Abstract:
An imaging unit includes an image sensor, an imaging optical system which includes a prism that is positioned on the object side of the image sensor, wherein the prism bends an optical path of light emanating from an object to lead the light to the image sensor; a housing which accommodates and supports the image sensor and the prism; a light shield frame, which forms a part of the housing, provided on the object side of an incident surface of the prism, wherein the light shield frame shields a part of the light incident on the incident surface of the prism; and a prism positioning surface which is formed on the housing and contacts the prism to position the prism with respect to the housing.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an imaging unit having an imaging optical system and an image sensor. 
     2. Description of the Related Art 
     In recent years, mobile electronic devices which are designed mainly for taking still/moving photographic images, such as digital cameras (still-video cameras) and digital camcorders (motion-video cameras), and other mobile electronic devices which are designed to have the capability of taking such photographic images as a subsidiary function, such as mobile phones equipped with a camera and personal digital assistants (PDAs) equipped with a camera, have become widespread. In these types of mobile electronic devices, it is common to provide the mobile electronic device therein with an imaging unit which is structured such that an image sensor and a photographing optical system, for leading light emanating from a photographic object (object to be photographed) to the image sensor, are accommodated in a hollow housing of the imaging unit. 
     In this type of imaging unit, it is sometimes the case that a prism which bends an optical path to lead this optical path to the image sensor is installed immediately in front of the image sensor (on the object side). Additionally, in recent years, the downsizing (slimming down) of the imaging unit has progressed, following the progress of the downsizing (slimming down) of the mobile electronic devices, and accordingly, if a prism is used as an element of this type of imaging unit, this prism needs to be miniaturized. 
     However, if the prism is miniaturized, stray light occurs by reflections caused by side surfaces of the prism, which increases the possibility of such stray light reaching the imaging surface of the image sensor. Therefore, in the case where a small prism is used, a light shield frame for preventing part of the light emanating from a photographic object from traveling toward the side surfaces of the prism needs to be provided on the incident surface side of the prism. 
     Japanese Unexamined Patent Publication No. 2010-026007 discloses a light shield frame that is provided on the incident surface of a prism. 
     Whereas, since the aforementioned prism has been miniaturized as described above and also since it is ideal to make the space in the imaging unit which is occupied by the aforementioned light shield frame as small as possible, the light shield frame needs to be miniaturized to produce a slimmed-down imaging unit. 
     A light shield sheet which can adhere to the incident surface of the prism is known in the art as an example of the light shield frame which is small in size. 
     However, since it is difficult to adhere a thin and small light shield sheet to the incident surface of the small prism, there is a possibility of the position of adhering the light shield sheet to the incident surface of the prism deviating from a desired position, which may consequently cause stray light to reach the imaging surface of the image sensor. 
     SUMMARY OF THE INVENTION 
     The present invention provides an imaging unit which is configured so that a small light shield frame can be easily installed with high positional precision on the incident surface side of a prism that is positioned on the object side of an image sensor even when the prism is miniaturized. 
     According to an aspect of the present invention, an imaging unit is provided, including an image sensor; an imaging optical system which includes a prism that is positioned on the object side of the image sensor, wherein the prism bends an optical path of light emanating from an object to lead the light to the image sensor; a housing which accommodates and supports the image sensor and the prism; a light shield frame, which forms a part of the housing, provided on the object side of an incident surface of the prism, wherein the light shield frame shields a part of the light incident on the incident surface of the prism; and a prism positioning surface which is formed on the housing and contacts the prism to position the prism with respect to the housing. 
     It is desirable for the imaging optical system to include an incident-side prism which is positioned closer to the object than the prism and bends the optical path to lead the light to the prism. 
     It is desirable for the imaging unit to include a cover glass which covers an imaging surface of the image sensor, and an image-sensor positioning surface which is formed on the housing and contacts the cover glass to position the image sensor with respect to the housing. 
     It is desirable for a plurality of the image-sensor positioning surfaces to lie on a common plane. 
     It is desirable for the prism positioning surface to include an inclined flat surface which is in contact with a flat reflecting surface that is formed on the prism to bend the optical path, wherein the inclined flat surface is inclined with respect to both an optical path of the light incident on the flat reflecting surface and an optical path of the light reflected by the flat reflecting surface; and an incident-side contact surface which contacts an outer edge of the incident surface of the prism and is orthogonal to the optical path of the light incident on the flat reflecting surface. 
     It is desirable for a plurality of the inclined flat surfaces to lie on a common plane. 
     It is desirable for the housing to include a frame member which faces the incident surface of the prism, and wherein the frame member includes a base portion which extends in a direction orthogonal to both the optical path of the light incident on the flat reflecting surface and the optical path of the light reflected by the flat reflecting surface and which is formed integrally with an inner surface of the housing; a pair of side portions which extend from both ends of the base portion in a direction parallel to the optical path of the light reflected by the flat reflecting surface; and a connecting portion which is parallel to the base portion and connects common ends of the pair of side portions. The light shield frame is formed on inner edges of the base portion, the pair of side portions and the connecting portion. The incident-side contact portion is formed on the frame member except on the connecting portion of the frame member. 
     According to the present invention, since the light shield frame is integrally configured on a part of the housing and also since the positioning of the prism with respect to the housing is achieved by the prism positioning surface that is formed on the housing, the light shield frame can be installed on the incident surface side of the prism with great positional precision even in the case where the prism and the light shield frame are miniaturized. 
     Moreover, the operation to install the light shield frame to the prism or the housing is not required since the light shield frame is formed integrally with the housing. Therefore, the light shield frame can be easily positioned on the incident surface side of the prism since the operation to position the light shield frame on the incident surface side of the prism only requires the prism to be brought into contact with the prism positioning surface. 
     The present disclosure relates to subject matter contained in Japanese Patent Application No. 2011-165137 (filed on Jul. 28, 2011) which is expressly incorporated herein by reference in its entirety. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be described below in detail with reference to the accompanying drawings in which: 
         FIG. 1  is a front perspective view of an embodiment of an imaging unit, according to the present invention, that is equipped with a lens unit, viewed obliquely from above; 
         FIG. 2  is a front exploded perspective view of the imaging unit, viewed obliquely from above; 
         FIG. 3  is a front elevational view of the imaging unit with a cover and a circuit board removed when an imaging optical system is set at the wide-angle extremity; 
         FIG. 4  is a view similar to that of  FIG. 3 , showing the imaging unit with the cover and the circuit board removed when the imaging optical system is set at the telephoto extremity; 
         FIG. 5  is a cross sectional view taken along the line V-V shown in  FIG. 1  when the imaging optical system is set at the telephoto extremity; 
         FIG. 6  is an enlarged view of a portion of the cross sectional view shown in  FIG. 5  which is surrounded by a single-dot chain line VI; 
         FIG. 7  is a perspective view of a housing that constitute a component of the imaging unit; 
         FIG. 8  is a cross sectional view taken along the line VIII-VIII shown in  FIG. 4 ; and 
         FIG. 9  is a cross sectional view showing the incident surface of the second prism shown in  FIGS. 2 ,  3 ,  5  and  6 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of an imaging unit according to the present invention will be hereinafter discussed with reference to the accompanying drawings. In the following descriptions, forward and rearward directions, leftward and rightward directions, and upward and downward directions are determined with reference to the directions of the double-headed arrows shown in the drawings. 
     As shown in  FIGS. 1 and 2 , the imaging unit  1  is provided with a first lens group unit  3 , a body module  15 , a board module  65  and a cover  76  that constitute the major components of the imaging unit  1 . 
     The first lens group unit  3  is provided with a holder  4  that is formed as a synthetic-resin molded product. The holder  4  is provided at the left end thereof with a pair of (upper and lower) lugs  5 , each of which has a through-hole  6 . As shown in  FIG. 5 , a prism accommodating space  7  which is open at the front and right sides is formed in the holder  4 . The holder  4  is provided at the front opening of the prism accommodating space  7  with a front lens holding hole (lens holding hole)  8  which is open at a left portion thereof, and is provided at the right opening of the prism accommodating space  7  with a right lens holding hole  9  which is noncircular in shape and open at the upper and lower ends thereof. As shown in  FIG. 5  and other drawings, the first lens group unit  3  is provided with a first prism (prism) LP 1  which is fit-engaged into the prism accommodating space  7  and fixed thereto. The first prism LP 1  is provided with a rectangular incident surface LP 1 - a , which is orthogonal to the forward/rearward direction and elongated in the upward/downward direction, and a rectangular exit surface LP 1 - b  which is orthogonal to the leftward/rightward direction and elongated in the upward/downward direction. The first lens group unit  3  is provided in the front lens holding hole  8  with a lens element (circular lens element) L 1  which has a circular outer shape and the optical axis of which extends in the forward/rearward direction. The lens element L 1  is fitted into the front lens holding hole  8  and fixed thereto so as to face the incident surface LP 1 - a  in the forward/rearward direction. 
     When the first lens group unit  3  is viewed from the front as shown in  FIG. 3 , the four corners of the incident surface LP 1 - a  of the first prism LP 1  are positioned radially outside the lens element L 1 . However, as shown in  FIGS. 2 and 4 , a front light shield mask (light shield member)  10  is fixedly adhered to a front surface (mask fixing surface) of the holder  4 . The front light shield mask  10  is provided with a lens opening  11  which is formed to avoid interference with the lens element L 1 . The four corners of the incident surface LP 1 - a  are covered by the four corners of the front light shield mask  10 , respectively, thus not being exposed to the front. 
     The first lens group unit  3  is provided with two plastic lens elements (lens elements) L 2  and L 3  which are fixedly fitted into the right lens holding hole  9  in a state of being cemented to each other (see  FIG. 5 ). 
     In addition, a right light shield mask  13  is fixedly adhered to a right end surface of the holder  4  to cover a right side surface of the lens element L 3  (see  FIG. 2 ). However, an opening  14  (see  FIG. 2 ) is formed in the right light shield mask  13 , and accordingly, a radially inner part of the lens element L 3  that is radially inside the outer edge of the lens element L 3  is exposed to the right side. 
     The lens element L 1 , the first prism LP 1 , the lens element L 2  and the lens element L 3  that have been described above are elements of a first lens group LG 1 . In addition, the holder  4 , the front light shield mask  10 , the first lens group LG 1  and the right light shield mask  13  are elements of the first lens group unit  3 . 
     The body module  15  is provided with a housing (casing)  16  made of synthetic resin. The housing  16  is provided at the left end thereof with a mounting recess  17 . In addition, the housing  16  is provided, on the front of a portion thereof which is positioned on the right-hand side of the mounting recess  17 , with an accommodating recess  18  which is substantially rectangular in cross section. The housing  16  is provided between the mounting recess  17  and the accommodating recess  18  with a partition wall  19  which separates the mounting recess  17  and the accommodating recess  18  from each other. The housing  16  is provided at a center of the partition wall  19  with a communication hole (through-hole)  20  (see  FIG. 3 ) via which the mounting recess  17  and the accommodating recess  18  are communicatively connected with each other. The housing  16  is provided, in the accommodating recess  18  at the right end thereof, with a positioning portion  22  having a substantially rectangular shape, in a front view, which projects forward from the base of the accommodating recess  18 . The positioning portion  22  is provided with three positioning spacers  23   a ,  23   b  and  23   c  which project forward, and the front end surfaces of the three spacers  23   a ,  23   b  and  23   c  are formed as flat positioning surfaces (image-sensor positioning surfaces)  24  which lie on a common plane orthogonal to the forward/rearward direction (i.e., the direction of the thickness of the housing  16 ). In addition, the housing  16  is provided in the positioning portion  22  with a prism mounting recess  25  which is open at the front and left sides. 
     The spacer  23   c  is provided with a connecting portion  23   c   1  which extends in the upward/downward direction and a bent end portion  23   c   2  which extends rightward from the lower end of the connecting portion  23   c   1  (see  FIG. 7 ). The connecting portion  23   c   1  connects the front ends (common ends) of upper and lower walls (side portions)  22   a  and  22   b  to each other, both of which project forward from the base of the accommodating recess  18  (both of which are integral with the base of the accommodating recess  18 ). The housing  16  is provided on the base of the accommodating recess  18  with a base portion  22   c  which connects the left ends of the upper wall  22   a  and the lower wall  22   b  to each other and which extends in the upward/downward direction. The upper wall  22   a , the lower wall  22   b , the base portion  22   c  and the connecting portion  23   c   1  constitute a frame member  21  that has a rectangular shape elongated in the upward/downward direction in a side view (see  FIG. 7 ). Light shield portions  22   a   1 ,  22   b   1 ,  22   c   1  and  23   c   3  are formed integrally with the inner edges of the upper wall  22   a , the lower wall  22   b , the base portion  22   c  and the connecting portion  23   c   1  as parts of the housing  16 , respectively (see  FIG. 8 ). In addition, the light shield portions  22   a   1 ,  22   b   1 ,  22   c   1  and  23   c   3  constitute a light shield frame  21   a  in the shape of a rectangle elongated in the upward/downward direction in a side view. The right sides of the light shield portions  22   a   1 ,  22   b   1  and  22   c   1  (i.e., the right side of the light shield frame  21   a  except the light shield portion  23   c   3  thereof) form an incident-side contact surface  21   b  (see  FIG. 6 ) that is formed as a flat surface orthogonal to the leftward/rightward direction. A prism support portion  26  is projected from the base of the accommodating recess  18  in the prism mounting recess  25 . The prism support portion  26  is positioned on the right-hand side of the light shield frame  21   a . The prism support portion  26  is provided on the front thereof with a right and left pair of inclined flat surfaces (prism positioning surfaces)  26   a  and  26   b  which are formed to be inclined at an angle of 45 degrees with respect to both a straight line extending in the forward/rearward direction and a straight line extending in the leftward/rightward direction. In addition, each of the upper wall  22   a  and the lower wall  22   b  is provided on the inner surface thereof with an inner wall projection  22   d , respectively, and each of these inner wall projections  22   d  is provided on a front surface thereof with an inclined flat surface (prism positioning surface)  22   d   1  which lies on a plane on which the inclined flat surfaces  26   a  and  26   b  lie (see  FIG. 8 ). Accordingly, the inclined flat surfaces  22   d   1  and the inclined flat surfaces  26   a  and  26   b  lie on a common plane. 
     The housing  16  is further provided, around the front edge of the inner peripheral surface of the accommodating recess  18 , with a board support surface  27  which is positioned one step behind the frontmost end of the housing  16  and lies on a plane orthogonal to the forward/rearward direction, and is further provided on the board support surface  27  with two lock projections  28  which project forward from the board support surface  27 . The housing  16  is provided, at the front end of the center of the upper surface of the housing  16 , with a first engaging recess  30  which is recessed one step downward from the upper surface (periphery) of the housing  16 . The housing  16  is also provided, at the front end of the upper surface of the housing  16  on the left-hand side of the first engaging recess  30  at the same vertical position as the first engaging recess  30 , with a second engaging recess  31  which is recessed downward from the upper surface (periphery) of the housing  16  by the same amount as the first engaging recess  30  and is wider than the first engaging recess  30 . The housing  16  is provided in the second engaging recess  31  with an engaging projection  32 . The housing  16  is also provided on the lower surface thereof with a first engaging recess  30 , a second engaging recess  31  and an engaging projection  32  which have the same configurations as those provided on the upper surface of the housing  16  (see  FIGS. 3 and 4 ). The positional relationship between the first engaging recess  30 , the second engaging recess  31  and the engaging projection  32  which are formed on the lower surface of housing  16  is identical to the positional relationship between the first engaging recess  30 , the second engaging recess  31  and the engaging projection  32  which are formed on the upper surface of housing  16 ; however, the positions of the first engaging recess  30 , the second engaging recess  31  and the engaging projection  32  in the leftward/rightward direction which are formed on the lower surface of the housing  16  are each shifted to the right with respect to those formed on the upper surface of the housing  16 . Additionally, the housing  16  is provided on the right side thereof with a pair of (upper and lower) engaging projections  34  (see  FIGS. 1 and 2 ). 
     The body module  15  is provided with a second prism (prism) LP 2  that is fit-engaged in the prism mounting recess  25  of the housing  16  and fixed thereto. The second prism LP 2  has an incident surface LP 2 - a  which is orthogonal to the leftward/rightward direction and an exit surface LP 2 - b  which is orthogonal to the forward/rearward direction. More specifically, the second prism LP 2  is fixed to the two inner wall projections  22   d , the prism support portion  26  and the light shield frame  21   a  by bringing the upper edge of a flat reflecting surface LP 2 - c  (see  FIG. 6 ) of the second prism LP 2  into surface contact with the upper inclined flat surfaces  22   d   1 ,  26   a  and  26   b , bringing the lower edge of the flat reflecting surface LP 2 - c  of the second prism LP 2  into surface contact with the lower inclined flat surfaces  22   d   1 ,  26   a  and  26   b , and bringing the upper, lower and rear edges of the incident surface LP 2 - a  into surface contact with the incident-side contact surface  21   b  (the hatched portion shown in  FIG. 9  shows a portion of the incident surface LP 2 - a  which is brought into contact with the incident-side contact surface  21   b ). The flat reflecting surface LP 2 - c  of the second prism LP 2  constitutes the right end surface of the second prism LP 2  and is inclined to both the incident surface LP 2 - a  and the exit surface LP 2 - b  at an angle of 45 degrees. Upon the second prism LP 2  being fitted into the prism mounting recess  25  and fixed thereto, the light shield frame  21   a  covers the entire peripheral edge of the incident surface LP 2 - a  as viewed from the left, a clearance is created between the front edge of the incident surface LP 2 - a  and a right side of the connecting portion  23   c   1 , and the incident surface LP 2 - a  faces the exit surface LP 1 - b  in the leftward/rightward direction. 
     The body module  15  is provided with a first rod (cylindrical rod)  36  and a second rod (cylindrical rod)  37  which are made of metal and extend linearly in the leftward/rightward direction. The first rod  36  and the second rod  37  are each fixed at both ends thereof to an inner surface of the right side wall of the housing  16  and the partition wall  19  so that the left ends of the first rod  36  and the second rod  37  are aligned in the upward/downward direction and so that the right ends of the first rod  36  and the second rod  37  are aligned in the upward/downward direction. 
     The body module  15  is provided with a second lens group frame  39  made of synthetic resin. An insertion hole  40  in which the first rod  36  is inserted is formed through the upper part of the second lens group frame  39 , and a rotation prevention groove  41  in which the second rod  37  is engaged is formed in the lower end of the second lens group frame  39 . The second lens group frame  39  is prevented from rotating about the first rod  36  by the engagement of the rotation prevention groove  41  with the second rod  37 , and accordingly, the second lens group frame  39  can slide on and along the first rod  36  and the second rod  37  in the leftward/rightward direction. The body module  15  is provided with a second lens group (image-side lens) LG 2  configured of two lens elements L 4  and L 5 , which are each supported by the second lens group frame  39  and which are adjacent to the exit surface LP 1 - b  of the first prism LP 1  and the incident surface LP 2 - a  of the second prism LP 2  in the leftward/rightward direction, respectively. In addition, a nut holding hole  42 , both ends of which in the leftward/rightward direction are formed as open ends, is formed at the upper end of the second lens group frame  39 , and a driven nut (upper driven nut)  44 , through which a female screw hole with the axis thereof extending in the leftward/rightward direction is formed, is fixedly fitted into the nut holding hole  42  in such a manner so as to be prevented from rotating about the axis of the aforementioned female screw hole. The body module  15  is provided with a first motor (stepping motor) M 1  which is fixed to the housing  16  in the upper part of the accommodating recess  18 . The first motor M 1  is provided with a rotary drive shaft M 1   a  which extends linearly leftwards, and a male screw thread formed on the rotary drive shaft M 1   a  in the vicinity of the end (left end with respect to  FIGS. 3 and 4 ) thereof is screw-engaged with the aforementioned female screw hole of the upper driven nut  44 . Accordingly, forward and reverse rotations of the rotary drive shaft M 1   a  that are caused by forward and reverse rotations of the first motor M 1  cause the second lens group frame  39  (the lens elements L 4  and L 5 ) to move linearly between the telephoto extremity position (shown in  FIG. 4 ) and the wide-angle extremity position (shown in  FIG. 3 ) in the leftward/rightward direction along the first rod  36  and the second rod  37 . 
     In addition, the body module  15  is provided with a third lens group frame  47  made of synthetic resin. An insertion hole  48  in which the second rod  37  is inserted is formed through the lower part of the third lens group frame  47 , and a rotation prevention groove  49  in which the first rod  36  is engaged is formed in the upper end of the third lens group frame  47 . Accordingly, the third lens group frame  47  can slide on and along the first rod  36  and the second rod  37  in the leftward/rightward direction (while being prevented from rotating about the second rod  37 ). The body module  15  is provided with a third lens group (image-side lens) LG 3  configured of a lens element L 6 , the optical axis thereof being coaxial with the optical axis of the second lens group LG 2 . The third lens group LG 3  is fixedly fitted into a lens holding hole which is formed through the third lens group frame  47  in the leftward/rightward direction. A nut holding hole  50 , both ends of which in the leftward/rightward direction are formed as open ends, is formed at the lower end of the third lens group frame  47 , and a driven nut (lower driven nut)  44 , through which a female screw hole with the axis thereof extending in the leftward/rightward direction is formed, is fixedly fitted into the nut holding hole  50  in such a manner so as to be prevented from rotating about the axis of the driven nut  44  that extends in the leftward/rightward direction. 
     The body module  15  is provided with a second motor M 2  which is identical in specifications to the first motor M 1  and fixed to the housing  16  in the lower part of the accommodating recess  18 . The second motor M 2  is provided with a rotary drive shaft M 2   a  (identical in specifications to the rotary drive shaft M 1   a ), and a male screw thread formed on the rotary drive shaft M 2   a  in the vicinity of the end (left end with respect to  FIGS. 3 and 4 ) is screw-engaged with the aforementioned female screw hole of the lower driven nut  44 . Therefore, forward and reverse rotations of the rotary drive shaft M 2   a  that are caused by forward and reverse rotations of the second motor M 2  cause the third lens group frame  47  (the third lens group LG 3 ) to move linearly between the telephoto extremity position (shown in  FIG. 4 ) and the wide-angle extremity position (shown in  FIG. 3 ) in the leftward/rightward direction along the first rod  36  and the second rod  37 . 
     The housing  16 , the first rod  36 , the second rod  37 , the second lens group LG 2  (the second lens group frame  39  and the driven nut  44 ), the third lens group LG 3  (the third lens group frame  47  and the driven nut  44 ), the second prism LP 2 , the first motor M 1  and the second motor M 2  are components of the body module  15 . 
     The first lens group unit  3  and the body module  15  are fixed together as one unit by inserting a pair of set screws B from the left side into the through-holes  6  of the upper and lower lugs  5  of the holder  4  and screwing the male thread portions of the pair of set screws B into a pair of female screw holes (not shown) formed in upper and lower left end surfaces of the housing  16  with a portion of the holder  4  which is positioned on the right-hand side of the upper and lower lugs  5  being fitted into the mounting recess  17  and with a pair of (upper and lower) spacers S held between the upper and lower lugs  5  of the holder  4  and the upper and lower left end surfaces of the housing  16  (that are the left end surfaces of a pair of projections of the housing  16  which are formed on the vertically opposite sides of the mounting recess  17 ). 
     Upon the first lens group unit  3  and the body module  15  being fixed together as one unit, the right end of the holder  4  (in which the right lens holding hole  9  is formed) is fitted into the communication hole  20  of the partition wall  19  of the housing  16 , and the optical axis A of the lens elements L 2  and L 3  aligns with the optical axis of the second lens group LG 2  and the third lens group LG 3 . 
     The first lens group LG 1  (L 1 , LP 1 , L 2  and L 3 ), the second lens group LG 2  (L 4  and L 5 ), the third lens group LG 3  (L 6 ) and the second prism LP 2  that have been described above are optical elements of an imaging optical system (bending optical system). A zooming operation is performed by moving the second lens group LG 2  (the lens elements L 4  and L 5 ) and the third lens group LG 3  (the lens L 6 ) along the first rod  36  and the second rod  37 , and a focusing operation is performed by moving only the third lens group LG 3  along the first rod  36  and the second rod  37 . 
     The board module  65  is provided with a circuit board  66 . The circuit board  66  has substantially the same (namely, corresponds to) front shape as the shape of the accommodating recess  18  of the housing  16  and is configured from a flat plate orthogonal to the forward/rearward direction. The circuit board  66  is provided on the rear surface thereof with a printed circuit (not shown), and two circular holes  67  are formed through two diagonally opposite corners of the circuit board  66 , respectively. 
     The board module  65  is provided with an image sensor (image pickup device)  69  (see  FIGS. 1 ,  2  and  5 ) which is fixed at the right end of the rear surface of the circuit board  66 . A plurality of terminals (not shown) of the image sensor  69  are fixedly connected to the aforementioned printed circuit by soldering. The image sensor  69  is provided on the rear thereof with an imaging surface (not shown) that is orthogonal to the forward/rearward direction. In addition, the image sensor  69  is provided, on the incident surface (the rear surface in the drawings) thereof, with a cover glass  70  which is made of a flat glass plate and fixedly installed so as to cover the entire imaging surface of the image sensor  69 . 
     A rear surface (rear end) of the image sensor  69  is covered with a packing  72  made of an elastic material such as rubber which is open at the front and left sides. The packing  72  is provided in the rear surface thereof with an exposing hole (through-hole)  73 , through which the entire imaging surface of the image sensor  69  is rearwardly exposed, and is further provided in the rear surface thereof with three through-holes  72   a ,  72   b  and  72   c  which allow the three positioning spacers  23   a ,  23   b  and  23   c  to penetrate and project forward therethrough, respectively. 
     The circuit board  66 , the image sensor  69  and the packing  72  that have been described above are components of the board module  65 . 
     The cover  76  is a press-molded sheet metal product and is integrally provided with a base  77 , two (upper and lower) short engaging lugs  78 , two (upper and lower) long engaging lugs (resilient engaging lug)  79  and a pair of (upper and lower) side engaging lugs (resilient engaging lugs)  81 . The base  77  is formed as a planar member orthogonal to the forward/rearward direction. The upper short engaging lug  78  and the upper long engaging lug  79  extend rearward from the upper edge of the base  77 , and the lower short engaging lug  78  and the lower long engaging lug  79  extend rearward from the lower edge of the base  77 . The pair of side engaging lugs  81  extend rearward from the right edge of the base  77 . Each side engaging lug  81  is in the shape of a letter T as viewed from a side thereof. The base  77  is slightly greater in size (dimensions) than the circuit board  66  and has a substantially rectangular shape. The base  77  is provided with three pressure leaves  84 ,  85  and  86 , each of which is resiliently deformable in the forward/rearward direction. 
     The pressure leaves  84 ,  85  and  86  are provided with pressure projections  84   a ,  85   a  and  86   a , respectively, which project rearwardly (portions of the front surfaces of the pressure leaves  84 ,  85  and  86  which respectively correspond to the pressure projections  84   a ,  85   a  and  86   a  are dented (recessed) rearward). The pressure leaves  84 ,  85  and  86  in a free state lie on a plane on which the remaining part of the base  77  lies. 
     An engaging hole  80  and an engaging hole  82  are formed through each long engaging lug  79  and each side engaging lug  81 , respectively. 
     To install the board module  65  and the cover  76  to the body module  15 , first the front opening of the accommodating recess  18  is closed by the circuit board  66  so that the outer edge of the rear surface of the circuit board  66  comes into surface contact with the board support surface  27  while the two circular holes  67  of the circuit board  66  are brought into engagement with the two lock projections  28  (thereupon, the front surface of the circuit board  66  and the front surface of the housing  16  substantially lie on a common plane). Thereupon, the three spacers  23   a ,  23   b  and  23   c  of the housing  16  forwardly pass through the three through-holes  72   a ,  72   b  and  72   c  of the packing  72  so that the flat positioning surfaces  24  of the three spacers  23   a ,  23   b  and  23   c  come in surface contact with a flat rear surface of the cover glass  70  (at the outer periphery of a rear surface portion immediately behind the imaging surface of the image sensor  69 ) to thereby create a clearance in the forward/rearward direction between the cover glass  70  and the second prism LP 2  (the exit surface LP 2 - b ). The imaging surface of the image sensor  69  faces the exit surface LP 2 - b  of the second prism LP 2  in the forward/rearward direction through the exposing hole  73 . Furthermore, a rear surface of the packing  72  comes in contact with the front surface of the positioning portion  22 . 
     Subsequently, the base  77  fully covers the front of the housing  16  to fix the cover  76  to the housing  16  by respectively engaging the upper and lower short engaging lugs  78  with the first engaging recesses  30 , respectively engaging the upper and lower engaging holes  80  of the upper and lower long engaging lugs  79  with the upper and lower engaging projections  32 , and respectively engaging the engaging holes  82  of the upper and lower side engaging lugs  81  with the upper and lower engaging projections  34 . 
     Upon the imaging unit  1  being assembled in the above described manner, the circuit board  66  and the image sensor  69  are pressed rearward due to the pressure projections  84   a ,  85   a  and  86   a  of the pressure leaves  84 ,  85  and  86  coming in contact with a right-hand side portion of the front surface of the circuit board  66  so that a pressing force (biasing force) in the rearward direction acts on the front side of the circuit board  66  from the pressure leaves  84 ,  85  and  86  (via the pressure projections  84   a ,  85   a  and  86   a ) that are slightly resiliently deformed forwardly. Thereupon, an integral combination of the circuit board  66  and the image sensor  69  is held from the front and the rear sides thereof between the flat positioning surfaces  24  of the three spacers  23   a ,  23   b  and  23   c  and the pressure projections  84   a ,  85   a  and  86   a  of the cover  76 , and accordingly, the circuit board  66  and the image sensor  69  are precisely positioned with respect to the housing  16  and the second prism LP 2  in the forward/rearward direction by the three spacers  23   a ,  23   b  and  23   c  of the housing  16  and the pressure projections  84   a ,  85   a  and  86   a  of the cover  76 . 
     If the imaging unit  1  is directed toward (pointed to) an object located in front of the imaging unit  1 , light reflected by the object (light emanating from a photographic object) enters the first prism LP 1  through the incident surface LP 1 - a  after passing through the lens element L 1  and is reflected at an angle of 90 degrees by an inner surface of the first prism LP 1  to travel toward the exit surface LP- 1   b . Subsequently, the reflected light that emerges from the exit surface LP 1 - b  enters the second prism LP 2  from the incident surface LP 2 - a  after passing through the lens elements L 2  through L 6 , and is reflected at an angle of 90 degrees by the flat reflecting surface LP 2 - c  of the second prism LP 2  to travel toward the exit surface LP- 2   b . Subsequently, the reflected light that emerges from the exit surface LP 2 - b  is captured (received) by the imaging surface of the image sensor  69  after passing through the exposing hole  73  and the cover glass  70 . Even if an external force is exerted on the imaging unit  1  or vibrations occur in the imaging unit  1  as described above, the position of the image sensor  69  (the imaging surface thereof) in the forward/rearward direction is held precisely at a predetermined design position, and accordingly, a sharp object image with no blur can be captured by the image sensor  69  even if an external force is exerted on the imaging unit  1  or vibrations occur in the imaging unit  1 . 
     Additionally, if the above described imaging optical system is driven to perform a zooming operation and a focusing operation by moving the second lens group LG 2  (the lens elements L 4  and L 5 ) and the third lens group LG 3  (the lens L 6 ) along the first rod  36  and the second rod  37  with the first motor M 1  and the second motor M 2 , an image capturing operation in a state where the imaging optical system is zoomed and focused on a photographic object image becomes possible. 
     In the above described embodiment of the imaging unit  1 , the light shield frame  21   a  (the light shield portions  22   a   1 ,  22   b   1 ,  22   c   1  and  23   c   3 ) is positioned immediately on the left-hand side of the peripheral edge of the incident surface LP 2 - a , and accordingly, light which emanates from a photographic object and is passed through the first lens group LG 1 , the second lens group LG 2  and the third lens group LG 3  is partly shielded by the light shield frame  21   a , so that the light which is passed through a through-hole formed through the inner side of the frame member  21  travels toward the incident surface LP 2 - a  of the second prism LP 2 . On the other hand, part of the same light that emanates from a photographic object and is passed through the first through third lens group LG 1 , LG 2  and LG 3  can be effectively prevented from traveling toward both the upper and lower surfaces of the second prism LP 2  by the light shield frame  21   a , so that stray light which occurs by reflections caused by both the upper and lower sides of the second prism LP 2  can be prevented from reaching the imaging surface of the image sensor  69 . 
     Additionally, since the light shield frame  21   a  is formed integrally with part of the housing  16  and also since the positioning of the second prism LP 2  with respect to the housing  16  is achieved by the incident-side contact surface  21   b  and the inclined flat surfaces  22   d   1 ,  26   a  and  26   b , which are formed integrally with the housing  16 , the light shield frame  21   a  can be installed on the incident surface LP 2 - a  side of the second prism LP 2  with great positional precision even when the second prism LP 2  and the light shield frame  21   a  are miniaturized. Moreover, since the light shield frame  21   a  is formed integrally with the housing  16 , an operation to install the light shield frame  21   a  to the housing  16  or the second prism LP 2  is not required. Therefore, only an operation to bring the second prism LP 2  into contact with the incident-side contact surface  21   b  and the inclined flat surfaces  22   d   1 ,  26   a  and  26   b  is required as an operation to position the light shield frame  21   a  on the incident surface LP 2 - a  side of the second prism LP 2 , so that the light shield frame  21   a  can be easily positioned on the incident surface LP 2 - a  side of the second prism LP 2 . 
     Furthermore, since the second prism LP 2  is brought into surface contact with the incident-side contact surface  21   b  and the inclined flat surfaces  22   d   1 ,  26   a  and  26   b  while a clearance is formed between the front edge of the incident surface LP 2 - a  and a right side of the connecting portion  23   c   1 , the level of accuracy of the flatness and dimensional control over the connecting portion  23   c   1  of the spacer  23  can be relaxed. 
     Although the present invention has been described based on the above illustrated embodiment, various modifications to the above illustrated embodiment are possible. 
     For instance, either the inclined flat surfaces  26   a  and  26   b  or the inclined flat surface  22   d   1  can be omitted. 
     In addition, the first prism LP 1  can be replaced by a mirror, or the first prism LP 1  can be removed from the imaging optical system. 
     Obvious changes may be made in the specific embodiment of the present invention described herein, such modifications being within the spirit and scope of the invention claimed. It is indicated that all matter contained herein is illustrative and does not limit the scope of the present invention.