the reference surface ss are reflected in the first direction d 1 . The second state includes a second reflection state in which electromagnetic waves incident on the reference surface ss are reflected in the second direction d 2 . 
     In the first embodiment, the first propagation unit  16  may more specifically include a reflection surface that reflects an electromagnetic wave for each pixel px. The first propagation unit  16  may switch the first reflection state and the second reflection state for each pixel px by changing an orientation of the reflection surface for each pixel px. 
     According to the first embodiment, the first propagation unit  16  may include, for example, a DMD (Digital Micromirror Device). The DMD can drive minute reflection surfaces constituting the reference surface ss such that the reflection surface for each of the pixels px is inclined at +12° or −12° with respect to the reference surface ss. The reference surface ss may be parallel to a plate surface of the substrate including the minute reflection surfaces of the DMD mounted thereon. 
     The first propagation unit  16  switches each of the pixels px between the first state and the second state, based on control by the controller  14 , as will be described below. For example, the first propagation unit  16  can propagate electromagnetic waves incident on a portion of pixels px in the first selection direction ds 1  by simultaneously switching the pixels px to the first state and propagate electromagnetic waves incident on another portion of pixels px in the second selection direction ds 2  by switching the pixels px to the second state. 
     The second propagation unit  17  is provided between the first imaging unit  15  and the first propagation unit  16 . The second propagation unit  17  separates electromagnetic waves propagated from the first imaging unit  15  and emits electromagnetic waves to the first detector  19  and the first propagation unit  16 . The second propagation unit  17  emits electromagnetic waves whose propagation direction is changed by the first propagation unit  16  to the second detector  20 . A detailed configuration of the second propagation unit  17  will be described below. 
     The second propagation unit  17  includes at least a first surface s 1 , a second surface s 2 , a third surface s 3 , a fourth surface s 4 , a fifth surface s 5 , and a sixth surface s 6 . 
     The first surface s 1  propagates electromagnetic waves incident on the second propagation unit  17  from the first direction d 1  in the second direction d 2 . The first surface s 1  may be perpendicular to a propagation axis of electromagnetic waves that are incident on the first surface s 1  from the first direction d 1 . Because the first direction d 1  is parallel to the principal axis of the first imaging unit  15  as described above, the principal axis of the first imaging unit  15  and the first surface s 1  may be vertical to each other, in other words, a principal plane of the first imaging unit  15  and the first surface s 1  may be parallel to each other. The first surface s 1  may propagate electromagnetic waves incident from the first direction d 1  in the second direction d 2  by transmitting or refracting the electromagnetic waves. 
     The first surface s 1  propagates electromagnetic waves propagated in a third direction d 3  from the second surface s 2  in a fourth direction d 4 , as will be described below. The first surface s 1  may subject the electromagnetic waves propagated in the third direction d 3  from the second surface s 2  to internal reflection and propagate the electromagnetic waves in the fourth direction d 4 . The first surface s 1  may subject the electromagnetic waves propagated in the third direction d 3  from the second surface s 2  to total internal reflection and propagate the electromagnetic waves in the fourth direction d 4 . An incident angle of electromagnetic waves propagated in the third direction d 3  from the second surface s 2  with respect to the first surface s 1  may be equal to or larger than a critical angle. 
     The second surface s 2  separates electromagnetic waves propagated in the second direction d 2  from the first surface s 1  and propagates the electromagnetic waves in the third direction d 3  and the fifth direction d 5 . Among the electromagnetic waves propagated in the second direction d 2 , the second surface s 2  may propagate electromagnetic waves having a particular wavelength in the third direction d 3  and propagate electromagnetic waves having other wavelengths in the fifth direction d 5 . Among the electromagnetic waves propagated in the second direction d 2 , the second surface s 2  may reflect the electromagnetic waves having the particular wavelength in the third direction d 3 , and may transmit or refract the electromagnetic waves having other wavelengths in the fifth direction d 5 . Among the electromagnetic waves propagated in the second direction, the second surface s 2  may subject the electromagnetic waves having the particular wavelength to total reflection and propagate in the third direction d 3 , and may transmit or refract the electromagnetic waves having other wavelengths in the fifth direction d 5 . An incident angle of the electromagnetic waves propagated in the second direction d 2  with respect to the second surface s 2  may be smaller than the critical angle. 
     The third surface s 3  emits electromagnetic waves propagating from the first surface s 1  in the fourth direction d 4  from the second propagation unit  17 . The third surface s 3  may be perpendicular to a propagation axis of the electromagnetic waves propagating from the first surface s 1  in the fourth direction d 4 , that is, perpendicular to the fourth direction d 4 . 
     The fourth surface s 4  emits electromagnetic waves propagated in the fifth direction d 5  from the second surface s 2  to the reference surface ss of the first propagation unit  16 . Further, the fourth surface s 4  propagates electromagnetic waves incident again from the reference surface ss of the first propagation unit  16  in a sixth direction d 6 . The fourth surface s 4  may be perpendicular to a propagation axis of electromagnetic waves propagated in the fifth direction d 5  from the second surface s 2 , that is, perpendicular to the fifth direction d 5 . The fourth surface s 4  may be parallel to the reference surface ss of the first propagation unit  16 . The fourth surface s 4  may transmit or refract electromagnetic waves incident again from the reference surface ss in the sixth direction d 6 . 
     The fifth surface s 5  propagates electromagnetic waves propagated in the sixth direction d 6  from the fourth surface s 4  in a seventh direction d 7 . The fifth surface s 5  may subject the electromagnetic waves propagated in the sixth direction d 6  from the fourth surface s 4  to internal reflection and propagate the electromagnetic waves in the seventh direction. The fifth surface s 5  may subject the electromagnetic waves propagated in the sixth direction d 6  from the fourth surface s 4  to total internal reflection and propagate the electromagnetic waves in the seventh direction. An incident angle of the electromagnetic waves propagated in the sixth direction d 6  from the fourth surface s 4  with respect to the fifth surface s 5  may be equal to or greater than the critical angle. The incident angle of the electromagnetic waves propagated in the sixth direction d 6  from the fourth surface s 4  with respect to the fifth surface s 5  may be different from the incident angle of the electromagnetic waves propagated in the second direction from the first surface s 1  with respect to the second surface s 2 . The incident angle of the electromagnetic waves propagated in the sixth direction d 6  from the fourth surface s 4  with respect to the fifth surface s 5  may be larger than the incident angle of the electromagnetic waves propagated in the second direction from the first surface s 1  with respect to the second surface s 2 . The fifth surface s 5  may be parallel to the second surface s 2 . 
     The sixth surface s 6  emits electromagnetic waves propagated in the seventh direction d 7  from the fifth surface s 5 . The sixth surface s 6  may be perpendicular to a propagation axis of the electromagnetic waves propagated in the seventh direction d 7  from the fifth surface s 5 , that is, perpendicular to the seventh direction d 7 . 
     Hereinafter, the first surface s 1  to the sixth surface s 6  in the first embodiment will be described with a detail description of a configuration of the second propagation unit  17 . 
     In the first embodiment, the second propagation unit  17  includes a first prism  21 , a second prism  22 , and a first intermediate layer  23 . 
     The first prism  21  may include the first surface s 1 , the second surface s 2 , and the third surface s 3  as different surfaces. The first prism  21  may include, for example, a triangular prism in which the first surface s 1 , the second surface s 2 , and the third surface s 3  intersect with one another. 
     The first prism  21  may be arranged such that the propagation axis of electromagnetic waves incident on the first surface s 1  from the first direction d 1  is perpendicular to the first surface s 1 . The first prism  21  may be arranged such that the second surface s 2  is located in the second direction d 2  of propagation through the first prism  21  after transmission or refraction at the first surface s 1  from the first direction d 1 . The first prism  21  may be arranged such that the first surface s 1  is located in the third direction d 3  in which electromagnetic waves reflected at the second surface s 2  propagate. Further, the first prism  21  may be arranged such that the third surface s 3  is located in the fourth direction d 4  in which electromagnetic waves having propagated in the third direction from the second surface s 2  and reflected by the first surface s 1  propagate. 
     The second prism  22  may include at least the fourth surface s 4 , the fifth surface s 5 , and the sixth surface s 6  as different surfaces. The second prism  22  may include, for example, a rectangular prism in which the fourth surface s 4 , the fifth surface s 5 , and the sixth surface s 6  intersect with one another. 
     The second prism  22  may be arranged such that the fifth surface s 5  is parallel to and opposes the second surface s 2  of the first prism  21 . The second prism  22  may be arranged such that the fourth surface s 4  is located in a propagation direction of electromagnetic waves propagating through the second prism  22  via the fifth surface s 5  after being transmitted from the second surface s 2  of the first prism  21 . The second prism  22  may be arranged such that the sixth surface s 6  is located in the second direction d 7 , which is a reflection angle equal to an incident angle of electromagnetic waves incident from the sixth direction d 6  with respect to the fifth surface s 5 . 
     The first intermediate layer  23  may be arranged between the first prism  21  and the second prism  22 . Further, the first intermediate layer  23  may be in contact with the second surface s 2  of the first prism  21  and may include the second surface s 2  along the boundary surface with the first prism  21 . The first intermediate layer  23  may be in contact with the fifth surface s 5  of the second prism  22  and may include the fifth surface s 5  along the boundary surface with the second prism  22 . The first intermediate layer  23  includes, for example, a visible light reflective coating, a half mirror, a beam splitter, a dichroic mirror, a cold mirror, a hot mirror, a meta surface, or a deflection element, which is attached to the second surface s 2 . 
     A refractive index of the first intermediate layer  23  may be smaller than a refractive index of the second prism  22 . Thus, electromagnetic waves that propagate through the second prism  22  and is incident at an incident angle equal to or larger than the critical angle is totally internally reflected by the fifth surface s 5 . Accordingly, the fifth surface s 5  internally reflects electromagnetic waves that propagate having a propagation axis in the fifth direction d 5  within the second prism  22 . In a configuration in which the incident angle of electromagnetic waves from the sixth direction d 6  is equal to or larger than the critical angle, the fifth surface s 5  subjects electromagnetic waves internally propagated in the sixth direction d 6  to total reflection and propagate the electromagnetic waves in the seventh direction d 7 . 
     The second imaging unit  18  may be provided on a path of electromagnetic waves that propagate in the seventh direction d 7  from the second propagation unit  17  and exit from the sixth surface s 6 . Further, the second imaging unit  18  may be provided such that the principal plane thereof is parallel to the sixth surface s 6 . 
     The second imaging unit  18  includes, for example, at least one of a lens and a mirror. The second imaging unit  18  may form a primary image on the reference surface ss of the first propagation unit  16  and propagate an image of the object ob as electromagnetic waves emitted from the sixth surface s 6  via the second propagation unit  17  towards the second detector  20  for image formation. 
     The first detector  19  detects electromagnetic waves emitted from the third surface s 3 . In order to detect electromagnetic waves emitted from the third surface s 3 , the first detector  19  may be provided on the path of electromagnetic waves that propagate in the fourth direction d 4  from the second propagation unit  17 . Further, the first detector  19  may be provided at or in the vicinity of the image forming position of the object ob by the first imaging unit  15  in the fourth direction d 4  from the second propagation unit  17 . 
     Thus, an image of electromagnetic waves of the object ob that reaches a detection surface of the first detector  19  via the second surface s 2 , the first surface s 1 , and the third surface s 3  may be formed. Also, a difference between a length of propagation path of electromagnetic waves propagated in the third direction d 3  from the second surface s 2  to the first detector  19  and a length of a propagation path of electromagnetic waves propagated in the fifth direction d 5  from the second surface s 2  to the reference surface ss may be equal to or smaller than a predetermined value, or these lengths of the propagation paths may be equal to each other. 
     The first detector  19  may be disposed such that the detection surface is parallel to the third surface s 3 . As described above, the third surface s 3  may be perpendicular to the propagation axis of electromagnetic waves propagated in the fourth direction d 4  to be emitted. The detection surface of the first detector  19  may be perpendicular to the propagation axis of electromagnetic waves emitted from the third surface s 3 . 
     In the first embodiment, the first detector  19  includes a passive sensor. In the first embodiment, the first detector  19  includes, in particular, an element array. For example, the first detector  19  may include an image sensor or an imaging array, capture an image of electromagnetic waves formed on the detection surface, and generate image information corresponding to the captured object ob. 
     In the first embodiment, the first detector  19  may capture, in particular, an image of visible light. The first detector  19  may generate the image information and transmits a signal representing the image information to the controller  14 . 
     Note that the first detector  19  may capture an image of infrared light, ultraviolet, radio waves, or the like rather than an image of visible light. The first detector  19  may include a distance measuring sensor. In this configuration, the electromagnetic wave detection apparatus  10  can acquire distance information in the form of an image using the first detector  19 . The first detector  19  may include a distance measuring sensor, a temperature sensor, or the like. In this configuration, the electromagnetic wave detection apparatus  10  can acquire temperature information in the form of an image using the first detector  19 . 
     The second detector  20  detects electromagnetic waves emitted from the sixth surface s 6  and passed through the second imaging unit  18 . In order to detect electromagnetic waves emitted from the sixth surface s 6 , the second detector  20  may be disposed in a path of electromagnetic waves that propagate via the second imaging unit  18  after being propagated in the second direction d 7  from the second propagation unit  17  and being emitted from the sixth surface s 6 . The second detector  20  may be disposed at or in the vicinity of the secondary image forming position by the second imaging unit  18  to form an image of electromagnetic waves formed on the reference surface ss of the first propagation unit  16 . 
     The second detector  20  may be disposed such that its detection surface is parallel to the sixth surface s 6 . As described above, the sixth surface s 6  may be perpendicular to the propagation axis of the electromagnetic waves propagated in the sixth direction d 6  to be emitted, and the detection surface of the second detector  20  may be perpendicular to a propagation axis of electromagnetic waves emitted from the sixth surface s 6 . The detection surface of the second detector  20  may be parallel to a principal plane of the second imaging unit  18 . 
     In the first embodiment, the second detector  20  may be an active sensor configured to detect electromagnetic waves reflected from the target ob after being emitted toward the object ob by the radiation unit  12 . In the first embodiment, the second detector  20  may detect electromagnetic waves that are reflected from the object ob after being emitted from the radiation unit  12 , reflected by the scanner  13 , and then propagate to the object ob. As will be described below, the electromagnetic waves emitted from the radiation unit  12  may be at least one of infrared light, visible light, ultraviolet, and radio waves. The second detector  20  is a sensor of a type that is the same as or a different from that of the first detector  19 , and detects electromagnetic waves of a different type or the same type. 
     In the first embodiment, the second detector  20  includes, in particular, an element constituting the distance measuring sensor. For example, the second detector  20  includes an element such as an APD (Avalanche PhotoDiode), a PD (PhotoDiode), an SPAD (Single Photon Avalanche Diode), a millimeter wave sensor, a submillimeter-wave sensor, or a distance image sensor. The second detector  20  may include an element array such as an APD array, a PD array, an MPPC (Multi Photon Pixel Counter), a distance measuring imaging array, or a distance measuring image sensor. 
     In the first embodiment, the second detector  20  transmits, as a signal, detection information indicating that electromagnetic waves reflected from the subject are detected to the controller  14 . The second detector  20  is, in particular, an infrared sensor configured to detect electromagnetic waves in the infrared spectrum. 
     The second detector  20  composed of one element constituting the distance measuring sensor as described above simply needs to be able to detect electromagnetic waves and does not need to form an image on the detection surface. Thus, the second detector  20  does not necessarily need to be arranged at or in the vicinity of the second image forming location where an image is formed by the second imaging unit  18 . That is, in this configuration, provided that electromagnetic waves from all angles of view can be incident on the detection surface of the second detector  20 , the second detector  20  may be disposed at any location on the path of electromagnetic waves propagating via the second imaging unit  18  after being emitted from the sixth surface s 6  of the second propagation unit  17 . 
     In  FIG. 1 , the radiation unit  12  may emit at least one of infrared light, visible light, ultraviolet, and radio waves. In the first embodiment, the radiation unit  12  emits infrared light. The radiation unit  12  may irradiate the object ob with electromagnetic waves, directly or indirectly via the scanner  13 . In the first embodiment, the radiation unit  12  may irradiate the object ob with electromagnetic waves indirectly via the scanner  13 . 
     In the first embodiment, the radiation unit  12  may emit a narrow beam of electromagnetic waves having a beam spread of, for example, 0.5°. In the first embodiment, the radiation unit  12  can emit electromagnetic waves in pulses. For example, the radiation unit  12  includes, for example, an LED (Light Emitting Diode) or an LD (Laser Diode). The radiation unit  12  may switch between radiating and not radiating electromagnetic waves, based on control by the controller  14 , as will be described below. 
     For example, the scanner  13  may include a reflector to reflect electromagnetic waves and change an irradiation location of electromagnetic waves which irradiate the object ob by reflecting electromagnetic waves emitted from the radiation unit  12  while changing the direction thereof. That is, the scanner  13  may scan the object ob using electromagnetic waves emitted from the radiation unit  12 . In the first embodiment, accordingly, the second detector  20  may constitute a scanning type distance measuring sensor, together with the scanner  13 . The scanner  13  may scan the object ob in one-dimension or in two-dimensions. In the first embodiment, the scanner  13  scans the object ob in two-dimensions. 
     The scanner  13  may be configured such that at least a portion of an irradiation region of electromagnetic waves that are emitted from the radiation unit  12  and reflected by the scanner  13  is included in a detection region of electromagnetic waves in the electromagnetic wave detection apparatus  10 . Thus, at least some of electromagnetic waves radiated to the object ob via the scanner  13  can be detected by the electromagnetic wave detection apparatus  10 . 
     In the first embodiment, the scanner  13  is configured such that at least a portion of the irradiation region of electromagnetic waves that is emitted from the radiation unit  12  and reflected by the scanner  13  is included in a detection region of the second detector  20 . In the first embodiment, thus, at least some of electromagnetic waves radiated to the object ob via the scanner  13  can be detected by the second detector  20 . 
     The scanner  13  includes, for example, a MEMS (Microelectromechanical systems) mirror, a polygon mirror, a galvanometer mirror, or the like. In the first embodiment, the scanner  13  includes the MEMS mirror. 
     The scanner  13  may change a reflection direction of electromagnetic waves, based on control by the controller  14 , which will be described below. The scanner  13  may include an angle sensor such as, for example, an encoder and notify the controller  14  of an angle detected by the angle sensor as direction information used for reflecting electromagnetic waves. In this configuration, the controller  14  can calculate the irradiation location, based on the direction information acquired from the scanner  13 . Alternatively, the controller  14  can calculate the irradiation location, based on a driving signal input to cause the scanner  13  to change the reflection direction of electromagnetic waves. 
     The controller  14  includes one or more processors and a memory. The processor may include a general purpose processor configured to read a specific program and perform a specific function, or a specialized processor dedicated for specific processing. The specialized processor may include an ASIC (Application Specific Integrated Circuit). The processor may include a PLD (Programmable Logic Device). The PLD may include an FPGA (Field-Programmable Gate Array). The controller  14  may include at least one of a SoC (System-on-a-Chip) that includes one or more cooperating processors or a SiP (System in a Package). 
     The controller  14  may acquire information regarding the surroundings of the electromagnetic wave detection apparatus  10 , based on electromagnetic waves respectively detected by the first detector  19  and the second detector  20 . The information regarding the surroundings is, for example, image information, distance information, temperature information, or the like. In the first embodiment, the controller  14  acquires electromagnetic waves detected as an image by the first detector  19  serving as the image information, as described above. In the first embodiment, further, the controller  14  acquires the distance information regarding the irradiation location irradiated by the radiation unit  12  using a ToF (Time-of-Flight) method, which will be described below, based on the detection information detected by the second detector  20 . 
     As illustrated in  FIG. 3 , the controller  14  causes the radiation unit  12  to emit electromagnetic waves in pulses by inputting an electromagnetic wave radiation signal to the radiation unit  12  (see “ELECTROMAGNETIC WAVE RADIATION SIGNAL” field). The radiation unit  12  emits electromagnetic waves, based on the electromagnetic wave radiation signal (see “RADIATION UNIT RADIATION AMOUNT” field). The electromagnetic waves that have been emitted from the radiation unit  12 , reflected by the scanner  13 , irradiate any irradiation region are reflected in the irradiation region. The controller  14  changes at least some of the pixels px within an image formation region of the first propagation unit  16  for an image of the reflected wave from the irradiation region formed by the first imaging unit  15  to the first state, and changes other pixels px to the second state. Then, when the second detector  20  detects electromagnetic waves reflected from the irradiation region (see “ELECTROMAGNETIC WAVE DETECTION AMOUNT” field), the second detector  20  notifies the controller  14  of the detection information, as described above. 
     The controller  14  includes, for example, a time measuring LSI (Large Scale Integrated circuit) and measures a time ΔT from a time T 1  at which the controller  14  causes the radiation unit  12  to emit electromagnetic waves to a time T 2  at which the detection information is acquired (see “ACQUISITION OF DETECTION INFORMATION”). The controller  14  calculates a distance to the irradiation location by multiplying the time ΔT by the speed of light and then dividing an acquired value by 2. The controller  14  calculates the irradiation location, based on the direction information acquired from the scanner  13  or the driving signal input to the scanner  13  by the controller  14 , as described above. The controller  14  calculates a distance to an irradiation location while changing the irradiation location, and thus generates the distance information in the form of an image. 
     In the first embodiment, the information acquisition system  11  is configured to generate the distance information employing a Direct ToF technique that directly measures the time period for radiated electromagnetic waves to return, as described above. However, the information acquisition system  11  is not limited to this configuration. For example, the information acquisition system  11  may be configured to generate the distance information employing a Flash ToF technique that emits electromagnetic waves in a constant cycle and indirectly measures the time period for the electromagnetic waves to return, based on a phase difference between the emitted electromagnetic waves and returned electromagnetic waves. The information acquisition system  11  may generate the distance employing another ToF technique such as, for example, a Phased ToF technique. 
     The electromagnetic wave detection apparatus  10  of the first embodiment configured as described above includes the second propagation unit  17  and the first detector  19 . The second propagation unit  17  includes the first surface s 1  that propagates electromagnetic waves incident from the first direction d 1  in the second direction d 2  and also propagates electromagnetic waves propagated in the third direction d 3  in the fourth direction d 4 , the second surface s 2  that separates electromagnetic waves propagated in the second direction d 2  and propagates electromagnetic waves in the third direction d 3  and the fifth direction d 5 , and the third surface s 3  that emits electromagnetic waves propagated in the fourth direction d 4 . The first detector  19  is configured to detect electromagnetic waves emitted from the third surface s 3 . This configuration enables, in the electromagnetic wave detection apparatus  10 , arrangement of the first detector  19  in a direction different from the third direction d 3  from the second surface s 2  that functions as a separation surface. Thus, the electromagnetic wave detection apparatus  10  can avoid the interference between the first imaging unit  15  and the first detector  19  even if the angle formed by the second direction d 2  and the second surface s 2  approximates 90°. Accordingly, the length of the back focus of the first imaging unit  15  can be reduced. In the electromagnetic wave detection apparatus  10 , as a result, a design restriction of the first imaging unit  15  is avoided, and image forming characteristics of the first imaging unit  15  can be secured. Such configuration and effect are applicable also to electromagnetic wave detection apparatuses according to second to eighth embodiments, which will be described below. 
     In addition, in the electromagnetic wave detection apparatus  10  of the first embodiment, the second propagation unit  17  includes the fourth surface s 4  that emits electromagnetic waves propagated in the fifth direction d 5  to the reference surface ss and propagates electromagnetic waves incident again from the reference surface ss in the sixth direction d 6  and the fifth surface s 5  that propagates electromagnetic waves propagated in the sixth direction d 6  in the seventh direction d 7 . In this configuration, because the electromagnetic wave detection apparatus  10  propagates electromagnetic waves propagated in a particular direction by the reference surface ss further in a different direction, the second imaging unit  18  can be arranged without interfering the first imaging unit  15 . In this configuration, further, because in the electromagnetic wave detection apparatus  10  the second imaging unit  18  can be arranged outside the path of the electromagnetic waves from the first imaging unit  15  to the first propagation unit  16 , the distance to the reference surface ss from the first imaging unit  15  and the distance of the propagation path of electromagnetic waves to the second imaging unit  18  from the reference surface ss can be reduced. Thus, the electromagnetic wave detection apparatus  10  can cause electromagnetic waves having changed its propagation direction after the formation of the primary image on the reference surface ss to be incident on the second imaging unit  18  before being widely spread while propagating. Accordingly, the electromagnetic wave detection apparatus  10  can prevent the occurrence of vignetting, even when the second imaging unit  18  is downsized. As a result, the electromagnetic wave detection apparatus  10  can homogenize the intensity of electromagnetic waves of a secondarily image formed on the second imaging unit  18 , without increasing the size of the apparatus as a whole. Such configuration and effect are applicable also to the electromagnetic wave detection apparatuses according to the second to eighth embodiments, which will be described below. 
     The electromagnetic wave detection apparatus  10  of the first embodiment separates electromagnetic waves incident from the first imaging unit  15  and propagate electromagnetic waves in the third direction d 3  and the fifth direction d 5 . This configuration enables the electromagnetic wave detection apparatus  10  to match the principal axis of the first imaging unit  15  with the propagation axis of electromagnetic waves propagated in the third direction d 3  and the propagation axis of electromagnetic waves propagated in the fifth direction d 5 . Accordingly, the electromagnetic wave detection apparatus  10  can reduce the deviation of coordinate systems between the first detector  19  and the second detector  20 . Such configuration and effect are applicable also to the electromagnetic wave detection apparatuses according to the second to eighth embodiments, which will be described below. 
     Further, in the information acquisition system  11  of the first embodiment, the controller  14  acquires the information regarding the surroundings, based on electromagnetic waves respectively detected by the first detector  19  and the second detector  20 . This configuration enables the information acquisition system  11  to provide useful information based on detected electromagnetic waves. Such configuration and effect are applicable also to the electromagnetic wave detection apparatuses according to the second to eighth embodiments, which will be described below. 
     Next, an electromagnetic wave detection apparatus according to the second embodiment of the present disclosure will be described. In the second embodiment, a configuration of the second propagation unit is different from that of the first embodiment. Hereinafter, the second embodiment will be described focusing on aspects different from the first embodiment. Note that elements having the same configurations of the elements of the first embodiment will be denoted by the same reference signs. 
     An electromagnetic wave detection apparatus  100  according to the second embodiment includes the first imaging unit  15 , the first propagation unit  16 , a second propagation unit  170 , the second imaging unit  18 , the first detector  19 , and the second detector  20 , as illustrated in  FIG. 4 . The information acquisition system  11  of the second embodiment includes the same configuration and function as those of the first embodiment, except for the electromagnetic wave detection apparatus  100 . The configurations and functions of the second embodiment are the same as those of the first embodiment, except for the second propagation unit  170 . 
     In the second embodiment, the second propagation unit  170  includes at least the first surface s 1 , the second surface s 2 , the third surface s 3 , the fourth surface s 4 , the fifth surface s 5 , and the sixth surface s 6 , in the same manner as the first embodiment. In the second embodiment, configurations and functions of the first surface s 1 , the second surface s 2 , the third surface s 3 , the fourth surface s 4 , the fifth surface s 5 , and the sixth surface s 6  are the same as those of the first embodiment. In the second embodiment, the configuration and function of a fifth surface s 50  are the same as those of the fifth surface s 5  of the first embodiment, except for an object to contact the fifth surface s 50 . 
     In the second embodiment, the second propagation unit  170  includes the first prism  21 , a second prism  220 , a first intermediate layer  230 , and a second intermediate layer  240 . The configuration and function of the first prism  21  are the same as those of the first embodiment. A configuration of the second prism  220  itself, an arrangement of the second prism  220  with respect to the first prism  21 , and the function of the second prism  220  are the same as those of the first embodiment. 
     The first intermediate layer  230  may be arranged between the first prism  21  and the second prism  220 , in the same manner as first embodiment. Further, the first intermediate layer  230  may be in contact with the second surface s 2  of the first prism  21  and may include the second surface s 2  along the boundary surface with the first prism  21 , in the same manner as the first embodiment. The first intermediate layer  230  includes, for example, a visible light reflective coating, a half mirror, a beam splitter, a dichroic mirror, a cold mirror, a hot mirror, a meta surface, or a deflection element, which is attached to the second surface s 2 , in the same manner as the first embodiment. 
     The second intermediate layer  240  may be arranged between the fifth surface s 50  of the second prism  220  and the first intermediate layer  230 . The second intermediate layer  240  may be in contact with the fifth surface s 50  of the second prism  220  and may include the fifth surface s 50  along the boundary surface with the second prism  220 . Further, the second intermediate layer  240  may be in contact with the surface of the first intermediate layer  230  opposite from the surface of the first intermediate layer  230  in contact with the first prism  21 . 
     The second intermediate layer  240  may have a refractive index smaller than that of the second prism  220  and include, for example, at least one of vacuum, a gas, a liquid, and a solid, which has a refractive index smaller than that of the second prism  220 . Thus, electromagnetic waves that propagate through the second prism  220  and is incident at an incident angle equal to or larger than the critical angle is totally internally reflected at the fifth surface s 50 . Accordingly, the fifth surface s 50  internally reflects electromagnetic waves that propagate having the propagation axis in the sixth direction d 6  within the second prism  220 . In a configuration in which the incident angle of electromagnetic waves from the sixth direction d 6  is equal to or larger than the critical angle, the fifth surface s 50  totally reflects electromagnetic waves that internally propagate in the sixth direction d 6  and propagates the electromagnetic waves in the seventh direction d 7 . In a configuration in which the second intermediate layer  240  is a gas or a liquid, the second intermediate layer  240  may be formed by providing spacers  250  on the respective peripheries of the first intermediate layer  230  and the fifth surface s 50  of the second prism  220  as illustrated in  FIG. 5  and filling the spacers  250  with a gas or a liquid. In the second embodiment, the second intermediate layer  240  may include an air layer or a prism. 
     In the electromagnetic wave detection apparatus  100  according to the second embodiment including the above configuration, the fifth surface s 50  includes the boundary surface between the second intermediate layer  240  and the second prism  220 . This configuration enables the electromagnetic wave detection apparatus  100  to adopt a configuration in which the first intermediate layer  230  may have the function to separate electromagnetic waves incident on the second surface s 2  and the second intermediate layer  240  may have the function to propagate electromagnetic waves propagated in the sixth direction d 6  in the seventh direction d 7 . Thus, a flexibility in selecting the material used for the first intermediate layer  230  can be improved. 
     Next, an electromagnetic wave detection apparatus according to a third embodiment of the present disclosure will be described. The third embodiment is different from the first embodiment, in terms of the configuration of the second propagation unit. Hereinafter, the third embodiment will be described focusing on aspects different from the first embodiment. Note that elements having the same configurations of the elements of the first embodiment or the second embodiment will be denoted by the same reference signs. 
     An electromagnetic wave detection apparatus  101  according to the third embodiment includes the first imaging unit  15 , the first propagation unit  16 , a second propagation unit  171 , the second imaging unit  18 , the first detector  19 , and the second detector  20 , as illustrated in  FIG. 6 . The configuration and function of the information acquisition system  11  according to the third embodiment are the same as those of the first embodiment, except for the electromagnetic wave detection apparatus  101 . Configurations and functions of the third embodiment are the same as those of the first embodiment, except for the second propagation unit  171 . 
     In the third embodiment, the second propagation unit  171  includes at least the first surface s 1 , the second surface s 2 , the third surface s 3 , the fourth surface s 4 , a fifth surface s 51 , and the sixth surface s 6 , in the same manner as the first embodiment. In the third embodiment, configurations and functions of the first surface s 1 , the second surface s 2 , the third surface s 3 , the fourth surface s 4 , and the sixth surface s 6  are the same as those of the first embodiment. In the third embodiment, the configuration and function of a fifth surface s 51  are the same as those of the first embodiment, except for an object to contact the fifth surface s 51 . 
     In the third embodiment, the second propagation unit  171  includes the first prism  21 , a second prism  221 , a third prism  261 , and a first intermediate layer  231 . The configuration and the function of the first prism  21  are the same as those of the first embodiment. The configuration of the second prism  221  itself, an arrangement of the second prism  221  with respect to the first prism  21 , and the function of the second prism  221  are the same as those of the first embodiment. 
     The third prism  261  may be arranged between the first intermediate layer  231  and the second prism  221 . The third prism may have a refractive index smaller than that of the second prism  211 . Thus, electromagnetic waves that propagate through the second prism  221  and is incident at an incident angle equal to or larger than the critical angle are totally internally reflected by the fifth surface s 51 . Accordingly, the fifth surface s 51  internally reflects electromagnetic waves that propagate having a propagation axis in the sixth direction d 6  within the second prism  221 . In a configuration in which the incident angle of electromagnetic waves from the sixth direction d 6  is equal to or larger than the critical angle, the fifth surface s 51  totally internally reflects the electromagnetic waves internally propagated in the sixth direction and propagates the electromagnetic waves in the seventh direction d 7 . 
     The third prism  261  may be a plate-like member and have one plate surface in contact with the first intermediate layer  231 . Another plate surface of the third prism  261  may be in contact with the fifth surface s 51  of the second prism  221  and may include the fifth surface s 51  along the boundary surface with the second prism  221 . 
     The first intermediate layer  231  may be arranged between the first prism  21  and the third prism  261 , in a manner different from the first embodiment. Further, the first intermediate layer  231  may be in contact with the second surface s 2  of the first prism  21  and may include the second surface s 2  along the boundary surface with the first prism  21 , in the same manner as the first embodiment. The first intermediate layer  231  includes, for example, a visible light reflection coating, a half mirror, a beam splitter, a dichroic mirror, a cold mirror, a hot mirror, a meta surface, or a deflection element, which is attached to the second surface s 2 , in the same manner as the first embodiment. 
     In the electromagnetic wave detection apparatus  101  of the third embodiment including the above configuration, the second propagation unit  171  includes the third prism  261  which includes the first intermediate layer  231  at the boundary with the first prism  21 , and the fifth surface s 51  includes the boundary surface between the second prism  221  and the third prism  261 . In order to prevent the first detector  19  from interfering with the first prism  21 , it is desired to arrange the second surface s 2  in the vicinity of the first imaging unit  15 . Also, in order to reduce the size of the second imaging unit  18 , it is desired to reduce the length of the propagation path of electromagnetic waves that propagate sequentially through the fifth surface s 51 , the fourth surface s 4 , the reference surface ss, the fourth surface s 4 , the fifth surface s 51 , and then the sixth surface s 6 . The fifth surface s 51  may be preferably arranged in the vicinity of the reference surface ss of the first propagation unit  16 . This configuration enables separation between the second surface s 2  and the fifth surface s 51  in the electromagnetic wave detection apparatus  101 , whereby the second surface s 2  can be arranged in the vicinity of the first imaging unit  15 , and the fifth surface s 51  can be arranged in the vicinity of the reference surface ss. As a result, in the electromagnetic wave detection apparatus  101  the size of the second imaging unit  18  can be reduced, while the interference with the first prism  21  by the first detector  19  can be suppressed. Such configuration and effect are applicable also to the electromagnetic wave detection apparatuses according to the fourth to eighth embodiments, which will be described below. 
     Next, an electromagnetic wave detection apparatus according to a fourth embodiment of the present disclosure will be described. The fourth embodiment is different from the first embodiment, in terms of the configuration of the second propagation unit. Hereinafter, the fourth embodiment will be described focusing on aspects different from the first embodiment. Note that elements having the same configurations of the elements of the first embodiment, the second embodiment, or the third embodiment will be denoted by the same reference signs. 
     An electromagnetic wave detection apparatus  102  according to the fourth embodiment includes the first imaging unit  15 , the first propagation unit  16 , a second propagation unit  172 , the second imaging unit  18 , the first detector  19 , and the second detector  20 , as illustrated in  FIG. 7 . The configuration and function of the information acquisition system  11  according to the fourth embodiment are the same as those of the first embodiment, except for the electromagnetic wave detection apparatus  102 . Configurations and functions of the fourth embodiment are the same as those of the first embodiment, except for the second propagation unit  172 . 
     In the fourth embodiment, the second propagation unit  172  includes at least the first surface s 1 , the second surface s 2 , the third surface s 3 , the fourth surface s 4 , the fifth surface s 50 , and the sixth surface s 6 , in the same manner as the first embodiment. In the fourth embodiment, configurations and functions of the first surface s 1 , the second surface s 2 , the third surface s 3 , the fourth surface s 4 , and the sixth surface s 6  are the same as those of the first embodiment. In the fourth embodiment, the configuration and function of the fifth surface s 50  are the same as those of the first embodiment, except for an object to contact the fifth surface s 50 . 
     In the fourth embodiment, the second propagation unit  172  includes, for example, a total internal reflection prism, and includes the first prism  21 , the second prism  220 , a third prism  262 , the first intermediate layer  231 , and a second intermediate layer  242 . The configuration and the function of the first prism  21  and the first intermediate layer  231  are the same as those of the third embodiment. The configuration of the second prism  221  itself, an arrangement thereof with respect to the first prism  21 , and the function of the second prism  221  are the same as those of the first embodiment. A configuration of the third prism  262  itself, an arrangement thereof with respect to the first prism  21 , and the function of the third prism  262  are the same as those of the first embodiment. Thus, the first intermediate layer  231  may be arranged between the first prism  21  and the second intermediate layer  242 . Further, the first intermediate layer  231  may include the second surface s 2  along the boundary surface with the first prism  21 . 
     The second intermediate layer  242  may be arranged between the second prism  220  and the third prism  262 . The second intermediate layer  242  may be in contact with the fifth surface s 50  of the second prism  220  and may include the second fifth surface s 50  along the boundary surface with the second prism  220 , in the same manner as the second embodiment. The second intermediate layer  242  may be in contact with the surface of the third prism  262  opposite from the surface of the third prism  262  in contact with the first intermediate layer  231 . 
     The second intermediate layer has the refractive index smaller than that of the second prism  220  in the same manner as the second embodiment and includes at least one of, for example, a gas, a liquid, or a solid that has a refractive index smaller than that of the second prism  220 . Thus, electromagnetic waves propagating through the second prism  220  and incident at an incident angle equal to or larger than the critical angle are totally internally reflected by the fifth surface s 50 . Accordingly, the fifth surface s 50  internally reflects electromagnetic waves propagating having the propagation axis in the sixth direction d 6  within the second prism  220 . In a configuration in which the incident angle of electromagnetic waves incident from the sixth direction d 6  is equal to or larger than the critical angle, the fifth surface s 50  totally internally reflects electromagnetic waves internally propagated in the sixth direction d 6  and propagates the electromagnetic waves in the seventh direction d 7 . In a configuration in which the second intermediate layer  242  is a gas or a liquid, the second intermediate layer  242  may be formed by providing spacers  250  on the respective peripheries of the third prism  262  and the fifth surface s 50  of the second prism  220  as illustrated in  FIG. 8  and filling the spacers  250  with a gas or a liquid. In the fourth embodiment, the second intermediate layer  242  may include an air layer or a prism. 
     Next, an electromagnetic wave detection apparatus according to a fifth embodiment of the present disclosure will be described. The fifth embodiment is different from the first embodiment, in terms of the configuration of the second propagation unit. Hereinafter, the fifth embodiment will be described focusing on aspects different from the first embodiment. Note that elements having the same configurations of the elements of the first embodiment, the second embodiment, the third embodiment, or the fourth embodiment will be denoted by the same reference signs. 
     An electromagnetic wave detection apparatus  103  according to the fifth embodiment includes the first imaging unit  15 , the first propagation unit  16 , a second propagation unit  173 , the second imaging unit  18 , the first detector  19 , and the second detector  20 , as illustrated in  FIG. 9 . The configuration and functions of the information acquisition system  11  according to the fifth embodiment are the same as those of the first embodiment, except for the electromagnetic wave detection apparatus  103 . The configuration and function of the fifth embodiment are the same as those of the first embodiment, except for the second propagation unit  173 . 
     In the fifth embodiment, the second propagation unit  173  includes at least the first surface s 1 , the second surface s 2 , the third surface s 3 , the fourth surface s 4 , a fifth surface s 53 , and the sixth surface s 6 , in the same manner as the first embodiment. In the fifth embodiment, configurations and functions of the first surface s 1 , the second surface s 2 , the third surface s 3 , the fourth surface s 4 , and the sixth surface s 6  are the same as those of the first embodiment. 
     The fifth surface s 53  may propagate electromagnetic waves propagated in the sixth direction d 6  from the fourth surface s 4  in the seventh direction d 7 , in the same manner as the first embodiment. The fifth surface s 53  may internally reflect the electromagnetic waves propagated in the sixth direction d 6  from the fourth surface s 4  and propagate the electromagnetic waves in the seventh direction d 7 , in the same manner as the first embodiment. The fifth surface s 53  may totally internally reflect the electromagnetic waves propagated in the sixth direction d 6  from the fourth surface s 4  and propagate the electromagnetic waves in the seventh direction d 7 , in the same manner as the first embodiment. The incident angle of the electromagnetic waves propagated in the sixth direction d 6  from the fourth surface s 4  with respect to the fifth surface s 53  may be equal to or larger than the critical angle, in the same manner as the first embodiment. 
     The incident angle of the electromagnetic waves propagated in the sixth direction d 6  from the fourth surface s 4  with respect to the fifth surface s 53  may be equal to or different from an incident angle of electromagnetic waves propagated in the second direction d 2  from the first surface s 1  with respect to the second surface s 2 , in a manner different from the first embodiment. The incident angle of the electromagnetic waves propagated in the sixth direction d 6  from the fourth surface s 4  with respect to the fifth surface s 53  may be larger or smaller than the incident angle of the electromagnetic waves propagated in the second direction d 2  from the first surface s 1  with respect to the second surface s 2 , in a manner different from the first embodiment. The fifth surface s 53  may not be parallel to the second surface s 2 , in a manner different from the first embodiment. 
     In the fifth embodiment, the second propagation unit  173  may include the first prism  21 , a second prism  223 , a third prism  263 , and the first intermediate layer  231 . The configurations and functions of the first prism  21  and the first intermediate layer  231  are the same as those of the third embodiment. 
     The second prism  223  may include the fourth surface s 4 , the fifth surface s 53 , and the sixth surface s 6  as separate different surfaces, in the same manner as the first embodiment. The second prism  223  includes, for example, a triangular prism in which the fourth surface s 4 , the fifth surface s 5 , and the sixth surface s 6  intersect with one another. 
     The second prism  223  is arranged such that the fifth surface s 53  opposes the second surface s 2  of the first prism  21 , in the same manner as the first embodiment. The second prism  223  is arranged such that the fourth surface s 4  is located in the propagation direction of electromagnetic waves propagating through the second prism  223  via the first surface s 5  after being transmitted through the second surface s 2  of the first prism  21 , in the same manner as the first embodiment. Further, the second prism  223  is arranged such that the sixth surface s 6  is located in the seventh direction d 7 , which is a reflection angle equal to the incident angle of electromagnetic waves incident from the sixth direction d 6  with respect to the fifth surface s 53 , in the same manner as the first embodiment. 
     The second prism  223  may be arranged such that an angle b formed by the second direction d 2  and the fifth surface s 53  is larger than an angle a formed by the second direction d 2  and the second surface s 2 , in a manner different from the first embodiment. 
     For example, the second prism  223  may be arranged with respect to the first prism  21 , in a manner in which the fifth surface s 53  is rotated in a direction remote from the first prism  21  about an intersection line of the fourth surface s 4  and the fifth surface s 53 , that is, in a manner in which the fifth surface s 53  is rotated in a direction approaching the fourth surface s 4 . 
     The third prism  263  is arranged between the first intermediate layer  231  and the second prism  223 , in the same manner as the third embodiment. The third prism  263  has a refractive index smaller than that of the second prism  223 , in the same manner as the third embodiment. Thus, electromagnetic waves that propagating through the second prism  223  and is incident at an incident angle equal to or larger than the critical angle after propagating through the second prism  223  are totally internally reflected by the fifth surface s 53 , in the same manner as the third embodiment. Accordingly, the fifth surface s 53  internally reflects the electromagnetic waves that propagate having the propagation axis in the sixth direction d 6  within the second prism  223 , in the same manner as the third embodiment. In a configuration in which the incident angle of the electromagnetic waves propagating from the sixth direction d 6  is equal to or larger than the critical angle, the fifth surface s 53  totally internally reflects electromagnetic waves internally propagated in the sixth direction d 6  and propagates the electromagnetic waves in the seventh direction d 7 , in the same manner as the third embodiment. 
     In the fifth embodiment, the third prism  263  includes, for example, a prism including surfaces inclined with respect to one another, in a manner different from the third embodiment. One surface of the third prism  263  may contact the first intermediate layer  231 , in a manner similar to the third embodiment. Another surface of the third prism  263  may be in contact with the fifth surface s 53  of the second prism  223  and may include the fifth surface s 53  along the boundary surface with the second prism  223 , in a manner similar to the third embodiment. 
     In the electromagnetic wave detection apparatus  103  of the fifth embodiment including the above configuration, the incident angle of electromagnetic waves propagated in the second direction d 2  with respect to the second surface s 2  is smaller than the incident angle of electromagnetic waves incident again from the reference surface ss via the fourth surface s 4  with respect to the fifth surface s 53 . This configuration enables the electromagnetic wave detection apparatus  103  to include the configuration in which the second prism  223  is arranged with respect to the first prism  21 , in a manner in which the fifth surface s 53  is rotated in a direction remote from the first prism  21  about the intersection line of the fourth surface s 4  and the fifth surface s 53 , that is, in a manner in which the fifth surface s  53  is rotated in a direction approaching the fourth surface s 4 . Thus, the electromagnetic wave detection apparatus  103  can reduce the distance between the reference surface ss of the first propagation unit  16 , which is arranged on the side of the third plane s 3  of the second plane s 2 , and the fifth plane s 53 . As a result, the electromagnetic wave detection apparatus  103  can reduce the length of the propagation path of electromagnetic waves that propagate through the fifth surface s 53 , the fourth surface s 4 , the reference surface ss, the fourth surface s 4 , the fifth surface s 53 , and then the sixth surface s 6 , whereby the second imaging unit  18  can be further downsized. Such configuration and effect are applicable also to the electromagnetic wave detection apparatus according to the sixth embodiment, which will be described below. 
     Next, an electromagnetic wave detection apparatus according to a sixth embodiment of the present disclosure will be described. The sixth embodiment is different from the first embodiment, in terms of the configuration of the second propagation unit. Hereinafter, the sixth embodiment will be described focusing on aspects different from the first embodiment. Note that elements having the same configurations of the elements of the first embodiment, the second embodiment, the third embodiment, the fourth embodiment, or the fifth embodiment will be denoted by the same reference signs. 
     An electromagnetic wave detection apparatus  104  according to the sixth embodiment includes the first imaging unit  15 , the first propagation unit  16 , a second propagation unit  174 , the second imaging unit  18 , the first detector  19 , and the second detector  20 , as illustrated in  FIG. 10 . The configuration and functions of the information acquisition system  11  according to the sixth embodiment are the same as those of the first embodiment, except for the electromagnetic wave detection apparatus  104 . The configuration and function of the sixth embodiment are the same as those of the first embodiment, except for the second propagation unit  174 . 
     In the sixth embodiment, the second propagation unit  174  includes at least the first surface s 1 , the second surface s 2 , the third surface s 3 , the fourth surface s 4 , a fifth surface s 54 , and the sixth surface s 6 , in the same manner as the first embodiment. In the sixth embodiment, configurations and functions of the first surface s 1 , the second surface s 2 , the third surface s 3 , the fourth surface s 4 , and the sixth surface s 6  are the same as those of the first embodiment. In the sixth embodiment, configurations and functions of the fifth surface s 54  are the same as those of the fifth embodiment, except for an object to contact the fifth surface s 54 . 
     In the sixth embodiment, the second propagation unit  174  includes a total internal reflection prism, and includes the first prism  21 , a second prism  224 , a third prism  264 , the first intermediate layer  231 , and the second intermediate layer  242 . The configuration and the function of the first prism  21  and the first intermediate layer  231  are the same as those of the third embodiment. The configurations of the second prism  224  and the third prism  264  themselves, arrangements of the second prism  224  and the third prism  264  with respect to the first prism  21 , and the functions of second prism  224  and the third prism  264  are the same as those of the fifth embodiment. Thus, the first intermediate layer  231  is arranged between the first prism  21  and the second intermediate layer  242 . Further, the first intermediate layer  231  may include the second surface s 2  along the boundary surface with the first prism  21 . 
     In the sixth embodiment, the configuration and function of the second intermediate layer  242  is the same as those of the fourth embodiment. Thus, the second intermediate layer  242  has a refractive index smaller than that of the second prism  224  in the same manner as the fourth embodiment, and includes at least any one of, for example, vacuum, a gas, a liquid, and a solid, which has a refractive index smaller than that of the second prism  224 . In a configuration in which the second intermediate layer  242  is a gas or a liquid, the second intermediate layer  242  may be formed by providing spacers  250  on the respective peripheries of the third prism  264  and the fifth surface s 50  of the second prism  224  as illustrated in  FIG. 11  and filling the spacers  250  with a gas or a liquid. In the sixth embodiment, the second intermediate layer  242  may include an air layer or a prism. 
     Next, an electromagnetic wave detection apparatus according to a seventh embodiment of the present disclosure will be described. The seventh embodiment is different from the first embodiment, in terms of the configuration of the second propagation unit. Hereinafter, the seventh embodiment will be described focusing on aspects different from the first embodiment. Note that elements having the same configurations of the elements of the first embodiment, the second embodiment, the third embodiment, the fourth embodiment, the fifth embodiment, or the sixth embodiment will be denoted by the same reference signs. 
     An electromagnetic wave detection apparatus  105  according to the seventh embodiment includes the first imaging unit  15 , the first propagation unit  16 , a second propagation unit  175 , the second imaging unit  18 , the first detector  19 , and the second detector  20 , as illustrated in  FIG. 12 . The configuration and functions of the information acquisition system  11  according to the seventh embodiment are the same as those of the first embodiment, except for the electromagnetic wave detection apparatus  105 . The configuration and function of the seventh embodiment are the same as those of the first embodiment, except for the second propagation unit  175 . 
     In the seventh embodiment, the second propagation unit  175  includes at least the first surface s 1 , the second surface s 2 , the third surface s 3 , the fourth surface s 4 , a fifth surface s 55 , and the sixth surface s 6 , in the same manner as the first embodiment. In the seventh embodiment, configurations and functions of the first surface s 1 , the second surface s 2 , the third surface s 3 , the fourth surface s 4 , and the sixth surface s 6  are the same as those of the first embodiment. 
     The fifth surface s 55  may propagate electromagnetic waves propagated in the sixth direction d 6  from the fourth surface s 4  in the seventh direction d 7 , in the same manner as the first embodiment. The fifth surface s 55  may internally reflect the electromagnetic waves propagated in the sixth direction d 6  from the fourth surface s 4  and propagate the electromagnetic waves in the seventh direction d 7 , in the same manner as the first embodiment. The fifth surface s 55  may totally internally reflect the electromagnetic waves propagated in the sixth direction d 6  from the fourth surface s 4  and propagate the electromagnetic waves to the seventh direction d 7 , in the same manner as the first embodiment. An incident angle of the electromagnetic waves propagated in the sixth direction d 6  from the fourth surface s 4  with respect to the fifth surface s 55  may be equal to or larger than the critical angle, in the same manner as the first embodiment. The incident angle of the electromagnetic waves propagated in the sixth direction d 6  from the fourth surface s 4  with respect to the fifth surface s 55  may be different from an incident angle of electromagnetic waves propagated in the second direction d 2  from the first surface s 1  with respect to the second surface s 2 . The incident angle of the electromagnetic waves propagated in the sixth direction d 6  from the fourth surface s 4  with respect to the fifth surface s 55  may be larger than the incident angle of the electromagnetic waves propagated in the second direction d 2  from the first surface s 1  with respect to the second surface s 2 . 
     The fifth surface s 55  may not be parallel to the second surface s 2 , in a manner different from the first embodiment. 
     In the seventh embodiment, the second propagation unit  175  includes the first prism  21 , a second prism  225 , a third prism  265 , and the first intermediate layer  231 . The configurations and functions of the first prism  21  and the first intermediate layer  231  are the same as those of the third embodiment. The function of the third prism  265  is similar to that of the fifth embodiment. 
     The second prism  225  may include the fourth surface s 4 , the fifth surface s 55 , and the sixth surface s 6  as separate different surfaces, in the same manner as the first embodiment. The second prism  225  may include, for example, a triangular prism in which the fourth surface s 4 , the fifth surface s 55 , and the sixth surface s 6  intersect with one another. 
     The second prism  225  may be arranged such that the fifth surface s 55  opposes the second surface s 2  of the first prism  21 , in the same manner as the first embodiment. The second prism  225  may be arranged such that the fourth surface s 4  is located in the propagation direction of electromagnetic waves propagating through the second prism  225  via the fifth surface s 55  after being transmitted from the second surface s 2  of the first prism  21 , in the same manner as the first embodiment. The second prism  225  may be arranged such that the sixth surface s 6  is located in the seventh direction d 7 , which is a reflection angle equal to the incident angle of electromagnetic waves incident from the sixth direction d 6  with respect to the fifth surface s 55 , in the same manner as the first embodiment. 
     The second prism  225  may be arranged such that an angle b formed by the second direction d 2  and the fifth surface s 55  is smaller than an angle a formed by the second direction d 2  and the second surface s 2 , in a manner different from the first embodiment. 
     For example, the second prism  225  may be arranged with respect to the first prism  21 , in a manner in which the fifth surface s 55  is rotated in a direction remote from the first prism  21  about an intersection line of the fifth surface s 55  and the sixth surface s 6 , that is, in a manner in which the fifth surface s 55  is rotated in a direction approaching the sixth surface s 6 . 
     In the electromagnetic wave detection apparatus  105  according to the seventh embodiment including the above configuration, the second prism  225  is arranged with respect to the first prism  21 , in the manner in which the fifth surface s 55  is rotated in a direction remote from the first prism  21  about the intersection line of the fifth surface s 55  and the sixth surface s 6 , that is, in the manner in which the fifth surface s 55  is rotated in a direction approaching the sixth surface s 6 . This configuration enables the electromagnetic wave detection apparatus  105  to further reduce the angle formed by the fifth surface s 55  and the sixth direction d 6 , which is the propagation axis of electromagnetic waves that propagating after being incident again on the fourth surface s 4  from the reference surface ss. Thus, the electromagnetic wave detection apparatus  105  can increase the incident angle of electromagnetic waves propagated in the sixth direction d 6  with respect to the fifth surface s 55 , and thus can increase components that do not pass through the surface s 55  but is reflected thereby, from among a bundle of electromagnetic waves that is emitted and propagated in the sixth direction d 6 . As a result, the electromagnetic wave detection apparatus  105  has a large ratio of electromagnetic waves incident on the second detector  20  and thus can improve detection sensitivity. Such configuration and effect are applicable also to the electromagnetic wave detection apparatus according to the eighth embodiment, which will be described below. 
     Next, an electromagnetic wave detection apparatus according to an eighth embodiment of the present disclosure will be described. The eighth embodiment is different from the first embodiment, in terms of the configuration of the second propagation unit. Hereinafter, the sixth embodiment will be described focusing on aspects different from the first embodiment. Note that elements having the same configurations of the elements of the first embodiment, the second embodiment, the third embodiment, the fourth embodiment, the fifth embodiment, the sixth embodiment, or the seventh embodiment will be denoted by the same reference signs. 
     An electromagnetic wave detection apparatus  106  according to the eighth embodiment includes the first imaging unit  15 , the first propagation unit  16 , a second propagation unit  176 , the second imaging unit  18 , the first detector  19 , and the second detector  20 , as illustrated in  FIG. 13 . The configuration and functions of the information acquisition system  11  according to the eighth embodiment are the same as those of the first embodiment, except for the electromagnetic wave detection apparatus  106 . The configuration and function of the eighth embodiment are the same as those of the first embodiment, except for the second propagation unit  176 . 
     In the eighth embodiment, the second propagation unit  176  includes at least the first surface s 1 , the second surface s 2 , the third surface s 3 , the fourth surface s 4 , a fifth surface s 56 , and the sixth surface s 6 , in the same manner as the first embodiment. In the eighth embodiment, configurations and functions of the first surface s 1 , the second surface s 2 , the third surface s 3 , the fourth surface s 4 , and the sixth surface s 6  are the same as those of the first embodiment. In the eighth embodiment, the configuration and function of the fifth surface s 56  are the same as those of the seventh embodiment, except for an object to contact the fifth surface s 56 . 
     In the eighth embodiment, the second propagation unit  176  includes a total internal reflection prism and includes the first prism  21 , a second prism  226 , a third prism  266 , the first intermediate layer  231 , and the second intermediate layer  242 . Configurations and functions of the first prism  21  and the first intermediate layer  231  are the same as those of the third embodiment. Configurations of the second prism  226  and the third prism  266  themselves, arrangements of the second prism  226  and the third prism  266  with respect to the first prism  21 , and functions of the second prism  226  and the third prism  266  are the same as those of the seventh embodiment. Thus, the first intermediate layer  231  may be arranged between the first prism  21  and the second intermediate layer  242 . Further, the first intermediate layer  231  may include the second surface s 2  along the boundary surface with the first prism  21 . 
     In the eighth embodiment, the configuration and function of the second intermediate layer  242  are the same as that of the fourth embodiment. Thus, the second intermediate layer  242  has a refractive index smaller than that of the second prism  226  in the same manner as the fourth embodiment, and includes at least any one of, for example, vacuum, a gas, a liquid, and a solid, which has a refractive index smaller than that of the second prism  226 . In a configuration in which the second intermediate layer  242  is a gas or a liquid, the second intermediate layer  242  may be formed by providing spacers  250  on the respective peripheries of the third prism  266  and the fifth surface s 56  of the second prism  226  as illustrated in  FIG. 14  and filling the spacers  250  with a gas or a liquid. In the eighth embodiment, the second intermediate layer  242  may include an air layer or a prism. 
     Although the present disclosure has been described based on the figures and the embodiments, it should be appreciated that those who are skilled in the art may easily perform variations or alterations based on the present disclosure. Accordingly, such variations and alterations are to be included in the scope of the present disclosure. 
     For example, although the radiation unit  12 , the scanner  13 , and the controller  14  constitute the information acquisition system  11  together with the electromagnetic wave detection apparatus  10 ,  100 ,  101 ,  102 ,  103 ,  104 ,  105 , or  106  in the first to eighth embodiments, the electromagnetic wave detection apparatus  10 ,  100 ,  101 ,  102 ,  103 ,  104 ,  105 , and  106  may include at least one of them. 
     Although the first propagation unit  16  can switch the propagation direction of the electromagnetic waves incident on the reference surface ss in the two directions: the first selection direction ds 1  and the second selection direction ds 2  in the first to eighth embodiments, the first propagation unit  16  can switch the propagation direction between three or more directions, rather than two directions. 
     Further, although the first state and the second state of the first propagation unit  16  in the first to eighth embodiments are respectively the state to reflect electromagnetic waves incident on the reference surface ss in the first selection direction ds 1  and the state to reflect the electromagnetic waves in the second selection direction ds 2 , these states may refer to other conditions. 
     For example, the second state may refer a transmitting state in which electromagnetic waves incident on the reference surface ss are caused to pass and propagate in the first direction d 2 , as illustrated in  FIG. 15 . In particular, the first propagation unit  167  may include a shutter that is provided for each of the pixels px and includes a reflection surface for reflecting electromagnetic waves in the first selection direction ds 1 . The first propagation unit  167  including this configuration can switch between the reflection state serving as the first state and the transmission state serving as the second state, by opening or closing the shutter for each of the pixels px. 
     The first propagation unit  167  including the above configuration may be, for example, a propagation unit that includes a MEMS shutter in which a plurality of shutters capable of opening and closing are arranged in an array. The first propagation unit  167  including the above configuration may be, for example, a propagation unit that includes a liquid crystal shutter that can be switched between the reflection state for reflecting electromagnetic waves and the transmission state for transmitting electromagnetic waves, in accordance with a liquid crystal alignment. The first propagation unit  167  including this configuration can switch between the reflection state serving as the first state and the transmission state serving as the second state for each of the pixels px by switching the liquid crystal alignment for each of the pixels px. 
     In the first to eighth embodiments, the information acquisition system  11  includes the configuration in which the scanner  13  scans a beam of an electromagnetic wave emitted from the radiation unit  12 , and the second detector  20  functions as a scanning type active sensor in cooperation with the scanner  13 . However, the information acquisition system  11  is not limited to this configuration. An effect similar to the first to eighth embodiments can be obtained by, for example, a configuration in which the radiation unit  12  including a plurality of radiation sources capable of radiating radial electromagnetic waves performs a phased-scanning method for radiating electromagnetic waves from each of the radiation sources at phased radiation timings. An effect similar to the first to eighth embodiments can be obtained by, for example, a configuration in which the information acquisition system  11  does not include the scanner  13 , the radiation unit  12  emits radial electromagnetic waves, and information is acquired without scanning. 
     In the first to eighth embodiments, the information acquisition system  11  includes the configuration in which the first detector  19  serves as a passive sensor and the second detector  20  serves as an active sensor. However, the information acquisition system  11  is not limited to this configuration. An effect similar to the first to eighth embodiments can be obtained by, for example, a configuration in which both the first detector  19  and the second detector  20  serve as active sensors or passive sensors. In a case in which both the first detector  19  and the second detector  20  serve as active sensors, either the radiation unit  12  or respective radiation units  12  may emit electromagnetic waves to the object ob. Further, the respective radiation units  12  may emit electromagnetic waves of the same type or different types. 
     Note that a system as disclosed herein includes various modules and/or units configured to perform a specific function, and these modules and units are schematically illustrated to briefly explain their functionalities and do not specify particular hardware and/or software. In that sense, these modules, units, and other components simply need to be hardware and/or software configured to substantially perform the specific functions described herein. Various functions of different components may be realized by any combination or subdivision of hardware and/or software, and each of the various functions may be used separately or in any combination. Further, an input/output device, an I/O device, or user interface configured as, and not limited to, a keyboard, a display, a touch screen, and a pointing device may be connected to the system directly or via an intermediate  110  controller. Thus, various aspects of the present disclosure may be realized in many different embodiments, all of which are included within the scope of the present disclosure. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10 ,  100 ,  101 ,  102 ,  103 ,  104 ,  105 ,  106  electromagnetic wave detection apparatus 
               11  information acquisition system 
               12  radiation unit 
               13  scanner 
               14  controller 
               15  first imaging unit 
               16 ,  167  first propagation unit 
               17 ,  170 ,  171 ,  172 ,  173 ,  174 ,  175 ,  176  second propagation unit 
               18  second imaging unit 
               19  first detector 
               20  second detector 
               21  first prism 
               22 ,  220 ,  221 ,  223 ,  224 ,  225 ,  226  second prism 
               23 ,  230 ,  231  first intermediate layer 
               240 ,  242  second intermediate layer 
               250  spacer 
               261 ,  262 ,  263 ,  264 ,  265 ,  266  third prism 
             d 1 , d 2 , d 3 , d 4 , d 5 , d 6 , d 7  first direction, second direction, third direction, fourth direction, fifth direction, sixth direction, seventh direction 
             ds 1 , ds 2  first selection direction, second selection direction 
             ob object 
             px pixel 
             s 1 , s 2 , s 3 , s 4 , s 6  first surface, second surface, third surface, fourth surface, sixth surface 
             s 5 , s 50 , s 51 , s 53 , s 54 , s 55 , s 56  fifth surface 
             ss reference surface