Patent Publication Number: US-2020293806-A1

Title: Camera system and facility

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a camera system and a facility. 
     Description of the Related Art 
     An inspection technique using a terahertz wave is known. The terahertz wave can be defined as an electromagnetic wave having a frequency of 30 (inclusive) GHz to 30 THz (inclusive). Japanese Patent Laid-Open No. 2004-286716 discloses a method of inspecting a prohibited drug such as a narcotic drug enclosed in a sealed letter. In this method, a characteristic absorption spectrum that a prohibited drug such as a narcotic drug has in the terahertz band is used to identify a substance in a sealed letter without breaking the seal. 
     SUMMARY OF THE INVENTION 
     Recently, dangerous items such as knives being taken into a railroad coach through a railroad station is a serious problem from the viewpoint of crime prevention. There is strong demand for a technique of detecting such a dangerous item. 
     Some embodiments of the present invention provide a technique advantageous in improving crime prevention by a camera system installed in a facility. 
     According to some embodiments, a camera system arranged to form a part of a monitoring system arranged in a place of a facility, where an inspection object lines up, comprising an imaging system configured to acquire an image formed by a terahertz wave reflected by the inspection object, is provided. 
     According to some other embodiments, a facility including a camera system, wherein the camera system arranged to form a part of a monitoring system arranged in a place of a facility, where an inspection object lines up, comprises an imaging system configured to acquire an image formed by a terahertz wave reflected by the inspection object, is provided. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are views showing an arrangement example of a ticket gate machine in which an imaging system included in a camera system according to an embodiment is arranged; 
         FIGS. 2A and 2B  are views showing a modification of the ticket gate machine shown in  FIGS. 1A and 1B ; 
         FIGS. 3A and 3B  are views showing an arrangement example of a partition in which the imaging system included in the camera system according to the embodiment is arranged; 
         FIGS. 4A to 4C  are views showing a modification of the partition shown in  FIGS. 3A and 3B ; 
         FIGS. 5A and 5B  are views showing an arrangement example of an escalator in which the imaging system included in the camera system according to the embodiment is arranged; 
         FIGS. 6A and 6B  are views showing an arrangement example of a staircase in which the imaging system included in the camera system according to the embodiment is arranged; 
         FIGS. 7A and 7B  are views showing an anangement example of a passage in which the imaging system included in the camera system according to the embodiment is arranged; 
         FIG. 8  is a view showing an anangement example of a station in which the imaging system included in the camera system according to the embodiment is arranged; 
         FIG. 9  is a view showing an anangement example when the camera system according to the embodiment and a railroad coach monitor an inspection object; 
         FIG. 10  is a flowchart showing an operation example of the camera system according to the embodiment; 
         FIG. 11  is a flowchart showing an operation example of the camera system according to the embodiment; 
         FIG. 12  is a flowchart showing an operation example of the camera system according to the embodiment; and 
         FIG. 13  is a flowchart showing an operation example of the camera system according to the embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments will be described in detail with reference to the attached drawings. Note, the following embodiments are not intended to limit the scope of the claimed invention. Multiple features are described in the embodiments, but limitation is not made an invention that requires all such features, and multiple such features may be combined as appropriate. Furthermore, in the attached drawings, the same reference numerals are given to the same or similar configurations, and redundant description thereof is omitted. 
     A camera system  200  according to some embodiments of the present invention will be described with reference to  FIGS. 1A to 13 . The camera system  200  according to this embodiment is installed in a facility. Examples of the facility are terminals such as a railroad station and an airport, a commercial facility, and an amusement facility. The camera system  200  according to this embodiment is installed in a structure of a railroad station or the like. Structures of a station include a station building in which a ticket gate, a ticket office, a waiting room, and the like are arranged, a platform at which a railroad coach arrives, a passage that connects the station building and the platform, and the like. Here, the passage can be not only a flat passage but also a place where passengers pass, such as a staircase, an escalator, or an elevator. In addition, a railroad coach is an example of a movable body. If the facility is an airport, the movable body is, for example, an airplane. The camera system  200  according to this embodiment can be a camera system arranged so as to form a part of the monitoring system of a station. 
     The camera system  200  includes an imaging system  201  configured to acquire an image formed by a terahertz wave reflected by an inspection object  250 . The imaging system  201  can include at least one illumination unit  202  configured to irradiate a terahertz wave, and at least one camera  203  configured to acquire an image formed by the terahertz wave. The illumination unit  202  is also referred to as the irradiation unit. To discriminate a plurality of illumination units  202  and a plurality of cameras  203  in the following explanation, a suffix is added to each reference numeral, like an illumination unit  202 “a” and a camera  203 “a”. If the illumination units and the cameras need not be discriminated, they are expressed simply as “illumination unit  202 ” and “camera  203 ”. This also applies to other constituent elements. 
     In this embodiment, the camera  203  that detects a terahertz wave is of a type called active camera, and can be used in combination with the illumination unit  202 . However, the camera is not limited to this, and may be a camera of a passive type. In this case, without illuminating the inspection object  250  with a terahertz wave irradiated from the illumination unit  202 , and an image can be acquired by a terahertz wave radiated from the inspection object  250 . 
     The imaging system  201  can be arranged to capture the inspection object  250  that uses the station. The inspection object  250  is normally a person but may be an animal other than a person or a robot. A terahertz wave passes through a fabric, a leather, and the like. For this reason, a processor (for example, a control system  310  to be described later) (not shown) connected to the camera system  200  can detect a dangerous item such as a firearm, a cutting tool, or an explosive based on an image provided from the imaging system  201  of the camera system  200 . 
       FIGS. 1A and 1B  are a plan view and a front view, respectively, showing an arrangement example of a ticket gate machine  211  in which the imaging system  201  included in the camera system  200  according to the present invention is arranged. The ticket gate machine  211  is installed in a ticket gate of a station, and separates the inside of the ticket gate and the outside of the ticket gate. Here, the inside of the ticket gate can be an area that needs a ticket such as a platform ticket or a boarding ticket to enter. The ticket gate machine  211  may be an automatic ticket gate machine. The imaging system  201  is arranged to acquire an image of the inspection object  250  that passes through a passage  240  of the ticket gate machine  211 . For example, the imaging system  201  may acquire the image of the inspection object  250  entering from the outside of the ticket gate to the inside of the ticket gate. A description will be made below assuming that the inspection object  250  passes from the outside of the ticket gate to the inside of the ticket gate in the direction of an arrow shown in  FIG. 1A . 
     In the arrangement shown in  FIGS. 1A and 1B , the ticket gate machine  211  includes a ticket gate machine  211   a  and a ticket gate machine  211   b , which are arranged to face each other across the passage. That is, the width and length of the passage  240  of the ticket gate machine  211  can be decided by the ticket gate machine  211   a  and the ticket gate machine  211   b . The imaging system  201  includes the illumination unit  202  arranged on the ticket gate machine  211   a , and the camera  203  arranged on the ticket gate machine  211   b . A terahertz wave irradiated from the illumination unit  202  can be specularly reflected by the inspection object  250  such as a person. For this reason, when the illumination unit  202  and the camera  203  are arranged on the ticket gate machine  211   a  and the ticket gate machine  211   b , which face each other across the passage  240 , respectively, the terahertz wave irradiated from the illumination unit  202  is readily reflected by the inspection object  250  and detected by the camera  203 . 
     As shown in  FIG. 1B , the illumination unit  202  can use a range  204  of almost the whole passage  240  as an irradiation range. The spread of the terahertz wave irradiated from the illumination unit  202  can be adjusted by using a lens or the like. In addition, the terahertz wave is reflected by a metal or the like. Hence, the inspection object  250  is illuminated even near the lower portion of the ticket gate machine  211   a  because the terahertz wave is reflected by the side surface of the ticket gate machine  211   b . Furthermore, to effectively use the reflection of the terahertz wave by each side surface of the ticket gate machine  211  on the side of the passage  240 , the illumination unit  202  and the camera  203  may be arranged near the end portion of the ticket gate machine  211  on the opposite side of the directions of the optical axes of the illumination unit  202  and the camera  203 , as shown in  FIG. 1A . That is, the illumination unit  202  and the camera  203  configured to capture the outer side of the ticket gate from the ticket gate machine  211  may be arranged near the end portion of the ticket gate machine  211  on the inner side of the ticket gate. 
     The arrangement of the illumination unit  202  and the camera  203  is not limited to the above-described arrangement. For example, the illumination unit  202  and the camera  203  may be arranged on the ticket gate machine  211   a . Alternatively, for example, the illumination unit  202  and the camera  203  may be arranged near the center of the ticket gate machine  211 , or may be arranged near the end portion on the outer side of the ticket gate in  FIG. 1A . For one camera  203 , the illumination unit  202  may be formed by a plurality of illumination devices. For example, the illumination unit  202  may be formed by a plurality of illumination devices whose terahertz wave irradiation directions are different. In addition, the illumination unit  202  and the camera  203  may be fixed to the ticket gate machine  211  in an immovable state, or may be arranged, for example, rotatably in accordance with the movement of the inspection object  250 . 
     If the imaging system  201  of the camera system  200  is used as a surveillance camera, in some cases, post-processing such as image processing by a processor (not shown) at the subsequent stage of the imaging system  201  of the camera system  200  is facilitated by capturing the person who is the inspection object  250  one by one. The ticket gate machine  211  passes the person who is the inspection object  250  one by one at a high possibility. Hence, when the imaging system  201  is arranged in the ticket gate machine  211 , the load of post-processing such as image processing can be suppressed. That is, the imaging system  201  can be arranged in a place where the inspection object  250  lines up. In addition, the time needed for the person that is the inspection object  250  to pass through the ticket gate machine  211  is about 1 sec. However, the imaging system  201  can acquire an image formed by a terahertz wave at a frame rate of 50 fps or more. For this reason, it is possible to perform capture one inspection object  250  at plurality of times. In the plurality of times of image capturing, the inspection object  250  may be captured wholly or may be captured only partially. 
     The imaging system  201  of the camera system  200  may include a sensor  260  configured to detect that the inspection object  250  approaches. For example, the ticket gate machine  211  may be provided with the sensor  260 , as shown in  FIG. 1B . Alternatively, for example, the sensor  260  may be added to the illumination unit  202  or the camera  203 . The illumination unit  202  is controlled based on the output of the sensor  260 . For example, the illumination unit  202  may start irradiating the terahertz wave in accordance with detection of the inspection object  250  by the sensor  260 . This can suppress the power consumed by the imaging system  201 . 
       FIG. 2A  shows an example in which as the imaging system  201 , two sets of illumination units  202  and cameras  203  are arranged on the ticket gate machine  211 . As shown in  FIG. 2A , illumination units  202   a  and  202   b  are arranged on the ticket gate machine  211   a , and cameras  203   a  and  203   b  are arranged on the ticket gate machine  211   b . At this time, as shown in  FIG. 2A , the illumination unit  202   a  and the camera  203   a  are arranged to illuminate and capture the inner side of the ticket gate from the ticket gate machine  211 , and the illumination unit  202   b  and the camera  203   b  are arranged to illuminate and capture the outer side of the ticket gate from the ticket gate machine  211 . With this arrangement, not only the front side but also the rear side of the inspection object  250  can be captured. At this time, as described above, to efficiently use terahertz waves irradiated from the illumination units  202   a  and  202   b , the illumination unit  202   a  and the camera  203   a  may be arranged on the inner side of the ticket gate with respect to the illumination unit  202   b  and the camera  203   b . However, the arrangement of the illumination units  202   a  and  202   b  and the cameras  203   a  and  203   b  is not limited to this, and they can freely be arranged, as described above. Additionally, as described above, each side surface of the ticket gate machine  211  on the side of the passage  240  may form a reflecting surface that reflects the terahertz wave. That is, each surface of the ticket gate machine  211  on the side of the passage  240  may be made of a metal, or may include a rough surface with an unevenness of about 1/10 of the wavelength of the terahertz wave. Additionally, for example, as shown in  FIG. 2B , the ticket gate machine  211  may have a gate-shaped structure including an upper structure  212  for more reflection of the terahertz wave. At this time, each surface of the upper structure  212  on the side of the passage  240  may be made of a metal, or may include a rough surface with an unevenness of about 1/10 of the wavelength of the terahertz wave. When the terahertz wave is reflected or scattered by the surfaces of the ticket gate machine  211 , the inspection object  250  is illuminated from various angles, and the quality of images obtained by the camera  203  can be improved. 
     Additionally, in  FIGS. 1A to 2B , the illumination unit  202  and the camera  203  are illustrated large as separated bodies on the ticket gate machine  211  to simplify the description. However, the present invention is not limited to this. The terahertz wave can pass through a material such as a resin. For this reason, a window made of a resin may be provided in a part of the ticket gate machine  211 , and the illumination unit  202  or the camera  203  may be arranged in the ticket gate machine  211 . As the resin, for example, an appropriate material such as high-density polyethylene or cyclic olefin copolymer can be used. In the following explanation as well, the illumination unit  202  and the camera  203  are illustrated large in the drawings. 
     An example in which the imaging system  201  is applied to a partition wall  213  on a platform  216  of a station will be described next with reference to  FIGS. 3A and 3B .  FIGS. 3A and 3B  are a plan view and a front view, respectively, showing an arrangement example of the partition wall  213  in which the imaging system  201  included in the camera system  200  according to the present invention is arranged. The imaging system  201  is arranged to be adjacent to the partition wall  213  configured to partition a platform  216  and a track-side area  217  and including a door portion  214  capable of opening and closing. The partition wall  213  is a so-called platform screen door installed on the platform  216 . In this embodiment, the imaging system  201  acquires an image of the inspection object  250  that passes through a passage  241  when the door portion  214  of the partition wall  213  opens. 
     The imaging system  201  includes the illumination unit  202   a  and the camera  203   a , which are arranged in the track-side area  217 . The illumination unit  202   a  and the camera  203   a  perform illumination and image capturing of the passage  241  from the track-side area  217  when the door portion  214  opens. In addition, the imaging system  201  includes the illumination unit  202   b  and the camera  203   b , which are arranged in the platform  216 . The illumination unit  202   b  and the camera  203   b  perform illumination and image capturing of the passage  241  from the platform  216  when the door portion  214  opens. When the illumination unit  202   a  and the camera  203   a , and the illumination unit  202   b  and the camera  203   b  are arranged, it is possible to acquire the front- and rear-side images of both the inspection object  250  that gets in a railroad coach  218  via a door  219  and the inspection object  250  that gets off the railroad coach  218  via the door  219 . However, the present invention is not limited to this, and only the illumination unit  202   a  and the camera  203   a  or only the illumination unit  202   b  and the camera  203   b  may be arranged. 
     Each of the illumination units  202   a  and  202   b  may include a plurality of illumination devices, as shown in  FIGS. 3A and 3B . As shown in  FIGS. 3A and 3B , the illumination units  202   a  and  202   b  may be arranged at an end portion, in the direction of opening and closing the door portion  214 , of a door pocket portion  215  in the partition wall  213 , which stores the door portion  214  when opening the door portion  214 . Additionally, as shown in  FIG. 3A , when the door portion  214  opens, a part of the door portion  214  is not stored in the door pocket portion  215  in some cases. In this case, the side surface of the part of the door portion  214 , which is not stored, may form a reflecting surface that reflects the terahertz wave, like the side surface of the above-described ticket gate machine  211  on the side of the passage  240 . This makes it possible to more efficiently use the terahertz waves irradiated from the illumination units  202   a  and  202   b . In addition, the vehicle body of the railroad coach  218  may be used as a reflecting surface that reflects the terahertz wave. When the door portion  214  of the partition wall  213  is opened to form the passage  241 , the railroad coach  218  may have arrived. The vehicle body of the railroad coach  218  can be made of a metal. For this reason, the vehicle body of the railroad coach  218  can be used as the reflecting surface that reflects the terahertz wave. 
     In addition, as shown in  FIG. 3B , the cameras  203   a  and  203   b  may be attached to a structure such as a pole  220 . As the camera  203   a  or  203   b , a plurality of image capturing devices may be used, as shown in  FIG. 3B . The cameras  203   a  and  203   b  may be arranged, for example, at the height of the waist of the inspection object  250  or at a position higher than the inspection object  250 , as shown in  FIG. 3B . When the cameras  203   a  and  203   b  are arranged at a high position, even if the interval of the inspection objects  250  in the front-and-rear direction is small, the possibility that the images can be acquired one by one becomes higher than in a case in which the cameras  203   a  and  203   b  are arranged at a low position. In addition, when the cameras  203   a  and  203   b  are arranged at an angle with respect to the passage  241 , the possibility of acquiring the front and rear images of the inspection object  250  becomes high. 
     The arrangement of the illumination units  202   a  and  202   b  and the cameras  203   a  and  203   b  is not limited to the arrangement shown in  FIGS. 3A and 3B . For example, as shown in  FIGS. 4A to 4C , the illumination units  202   a  and  202   b  may be attached to a structure such as the pole  220 . At this time, as shown in  FIGS. 4A and 4B , the illumination unit  202  and the camera  203  may be attached to separate poles  220   a  and  220   b . Additionally, for example, as shown in  FIG. 4C , the illumination unit  202  and the camera  203  may be attached to the same pole  220 . 
     As described above, if the imaging system  201  is used as a surveillance camera, the load of post-processing such as image processing can be reduced by capturing the person who is the inspection object  250  one by one. Hence, the imaging system  201  included in the camera system  200  may be applied to the partition wall  213  installed in a station where a bullet train or a limited express for which persons line up and get in one by one stops. In this case, the width of the door  219  of the railroad coach  218  used for the bullet train or limited express is about 700 mm to 1,000 mm. Hence, in the arrangement shown in  FIG. 3A or 4A , the maximum distance between the illumination unit  202  and the camera  203  can be decreased to, for example, about 1,100 mm (inclusive) to 2,000 mm (inclusive). This makes it possible to efficiently use the terahertz wave irradiated from the illumination unit  202 . The bullet train or limited express often has a fixed train formation. Hence, the size of the door portion  214  can be changed in accordance with the size of the door  219  of the railroad coach  218 . Hence, for example, the distance between the illumination unit  202  and the camera  203  adjacent to the door portion  214  corresponding to the door  219  having a small width (for example, 700 mm), may be, for example, 700 mm (inclusive) to 1,000 mm (inclusive). In this case, the maximum distance between the illumination unit  202  and the camera  203  may be, for example, 850 mm or more. 
     For example, the imaging system  201  may be applied to, for example, the partition wall  213  in a railroad station of a commuter train or the like. In this case, it may be possible to acquire images of persons one by one except during rush hours. Even in a case in which a plurality of inspection objects  250  simultaneously get in or get off, the inspection objects  250  often line up in two or three lines and get in. Each inspection object  250  can be distinguished by image processing or the like using a processor included in the camera system  200 . In a commuter train or the like, the width of a door is about 1,300 mm to 2,000 mm. For this reason, in the arrangement shown in  FIG. 3A or 4A , the maximum distance between the illumination unit  202  and the camera  203  may be, for example, 1,500 mm (inclusive) to 3,000 mm (inclusive). The output of the terahertz wave irradiated from the illumination unit  202  may be changed in accordance with the distance between the illumination unit  202  and the camera  203 . In this case, the illumination unit  202  whose distance to the camera  203  is longer may irradiate the terahertz wave at an output higher than the illumination unit of a shorter distance. 
     In some cases, the platform  216  is arranged outdoors. Hence, the imaging system  201  arranged on the platform  216  or the track-side area  217  is readily affected by the external environment. A terahertz wave is readily absorbed by water, and it may be impossible to obtain images with sufficient image quality in a highly humid environment such as a rainfall. Hence, the imaging system  201  may include a sensor  261  configured to detect the external environment, as shown in  FIG. 3B . The illumination unit  202  is controlled based on the output of the sensor  261 . For example, if the sensor  261  detects the information of humidity, and the humidity is high, the output of the illumination unit  202  to irradiate the terahertz wave may be increased. This can improve the quality of the image acquired by the camera  203 . 
     In addition, for example, the illumination unit  202  or the camera  203  may be attached to the vehicle body of the railroad coach  218 . That is, the camera system  200  may include an illumination unit or a camera mounted on the railroad coach  218 . In this case, the camera system  200  can include a communication unit between the imaging system  201  arranged in a station and the imaging system including the illumination unit or the camera included in the railroad coach  218 . 
     In addition, for example, the illumination unit  202  and the camera  203  may start operations when the door portion  214  of the partition wall  213  opens. For example, the imaging system  201  may synchronize with the operation of the door portion  214 , or may include a sensor configured to detect that the door portion  214  has opened. This can suppress power consumption of the imaging system  201 . 
     An example in which the imaging system  201  is applied to an escalator  221  will be described next with reference to  FIGS. 5A and 5B .  FIGS. 5A and 5B  are a side view and a plan view, respectively, showing an arrangement example of the escalator  221  in which the imaging system  201  included in the camera system  200  according to the present invention is arranged. 
     The imaging system  201  is arranged to be adjacent to the escalator  221  to acquire the image of the inspection object  250  that passes through the escalator  221 . In the arrangement shown in  FIG. 5A , the imaging system  201  includes the illumination unit  202  and the camera  203 . The illumination unit  202  may include a plurality of illumination devices, and the camera  203  may include a plurality of image capturing devices. In addition, the illumination unit  202  and the camera  203  may be arranged on separate structures such as the poles  220 , as shown in  FIG. 5A , or may be arranged on the same structure such as the pole  220 . For example, as shown in  FIG. 5A , the illumination unit  202  and the camera  203  may be arranged to face a direction opposite to the advancing direction of the escalator  221  and perform illumination and image capturing. This makes it possible to acquire an image of the inspection object  250  on the front side. 
     For example, as shown in  FIG. 5B , the illumination unit  202  and the camera  203  may be arranged in a direction crossing the advancing direction of the escalator  221  while sandwiching the escalator  221 . It is possible to obtain the same effects as the illumination unit  202  arranged on the ticket gate machine  211   a  and the camera  203  arranged on the ticket gate machine  211   b  described above. In addition, at this time, the illumination unit  202   a  and the camera  203   a  may be arranged to face the direction opposite to the advancing direction of the escalator  221  and perform illumination and image capturing, and the illumination unit  202   b  and the camera  203   b  may be arranged to face the advancing direction of the escalator  221  and perform illumination and image capturing. This makes it possible to capture not only the front side but also the rear side of the inspection object  250 . 
     As described above, if the imaging system  201  is used as a surveillance camera, it may be advantageous that the person who is the inspection object  250  can be captured one by one. Since the escalator  221  operates at a predetermined speed, the possibility that the image of the inspection object  250  can be acquired one by one is high. Additionally, as shown in  FIG. 5A , the escalator  221  includes steps. When the camera  203  is arranged at a high position, the possibility that the image can be acquired one by one becomes higher. On the escalator  221 , the inspection objects  250  normally line up in one or two lines. For example, in the escalator  221  on which the inspection objects line up in two lines, the images of the inspection objects  250  may be acquired using two cameras  203 . This raises the possibility that the images can be acquired one by one. As a result, the load of image processing at the subsequent stage of the imaging system  201  of the camera system  200  can be reduced. 
     In addition, as described above, a terahertz wave can pass through a resin or the like. For this reason, the illumination unit  202  or the camera  203  may be embedded in the floor, wall, or ceiling of the portion where the escalator  221  is arranged. For example, the illumination unit  202  may be installed on the deck board of the escalator  221  together with a normal illumination. 
     An example in which the imaging system  201  is applied to a staircase  222  will be described next with reference to  FIGS. 6A and 6B .  FIGS. 6A and 6B  are a plan view and a sectional view, respectively, showing an arrangement example of the staircase  222  in which the imaging system  201  included in the camera system  200  according to the present invention is arranged. 
     The imaging system  201  is arranged in the staircase  222  to acquire the image of the inspection object  250  that passes through the staircase  222 . In the arrangement shown in  FIGS. 6A and 6B , the imaging system  201  includes the illumination unit  202  and the camera  203 . The illumination unit  202  and the camera  203  are embedded in the staircase  222  and arranged to illuminate and capture the inspection object  250  from a window  224  of a riser portion  223  of the staircase  222 . As shown in  FIGS. 6A and 6B , the illumination unit  202  and the camera  203  can be arranged in the same riser portion  223  of the staircase  222 . 
     Since the person who is the inspection object  250  goes up or down the staircase  222  one by one (or by about two steps), the image of the inspection object  250  can sequentially be acquired from the head (or foot) of the inspection object  250  to the foot (or head). 
     For the window  224  provided in the riser portion  223  of the staircase  222 , various kinds of resins that pass a terahertz wave can be used, as described above. When an appropriate resin material is selected in accordance with the material used for the riser portion  223  or a tread portion  225  of the staircase  222 , where the imaging system  201  is not arranged, the imaging system  201  can be made unnoticeable (its existence can be hidden). 
     An example in which the imaging system  201  is applied to a passage  242  will be described next with reference to  FIGS. 7A and 7B .  FIGS. 7A and 7B  are side views showing an arrangement example of the passage  242  in which the imaging system  201  included in the camera system  200  according to the present invention is arranged. 
     The imaging system  201  is arranged in the passage  242  to acquire the image of the inspection object  250  that passes through the passage  242 . The imaging system  201  includes the illumination unit  202  and the camera  203 . At this time, one of the illumination unit  202  and the camera  203  is arranged on a ceiling  227  of the passage  242 , and the other of the illumination unit  202  and the camera  203  is embedded in a floor  226  of the passage  242 . In the arrangement shown in  FIGS. 7A and 7B , the camera  203  is arranged on the ceiling  227  of the passage  242 , and the illumination unit  202  is embedded in the floor  226  of the passage  242 . However, the present invention is not limited to this. The illumination unit  202  may be arranged on the ceiling  227  of the passage  242 , and the camera  203  may be embedded in the floor  226  of the passage  242 . 
     In the arrangement shown in  FIGS. 7A and 7B , the illumination unit  202   a  and the camera  203   a  are arranged to perform illumination and image capturing from one side of the passage  242  in the passing direction to the other side. In addition, the illumination unit  202   b  and the camera  203   b  are arranged to perform illumination and image capturing from the other side of the passage  242  in the passing direction to the one side. This makes it possible to acquire the front- and rear-side images of the inspection object  250  that advances in both the two passing directions of the passage  242 . However, the present invention is not limited to this, and only the illumination unit  202   a  and the camera  203   a  may be arranged on the passage  242 . 
       FIG. 7B  shows an example in which the cameras  203   a  and  203   b  are embedded in the ceiling  227  of the passage  242 . Accordingly, the cameras  203   a  and  203   b  can be made unnoticeable (their existence can be hidden) as compared to a case in which the cameras  203   a  and  203   b  are suspended from the ceiling  227 , as shown in  FIG. 7A . In addition, the optical axis of the illumination unit  202  or the camera  203  is set at a larger angle with respect to the advancing direction of the inspection object  250  in the arrangement shown in  FIG. 7B  than in the arrangement shown in  FIG. 7A . When the angles of the optical axes are set large, as indicated by dotted lines shown in  FIGS. 7A and 7B , the possibility that the images of the inspection object  250  can be acquired one by one can become high. 
     Additionally, in the arrangement shown in  FIGS. 7A and 7B , one of the illumination unit  202  and the camera  203  is arranged in the floor  226 , and the other is arranged on the ceiling  227 . As described above, a terahertz wave can be specularly reflected by the inspection object  250 . Hence, when the illumination unit  202  and the camera  203  are arranged to face each other, the terahertz wave irradiated from the illumination unit  202  can readily be detected by the camera  203 . 
     However, the arrangement of the illumination unit  202  and the camera  203  on the passage  242  is not limited to the arrangement shown in  FIGS. 7A and 7B . For example, the illumination unit  202  and the camera  203  may be arranged on a side wall of the passage  242  or the like. Both the illumination unit  202  and the camera  203  may be arranged on the floor  226  or the ceiling  227 . In this case, the floor  226  or the ceiling  227  on which the illumination unit  202  and the camera  203  are not arranged may function as a reflecting surface that reflects the terahertz wave. For example, the entire interior of the passage  242  except a portion of the illumination unit  202  or the camera  203 , which functions as a window to pass the terahertz wave, may function as a reflecting surface that reflects the terahertz wave. Additionally, for example, the illumination unit  202  may include a plurality of illumination devices. In this case, the plurality of illumination devices included in the illumination unit  202  may be arranged in an appropriate number in an appropriate place such as the floor  226 , the ceiling  227 , or the side wall. 
     As described above, when the imaging system  201  is arranged in the staircase  222  or the passage  242 , a plurality of cameras  203  may be arranged in the widthwise direction of the staircase  222  or the passage  242 . Accordingly, the possibility that the image of the inspection object  250  can be captured one by one becomes high. In addition, the imaging system  201  may be arranged in a portion of the staircase  222  or the passage  242 , where the width decreases. In the portion of the staircase  222  or the passage  242 , where the width decreases, the inspection object  250  can easily line up. 
       FIG. 8  is a view showing an arrangement example of a station  245  in which the imaging system  201  included in the camera system  200  is arranged. As described above, the imaging system  201  can be arranged on the ticket gate machine  211  at the ticket gate, the passage  242 , the escalator  221 , the staircase  222 , the partition wall  213 , or the like. The place to arrange the imaging system  201  including the illumination unit  202  and the camera  203  included in the camera system  200  according to this embodiment is not limited to the above-described places. For example, the imaging system  201  may be arranged in another place such as the entrance or hand wash basin of a restroom, where it is considered that the image of the inspection object  250  can be acquired one by one. The above-described imaging system  201  including the illumination unit  202  and the camera  203  and configured to acquire an image using the terahertz wave may be arranged in a place where a normal surveillance camera using visible light is installed. 
     Additionally, the camera system  200  according to this embodiment can monitor the inspection object  250  shown in  FIG. 8  in cooperation with the railroad coach  218 .  FIG. 9  shows an arrangement example of a monitoring system in which the camera system  200  monitors the inspection object  250  in cooperation with the railroad coach  218 . The camera system  200  can include the control system  310  and a communication unit  315  in addition to the above-described imaging system  201  arranged in the station. The control system  310  processes a signal output from the imaging system  201 . The processing can include deciding a risk concerning the inspection object  250 . The processing can include specifying the position of the inspection object  250  having a predetermined risk. Alternatively, the processing can include specifying the seat of the inspection object  250  that has got in the railroad coach  218  based on, for example, a ticket that the inspection object  250  with the predetermined risk has put into the ticket gate machine  211  when passing through the ticket gate machine  211 . The control system  310  can be formed by, for example, a PLD (short for Programmable Logic Device) such as an FPGA (short for Field Programmable Gate Array), a processor such as an ASIC (short for Application Specific Integrated Circuit), or a general-purpose or dedicated computer in which a program is installed, or a combination of some or all of them. 
     The control system  310  can specify the inspection object  250  based on the feature information of the inspection object  250  and correspondence information that associates the feature information with seat information assigned to a passenger having a feature corresponding to the feature information. The feature information can be information extracted by the control system  310  from an image obtained by the imaging system  201 . The feature information may be, for example, a feature amount specified based on the shape, the size, and the like of a partial image extracted from an image obtained by the imaging system  201 , may be information that specifies the type of a dangerous item, or may be information representing another feature. Alternatively, the feature information may be information representing the above-described risk. Extraction of the partial image from the image acquired by the imaging system  201  can include, for example, extracting a portion having a brightness more than a predetermined brightness. AI (Artificial Intelligence) can be used to extract the feature information. More specifically, AI that has undergone deep learning is installed in the control system  310 , and the feature information can be extracted by the AI. For example, information representing a risk in an image captured by the camera  203  appears in a different manner depending on the position and orientation of the camera  203 . Hence, deep learning can be executed based on images captured by a plurality of cameras  203 . 
     The control system  310  can transmit the result of the above-described processing to a terminal  320  set in advance via the communication unit  315 . The terminal  320  can be carried by, for example, a conductor in the railroad coach  218 . The terminal  320  may include a terminal carried by a person other than the conductor in the railroad coach  218 , a terminal provided in a security office arranged in a station or the like, and a terminal provided in an administrative body such as a police station. 
     The imaging system  201  can acquire the image of the inspection object  250  that passes through the ticket gate machine  211  and transmit the obtained image to the control system  310 . The control system  310  can decide the risk of the inspection object based on the image received from the imaging system  201 . In addition, the control system  310  can extract the feature information of the inspection object from the image received from the imaging system  201 . 
     The control system  310  generates correspondence information that associates the feature information of the inspection object  250  extracted from the image received from the imaging system  201  with seat information read by the ticket gate machine  211 . For example, the feature information can be information strongly suggesting holding of a gun, and the seat information can be seat information read by the ticket gate machine  211  from a ticket held by the inspection object  250  that holds the gun. The correspondence information can be transmitted from the control system  310  to the terminal  320  in the railroad coach  218 . The feature information may include information that identifies the ID of the inspection object  250  (that is, information that specifies an individual). The imaging system  201  may include a visible light camera, and the ID of the inspection object  250  can be identified from the visible light image of the inspection object  250  or from the visible light image by AI or the like. The visible light image of the inspection object having a predetermined risk can be transmitted to the railroad coach  218  together with the above-described correspondence information and can further be transmitted to the terminal  320 . A case in which the image of the inspection object  250  is acquired by the imaging system  201  arranged on the ticket gate machine  211  has been described here. However, tracking of the inspection object  250  may be started or continued based on an image obtained from the imaging system  201  arranged on the partition wall  213 , the escalator  221 , the staircase  222 , or the passage  242 . In addition, tracking of the inspection object  250  may be started based on an image obtained by the imaging system  201  arranged on the ticket gate machine  211 , and after that, tracking of the inspection object  250  may be continued using a surveillance camera using visible light. 
     The operation of the camera system  200  will be described next with reference to  FIG. 10 .  FIG. 10  is a flowchart showing an example of the operation of the camera system  200  according to this embodiment. As the arrangement of the camera system  200 , the arrangement of the above-described embodiment can be applied. In this embodiment, an operation after an image (to be sometimes referred to as a terahertz image) based on a terahertz wave is acquired by the camera system  200  will be described. In  FIG. 10 , the camera system  200  evaluates the acquired image, and if the quality is less than desired quality, performs an operation of capturing an image again (recapturing). 
     First, in step S 1001 , the illumination unit  202  irradiates the inspection object  250  with a terahertz wave under a desired condition. Next, in step S 1002 , the camera  203  detects the terahertz wave reflected by the inspection object  250  and acquires information based on the terahertz wave. In step S 1003 , a control unit performs processing of converting the information based on the terahertz wave into an image. Here, the control unit can be, for example, the control system  310  shown in  FIG. 9 . 
     Next, the control unit evaluates the quality of the acquired terahertz image (step S 1004 ). As the evaluation items, items representing whether an appropriate terahertz image according to the inspection object  250  can be acquired, whether an article can be detected from the terahertz image of the image quality, and the like can appropriately be set. In the image quality evaluation, if desired image quality is not satisfied, the camera system  200  performs the operation of step S 1005 . In step S 1005 , the control unit supplies, to the illumination unit  202 , a control signal for changing the wavelength and increasing the power of the irradiated terahertz wave. The illumination unit  202  performs terahertz wave irradiation (step S 1001 ) again. With the series of operations, a desired article can appropriately be detected. 
     Note that in the image evaluation, upon determining that the desired image quality can be obtained, the control unit judges the presence and absence of a detected article, and in some cases, the type of the article (step S 1006 ). If an article is detected, the control unit causes a monitor system to display an alert. Alternatively, the control unit outputs an instruction to perform an operation of adding a flag to an article or person of high risk (step S 1007 ). If no article is detected, the control unit may add a flag to the confirmed person of low risk (step S 1008 ). 
     For the series of operations, the illumination unit  202  and the camera  203  can be used in the following combinations. If there are the illumination unit  202  and the camera  203  used in the first capturing, second and subsequent capturing may be executed using the same illumination unit  202  and the same camera  203 . In addition, if there are the illumination unit  202  and the camera  203  used in the first capturing, second and subsequent capturing may be executed using the same illumination unit  202  as in the first capturing and the camera  203  different from that in the first capturing. Furthermore, if there are the illumination unit  202  and the camera  203  used in the first capturing, second and subsequent capturing may be executed using the illumination unit  202  different from that in the first capturing and the same camera as in the first capturing. Furthermore, if there are the illumination unit  202  and the camera  203  used in the first capturing, second and subsequent capturing may be executed using the illumination unit  202  and the camera  203  which are different from those in the first capturing. 
     Another operation of the camera system  200  will be described next with reference to  FIG. 11 . In this example, an operation of capturing an image in synchronism with opening and closing of a boarding door of a railroad coach, an entrance door to a carriage, or a platform screen door will be described.  FIG. 11  is a flowchart showing an operation of capturing an image in synchronism with a door. In this embodiment as well, the control unit can be, for example, the control system  310  shown in  FIG. 9 . A description of the same arrangements and operations as in the other embodiments will be omitted. In addition, the door can be, for example, the door  219  of the railroad coach  218  or the door portion  214  shown in  FIGS. 3A to 4C . 
     First, in the camera system  200 , the illumination unit  202  is in a standby state (step S 1101 ). At this time, the camera  203  may also be in the standby state. The control unit detects a door opening signal (step S 1102 ). Upon detecting the door opening signal, the control unit supplies a control signal for irradiating the inspection object  250  to the illumination unit  202 , and supplies a control signal for capturing the inspection object  250  to the camera  203 . The illumination unit  202  starts terahertz wave irradiation in accordance with the control signal from the control unit (step S 1103 ). The camera  203  starts detecting the terahertz wave in accordance with the start of illumination by the illumination unit  202  (step S 1104 ). If the control unit does not detect the door opening signal, the standby state is maintained (step S 1101 ). Upon detecting the door opening signal in step S 1102 , the control unit is set in a state in which it can always detect the door closing signal (step S 1106 ). Upon detecting a door closing signal in step S 1106 , the control unit supplies a control signal for stopping terahertz wave irradiation to the illumination unit  202 , and supplies a control signal for stopping terahertz wave detection to the camera  203  (steps S 1107  and S 1108 ). Here, if the control unit detects the door closing signal, at least one of steps S 1107  and S 1108  is performed. If the door closing signal is not detected, irradiation and detection of the terahertz wave are continued, and the camera system  200  continues capturing. By the series of operations of monitoring the open and close state of the door, power of the camera system  200  can be saved. In addition, by the series of operations, reliable capturing can be performed at a necessary timing. 
     Still another operation of the camera system  200  will be described next with reference to  FIG. 12 . In this example, a case in which an image is captured in synchronism with the operation of the ticket gate machine  211  will be described.  FIG. 12  is a flowchart showing an operation of capturing an image in synchronism with the ticket gate machine  211 . In this embodiment as well, the control unit can be, for example, the control system  310  shown in  FIG. 9 . A description of the same arrangements and operations as in the other embodiments will be omitted. 
     First, in the camera system  200 , the illumination unit  202  is in a standby state (step S 1201 ). At this time, the camera may also be in the standby state. The control unit is in a state in which it can detect a door opening signal that notifies that the door provided in the ticket gate machine  211  opens (step S 1202 ). If the control unit detects the door opening signal, the illumination unit  202  starts terahertz wave irradiation (step S 1203 ). In addition, the camera  203  starts detecting the terahertz wave in accordance with the start of illumination by the illumination unit  202  (step S 1204 ). If the control unit does not detect the door opening signal, the standby state is maintained (step S 1201 ). To detect the door opening signal in step S 1202 , a signal generated by a ticket put into the ticket gate machine  211 , a signal generated by bringing a ticket such as an IC card into contact with the ticket gate machine  211 , or a signal for detecting the presence and absence of a ticket such as an IC card using a millimeter wave in the ticket gate machine  211  can be used. When the open and close state of the door of the ticket gate machine  211  is monitored in this way, power can be saved. In addition, by the operation, reliable capturing can be performed. 
     Still another operation of the camera system  200  will be described next with reference to  FIG. 13 . In this embodiment, an operation performed in a case in which the sensor  260  described with reference to  FIG. 1B  is used will be described.  FIG. 13  is a flowchart showing the capturing operation in the ticket gate machine  211 . In this embodiment as well, the control unit can be, for example, the control system  310  shown in  FIG. 9 . A description of the same arrangements and operations as in the other embodiments will be omitted. 
     As described above, the sensor  260  detects the inspection object  250 . Here, the sensor  260  may be, for example, a motion sensor using infrared rays or a camera using visible light. First, in the camera system  200 , the illumination unit  202  is in a standby state (step S 1301 ). At this time, the camera may also be in the standby state. Next, the inspection object  250  is detected using the sensor  260  (step S 1302 ). The signal from the sensor  260  is sent to the control unit. Upon determining that the inspection object  250  is detected, the control unit supplies a control signal for irradiating the inspection object  250  to the illumination unit  202 . The illumination unit  202  starts terahertz wave irradiation in accordance with the control signal from the control unit (step S 1303 ). In addition, the control unit supplies a control signal for capturing the inspection object  250  to the camera  203 . The camera  203  starts detecting the terahertz wave in accordance with the control signal from the control unit (step S 1304 ). If the control unit does not determine that the inspection object  250  is detected, the standby state is maintained (step S 1301 ). 
     By this operation, power can be saved. Additionally, with this operation, reliably capturing can be performed. In this embodiment, a case in which the inspection object  250  is a person has been described. However, the inspection object  250  may be an object. This arrangement can also be applied to the escalator  221  shown in  FIGS. 5A and 5B , or the like. If the escalator includes a motion sensor, the sensor can be shared. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application Nos. 2019-047788, filed on Mar. 14, 2019, and 2020-032193, filed on Feb. 27, 2020, which are hereby incorporated by reference herein in their entirety.