Patent Publication Number: US-2005134977-A1

Title: Image sensing device and image sensing apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2003-422872, filed Dec. 19, 2003, the entire contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an image sensing device used in a digital camera or a cellular phone with a camera and, more particularly, to an image sensing device using a prism having a free-form surface as a reflecting surface, and an image sensing apparatus using the image sensing device.  
      2. Description of the Related Art  
      A number of applications for image sensing apparatuses using a coaxial optical system have been filed as image sensing apparatuses used in digital cameras or cellular phones with a camera. In a coaxial optical system, optical elements such as a lens are rotationally symmetrical with respect to the optical axis (an axis which connects the center of the aperture of the image sensing system and the center of the image sensing screen) of the optical system. Image sensing apparatuses having a coaxial system are disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publications No. 2001-272587 (reference 1) No. 2002-267928 (reference 2), and No. 2002-320122 (reference 3).  
      Recent digital cameras and cellular phone with a camera are required to be compact and thin and have high performance. In these devices, if the image sensing device using a coaxial optical system should be compact, the number of lenses must be decreased. However, when the number of lenses is decreased, aberrations generated in the optical system can hardly be suppressed, resulting in poor image quality. To obtain a high image quality, the number of lenses must be increased. As a result, the image sensing device becomes bulky.  
      As means for solving these problems, image sensing apparatus using an eccentric optical system has been proposed. Image sensing apparatuses using an image sensing optical system using a prism a free-form surface are disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publications No. 11-326766 (reference 4), No. 2002-196243 (reference 5), and No. 2003-84200 (reference 6).  
      In the present specification, a term “eccentric optical system” means an optical system in which an optical axis of a luminous flux made incident to an optical system and an optical axis of a luminous flux emerged from this optical system do not exist coaxially. A term “free-form surface” means a curved surface which is rotationally asymmetrical to an optical axis of a luminous flux incident to the surface or an optical axis of a luminous flux emerged from the surface and which has only one mirror image plane along these optical axes.  
      The techniques described in references 4 to 6 aim at obtaining a compact device and a high-quality image by forming an image sensing optical system by using a prism having a free-form surface as a light incident surface, light emergent surface, or reflecting surface. Especially, in references 5 and 6, two prisms are combined. The light incident surface, reflecting surface, and light emergent surface of the first prism close to the object and the light incident surface, two reflecting surface, and the light emergent surface of the second prism close to the image sensing surface, i.e., a total of seven surfaces are formed as free-form surfaces.  
      The characteristic features of such an optical system are as follows.  
      (1) The three reflecting surfaces are formed from free-form surfaces having a power (reflecting power). These reflecting surfaces can obtain a large power and are rarely affected by chromatic aberration as compared to a refractive optical system such as a lens.  
      (2) The seven optical surfaces can be formed in a compact space. Hence, the optical elements are concentratedly set in the limited space.  
      (3) To obtain high optical performance, the optical path length of the entire optical system is preferably long to some extent. The optical path is bent by using such a prism optical system. Hence, image sensing device having a long optical path and a small outward size can be manufactured.  
      For these reasons, the image sensing device can be raised the quality of an image in spite of the size.  
      The optical system described in Jpn. Pat. Appln. KOKAI Publication No. 7-333505 (reference 7) includes a reflecting mirror, a coaxial optical system by a lens, and a reflecting mirror sequentially from the object side. As compared to this system, the optical system described in reference 5 or 6 can reduce the width. For this reason, a more compact image sensing device can be provided by using this optical system.  
      As electrical image sensing devices such as a digital cameras or cellular phones with a camera are prevalent, there is a demand for higher quality image sensing. The number of pixels of a CCD (Charge-coupled device) as an image sensing element for converting an object image into image data has trended to increase. Many of the CCDs with a large number of pixels are of interlace type. The interlace type CCDs read out image data by dividing it into an odd numbered field and an even numbered field.  
      The image data stored in these two fields cannot be read out at one time and at the same time. If the image data is sequentially read out from the two fields without light interrupting the CCD, exposure times of the odd numbered field and even numbered field become different from each other. In order to make identical the exposure times of both of the fields, it is necessary to light interrupt the CCD so as not to ensure that light is incident to another field while reading out the image data in one field. Therefore, while the image data is read out, a mechanical shutter must be provided to light interrupt the CCD.  
      In addition, the luminance of objects covers a wide range. If the number of pixels of the image sensing element is increased, fine graduation in one item of image data can be provided. However, under the luminance conforming to a variety of conditions, it is difficult to carry out image sensing for an optimal exposure time only with a shutter opening time and a dynamic range that the image sensing element has. In order to solve this problem, there is a need for an aperture for changing an amount of light projected to the image sensing element.  
      However, if coaxial optical systems disclosed in references 1 to 3 are provided with the mechanism of shutter or the aperture, the optical system become bulky in capacity, respectively. Thus, it is difficult to downsize and thin the image sensing device. On the other hand, references 4 to 6 discloses an image sensing optical system using a prism having a free-form surfaces. However, there is no reference describing specifically mounting the mechanical shutter and the aperture.  
     BRIEF SUMMARY OF THE INVENTION  
      It is an object of the present invention to provide an image sensing device capable of downsizing the entire device and capable of acquiring a high quality image and an image sensing apparatus comprising the image sensing device.  
      The image sensing device according to the present invention includes a first prism, a second prism, a shutter mechanism, and an image sensing element. The first prism receives at a first incident surface a luminous flux radiated from an object, and outputs the luminous flux at a first emergent surface after reflecting the luminous flux on at least one of a first reflecting surface formed in the shape of a free-form surface. The second prism receives at a second incident surface the luminous flux emerging from the first emergent surface, and outputs the luminous flux at a second emergent surface after reflecting the luminous flux on at least one of a second reflecting surface formed in the shape of a free-form surface. The shutter mechanism is arranged between the first emergent surface and the second incident surface. The image sensing element is arranged on an image focusing surface of an optical system including the first prism and the second prism, and converts an object image formed by such an optical system into an electrical signal.  
      In this case, the shutter mechanism includes a shutter blade which is selectively switched to either of an open state and a closed state. In the open state, the luminous flux emerging from the first emergent surface is passed toward the second incident surface. In the closed state, the luminous flux emerging from the first emergent surface is interrupted. Alternatively, the shutter mechanism has at least two shutter blades moving together. These shutter blades are selectively switched to an open state in which the luminous flux emerging from the first emergent surface is passed toward the second incident surface and a closed state in which the luminous flux emerging from the first emergent surface is interrupted.  
      In addition, in order for the shutter mechanism to have an aperture function, a blade drive mechanism is provided in the shutter mechanism. This blade drive mechanism moves and holds the shutter blades in a direction which crosses the luminous flux emerging from the first emergent surface. The blade drive mechanism stops the shutter blades in a range between the closed state and the open state in order to change the size of an opening formed by the shutter blades.  
      In addition, in order to adjust an amount of light made incident to the image sensing element, an aperture is arranged between the first emergent surface and the second incident surface. This aperture has an opening which is smaller than an external diameter of the luminance flux emerging from the first emergent surface. In this case, in order to actuate the aperture as required, the aperture is selectively held in either of an insert position and a retracted position. At the insert position, the aperture crosses the luminous flux emerging from the first emergent surface between the first emergent surface and the second incident surface. At the retracted position, the aperture is deviated from the luminous flux. In addition, in the aperture, it is preferable to the center of the opening be arranged coaxially to a center axis of the luminance flux in the insert state. In addition, in order to reduce the bulkiness of the image sensing device, the aperture is incorporated in the shutter mechanism.  
      Instead of providing the aperture, a light reducing filter which reduces an amount of light may be provided between the first prism and the second prism. The light reducing filter is selectively held in either of an insert position and a retracted position. The light reducing filter crosses the luminous flux emerging from the first emergent surface between the first emergent surface and the second incident surface in the insert position. The light reducing filter is deviated from the luminous flux in the retracted position.  
      In addition, in order to improve assembling precision of an optical system, the first prism and the second prism are provided with an engagement portion for keeping mutual relative positions. In this case, more preferably, an engagement portion provided in the first prism side and an engagement portion provided at the second prism side are directly engaged with each other.  
      In addition, it is also preferable that the image sensing device comprises a frame which holds the first prism, the second prism, the shutter mechanism and the image sensing element in a specific positional relation. In this case, the frame has a first wall which positions and holds the shutter mechanism between the first prism and the second prism and a second wall which positions and holds the image sensing element on an image focusing surface. In addition, the first wall and the second wall are integrally formed.  
      The image sensing apparatus according to the present invention has processing means to obtain image data by executing predetermined electrical processing to an electrical signal obtained by the image sensing device described above and recording means to record image data from the processing means in an applied information recording medium.  
      The image sensing device according to the present invention can interrupt the light incident to the image sensing element while reading out image data from the image sensing element. Therefore, the exposure times of the odd numbered field and the even numbered field of the interlace type CCD can be identical to each other. In addition, by using this image sensing device, there can be provided an image sensing apparatus capable of acquiring a good quality image while compactly maintaining the size of the entire apparatus.  
      In addition, according to an invention in which a light reducing filter of open and close type is provided in a shutter mechanism, an amount of light can be regulated. Thus, in a state such as a object condition with large difference in brightness and darkness, in which an electronic shutter function having an image sensing element is insufficient, by properly inserting the light reducing filter, a good quality image can be obtained even by an electronic shutter which the image sensing element has.  
      Advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
      The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.  
       FIG. 1  is a perspective view of a digital camera having an image sensing device according to a first embodiment of the present invention;  
       FIG. 2  is a sectional view schematically showing an inside of the digital camera shown in  FIG. 1 ;  
       FIG. 3  is an exploded view of an image sensing device of the digital camera shown in  FIG. 1 ;  
       FIG. 4  is a partially sectional side view of the image sensing device shown in  FIG. 3 ;  
       FIG. 5  is an enlarged view showing an engagement portion of a first prism and an engagement portion of a second prism shown in  FIG. 4 ;  
       FIG. 6  is an exploded view showing a shutter mechanism and a filter mechanism of the image sensing device shown in  FIG. 3 ;  
       FIG. 7  is a sectional view selectively passing an engagement portion of the shutter mechanism and filter mechanism of the image sensing device shown in  FIG. 3 ;  
       FIG. 8  is a front view showing a shutter mechanism in an open state taken along the line A-A in  FIG. 6 ;  
       FIG. 9  is a front view showing a shutter mechanism in a closed state taken along the line A-A in  FIG. 6 ;  
       FIG. 10  is a front view showing a filter mechanism in a non-light reducing state taken along the line B-B in  FIG. 6 ;  
       FIG. 11  is a front view showing a filter mechanism in a light reducing state taken along the line B-B in  FIG. 6 ;  
       FIG. 12A  is a sectional view showing another embodiment of the engaging portion shown in  FIG. 5 ;  
       FIG. 12B  is a sectional view showing another embodiment of the engaging portion shown in  FIG. 5 ;  
       FIG. 12C  is a sectional view showing another embodiment of the engaging portion shown in  FIG. 5 ;  
       FIG. 12D  is a sectional view showing another embodiment of the engaging portion shown in  FIG. 5 ;  
       FIG. 13  is a sectional view showing a joint portion of a frame in another embodiment in which a first wall and a second wall are provided at another member;  
       FIG. 14  is a front view showing another embodiment in which an aperture plate is provided instead of an ND filter;  
       FIG. 15  is a disassembled perspective view showing an image sensing device according to a second embodiment of the present invention;  
       FIG. 16  is a partially sectional side view showing the image sensing device shown in  FIG. 15 ;  
       FIG. 17  is a sectional view showing a joint portion of a frame in another embodiment in which the first wall and the second wall of the frame of  FIG. 15  are provided independently;  
       FIG. 18  is a perspective view showing a cellular phone with a camera as another example of an image sensing apparatus according to the present invention;  
       FIG. 19  is a sectional view schematically showing another example of a prism optical system; and  
       FIG. 20  is a sectional view schematically showing still another example of the prism optical system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      An image sensing device according to a first embodiment of the present invention will be described by way of one example of a digital camera  1  with reference to  FIG. 1  to  FIG. 4 . As shown in  FIG. 1 , the digital camera  1  has a release button  3 , a flash  4 , a finder optical system  5 , an image sensing optical system  6 , and an image display unit  7  (refer to  FIG. 2 ), which are arranged on the outer surface of a housing  2 . The release button  3  is one of operating portions.  
      As shown in  FIG. 2 , the housing  2  incorporates devices: such as, an image sensing device  10  which configures main parts of an image sensing optical system  6 ; an image processing circuit  21  serving as processing means; and a recording unit  22  serving as recording means. The image processing circuit  21  has a function of executing predetermined electrical processing to an electrical signal obtained by the image sensing device  10 , thereby obtaining image data. The recording unit  22  functions as recording means for temporarily storing the image data from the image processing circuit  21  and recording the image data in an applied recording medium. An incident window  6   a  through which a luminous flux from an object made incident to the image sensing device  10  passes is provided in the housing  2 .  
      As shown in  FIG. 2  and  FIG. 3 , the image sensing device  10  has a first prism  11 , a second prism  12 , a shutter mechanism  13 , a filter mechanism  14 , and an image sensing element  15 . The first prism  11  is an eccentric prism including a first incident surface  11   a , a first reflecting surface  11   b , and a first emergent surface  11   c . The first reflecting surface  11   b  is formed in the shape of a rotationally asymmetric free-form surface. The second prism  12  is an eccentric prism including a second incident surface  12   a , two second reflecting surface  12   b ,  12   c , and a second emergent surface  12   d . The two second reflecting surfaces  12   b ,  12   c  are formed in the shape of a rotationally asymmetric free-form surface, respectively.  
      As shown in  FIG. 4 , a luminous flux λi reflected from an object is incident from the first incident face  11   a , and emerges from the first emergent surface  11   c , after having been reflected on the first reflecting surface  11   b . A luminous flux λm emerged from the first emergent surface  11   c  of the first prism  11  is incident from the second incident surface  12   a , and emerged from the second emergent surface  12   d  after being reflected on the two second reflecting surfaces  12   b ,  12   c  respectively. Then a luminous flux λo emerged from the second emergent surface  12   d  forms an object image on an image focusing surface  45 .  
      As shown in  FIG. 4 , the shutter mechanism  13  and the filter mechanism  14  are arranged between the first emergent surface  11   c  of the first prism  11  and the second incident surface  12   a  of the second prism  12 . As shown in  FIG. 6 , the shutter mechanism  13  has: two shutter blades  33 ,  34 ; and a blade drive ring  32  which is a part of a blade drive mechanism. The filter mechanism  14  has an ND (Natural Density) filter  36  serving as a light reducing filter. A shutter actuator  27  is linked with the shutter blades  33 ,  34  via a blade drive ring  32 . A filter actuator  28  is linked with the ND filter  36 . A detailed description of these elements will be given later.  
      The image sensing element  15  is mounted on a substrate  23 . A light-receiving surface  15   a  of the image sensing element  15  is arranged on an image focusing surface  45 . The image sensing element  15  is a CCD (Charge-coupled device) in which semiconductor elements for converting light into an electrical signal are arranged in plurality on the light-reserving surface  15   a . As shown in  FIG. 4 , the filter  15   b  is mounted between the second emergent surface  12   d  and the light-receiving surface  15   a . A cover glass is mounted instead of the filter  15   b . On the substrate  23 , an image element IF (interface) circuit  24  communicating between the image sensing element  15  and the image processing circuit  21  is mounted. In addition, a driving circuit  25  for actuating a shutter actuator  27  and a filter actuator  28  may be mounted on the substrate  23 .  
      The first prism  11 , the second prism  12 , the shutter mechanism  13 , and the image sensing element  15  are mounted on a frame  30 . The frame  30  has a first wall  30   a  and a second wall  30   b . As shown in  FIG. 4 , the first wall  30   a  is sandwiched between the first prism  11  and the second prism  12 , and is arranged in a direction crossing the luminous flux λm emerging from the first emergent surface  11   c . The second wall  30   b  is arranged in a direction crossing the luminous flux λo emerging from the second emergent surface  12   d , and extends to the first prism  11  side. The frame  30  is formed continuously in a T shape such that the first wall  30   a  abuts against the second wall  30   b . The opening portions  30   c ,  30   d  through which the luminous fluxes λm, λo pass are provided on the first wall  30   a  and the second wall  30   b , respectively.  
      In addition, mount holes  31   x ,  31   y ,  31   z  communicating with the first prism  11  side and the second prism  12  side are provided at three portions surrounding an opening portion  30   c  of the first wall  30   a . In the present embodiment, two mount holes  31   x ,  31   y  are provided at corner portions spaced from the second wall  30   b  with respect to the opening portion  30   c , and one mount hole  31   z  is provided at a position close to the second wall  30   b  with respect to the opening portion  30   c.    
      In the first prism  11  and the second prism  12 , columnar engagement portions  11   x ,  11   y ,  11   x ,  12   x ,  12   y ,  12   z  are formed at positions corresponding to the mount holes  31   x ,  31   y ,  31   z . At the engagement portions  11   x ,  11   y ,  11   z ,  12   x ,  12   y ,  12   z , there are provided press-fit portions  11   r ,  12   r  which is formed by one turn more thinly than the engagement portions  11   x ,  11   y ,  11   z ,  12   x ,  12   y ,  12   z , and inserts to the mount holes  31   x ,  31   y ,  31   z . In the present embodiment, as shown in  FIG. 5 , the press-fit holes  11   r  of the first prism  11  is engagingly fitted to the mount holes  31   x ,  31   y ,  31   z , and the press-fit portion  12   r  of the second prism  12  is slightly spaced from the mount holes  31   x ,  31   y ,  31   z . Therefore, at least the second prism  12  is fixedly bonded to the first wall  30   a.    
      A positioning protecting portion  11   s  is provide at a tip end of the press-fit portion  11   r  of the first prism  11 . A positioning projecting portion  12   t  is provided at a tip end of the press-fit portion  12   r  of the second prism  12  opposed to the press-fit portion  11   r . The first prism  11  and the second prism  12  are directly abutted by the projecting portion  11   s  and the recessed portion  12   t , and are relatively positioned. In addition, the press-fit portion  11   r  of the first prism  11  is engagingly fitted to mount holes  31   x ,  31   y ,  31   z , whereby the frame  30 , the first prism  11 , and the second prism  12  are relatively positioned.  
      These shapes are intended to ensure relative positioning between the first prism  11  and the second prism  12 , and may be formed in a reversed manner. Further, the function is identical even if a combination of the press-fit portion  11   r ,  12   r  and the projection portion  11   s  and the recessed portion  12   t  is changed. Therefore, for example, the press-fit portion  12   r  of the second prism  12  is engagingly fitted to the mount holes  31   x ,  31   y ,  31   z , and the recessed portion  12   t  may be provided at its tip end. In addition, as shown in  FIG. 3 , a viewing window  30   e  for verifying that the press-fit portion  11   r  of the first prism  11  is correctly engagingly fitted to the mount hole  31   z  provided close to the second wall  30   b  is provided on the second wall  30   b  which extends to the first prism  11  side.  
      As shown in  FIG. 6 , a shutter/filter holding portion  31  is provided on the first wall  30   a  of the first prism  11  side. The shutter mechanism  13  including the shutter blades  33 ,  34  and the filter mechanism  14  including the ND filter  36  are incorporated in the shutter/filter holding portion  31 . As shown in  FIG. 4 , a substrate mount portion  30   f  is provided on the second wall  30   b  at the opposite side of the second prism  12 . The substrate  23  is fixed to the substrate mount portion  30   f  by a lock screw  18  by properly sandwiching a spacer  17  so that the light-receiving surface  15   a  of the image sensing element  15  is positioned on the image focusing surface  45 . As shown in  FIG. 3 , the lock screw  18  is set so that an angle of the image sensing element  15  can be adjusted in a direction taken along a surface on which optical axes of the luminous fluxes λi, λm, λo looped by the first prism  11  and the second prism  12  pass and in a direction perpendicular thereto.  
      As has been described above, the first prism  11 , the second prism  12 , the shutter mechanism  13 , the filter mechanism  14 , and the image sensing element  15  are fixed to the frame  30 , whereby a mutual relative position is held with respect to the first prism  11 , the second prism  12 , the shutter mechanism  13 , the filter mechanism  14 , and the image sensing element  15 , respectively.  
      Next, a detailed configuration of the shutter mechanism  13  and filter mechanism  14  incorporated in the shutter/filter holding portion  31  provided on the first wall  30   a  of the frame  30  will be described with reference to  FIG. 6  and  FIG. 7 .  
      In the shutter/filter holding portion  31 , as shown in  FIG. 6 , a blade drive ring  32  of a shutter mechanism  13 ; two shutter blades  33 ,  34 ; a spacer  35  for spacing the shutter mechanism  13  and the filter mechanism  14  from each other; an ND filter  36  of the filter mechanism  14 ; and a cover  37  for covering the shutter/filter holding section  31  are mounted to be superimposed in order to the first wall  30   a.    
      In addition, an image sensing device  10  has a shutter actuator  27  and a filter actuator  28  at the second prism  12  side of the first wall  30   a . The shutter actuator  27  is a part of the blade drive mechanism linked with the shutter blades  33 ,  34  via the blade drive ring  32 . The filter actuator  28  is a filter drive mechanism linked with the ND filter  36 . The shutter actuator  27  and the filter actuator  28  are actuators of rotary solenoid type, each of which incorporates rotary shafts  27   a ,  28   a  and a coil in main body cases  27   b    28   b.    
      In the present embodiment, a rotary shaft  27   a  of the shutter actuator  27  and a rotary shaft  28   a  of the filter actuator  28  are arranged coaxially in parallel to a direction taken along an optical axis of the luminous flux λm emerging from the fist emergent surface  11   c  along a direction in which the first prism  11  and the second prism  12  are arranged. A main body case  27   b  of the shutter actuator  27  and a main body case  28   b  of the filter actuator  28  are integrally formed, and a screw hole  27   q  is provided on an end face of the shutter actuator  27  side arranged close to the first wall  30   a . A screw through hole  31   q  is provided on the first wall  30   a . The shutter actuator  27  and the filter actuator  28  are fixed to the first wall  30   a  by a screw  38  spirally fitted to the screw hole  27   q  through the screw through hole  31   q . That is, the shutter actuator  27  is arranged closer to the positions of the shutter blades  33 ,  34  and the ND filter  36 .  
      A blade drive arm  41  extending in a radial direction is fixedly attached to the rotary shaft  27   a  of the shutter actuator  27 . At a tip end of the blade drive arm  41 , the ring drive pin  41   e  extending to the first prism  11  side is mounted along an optical axis direction of the luminous flux οm. The ring drive pin  41   e  is passed through an arc shaped elongated hole  31  provided on the first wall  30   a  along the rotational direction of the blade drive arm  41 .  
      A filter drive arm  42  is fixedly attached to a rotary shaft  28   a  of the filter actuator  28 . The filter drive arm  42  has a proximal portion  42   a , an extension portion  42   b , and a tip end portion  42   c . The proximal portion  42   a  extends in a radial direction from the rotary shaft  28   a . The extension portion  42   b  extends from the rotary end of the proximal portion  42   a  toward the first wall  30   a  parallel to the optical axis of the luminous flux λm. A tip end portion  42   c  is bent along the first wall  30   a  at a side end of the first wall  30   a  of the extension portion  42   b . A filter drive pin  42   f  extending along the first prism  1  side along the optical axis direction of the luminous flux λm is mounted on the tip end portion  42   c . The filter drive pin  42   f  is passed through the arc shaped elongated hole  31   f  and an arc shaped filter drive pin through hole  35   f . The elongated hole  31   f  is provided on the first wall  30   a  along the rotational direction of the filter drive arm  42 . The filter drive pin through hole  35   f  is provided in the spacer  35  along the rotational direction of the filter drive arm  42 .  
      The filter drive arm  42  is looped in complex as described above because this arm bypasses the second prism  12  so as to avoid interference with the second prism  12 . Therefore, the rotary shaft  27   a  of the shutter actuator  27  and the rotary shaft  28  of the filter actuator  28  are arranged in parallel, and are arranged on the first wall  30   a  serving as a position which does not interference with the second prism  12 , whereby the shape of the filter drive arm  42  can be simplified.  
      As shown in  FIG. 6 , an engagement opening  31   b , support pins  31   c ,  31   d , elongated holes  31   e ,  31   f , a first plane portion  31   k , a second plane portion  31   m , a third plane portion  31   n , a fourth plane portion  31   p , and a screw through hole  31   q  are provided in the shutter/filter holding portion  31 . The engagement opening  31   b  penetrates the first wall  30   a  around the optical axis of the luminous flux λm.  
      The first plane portion  31   k  spreads in a direction crossing the optical axis of the luminous flux λm on the first prism  11  side of the engagement opening  31   b . At the first plane portion  31   k , there are provided: an elongated hole  31   e  through which the ring drive pin  41   e  is passed; an elongated hole  31   f  through which the filter drive pin  42   f  is passed; and a through hole  31   q  through which a screw  38  for fixing the shutter actuator  27  and the filter actuator  28  to the first wall  30   a  is passed. The elongated hole  31   e  is provided in an arc shaped along a trajectory in which the ring drive pin  41   e  moves. The elongated hole  31   f  is provided an arc shape along a trajectory in which the filter drive pin  42   f  moves.  
      The first plane portion  31   k , the second plane portion  31   m , the third plane portion  31   n , and the fourth plane portion  31   p  are arranged in parallel to each other. The second plane portion  31   m  is provided at the first prism  11  side rather than the first plane portion  31   k . The third plane portion  31   n  is provided at the first prism  11  side rather than the second plane portion  31   m . The fourth plane portion  31   p  is provided at the first prism  11  side rather than the third plane portion  31   n . Support pins  31   c ,  31   d  are arranged at a position which is symmetrical to the second plane portion  31   m  around the optical axis of the luminous flux λm.  
      In the present embodiment, the fourth plane portion  31   p  is a side face of the first prism  11  of the first wall  30   a . That is, the mount holes  31   x ,  31   y ,  31   z  into which an engagement portion of the first prism  11  and the second prism  12  is inserted are provided to penetrate from the fourth plane portion  31   p  to a face at the second prism  12  side.  
      A blade drive ring  32  has a ring opening  32   a , an engagement projecting portion  32   b , a flange portion  32   k , an arm portion  32   d , an elongated hole  32   e , and blade drive pins  32   g ,  32   h . The ring opening  32   a  is provided in a circular shape around the optical axis of the luminous flux λm. The engagement projecting portion  32   b  is formed in a cylindrical shape whose external diameter is slightly smaller than an internal diameter of the engagement opening  31   b , and is engagingly inserted into the engagement opening  31   b . The flange portion  32   k  spreads in a jaws shape from the first prism  11  side of the engagement projecting portion  32   b  along the first plane portion  31   k , and slidably comes into contact with the first plane portion  31   k.    
      The arm portion  32   d  extends in a radiation direction from the flange portion  32   k  toward a position which communicates with the elongated hole  31   f  provided at the first plane portion. The elongated hole  32   e  is provided at the arm portion  32   d , and an elongated diameter is arranged in a radial direction around the optical axis of the luminous flux λm. The elongated hole  32   e  penetrates the first wall  30   a , and is engaged with the ring drive pin  41   e  projected to the first prism  11  side. Therefore, when the blade drive arm  41  is rotated by the shutter actuator  27 , the blade drive ring  32  rotates around the optical axis of the luminous flux λm. The blade drive pins  32   g ,  32   h  are arranged at the flange portion  32   k  rotationally symmetrically around the optical axis of the luminous flux λm, and extends toward the first prism  11  side.  
      Shutter blades  33 ,  34  are formed in a new moon shape or in a sickle shape, and at one end, pin holes  33   c ,  33   d  and sliding elongated holes  33   g ,  33   h  are provided, respectively. The shutter blade  33  is mounted on a shutter/filter holding portion  31  in a state in which the support pin  31   c  is inserted into the pin hole  33   c . The shutter blade  34  is mounted on the shutter/filter holding portion  31  in a state in which the support pin  31   d  is inserted into the pin hole  34   d.    
      The shutter blades  33 ,  34  are arranged rotationally symmetrically around the optical axis of the luminous flux λm in a state in which a part of these shutters is superimposed in a direction along the optical axis of the luminous flux λm with the inside of an arc toward the optical axis side of the luminous flux λm. The sliding elongated holes  33   g ,  34   h  are engaged with the blade drive pins  32   g ,  32   h , respectively. In this manner, when the blade drive ring  32  is rotated by the shutter actuator  27 , the shutter blades  33 ,  34  rotate around the support pins  31   c ,  31   d , respectively.  
      A spacer  35  is mounted on the shutter/filter holding portion  31  at the more inside of the fourth plane portion  31   p . The spacer  35  includes: an aperture opening  35   a ; support pin through holes  35   c ,  35   d ; a filter drive pin through hole  35   f ; and blade drive pin through holes  35   g ,  35   h . The aperture opening  35   a  is a circular hole around the optical axis of the luminous flux λm. An opening diameter of the aperture opening  35   a  is slightly smaller than the ring opening  32 .  
      The spacer  35  is mounted on the shutter/filter holding portion  31  in a state in which the spacer abuts against the third plane portion  31   n  while the support pins  31   c ,  31   d  are passed through the support pin through holes  35   c ,  35   d.    
      The support pin through holes  35   c ,  35   d  are provided as release holes of the blade drive pins  32   g ,  32   h . These support pin through holes are formed in an arc shaped elongated hole which corresponds to a trajectory in which the blade drive ring  32  rotates, whereby the blade drive pins  32   g ,  32   h  move. The filter drive pin through hole  35   f  is formed in an arc shaped elongated hole which corresponds to a trajectory in which the filter actuator  28  rotates the filter drive arm  42 , whereby the filter drive pin  42   f  moves.  
      The spacer  35  ensures a rotational gap in a direction along the optical axis of the luminous flux λm of the shutter blades  33 ,  34 . In addition, this spacer separates the shutter blades  33 ,  34  and the ND filter  36  from each other so as to rotate independently.  
      As shown in  FIG. 11 , the ND filter  36  has a sufficient size which covers the aperture opening  35   a . This ND filter has a support pin through hole  35   c , a filter drive pin through hole  36   f , and a cutout portion  36   g . The ND filter  36  is mounted in a state in which the support pin through hole  35   c  and the filter drive pin through hole  36   f  are mounted with the support pin  31   c  and the filter drive pin  42   f  which project from the spacer  35  to the first prism  11  side, respectively.  
      As shown in  FIG. 11 , a cutout portion  36   g  is provided in a shape such that the cutout portion  36  is not superimposed on the blade drive pin through hole  35   g  in a direction taken along the optical axis of the luminous flux λm so that the ND filter  36  and the blade drive pin  32   g  do not interface with each other. When the blade drive pins  32   g ,  32   h  do not project to the first prism  11  side beyond the spacer  35 , the cutout portion  36   g  is not required.  
      The ND filter  36  rotates along the support pin  31   c  by the filter actuator  28  rotating the filter drive arm  42 . In addition, the ND filter  36  is selectively positioned and held in either one of an insert position ( FIG. 10 ) crossing the luminance flux λm emerging from the first emergent surface  11   c  and a retracted position ( FIG. 11 ) coming out of the luminous flux λm.  
      A cover  37  abuts against a fourth plane portion  31   p , and includes an opening portion  37   a , support pin engagement holes  37   c ,  37   d , a filter drive pin through hole  37   f , and an locking pieces  37   g ,  37   h . The opening portion  37   a  is provided in a circular shape around the optical axis of the luminous flux λm. The support pin engagement holes  37   c ,  37   d  are engaged with a tip end of the support pins  31   c ,  31   d . Thus, the support pins  31   c ,  31   d  function as a rotary center shaft of the shutter blades  33 ,  34 , and functions as a fixing member of the spacer  35  and cover  37 .  
      The filter drive pin through hole  37   f  is formed in an arc shaped elongated hole which corresponds to trajectory in which the filter drive pin  42   f  is moved by the filter actuator  28  rotating the filter drive arm  42 . The locking pieces  37   g ,  37   h  are looped back to the first prism  11  side so as to overlap on the outer periphery of the first wall  30   a . The locking pieces  37   a ,  37   h  have locking holes  37   i ,  37   j . The locking holes  37   i ,  37   j  are locked with locking projections  31   i ,  31   j  formed at the outer periphery of the first wall  30   a.    
      The shutter mechanism  13  and filter mechanism  14  assembled as described above are incorporated in the shutter/filter holding portion  31  of the first wall  30   a  via the spacer  35 . The shutter mechanism  13  holds the shutter blades  33 ,  34 , and the filter mechanism  14  holds the ND filter  36  in a state in which they can be rotated respectively independently.  
      The shutter mechanism  13  and the filter mechanism  14  are arranged to be housed in a projection area of the first wall  30   a  in a direction taken along the optical axis of the luminous flux λm. In this manner, an occupying area of the image sensing device  10  on a perpendicular surface with respect to the optical axis of the luminous flux λm is determined depending on the size of the frame  30 .  
      An operation of the shutter blades  33 ,  34  of the above described shutter mechanism  13  will be described with reference to  FIG. 8  and  FIG. 9 .  FIG. 8  and  FIG. 9  are plan views when the shutter mechanism  13  is seen from the first prism  11  side along the line A-A in  FIG. 6 . An open state of the shutter mechanism  13  is shown in  FIG. 8 , and a closed state of the shutter mechanism  13  is shown in  FIG. 9 .  
      A shutter actuator  27  is turned OFF in an open state. A rotary shaft  27   a  is biased in the counterclockwise direction in  FIG. 8  by a coil spring or the like, for example, incorporated in a main body case  27   b . Therefore, the blade drive ring  32  engaged with the ring drive pin  41   e  of a blade drive arm  41  is biased in the clockwise direction in  FIG. 8 . As a result, the shutter blades  33 ,  34  are held at a release position hidden in a projection area of a spacer  35 , as shown in  FIG. 8 , in an open state, and the luminous flux λm emerging from the first emergent surface  11   c  is passed to the second incident surface  12   a . Then, the shutter blades  33 ,  34  abut against an internal wall of the shutter/filter holding portion  31 , the ring drive pin  41   e  abuts against the elongated hole  31   e , or the arm portion  32   d  of the blade drive ring  32  abuts against the internal wall of the shutter/filter holding section  31 , whereby the open state shown in  FIG. 8  is maintained. The shutter mechanism  13  enters the open state shown in  FIG. 8  in a normal state.  
      The shutter actuator  27  passing the light radiated from an object image during a predetermined time after an image sensing operation is made is turned ON, and the blade drive arm  41  is rotated in the clockwise direction together with the rotary shaft  27   a . In this manner, the blade drive ring  32  engaged with the blade drive arm  41  by the ring drive pin  41   e  is rotated in the counterclockwise direction around the optical axis of the luminous flux λm. Then, the shutter blades  33 ,  34  engaged with the blade drive pins  32   g ,  32   h  are rotated around the support pins  31   c ,  31   d  at an interruption position crossing the luminous flux λm as shown in  FIG. 9 .  
      As a result, the shutter mechanism  13  enters a closed state for interrupting the luminous flux λm emerging from the first emergent surface  11   c  which covers the ring opening  32   a . Therefore, continuous impinging of the luminous flux from an object to the image sensing element  15  can be prevented in duration while image data has been picked up from the image sensing element  15 . Therefore, exposure times of the odd numbered field and the even numbered field of the interlace type CCD can be made identical to each other.  
      In addition, an operation of the ND filter  36  of the above described filter mechanism  14  will be described with reference to  FIG. 10  and  FIG. 11 .  FIG. 10  and  FIG. 11  are plan views when the filter mechanism  14  is seen from the first prism  1  side along the line B-B in  FIG. 6 .  FIG. 10  shows a non-light reducing state in which the ND filter  36  is held in a retracted state.  FIG. 11  shows a state in which the ND filter  36  is held at an insert position.  
      The filter actuator  28  turns OFF in the non-light reducing state. The rotary shaft  28   a  is biased in the clockwise direction in  FIG. 10  by a torsion coil spring or the like, for example, incorporated in the main body case  28   b . Therefore, the ND filter  36  engaged through the filter drive pin through hole  36   f  with respect to the filter drive pin  42   f  of the filter drive arm  42  is biased in a direction rotating in the counterclockwise direction in  FIG. 10  around the support pin  31   c.    
      As a result, the ND filter  36 , in the non-light reducing state, is held at a retracted position coming out of an aperture opening  35   a  which is a position hidden in the projection area of the spacer  35 , as shown in  FIG. 10 . Then, the luminous flux λm emerging from the first emergent surface  11   c  of the first prism  11  passes through the aperture opening  35   a  without passing through the ND filter  36 , and is made incident to the second incident surface  12   a  of the second prism  12 . As shown in  FIG. 10 , the ND filter  36  abuts against any one of the elongated hole  31   f  at which the filter drive pin  42   f  is provided on the first wall  30   a ; the filter drive pin through hole  35   f  of the spacer  35 ; and the filter drive pin through hole  37   f  of the cover  37 , whereby the ND filter is maintained at a retracted position. The filter mechanism  14  enters the non-light reducing state shown in  FIG. 10  in a normal state.  
      Based on a luminescence gauge provided independently in the digital camera  1  which is an image sensing apparatus, or based on luminescence detected by the image sensing element  15 , if the light from an object is too strong, the filter actuator  28  is turned ON. In this manner, the filter drive arm  42  is rotated in the counterclockwise direction together with the rotary shaft  28   a . The ND filter  36  engaged with the filter drive arm  42  by the filter drive pin  42   f  is rotated in the clockwise direction from the retracted position around the support pin  31   c , and is held at an insert position crossing the luminous flux λm, as shown in  FIG. 11 .  
      As a result, the luminous flux λm emerging from the first emergent surface  11   c  passes through the ND filter  36 , whereby light is reduced at a ratio which this NF filter  36  has, and then the reduced light is incident to the second incident surface  12   a  of the second prism  12 . Therefore, even when the luminescence of an object covers a wide region, an image can be sensed with fine gradation.  
       FIG. 12A ,  FIG. 12B ,  FIG. 12C , and  FIG. 12D  show another embodiment a mount configuration between an engagement portion of the first prism  11  and the second prism  12  and mount holes  31   x ,  31   y ,  31   z  provided on the first wall  30   a , respectively. As shown in the figures, the first prism  11  and the second prism  12  may not be shaped so as not to be directly in contact with each other. In any embodiment of  FIG. 12A ,  FIG. 12B ,  FIG. 12C , and  FIG. 12D , the first prism  11  and the second prism  12  can be relatively positioned via the first wall  30   a.    
      In the embodiment shown in  FIG. 12A , the engagement portions  11   x ,  11   y ,  11   z ,  12   x ,  12   y ,  12   z  provided in the first prism  1  and the second prism  12  include press-fit portions  11   r ,  12   r  to be engagingly fitted to the mount holes  31   x ,  31   y ,  31   z  provided on the first wall  30   a  respectively. The proximal portions  11   u ,  12   u  of the respective press-fit portions  11   r ,  12   r  abut against an external surface of the first wall  30   a , whereby the first prism  11  and the second prism  12  are relatively positioned via the first wall  30   a.    
      In the embodiment shown in  FIG. 12B , the engagement portions  11   x ,  11   y ,  11   z ,  12   x ,  12   y ,  12   z  provided in the first prism  11  and the second prism  12  includes recessed portions  11   v ,  12   v  externally engaged with a mount boss  30   v  provided on the first wall  30   a , respectively. Tip ends of the engagement portions  11   x ,  11   y ,  11   z ,  12   x ,  12   y ,  12   z  abut against the external surface of the first wall  30   a , whereby the first prism  11  and the second prism  12  are relatively positioned via the first wall  30   a , respectively.  
      In the embodiment shown in  FIG. 12C , the engagement portions  11   x ,  11   y ,  11   z  provided in the first prism  11  is formed in a shape similar to the engagement portions  11   x ,  11   y ,  11   z  of the first prism  11  shown in  FIG. 12B . The engagement portions  12   x ,  12   y ,  12   z  provided in the first prism  12  is formed in a shape similar to the engagement portions  12   x ,  12   y ,  12   z  of the second prism  12  shown in  FIG. 12A . The press-fit portion  12   r  of the engagement portions  12   x ,  12   y ,  12   z  is engagingly inserted into the engagement hole  30   r  provided on the first wall  30   a.    
      In the embodiment shown in  FIG. 12D , the engagement portions  11   x ,  11   y ,  11   z  provided in the first prism  11  is formed in a shape similar to the engagement portions  11   x ,  11   y ,  11   z  of the first prism  11  shown in  FIG. 12A . The engagement portions  12   x ,  12   y ,  12   z  provided in the first prism  12  is formed in a shape similar to the engagement portions  12   x ,  12   y ,  12   z  of the first prism  12  shown in  FIG. 12B . The press-fit portion  11   r  of the engagement portions  11   x ,  11   y ,  11   z  is engagingly inserted into the engagement hole  30   r  provided on the first wall  30   a.    
      In addition, with respect to the frame according to an embodiment in which the first wall  30   a  and the second wall  30   b  are composed of another member, respectively, a configuration of these joint portion  50  is shown in  FIG. 13  in an enlarged manner. As shown in  FIG. 13 , a joint end  51  of the first wall  30   a  has a screw hole  53  spirally fitted with a fixing screw  52 . A groove  55  is formed on a joint face  54  of the second wall  30   b . The width of the groove  55  is formed to be wider than that of the joint end  51 . A screw through hole  55   b  through which the fixing screw  52  is inserted is provided at a bottom part  55   a . The screw through hole  55   b  is provided in a position superimposed on the screw hole  53  in a state in which the joint end  51  is pressed against one side wall of the groove  55 . As shown in  FIG. 13 , the first wall  30   a  and the second wall  30   b  are positioned each other by two surfaces.  
       FIG. 14  shows a state in which an aperture plate  100  is mounted instead of the ND filter  36 . The aperture plate  100  has an aperture hole  100   a  whose opening diameter is smaller than the aperture opening  35   a  provided in the spacer  35  to the coaxial optical axis of the luminous flux λm. The aperture plate  100  is pivoted on a support pin  31   c  by a support pin through hole  100   c  provided in the same manner as in the ND filter  36 . In addition, this aperture plate is rotated by the filter drive pin  42   f  engaged with a drive pin through hole  10   f , and is positioned at either of the insert position and the retracted position. In addition, as in the ND filter  36 , a cutout portion  100   g  which avoids interference with the blade drive pin  32   g  is provided.  
      This aperture plate  100  is driven in the same manner as the ND filter  36 , whereby an amount of light transmitted to the image sensing element  15  can be changed. The opening diameter of the aperture hole  100   a  is properly determined according to use of an image sensing device, and is not limited to a ratio based on a relationship between the aperture opening  35   a  and the aperture hole  100   a.    
      In addition, in the ND filter  36  in the present embodiment, the shutter mechanism  13  and the filter mechanism  14  can be functionally switched from each other by changing the filter to a light interrupting member. For example, a light interrupting member provided instead of the ND filter  36  is used as a mechanical shutter for switching an open state into a closed state and vice versa. In addition, a restriction is applied to a rotational range of the blade drive arm  41 , a restriction is applied to a rotational range of the blade drive ring  32 , or a restriction is applied to a rotational range of the shutter blades  33 ,  34 , thereby holding the shutter blades  33 ,  34  in an aperture state in which an opening smaller than the aperture opening  35   a  of the spacer  35  is left.  
      In addition, if the shutter blades  33 ,  34  are driven and positioned at the shutter actuator  27  at a plurality of stages by using a stepping motor, there can be provided an aperture mechanism capable of setting a plurality of aperture values. With respect to the number of shutter blades, a more circular opening can be produced by increasing the number. In addition, the shutter actuator  27  and the filter actuator  28  can be replaced with a hollow motor provided coaxially to the optical axis of the luminous flux λm.  
      In an image sensing condition such that an opening of an aperture formed of the shutter blades  33 ,  34  or an opening of the aperture hole  100   a  of the aperture plate  100  must be extremely small, there is a case in which a diffraction phenomenon occurs because of its small opening. In such an image sensing condition, it is proper to use the ND filter  36  capable of reducing intensity of light without changing a relative spectroscopy distribution of energy.  
      An image sensing device  10   a  according to a second embodiment of the present invention will be described with reference to  FIG. 15  to  FIG. 17 . Like constituent elements having identical functions to those of the image sensing device  10  shown in the first embodiment are designated by like reference numerals. A duplicate description is not repeated here.  
      In the image sensing device  10   a  shown in  FIG. 15 , a second wall  30   b  extends from a first wall  30   a  to the second prism  12  side, and does not extend to the first prism  11  side. That is, a frame  30  is formed continuously in an L shape surrounding the second incident surface  12   a  side and the second emergent surface  12   d  side of the second prism  12 . In addition, a substrate  23  on which an image sensing element  15  is mounted is formed in size corresponding to a substrate mount portion  30  provided on a second wall  30   b.    
      As shown in  FIG. 16 , the substrate  23  is held on a substrate mount portion  30   f  by an adjustment screw  20  in a state in which a coil spring  19  being one embodiment of an elastic member is sandwiched between the substrate and the substrate mount portion  30   f . The substrate  23  is biased in a direction spaced from the substrate mount potion  30   f  by the coil spring  19 . The location of a light-receiving surface  15   a  of the image sensing element  15  with respect to an image focusing surface  45  can be finely adjusted by adjusting a threading amount of the adjustment screw  20 . Therefore, the light-receiving surface  15   a  can be easily positioned with respect to the image focusing surface  45 .  
      Instead of the coil spring  19 , a rubber sheet or a spring washer may be used as an elastic member. In addition, instead of providing the coil spring  19 , the light-receiving surface  15   a  of the image sensing element  15  may be positioned and adjusted with respect to the image focusing surface  45  while the spacer  17  is sandwiched, in the same manner as in the first embodiment. The coil spring  19  or rubber sheet, a spring washer and the like may be used instead of the spacer  17  in the first embodiment.  
      In addition, in the second embodiment as well, the engagement portions  11   x ,  11   y ,  11   z  of the first prism  11  and the engagement portions  12   x ,  12   y ,  12   z  of the second prism  12  may be shaped in the embodiment as shown in  FIG. 12A ,  FIG. 12B ,  FIG. 12C , and  FIG. 12D , as shown in the first embodiment. In addition, the aperture plate  100  shown in  FIG. 14  may be provided instead of providing the ND filter  36 .  
      Further, with respect to the frame  30  in an embodiment in which the first wall  30   a  and the second wall  30   b  are composed of another member, respectively, a configuration of these joint portions  60  is shown in  FIG. 17  in an enlarged manner. As shown in  FIG. 17 , the first wall  30   a  has an abutment portion  61  facing the second prism  12  side. A screw hole  64  spirally fitted with the fixing screw  63  is provided on an end face  62  of the second wall  30   b  facing the first prism  11 . A screw through hole  65  through which the fixing screw  63  is inserted is provided at an abutment portion  61  of the first wall  30   a  corresponding to the screw hole  64  in a state in which the abutment portion  61  of the first wall  30   a  and the end face  62  of the second wall  30   b  are abutted against each other.  
       FIG. 18  shows an example of incorporating an image sensing unit in a cellular phone  160  with a camera as an example of an image sensing apparatus according to the present embodiment. In this cellular phone  160  with a camera, the image sensing device (for example, image sensing device  10 ) explained in the embodiments each described previously is incorporated as an image sensing optical system  6 , thereby making it possible to compactly and thinly produce the cellular phone  160  with a camera and produce an image with high quality.  
       FIG. 19  and  FIG. 20  show an example in which a different image sensing device is applied to the image sensing apparatus according to the present invention respectively.  
      In the image sensing device  10   c  shown in  FIG. 19 , the first prism  111  and the second prism  112  are composed of surfaces  201  to  206 , each of which is fully formed on a free-form surface, respectively. The luminous flux λ1 incident from the first surface  201  is diffracted on a first surface  201 , the diffracted luminous flux is fully reflected on a second surface  202 , and then the fully reflected luminous flux is diffracted on and emerged from a third surface  203 . The luminous flux λm emerged from the first prism  111  is made incident and diffracted at a fourth surface  204 , the diffracted luminous flux is fully reflected on a fifth surface  205  and a sixth surface  206 , and then the fully reflected luminous flux is diffracted and emerged on the fifth surface  205 . The luminous flux λo emerged from the second prism  112  is focused as an image on the image focusing surface  45 .  
      In an image sensing device  10   d  shown in  FIG. 20 , a first prism  221  and a second prism  222  are composed of surfaces  231  to  238  fully formed on a free-form surface. The luminous flux λi incident from a first surface  231  is diffracted on the first surface  231 , and the diffracted luminous flux is fully reflected on a second face  232  and a third face  233 . Then, the fully reflected luminous flux is diffracted on a fourth surface  234 , and is emerged wherefrom. The luminous flux emerged from the first prism  221  is incident to a fifth surface  235  of the second prism  222 , and is diffracted there. The diffracted luminous flux is fully reflected on a sixth surface  236  and a seventh surface  237 , and then the fully reflected luminous flux is diffracted on an eighth surface  238 , and is emerged wherefrom. The luminous flux λo emerged from the second prism  222  is focused as an image on the image focusing surface  45 .  
      When carrying out the present invention, of course, constituent elements of the invention including a prism optical system or an image sensing element can be variously modified and carried out without deviating the spirit of the invention.  
      Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the inventive as defined by the appended claims and their equivalents thereof.