Patent Publication Number: US-2016235282-A1

Title: Capsule endoscope

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of International Application PCT/JP2015/060931, with an international filing date of Apr. 8, 2015, which is hereby incorporated by reference herein in its entirety. 
     This application is based on Japanese Patent Application No. 2014-082863, the contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to capsule endoscopes. 
     BACKGROUND ART 
     In the related art, there are known capsule-type endoscopes that are swallowed by a subject and that capture images of the inside of the subject&#39;s body while moving inside the subject&#39;s body (for example, see Patent Literature 1 and Patent Literature 2). 
     It is difficult to control the position and orientation of this type of capsule endoscope inside the patient&#39;s body, and therefore, it is preferable that the viewing area of the image-acquisition optical system in the capsule endoscope be made as wide as possible so that as large an area as possible can be acquired even though the capsule endoscope does not change its orientation. 
     CITATION LIST 
     Patent Literature 
     {PTL 1} 
     Japanese Unexamined Patent Application, Publication No. 2004-357933 
     {PTL 2} 
     Japanese Unexamined Patent Application, Publication No. 2008-43626 
     SUMMARY OF INVENTION 
     Technical Problem 
     The present invention provides a capsule endoscope with which it is possible to sufficiently enlarge the viewing area and also to distribute a sufficient amount of illumination light in the enlarged viewing area. 
     Solution to Problem 
     The present invention provides the following solutions. 
     A first aspect of the present invention is a capsule endoscope including a case, and an objective lens unit, an image acquisition device, and an illumination unit accommodated in the case; wherein the first lens, which is disposed on the extreme object side of the objective lens unit, is a meniscus lens having negative power and with a convex surface thereof facing the object side and is disposed so that at least the convex surface thereof faces the exterior of the case; the illumination unit includes a light source and an illumination window member that faces the exterior of the case and guides illumination light emitted from the light source to the exterior; the light source is disposed so as to surround the circumference of the first lens; the illumination window member is formed in an approximately circular ring shape that surrounds the circumference of the first lens and is positioned closer to the image side than an outermost diameter section of the convex surface of the first lens through which a light flux passes; a light-blocking member that blocks the entry of illumination light into the objective lens unit is disposed between the illumination window member and the first lens; and conditional expressions (1) and (2) below are satisfied 
       −10&lt; f 1/ f&lt;− 4  (1)
 
       0.7&lt; d 1/ f&lt; 2  (2)
 
     where f1 is the focal length of the first lens, f is the focal length of the objective lens unit, and d1 is the distance between the first lens and a lens disposed adjacent to the image side of the first lens. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view showing the overall configuration of a capsule endoscope according to a first embodiment of the present invention. 
         FIG. 2  is a cross-sectional view showing the overall configuration of an objective optical system used in the capsule endoscope according to the first embodiment of the present invention. 
         FIG. 3  is a cross-sectional view showing the overall configuration of a lens holding frame used in the capsule endoscope according to the first embodiment of the present invention. 
         FIG. 4  is a front view showing the overall configuration of a light source unit used in the capsule endoscope according to the first embodiment of the present invention. 
         FIG. 5  is a cross-sectional view showing the overall configuration of an illumination window member used in the capsule endoscope according to the first embodiment of the present invention. 
         FIG. 6  is a cross-sectional view showing the overall configuration of an illumination window member used in the capsule endoscope according to another example of the first embodiment of the present invention. 
         FIG. 7  is a cross-sectional view showing the overall configuration of an illumination window member used in a capsule endoscope according to a modification of the first embodiment of the present invention. 
         FIG. 8  is a cross-sectional view showing the overall configuration of a lens holding frame used in the capsule endoscope according to the modification of the first embodiment of the present invention. 
         FIG. 9  is a cross-sectional view showing the overall configuration of a capsule endoscope according to a second embodiment of the present invention. 
         FIG. 10  is a cross-sectional view showing the overall configuration of an objective optical system used in the capsule endoscope according to the second embodiment of the present invention. 
         FIG. 11  is a cross-sectional view showing the overall configuration of an example of a capsule endoscope that can observe in two directions. 
         FIG. 12  is a reference perspective view showing the concept of an example of a capsule endoscope that can observe in six directions. 
         FIG. 13  is a cross-sectional view showing the overall configuration of the example of the capsule endoscope in  FIG. 12 , which can observe in six directions. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     A capsule endoscope according to a first embodiment of the present invention will be described below with reference to the drawings. 
       FIG. 1  is a cross-sectional view showing the overall configuration of a capsule endoscope according to this embodiment. The capsule endoscope includes a case  10 , an objective lens unit  11  that is accommodated in the case  10  and  12  that forms an image of a subject, an image acquisition device that acquires the image of the subject formed by the objective lens unit  11 , and an illumination unit  13  that irradiates the subject with illumination light. 
     The objective lens unit  11  includes an objective optical system  15  formed of a plurality of lenses and a lens holding frame  16  that holds the objective optical system. 
     As shown in  FIG. 2 , the objective optical system  15  is formed of a first lens L 1 , a second lens L 2 , an aperture stop S, a third lens L 3 , and a fourth lens L 4 , in this order from the object side, and the first lens L 1 , which is disposed at the extreme object side of the objective optical system  15 , is a meniscus lens in which the convex surface faces the object side and which has negative power. 
     The objective optical system  15  is configured so as to satisfy conditional expressions (1) and (2) below: 
       −10&lt; f 1/ f&lt;− 4  (1)
 
       0.7&lt; d 1/ f&lt; 2  (2)
 
     where f1 is the focal length of the first lens, f is the focal length of the objective lens unit, and d1 is the distance between the first lens and the lens disposed adjacent to the image side of the first lens. 
     If the lower limit of conditional expression (1) is exceeded, due to the fact that the power of the first lens becomes weaker, in order to reduce the overall focal length so as to ensure a sufficient viewing angle, from paraxial calculations, the overall length of the objective lens unit increases since it is necessary to increase the spacing between components other than the first lens. If the upper limit of conditional expression (1) is exceeded, the power of the first lens and the curvature of the surface on the image-plane side of the first lens become stronger, and it becomes difficult to ensure, on the surface at the image side of the first lens, a region through which the required light flux passes, and therefore, a viewing angle of 180 degrees or more becomes impossible, and aberrations deteriorate. 
     If the lower limit of conditional expression (2) is exceeded, the power of the first lens becomes too strong, degrading the performance, and if the upper limit is exceeded, the overall length increases, and the outer diameter becomes large. 
     In addition, conditional expression (3) below is satisfied: 
       1.1&lt; fm/f≦ 1.3  (3)
 
     where fm is the focal length of the components of the objective lens, excluding the first lens. 
     If the lower limit of conditional expression (3) is exceeded, the power of the first lens becomes weaker, and therefore, the overall length of the objective lens unit increases. If the upper limit of conditional expression (3) is exceeded, the aberrations become worse, and the power of the first lens becomes stronger, and accordingly, the power of the components in the objective lens unit except for the first lens also becomes stronger. Because of this, there are some disadvantages, such as the need to increase the number of lenses to correct the aberrations, an increase in the overall length, and a rise in costs. 
     As shown in  FIG. 3 , the lens holding frame  16  has a first holding section  16 A that is formed in a ring shape when viewed from the front and that holds the first lens L 1 , and a second holding section  16 B that holds the second lens L 2 , the third lens L 3 , and the fourth lens L 4 . The first holding section  16 A has a side-surface holding section  17 A that holds a side surface of the first lens L 1  and a flat-surface holding section  17 B that holds the surface at the image plane side of the first lens L 1 . 
     When held in the first holding section  16 A, the first lens is disposed so that the convex surface, which is the surface that faces the object side of the first lens L 1 , faces the outside of the case  10 , so that the convex surface of the first lens L 1  functions as a portion of the case. 
     The second holding section  16 B holds the side surfaces of the second lens L 2 , the third lens L 3 , and the fourth lens L 4  and is configured so that the second lens L 2 , the third lens L 3 , and the fourth lens L 4  are accommodated in a space formed by the first lens L 1  and the second holding section  16 B. 
     By forming the lens holding frame  16  of a light blocking material, the lens holding frame functions as a light-blocking member. In other words, a light-blocking member that blocks the entry of the illumination light into the objective lens unit  11  is disposed between a unit  18  described later and the first lens L 1 . Therefore, the entry of illumination light from the illumination unit  13  disposed outside the lens holding frame  16  into the objective optical system  15  accommodated in the lens holding frame  16  can be blocked. 
     The illumination unit  13  is provided with the light source unit  18  and an illumination window member  19  that guides the illumination light emitted from the light source unit  18  to the outside. 
     As shown in  FIG. 4 , the light source unit  18  is provided with a flexible substrate  18 A formed in an approximately circular ring shape that surrounds the circumference of the first lens L 1  when viewed from the front and a plurality of LEDs  18 B disposed uniformly on this flexible substrate.  FIG. 4  shows an example in which four LEDs are disposed at 90° intervals, and are disposed so as to surround the first lens L 1 . 
     As shown in  FIG. 5 , the illumination window member  19  is formed in an approximately circular ring shape that surrounds the circumference of the first lens when viewed from the front, is fitted on the end of the case  10 , and is formed so that the light source unit  18  and the lens holding frame  16  are accommodated at the inner side of the illumination window member  19 , so as to be positioned closer to the image side than the outermost diameter section of the convex surface of the first lens L 1  through which the light flux passes. The outer surface of the illumination window member  19  faces outside the case  10 , and the convex surface of the first lens, the end surface of the side-surface holding section  17 A of the lens holding frame, which holds the first lens, and the outer surface of the illumination window member are disposed to form a single surface, so as to function as a portion of the case. 
     According to this embodiment, with the object-side surface of the first lens assumed to be the convex surface, negative distortion becomes stronger, and from a paraxial calculation in which the powers of components of the objective lens unit other than the first lens are combined, the lenses are configured so that, with the power of the first lens assumed to be negative, conditional expressions (1) and conditional expression (2) are satisfied. By doing so, a viewing angle of 180° and higher, with which it is possible to perform rearward observation, can be ensured. 
     In addition, by disposing the illumination window, which is formed in an approximately circular ring shape that surrounds the first lens, around the circumference of the first lens, it is possible to ensure a suitable light distribution and intensity for covering a viewing angle of 180° or more. 
     By forming the lens holding frame  16  of a light blocking material, the light-blocking member that blocks the entry of illumination light into the objective lens unit is disposed between the illumination window member and the first lens, and it is possible to prevent unwanted light such as direct illumination light or flare reflected at a surface of the case or the like from entering the objective lens. 
     In addition, by disposing the convex surface of the first lens so as to face the exterior of the case, the convex surface of the first lens functions as a portion of the case, which is the boundary with the exterior, and therefore, it is possible to reduce the overall length of the capsule endoscope. 
     Modification 1 
     In the first embodiment described above, the lens holding frame  16  and the illumination window member  19  are formed to have approximately circular ring shapes in which the inner side and the outer side are both circular shapes so as to be able to hold the first lens, which has a circular shape when viewed from the front; however, it is not necessarily limited thereto. Specifically, due to the fact that the image-acquisition device that forms an image of the subject has a rectangular shape, the first lens can be made to have a rectangular shape that matches the long side and short side of the image acquisition device to the extent that the light flux required for forming an image is not blocked, and the inner shapes of the lens holding frame  16  and the illumination window member  19  can be made to have approximately rectangular shapes so as to match the shape of this first lens (see  FIG. 7  and  FIG. 8 ). 
     By doing so, the projection area of the illumination window member can be enlarged, and the light distribution in the forward direction can be further improved. 
     In the above-described first embodiment and the first modification thereof, the illumination window member  19  is formed in an approximately circular shape that surrounds the circumference of the first lens when viewed from the front; however, it is not necessarily limited thereto. For example, as shown in  FIG. 6 , a configuration in which a plurality of circular arc-shaped illumination window members  20  are disposed in the form of an approximately circular ring is possible. 
     EXAMPLES 
     Lens data for the objective optical systems according to Example 1 and Example 2 of the aforementioned first embodiment and modification 1 thereof are shown below. In the lens data below, r indicates the radius of curvature (unit, mm), d indicates the inter-surface distance (mm), Nd indicates the refractive index at the d-line, and Vd indicates the Abbe number at the d-line. Aspheric surfaces are indicated by a * on the surface numbers in the lens data, and the paraxial radius of curvature r, the conic constant K, and the aspheric coefficients Ai (i=2, 4, 6, 8, 10) of the aspheric surface shape expressed by the following equation are shown in the aspheric surface data. Note that, in the following equation, the optical axis direction is z, and the direction perpendicular to the optical axis is y. 
         z =( y   2   /r )/[1+{1−(1+ K )( y/r ) 2 } 1/2   ]+A 2 y   2   +A 4 y   4   + . . . +A 8 y   8   +A 10 y   10  
 
     Example 1 
     Lens data for the objective optical system in Example 1 according to the above-mentioned first embodiment and modification 1 thereof are shown below. 
     
       
         
           
               
             
               
                   
               
             
            
               
                 Lens Data 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Surface number 
                 r 
                 d 
                 Ne 
                 Vd 
               
               
                   
                   
               
               
                   
                 Object plane 
                 ∞ 
                 ∞ 
                 1. 
               
               
                   
                  1 
                 9.5885 
                 0.6480 
                 1.53296 
                 55.69 
               
               
                   
                  2 
                 2.5855 
                 1.2138 
                 1. 
               
               
                   
                  3 
                 51.8397 
                 0.3888 
                 1.53296 
                 55.69 
               
               
                   
                  4 
                 0.9694 
                 0.7776 
                 1. 
               
               
                   
                  5 Aperture stop 
                 ∞ 
                 0.0959 
                 1. 
               
               
                   
                  6 
                 2.0723 
                 1.2182 
                 1.53296 
                 55.69 
               
               
                   
                  7* 
                 −1.2571 
                 0.2074 
                 1. 
               
               
                   
                  8 
                 2.8395 
                 1.0368 
                 1.53296 
                 55.69 
               
               
                   
                  9 
                 −21.5783 
                   
                 1.3396 
                 1. 
               
               
                   
                 10 (image plane) 
                 ∞ 
               
               
                   
                   
               
            
           
           
               
            
               
                 Aspheric surface data 
               
               
                 Surface 7 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 r = −1.2571 
               
               
                   
                 K = −1.8390 
               
               
                   
                 A2 = 0.0 
               
               
                   
                 A4 = 4.0166e−03 
               
               
                   
                 A6 = 4.9015e−02 
               
               
                   
                 A8 = 8.1832e−02 
               
               
                   
                 A10 = −3.0745e−02 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Viewing angle (2ω) 
                 200.8° 
               
               
                   
                 Maximum image height 
                 1.264 
               
               
                   
                 Fno. 
                 4.625 
               
               
                   
                 Focal length 
                 1.000 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 f1/f: −6.86 
               
               
                   
                 d1/f: 1.21 
               
               
                   
                 fm/f: 1.22 
               
               
                   
                   
               
            
           
         
       
     
     Example 2 
     Lens data for the objective optical system in Example 2 according to the above-mentioned first embodiment and modification 1 thereof are shown below. 
     
       
         
           
               
             
               
                   
               
             
            
               
                 Lens Data 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Surface number 
                 r 
                 d 
                 Ne 
                 Vd 
               
               
                   
                   
               
               
                   
                 Object plane 
                 ∞ 
                 ∞ 
                 1. 
               
               
                   
                  1 
                 7.4466 
                 0.7195 
                 1.53296 
                 55.69 
               
               
                   
                  2 
                 2.0518 
                 1.1231 
                 1. 
               
               
                   
                  3 
                 −62.9078 
                 0.4317 
                 1.53296 
                 55.69 
               
               
                   
                  4 
                 0.9371 
                 0.9144 
                 1. 
               
               
                   
                  5 Aperture stop 
                 ∞ 
                 0.1284 
                 1. 
               
               
                   
                  6 
                 1.8545 
                 1.3528 
                 1.53296 
                 55.69 
               
               
                   
                  7* 
                 −1.2470 
                 0.2303 
                 1. 
               
               
                   
                  8 
                 4.9190 
                 1.1513 
                 1.53296 
                 55.69 
               
               
                   
                  9 
                 −65.3777 
                 1.4650 
                 1. 
               
               
                   
                 10 (image plane) 
                 ∞ 
               
               
                   
                   
               
            
           
           
               
            
               
                 Aspheric surface data 
               
               
                 Surface 7 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 r = −1.2470 
               
               
                   
                 K = −1.9262 
               
               
                   
                 A2 = 0.0 
               
               
                   
                 A4 = −8.1063e−03 
               
               
                   
                 A6 = 2.2254e−02 
               
               
                   
                 A8 = 1.1724e−01 
               
               
                   
                 A10 = −1.7269e−02 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Viewing angle (2ω) 
                 201.4° 
               
               
                   
                 Maximum image height 
                 1.403 
               
               
                   
                 Fno. 
                 4.581 
               
               
                   
                 Focal length 
                 1.000 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 f1/f: −5.57 
               
               
                   
                 d1/f: 1.12 
               
               
                   
                 fm/f: 1.26 
               
               
                   
                   
               
            
           
         
       
     
     Second Embodiment 
     Next, a second embodiment of the present invention will be described. In this embodiment, configurations that are identical to those in the first embodiment described above are assigned the same reference signs, and descriptions thereof are omitted. The differences between this embodiment and the capsule endoscope according to the first embodiment and modification 1 thereof are as follows. 
     Specifically, as shown in  FIGS. 9 and 10 , the objective optical system is formed of a first lens L 1 , an aperture stop S, a second lens L 2 , a third lens L 3 , a fourth lens L 4 , a fifth lens L 5 , and a sixth lens L 6 , in this order from the object side, where the fourth lens and the fifth lens are combined to form a combined lens CL 1 . Chromatic aberration of magnification can be corrected more effectively by using the combined lens CL 1 , field curvature can be corrected by using a meniscus lens at the sixth lens L 6 , and in addition, by using aspheric surfaces, aberrations can be corrected more effectively. Accordingly, an objective optical system that can also be used for a high-precision image acquisition device becomes possible. 
     In addition, the lens holding frame  16  and the light-blocking member  22  are separate, independent structures rather than being combined in a single unit. 
     The lens data of the objective optical system according to the second embodiment described above is as follows. 
     
       
         
           
               
             
               
                   
               
             
            
               
                 Lens data 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Surface number 
                 r 
                 d 
                 Ne 
                 Vd 
               
               
                   
                   
               
               
                   
                 Object 
                 ∞ 
                 ∞ 
                 1. 
               
               
                   
                  1 
                 7.0056 
                 0.6369 
                 1.58959 
                 30.00 
               
               
                   
                  2 
                 1.9106 
                 1.8469 
                 1. 
               
               
                   
                  3 Aperture stop 
                 ∞ 
                 0.0319 
                 1. 
               
               
                   
                  4 
                 −0.7192 
                 0.5581 
                 1.53336 
                 56.00 
               
               
                   
                  5* 
                 −0.6479 
                 0.0274 
                 1. 
               
               
                   
                  6 
                 −10.8727 
                 0.6858 
                 1.53336 
                 56.00 
               
               
                   
                  7* 
                 −3.4092 
                 0.0274 
                 1. 
               
               
                   
                  8 
                 2.5028 
                 0.1824 
                 1.93429 
                 18.90 
               
               
                   
                  9 
                 1.5989 
                 1.2252 
                 1.73234 
                 54.68 
               
               
                   
                 10 
                 −4.7961 
                 0.0274 
                 1. 
               
               
                   
                 11* 
                 3.8546 
                 0.1824 
                 1.53336 
                 56.00 
               
               
                   
                 12 
                 2.5989 
                 0.8211 
                 1. 
               
               
                   
                 13 (image plane) 
                 ∞ 
               
               
                   
                   
               
            
           
           
               
            
               
                 Aspheric surface data 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 Surface 5 
               
               
                   
                 r = −0.6479 
               
               
                   
                 K = −0.5730 
               
               
                   
                 A2 = 0.0 
               
               
                   
                 A4 = 1.6518e−01 
               
               
                   
                 A6 = −4.9711e−02 
               
               
                   
                 Surface 7 
               
               
                   
                 r = −3.4092 
               
               
                   
                 K = 0.3236 
               
               
                   
                 A2 = 0.0 
               
               
                   
                 A4 = −2.7518e−01 
               
               
                   
                 A6 = 8.9225e−02 
               
               
                   
                 A8 = −5.3971e−02 
               
               
                   
                 Surface 11 
               
               
                   
                 r = 3.8546 
               
               
                   
                 K = −0.7326 
               
               
                   
                 A2 = 0.0 
               
               
                   
                 A4 = −5.8668e−02 
               
               
                   
                 A6 = −1.5556e−02 
               
               
                   
                 A8 = 9.0343e−03 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Viewing angle (2ω) 
                 200.1° 
               
               
                   
                 Maximum image height 
                 1.14 
               
               
                   
                 Fno. 
                 7.591 
               
               
                   
                 Focal length 
                 1.000 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 f1/f: −5.57 
               
               
                   
                 d1/f: 1.12 
               
               
                   
                 fm/f: 1.26 
               
               
                   
                   
               
            
           
         
       
     
     In the above embodiments, although a configuration in which the objective optical system and so forth are provided only in one direction in the capsule endoscope has been described, it is not limited thereto; for example, as shown in  FIG. 11 , a configuration that enables observation in two directions, namely, the forward and rearward directions, of the capsule endoscope is also possible, and as shown in  FIG. 12  and  FIG. 13 , a configuration that enables observation in six directions by using a substantially cube shaped capsule endoscope is also possible. 
     As described above, with the object-side surface of the first lens assumed to be the convex surface, negative distortion becomes stronger, and from a paraxial calculation in which the powers of components of the first objective lens unit except for the first lens are combined, the power of the first lens is set to be negative, the lenses are configured so as to satisfy conditional expressions (1) and (2). By doing so, it is possible to ensure a viewing angle of 180° or more, which enables rearward observation. 
     If the lower limit of conditional expression (1) is exceeded, due to the fact that the power of the first lens becomes weaker, in order to reduce the overall focal length so as to ensure a sufficient viewing angle, from paraxial calculations, the overall length of the objective lens unit increases since it is necessary to increase the spacing between components other than the first lens. If the upper limit of conditional expression (1) is exceeded, the power of the first lens and the curvature of the surface on the image-plane side of the first lens become stronger, and it becomes difficult to ensure, on the surface at the image side of the first lens, a region through which the required light flux passes, and therefore, a viewing angle of 180 degrees or more becomes impossible, and aberrations deteriorate. 
     If the lower limit of conditional expression (2) is exceeded, the power of the first lens becomes too strong, degrading the performance, and if the upper limit is exceeded, the overall length increases, and the outer diameter becomes large. 
     In addition, by disposing, around the circumference of the first lens, the illumination window formed in an approximately circular ring shape that surrounds the first lens, it is possible to ensure a suitable light distribution and intensity for covering a viewing angle of 180° or more. 
     Since the light-blocking member that blocks the entry of illumination light into the objective lens unit disposed between the illumination window member and the first lens, it is possible to prevent unwanted light such as direct illumination light or flare reflected at a surface of the case or the like from entering the objective lens. 
     It is preferable to dispose the convex surface, which serves as the object-side surface of the first lens, so as to face the exterior of the case, and by doing so, the convex surface of the first lens functions as a portion of the case, which is the boundary with the exterior, and therefore, it is possible to reduce the overall length of the capsule endoscope. 
     As described above, conditional expression (3) below may be satisfied: 
       1.1&lt; fm/f≦ 1.3  (3)
 
     where fm is the focal length of components of the objective lens excluding the first lens. 
     By doing so, the number of lenses can be suppressed, and manufacturing costs can be controlled. If the lower limit of conditional expression (3) is exceeded, the power of the first lens becomes weaker, and therefore, the overall length of the objective lens unit increases. If the upper limit of conditional expression (3) is exceeded, the aberrations become worse, and the power of the first lens becomes stronger, and accordingly, the power of the components in the objective lens unit except for the first lens also becomes stronger. Because of this, there are some disadvantages, such as the need to increase the number of lenses to correct the aberrations, an increase in the overall length, and a rise in costs. 
     As described above, a front surface shape as viewed from the object side of the light-blocking member may be a substantially rectangular shape that matches the outer shape of the image acquisition device. 
     By doing so, the projection area of the illumination window member in the forward direction can be enlarged, and therefore, the light distribution in the forward direction can be further improved. 
     As described above, the light-blocking member may be formed as a single unit together with a lens frame that holds the objective lens unit. 
     By doing so, manufacturing costs can be reduced, and assembly of the objective lens unit, and by extension, the capsule endoscope, is facilitated. In addition, it is possible to further improve the blocking of unwanted light on the objective lens unit. 
     REFERENCE SIGNS LIST 
     
         
           10  Case 
           11  Objective lens unit 
           12  Image acquisition device 
           13  Illumination unit 
           15  Objective optical system 
           16  Lens holding frame 
           18  Light source unit 
           19 ,  20  Illumination window member 
           22  Light-blocking member 
         L 1  First lens 
         L 2  Second lens 
         L 3  Third lens 
         L 4  Fourth lens 
         L 5  Fifth lens 
         L 6  Sixth lens 
         CL 1  Combined lens