Abstract:
In an image-forming lens, an inclined slope or section is provided at a marginal annular region on a surface of an optical member and a light shield is provided on this inclined slope or section. The optical member is mounted longitudinally so that the inclined slope or section is located near where off-field fight (i.e, fight from outside the designed field of view of the lens) reflects off an inner surface of a holding member or at the periphery of an optical member. Thus, the inclined slope or section with light shield acts to shield the reflected off-field light from passing to the image. The invention finds particular advantage in very small lenses that include optical members less than 3 mm in diameter.

Description:
BACKGROUND OF THE INVENTION 
     An endoscope is inserted into narrow spaces such as human internal body parts or the interior of an engine. Therefore it is desirable that the outer diameter of an endoscope be as small as possible, and this requires that the diameter of the lens that is arranged at the tip of an endoscope be as small as possible. An example of a lens of an endoscope that has a comparatively small diameter and wherein the lens is made using only a few lens elements is described in Japanese Laid-Open Patent Applications H9-68647 and H7-84179. 
     FIG. 9 illustrates the same objective lens element structure as that of Example 1 of Japanese Laid-Open Patent Application H9-68647, but includes different labels and, in addition to showing the objective lens elements themselves, FIG. 9 also illustrates conjectured support structure and flare-reducing structure for these lens elements. The lens comprises, in order from the object side, a first lens element lens L 1 , an aperture stop  5  (ie., a brightness diaphragm), a second lens element L 2  of positive refractive power and a third lens element L 3  of positive refractive power. In one embodiment of Japanese Laid-Open Patent Application H9-68647, lens element L 1 , which in all other embodiments is piano-concave, is replaced by a plane-parallel cover plate. Although no support structure whatsoever for these lens elements is actually shown in Japanese Laid-Open Patent Application H9-68647, FIG. 9 illustrates the assumed support structure for these components based on standard practices in the art, as will now be discussed. Because the first lens element L 1  (or cover plate, as the case may be) and the second lens element L 2  have the same outer diameter, they have been illustrated in FIG. 9 as being fixed to a first support frame SF 1 . An aperture stop  5  lies between the first lens element L 1  and the second lens element L 2 . Thus, the first support frame SF 1 , is shown supporting these three elements. Further, it is likely that the outer diameter of the first support frame SF 1  would be sized to equal the outer diameter of the third lens element L 3 , which in Japanese Laid-Open Patent Application H9-68647 is shown as having an outer diameter that is slightly larger than the outer diameter of the other two lens elements. Having their outer diameters equal would enable the first support frame SF 1  and the second lens element L 2  to each be inserted into a second support frame SF 2 , as illustrated in FIG.  9 . Adjacent lens element L 3  in FIG. 9 is illustrated a conjectured flare diaphragm  7  and a diaphragm receiving member  8 , as would commonly be used in the prior art to form a flare stop. 
     FIG. 10 illustrates the lens element structure of Example 6 of Japanese Laid Open Patent Application H7-84179. The lens of this example includes, in order from the object side, a plano-concave lens L 1 , an aperture stop  5 , a positive meniscus lens element L 2 , and a positive lens element L 3 . 
     In image formation lenses generally, in order to remove the light which does not contribute to image formation, a flare stop is often provided. There is no known description in the above-mentioned two patents which relates to a flare stop. However, as shown in prior art FIG. 9 for optical diaphragms generally, a flare diaphragm  7  and a diaphragm receiving member  8  are often provided on the object side of lens element L 3 . In many cases, such a flare stop is provided at this position in order to prevent stray light that has been reflected off the inner surface of the frame from passing through to the observer. The flare stop is positioned before lens element L 3  because stray light rays (i.e., those from a larger angle of view than for which the lens was designed to image) in many cases are incident on the frame between positive lens elements L 2  and L 3 . 
     However, if the outer diameter of a lens element is made smaller to achieve size reduction, the associated diaphragm receiving member and flare stop member must also be made smaller. Because these members are each shaped as an annular ring which circumscribes an aperture having a diameter that is nearly the same diameter as the lens element itself, as the lens element becomes smaller, the width of the annular ring becomes so small that manufacture and assembly of the ring become difficult. 
     Although it is known to bevel the edge of a lens element so as to form a sloped surface near the circumference of the lens element and to use this surface to self-center a lens element in lieu of using a spacer ring (as taught, for example, in U.S. Pat. No. 5,589,989) there is no teaching in this reference or any other reference known to applicant which suggests beveling an edge of a lens element and applying a light shielding material (i.e., one which absorbs or attenuates light) to a sloped surface created by the beveling process in order to reduce flare. Nor is there any suggestion that such an approach to reducing flare will result in fewer components and result in an overall cost reduction for assembling a lens system having small diameter components. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention forms an inclined or sloped surface on an optical member (such as a lens element, plane-parallel plate of a filter or cover glass, etc.), near the outer diameter of the optical member and applies a light shielding substance to the sloped surface in order to achieve flare reduction. The sloped or inclined surface, which may be the lens element surface itself, is immediately adjacent a circumferential edge of the optical element, and the circumferential edge is supported by a holding member. Alternatively, the present invention omits forming a separate, planar, sloped surface for the light shield, and instead designs the optical member so a light shielding substance may be easily applied to a sloped surface of a convex lens element near its periphery in order to shield flare from reaching the image. Flare, as used herein, refers to unwanted light which enters an optical system outside the intended field of view and is reflected toward the image plane by one or more surfaces of the optical system. A first object of the invention is to avoid using an annular ring for flare reduction in the case where the diameter of the optical member is small A second object of the invention is to enable the overall manufacturing and assembly cost of an optical system to be reduced by achieving the first object of the invention, while still maintaining adequate flare reduction. Thus, the present invention relates particularly to very small optical members, such as lens elements, filters or cover plates as may be found in endoscopes, or in other applications (e.g., in small electronic cameras, small cameras for monitoring, etc.) where size reduction causes the prior art structures and methods for flare reduction (discussed above) to become unsatisfactory. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given below and the accompanying drawings, which are given by way of illustration only and thus are not limitative ofthe present invention, wherein: 
     FIG.  1 ( a ) is a side, sectional view of a first embodiment of the invention, 
     FIG.  1 ( b ) is an end view of the lens element L 2  shown in FIG. 1 ( a ), 
     FIG.  2 ( a ) is a side, sectional view of a second embodiment of the invention, 
     FIG.  2 ( b ) is a different sectional view of the second embodiment of the invention, in the case where a square detector is used that eats into the optical surface of lens L 2  from four sides. 
     FIG. 3 is a side, sectional view of a third embodiment of the invention, 
     FIG. 4 is a sectional view of a fourth embodiment of the invention, 
     FIG. 5 is a sectional view of a fifth embodiment of the invention, less the support structure for the optical elements, 
     FIG. 6 is a sectional view of a sixth embodiment of the invention, 
     FIG. 7 is a sectional view of a seventh embodiment of the invention, 
     FIG.  8 ( a ) is a sectional view of an eighth embodiment of the invention, 
     FIG.  8 ( b ) is an expanded view of the lens element L 2  shown in FIG.  8 ( a ), 
     FIG. 9 is a sectional view of a conventional lens, and 
     FIG. 10 is a sectional drawing of another conventional lens, less the support structure for the optical elements. 
    
    
     DETAILED DESCRIPTION 
     As used herein, the term “optical member” refers to either a single lens element, two lens elements that are cemented together, a plane-parallel plate such as a filter or cover glass, or any combinations of these elements. The present invention relates particularly to optical systems having small diameter optical members. However, the invention may also find utility in other optical systems as well. In optical systems having small diameter optical members, the invention enables such systems to be made and assembled less expensively than previously possible while retaining the same capability of preventing unwanted light from outside the design field of view of the lens from reaching the image plane. Such light is termed flare and, if not shielded, may reach the image surface as a result of undesired reflections of off-axis rays within the lens and cause artifacts in the image. 
     In the present invention, it is preferable if the following Conditions (1) and (2), below, are satisfied. 
     
       
         Φ1&lt;3 mm  Condition (1) 
       
     
     
       
         0.6&lt;Φ2/Φ1&lt;0.95  Condition (2) 
       
     
     where 
     Φ1 is the outer diameter of an optical member 
     Φ2 is the maximum clear aperture diameter of the optical member (excluding a peripheral region, to which is applied a light shielding substance). 
     If the upper limit value of 0.95 of Condition (2) is exceeded, there will be too little width to adequately shield the rays that cause flare at the image plane. In other words, the slope becomes smaller, and the effect of flare reduction decreases. Moreover, if the lower limit value (0.6) of Condition (2) is not satisfied, the outer diameter for the portion of the optical member other than the slope will become large. Thus, although flare reduction will be obtained, light throughput of the optical member will become small and the image brightness will decrease. 
     Moreover, it is better if either of the stricter conditions (3) or (4) is satisfied by at least one of the optical members in the lens of the invention. 
     
       
         Φ1&lt;2 mm  Condition (3) 
       
     
     
       
         0.6&lt;Φ2/Φ1&lt;0.85  Condition (4) 
       
     
     In other words, instead of merely satisfying Conditions (1) and (2), it is better if one of the stronger Conditions (3) and (4) is also satisfied by at least one of the optical members in the lens of the invention. Further, it is best if both of the stronger Conditions (3) and (4) are satisfied by at least one of the optical members in the lens of the invention. 
     A lens according to the invention includes, in order from the object side, an aperture stop, a positive lens element having a convex surface on the image side, and a positive lens element having a convex surface on the object side. Further, the positive lens element with the convex surface on the object side is held by a holding member, a region of the convex surface near the edge of this positive lens has a slope, the slope has a light attenuating or light absorbing material thereon which reduces flare by the slope being near the region (and thus in the path) where a light ray from outside the visual field reflects at the interior of the holding member and is incident on the convex surface side of a positive lens element. 
     The lens of this invention includes a lens element, plane-parallel plate (such as a filter or cover glass) or other optical member. A slope is provided on the circumference edge of at least one optical member. A fight shield is provided on the slope, and the following condition is satisfied. 
     
       
         0.5&lt;D/f&lt;2  Condition (5) 
       
     
     where 
     D is the air path equivalent length (i.e., the sum of the values d/n) from the on-axis position of the surface of the optical member having the slope formed thereon to an image surface, and 
     f is the focal length of the lens. 
     Condition (5) defines the distance from the on-axis position of the surface which includes the slope to the image surface. In other words Condition (5) defines an effective axial position for the slope which removes flare, which also corresponds to the axial position at which flare tends to be generated by light being reflected off a framework surface. 
     In the objective optical system of the above-mentioned type with two convex surfaces, the principal ray has a tendency to become farther removed from the optical axis as it progresses past the aperture stop. For this reason, since the image surface tends to be close to the aperture stop at the position where the upper limit of Condition (5) is exceeded, the light ray height is low. Removal of flare light, on the other hand, is best achieved where the ray height is high, i.e., where it is so high that it reflects from the frame. For this reason, axial positions near an aperture stop are not suitable as a position for providing a flare stop. 
     On the other hand, if the lower limit of Condition (5) is exceeded, the flare stop must be too near the image surface, making it impossible to remove flare that is generated before the slope. Moreover, there is a possibility that the image-formation beam for which the height of the light-rays becomes high may be scattered. 
     The lens of this invention relates particularly to optical members which are inserted and held inside a holding member. A slope is formed on an annular surface of an optical member in a section adjacent to the edge of the optical surface that is held by the holding member. Usually, the optical member selected for having the flare-reducing slope is the one nearest the image side of the holding member. In the case where a lens has several optical members and holding members, and the optical members are held by different holding members, at least one of the optical members held by the holding member which is nearest the image side should have a flare-reducing slope (either a planar or convex slope) provided. 
     An important characteristic of the present invention is that a flare stop that is an integral part of the optical member is provided. A conventional flare stop is in many cases inserted and held between optical members. Many optical members, such as a lens element provided at the end of a support frame of a lens, do not have another optical member attached in the same support frame (on one side). Thus, using a conventional flare stop becomes difficult, since there are not the necessary optical components within which the flare stop may be inserted. For this reason, when flare removal is necessary at such a position, it is preferable to employ the slope structure of the present invention for flare reduction. 
     Furthermore, in forming a light shielding means using adhesives while fixing a lens element to a frame, a specially formed (i.e., planar) slope for supporting the light absorbing or light attenuating adhesive material to shield against flare is not required, provided the following Condition (6) is satisfied: 
     
       
         Φ1/(2×|R|)&gt;0.34  Condition (6) 
       
     
     where Φ1 and R are as defined above. 
     Condition (6) provides that the angle between a normal to the lens element surface at the periphery of the lens and the optical axis is greater than 20°, thereby ensuring that a groove  4 , as illustrated in FIG.  8 ( b ), is formed at the periphery of the lens element in which a light shielding adhesive may be applied to the sloped, convex surface of a positive lens at the lens periphery for flare reduction. 
     Several embodiments of the invention will now be described in detail, with reference to the drawings. 
     Embodiment 1 
     FIG.  1 ( a ) is a side, sectional view of a first embodiment of the invention. The lens of this embodiment includes two lens elements, and is formed of the following components, in order from the object side: a cover glass C 1 , a film (not visible in this side view), a first lens element L 1  of positive power, a second lens element L 2  of positive power, and a second cover glass C 2 . The cover glass C 1  has film (not visible in this view) deposited on a portion of its image-side surface, and the film thus serves as an aperture stop. A first lens element L 1  is cemented to the film and the remaining portion of the image-side surface of the cover glass C 1 , and all of these elements are inserted within and held by a first frame  1 . The first frame  1  is partially inserted into one end of a second frame  2 , and all of these components are fixed by bonding. A second lens element L 2  having an outer diameter the same as, or nearly the same as, that of the outer diameter of the first frame  1  is inserted within the other end of the second frame  2  and is fixed by being bonded. Second frame  2  maintains a cover glass C 2  of an image pick-up element against its image-side end. Moreover, the image side of the cover glass C 2  is at the position of the image plane I, which coincides with the pick-up face of a detector, such as a CCD array. 
     For the lens of this first embodiment, the circumference edge of the convex surface on the object side of the second positive lens element L 2  is made to have a planar slope A 1 , as illustrated. The slope A 1  extends to the side of the lens element and provides an annular cone-like surface. The slope A 1  itself provides some assistance in reducing flare by reason of its ability to reflect some of the flare fight reflected off the second frame  2  in a direction away from the image. However, in those embodiments of the present invention which use a separately-formed, planar slope to reduce flare, a light shielding material (not visible in this view) is applied to this slope to prevent the passage of light reflected from second frame  2  from progressing to the image plane at I. 
     FIG.  1 ( b ) is an end view of the lens element L 2  shown in FIG.  1 ( a ). The outer diameter of this lens element is Φ1 and the maximum clear aperture diameter is Φ2, as illustrated. In this embodiment Φ1 is 1 mm and Φ2 is 0.75 mm; thus, the ratio Φ2/Φ1 is 0.75. This embodiment prevents the sloping surface from eclipsing part of the image-forming beam, and thus enables a maximum image height to pass to the image plane unhindered. 
     Also, the air path equivalent length D from the on-axis position of the object side surface of the second positive lens element L 2  to the image plane at I is 0.6 mm. The focal distance of the lens is 0.626 mm. Therefore, the ratio of D/f is 0.96, which satisfies Condition (5). 
     The slope angle θ1 (illustrated in FIG.  1 ( a )) of the second positive lens element L 2  is 45 degrees. Moreover, this amount is less than 70 degrees, which is advantageous. 
     In this embodiment, the second positive lens element L 2  is attached to the second frame  2  with a black adhesive. The black adhesive is coated on the periphery surface of the lens (ie., the part supported by the frame) as well as on the slope A 1 . Thus, the adhesive serves to block light from passing through the lens portion having slope A 1 . Alternatively, the bonding of the lens elements to the lens frame can use a more common adhesive that is transparent or translucent, and instead of using adhesive on the slope A 1 , this portion can be coated with black paint to thereby enable the slope A 1  to block off-field light that has been reflected by the frame  2  or the periphery surface of a lens element from passing through to the image plane at I. 
     The lens of this embodiment can be used for an endoscope, an electronic camera, a surveillance camera, etc. 
     Table 1 lists the surface # in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis surface spacing d (in mm) of each surface, as well as the index of refraction N d  and the Abbe constant V d  (for the d-line) of each optical member of Embodiment 1. In the bottom portion of the table are listed some of the performance values of the lens of this embodiment. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 # 
                 R 
                 d 
                 N d   
                 V d   
               
               
                   
               
             
             
               
                 1 
                 ∞ 
                 0.2000 
                 1.51633 
                 64.15 
               
               
                 2 
                 ∞ 
                 0.4500 
                 1.51633 
                 64.15 
               
               
                 3 
                 0.4420 
                 0.1600 
               
               
                 4 
                 0.9770 
                 0.5000 
                 1.51633 
                 64.15 
               
               
                 5 
                 ∞ 
                 0.4000 
                 1.51633 
                 64.15 
               
               
                 6 
                 ∞ 
               
             
          
           
               
                   
                 F N0  = 5.2 
                 Image height = 0.45 
                 f = 0.626 
                 D = 0.594 
               
               
                   
                 D/f = 0.95 
               
               
                   
                   
               
             
          
         
       
     
     Embodiment 2 
     FIG.  2 ( a ) illustrates a second embodiment of the invention. The lens of this embodiment includes a first cover glass C 1 , a first lens element L 1  of positive power, a second lens element L 2  of positive power, and a charge-coupled device (hereinafter CCD) cover glass C 2 . These optical members are inserted in a single frame  3  having one end bored to a larger diameter than the other end. In this embodiment, a square image pick-up surface of an image pick-up element (not shown) is used. As shown in FIG.  2 ( b ), four sloped surfaces A 1  (which correspond to segments ofthe annular ring A 1  illustrated in FIG.  1 ( b )) are visible and a light shield is provided on each of these sloped surfaces. 
     The outer diameter Φ1 of the second lens element L 2  of this embodiment is 1.2 mm, and the diameter Φ2 the part except for the slope (i.e., the maximum clear aperture) is 0.84 mm. Further, Φ2/Φ1 equals 0.7. Moreover, the air equivalent path length D from the on-axis position of the second lens element L 2  to the image plane I is 0.466 mm. The focal distance of the lens is 0.66 mm Therefore, D/f is 0.71. 
     Table 2 lists the surface # in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis surface spacing d (in mm) of each surface, as well as the index of refraction N d  and the Abbe constant V d  (for the d-line) of each optical member of Embodiment 2. In the bottom portion of the table are listed some of the performance values of the lens of this embodiment. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 # 
                 R 
                 d 
                 N d   
                 V d   
               
               
                   
               
             
             
               
                 1 
                 ∞ 
                 0.2000 
                 1.51633 
                 64.15 
               
               
                 2 
                 ∞ 
                 0.6100 
                 1.88300 
                 40.76 
               
               
                 3 
                 −0.8170 
                 0.4100 
               
               
                 4 
                  1.1310 
                 0.3800 
                 1.88300 
                 40.76 
               
               
                 5 
                 ∞ 
                 0.4000 
                 1.51633 
                 64.15 
               
               
                 6 
                 ∞ 
               
             
          
           
               
                   
                 F N0  = 5.5 
                 Image height = 0.45 
                 f = 0.66 
                 D = 0.466 
               
               
                   
                 D/f = 0.71 
               
               
                   
                   
               
             
          
         
       
     
     Embodiment 3 
     FIG. 3 illustrates a third embodiment of the lens of the present invention. In this embodiment the lens includes a cover glass C 1 , a first lens element L 1  of positive power, a second lens element L 2  of positive power, an infrared cut-off filter F, and a CCD cover glass C 2 . The cover glass C 1  and the first lens L 1  of positive power are held by the first frame  1 . The second positive lens element L 2  and the infrared cut-off-filter F are joined and held by the second frame  2 , and C 2  is the CCD cover glass. 
     In this third embodiment, the second positive lens element L 2  and the infrared cut-off filter F are joined to form an integrated part. A slope A 2  is provided on the object side of this optical member, and a light shading means, which may be formed of a light absorbing substance or film, is provided on the slope A 2 . Therefore, the second lens element L 2  does not provide a side for attaching to a support frame  2 . However, if the integrated optical member provides the side, installation is not affected. 
     In this third embodiment, the outer diameter Φ1 of the optical member is 2.85 mm, and the maximum clear aperture diameter Φ2 is 2.28 mm. Therefore, the ratio of Φ2/Φ1 is 0.8. Moreover, the air path equivalent length from the on-axis convex surface with the slope to the image surface is 1.173 mm. The focal-distance ofthe lens is 1.239 mm. Therefore, the ratio D/f is 0.95. 
     Table 3 fists the surface # in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis surface spacing d (in mm) of each surface, as well as the index of refraction N d  and the Abbe constant V d  (for the d-line) of each optical member of Embodiment 3. In the bottom portion of the table are listed some of the performance values of the lens of this embodiment. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 # 
                 R 
                 d 
                 N d   
                 V d   
               
               
                   
               
             
             
               
                 1 
                 ∞ 
                 1.5000 
                 1.88300 
                 40.76 
               
               
                 2 
                 ∞ 
                 1.3875 
                 1.88300 
                 40.76 
               
               
                 3 
                 −1.5367 
                 0.3750 
               
               
                 4 
                  2.5485 
                 0.8325 
                 1.75500 
                 52.32 
               
               
                 5 
                 ∞ 
                 0.7500 
                 1.49400 
                 75.00 
               
               
                 6 
                 ∞ 
                 0.3000 
                 1.52287 
                 59.89 
               
             
          
           
               
                   
                 F N0  = 6.9 
                 Image height = 1.23 
                 f = 1.239 
                 D = 1.173 
               
               
                   
                 D/f = 0.95 
               
               
                   
                   
               
             
          
         
       
     
     Embodiment 4 
     FIG. 4 illustrates a fourth embodiment of the lens of the present invention. This embodiment of the invention is an improvement over the conventional optical system shown in FIG.  9 . The lens of FIG. 4 includes a cover glass C 3 , an aperture stop S, a first lens element L 1  of positive power, and a second lens element L 2  of positive power. The cover glass C 3 , the aperture stop S, and the first lens element L 1  of positive power are held by the first frame  1 . The second lens element L 2  of positive power as well as the first frame  1  are held by the second frame  2 . 
     In this embodiment, a slope is formed on the circumference part of the surface on the object side of the second lens element L 2  of positive power, without employing a flare stop as in the prior art. Black paint or black ink is applied at this slope so as to form a light blocking region or shade. 
     The outer diameter Φ1 of the second lens element L 2  of this embodiment is 0.5 mm, and the maximum clear aperture diameter Φ2 is 0.4 mm. Therefore, Φ2/Φ1=0.8. The air path equivalent length D from the object side of the positive lens element L 2  to the image surface at I is 0.214 mm. The focal distance ofthe lens is 0.32 mm. Therefore, the ratio D/f is 0.67. 
     Table 4 lists the surface # in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis surface spacing d (in mm) of each surface, as well as the index of refraction N d  and the Abbe constant V d  (for the d-line) of each optical member of Embodiment 4. In the bottom portion of the table are listed some of the performance values of the lens of this embodiment. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 # 
                 R 
                 d 
                 N d   
                 V d   
               
               
                   
               
             
             
               
                 1 
                 ∞ 
                 0.30 
                 1.88300 
                 40.8 
               
               
                 2 
                 ∞ 
                 0.03 
               
               
                 3 
                 ∞ 
                 0.37 
                 1.88300 
                 40.8 
               
               
                 4 
                 −0.346 
                 0.15 
               
               
                 5 
                  0.725 
                 0.37 
                 1.72916 
                 54.7 
               
               
                 6 
                 ∞ 
               
             
          
           
               
                   
                 F N0  = 1.75 
                 Image height = 0.24 
                 f = 0.32 
                 D = 0.214 
               
               
                   
                 D/f = 0.67 
               
               
                   
                   
               
             
          
         
       
     
     Embodiment 5 
     FIG. 5 illustrates a fifth embodiment of the lens of the present invention, which applies the invention to the prior art example shown in FIG.  10 . That is, the lens is formed of, in order from the object side: a negative lens element L 1 , a diaphragm S, a first lens element L 2  of positive power and a second lens element L 3  of positive power. A slope is provided on the object-side surface of the second lens element L 3 , and a light shield is again provided in this slope. 
     For this embodiment, the outer diameter Φ1 of the second positive lens element L 3  is 1 mm The maximum clear aperture diameter Φ2 of this lens element is 0.85 mm. Therefore, Φ2/Φ1=0.85. The air path equivalent length D from the optical member with the flare stop to the image surface I is 0.798 mm. Further, the focal distance f of the lens is 0.52 mm Therefore, D/f=1.53. 
     Table 5 lists the surface # in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis surface spacing d (in mm) of each surface, as well as the index of refraction N d  and the Abbe constant V d  (for the d-line) of each optical member of Embodiment 5. In the bottom portion of the table are listed some of the performance values of the lens of this embodiment. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                 # 
                 R 
                 d 
                 N d   
                 V d   
               
               
                   
               
             
             
               
                 1 
                 ∞ 
                 0.32 
                 1.51633 
                 64.1 
               
               
                 2 
                  0.817 
                 0.13 
               
               
                 3 
                 ∞ (aperture stop) 
                 0.03 
                 l.88300 
                 40.8 
               
               
                 4 
                 −1.454 
                 0.58 
                 l.88300 
                 40.8 
               
               
                 5 
                 −0.538 
                 0.06 
               
               
                 6 
                  1.653 
                 0.58 
                 1.88300 
                 40.8 
               
               
                 7 
                 ∞ 
               
             
          
           
               
                   
                 F N0  = 2.2 
                 Image height = 0.45 
                 f = 0.52 
                 D = 0.798 
               
               
                   
                 D/f = 1.53 
               
               
                   
                   
               
             
          
         
       
     
     Embodiment 6 
     FIG. 6 shows the lens element structure of the sixth embodiment of the present invention. In this embodiment, the lens is formed of a first lens element L 1  that is a plano-concave lens, a plane-parallel plate P 1 , a second lens element L 2  that is a biconvex lens, two plane-parallel plates P 2  and P 3 , a third lens element L 3  that is a piano-convex lens, a plane-parallel plate P 4 , and CCD cover glass C. The image-side surface of plane-parallel plate P 4  is attached to the CCD cover glass C and bonded within the frame  2  to form the CCD sub-assembly. A frame I supports lens element L 1  and plane parallel plate P 1 , which are inserted into the object-side end of the frame  1 , which has a larger bore at the object side end The frame  1  also supports the second lens element L 2 , the plane parallel plates P 2 , P 3  as well as the third lens element L 3  at the image side of the frame  1 , within the smaller bore. The optical members illustrated in FIG. 6 that are held by the frame  1  are bonded thereto after being inserted in the appropriate end of frame  1  so as to form another sub-assembly. These two sub-assemblies can be attached to one another as illustrated and bonded to form the lens. 
     The light-rays near the CCD sub-assembly are located far from the optical axis. Therefore, flare is easily generated in this vicinity, and accordingly it is necessary to remove flare near the CCD subassembly. Thus, the present invention can be applied in this vicinity to reduce flare. Moreover, if the plane-parallel plate P 4  is provided with a slope (as illustrated) near its circumferential edge, any flare resulting from off-field rays being reflected by the inside surface of the frame  2  can be removed. If a slope is provided on an attached cover glass or on a lens element flare generated by off-field rays being reflected by the circumferential edge of these elements or the inside surface of the frame  1  can be removed 
     As illustrated, a flare stop FS 1  is arranged on the object side of a plane-parallel plate P 1 . Aperture stop S is arranged on the object side of plane-parallel plate P 2 . A flare stop FS 2  is arranged between the two plane-parallel plates P 2  and P 3 , and a flare stop FS 3  is arranged between plane-parallel plate P 3  and the third lens element L 3 . 
     For this sixth embodiment, a slope is provided at the image-side surface of the third lens element L 3 , i.e., on the surface supported by frame  1  nearest the image-side. A shading means, such as a light absorbing substance, is provided on this slope. In addition, a slope is also provided on the object-side surface of plate P 4 , and a shading means, such as a light absorbing substance, is also provided on this slope. Thus, the present invention is advantageous if a flare stop is to be provided at a location where, using a conventional flare stop formed of an annular diaphragm, it would be difficult to locate the support for the annular diaphragm. Thus the present invention is especially suitable at locations, such as the image-side end of frame  1 , where it would be especially difficult to hold a conventional flare stop. 
     The outer diameter Φ1 of the third lens element L 3 , which is the optical member with the slope in this embodiment, is 1 mm. The maximum clear aperture diameter Φ2 (except for the slope) of this optical member is 0.9 mm. Therefore, Φ2/Φ1=0.9. Moreover, the air path equivalent length D from the image side of the third lens element L 3  to the image plane I (i.e., measured on-axis from surface #14 to surface #17) is 0.819. The focal distance f of the lens is 0.573 mm; thus, the ratio D/f is 1.43. Moreover, the angle θ1 of the slope provided at the image side of the third lens element L 3  is 40 degrees. 
     When the third lens element L 3  is inserted into the frame  1 , the third lens element L 3  is fixed by applying an adhesive to which India ink has been added and mixed-in. The adhesive is applied not only to the circumference edge of the third lens element L 3 , but is also applied to the slope. In this way the adhesive serves as the shading means on the slope. The other optical member having a slope provided for flare reduction is plane parallel plate P 4 . The outer diameter Φ1 of plane-parallel plate P 4  in this embodiment is 1.3 mm, and the maximum clear aperture diameter Φ2 is 1 mm Therefore, the ratio Φ2/Φ1 is 0.77. Moreover, the air path equivalent length D from object side surface of plane-parallel plate P 4  to the image surface I (measured from surface #15 to surface #17) is 0.529 mm. Thus, D/f is 0.92. The angle of the slope θ1 on this optical member is 60 degrees. 
     Table 6 lists the surface # in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis surface spacing d (in mm) of each surface, as well as the index of refraction N d  and the Abbe constant V d  (for the d-fine) of each optical member of Embodiment 6. In the bottom portion of the table are listed some of the performance values of the lens of this embodiment. 
     
       
         
               
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                 # 
                 R 
                 d 
                 N d   
                 V d   
               
               
                   
               
             
             
               
                 1 
                 ∞ 
                 0.2000 
                 1.88300 
                 40.76 
               
               
                 2 
                  0.4270 
                 0.2400 
               
               
                 3 
                 ∞ (flare stop) 
                 0.0300 
               
               
                 4 
                 ∞ 
                 0.4000 
                 1.52287 
                 59.89 
               
               
                 5 
                 ∞ 
                 0.1000 
               
               
                 6 
                  2.4060 
                 0.57 
                 1.90135 
                 31.55 
               
               
                 7 
                 −0.9770 
                 0.1100 
               
               
                 8 
                 ∞ (aperture stop) 
                 0.0300 
               
               
                 9 
                 ∞ 
                 0.4000 
                 1.51400 
                 75.00 
               
               
                 10 
                 ∞ (flare stop) 
                 0.0300 
               
               
                 11 
                 ∞ 
                 0.4000 
                 1.52287 
                 59.89 
               
               
                 12 
                 ∞ (flare stop) 
                 0.0300 
               
               
                 13 
                 ∞ 
                 0.4500 
                 1.88300 
                 40.76 
               
               
                 14 
                 −2.6340 
                 0.2900 
               
               
                 15 
                 ∞ 
                 0.4000 
                 1.51633 
                 64.15 
               
               
                 16 
                 ∞ 
                 0.4000 
                 1.51100 
                 64.14 
               
               
                 17 
                 ∞ 
               
             
          
           
               
                   
                 F N0  = 3.6 
                 Image height = 0.5 
                 f = 0.573 
               
               
                   
                 For L3: 
                 D = 0.819 
                 D/f = 1.43 
               
               
                   
                 For P4: 
                 D = 0.529 
                 D/f = 0.92 
               
               
                   
                   
               
             
          
         
       
     
     Embodiment 7 
     FIG. 7 illustrates the seventh embodiment of the invention. The lens of this embodiment includes a first lens element L 1  formed of a piano-concave lens with its concave surface on the image side, a plane-parallel plate P 1 , a second lens element L 2  that is biconvex, three plane-parallel plates P 2 -P 4 , a third lens element L 3  formed of a piano-convex lens with its convex surface on the object side, and a CCD cover plate C. The third lens element L 3  and the CCD cover glass C are attached to one another. And, the third lens element L 3 , the CCD cover glass C and the CCD array are integrated to form a CCD sub-assembly. An aperture stop S is arranged at the object-side surface of plane-parallel plate P 2 , a flare stop FSI is arranges at the object-side of plane-parallel plate P 1 , and flare stops FS 2  and FS 3  are arranged and pressed between plane-parallel plates P 2 -P 3  and between plates P 3 -P 4 , respectively. 
     In this embodiment, the flare-reducing slope is formed on the image-side surface of the plane-parallel plate P 4  , which is the optical member at the image-side of frame  1 . The outer diameter Φ1 of plane-parallel plate P 4  is 1 mm The maximum clear aperture diameter Φ2 is 0.9 mm. Therefore, Φ2/Φ1 is 0.9. The air path equivalent length D from the optical element to the image plane (measured on-axis from surface #14 to #17) is 0.879 mm. The focal-distance of the lens is 0.564 mm. Therefore, D/f=1.56. In addition, flare reduction can be maximized if a slope is also provided on the object side of the third lens element L 3 . Moreover, the inclination angle of the slope at the image side of plane-parallel plate P 4  in this embodiment is 50 degrees. 
     Table 7 lists the surface # in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis surface spacing d (in mm) of each surface, as well as the index of refraction N d  and the Abbe constant V d  (for the d-line) of each optical member of Embodiment 7. In the bottom portion of the table are listed some of the performance values of the lens of this embodiment. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 7 
               
               
                   
               
               
                 # 
                 R 
                 d 
                 N d   
                 V d   
               
               
                   
               
             
             
               
                 1 
                 ∞ 
                 0.2000 
                 1.88300 
                 40.76 
               
               
                 2 
                  0.4620 
                 0.2100 
               
               
                 3 
                 ∞ (flare stop) 
                 0.0300 
               
               
                 4 
                 ∞ 
                 0.4000 
                 1.51633 
                 64.14 
               
               
                 5 
                 ∞ 
                 0.0900 
               
               
                 6 
                  2.0490 
                 0.5700 
                 1.90135 
                 31.55 
               
               
                 7 
                 −1.0090 
                 0.0200 
               
               
                 8 
                 ∞ (aperture stop.) 
                 0.0300 
               
               
                 9 
                 ∞ 
                 0.4000 
                 1.52287 
                 59.89 
               
               
                 10 
                 ∞ (flare stop) 
                 0.0300 
               
               
                 11 
                 ∞ 
                 0.4000 
                 1.52287 
                 59.89 
               
               
                 12 
                 ∞ (flare stop) 
                 0.0300 
               
               
                 13 
                 ∞ 
                 0.4000 
                 1.51400 
                 75.00 
               
               
                 14 
                 ∞ 
                 0.3700 
               
               
                 15 
                  1.3920 
                 0.4600 
                 1.88300 
                 40.76 
               
               
                 16 
                 ∞ 
                 0.4000 
                 1.51100 
                 64.10 
               
               
                 17 
                 ∞ 
               
             
          
           
               
                   
                 F N0  = 3.5 
                 Image height = 0.5 
                 f = 0.564 
                 D = 0.879 
               
               
                   
                 D/f = 1.56 
               
               
                   
                   
               
             
          
         
       
     
     Embodiment 8 
     FIG.  8 ( a ) illustrates an eighth embodiment of the lens of the present invention. Embodiment 8 includes, in order from the object side, a field transforming optical element Pr, a first lens element L 1  of positive power, aperture stop S, a plane-parallel cover glass P 1  held in a frame  1 , a plane-parallel cover glass P 2 , a second lens element L 2  of positive power, a plane-parallel absorption-type infrared blocking filter P 3  and a CCD cover glass C held in a frame  2 . 
     This embodiment has a structure wherein the front and back of an optical system can be separated between the two plane-parallel cover glasses P 1  and P 2 . An adapter unit is placed before the plane-parallel cover glass P 1  and a master unit is placed after the plane-parallel cover glass P 2 . The adapter unit may be selected in accordance with the intended purposes. 
     The marginal section of a surface of the positive lens element L 2  on the image side is fixed to the frame  2  with adhesives A. The adhesives contain a black, light-absorbing material such as carbon so as to form a light shield. In this embodiment, in lieu of there being an inclined section at the marginal periphery of the positive lens element L 2 , the inclination angle of the lens surface itself at the periphery is made large, so that a groove  4  (FIG.  8 ( b )) is formed between the optical surface and the frame. The light blocking adhesive is then applied to this slot. 
     The positive lens element L 2  of this embodiment is arranged nearest the image side among those lens elements of the lens having curvature. It is preferable to provide light shielding at the lens element arranged closest to the image side. This is because, as the position of the lens element approaches the image side of the lens, the ray height of a principal ray gets larger (i.e, the ray is farther from the optical axis) and thus it is more likely that flare is generated. 
     In this embodiment, the outer diameter Φ1 of the second positive lens element L 2  is 1.4 mm and the radius of curvature R ofthe optical surface on the image side is −1.2362. Thus the right side of Condition (6) is 0.57. The air path equivalent length (D) from the on-axis position of the image side of positive lens element L 2  to the image plane I is 1.34 mm and the focal length ofthe lens is 1.98 mm. Therefore, the ratio D/f equals 0.68. 
     FIG.  8 ( b ) is an expanded view of the lens element L 2  shown in FIG.  8 ( a ). When the angle a between the normal line at the marginal section of an optical surface and the optical axis is larger than 20°, the groove  4  is formed between the optical surface and a frame, so that it will be easy to apply an adhesive to the marginal section without need for forming a separate annular inclined surface near the circumferential edge of the lens, as done for the previous embodiments. Condition (4) is specifies that the angle α exceeds 20°, and the constant 0.34 on the right side of this condition represents the value of sin 20°. In this embodiment, the adhesive is mixed with a black light absorbing material, such as carbon. 
     Moreover, in forming a light shield using adhesive mixtures while fixing a lens element to a frame, it is preferable to form the light shield on the lens element, among those lens elements having curvature, that is arranged in the objective portion of the lens nearest the image side. This is because the ray height (i.e., distance from the optical axis) becomes higher as the lens element position approaches the image side, and thus flare is more likely to occur. Furthermore, in forming a light shield using an adhesive mixture while fixing a lens to a frame, it is preferable to satisfy Condition (3) even when Condition (6) is satisfied and an inclined section is not separately formed at the marginal section of an optical surface. 
     Table 8 lists the surface # in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis surface spacing d (in mm) of each surface, as well as the index of refraction N d  and the Abbe constant V d  (for the d-line) of each optical member of Embodiment 8. In the bottom portion ofthe table are listed some of the performance values of the lens of this embodiment. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 8 
               
               
                   
               
               
                 # 
                 R 
                 d 
                 N d   
                 V d   
               
               
                   
               
             
             
               
                 1 
                 ∞ 
                 1.0000 
                 1.88300 
                 40.76 
               
               
                 2 
                 ∞ 
                 1.0000 
                 1.88300 
                 40.76 
               
               
                 3 
                 ∞ 
                 0.0500 
               
               
                 4 
                  1.5896 
                 1.2382 
                 1.88300 
                 40.76 
               
               
                 5 
                  1.3113 
                 0.0492 
               
               
                 6 
                 ∞ (aperture stop) 
                 0.0300 
               
               
                 7 
                 ∞ 
                 0.3500 
                 1.51633 
                 40.76 
               
               
                 8 
                 ∞ 
                 0.0500 
               
               
                 9 
                 ∞ 
                 0.3500 
                 1.51633 
                 64.14 
               
               
                 10 
                 ∞ 
                 0.0300 
               
               
                 11 
                 ∞ 
                 0.3974 
                 1.88300 
                 64.14 
               
               
                 12 
                 −1.2362 
                 0.4069 
               
               
                 13 
                 ∞ 
                 1.0000 
                 1.49400 
                 75.00 
               
               
                 14 
                 ∞ 
                 0.4000 
                 1.51633 
                 64.14 
               
               
                 15 
                 ∞ 
                 0.0000 
               
             
          
           
               
                   
                 F N0  = 4.7 
                 Image height = 0.9 
                 f = 1.98 
                 D = 1.34 
               
               
                   
                 D/f = 0.68 
               
               
                   
                   
               
             
          
         
       
     
     The invention being thus described, it will be obvious that the same may be varied in many ways. For example, the light shield could be modified to be reflective, since the flare rays would in most instances be reflected to an absorbing surface and thereby be greatly attenuated. Such variations are not to be regarded as a departure from the spirit and scope of the invention. Rather the scope of the invention shall defined as set forth in the following claims and their legal equivalents. A 1 l such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.