Abstract:
A composition has a durability to a sterilization treatment using vapor under high temperature and pressure and capable of transmitting or absorbing light. The composition comprises a group A including 15 to 23 mol % of lanthanum oxide, 3 to 8 mol % of gadolinium oxide, 3 to 8 mol % of tantalum oxide, and 8 mol % or less of yttrium oxide; a group B including 30 to 45 mol % of boron oxide, 20 mol % or less of silicon oxide, and 20 mol % or less of germanium oxide; zero mol % of alkaline metal oxide or alkaline earth metal oxide; and a group D including 0.3 to 15 mol % of niobium oxide and 3 to 15 mol % of zirconium oxide. A mol % ratio A/B between the group A and group B is 80% or more, and a dissolving-out rate of metal ion of the composition is 0.002×10 −6  mol/hour or less per square centimeter. In case of applying the composition to an endoscope including an observation system and an illuminating system, an optical element disposed at the outer surface of the observation system and an optical element disposed at the outer surface of the illuminating system are formed of the composition.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a composition excellent in durability to a sterilization process and an endoscope optical system using the composition. 
     BACKGROUND OF THE INVENTION 
     The durability to the sterilization process or treatment herein includes a durability under harsh conditions, for example, to so-called autoclave sterilization using saturated vapor under high temperature and pressure at about 140° C. In these years, the autoclave sterilization involving the harsh conditions has been dominant in endoscopes which are required to be rinsed out, disinfected, and sterilized after use. 
     In disinfection treatments using chemicals, even if an ordinary optical glass is used for an endoscope, such a glass has no possibility of suffering from corrosion which leads to deteriorated opacity and light transmittance, because the condition of this treatment is not so harsh. In contrast, the harsh condition of the autoclave sterilization causes undesirable corrosion of the optical glass, resulting in deteriorated opacity and inability to illuminate or observe. On the other hand, no chemicals are used in the autoclave sterilization. Thus, the autoclave sterilization is not harmful to the environment and excellent in economical efficiency. 
     Sapphire has been known as an optical material having an excellent corrosion resistance under any harsh conditions. U.S. Pat. No. 6,038,079 discloses an example of applying sapphire to an optical system of an endoscope. In this patent, sapphire is used in a part or all of lenses for the objective optical system at the front end of the endoscope. 
     While sapphire advantageously has an excellent durability under harsh conditions as described above, it has the following disadvantages. 
     1) The high material cost of sapphire leads to a high product cost. 
     2) The high hardness of sapphire requires a long time for polishing to process it into an optical lens. This is not adequate to mass production. 
     3) The high: melting temperature of sapphire precludes the possibility of applying a postforming operation in its molten state to process it into an optical lens. This disenables sapphire to process into a complex shape such as an aspheric lens. 
     4) Sapphire has only one value for each of refractive index and Abbe number in optical properties thereof. Thus, it is difficult to correct an aberration sufficiently in optical design processes, resulting in deteriorated performance of the optical system. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an improved composition free from the disadvantages of sapphire (in cost, hardness, workability, melting temperature, optical properties, etc) and having a sophisticated durability to a sterilization treatment. 
     It is another object of the present invention to provide an endoscope using the improved composition in an optical system thereof and having an effective structure capable of yielding a sufficient durability to a sterilization treatment. 
     In order to achieve the above object, according to the present invention, there is provided a composition having a durability to a sterilization treatment using vapor under high temperature and pressure and capable of transmitting or absorbing light, the composition comprising: a group A including 15 to 23 mol % of lanthanum oxide, 3 to 8 mol % of gadolinium oxide, 3 to 8 mol % of tantalum oxide, and 8 mol % or less of yttrium oxide; a group B including 30 to 45 mol % of boron oxide, 20 mol % or less of silicon oxide, and 20 mol % or less of germanium oxide; zero mol % of alkaline metal oxide or alkaline earth metal oxide; and a group D including 0.3 to 15 mol % of niobium oxide and 3 to 15 mol % of zirconium oxide, wherein a mol % ratio A/B between the group A and group B is 80% or more, and a dissolving-out rate of metal ion of the composition is 0.002×10 −6  mol/hour or less per square centimeter, wherein in case of applying the composition to an endoscope including an observation system and an illuminating system, an optical element disposed at the outer surface of the observation system and an optical element disposed at the outer surface of the illuminating system are formed of the composition. 
     The objects of the present invention are also achieved by providing a composition having a durability to a sterilization treatment using vapor under high temperature and pressure and capable of transmitting or absorbing light, the composition comprising: a group A including at least either one of lanthanum oxide, gadolinium oxide, tantalum oxide and yttrium oxide, a total mol % of the group A being 25 mol % or more; and a group B including at least either one of boron oxide, silicon oxide and germanium oxide, a total mol % of the group B being 60 mol % or less. 
     The objects of the present invention are also achieved by providing a composition wherein a mol % ratio A/B between the group A and group B is 80% or more. 
     The objects of the present invention are also achieved by providing a composition having a durability to a sterilization treatment using vapor under high temperature and pressure and capable of transmitting or absorbing light, the composition comprising: a group A including at least either one of lanthanum oxide, gadolinium oxide, tantalum oxide and yttrium oxide, a total mol % of the group A being 25 mol % or more; a group B including at least either one of boron oxide, silicon oxide and germanium oxide, a total mol % of the group B being 60 mol % or less; and a group C including 5 mol % or less of alkaline metal oxide or alkaline earth metal oxide. 
     The objects of the present invention are also achieved by providing a composition wherein a mol % ratio A/B between the group A and group B is 80% or more. 
     The objects of the present invention are also achieved by providing a composition having a durability to a sterilization treatment using vapor under high temperature and pressure and capable of transmitting or absorbing light, the composition comprising: a group A including at least either one of lanthanum oxide, gadolinium oxide, tantalum oxide and yttrium oxide, a total mol % of the group A being 25 mol % or more; a group B including at least either one of boron oxide, silicon oxide and germanium oxide, a total mol % of the group B being 60 mol % or less; and zero mol % of alkaline metal oxide or alkaline earth metal oxide. 
     The objects of the present invention are also achieved by providing a composition wherein a mol % ratio A/B between the group A and group B is 80% or more. 
     The objects of the present invention are also achieved by providing a composition having a durability to a sterilization treatment using vapor under high temperature and pressure and capable of transmitting or absorbing light, the composition comprising: a group A including at least either one of lanthanum oxide, gadolinium oxide, tantalum oxide and yttrium oxide, a total mol % of the group A being 25 mol % or more; and a group B including at least boron oxide, a total mol % of the group B being 60 mol % or less, wherein silicon oxide and germanium oxide are not included in the group B. 
     The objects of the present invention are also achieved by providing a composition wherein a mol % ratio A/B between the group A and group B is 80% or more. 
     The objects of the present invention are also achieved by providing a composition having a durability to a sterilization treatment using vapor under high temperature and pressure and capable of transmitting or absorbing light, the composition comprising: a group A including at least either one of lanthanum oxide, gadolinium oxide, tantalum oxide and yttrium oxide, a total mol % of the group A being 25 mol % or more; a group B including at least boron oxide, a total mol % of the group B being 60 mol % or less, wherein silicon oxide and germanium oxide are not included in the group B; and zero mol % of alkaline metal oxide or alkaline earth metal oxide. 
     The objects of the present invention are also achieved by providing a composition wherein a mol % ratio A/B between the group A and group B is 80% or more. 
     The objects of the present invention are also achieved by providing a composition having a durability to a sterilization treatment using vapor under high temperature and pressure and capable of transmitting or absorbing light, the composition comprising: a group A including at least either one of 13 to 35 mol % of lanthanum oxide, 16 mol % or less of gadolinium oxide, 16 mol % or less of tantalum oxide, and 16 mol % or less of yttrium oxide, a total mol % of the group A being 25 mol % or more; a group B including at least either one of 22 to 60 mol % of boron oxide, 25 mol % or less of silicon oxide, and 25 mol % or less of germanium oxide, a total mol % of the group B being 60 mol % or less; a group C including 5 mol % or less of alkaline metal oxide or alkaline earth metal oxide; and a group D including at least either one of 0.3 to 15 mol % of niobium oxide and 3 to 15 mol % of zirconium oxide. 
     The objects of the present invention are also achieved by providing a composition wherein a mol % ratio A/B between the group A and group B is 80% or more. 
     The objects of the present invention are also achieved by providing an endoscope using the composition wherein an optical element disposed at an outer surface of an optical system of the endoscope is formed of the composition. 
     The objects of the present invention are also achieved by providing an endoscope wherein the optical system includes an observation system and an illuminating system, wherein at least either one of an optical element disposed at the outer surface of the observation system and an optical element disposed at the outer surface of the illuminating system is formed of the composition. 
     The objects of the present invention are also achieved by providing an endoscope wherein the optical system includes an observation system and an illuminating system, wherein all of an optical element disposed at the outer surface of the observation system and an optical element disposed at the outer surface of the illuminating system are formed of the composition. 
     The objects of the present invention are also achieved by providing an endoscope using the composition wherein a surface of an optical element disposed at an outer surface of an optical system of the endoscope is formed of the composition. 
     The objects of the present invention are also achieved by providing an endoscope wherein the optical system includes an observation system and an illuminating system, wherein at least either one of a surface of an optical element disposed at the outer surface of the observation system and a surface of an optical element disposed at the outer surface of the illuminating system is formed of the composition. 
     The objects of the present invention are also achieved by providing an endoscope wherein the optical system includes an observation system and an illuminating system, wherein all of a surface of an optical element disposed at the outer surface of the observation system and a surface of an optical element disposed at the outer surface of the illuminating system are formed of the composition. 
     The objects of the present invention are also achieved by providing an endoscope using the composition which includes an objective lens unit for imaging an object on a front end of an insertion section of the endoscope, wherein an optical element located proximal to the object side of the objective lens unit and an optical element located proximal to the image side of the objective lens unit are formed of the composition. 
     The objects of the present invention are also achieved by providing an endoscope wherein all of optical elements of the objective lens unit are formed of the composition. 
     The objects of the present invention are also achieved by providing an endoscope wherein the optical element located proximal to the image side of the objective lens unit is a convex lens. 
     The objects of the present invention are also achieved by providing an endoscope wherein the optical element located proximal to the image side of the objective lens unit is either one of an absorption type infrared cut-off filter and an interference type infrared cut-off filter. 
     The objects of the present invention are also achieved by providing an endoscope using the composition which includes, in order from the object side of the endoscope, an image guide and an ocular lens unit, wherein an optical element located proximal to the image guide side of the objective lens unit and an optical element located proximal to the observation side of the objective lens unit are formed of the composition. 
     The objects of the present invention are also achieved by providing an endoscope wherein all of optical elements of the ocular lens unit are formed of the composition. 
     The objects of the present invention are also achieved by providing an endoscope wherein at least a part of the image guide is formed of the composition. 
     The objects of the present invention are also achieved by providing an endoscope using the composition which includes an image pickup device for imaging an object on a front end of an insertion section of the endoscope, wherein an optical element located proximal to the object side of the image pickup device is formed of the composition. 
     The objects of the present invention are also achieved by providing an endoscope using the composition which includes, in order from the object side at a front end of an insertion section of the endoscope, an illuminating unit and a light guide, wherein an optical element located proximal to the object side of the illuminating unit and an optical element located proximal to the light guide side of the illuminating unit are formed of the composition. 
     The objects of the present invention are also achieved by providing an endoscope wherein all of optical elements of the illuminating unit are formed of the composition. 
     The objects of the present invention are also achieved by providing an endoscope wherein at least a part of the light guide is formed of the composition. 
     The objects of the present invention are also achieved by providing an endoscope using the composition which includes, in order from the light source side of the endoscope, an incident unit and a light guide, wherein an optical element located proximal to the light source side of the incident unit and an optical element located proximal to the light guide side of the incident unit are formed of the composition. 
     The objects of the present invention are also achieved by providing an endoscope wherein all of optical elements of the incident unit are formed of the composition. 
     The objects of the present invention are also achieved by providing an endoscope, wherein the incident unit includes a columnar rod-shaped optical element, wherein the rod-shaped optical element is formed of the composition. 
     The objects of the present invention are also achieved by providing a composition having a durability to a sterilization treatment using vapor under high temperature and pressure and capable of transmitting or absorbing light, wherein a dissolving-out rate of metal ion of the composition is 0.002×10 −6  mol/hour or less per square centimeter. 
     The objects of the present invention are also achieved by providing a composition wherein the dissolving-out rate of metal ion of the composition is 0.0005×10 −6  mol/hour or less per square centimeter. 
     Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fundamental conceptual view showing an objective optical system according to an embodiment of the present invention. 
     FIGS. 2 to  5  show objective optical systems according to other embodiments of the present invention, respectively. 
     FIG.  6 ( a ) is a general view showing an endoscope according to another embodiment of the present invention. 
     FIG.  6 ( b ) shows an illuminating optical system using a negative lens, of the endoscope shown in FIG.  6 ( a ). 
     FIG.  6 ( c ) shows an illuminating optical system using a positive lens, of the endoscope shown in FIG.  6 ( a ). 
     FIG.  6 ( d ) shows an ocular optical system of the endoscope shown in FIG.  6 ( a ). 
     FIG. 7 shows an illuminating objective optical system having an outer surface formed of the composition of the present invention, according to another embodiment of the present invention. 
     FIG. 8 shows an objective optical system having an outer surface formed of the composition of the present invention, according to another embodiment of the present invention. 
     FIG. 9 shows an ocular optical system having an outer surface formed of the composition of the present invention, according to another embodiment of the present invention. 
     FIG. 10 shows an objective lens unit according to another embodiment of the present invention. 
     FIG. 11 shows an objective lens unit composed only of convex lenses, according to another embodiment of the present invention. 
     FIG. 12 shows a retrofocus type of objective lens unit according to another embodiment of the present invention. 
     FIG. 13 shows an objective lens unit in which a convex lens is used as a lens located proximal to the image side of the objective lens unit, according to another embodiment of the present invention. 
     FIGS. 14 and 15 show objective lens units each having an infrared cut-off filter, according to other embodiments of the present invention, respectively. 
     FIG. 16 shows an ocular lens unit according to another embodiment of the present invention. 
     FIG. 17 shows a Ramsden type of ocular lens unit according to another embodiment of the present invention. 
     FIGS. 18 to  20  are end and sectional views showing image guides according to other embodiments of the present invention, respectively. 
     FIG. 21 shows an image guide having an incident end or outgoing end formed of the composition of the present invention, according to another embodiment of the present invention. 
     FIG. 22 shows an image pickup device according to another embodiment of the present invention, in which an optical element located proximal to the object side of the image pickup device is formed of the composition of the present invention. 
     FIG. 23 shows an illuminating unit according to another embodiment of the present invention. 
     FIG. 24 shows an illuminating unit according to another embodiment of the present invention. 
     FIGS. 25 and 26 are end and sectional views showing light guides according to other embodiments of the present invention, respectively. 
     FIG. 27 shows a light guide having an incident end or an outgoing end formed of the composition of the present invention, according to another embodiment of the present invention. 
     FIG. 28 shows a connecting optical system according to another embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Table 1 shows examples 1 to 16 of a composition having durability to a sterilization treatment according to the present invention. In Table 1, each ratio of components is defined by mol % based on 100 mol % of oxides. 
     Raw materials were prepared by oxides, carbonates, nitrates or the like of respective elements, and all of the raw materials were weighed by each given amount to mix with each other in a conventional manner. This mixed powder was fused in a platinum crucible by an electric furnace at 1350° C. to 1450° C. for 1 to 3 hours to homogenize the molten product. Then, the molten product was cooled down to a certain temperature corresponding to a suitable viscosity for casting the molten product. After casting, the distortion of the cast product was eliminated to form an optically homogeneous composition. The obtained composition was processed into an optical lens in a conventional manner (polishing, forming). 
     The optical lens was applied to an endoscope optical system, and subjected to the autoclave sterilization. One cycle of the autoclave sterilization was conducted under a saturated vapor pressure at the temperature 135° C. for 5 minutes, and after conducting a plurality of cycles, roughness on the surface of each composition was observed by an optical microscope. 
     A polished optical glass was subjected to 30 cycles of a sterilization treatment in which one cycle of the sterilization treatment was conducted under a saturated vapor pressure at the temperature 135° C. for 250 minutes. During this treatment, dissolving-out components from the glass were sampled to determine each element and amount of the dissolving-out components by inductively coupled mass spectrometry (ICP-MS). 
     The composition according to the present invention essentially includes B 2 O 3  and La 2 O 3  as primary components. Preferably, the composition includes another substantial component, such as Ta 2 O 5  or Gd 2 O 3  to provide high refractive index and enhance autoclave durability, or ZrO 2  or Nb 2 O 5  to stabilize a range of vitrification. In particular, it is preferable to include B-La-Gd-Ta-Zr-Nb series having a stable range of vitrification or B-Si-La-Gd-Ta-Zr-Nb series composed of the B-La-Gd-Ta-Zr-Nb series and SiO 2  added for stabilizing vitrification. If necessary, another element, such as Y 2 O 3 , GeO 2 , TiO 2  or Al 2 O 3 , may be included. 
     Table 1 shows a result of the autoclave test for each composition of the examples each having a different ratio of a group A to a group B, wherein the group A comprises oxides of rare earth elements consisting of La 2 O 3 , Gd 2 O 3 , Ta 2 O 5  and Y 2 O 3 , and the group B comprises oxides of creating glass network-former.consisting of B 2 O 3 , SiO 2  and GeO 2 . 
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 Example 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
               
               
                   
               
             
          
           
               
                 Refractive index 
                 d-line 
                 1.95 
                 2 
                 1.9 
                 1.95 
                 2.1 
                 1.98 
                 1.9 
                 1.84 
               
               
                 Abbe number 
                 d-line 
                 38 
                 36 
                 35 
                 30 
                 40 
                 40 
                 45 
                 45 
               
               
                 Autoclave durability 
                 (cycle) 
                 2000 
                 2000 
                 2000 
                 2000 
                 3000 
                 2000 
                 2000 
                 1000 
               
               
                 Dissolving-out test 
                 (10-6 mol/cm2 · h) 
                 0.0002 
                 0.0002 
                 0.0002 
                 0.0002 
                 0.0001 
                 0.0002 
                 0.0002 
                 0.0008 
               
               
                 Component 
                 SiO 2  (mol %) 
                   
                   
                 9.6 
                 11.2 
                 4.0 
                 4.0 
                 6.0 
                 8.0 
               
               
                   
                 B 2 O 3   
                 46.9 
                 28.0 
                 39.0 
                 26.6 
                 22.0 
                 35.0 
                 34.5 
                 50.0 
               
               
                   
                 GeO 2   
               
               
                   
                 La 2 O 3   
                 25.0 
                 31.0 
                 25.0 
                 27.2 
                 34.9 
                 27.0 
                 25.0 
                 23.0 
               
               
                   
                 Ta 2 O 5   
                 11.0 
                 16.0 
                 7.3 
                 11.4 
                 16.0 
                 11.4 
                 8.3 
                 4.0 
               
               
                   
                 Gd 2 O 3   
                 8.0 
                 14.0 
                 8.5 
                 11.7 
                 14.0 
                 11.5 
                 9.0 
               
               
                   
                 Y 2 O 3   
                   
                   
                   
                   
                   
                   
                   
                 4.3 
               
               
                   
                 WO 3   
               
               
                   
                 TiO 2   
                   
                   
                 4.0 
                 5.0 
               
               
                   
                 ZrO 2   
                 7.0 
                 8.9 
                 3.9 
                 4.2 
                 7.0 
                 9.0 
                 14.5 
                 8.0 
               
               
                   
                 Nb 2 O 5   
                 1.9 
                 1.9 
                 2.5 
                 2.5 
                 1.9 
                 1.9 
                 2.5 
                 2.5 
               
               
                   
                 Al 2 O 3   
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
               
               
                   
                 ZnO 
               
               
                   
                 BaO 
               
               
                   
                 Li2O 
               
               
                   
                 Sb 2 O 3   
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
               
               
                 Total 
                 (mol %) 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
               
               
                 Group A 
                 (mol %) 
                 44.0 
                 61.0 
                 40.8 
                 50.3 
                 64.9 
                 49.9 
                 42.3 
                 31.3 
               
               
                 Group B 
                 (mol %) 
                 46.9 
                 28.0 
                 48.6 
                 37.8 
                 26.0 
                 39.0 
                 40.5 
                 58.0 
               
               
                 A/B 
                   
                 94% 
                 218% 
                 84% 
                 133% 
                 250% 
                 128% 
                 104% 
                 54% 
               
               
                   
               
             
          
           
               
                 Example 
                 9 
                 10 
                 11 
                 12 
                 13 
                 14 
                 15 
                 16 
               
               
                   
               
             
          
           
               
                 Refractive index 
                 d-line 
                 1.88 
                 1.82 
                 1.83 
                 2.01 
                 1.75 
                 1.8 
                 2.1 
                 1.94 
               
               
                 Abbe number 
                 d-line 
                 40 
                 47 
                 37 
                 28 
                 50 
                 35 
                 25 
                 39 
               
               
                 Autoclave durability 
                 (cycle) 
                 1500 
                 1000 
                 500 
                 1000 
                 250 
                 500 
                 1000 
                 500 
               
               
                 Dissolving-out test 
                 (10-6 mol/cm2 · h) 
                 0.0005 
                 0.0008 
                 0.0013 
                 0.0008 
                 0.002 
                 0.0013 
                 0.0008 
                 0.0013 
               
               
                 Component 
                 SiO 2 (mol %) 
                 18.0 
                 17.0 
                 12.5 
                 4.3 
                 22.0 
                 12.0 
                 3.2 
               
               
                   
                 B 2 O 3   
                 32.0 
                 41.0 
                 35.0 
                 25.0 
                 37.0 
                 35.0 
                 21.2 
                 46.5 
               
               
                   
                 GeO 2   
                   
                   
                   
                 20.4 
                   
                   
                 25.0 
               
               
                   
                 La 2 O 3   
                 22.1 
                 15.0 
                 13.4 
                 19.5 
                 14.0 
                 13.0 
                 19.5 
                 22.6 
               
               
                   
                 Ta 2 O 5   
                 11.0 
                 3.5 
                   
                 4.1 
                 6.0 
                 1.0 
                 5.0 
                 11.0 
               
               
                   
                 Gd 2 O 3   
                 8.0 
                 14.0 
                   
                   
                   
                   
                   
                 9.3 
               
               
                   
                 Y 2 O 3   
                   
                   
                   
                   
                 6.0 
               
               
                   
                 WO 3   
                   
                   
                 1.7 
                   
                   
                 2.0 
               
               
                   
                 TiO 2   
                   
                   
                   
                 4.1 
                   
                   
                 6.0 
               
               
                   
                 ZrO 2   
                 8.0 
                 8.5 
                 5.2 
                 7.8 
                 3.5 
                 4.8 
                 7.3 
                 5.3 
               
               
                   
                 Nb 2 O 5   
                 0.7 
                 0.8 
                 7.4 
                 14.5 
                 0.3 
                 3.0 
                 12.5 
                 1.9 
               
               
                   
                 Al 2 O 3   
                 0.1 
                 0.1 
                 0.1 
                 0.2 
                 0.1 
                 0.1 
                 0.2 
                 0.1 
               
               
                   
                 ZnO 
                   
                   
                 21.1 
                   
                 4.0 
                 26.0 
               
               
                   
                 BaO 
                   
                   
                 2.8 
                   
                 3.0 
               
               
                   
                 Li2O 
                   
                   
                 0.7 
                   
                 4.0 
                 3.0 
                   
                 3.2 
               
               
                   
                 Sb 2 O 3   
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
               
               
                 Total 
                 (mol %) 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
               
               
                 Group A 
                 (mol %) 
                 41.1 
                 32.5 
                 13.4 
                 23.6 
                 26.0 
                 14.0 
                 24.5 
                 42.9 
               
               
                 Group B 
                 (mol %) 
                 50.0 
                 58.0 
                 47.5 
                 49.7 
                 59.0 
                 47.0 
                 49.4 
                 46.5 
               
               
                 A/B 
                   
                 82% 
                 56% 
                 28% 
                 47% 
                 44% 
                 30% 
                 50% 
                 92% 
               
               
                   
               
             
          
         
       
     
     As the result of analyzing the dissolving-out components from each composition, the following points have been come into focus. For a glass including alkaline metal element and alkaline earth metal element, these components are liable to dissolve out due to their property apt to be ionized. For a glass including no alkaline metal element and no alkaline earth metal element, Si, Ge and B serving as glass network-former is liable to dissolve out, and rare earth elements such as La and other elements such as Zr do not substantially dissolve out. Among the glass network-former, Si and Ge dissolve out in preference to B. 
     Based on the above knowledge, it has been concluded that a particular composition including no alkaline glass and no alkaline earth glass, and including a larger amount or ratio of rare earth components and a less amount or ratio of glass network-formers, particularly silicon (Si) and germanium (Ge), would provide an excellent autoclave durability. 
     For the composition including no alkaline metal element, the autoclave durability of each composition can be assured in the range of about 1000 cycles when the ratio A/B is about 50%, in the range of about 1500 cycles when the ratio A/B is 80% or more, and in the range of about 2000 cycles when the ratio A/B is 100% or more. Thus, the autoclave durability is enhanced as the ratio A/B is increased. In a practical standpoint, an adequate autoclave durability can be obtained from the ratio A/B of 80% (0.8) or more. 
     A high refractive index of the composition may be obtained by increasing the amount or ratio of rare earth elements, or using GeO 2  as a substitute for B 2 O 3 . However, in view of the above result showing that the glass network-formers are hydrolyzed by the vapor, the former approach capable of reducing the ratio of the glass network-formers is more effective. 
     Other components will be described which may be included in the composition in addition to the primary components. Alkaline element oxides such as Li 2 O, Na 2 O, K 2 O and Cs 2 O, or alkaline earth element oxides such as BaO, CaO and SrO significantly deteriorate the durability to the sterilization treatment under high temperature and pressure. Thus, it is preferable to mix these components as less as possible. However, considering that it is necessary to facilitate the vitrification and these oxides are included in the raw materials as inevitable impurities, it is practically required to control the amount of these components in a suitable range not to neutralize the effects of the present invention. 
     The autoclave durability is determined by and varied depending on the plurality of factors as described above. However, as shown in the examples 11, 14 and 16, when alkaline and alkaline earth metals are included by about 3 mol %, the autoclave durability is deteriorated down to about 500 cycles, and when included by about 6 mol %, the autoclave durability is deteriorated down to about 250 cycles. Thus, the amount of alkaline metal or alkaline earth metal oxides may be 5 mol % or less, preferably 3 mol % or less, more preferably not be included. 
     As long as having no adverse effect against the present invention, it is apparent that any other elements other than those shown in the above examples may be added as raw materials. For example, Sb 2 O 3  serving as clarificant for glass, WO 3  or TeO 2  as a component yielding high refractive index, or fluorine having an effect of improving chemical properties or transmission factor may be included. However, it is desirable not to use PbO or ThO 2  as a component yielding high refractive index, or As 2 O 3  serving as clarificant, because these oxides have significant adverse effects on the environment and on the biocompatibility. 
     FIG. 1 shows an objective optical system according to a first embodiment of the present invention. According to a fundamental concept of the present invention, an objective cover glass  1  disposed, on the object side of the objective optical system and exposed to the outside air is formed of the composition having the improved durability to the sterilization treatment according to the present invention (hereinafter referred to as “the composition having the improved sterilization durability”). 
     FIGS. 2 to  5  show objective optical systems according to a second to fifth embodiments of the present invention, respectively, and the dimensional data of these objective optical systems are shown in Tables 2 to 5. In Tables 2 to 5, “S” indicates a number of each lens surface, “R” indicating a curvature radius of each lens surface, “D” indicating a thickness of or distance between each lens, “Nd” indicating a refractive index of each lens, and “Vd” indicating an Abbe number of each lens. In all of the above embodiments, a lens  2  located proximal to the object side of the objective optical system and exposed to the outside air is formed of the composition having the improved sterilization durability. These embodiments have differences in types of optical system, respectively, and may be selected depending on the product specification. 
     The types of optical system will be described as follows. In the following description, “−” (minus) indicates a lens having a negative power, and “+” (plus) indicates a lens having a positive power. The term “lens” herein includes a single lens, a combination of plural single lens, and a cemented lens. For the combination of plural single lens or the cemented lens, the term “power” means a combined or resultant power. An aperture diaphragm is indicated by the symbol “S”, an infrared cut-off filter being indicated by the symbol “F”, and a cover glass being indicated by the symbol “C”. 
     As shown in FIG. 2, the objective optical system of the second embodiment comprises, in order from the object side, −, F, +, S, F, +, C. An objective cover glass  3  located proximal to the object side of the objective optical system is closely contacted with an image pickup device unit. It is herein defined that this objective cover glass  3  is one of components of the image pickup device unit, and not included in components of the objective optical system. 
     As shown in FIG. 3, the objective optical system of the third embodiment comprises, in order from the object side, −, F, +, S, +, C. As shown in FIG. 4, the objective optical system of the fourth embodiment comprises, in order from the object side, −, F, +, S, F, +, +. As shown in FIG. 5, the objective optical system of the fifth embodiment comprises, in order from the object side, −, F, +, F, S, +, +, C. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Focal Length: 0.57631 F number: 4.992 
               
               
                 Field of View: 112.2 degree 
               
             
          
           
               
                 S 
                 R 
                 D 
                 Nd 
                 Vd 
                 Comment 
               
               
                   
               
             
          
           
               
                 Object 
                 INF 
                 8 
                 1 
                   
                   
               
               
                  1 
                 INF 
                 0.2 
                 1.9 
                 35 
                 Autoclavable Optical 
               
               
                   
                   
                   
                   
                   
                 Glass 
               
               
                  2 
                 0.4939 
                 0.2477 
                 1 
               
               
                  3 
                 INF 
                 0.5 
                 1.494 
                 75 
                 IR Cut Filter with 
               
               
                   
                   
                   
                   
                   
                 Laser cut Multi-Layer 
               
               
                   
                   
                   
                   
                   
                 Coating 
               
               
                  4 
                 INF 
                 0.03 
                 1 
               
               
                  5 
                 1.8448 
                 0.9078 
                 1.816 
                 46.62 
               
               
                  6 
                 −1.1932  
                 0.03 
                 1 
               
               
                  7 
                 INF 
                 0.06 
                 1 
                   
                 Aperture Stop 
               
               
                  8 
                 INF 
                 0.5 
                 1.494 
                 75 
                 IR Cut Filter with 
               
               
                   
                   
                   
                   
                   
                 Laser cut Multi-Layer 
               
               
                   
                   
                   
                   
                   
                 Coating 
               
               
                  9 
                 INF 
                 0.03 
                 1 
               
               
                 10 
                 1.0192 
                 0.6014 
                 1.51633 
                 64.14 
               
               
                 11 
                 −0.6831  
                 0.2 
                 1.92286 
                 18.9 
               
               
                 12 
                 −1.9523  
                 0.2131 
                 1 
               
               
                 13 
                 INF 
                 0.4 
                 1.51633 
                 64.14 
                 Cover Glass (anti-dust) 
               
               
                 14 
                 INF 
                 0.01 
                 1.51 
                 64.14 
                 Optical Cement 
               
               
                 15 
                 INF 
                 0.4 
                 1.52 
                 64.14 
                 CCD Cover Glass 
               
               
                 16 
                 INF 
                 0.01 
                 1.56 
                 64.14 
                 Optical Cement 
               
               
                 17 
                 INF 
                 0.019 
                 1 
               
               
                 Image 
                 INF 
                 0 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Forcal Length: 0.60956 F number: 3.338 
               
               
                 Field of View: 110.3 degree 
               
             
          
           
               
                 S 
                 R 
                 D 
                 Nd 
                 Vd 
                 Comment 
               
               
                   
               
             
          
           
               
                 Object 
                 INF 
                 12 
                 1 
                   
                   
               
               
                  1 
                 INF 
                 0.2 
                 1.95 
                 30 
                 Autoclavable Optical 
               
               
                   
                   
                   
                   
                   
                 Glass 
               
               
                  2 
                  0.4918 
                 0.284 
                 1 
               
               
                  3 
                 INF 
                 0.62 
                 1.514 
                 75 
                 IR Cut Filter 
               
               
                  4 
                 INF 
                 0.067 
                 1 
               
               
                  5 
                  1.6264 
                 0.8858 
                 1.7725 
                 49.6 
               
               
                  6 
                 −1.2395 
                 0.03 
                 1 
               
               
                  7 
                 INF 
                 0.06 
                 1 
                   
                 Aperture Stop 
               
               
                  8 
                  1.1794 
                 0.7424 
                 1.51633 
                 64.14 
               
               
                  9 
                 −0.7151 
                 0.2064 
                 1.84666 
                 23.78 
               
               
                 10 
                 −6.7749 
                 0.3808 
                 1 
               
               
                 11 
                 INF 
                 0.4 
                 1.51633 
                 64.14 
                 Covere Glass 
               
               
                   
                   
                   
                   
                   
                 (anti-dust) 
               
               
                 12 
                 INF 
                 0.01 
                 1.51 
                 64.14 
                 Optical Cement 
               
               
                 13 
                 INF 
                 0.4 
                 1.52 
                 64.14 
                 CCD Cover Glass 
               
               
                 14 
                 INF 
                 0.034 
                 1 
               
               
                 Image 
                 INF 
                 0 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Focal Length: 0.95018 F number: 4.190 
               
               
                 Field of View: 132.4 degree 
               
             
          
           
               
                 S 
                 R 
                 D 
                 Nd 
                 Vd 
                 Comment 
               
               
                   
               
             
          
           
               
                 Object 
                 INF 
                 14.3 
                 1 
                   
                   
               
               
                  1 
                 INF 
                 0.3 
                 1.95 
                 30 
                 Autoclavable Optical 
               
               
                   
                   
                   
                   
                   
                 Glass 
               
               
                  2 
                 0.7412 
                 0.41 
                 1 
               
               
                  3 
                 INF 
                 0.4 
                 1.52287 
                 59.89 
                 Laser cut filter 
               
               
                  4 
                 INF 
                 0. 0589 
                 1 
               
               
                  5 
                 3.163  
                 1.356 
                 1.8061 
                 40.92 
               
               
                  6 
                 −1.662  
                 0.2545 
                 1 
               
               
                  7 
                 INF 
                 0.03 
                 1 
                   
                 Aperture Stop 
               
               
                  8 
                 INF 
                 1 
                 1.494 
                 75 
                 IR Cut Filter 
               
               
                  9 
                 INF 
                 0.0687 
                 1 
               
               
                 10 
                 2.2215 
                 1.16 
                 1.72916 
                 54.68 
               
               
                 11 
                 −1.232  
                 0.2187 
                 1.92286 
                 18.9 
               
               
                 12 
                 INF 
                 0.4787 
                 1 
               
               
                 13 
                 2.9114 
                 0.8 
                 1.51633 
                 64.14 
               
               
                 14 
                 INF 
                 0.01 
                 1.51 
                 63 
                 Optical Cement 
               
               
                 15 
                 INF 
                 0.4 
                 1.6109 
                 50.2 
                 CCD Cover Glass 
               
               
                 16 
                 INF 
                 0.01 
                 1.52 
                 63 
                 Optical Cement 
               
               
                 17 
                 INF 
                 0.02 
                 1 
               
               
                 Image 
                 INF 
                 0 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Focal Length: 1.16211 F number: 12.523 
               
               
                 Field of View: 114.7 degree 
               
             
          
           
               
                 S 
                 R 
                 D 
                 Nd 
                 Vd 
                 Comment 
               
               
                   
               
             
          
           
               
                 Object 
                 INF 
                 13 
                 1 
                   
                   
               
               
                  1 
                 INF 
                 0.34 
                 1.9 
                 35 
                 Autoclavable Optical 
               
               
                   
                   
                   
                   
                   
                 Glass 
               
               
                  2 
                  0.874 
                 0.33 
                 1 
               
               
                  3 
                 INF 
                 0.31 
                 1.514 
                 75 
                 IR Cut Filter with 
               
               
                   
                   
                   
                   
                   
                 Laser cut Multi-Layer 
               
               
                   
                   
                   
                   
                   
                 Coating 
               
               
                  4 
                 INF 
                 0.09 
                 1 
               
               
                  5 
                  1.392 
                 0.68 
                 1.84666 
                 23.78 
               
               
                  6 
                 INF 
                 0.03 
                 1 
               
               
                  7 
                 INF 
                 0.31 
                 1.514 
                 75 
                 IR Cut Filter with 
               
               
                   
                   
                   
                   
                   
                 Laser cut Multi-Layer 
               
               
                   
                   
                   
                   
                   
                 Coating 
               
               
                  8 
                 INF 
                 0.03 
                 1 
                   
                 Aperture Stop 
               
               
                  9 
                 INF 
                 0.45 
                 1.72916 
                 54.68 
               
               
                 10 
                 −2.942 
                 0.12 
                 1 
               
               
                 11 
                 INF 
                 0.65 
                 1.51633 
                 64.14 
               
               
                 12 
                 −1.581 
                 0.45 
                 1 
               
               
                 13 
                 INF 
                 1.15 
                 1.51633 
                 64.14 
                 Cover Glass (anti-dust) 
               
               
                 14 
                 INF 
                 0.01 
                 1.51 
                 64.1 
                 Optical Cement 
               
               
                 15 
                 INF 
                 0.8 
                 1.52 
                 64.1 
                 CCD Cover Glass 
               
               
                 16 
                 INF 
                 0.01 
                 1.56 
                 64.1 
                 Optical Cement 
               
               
                 17 
                 INF 
                 0 
                 1 
               
               
                 Image 
                 INF 
                 0 
               
               
                   
               
             
          
         
       
     
     FIGS.  6 ( a ) to ( d ) show a sixth embodiment. FIG.  6 ( a ) is a general view showing an endoscope provided with an objective optical system  10  and an illuminating optical system  20  at a front end of an insertion section thereof. The insertion section has an image transmitting system  11  (i.e. signal transmitting means using a relay lens, an image guide fiber, or image pickup device) built-in. When the image transmitting system  11  is composed of a hard lens or a fiberscope using the relay lens or the image guide fiber, an ocular optical system  12  is additionally provided. Further, a connecting optical system is provided to connect a light source  30  to a light guide  21  for guiding light from the light source  30  to the illuminating optical system  20 . All or a part of optical systems exposed to the outside air in the objective optical system  10 , the illuminating optical system  20 , the ocular optical system  12  and the connecting optical system  22  are formed of the composition having the improved sterilization durability. Thus, even if the whole of the endoscope is subjected to the sterilization treatment, a sufficient durability can be assured. 
     Each component will be described in detail. The details of the objective optical system  10  are the same as those described in conjunction with the first to fifth embodiments. 
     The details of the illuminating optical system  20  are shown in FIGS.  6 ( b ) and ( c ) in which FIGS.  6 ( b ) shows an illuminating optical system  200  constructed by a negative lens and FIG.  6 ( c ) shows an illuminating optical system  201  constructed by a positive lens. Each lens in FIGS.  6 ( b ) and ( c ) may have a roughened surface or may include a light-diffusing portion to improve light distribution. 
     The details of the ocular optical system  12  are shown in FIGS.  6 ( d ). The ocular optical system  12  is composed of one modification of the Gaussian ocular lens system in which an image transmitted through the image guide fiber  11  is magnified to observe the image. A cover glass  120  located proximal to the observation side of the ocular optical system is formed of the composition having the improved sterilization durability. 
     FIGS. 7 to  9  show seventh to ninth embodiments. In these embodiments, all or a part of the outer surface of the illuminating optical system  20 , the objective optical system  10  and the ocular optical system  12  are formed of the composition having the improved sterilization durability. In this case, the composition having the improved sterilization durability is preferably formed in a thin film. Thus, even if the whole of the endoscope is subjected to the sterilization treatment, a sufficient durability can be assured. In the seventh embodiment shown in FIG. 7, the composition  202  having the improved sterilization durability is provided on the outer surface of the illuminating optical system  20 . In the eighth embodiment shown in FIG. 8, the composition  100  having the improved sterilization durability is provided on the outer surface of the objective optical system  10 . In the ninth embodiment shown in FIG. 9, the composition  121  having the improved sterilization durability is provided on the outer surface of the ocular optical system  12 . 
     FIG. 10 shows another objective optical system according to a tenth embodiment. When it is difficult to assure the gas and pressure tightness of the overall endoscope, in order to yield the durability to the autoclave sterilization only to an objective lens unit, an optical element  101  located proximal to the object side of the objective lens unit and an optical element  102  located proximal to the image side of the objective lens unit are formed of the composition having the improved sterilization durability. 
     FIG. 11 shows an eleventh embodiment. As shown in FIG. 11, this embodiment comprises, in order from the object side, a cover glass having parallel surfaces and a pair of convex lenses. An objective lens unit is composed only of the pair of convex lenses. In this embodiment, all of the above optical elements are formed of the composition having the improved sterilization durability. An aperture diaphragm is provided between the cover glass having parallel surfaces and one of the convex lenses located proximal to the object side of the objective lens unit. When the aperture diaphragm has no sterilization durability, it is effective to seal the aperture diaphragm by the cover glass and the one of the convex lenses. 
     FIG. 12 shows a twelfth embodiment. This embodiment is a so-called retrofocus type of objective lens unit consisting of a concave lens and a pair of convex lenses. In this embodiment, all of the above optical elements are formed of the composition having the improved sterilization durability. 
     FIG. 13 shows a thirteenth embodiment. This embodiment is an objective lens unit including a convex lens serving as an optical element  104  located proximal to the image side of the objective lens unit. An optical element  103  located proximal to the object side of the objective lens unit and the optical element  104  located proximal to the image side of the objective lens unit are formed of the composition having the improved sterilization durability. 
     FIGS. 14 and 15 show fourteenth and fifteenth embodiments, respectively. In the fourteenth embodiment, an optical element  105  located proximal to the object side of the objective lens unit and an optical element  106  located proximal to the image side of the objective lens unit are formed of the composition having the improved sterilization durability. The optical element  106  located proximal to the image side of the objective lens unit is composed of an absorption type infrared cut-off filter or interference type infrared cut-off filter. The fifteenth embodiment is different in lens type from the fourteenth embodiment. In the fifteenth embodiment, an optical element  107  located proximal to the object side of the objective lens unit and an optical element  108  located proximal to the image side of the objective lens unit are formed of the composition having the improved sterilization durability. The optical elements  106  and  108  of the fourteenth and fifteenth embodiments located proximal to the image side of the objective lens unit may be commonly used as a dust-protection cover glass disposed in front of a CCD cover glass of the image pickup device unit. 
     FIG. 16 shows an ocular optical system according to a sixteenth embodiment. When it is difficult to assure the gas and pressure tightness of the overall endoscope, in order to yield the durability to the autoclave sterilization only to an ocular lens unit, the following stricture is employed. That is, the sixteenth embodiment comprises, in order from the object side, an image guide  11  and the ocular lens unit. An optical element  121  located proximal to the image guide side of the ocular lens unit and an optical element  122  located proximal to the observation side of the ocular lens unit are formed of the composition having the improved sterilization durability. 
     FIG. 17 shows another ocular optical system according to a seventeenth embodiment. In the seventeenth embodiment, all of optical elements are formed of the composition having the improved sterilization durability. This ocular lens unit is a Ramsden type of ocular lens unit which has a simplest structure without any bonded surface. Thus, this embodiment may be effectively used when an applied adhesive has insufficient sterilization durability. However, in the present invention, the lens unit is not limited to a particular type. 
     FIGS. 18 to  20  show various image guides according to eighteenth to twentieth embodiments, respectively. A part of each image guide of these embodiments is formed of the composition having the improved sterilization durability. Specifically, in the eighteenth embodiment, a clad  110  of the image guide is formed of the composition having the improved sterilization durability. In the nineteenth embodiment, a core  111  of the image guide is formed of the composition having the improved sterilization durability. In the twentieth embodiment, the outside section  112  of the clad  110  is formed of the composition having the improved sterilization durability. 
     FIG. 21 shows a twenty-first embodiment. In this embodiment, an incident or outgoing end element  113  of an image guide is formed of the composition having the improved sterilization durability. 
     FIG. 22 shows a twenty-second embodiment. This embodiment includes an image pickup device for imaging an object on the front end of the insertion section of the endoscope. An optical element  109  located proximal to the object side of the image pickup device is formed of the composition having the improved sterilization durability. 
     FIG. 23 shows a twenty-third embodiment. This embodiment includes an illuminating unit and a light guide, and an optical element  203  located proximal to the object side of the illuminating unit and an optical element  204  located proximal to the light guide side of the illuminating unit are formed of,the composition having the improved sterilization durability. 
     FIG. 24 shows a twenty-fourth embodiment. This embodiment includes an illuminating unit and a light guide, and all of optical elements of the illuminating unit are formed of the composition having the improved sterilization durability. 
     FIGS. 25 and 26 shows twenty-fifth and twenty-sixth embodiments, respectively. In these embodiments, a part of each light guide is formed of the composition having the improved sterilization durability. Specifically, in the twenty-fifth embodiment, a clad  210  of the light guide is formed of the composition having the improved sterilization durability. In the twenty-sixth embodiment, a core  211  of the light guide is formed of the composition having the improved sterilization durability. 
     FIG. 27 shows a twenty-seventh embodiment. In this embodiment, an incident or outgoing end element  212  of a light guide is formed of the composition having the improved sterilization durability. 
     FIG. 28 shows a twenty-eighth embodiment. This embodiment includes a light source and a connecting optical system  22 . The connecting optical system  22  is composed of an incident unit adjacent to a light guide  21 , and an optical element  220  located proximal to the light source side of the incident unit or an optical element  221  located proximal to the light guide side of the incident unit is formed of the composition having the improved sterilization durability. Otherwise, all of the optical elements of the incident unit may be formed of the composition having the improved sterilization durability. Further, the optical element  221  of the incident unit may be a columnar rod-shaped optical element formed of the composition having the improved sterilization durability. 
     The invention has now been explained with reference to specific embodiments. Other embodiments will be apparent to those of ordinary skill in the art. Therefore, it is not intended that the invention be limited, except as indicated by the appended claims, which form a part of this invention description.