Patent Publication Number: US-2021181125-A1

Title: Radiation transmission inspection method and device, and method of manufacturing microporous film

Description:
TECHNICAL FIELD 
     This disclosure relates to a radiation transmission inspection method and device for inspecting a foreign body mixed in a film reel on which a film has been wound, and a method of manufacturing a microporous film including such a radiation transmission inspection method. 
     BACKGROUND 
     In general, films such as various polymer films are supplied as a film raw material in a state of being wound on a cylindrical core. When a foreign body such as a small metal piece is mixed in such a film reel, the foreign body may cause a defect in a product manufactured using the film. For example, when a film unwound from a film reel is used as a battery separator film inserted between a positive electrode and a negative electrode of a lithium ion secondary battery, when a foreign body mixed in the film reel is a small metal piece, it causes a short circuit between a positive electrode and a negative electrode in a lithium ion secondary battery, or the metal piece is dissolved in an electrolytic solution to deteriorate battery characteristics. Therefore, it is necessary to detect whether or not a small metal foreign body has been mixed in the film reel. From the viewpoint of quality assurance in the manufacture of a battery separator film, it is preferable to inspect the film reel, which is the final product, for the presence or absence of a foreign body, rather than during the manufacture of the film. 
     Films such as battery separator films are polymer films, and a foreign body to be detected is metal. Therefore, as a method of detecting a metal foreign body in the film reel, a radiation transmission inspection method in which radiation such as X-rays or y-rays is emitted from the outside of the film reel to detect, as an image, a shadow of the foreign body that does not easily transmit the radiation is an effective method. When detecting a foreign body, it is preferable to detect the presence position of the foreign body in the film reel in addition to the presence or absence of the foreign body. 
     Japanese Patent Laid-open Publication No. 2015-44602 does not relate to detection of a foreign body in a film reel, but is a method to detect the number of seams in a state in which a label continuous body formed by joining two or more long label base materials by a metal coupling member is wound in the form of a roll. It discloses that the number of seams is detected based on the difference in the amount of X-ray transmission between the label base material portions and the coupling member portion by emitting X-rays from a side end of the roll. 
     On the other hand, as a method of determining a spatial arrangement of an object using X-rays, an X-ray CT (computed tomography) method is known. In the X-ray CT method, X-rays are emitted from various directions to take an image, and a three-dimensional image is obtained by image synthesis technology. Therefore, it takes a long time for measurement. Japanese Patent Laid-open Publication No. 63-21039 discloses a technique of shortening the measurement time by X-ray CT by arranging the same number of multiple sources and detectors in a translational scanning direction and performing translational scanning between adjacent sources to reduce the distance of the translation scanning, thereby shortening the measurement time. 
     In X-ray transmission inspection, an inspection object is arranged between an X-ray source and a detector such as an imaging plate, and X-rays are discharged from the X-ray source in a conical shape or a pyramidal shape around the optical axis (irradiation center axis). When the inspection object is larger than an X-ray irradiation field of view (irradiation range emitted from the X-ray source, the X-ray source and detector need to be scanned depending on the inspection object so that the inspection object is entirely inspected. When the inspection target has a thickness in the X-ray transmission direction, depending on the position in thickness direction of a foreign body of the same size, the size of an image on the detector varies, and the detection sensitivity varies depending on the position of the foreign body. This is because the size of the image of the foreign body projected on the detector is enlarged as it is closer to the X-ray source and farther from the detector, and the image is largely projected on the detector. Since the detection sensitivity of the detector becomes capable of detection when the number of pixels of the image exceeds a predetermined value, it depends on the size of the image projected on the detector. Therefore, when the position of the foreign body is close to the X-ray source, the detection is easy, and the detection sensitivity increases, and when the position of the foreign body is far from the X-ray source, the detection is difficult, and the detection sensitivity is reduced. 
     To reduce the variation in detection sensitivity depending on the position in the thickness direction, it is sufficient if the ratio of the distance from the radiation source to the inspection object to the distance from the radiation source to the detector is increased. However, in consideration of the thickness of the inspection object, to increase this ratio, it is necessary to increase the distance itself from the radiation source to the detector and, as a result, the attenuation of X-rays can increase and the required sensitivity cannot be obtained. That is, in an inspection object having a thickness in the X-ray transmission direction, it was not possible to suppress sensitivity variations due to the position of a foreign body in the thickness direction with high detection sensitivity. Therefore, there was a problem that the size of the foreign body detected varies depending on the position of the foreign body in the thickness direction (this is referred to as sensitivity variation), and it is not possible to specify the position of the foreign body in the inspection object and the actual size of the foreign body. 
     On the other hand, inspection by X-ray CT can easily specify the shape and position of a foreign body, but there is a problem that it requires a complicated rotating mechanism and image processing system and takes much longer measurement time and processing time than X-ray transmission inspection. 
     It could therefore be helpful to provide an X-ray transmission inspection method and device capable of reliably detecting a foreign body by reducing the influence of variations in detection sensitivity depending on the position of the foreign body when an X-ray transmission inspection is performed using a film reel as an inspection object, and a method of manufacturing a microporous film manufactured using the X-ray transmission inspection method. 
     SUMMARY 
     We thus provide: 
     (1) A radiation transmission inspection method of inspecting a film reel including a long film wound on an outer peripheral surface of a core a plurality of times, wherein one side surface of the reel is a side end A and another side surface is a side end B, the method comprising: 
     a first foreign body detection process in which radiation emitted from a first radiation source, incident from the side end A of the film reel, transmitted through the film reel, and exited from the side end B is detected by a first detector, and information regarding a foreign body is obtained; and 
     a second foreign body detection process in which radiation emitted from a second radiation source, incident from the side end B of the film reel, transmitted through the film reel, and exited from the side end A is detected by a second detector, and information regarding a foreign body is obtained. 
     (2) Furthermore, it is a radiation transmission inspection method wherein a distance (FID) between the first radiation source and the first detector is equal to a distance (FID) between the second radiation source and the second detector, and 
     a distance (FOD) between the first radiation source and the side end A is equal to a distance (FOD) between the second radiation source and the side end B. Furthermore, it is the radiation transmission inspection method characterized by satisfying Formula (1) below: 
       0.2≤( T+ 2 FOD )/2 FID≤ 0.5  (1)
 
     where T represents a thickness of the film reel.
 
(3) Furthermore, it is a radiation transmission inspection method according to (1) or (2), determining position information and a size of the foreign body mixed in the film reel.
 
(4) Furthermore, it is a radiation transmission inspection method wherein position information of the foreign body and a size of the foreign body mixed in the film reel are determined from foreign body information obtained from the first foreign body detection process and foreign body information obtained from the second foreign body detection process.
 
(5) A radiation transmission inspection device is capable of inspecting a film reel including a long film wound on an outer peripheral surface of a core a plurality of times, wherein one side surface of the reel is a side end A and another side surface is a side end B, the device comprising: a holding portion configured to grip the core of the film reel; a first measurement portion including a first radiation source for emitting radiation arranged to be incident from the side end A of the film reel, transmitted through the reel, and exited from the side end B, and a first detector for detecting the radiation exited from the side end B; and a second measurement portion including a second radiation source that is provided at a position separated from the first detector and is arranged so that radiation is incident from the side end B of the film reel, transmitted through the reel, and exited from the side end A, and a second detector for detecting radiation exited from the side end A.
 
(6) Furthermore, the radiation transmission inspection device can further comprise: an adjustment portion for adjusting a position of the radiation source and the detector of the first measurement portion and a position of the radiation source and the detector of the second measurement portion; and a control portion for adjusting positions such that a distance (FOD) between the first radiation source and the side end A and a distance (FOD) between the second radiation source and the side end B become equal, and a distance (FID) between the first radiation source and the detector and a distance (FID) between the second radiation source and the detector become equal. Furthermore, there is provided the radiation transmission inspection device further comprising a movement portion for moving the first measurement portion and the second measurement portion in a radial direction of the film reel.
 
(7) A method of manufacturing a microporous film, the method comprises: a process of kneading a polyolefin resin and a plasticizer to prepare a polyolefin solution; a process of discharging the polyolefin solution from a die and cooling the polyolefin solution to obtain a gel-like sheet; a process of stretching the gel-like sheet to form a stretched sheet; a process of removing the plasticizer from the stretched sheet to obtain a microporous film; a process of winding the microporous film on a core to obtain a film reel; and a process of inspecting a foreign body contained in the film reel by a radiation transmission inspection method. That is, it is a method of manufacturing a film reel, the method comprising obtaining a film reel by winding a long film on a core; and then a foreign body detection process of inspecting a foreign body contained in the film reel by the aforementioned radiation transmission inspection method.
 
     Since a film product reel is irradiated with radiation from a side end on the other side toward each of both side ends, it is sufficient if each irradiation can detect a foreign body in a region from an intermediate position in the thickness direction of the film product reel to the side end on the irradiation side. This means that the thickness of the film product reel, which is the inspection object, has been substantially reduced by half in terms of radiation transmission inspection. Thus, the sensitivity variation is reduced and the size of the image formed on the detector is also enlarged. As a result, the foreign body can be reliably detected. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram explaining a basic principle of radiation transmission inspection. 
         FIG. 2( a )  is a diagram explaining sensitivity variation based on the position of a foreign body in a thickness direction. 
         FIG. 2( b )  is a diagram explaining sensitivity variation based on the position of a foreign body in a thickness direction. 
         FIG. 2( c )  is a diagram explaining sensitivity variation based on the position of a foreign body in a thickness direction. 
         FIG. 3( a )  is a diagram explaining a principle of a conventional radiation transmission inspection method. 
         FIG. 3( b )  is a diagram explaining a principle of a radiation transmission inspection method. 
         FIG. 4  is a diagram explaining processing for obtaining a position and a size of a foreign body in a thickness direction. 
         FIG. 5  is a graph explaining conditions under which the same foreign body can be detected by irradiating each of both side ends with X-rays. 
         FIG. 6( a )  is a diagram explaining that a foreign body is detected by dividing a region between a side end and an intermediate position in the thickness direction into a plurality of regions in the thickness direction. 
         FIG. 6( b )  is a diagram explaining that a foreign body is detected by dividing a region between a side end and an intermediate position in the thickness direction into a plurality of regions in the thickness direction. 
         FIG. 7( a )  is a plan view showing a first example of a radiation transmission inspection device. 
         FIG. 7( b )  is a front view showing a first example of a radiation transmission inspection device. 
         FIG. 8  is a front view showing a second example of a radiation transmission inspection device. 
         FIG. 9  is a side view showing a third example of a radiation transmission inspection device. 
         FIG. 10  is a front view showing a fourth example of a radiation transmission inspection device. 
     
    
    
     DESCRIPTION OF REFERENCE SIGNS 
     
         
           10 : Film reel 
           11 : Core 
           12 : Film 
           13 : Axis of core 
           14 : Side end A of film reel 
           15 : Side end B of film reel 
           16 : Side end D of film reel 
           21 : First radiation source 
           22 : Second radiation source 
           23 : Third radiation source 
           24 : Fourth radiation source 
           26 : First detector 
           27 : Second detector 
           28 : Third detector 
           29 : Fourth detector 
           31 : Optical axis 
           32 : X-ray irradiation range 
           41 : Foreign body 
           42 : Image 
           46 : Holding portion 
           47 : Rotation drive portion 
           50 : Control portion 
           51 ,  52 ,  56 ,  57 : Adjustment stage 
           61 ,  62 : Movement stage 
           65 : Processing portion 
           66 ,  67 : Attachment member 
           71 ,  72 ,  76 ,  77 : Adjustment member 
           81 : Up-and-down movement portion 
           82 : Switching portion 
         C: Center position in thickness direction of film reel 
         T: Thickness of film reel 
       
    
     DETAILED DESCRIPTION 
     First, terms related to radiation transmission inspection using X-rays are shown below: 
     Optical axis of X-ray source: The central axis of X-ray irradiation. X-rays are emitted to spread in a conical shape or a pyramidal shape about the optical axis. 
     Field of view: The range that can be irradiated with X-rays. Expressed in area. The closer to the radiation source, the narrower the field of view. 
     Irradiation range: The range that is irradiated with X-rays. This is the range where X-rays spread in a conical shape or a pyramidal shape about the optical axis hit. 
     Scanning: To move a set of a radiation source and a detector along the inspection object in an axial direction or the like. 
     FID: The separation distance between the radiation source and the detector. 
     FOD: The separation distance between the radiation source and the shortest side end of the film reel from the radiation source. 
     Detection sensitivity: Detectable size of the inspection object. Expressed by the minimum size. 
     Sensitivity variation: Difference in the size of the detectable inspection object depending on the position of the inspection object in the thickness direction.
 
Foreign body detection process: It represents one processing step of a foreign body inspection process of irradiating an object with radiation from a radiation source and detecting radiation that has passed through the object. When there are a plurality of processing steps, they are called first, second, third, . . . .
 
     Radiation Transmission Inspection Method 
     Next, a preferred example will be described with reference to the drawings.  FIG. 1  shows a basic principle of a general radiation transmission inspection using a film reel  10  as an inspection object. The film reel  10  is shown as a cross-section in a plane including a central axis  13  in a length direction of a cylindrical core  11 , and hatching is partially omitted. The film reel  10  is configured by winding a long film around the outer peripheral surface of the core  11  a plurality of times around the central axis  13  of the core  11  as a rotation axis. Reference numeral  12  denotes a layer of a film wound on the outer peripheral surface of the core  11 . The surface of the film reel  10  facing the direction in which the central axis  13  of the core  11  extends is referred to as a side end of the film reel  10 . The side ends are circular surfaces corresponding to both ends of the cylindrical film reel, and are located on the side surfaces of the film reel. Therefore, a side surface on one side is referred to as a side end A (reference numeral  14  in  FIG. 1 ), and a side surface on the other side is referred to as a side end B (reference numeral  15  in  FIG. 1 ). The dimension between both side ends in the length direction of the core  11  (that is, the distance between the side end A and the side end B) substantially matches the width dimension of the film wound around the core  11 . The side end of the film reel  10  is also a surface where the side end in the width direction of the film wound around the core  11  is exposed. In the drawing, T indicates the thickness of the film reel  10 , which is equal to the width of the film wound around the core  11 . The direction parallel to the direction in which the central axis  13  of the core  11  extends is referred to as the thickness direction of the film reel  10 . 
     To detect whether or not a foreign body such as a small metal piece is mixed in the film reel  10 , a radiation source  21  such as an X-ray source is arranged at a position facing one side end of the film reel  10 . In the following, it is assumed that X-rays are emitted from the radiation source  21 , but other radiation such as y-rays may be used instead of X-rays. The radiation source  21  can be generally considered as a point light source. From the radiation source  21 , X-rays are emitted to spread out in a conical shape or a pyramidal shape along the optical axis  31  such that the optical axis  31  is perpendicular to one side end of the film reel  10 . In the drawing, reference numeral  32  indicates a range (irradiation range) in which the X-ray spreads. Then, to detect X-rays transmitted through the film reel  10 , the detector  26  including a two-dimensional X-ray detector such as an imaging plate is arranged at a position facing the other side end of the film reel  10  such that the center position of the detector  26  is positioned on an extension of the optical axis  31 . 
     If there is a foreign body such as metal in the film reel  10 , the X-ray is blocked by the foreign body. Therefore, the X-ray intensity at the position corresponding to the foreign body in the detector  26  is reduced. By detecting, as an image, the position where the X-ray intensity is reduced, the foreign body in the film reel  10  can be detected including the position in the film reel  10 . The position here is, when the film reel is viewed from the side end face, a two-dimensional coordinate on a circular plane projected in that direction. 
     In accordance with the general terminology of radiation transmission inspection techniques, the separation distance between the radiation source  21  and the detector  26  is called FID (Focus to Image Distance). Moreover, the separation distance between the radiation source  21  and the side end of the film reel  10 , which is an inspection object, on the radiation source  21  side is called an FOD (Focus to Object Distance). The FOD is originally the separation distance between the radiation source  21  and the foreign body, which is a detection target. However, the position of the foreign body is unknown at a stage before execution of the radiation transmission inspection. Therefore, the separation distance between the radiation source  21  and the side end of the film reel  10  on the radiation source  21  side is defined as the FOD. 
     Detection Sensitivity and Sensitivity Variation 
     Next, the detection sensitivity and sensitivity variation of a foreign body will be described with reference to  FIG. 2 . Specific numerical values are used for the sake of easy understanding, but this disclosure is not limited to such specific dimensions. It is assumed that the FID is 200 mm, the FOD is 20 mm, and the thickness T of the film reel  10  is 60 mm. 
     In  FIG. 2 , X-rays are emitted along the thickness direction of the film reel  10  from the radiation source  21  present above in the drawing to the detector  26  present below in the drawing. To reliably detect a small foreign body  41 , it is necessary to make the foreign body image  42  on the detector  26  as large as possible. The projection magnification is increased such that the distance from the radiation source  21  to the foreign body  41  is smaller than the FID. The projection magnification is equivalent to a value obtained by dividing the size of the foreign body image  42  by the actual size of the foreign body  41 , and the projection magnification is a value obtained by dividing FID by FOD (FID/FOD). 
       FIG. 2( a )  shows when the foreign body  41  exists at the side end A (reference numeral  14  in  FIG. 2 ) of the film reel  10  on the radiation source  21  side. At this time, the separation distance between the radiation source  21  and the foreign body  41  is 20 mm, and the projection magnification is 10 (=200÷20). Therefore, assuming that the size of the foreign body  41  is 100 μm, the size (projection size) of the foreign body image  42  on the detector  26  is 1000 μm. Conversely, the size of the foreign body, which is the lower limit of the detection sensitivity, is determined by the size of the detectable image  42  as described below. When the size of the image  42  that can be detected by the detector  26  is 400 μm or more, the lower limit of the size of the detectable foreign body  41 , that is, the detection sensitivity, the size of the foreign body, is 40 μm. Similarly,  FIG. 2( b )  shows when the foreign body  41  exists at the center of the film reel  10  in the thickness direction. At this time, the separation distance between the radiation source  21  and the foreign body  41  is 50 mm.  FIG. 2( c )  shows when the foreign body  41  exists at the side end B (reference numeral  15  in  FIG. 2 ) of the film reel  10  on the detector  26  side. At this time, the separation distance between the radiation source  21  and the foreign body  41  is 80 mm. In (b) and (c), similar to (a), the projection magnification, the projection size at the detector  26  when the size of the foreign body  41  is 100 μm, and the size of the foreign body  41  at the time when the size of image  42  is 400 μm , that is, the detection sensitivity (lower limit) can be obtained. Table 1 shows projection magnification, projection size, and detection sensitivity (lower limit). 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 (a) 
                 (b) 
                 (c) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Foreign matter location 
                 Side end on 
                 Center in 
                 Side end on 
               
               
                   
                 radiation source 
                 thickness 
                 detector side 
               
               
                   
                 side 
                 direction 
                   
               
               
                 Separation distance (FID) 
                  200 mm 
                 200 mm 
                 200 mm 
               
               
                 between radiation source 21 
                   
                   
                   
               
               
                 and detector 26 
                   
                   
                   
               
               
                 Separation distance (FOD) 
                  20 mm 
                  50 mm 
                  80 mm 
               
               
                 between radiation source 21 
                   
                   
                   
               
               
                 and foreign matter to be 
                   
                   
                   
               
               
                 detected 
                   
                   
                   
               
               
                 Projection magnification 
                 10 
                 4 
                 2.5 
               
               
                 Projection size of 100 μm 
                 1000 μm 
                 400 μm 
                 250 μm 
               
               
                 foreign matter 
                   
                   
                   
               
               
                 Detection sensitivity (lower 
                  40 μm 
                 100 μm 
                 160 μm 
               
               
                 limit) 
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, when the thickness T of the film reel  10  is 60 mm, the sensitivity variation is as large as 40 to 160 μm. Further, for example, in (c), since the detection sensitivity is 160 μm, a foreign body having a size of 100 μm cannot be detected. Further, the projection size of the foreign body mixed in the film reel is obtained by multiplying the size of the foreign body by the projection magnification. That is, since the position of the foreign body is unknown from the projection size, the size of the foreign body cannot be specified. 
     As described above, the size of the foreign body that can be detected is determined by the position of the foreign body in the thickness direction, that is, where the foreign body exists at what ratio with respect to the radiation source and the detector. Since the projection magnification is FID/FOD, the image size at the detector is proportional to the FID and inversely proportional to the FOD. Therefore, the detection sensitivity for a foreign body of the same size is inversely proportional to the FOD. By increasing the FOD and increasing the ratio of the distance from the radiation source to the inspection object to the distance from the radiation source to the detector, the difference in the size of the inspection object that can be detected depending on the position in the thickness direction, that is, sensitivity variation is reduced. 
     Specifically, the distance of the FID is set to 1, the position of the radiation source is set to 0, the position of the detector is set to 1, and the sensitivity variation when the ratio of the thickness of the film reel, which is an inspection target, to the distance of the FID is 0.3 (the measurement range is 0.3 width) is described below: 
     When the distance (FOD) from the radiation source to the side end face A is 0.2
         The thickness range of the film reel is 0.2 to 0.5       

       In 0.2,1/0.2=5 
       In 0.5,1/0.5=2         Therefore, the sensitivity variation is 5/2=2.5 (times).       
     When the distance (FOD) from the radiation source to the side end face A is 0.5 
     The thickness range of the film reel is 0.5 to 0.8 
       In 0.5,1/0.5=2 
       In 0.8,1/0.8=1.25         Therefore, the sensitivity variation is 2/1.25=1.6 (times).       
     On the other hand, the detection sensitivity (detecting a small foreign body) is to increase the FID or reduce the FOD. That is, the detection sensitivity and the sensitivity variation have opposite characteristics, and the sensitivity variation increases to detect a small foreign body. When the foreign body in the film reel is detected by transmitting the radiation from one of the side end faces, the small foreign body in the film reel may not be detected in some places. Moreover, it is difficult to specify the actual size of the foreign body from the detection result without knowing where in the thickness direction it exists. 
     Method of Inspecting Radiation Transmission from the Front and Back of Film Reel when FOD is Large 
     To be able to detect a foreign body  41  having a size of 100 μm at the side end of the film reel  10  on the detector  26  side in the conventional art, if the FOD is left as it is, the FID needs to be longer than the above conditions. This results in a reduction in the X-ray intensity on the detector  26  side, and it is necessary to increase the cumulative X-ray irradiation time, which increases the measurement time. 
     Furthermore, when the FID becomes longer, the spread of radiation becomes larger than the area of the detector  26 , and the spread of X-rays incident on the detector  26  becomes narrower. The measurement range in which foreign body detection can be performed by one X-ray shot is also narrowed so that the number of X-ray shots for inspecting the entire film reel  10  is increased, and the measurement time is further increased. On the other hand, to be able to detect the foreign body  41  having a size of 100 μm attached on the detector  26  side, when the FID is set to the above-described conditions, the FOD is cannot be set to zero or less, and the film reel  10  having a large thickness T cannot be measured. 
       FIG. 3  is a diagram explaining the principle of the radiation transmission inspection method.  FIG. 3( a )  shows an inspection according to the conventional method, and  FIG. 3( b )  shows an example of my inspection. Using the film reel  10  having the same thickness T of 60 mm as that shown in  FIG. 2  as an inspection object, a foreign body having a size of 100 μm or more is detected. As is clear from the description using  FIG. 2 , it is more difficult to detect a foreign body on the detector  26  side. Therefore, according to the conventional method shown in  FIG. 3( a ) , to enable detection of a foreign body having a size of 100 μm present in the side end on the detector  26  side (that is, the detection sensitivity at the side end on the detector  26  side is set to 100 μm), the FOD is set to 15 mm. At this time, the foreign body  41  can be detected with a detection sensitivity of 20 μm at the side end of the film reel  10  on the radiation source  21  side. That is, in this example, the detection sensitivity varies between 20 μm and 100 μm, and the sensitivity variation is large. 
     Moreover, considering the spread of X-rays, in one shot inspection, for example, at the side end on the radiation source  21  side, the foreign body  41  can be detected in a region with a field of view of 3.5 mm×2 mm, and at the side end on the detector  26  side, the foreign body  41  can be detected in a region with a field of view of 17.5 mm×10 mm. To inspect the foreign body  41  over the entire film reel  10 , the film reel  10  needs to be completely scanned to be irradiated with X-rays as shown by the broken lines in the drawing on the basis of the minimum field of view (X-ray field of view at the side end on the radiation source  21  side). 
     On the other hand, in my method shown in  FIG. 3( b ) , in one X-ray shot, a foreign body  41  having a size of 100 μm or more in the region from the side end A of the film reel  10  on the radiation source  21  side to an intermediate position C (dashed line) of the film reel  10  in the thickness direction is detected. Then, although not shown, the region from the intermediate position C is irradiated with radiation from the other face to perform detection. Assuming that the radiation source  21 , the detector  26 , the FID and the like are the same as in  FIG. 3( a ) , it is only necessary to detect a foreign body  41  having a size of 100 μm at the intermediate position C of the film reel  10  in the thickness direction, the FOD can be 45 mm. 
     At this time, the detection sensitivity at the side end A on the radiation source  21  side is 60 and the field of view is 10.5 mm×6 mm. The detection sensitivity at the intermediate position C in the thickness direction is 100 and the field of view is 17.5×10 mm. The detection sensitivity varies between 60 μm and  100  but the variation is much smaller than in  FIG. 3( a ) . At this time, the minimum field of view is 10.5×6 mm, which is nine times in area ratio as compared with the conventional configuration shown in  FIG. 3( a ) . Therefore, the method can perform inspection nine times faster than the conventional method. 
     In the method shown in  FIG. 3( b ) , the detection sensitivity is lower than 100 μm in a region between the intermediate position C of the film reel  10  in the thickness direction and the side end B on the detector  26  side. Therefore, to reliably detect the foreign body  41  having a size of 100 μm or more, the film reel  10  is turned over relative to a measurement portion including the radiation source  21  and the detector  26 , that is, a first inspection process portion such that the region that could not be inspected faces the radiation source  21  side, and the radiation transmission inspection is performed again (second inspection process). After all, in the method, a first foreign body detection process of irradiating the film reel  10  with radiation from the side end A, which is one side thereof, and a second foreign body detection process of irradiating the film reel  10  with radiation from the side end B, which is the other side are performed. 
     Regarding the time required for the measurement, the area of the minimum field of view is nine times as large as that in  FIG. 3( a )  in which all the foreign bodies  41  are detected in a single foreign body detection process. Therefore, the number of X-ray shots can be reduced to about 1/5 (2/9) even in consideration of performing the foreign body detection process twice. That is, foreign bodies can be detected in a shorter time compared to the method based on a conventionally known technique. Two sets of measurement portions including the radiation source  21  and the detector  26  are prepared not to interfere with each other. When one of the measurement portions emits radiation from the side end A, which is one side, of the film reel  10  and the other of the measurement portions emits radiation from the side end B, which is the other side, the first foreign body detection process and the second foreign body detection process can be proceeded simultaneously. Thus, the time required to inspect the entire surface of the film reel  10  can be further reduced. Further, in the method shown in  FIG. 3( b ) , the sensitivity variation is reduced compared to the method of  FIG. 3( a ) . Therefore, the film reel  10  having a large thickness can be inspected accordingly. Further, the film reel  10  can be largely separated from the radiation source  21 . Therefore, the types of film reel  10  for which inspection can be executed also are increased. 
     Regarding the Position and Size of a Foreign Body by Radiation Transmission Inspection from the Front and Back 
     By the way, in the radiation transmission inspection method described with reference to  FIG. 3( b ) , when the film reel  10  is irradiated with radiation from the side end A, which is one side, and when the film reel  10  is irradiated with radiation from the side end B, which is the other side, the same foreign body  41  is detected in some examples. In that instance, the position and size of the foreign body  41  in the thickness direction can be determined.  FIG. 4  is a diagram explaining the processing of determining the position and size of the foreign body  41  in the thickness direction in such an instance. 
     In the film reel  10  shown in  FIG. 4( a ) , one side surface is a side end A (reference numeral  14 ), and the other side surface is a side end B (reference numeral  15 ). Then, the first detector  26  detects the radiation emitted from the first radiation source  21 , incident from the side end A of the film reel  10 , transmitted through the reel, and exited from the side end B, to obtain information regarding the foreign body  41 . The foreign body  41  projects an image  42  on the detector  26 , and the position of the image  42  is recorded as specific position information of the detector  26 . For example, the position of the foreign body is mapped on an XY coordinate. Then, the distance between the first radiation source  21  and the first detector  26  or the side ends A, that is, the FID and the FOD are fixed, the inside of the side surface of the film reel is scanned, and the coordinate information of the foreign body  41  across the entire film reel is obtained. 
     In the scanning method, the first radiation source  21  and the first detector  26  may be moved in two axes of XY, or the film reel may be rotated while moving in the radial direction of the film reel. The scanning may be performed stepwise, and after emission of a predetermined amount of radiation required for the inspection, they may be moved a predetermined distance so that the irradiation areas do not overlap. Alternatively, recording may be performed as the position of the side surface of the film reel while continuously movement is performed at a very low speed. If the irradiation area per one time is small, the number of times of irradiation increases. By performing such scanning, the position of the foreign body on the side surface of the film reel and a size A 1  of the image  42  can be obtained. Regarding the size of the image  42 , because the radiation is reduced because the radiation is blocked by a metallic foreign body or the like, it is preferable to use a method of counting the number of pixels. 
     In  FIG. 4( b ) , the inspection of the film reel  10  is performed from the surface (back surface) on the opposite side to  FIG. 4( a ) . The second detector  27  detects the radiation emitted from the second radiation source  22 , incident from the side end B of the film reel, transmitted through the reel, and exited from the side end A, to obtain information regarding the foreign body. The foreign body  41  projects an image  42  on the detector  27 , and the position of the image  42  is recorded as specific position information of the detector  27 . For example, the position of the foreign body is mapped on an XY coordinate. Then, similar to the first radiation source  21  and the first detector  27 , the FID and the FOD are fixed, and the inside of the side surface of the film reel is scanned, and the coordinate information of the foreign body  41  across the entire film reel is obtained. Further, a size A 2  of the image  42  is obtained. 
     Since the foreign bodies  41  are the same in  FIGS. 4( a ) and 4( b ) , the position information (for example, XY coordinate) of the inside of the side surface of the film reel is the same. However, the sizes A 1  and A 2  of the image  42  in the first detector  26  and the second detector  27  differ from each other due to the distance relationship between the radiation sources  21  and  22 . 
     The ratio between the FID and the FOD and the detection sensitivity (the minimum detectable size) will be described while the first foreign body detection process and the second foreign body detection process are compared. The first foreign body detection process includes the first radiation source  21  and the first detector  26 , and performs inspection by transmitting radiation from the side end A to the side end B of the film reel. The second foreign body detection process includes the second radiation source  22  and the second detector  27 , and performs inspection by transmitting radiation from the side end B to the side end A of the film reel. 
     The distance of the FID in the first foreign body detection process and the distance of the FID in the second foreign body detection process are adjusted to be equal. Then, the distance of the FOD in the first foreign body detection process and the distance of the FOD in the second foreign body detection process are adjusted to be equal. However, in the first foreign body detection process, it is the distance between the radiation source  21  and the side end A, and in the second foreign body detection process, it is the distance between the radiation source  22  and the side end B. As described above, in one example of  FIGS. 4( a ) and 4( b ) , the FID is 200 mm and the FOD is 45 mm. 
     The detection sensitivities and sensitivity variations in the first foreign body detection process and the second foreign body detection process are as described above. 
     Detection Sensitivity 
     At a distance in the thickness direction between the side end A in the first foreign body detection process and the side end B in the second foreign body detection process, a foreign body of about 90 μm or more can be detected, at a center position of the film reel (thickness T is 60 mm), about 150 μm or more can be detected, and further at a distance in the thickness direction between the side end B in the first foreign body detection process and the side end B in the second foreign body detection process, about 210 μm or more can be detected. 
     Then, regarding the size of the image  42  projected on each detector, a foreign body of 100 μm square is 444 μm square, about 4.44 times at a distance in the thickness direction between the side end A in the first foreign body detection process and the side end B in the second foreign body detection process, 267 μm square, about 2.67 times at a center position of the film reel (thickness T is 60 mm), and further 211 μm square, about 2.11 times at a distance between the side end B in the first foreign body detection process and the side end B in the second foreign body detection process. 
     It is assumed that a foreign body  41  whose actual dimension is a exists in the film reel  10 , and a distance from the intermediate position C in the thickness direction of the film reel  10  to the foreign body  41  is z. The thickness of the film reel  10  is T as described above. The sizes of the images  42  on the detector  26  due to the foreign body  41  when the film reel  10  is irradiated with X-rays from one side end side and irradiated with X-rays from the other side end side are respectively A 1  and A 2 . Since generality is not lost even if A 1 ≥A 2  is assumed, A 1 ≥A 2  is set. Then, the foreign body  41  is present at a position separated by z in the direction of the radiation source  21  when the size of the image is A 1  when viewed from the intermediate position C in the thickness direction of the film reel. The projection magnification A 1 /a when the size of the image  42  is A 1  as shown in  FIG. 4( a )  and the projection magnification A 2 /a when the size of the image  42  is A 2  as shown in  FIG. 4( b )  are represented by Formulae (1) and (2), respectively: 
         A   1   /a=FID /( FOD+T/ 2− z )  (2)
 
         A   2   /a=FID /( FOD+T/ 2+ z )  (3).
 
     From Formulae (2) and (3), the actual dimension a of the foreign body  41  and the distance z from the intermediate position C can be determined as shown in Formulae (4) and (5): 
         z =( A   1   −A   2 )/( A   1   +A   2 )×( FOD+T/ 2)  (4)
 
         a= 2× A   1   ×A   2 /( A   1   +A   2 )×( FOD+T/ 2)/ FID   (5).
 
     As described above, by obtaining the foreign body information by using the first foreign body detection process and the second foreign body detection process in which radiation is incident from both sides of the film reel, the distance in the thickness direction of the film reel  10  and the actual size of the foreign body can be obtained. Because the actual size of the foreign body can be determined regardless of the distance in the thickness direction, that is, the presence position in the film width direction in the film reel by determination of the foreign body mixed in the film reel, it is possible to determine, for example, whether or not the size exceeds the size of the foreign body that becomes a problem, and the inspection accuracy can be improved. 
     Thus, when the radiation is incident from both sides of the film reel and the inspection including the first foreign body detection process and the second foreign body detection process is performed, the distance from the radiation source on both sides of the film reel to the inspection object is preferably set to conditions that a foreign body existing at least up to half the thickness (T) of the film reel can be easily detected. That is, the maximum distance from the radiation source to the foreign body is FOD+1/2T (where T represents the thickness of the film reel). Since the detection sensitivity increases as the distance from the radiation source decreases, the value obtained by dividing FOD+1/2T by FID is preferably 0.5 or less. On the other hand, since the sensitivity variation increases as the distance from the radiation source decreases, the FOD is preferably 20 mm or more. Depending on the thickness (T) of the film reel, the value obtained by dividing FOD+1/2T by FID is preferably 0.25 or more when T=60 mm, and preferably 0.2 or more when T=40 mm. That is, a condition satisfying Formula (1) is a preferable aspect: 
       0.2≤( T+ 2 FOD )/2 FID≤ 0.5  (1).
 
     In addition, regarding the inspection time, the measurement time is doubled because the measurement is performed twice from the front and back of the film reel. When the measurement area per time is doubled or more, the total time required for inspection is reduced. The measurement area is proportional to the square of the distance. In a preferred aspect, the ratio of the measurement area corresponding to the distance of FOD+1/2T to the measurement area corresponding to the distance of FOD, that is, the ratio of (FOD+1/2T)/FOD exceeds √2. 
     When the foreign body  41  can be detected only from one side end of the film reel  10 , it is sufficient if an assumption is made regarding the actual dimension of the foreign body  41  on the basis of the measured size of the image  42 . The radiation transmission inspection method is used, for example, to remove the film reel  10  as a defective product when the foreign body  41  having a predetermined size or more is mixed in the film reel  10 . Therefore, the significance of performing radiation transmission inspection will not be lost even when such an assumption is made regarding the actual dimension. 
       FIG. 5  is a characteristic diagram showing, in the thickness direction of the film reel, the presence/absence of foreign body detection in the first foreign body detection process and the second foreign body detection process. Using a configuration shown in  FIG. 2( b )  (FID=200 mm, FOD=20 mm), when inspecting the film reel  10  having a thickness of 60 mm, depending on the position of the foreign body  41  in the thickness direction and the actual dimension, it is indicated whether the foreign body  41  can be detected by X-ray irradiation from either side of both side ends (described as “detectable from both side ends”), it can be detected only by X-ray irradiation from one side end (described as “detectable from only one side end”), or it cannot be detected by X-ray irradiation from either side end (“undetectable from either side end”). 
     As described above, by the detection in the first foreign body detection process and the second foreign body detection process, the radiation source and the detector are scanned on the side surface of the film reel, and an obtained foreign body defect map is overwritten, and a smaller foreign body can be detected. Further, it is possible to reduce the time required for calculating the size and the position of the foreign body. When a plurality of foreign bodies  41  exists along the optical axis of the X-ray in the film reel  10 , the images of these foreign bodies  41  can be overlapped and detected as if there is only one foreign body. Such overlapping of the images can be resolved by slightly moving the X-ray irradiation position, for example, at a smaller interval than half of the minimum field of view described above, and the plurality of foreign bodies  41  can be detected independently. However, when a radiation transmission inspection is performed to determine a defective product by detecting a foreign body, it is not necessary to resolve the image  42  of such a foreign body. In addition, to separate the images, regarding the incident angle of each radiation on the optical axis and the side surface of the film reel, the optical axis may be obliquely, not perpendicularly, incident on the film reel to separate and detect the foreign bodies. 
     In the radiation transmission inspection method described above, the required number of times of X-ray shot is determined according to the minimum visual angle defined at the side end of the film reel  10  on the radiation source  21  side, and the inspection time is determined. Since the minimum visual angle can be increased by reducing the thickness of the inspection target, the required number of times of X-ray shot is inversely proportional to the area based on the minimum visual angle. Therefore, to further reduce the inspection time, the range between the intermediate position C on the film reel  10  and the side end on the radiation source  21  side is divided into a plurality of regions in the thickness direction, and the inspection of the foreign body  41  is performed for each region. That is, it is possible to perform a plurality of times of detection scanning with different FODs. Since the X-ray irradiation is performed in the same direction, the detection of the foreign body  41  can be overlapped. It is thus sufficient to determine the detection of one foreign body  41  as a defective product. 
     Regarding when the Thickness of the Film Reel is Large 
     The greater the thickness of the film reel, the more difficult it is to inspect a foreign body mixed in the film reel.  FIG. 6  is a diagram explaining that a film reel  10  having a thickness T of, for example, 120 mm is further divided to two regions in the thickness direction from the intermediate position C in the thickness direction of the film reel  10  to a side end D on the radiation source  21  side, and the inspection of the foreign body  41  is performed for each region. That is, this is a method in which the inspection is performed by dividing the film reel into four parts in the thickness direction. This is to increase the ratio of the distance from the radiation source to the inspection object to the distance from the radiation source to the detector, thereby reducing the sensitivity variation and obtaining the detection sensitivity for detecting a required (100 μm size) foreign body. 
     As the number of divisions in the thickness direction of the film reel, an example when the region from one side end D (reference numeral  16 ) of the film reel  10  to the intermediate position C is divided into two will be described. The region from the other side end of the film reel  10  to the intermediate position C can also be divided into two and detection of the foreign body  41  can be performed similarly. In practice, it is preferable that the film reel  10  be divided into two regions in the thickness direction on one side end side and the other side end side so that the film reel  10  is divided into four regions in the thickness direction in total and inspection of the foreign body  41  is performed for each. By applying the concept described here, it is also possible to divide the region from any side end of the film reel  10  and to intermediate position C into three or more regions and inspect the foreign body  41  for each region. 
     In  FIG. 6 , the position of the side end of the film reel  10  on the radiation source  21  side is indicated at D, and the position, which is a middle point, between the side end D (reference numeral  16 ) and the intermediate position C in the thickness direction of the film reel  10  is indicated at E. Assuming that the thickness T of the film reel  10  is 120 mm, the distance between the side end D and the position E is 30 mm, and the distance between the position E and the intermediate position C is also 30 mm.  FIG. 6( a )  shows an arrangement of the radiation source  21  and the detector  26  for detecting a foreign body  41  having a size of, for example, 160 μm or more in the region between the position E and the intermediate position C. The separation distance FID between the radiation source  21  and the detector  26  is 200 mm, and the separation distance FOD between the radiation source  21  and the side end D on the radiation source  21  side is 20 mm. Then, as shown in a table form in  FIG. 6( a ) , the detection sensitivity at the intermediate position C is 160 μm, and the field of view size at the position E is 50 mm. If the X-ray irradiation range is a square, the minimum field of view when detecting the foreign body  41  between the position E and the intermediate position C is 50 mm×50 mm. In  FIG. 6( a ) , an obliquely hatched region is a region where a foreign body having a size of 160 μm can be detected by scanning using a minimum range of 50 mm×50 mm (a region having a detection sensitivity of less than 160 μm). As shown in  FIG. 6 , a foreign body  41  having a size of 160 μm can also be detected in a part of the region between the side end D and the position E. In the region between the side end D and the position E, the portion shown in black is an uninspected region not irradiated with X-rays. 
       FIG. 6( b )  shows an arrangement that detects a foreign body  41  having a size of 160 μm or more in a region between the side end D on the radiation source  21  side and the position E. The separation distance FID between the radiation source  21  and the detector  26  is 200 mm as in  FIG. 6( a ) , and the separation distance FOD between the radiation source  21  and the side end D of the radiation source  21  side is 50 mm. That is, compared to  FIG. 6( a ) , the FID is the same, but the FOD is increased by 30 mm. At this time, the detection sensitivity at the position E is 160 μm, the field of view size at the position D is 50 mm, and assuming a square irradiation field, the minimum field of view when detecting the foreign body  41  between the side end D and the position E is 50 mm×50 mm. In  FIG. 6( b ) , an obliquely hatched region is a region where a foreign body  41  having a size of 160 μm can be detected by scanning using a minimum field of view of 50 mm×50 mm (a region having a detection sensitivity of less than 160 μm). 
     In the method described with reference to  FIG. 6 , scanning using a field of view of 50 mm×50 mm is performed twice. On the other hand, to detect a foreign body  41  having a size of 160 μm or more between the side end D and the intermediate position C in one inspection using the device configuration shown here, similar to  FIG. 6( a ) , the FOD needs to be 20 mm. At this time, since the field of view size at the side end D is 20 mm, it is necessary to perform scanning once using a field of view of 20 mm×20 mm. The area of the field of view of 50 mm×50 mm is 2500 mm 2 , which is six times or wider than 400 mm 2  which is the area of the field of view of 20 mm×20 mm. Therefore, the method described with reference to  FIG. 6 , even if the scanning is performed twice, can reduce the overall measurement time compared to when a foreign body between the side end D and the intermediate position C is detected by one scanning. 
     Further, in  FIGS. 6( a ) and 6( b ) , the description has been given from the side end D (reference numeral  16 ) to the intermediate position C of the film reel, but it is preferable to similarly perform inspection from the other side end to the intermediate position C of the film reel. At this time, it is preferable to include a third foreign body detection process and a fourth foreign body detection process in addition to the first foreign body detection process and the second foreign body detection process. The third foreign body detection process is configured to include a third radiation source  23  and a third detector  28 , and the fourth foreign body detection process is configured to include a fourth radiation source  24  and a third detector  29 . Then, the FID and FOD of the third foreign body detection process and the fourth foreign body detection process are adjusted to be different values from those of the first and second foreign body detection processes, and that the FID and FOD have the same value. An inspection method is provided that covers the entire film reel in the thickness direction using two sets of foreign body detection processes from both sides of the film reel. 
     Foreign Body 
     Examples of the material of the foreign body that can be detected include metals (Cu, SUS, Fe and the like) and an oxide thereof, silica and the like. When there is a significant difference (=if the S/N ratio is high) in the X-ray intensity transmitted through a portion where a foreign body is present as compared with the X-ray intensity transmitted through a portion where a foreign body is absent (including a variation), detection is possible in addition to the above. In general, as the specific gravity of a foreign body increases, the X-ray intensity after transmission decreases, and the S/N ratio increases, which tends to facilitate detection. Further, as the thickness T is larger, the X-ray intensity variation after transmission through the film is integrated and becomes larger. Therefore, even for the same foreign body, the S/N ratio tends to be small and the foreign body tends to be difficult to detect. 
     First Example of Radiation Transmission Inspection Device 
     Next, description will be given of a radiation transmission inspection device used that performs the above-described radiation transmission inspection method.  FIG. 7  is a diagram showing a first example of the radiation transmission inspection device, wherein  FIG. 7( a )  is a plan view and  FIG. 7( b )  is a front view. A holding portion  46  for detachably holding the film reel  10  as an inspection object on which a long film is wound a plurality of times on the outer peripheral surface of a core  11  is provided. The holding portion  46  holds the film reel  10  via the core  11  such that the central axis  13  of the core  11  is horizontal. The holding portion  46  is also provided with a rotation drive portion  47  for rotating the film reel  10  around the central axis  13 . 
     With one side surface of the film reel as the side end A and the other side surface as the side end B, a radiation source  21  that emits X-rays toward the film reel  10  is provided at a position facing the one side end of the film reel  10 , and a detector  26  that detects the X-ray transmitted through the film reel  10  is provided at a position facing the other side end of the film reel  10  and on an extension of the optical axis  31  of the X-ray from the radiation source  21 . The radiation source  21  and the detector  26  constitute a first measurement portion. In other words, the first measurement portion includes a first radiation source that emits radiation arranged to be incident on from the side end A of the film reel, transmitted through the reel, and exited from the side end B, and a first detector that detects the radiation exited from the side end B. Similarly, a radiation source  22  that irradiates the film reel  10  with X-rays is provided at a position facing the other side end of the film reel  10  and at a position separated from the first detector  21 , and a detector  27  that detects the X-ray transmitted through the film reel is provided at a position facing one side end of the film reel  10  and on an extension of the optical axis  31  of the X-ray from the radiation source  22 . The radiation source  22  and the detector  27  constitute a second measurement portion. In other words, the second measurement portion includes a second radiation source that emits radiation arranged to be incident on from the side end B of the film reel, transmitted through the reel, and exited from the side end A, and a second detector that detects the radiation exited from the side end A. 
     Each of the detectors  26  and  27  is configured by a two-dimensional detection device such as an imaging plate. The optical axis  31  of the X-ray in the first measurement portion and the optical axis  31  of the X-ray in the second measurement portion are both parallel to the central axis  13  of the core  11 , and these optical axes  31  and the central axis  13  of the core  11  are in the same horizontal plane. 
     In the following description, a direction parallel to the central axis  13  of the core  11  is referred to as an x direction, and a direction orthogonal to the x direction in a horizontal plane is referred to as a y direction. The radiation sources  21  and  22  are attached respectively to adjustment stages  51  and  52  that move the radiation sources  21  and  22  in the x direction in a horizontal plane while maintaining the heights of the radiation sources  21  and  22 . Similarly, the detectors  26  and  27  are attached respectively to adjustment stages  56  and  57  that move the detectors  26  and  27  in the x direction in a horizontal plane while maintaining the heights of the detectors  26  and  27 . In the first measurement portion, the FOD (the separation distance between the radiation source and the side end of the film reel  10  facing the radiation source) can be changed by moving the radiation source  21  in the x direction by the adjustment stage  51 . By performing at least one of the movement of the radiation source  21  in the x direction by the adjustment stage  51  and the movement of the detector  26  in the x direction by the adjustment stage  56 , the FID (the separation distance between the radiation source and the detector) can be changed. Similarly, the FID and the FOD of the second measurement portion can be adjusted. A control portion  50  (not shown in  FIG. 7( a ) ) that controls the amount of movement of the adjustment stages  51 ,  52 ,  56 ,  57  is provided, and the control portion  50  preferably performs control so that the FID and the FOD in the first measurement portion and the FID and the FOD in the second measurement portion are equal. 
     Movement stages  61  and  62  (not shown in  FIG. 7( b ) ) are provided to change the X-ray irradiation position in the radial direction on the film reel  10 . The adjustment stages  51  and  56  are attached to the movement stage  61 , and the movement stage  61  integrally moves the adjustment stages  51  and  56 , to which the radiation source  21  and the detector  26  of the first measurement portion are respectively attached, in the y direction. Similarly, the adjustment stages  52  and  57  are attached to the movement stage  62 , and the movement stage  62  integrally moves the adjustment stages  52  and  57 , to which the radiation source  22  and the detector  27  of the second measurement portion are respectively attached, in the y direction. At this time, the movement stages  61  and  62  preferably move relative to each other such that the distance from the center of the film reel  10  (that is, the position of the central axis  13  of the core  11 ) to the optical axis  31  of the radiation in the first measurement portion and the distance  31  to the second measurement portion are always the same. 
     The radiation transmission inspection device further includes a processing portion  65  that calculates the size of the foreign body detected in the film reel  10  based on the detection results of the detectors  26  and  27  using the principle described with reference to  FIGS. 4 and 5 . 
     In the radiation transmission inspection device shown in  FIG. 7 , the FID and the FID of the first measurement portion and the second measurement portion are adjusted by the adjustment stages  51 ,  52 ,  56 , and  57 , the film reel  10  is rotated by the rotation drive portion  47 , and furthermore the X-ray irradiation positions in the radial direction of the film reel  10  are changed by the movement stages  61  and  62  such that the radiation transmission inspection method can be performed over the entire film wound on the film reel  10 . In this device, the first measurement portion and the second measurement portion whose X-ray irradiation directions are opposite to each other are used, and by simultaneously performing the radiation transmission inspection, without turning over the one side end and the other side end, i.e., the front surface and the back surface, of the film reel  10 , the foreign body can be inspected over the entire film wound around the film reel  10  in a short time. In addition, since there is no member that inhibits or attenuates the transmission of X-rays except for the film reel  10  between the radiation source  21  ( 22 ) and the detector  26  ( 27 ), it is possible to obtain a clear image while suppressing the influence of noise. 
     Second Example of Radiation Transmission Inspection Device 
     In the radiation transmission inspection device described with reference to  FIG. 7 , the film reel  10  is held so that the central axis  13  of the core  11  is horizontal, but the film reel  10  can also be configured such that the central axis  13  is vertical. In the radiation transmission inspection device whose front view is shown in  FIG. 8 , the film reel  10  is detachably held by the holding portion  46  so that the central axis  13  of the core  11  is vertical. At this time, since the optical axis of the X-ray is also vertical, the positions of the radiation sources  21  and  22  and the detectors  26  and  27  cannot be adjusted using the adjustment stages. Therefore, in the radiation transmission inspection device shown in  FIG. 8 , with respect to the first measurement portion, the radiation source  21  and the detector  26  are attached to face each other via adjustment members  71  and  76 , respectively, to both ends of an attachment member  66  formed in a C-shape or U-shape. Similarly, regarding the second measurement portion, the radiation source  22  and the detector  27  are attached via adjustment members  72  and  77 , respectively, to both ends of a C-shaped or U-shaped attachment member  67 . The adjustment members  71 ,  72 ,  76 , and  77  adjust the FID and the FOD, and are controlled by the control portion  50  (not shown in  FIG. 8 ) as in the device of  FIG. 7 . Then, the movement stages  61  and  62  move the attachment members  66  and  67  in the radial direction of the film reel  10 , respectively. Also in the radiation transmission inspection device shown in  FIG. 8 , a foreign body in the film reel  10  can be detected in the same manner as in the radiation transmission inspection device shown in  FIG. 7 . Further, a processing portion that calculates the size of the foreign body based on the detection results of the detectors  26  and  27  may be provided. Also in this example, since there is no member that inhibits or attenuates the transmission of X-rays except for the film reel  10  between the radiation source  21  ( 22 ) and the detector  26  ( 27 ), it is possible to obtain a clear image while suppressing the influence of noise. Accordingly, in the second example, a table-shaped member on which a portion of the film reel  10  through which the X-rays transmit is placed may be used as the holding portion  46 . In this example, however, the detector  26  ( 27 ) detects a transmitted image of the table as a background signal, which leads to a reduction in the S/N ratio. Therefore, the configuration of holding the central axis  13  of the film reel  10  as described above is preferable. 
     Third Example of Radiation Transmission Inspection Device 
     The radiation transmission inspection device shown in  FIG. 7  includes two measurement portions: the first measurement portion including the radiation source  21  and the detector  27 , and the second measurement portion including the radiation source  22  and the detector  27 . However, with the radiation transmission inspection device, the inspection time can be further reduced by further increasing the number of measurement portions and simultaneously performing the foreign body detection process.  FIG. 9  shows a radiation transmission inspection device having additional two measurement portions with respect to the device shown in  FIG. 7  to have a total of four measurement portions. In  FIG. 9 , to clarify the arrangement of the radiation sources and the detectors, as a side view viewed from one side end of the film reel  10 , only the film reel  10  including the core  11 , the radiation sources  21  to  24 , and the detectors  26  to  29  are shown. The elements shown by the broken lines in the drawing are located on the other side end side of the film reel  10  and are hidden behind the film reel  10  when viewed from one side end side. 
     In the radiation transmission inspection device shown in  FIG. 9 , it is assumed that the first measurement portion and the second measurement portion are already provided as shown in  FIG. 7 , and at a position facing one side end of the film reel  10  and at a position separated from the radiation source  21  and the second detector  27 , the radiation source  23  for irradiating the film reel  10  with X-rays is provided. The detector  28  that detects X-rays transmitted through the film reel  10  is provided at a position facing the other side end of the film reel  10  and on an extension of the optical axis of the X-rays from the radiation source  23 . The radiation source  23  and the detector  28  constitute the third measurement portion. Further, at a position facing the other side end of the film reel  10  and at a position separated from the radiation source  22  and the detectors  26  and  28 , the radiation source  24  that irradiates the film reel  10  with radiation is provided. The detector  29  that detects X-rays transmitted through the film reel  10  is provided at a position facing one side end of the reel  10  and on an extension of the optical axis of the X-rays from the radiation source  24 . The radiation source  24  and the detector  29  constitute the third measurement portion. The first measurement portion, the second measurement portion, the third measurement portion, and the fourth measurement portion are configured to have the same FID. 
     In particular, in the radiation inspection device shown in  FIG. 9 , each measurement portion may have the same FOD to narrow the scanning range of each measurement portion and shorten the inspection time as a whole. However, when this device is configured such that the first measurement portion and the second measurement portion have the same FOD, the third measurement portion has a larger FOD than the first measurement portion, and the fourth measurement portion has a larger FOD than the second measurement portion, it becomes possible to perform a method of detecting a foreign body by dividing the range between the side end and the intermediate position in the thickness direction into a plurality of regions in the thickness direction described with reference to  FIG. 6 . 
     Fourth Example of Radiation Transmission Inspection Device 
     The radiation transmission inspection device shown in  FIGS. 7, 8, and 9  has a plurality of measurement portions each including a radiation source and a detector. However, in some examples, a plurality of measurement portions cannot be used. If only one measurement portion can be used, some switching mechanism is required to switch between X-ray irradiation from one side end of the film reel  10  and X-ray irradiation from the other side end.  FIG. 10  shows a radiation transmission inspection device including one measurement portion and a switching mechanism. 
     A holding portion  46  that detachably holds the film reel  10  via the core  11  is provided so that the central axis  13  of the core  11  is horizontal. The holding portion  46  is also provided with a rotation drive portion  47  to rotate the film reel  10  around the central axis  13 . A radiation source  21  that irradiates the film reel  10  with X-rays is provided at a position facing one side end of the film reel  10 , and a detector  26  that detects X-rays transmitted through the film reel  10  is provided at a position facing the other side end of the film reel  10  and on an extension of the optical axis  31  of the X-ray from the radiation source  21 . The optical axis  31  is set to be parallel to the central axis  13  of the core  11 . The radiation source  21  and the detector  26  constitute a measurement portion. The radiation source  21  and the detector  26  are attached to face each other via adjustment members  71  and  76 , respectively, to both ends of an attachment member  66  formed in a C-shaped or U-shaped. The adjustment members  71  and  76  adjust the FID and the FOD. To move the position of the optical axis  31  in the radial direction of the film reel  10 , an up-and-down movement portion  81  that moves the attachment member  66  in an up-and-down direction in the drawing is provided, and the attachment member  66  is connected to be suspended from the up-and-down movement portion  81 . Further, a switching portion  82  is provided to rotate the radiation source  21  by 180° relative to the film reel  10  about an axis perpendicular to the central axis  13  of the core  11 . For example, the switching portion  82  is attached to the ceiling of the space where the radiation transmission inspection device is provided, and the upper end of the up-and-down movement portion  81  is connected to the switching portion  82 . 
     With the radiation transmission inspection device shown in  FIG. 10 , the FOD and the FID are adjusted by the adjustment members  71  and  76 , the film reel  10  is rotated by the rotation drive portion  47 , and the X-ray irradiation position of the film reel  10  is changed in the radial direction by the up-and-down movement portion  81  such that the entire film wound on the film reel  10  can be operated with X-rays. To carry out the radiation transmission inspection method, the side on which the X-rays are incident on the film reel  10  must be turned over. For that purpose, it is sufficient if the attachment member  66  is pulled up by the up-and-down movement portion  81  to a position where the radiation source  21  and the detector  26  do not mechanically interfere with the film reel  10 , then the orientation of the attachment member  66  is rotated by 180° in a horizontal plane by the switching portion  82 , and after the rotation, the attachment member  66  is again lowered and next irradiation is performed. 
     The radiation transmission inspection device shown in  FIG. 10  requires only one radiation source and one detector, and is an effective device when the cost of the radiation source and the detector becomes an issue. 
     Method of Manufacturing Microporous Film 
     Next, a method of manufacturing a microporous film to determine the quality by the above-described radiation transmission inspection method will be described. When manufacturing a polyolefin microporous film as a microporous film, first, a plasticizer such as liquid paraffin is added to a polyolefin resin, and these are melt-kneaded by a twin-screw extruder or the like to obtain a polyolefin solution. Then, the polyolefin solution is discharged using a die such as a T-die, and cooled by a cast cooling device or the like to obtain a gel-like sheet. The gel-like sheet is stretched in the machine direction (MD) and the width direction (TD) to form a stretched sheet and, thereafter, the plasticizer is dissolved and removed from the stretched sheet using a cleaning solvent or the like to obtain a microporous film. The microporous film is obtained as a long film by performing the continuous process from the discharge of the polyolefin solution to the dissolution and removal of the plasticizer. The microporous film is wound around the outer peripheral surface of the core  11 , and the film reel  10  is obtained. Then, any one of the above-described radiation transmission inspection methods is performed to inspect a foreign body contained in the film reel  10 . Products determined to be acceptable products as a result of the inspection are shipped. 
     INDUSTRIAL APPLICABILITY 
     The manufacturing process to which the inspection method is applied is not limited to a polyolefin battery separator film, but it is also suitable for a manufacturing process of a coating separator, a nonwoven fabric battery separator, a capacitor film, an MLCC release film, a polyolefin microporous film used for high precision filtration applications, and the like.