Patent Publication Number: US-2003233024-A1

Title: Electronic endoscope for stereoscopic endoscope system

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to an electronic endoscope for a stereoscopic endoscope system and, more particularly, to an electronic endoscope suitably used as a laparoscope in a stereoscopic endoscope system that essentially includes an elongated rigid sheath having a reduced outer diameter for insertion into a human or animal body to view therein or to perform surgical procedures.  
       [0003] 2. Description of the Related Art  
       [0004] In recent years, endoscopic surgical operations on or endoscopic direct visual examinations of abdominal organs within a human or animal body are widely performed using laparoscopes. Such an endoscopic surgical operation or an endoscopic direct visual examination is performed by introducing a laparoscope and, if necessary, forceps into an abdominal cavity through a relatively small incision in the skin.  
       [0005] In the endoscopic surgical operation or the endoscopic direct visual examination, in light of the fact that stereoscopic visual images of the interior of a human body are more accurate and realistic than conventional two-dimensional images, many stereoscopic endoscope systems including laparoscopes that provide stereoscopic vision of the internal of the body have been proposed.  
       [0006] Such a stereoscopic endoscope system typically includes an electronic endoscope such as an electronic laparoscope which has a elongated rigid tubular sheath provided with a pair of objective lens systems positioned at a distal end thereof and a pair of relay lens systems as image guides for transmitting right and left images of the interior of a patient body formed by the objective lens systems to a pair of video camera set up separately from the electronic endoscope. The images of the interior of the patient body are displayed as a three-dimensional image on a monitor through the video camera. This type of electronic endoscope is unfavorable because the viewer is provided with poor visual perception.  
       [0007] In the meanwhile, with recent advance in miniaturization of solid state charge coupled devices (CCDs) as an imaging device, electronic endoscopes that include a pair of image pickup means each of which comprises an objective lens system and a CCD image sensor disposed, basically in place of the conventional relay lens system and video camera, in the distal end of the elongated rigid tubular sheath have been put into practical use and basically prevails in the field of endoscopic surgical operations and endoscopic direct visual examinations. Such an electronic endoscope for a stereoscopic endoscope system produces high quality images (e.g., on a monitor connected to the system) which a properly-equipped user/viewer of the system will perceive as three-dimensional images having stereoscopic depth. With this system, the viewer is provided with visual perception because of stereopsis.  
       [0008] Referring is made to FIGS. 8 and 9 for the purpose of providing a brief background that enhances an understanding of a stereoscopic endoscope system including an electronic endoscopic instrument. Referring to FIG. 8 showing a stereoscopic endoscope system exemplified as a conventional system, the stereoscopic endoscope system comprises an electronic laparoscope  1  as an endscopic instrument, a light source unit  2 , a video signal processing unit  3  and a video display or monitor  4 . The electronic laparoscope  1  generally includes an elongated rigid cylindrical sheath  1 A, that has a double-barreled structure such as described in detail later, for insertion into an interior of a body cavity of a patient, i.e. a human body or animal body, to view therein or to perform surgical procedures, a pair of or right and left image pickup devices  15 R and  15 L disposed in close proximity to an distal end  11  of the sheath  1 A for generating and transmitting images of the cavity from within the body and an illumination or light guide  6  comprising a generally cylindrically tubular-shaped bundle of optical fibers and extending through the sheath  1 A. The electronic laparoscope  1  at a proximal end further includes a handling unit  1 B provided with a light guide connector  7  for optically connecting the light guide  6  to the light source unit  2  and a relay connector  9  for electrically connecting the image pickup devices  15 R and  15 L to the video signal processing unit  3  through respective signal lines  8 .  
       [0009] The right and left image pickup devices  15 R and  15 L, that correspond to right and left eyes of the viewer respectively, are located within the sheath  1 A in close proximity to the distal end  11  opposite to the proximal end where the handling unit  1 B is attached. Each image pickup device  15 L or  15 R comprises an objective lens system  13 R or  13 L which gathers a right or a left optical image of an object, i.e. the cavity of the patient body, illuminated with light from the light guide  6  and a CCD image sensor  14 L or  14 R as a solid state image sensor which converts the optical image incident thereupon into video signals. The right and left video signals are transmitted to and processed by the signal processing unit  3  in an ordinary manner. The processed right and left video signals are distributed to a remote monitoring unit  4  to display right and left images which can be perceived as a three-dimensional image having stereoscopic depth when observed through a viewing device (not shown) such as a specially-designed eyeglasses which are known in various forms and may take any form well known in the art.  
       [0010] Referring to FIG. 9 showing an exemplary configuration of the distal end  11  of the laparoscope shown in FIG. 8, the sheath  1 A of the electronic laparoscope comprises internal and external rigid cylindrical tubular barrels  21  and  22  which have outer diameters D 1  and D 2 , respectively, different sufficiently to provide a cylindrical tubular space therebetween for receiving the light guide  6  in the form of a tubular-shaped bundle of fibers therein. The right and left image pickup devices  15 R and  15 L are fixedly mounted in the internal cylindrical barrel  21  in close proximity to the distal end  11 . In this example, the interior of an abdominal cavity of the patient body is illuminated with light diverging in the form of an annular ring from the light guide  6 .  
       [0011] Although the electronic laparoscope shown in FIGS. 8 and 9 is desirable for physicians and surgeons in the viewpoint of homogeneous or uniformly distributed illumination over the interior of an abdominal cavity of a patient body with light, nevertheless, it is distasteful to patients to make a relatively large incision in the skin for introducing the electronic laparoscope, stereoscopic or non-stereoscopic, whose sheath  1 A has to unavoidably have a large outer diameter (the outer diameter D 2  of the external cylindrical barrel  22 ) due to the configuration of the electronic laparoscope in which the cylindrically tubular-shaped light guide  6  surrounding the inner cylindrical barrel  21  is covered with the outer cylindrical barrel  22  as seen in FIG. 9.  
       SUMMARY OF THE INVENTION  
       [0012] It is therefore an object of the present invention to provide an electronic endoscope for stereoscopic endoscope systems which includes an elongated cylindrical sheath for insertion into the interior of a body cavity or a hollow organ of a human or animal body made significantly thin.  
       [0013] It is another object of the present invention to provide an electronic endoscope for stereoscopic endoscope systems which produces images on a monitor connected to the system perceived as three-dimensional images having stereoscopic depth by a properly-equipped viewer of the system even though an elongated cylindrical sheath of the endoscope for insertion into the interior of a body cavity or a hollow organ of a human or animal body is made as thin as possible.  
       [0014] The above objects of the present invention are achieved by an electronic endoscope, suitably used as a laparoscope, for stereoscopic endoscope systems that includes illumination guide means such as a optical fiber bundle optically connected to a light source for illuminating an interior of a hollow body object and a pair of right and left image pickup means, each comprising an objective lens system and a solid state image sensor such as a chip of charge coupled device (CCD), for gathering right and left optical images of the hollow body object, respectively, and converting the right and left optical images into video signals to display the left and right images on a monitor connected to the system which a properly-equipped viewer of the system perceives as a three-dimensional image having a stereoscopic effect. The electronic endoscope comprises an elongated cylindrical rigid sheath for insertion into the interior of the hollow body object and a mounting fixture that is fitted in the elongated cylindrical sheath in proximity to an distal end of the elongated cylindrical sheath and has a pair of mounting bores arranged side by side to hold the right and left solid state image pickup means side by side therein in a diametrical direction of the elongated cylindrical sheath. The mounting fixture is cut off partially at either one side, more preferably opposite sides, of an axial plane including both center axes of the mounting bores so as to define an arched space or spaces between the mounting fixture and the elongated cylindrical sheath on one or both sides of the mounting fixture for receiving the illumination guide means therethrough. In this instance, the light guide means comprising a number of optical fibers is bundled so as to have an exit end in conformity with the arched space.  
       [0015] The electronic endoscope is preferably provided with an anterior optical element fixedly installed in the elongated cylindrical sheath at the distal end commonly through which the right and left image pickup means gather right and left optical images of the hollow body object. The anterior optical element comprises a converging lens forming a common part of the objective lens systems of the left and light image pickup means and is configured in conformity with the front of the mounting fixture.  
       [0016] The electronic endoscope may further be provided a transparent protective plate configured in conformity with the arched space and installed in the elongated cylindrical sheath at the distal end for protecting a distal end of the illumination guide means. The transparent protective plate or a pair of the transparent protective plates are complementary in shape to the single converging lens to form a substantially completely circular anterior optical element capable of being snugly fitted into the elongated cylindrical sheath at the distal end. The single converging lens and the transparent protective plate or plates are cemented to each other to form an integral anterior optical element.  
       [0017] The electronic endoscope may further be provided with light blocking means disposed at an interface between the converging lens and the illumination guide means or between converging lens and the transparent protective plate or plates for preventing stray light emanating from the illumination light guide from entering the converging lens. The light blocking means may comprise one of a light blocking film disposed at the interface, a black coating applied to the cut-off sides of the conversing lens, and a black adhesive for cementing the converging lens and the illumination guide means or the transparent protective plates together.  
       [0018] The circular anterior optical element, that comprises the single converging lens forming a common part of both right and left objective lens systems and the transparent protective plate or plates, may be borne down by a peripheral edge of the elongated cylindrical sheath clinched radially inwardly at the distal end. Alternatively, the elongated cylindrical sheath may be provided with an annular flange extending radially inwardly from the distal end thereof. In this instance, the mounting fixture with the right and left image pickup means attached thereto is installed into the elongated cylindrical sheath only from the proximal end.  
       [0019] With the electronic endoscope of the present invention, as the mounting fixture has a shape of substantial circle cut off at least partially at either one side, more preferably opposite sides, there is provided an arched space or spaces between the mounting fixture and the elongated cylindrical sheath at either one or both sides of the mounting fixture in which the illumination guide means is received. This configuration effectively improves the availability of space of the interior of the elongated cylindrical sheath at the distal end and, consequently, the elongated cylindrical sheath of the electronic endoscope is allowed to be significantly thin.  
       [0020] The circular anterior optical element comprising the single converging lens and the transparent protective plate or plates are borne down by a clinched peripheral edge of the elongated cylindrical sheath clinched at the distal end or an annular flange formed at the distal end of the elongated cylindrical sheath. Alternatively, the circular anterior optical element may be stopped an annular flange extending radially inwardly from the distal end of the elongated cylindrical sheath. The circular anterior optical element prevents an adhesive with which a great number of fibers forming the light guide are bonded together from deteriorating on contact with chemical solutions such as cleaning and disinfectant solutions whenever the electronic endoscope is washed in disinfectant after use. This is because, adhesive deterioration often leads to optical and/or mechanical local damages and/or deterioration of air-tightness of the distal ends of the light guides which cause failure of image quality and a problem of reliability of the electronic endoscope depending on the degree of adhesive deterioration.  
       [0021] The light blocking means disposed at an interface between the converging lens and the illumination guide means or between the converging lens and the transparent protective plate or plates prevents stray light emanating from the illumination light guide from entering the converging lens even in the case where the light guide is located in close proximity to either one or both of the right and left image pickup means in the elongated cylindrical sheath made thin. This results in high quality three-dimensional images. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0022] The above and other objects and features of the present invention will be clearly understood from the following detailed description when read with reference to the accompanying drawings, wherein the same numerical symbols have been used to denote same or similar parts or mechanisms throughout the drawings, and in which:  
     [0023]FIG. 1 is a perspective view, partly broken away, showing a configuration of a portion of an electronic endoscope in proximity to the distal end thereof for a stereoscopic endoscope system according to an embodiment of the present invention;  
     [0024]FIG. 2 is an end view of the electronic endoscope seen from the distal end in which an anterior optical lens element is removed;  
     [0025]FIG. 3 is a longitudinal cross-sectional view of the electronic endoscope according to the embodiment in a plane along line III-III of FIG. 1 that includes a center line of an elongated cylindrical sheath of the electronic endoscope and is perpendicular to an axial plane including optical axes of a pair of image objective systems of the electronic endoscope;  
     [0026]FIG. 4 is a perspective view, similar to FIG. 1, showing a configuration of an electronic endoscope according to an alternative embodiment of the present invention;  
     [0027]FIG. 5 is a perspective exploded view of an anterior optical element used in the electronic endoscope of the alternative embodiment;  
     [0028]FIG. 6 is a longitudinal cross-sectional view, similar to FIG. 3, of the electronic endoscope according to the alternative embodiment in a plane taken along line VI-VI of FIG. 4 that includes a center line of an elongated cylindrical sheath of the electronic endoscope and is perpendicular to an axial plane including optical axes of a pair of image objective systems of the electronic endoscope; and  
     [0029]FIG. 7 is a cross-sectional view, similar to FIG. 6, showing a variant of the electronic endoscope according to the alternative embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0030] In the following description, parts or units which are not of direct importance to the invention and parts or units which are purely of conventional construction will not be described in detail. For example, details of the light source unit, the video signal processing unit, the video display or monitor unit, etc., which necessary to the stereoscopic endoscope system will not set out in detail since their construction and operation can easily be arrived at by those skilled in the art.  
     [0031] Referring now to the drawings in detail and, in particular, FIGS. 1 through 3 showing a front portion of an electronic endoscope  30  used typically as a laparoscope for a stereoscopic endoscope system according to a preferred embodiment of the present invention, the electronic endoscope  30  basically comprises an elongated hollow cylindrical sheath  35  (which is hereafter referred to as a cylindrical sheath for simplicity) made of a rigid tubular barrel for insertion into the interior of an abdominal cavity of a human or animal body, a pair of image pickup devices  34 R and  34 L operative as right and left eyes of a viewer, respectively, (which are referred to as right and left image pickup devices) for gathering right and left images of the abdominal cavity, and a pair of light guides  37 D and  37 U for transmitting light generated by a light source and illuminating the interior of the abdominal cavity. The left image pickup device  34 L comprises an objective lens system  31 LL built in a lens barrel  31 L operative to produce a left optical image of the abdominal cavity and a solid state image sensor such as a CCD image sensor  33 L operative to convert the left optical image incident thereupon into video signals. The lens barrel  31 L and the CCD image sensor  33 L are assembled as one integral unit. Specifically, the lens barrel  31 L is fixedly mounted to a lens holder  32 L, and the CCD image sensor  33 L is secured to the back of the lens holder  32 L. Similarly, the right image pickup device  34 R comprises an objective lens system  31 LR built in a lens barrel  31 R operative to produce a right optical image of the abdominal cavity and a solid state image sensor such as a CCD image sensor  33 R operative to convert the right optical image incident thereupon into video signals. The lens barrel  31 R and the CCD image sensor  33 R are assembled as one integral unit such that the lens barrel  31 R is fixedly mounted to a lens holder  32 R and the CCD image sensor  33 R is secured to the back of the lens holder  32 R.  
     [0032] As is well known in the art, if the right and left image pickup devices  34 R and  34 L are fixedly installed side by side in the cylindrical sheath  35  with the respective CCD image sensors  33 L and  33 R put out of alignment in horizontal scanning direction, the properly-equipped viewer will perceive the image on a monitor connected to the system as a three-dimensional image with sensuously offensive stereopsis. For this reason, it is necessary to adjust the left image pickup devices  34 L and  34 R installed in the cylindrical sheath  35  so as to put the respective CCD image sensors  33 L and  33 R well aligned in horizontal scanning direction. Further, in the case where an electronic endoscope is provided with a single image pickup device like a laparoscope which produces a two-dimensional image for observational purposes only, it is desirable in light of even illumination or non-irregularity of illumination to set out a cylindrically tubular-shaped light guide surrounding the image pickup device that is positioned in alignment with a longitudinal center axis of the sheath of the endoscope such as shown in FIG. 9. However, since it is essential for the electronic endoscope  30  used in the stereoscopic endoscope system to arrange the right and left image pickup devices  34 R and  34 L side by side in a diametrical direction or straight line passing through both optical axes of the right and left image pickup devices  34 R and  34 L, the electronic endoscope  30  has to have a cylindrical sheath  35  increased in its outer diameter if employing a cylindrically tubular-shaped light guide surrounding the right and left image pickup devices  34 R and  34 L similarly to the electronic endoscope that produces a two-dimensional image for observational purposes. This configuration comes into conflict with the demand that the cylindrical sheath  35  be as thin as possible.  
     [0033] The electronic endoscope  30  with a cylindrical sheath made as thin as possible is realized by employing a mounting structure for mounting the right and left image pickup devices  34 R and  34 L in the cylindrical sheath  35  at the distal end  11 . The mounting structure includes a mounting fixture  36  basically comprising a solid cylindrical block having a relatively small thickness and approximately the same outer diameter as the inner diameter of the cylindrical sheath  35 . The mounting fixture  36  has a pair of mounting bores  36 H arranged side by side in a diametrical direction and is partially cut off in a plane parallel to an axial plane including center axes of the mounting bores  36 H at each of opposite sides of the mounting bores  36 H in a direction perpendicular to the axial plane. It is preferable to determine the outer diameter of the mounting fixture  36 H such that the mounting fixture  36 H is dovetailed or engaged with the cylindrical sheath  35 . In an assembling process of the electronic endoscope  30 , the right and left lens barrels  31 R and  31 L are fitted in and secured within the mounting bores  36 H, respectively, of the mounting fixture  36  in advance and, then, the mounting fixture  36  is fitted into the cylindrical sheath  35  from the distal end  11 .  
     [0034] When the mounting fixture  36  partially cut off in planes parallel to the axial plane at the opposite sides of the mounting bores  36 H is tightly fitted in the cylindrical sheath  35 , there are left arched spaces  35 U and  35 D, smaller in depth than a semicircular arch, between the cylindrical sheath  35  and the mounting fixture  36  on opposite sides of the mounting fixture  36 . A light guide  37 U in a form having a cross-section in conformity or similar to the arched space  35 U extends through the cylindrical sheath  35  and is fitted in and secured within the arched space  35 U at the distal end  11 . Similarly, a light guide  37 D a form having a cross-section in conformity or similar to the arched space  350  extends through the cylindrical sheath  35  and is fitted in and secured within the arched space  35 D at the distal end  11 . The light guides  37 U and  37 D are completely identical in structure and function. Each of the light guides  37 U and  37 D is made up of a great number of fibers bundled and bonded together with, for example, an epoxy resin adhesive so as to form an arched cross-section and completely identical in structure and function. In this embodiment, it is preferred to form a fiber bundle by shaping and firmly bonding fibers in an arched shape in an end region of the fiber bundle spatially overlapping the image pickup devices  34 R and  34 L and leaving them bundled but separate from one another in the remaining region.  
     [0035] As clearly seen in FIGS. 1 and 3, the light guides  37 U and  37 D at their distal ends are put substantially in plane with the distal end  11  of the cylindrical sheath  35 .  
     [0036] The electronic endoscope  30  may or may not be provided with two light guides separately on opposite sides of the mounting fixture  36 . In the case where the electronic endoscope  30  is provided with a single light guide, the mounting fixture  36  is partially cut off in a plane parallel to the axial plane at either one of the opposite sides of the mounting bores  36 H so as to leave an arched space between the cylindrical sheath  35  and the mounting fixture  36  on the corresponding side of the mounting fixture  36 .  
     [0037] Basically, as is known in the art, the stereoscopic endoscope system including the electronic endoscope having a pair of right and left image pickup devices can produce a three-dimensional image having stereoscopic depth when the right and left image pickup devices are positioned with their optical axes inclined with respect to each other so as to provide a predetermined angle of convergence. However, it is not always asseverated that such an inclined formation of the right and left image pickup devices is advantageous from the aspect of space availability and/or configuration.  
     [0038] In these circumstances, the electronic endoscope of the present invention is provided with an anterior lens element operative to provide an angle of convergence according to distance from the right and left objective lens systems  31 LR and  31 LL to an object to be viewed that is appropriate to provide the viewer with appropriate perceptible stereopsis on the image on the monitor. The anterior optical element comprises a converging lens  38  forming a part of both of the right and left objective lens systems  31 LR and  31 LL, which is preferably a convex lens and more preferably a plano-convex lens, secured to the mounting fixture  36 . Specifically, as shown in FIG. 1, the converging lens  38 , that is partly cut off in substantially conformity with a front shape of the mounting fixture  36 , is firmly fitted between and cemented, or otherwise secured, to the light guides  37 U and  37 D at the distal end  11  of the cylindrical sheath  35 . In this instance, light blocking means  100 U and  100 D are applied at interfaces between the conversion lens  38  and the light guides  37 U and  37 D, respectively, to avoid a causes of an occurrence of flair due to stray light from the light guides  37 U and  37 D entering the objective lens systems  31 LR and  31 LL through the periphery of the conversion lens  38 . The application of light blocking means  100 U and  100 D has a significant effect on improving image quality. The light blocking means  100 U and  100 D may comprise an adhesive containing a light shielding material such as a black pigment that is used to cement the converging lens  38  and the light guides  37 U and  37 D together or a light shielding film. The light shielding material or the light shielding film are known in various forms and may take any form well known in the art.  
     [0039] According to the configuration of the electronic endoscope  30  described above, it is allowed to employ a cylindrical sheath  35  significantly reduced in diameter although the cylindrical sheath  35  has a pair of the right and left image pickup devices  34 R and  34 L each comprising the objective lens system  31 LR or  31 LL built in the lens barrel  31 R or  31 L and the CCD image sensor  33 R or  33 L incorporated therein. For example, when comparing with the conventional electronic endoscope with a pair of image pickup devices which has a double-barreled sheath having inner and outer diameters D 1  and D 2  as shown in FIGS. 9 and 10, the cylindrical sheath  35  of the electronic endoscope  30  can be reduced in outer diameter to the inner diameter D 1 .  
     [0040] FIGS.  4  to  6  show an electronic endoscope  40  for stereoscope systems according to alternative embodiment of the present invention in which transparent protective plates for protecting the light guides  37 U and  37 D are incorporated with an intention to avoid drawbacks inherent to the light guides  37 U and  37 D resulting from that it is often the case of using resin adhesives such as epoxy resin adhesives to bond a great number of fibers together and, in this instance, the adhesive has a comparatively high proportion of area in a cross section of the fiber bundle. That is, when the electronic endoscope  40  with the light guides  37 U and  37 D whose distal end surfaces are barely formed or exposed as shown in, for example, FIG. 1 is used as a laparoscope, at least the elongated cylindrical sheath is always cleaned and disinfected after use of the electronic endoscope. Consequently, the adhesive in the light guide  37 U and  37 D deteriorates on contact with chemical solutions such as cleaning and disinfectant solutions. Such adhesive deterioration leads to optical and/or mechanical local damages and/or deterioration of air-tightness of the distal ends of the light guide  37 U and  37 D which cause failure of image quality and a problem of reliability of the electronic endoscope depending on how the adhesive deterioration progresses.  
     [0041] Specifically, the electronic endoscope according to the alternative embodiment  40  includes light guides  37 U and  37 D and a mounting fixture  36  with right and left image pickup devices  34 R and  34 L fitted in and secured within mounting bores  36 H thereof, respectively, similar to those of the electronic endoscope  40  of the previous embodiment shown in FIGS.  1  to  3 . The right image pickup device  34 R comprises an objective lens system  31 LR built in a lens barrel  31 R and a CCD image sensor  33 R optically connected to each other by means of a lens holder  32 R. Similarly, the left image pickup device  34 L comprises an objective lens system  31 LL built in a lens barrel  31 L and a CCD image sensor  33 L optically connected to each other by means of a lens holder  32 L. The light guides  37 U and  37 D are fitted in and secured within arched spaces  35 U and  35 D, respectively, formed between a cylindrical sheath  41  and the mounting fixture  36 . However, in this instance, the light guides  37 U and  37 D at their distal end and the mounting fixture  36  at its front end are located in plane with each other and kept away from but in close proximity to the distal end of the elongated cylindrical sheath  41  so as thereby to leave an anterior annular space in the cylindrical sheath  41  before the mounting fixture  36  and the light guides  37 U and  37 D. The electronic endoscope  40  further includes a circular anterior optical element  45  fitted in the annular space of the cylindrical sheath  41  as shown in FIG. 6. The circular anterior optical element  45  comprises a converging lens  44  in conformity with the mounting mixture  36  and transparent protective parallel plates  43 U and  43 D in conformity with the arched spaces  35 U and  35 D, respectively, as clearly shown in FIG. 5. These converging lens  44  and arched transparent protective parallel plates  43 U and  43 D are complementary to form a substantially complete circular element and cemented to one another with an adhesive so as thereby to form an integral anterior optical element  45 . The circular anterior optical element  45  is fitted in the annular space of the cylindrical sheath  41  from the distal end and secured to the cylindrical sheath  41  with an adhesive. It is preferred to mechanically bear down the anterior optical element  45  with a peripheral edge  41 G of the cylindrical sheath  41  clinched at least partly, and desirably entirely, along the periphery of the distal end radially inwardly as shown in FIG. 6. In this instance, it is desirable for the anterior optical element  45  to have a chamfer formed along its frontal periphery. Otherwise, the elongated cylindrical sheath  41  may be provided with an annular flange extending radially inwardly from the distal end thereof. In this instance, the mounting fixture with the right and left image pickup means attached thereto is installed into the elongated cylindrical sheath only from the proximal end.  
     [0042] In this instance, light blocking means  101 U and  101 D are applied at interfaces between the conversion lens  44  and the transparent protective parallel plates  43 U and  43 D, respectively, to prevent light emanating from the light guides  37 U and  37 D from directly entering the converging lens  44  through its periphery. The light blocking means  101 U and  101 D may comprise a light shielding material such as a black pigment contained in the adhesive that is used to cement the converging lens  44  and the transparent protective parallel plates  43 U and  43 D together or a light shielding film. The light shielding material or the light shielding film are known in various forms and may take any form well known in the art.  
     [0043] While the configuration of the electronic endoscope  40  according to the alternative embodiment is of great advantage in employing the cylindrical sheath  35  significantly reduced in outer diameter resulting from high space availability for the light guide  37 U and  37 D. Besides this advantage, the electronic endoscope  40  avoids possible deterioration of the adhesive in the light guide  37 U and  37 D at their distal end due to direct contact with chemical solutions such as cleaning and disinfectant solutions. This is of great advantage in enhancing the durability of the light guide  37 U and  37 D, and hence the electronic endoscope  40 . Furthermore, although axes of the image pickup devices come close to each other due to a reduced diameter of the cylindrical sheath  41 , nevertheless, illumination light and image bearing light are finely isolated from each other by the light shielding means. Consequently, the electronic endoscope  40  produces high quality images.  
     [0044] Furthermore, the configuration that the anterior optical element is mechanically borne down by clinching partially a peripheral edge of the sheath at distal end radially inwardly prevents the adhesive with which the anterior optical element is cemented to the sheath from brought into contact with chemical solutions during cleaning and/or disinfection, thereby protecting the distal ends of the light guides against damages, and besides, provides significantly reliable and enhanced supporting strength.  
     [0045]FIG. 7 shows a variant of the electronic endoscope  40  of the alternative embodiment which has a modified distal end configuration. As shown, light guides  37 U and  37 D are located in plane with each other and separately from but in close proximity to the distal end of the cylindrical sheath  41  so as thereby to leave an anterior annular space in the cylindrical sheath  35  for flat transparent protective parallel plates  43 U and  43 D in the cylindrical sheath  35 . Besides the light guides  37 U and  37 D, a mounting fixture  36  is set back toward the proximal end from but in close proximity to the distal ends of the light guides  37 U and  37 D. The variant configuration enables the use of a converging lens  48  having an increased axial thickness and allows an anterior optical element  45 A to be fixedly supported between the light guides  37 U and  37 D additionally by the cylindrical sheath  41 . Consequently, positioning and fixing the anterior optical element  45 A is more easily be performed.  
     [0046] In the embodiments described above, though the converging lenses  38 ,  44  and  48  as the anterior optical element or forming part of the anterior optical element have been exemplified in the form of a plano-convex lens, it may be of a meniscus type having a large curvature radius at an exit side. The transparent protective plates  43 U and  43 D which have been exemplified in the form of a parallel plate may be replaced with a simple projection lens. Further, the electronic endoscope  30 ,  40  may incorporate at least one light guide disposed in an arched space defined between the mounting fixture and the sheath on either one side of the mounting fixture in a direction perpendicular to the axial plane including center axes of the mounting bores of the mounting fixture.  
     [0047] Although the present invention has been described with reference to preferred embodiments thereof, it will be appreciated that variants and other embodiments can be effected by person of ordinary skill in the art without departing from the scope of the invention.