Abstract:
A microscope objective that is achromatic, so that deviations in focus do not arise with visible light and ultraviolet light (particularly light in the vicinity of wavelength=266 nm), and which can satisfactorily observe a specimen. The microscope objective is made of at least three types of glass materials, of which one type is barium fluoride.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a microscope objective, and more particularly relates to a microscope objective that uses visible light and ultraviolet light. 
     2. Description of the Related Art 
     In recent years, specimens have not only been observed by illumination with visible light in the wavelength region of approximately 400 to 750 nm, but they have also been observed and processed by illumination with light of short wavelengths, such as ultraviolet light. 
     In viewing specimens through a microscope it is important to correct chromatic aberration simultaneously for both visible light and ultraviolet light in an objective of a microscope that observes a specimen using visible light and ultraviolet light in combination. This correction is called achromatism. However, there is a limited number of glass materials available that satisfy the achromatism condition for visible light and ultraviolet light. Consequently, even though a so-called parfocal length is increased to achieve achromatism, there is a problem in that chromatic aberration cannot be sufficiently corrected. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to overcome the above problems, and an object to provide a microscope objective that is achromatic, so that deviations in focus do not arise with visible light and ultraviolet light (particularly light in the vicinity of wavelength 266 nm), and which can satisfactorily observe a specimen. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the invention will become apparent and more readily appreciated for the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
     FIG. 1 illustrates the lens configuration of the first embodiment for carrying out the present invention. 
     FIGS. 2A to  2 D illustrates aberration plots of the lens according to the first embodiment for carrying out the present invention. 
     FIG. 3 illustrates the lens configuration of the second embodiment for carrying out the present invention. 
     FIGS. 4A to  4 D illustrates aberration plots of the lens according to the second embodiment for carrying out the present invention. 
     FIG. 5 illustrates the configuration of the imaging lens. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In accordance with the preferred embodiments of the invention there is provided a microscope objective solving the above mentioned problems by providing a microscope objective made of at least three types of glass materials, of which one type is barium fluoride. 
     Furthermore, the present invention is preferably a microscope objective, includes, objectwise to imagewise: first lens group G 1  having a cemented lens and having positive refractive power; second lens group G 2  having a cemented lens and having positive refractive power; third lens group G 3  having a cemented lens group and having negative refractive power; fourth lens group G 4  having positive refractive power; and fifth lens group G 5  having negative refractive power. 
     In addition, the present invention is preferably a microscope objective that satisfies each of the following conditions: (1) 1.5&lt;|f1/F|&lt;5; (2) 5&lt;|f3/F|&lt;15; (3) 3&lt;|f4/F|&lt;45; (4) 1&lt;|f5/F|&lt;10; and (5) (W×h)/(l×F)&gt;0.015), wherein, F is the focal length of the entire microscope objective system, f1 is the focal length of the first lens group, f3 is the focal length of the third lens group, f4 is the focal length of the fourth lens group, f5 is the focal length of the fifth lens group, W is the working distance of the microscope objective, h is the maximum height of the principal ray that enters the lens most objectwise, and l is the parfocal length. 
     The microscope objective according to preferred embodiments of the present invention uses glass materials having a high transmittance with respect to a visible light wavelength of approximately 400 to 750 nm, and ultraviolet light wavelength of approximately 266 nm. In addition, at least three types of glass materials are used in order to achieve achromatism with respect to both lights, one type of glass material among these being barium fluoride (BaF 2 ). Furthermore, since barium fluoride is highly deliquescent, it is preferable to use a lens that does not contact air inside the cemented lens. Examples of preferred glass materials besides barium fluoride include quartz, fluorite and lithium fluoride. 
     The aforementioned condition (1) specifies the condition for reducing secondary dispersion while maintaining an appropriate relationship between the numerical aperture and the working distance. If |f1/F| exceeds the upper limit in condition (1), it adversely degrades the balance of secondary dispersion over the entire microscope objective system. Conversely, if |f1/F| falls below the lower limit in condition (1), first lens group G 1  exceeds the limit in which chromatic aberration can be corrected; consequently, the shape of the other lens groups G 2  to G 5  changes, and chromatic aberration cannot be satisfactorily corrected. Furthermore, even better results maybe obtained by setting the upper limit to 3.2. 
     Condition (2) specifies an appropriate range for the focal length of third lens group G 3 . If |f3/F| exceeds the upper limit in condition (2), secondary dispersion increases, the effect of using a glass material having a high refractive index in third lens group G 3  is lost, and chromatic aberration cannot be sufficiently corrected. Conversely, if |f3/F| falls below the lower limit in condition (2), aberrations can no longer be corrected by fifth lens group G 5  as one group, since the secondary dispersion, which includes the refractive index differential of third lens group G 3 , adversely increases in a reverse direction compared to when |f3/F| exceeds the upper limit. 
     Condition (3) specifies an appropriate range for the focal length of fourth lens group G 4 . If |f4/F| exceeds the upper limit in condition (3), secondary dispersion increases, the effect of using a glass material having a high refractive index in fourth lens group G 4  is lost and chromatic aberration cannot be sufficiently corrected. Conversely, if |f4/F| falls below the lower limit in condition (3), secondary dispersion, including the refractive index differential of fourth lens group G 4 , exceeds the capability to be reduced, and therefore aberrations cannot be corrected by the subsequent lens group. 
     Condition (4) specifies an appropriate range for the focal length of fifth lens group G 5 , and is a condition for arranging the balance of achromatism of the entire microscope objective system. If |f5/F| exceeds the upper limit in condition (4), chromatic aberration is undercorrected. Conversely, if |f5/F| falls below the lower limit in condition (4), chromatic aberration is overcorrected. 
     Condition (5) describes the relationship between the parfocal length, N.A. and the working distance. The interchangeability of an objective with other objectives in a microscope is improved by being parfocal. By increasing the parfocal length, the working distance can also be lengthened. If (W×h)/(l×F) falls below the lower limit in condition (5), the working distance or N.A. is reduced. 
     In addition, embodiments of the present invention are preferably directed toward a microscope objective, wherein at least one cemented lens in third lens group G 3  and in fourth lens group G 4  is a cemented lens that has, objectwise to imagewise, a first lens element, second lens element L 31  or L 41 , and a third lens element. 
     In addition, the present invention preferably satisfies each of the following conditions: (6) 1.45&lt;n; and (7) 70&lt;v&lt;90, wherein, n is the refractive index with respect to the d-line (X=587.56 nm) of the second lens element L 31  or L 41 , and v is the Abbe number. 
     Condition (6) specifies an appropriate range for the refractive index of second lens element L 31  or L 41 , for reducing secondary dispersion. If n falls below the lower limit in condition (6), the refractive index differential at the cemented surface of the cemented lens unfortunately decreases, and secondary dispersion cannot be reduced. 
     Condition (7) specifies an appropriate range for the Abbe number of second lens element L 31  or L 41 , also for reducing secondary dispersion. If v falls below the lower limit or if v exceeds the upper limit in condition (7), chromatic aberration cannot be sufficiently corrected. 
     Table 1 and Table 2 list the specifications of the microscope objective according to embodiments of the present invention. In both tables, f is the focal length, N.A. is the numerical aperture, B is the magnification, and W.D. is the working distance. In addition, the Surface No. is representative of the objectwise order of the lens surface, r is the radius of curvature, d is the lens surface spacing, vd is the Abbe number, and nd is the refractive index with respect to the d-line (λ=587.56 nm). 
     FIGS. 2A to  2 D and FIGS. 4A to  4 D illustrate aberration plots of the microscope objective according to embodiments of the present invention. In the spherical aberration plots, C is the C-line (λ=656.28 nm), D is the d-line (λ=587.56 nm), F is the F-line (λ=486.13 nm), and 266 is the aberration with respect to a ray having a wavelength of λ=266 nm. In addition, the astigmatism plots, coma plots and distortion plots are with respect to the d-line; in the astigmatism plots, the solid line is the sagittal image plane, and the broken line is the meridional image plane. 
     FIG. 1 illustrates the configuration of the microscope objective according to a first embodiment of the present invention. Therein, at least one cemented lens in third lens group G 3  is a cemented lens including, objectwise to imagewise, a first lens element, lens element L 31  (barium fluoride) and a third lens element. Furthermore, at e cemented lens in fourth lens group G 4  is a cemented lens including, se to imagewise, a first lens element, second lens element L 41  (barium and a third lens element. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Surface No. 
                 r 
                 d 
                 vd 
                 nd 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                  1 
                 −24 
                 0.9 
                 67.8 
                 1.45850 
                 G1 
                   
               
               
                  2 
                 15 
                 3.3 
                 95.2 
                 1.43385 
               
               
                  3 
                 −7.4 
                 0.2 
               
               
                  4 
                 −100 
                 2.4 
                 95.2 
                 1.43385 
               
               
                  5 
                 −9.5 
                 0.2 
               
               
                  6 
                 −124.5 
                 0.9 
                 67.8 
                 1.45850 
                 G2 
               
               
                  7 
                 9.8 
                 3.7 
                 95.2 
                 1.43385 
               
               
                  8 
                 −16.8 
                 0.8 
               
               
                  9 
                 4.0 
                 2.9 
                 95.2 
                 1.43385 
               
               
                 10 
                 −10 
                 1 
                 67.8 
                 1.45850 
               
               
                 11 
                 9.7 
                 3.5 
                 95.2 
                 1.43385 
               
               
                 12 
                 −15 
                 0.7 
               
               
                 13 
                 −12 
                 1 
                 67.8 
                 1.45850 
                 G3 
               
               
                 14 
                 10.5 
                 2.2 
                 95.2 
                 1.43385 
               
               
                 15 
                 100 
                 0.2 
               
               
                 16 
                 33 
                 2.7 
                 95.2 
                 1.43385 
               
               
                 17 
                 −9.8 
                 1.4 
                 81.6 
                 1.47460 
                   
                 L31 
               
               
                 18 
                 10.5 
                 2.5 
                 95.2 
                 1.43385 
               
               
                 19 
                 −41.3 
                 0.2 
               
               
                 20 
                 14 
                 2.6 
                 95.2 
                 1.43385 
                 G4 
               
               
                 21 
                 −14.8 
                 1.9 
                 81.6 
                 1.47460 
                   
                 L41 
               
               
                 22 
                 7 
                 2 
                 67.8 
                 1.45850 
               
               
                 23 
                 20 
                 16.7 
               
               
                 24 
                 900 
                 1 
                 95.2 
                 1.43385 
                 G5 
               
               
                 25 
                 4.1 
                 2.8 
                 67.8 
                 1.45850 
               
               
                 26 
                 −4.1 
                 1 
                 95.2 
                 1.43385 
               
               
                 27 
                 7.2 
               
               
                   
               
               
                 f = 4 mm  
               
               
                 N. A. = 0.5  
               
               
                 B = −50X  
               
               
                 W. d = 5 mm  
               
               
                 Values Corresponding to Conditions (1)-(7)  
               
               
                 (1) f1/F = 3.0  
               
               
                 (2) f3/F = 9.2  
               
               
                 (3) f4/F = 40.1  
               
               
                 (4) f5/F = 5.4  
               
               
                 (5) (W × h)/(I × F) = 0.06  
               
               
                 (6) n = 1.47460  
               
               
                 (7) v = 81.6  
               
             
          
         
       
     
     FIGS. 2A to  2 D shows the various aberrations of the microscope objective according to the first embodiment of the present invention. As is clear from the drawings, the various aberrations are satisfactorily corrected. 
     FIG. 3 shows the configuration of the microscope objective according to a second embodiment of the present invention. Therein, at least one cemented lens in third lens group G 3  is a cemented lens having, objectwise to imagewise, a first lens element, second lens element L 31  (barium fluoride) and a third lens element. Furthermore, at least one cemented lens in fourth lens group G 4  is a cemented lens having, objectwise to imagewise, a first lens element, second lens element L 41  (barium fluoride) and a third lens element. Table 2 lists the various specifications of the microscope objective according to the second embodiment. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Surface No. 
                 r 
                 d 
                 vd 
                 nd 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                  1 
                 −64.8 
                 1.2 
                 67.8 
                 1.45850 
                 G1 
                   
               
               
                  2 
                 9.1 
                 3.3 
                 95.2 
                 1.43385 
               
               
                  3 
                 −6.6 
                 0.2 
               
               
                  4 
                 −150.8 
                 2.5 
                 95.2 
                 1.43385 
               
               
                  5 
                 −9.8 
                 0.2 
               
               
                  6 
                 45.1 
                 1 
                 67.8 
                 1.45850 
                 G2 
               
               
                  7 
                 7.6 
                 4.5 
                 95.2 
                 1.43385 
               
               
                  8 
                 −16.1 
                 0.2 
               
               
                  9 
                 24.2 
                 3.5 
                 95.2 
                 1.43385 
                 G3 
               
               
                 10 
                 −10.9 
                 1 
                 81.6 
                 1.47460 
                   
                 L31 
               
               
                 11 
                 7.6 
                 3.4 
                 95.2 
                 1.43385 
               
               
                 12 
                 −17.0 
                 0.7 
               
               
                 13 
                 −8.4 
                 1 
                 67.8 
                 1.45850 
               
               
                 14 
                 6.4 
                 2.5 
                 95.2 
                 1.43385 
               
               
                 15 
                 80.2 
                 0.2 
               
               
                 16 
                 8.9 
                 2.8 
                 95.2 
                 1.43385 
                 G4 
               
               
                 17 
                 −9.3 
                 1.0 
                 81.6 
                 1.47460 
                   
                 L41 
               
               
                 18 
                 6.9 
                 2.7 
                 95.2 
                 1.43385 
               
               
                 19 
                 −43.2 
                 14.8 
               
               
                 20 
                 −15.7 
                 1 
                 95.2 
                 1.43385 
                 G5 
               
               
                 21 
                 −18.8 
                 2 
                 67.8 
                 1.45850 
               
               
                 22 
                 −11.1 
                 0.2 
               
               
                 23 
                 −31.8 
                 1.5 
                 67.8 
                 1.45850 
               
               
                 24 
                 9.8 
                 1 
                 95.2 
                 1.43385 
               
               
                 25 
                 9 
               
               
                   
               
               
                 f = 4 mm  
               
               
                 N. A. = 0.5  
               
               
                 B = −50X  
               
               
                 W. D = 5 mm  
               
               
                 Values Corresponding to Conditions (1)-(7)  
               
               
                 (1) f1/F = 2.5  
               
               
                 (2) f3/F = 10.0  
               
               
                 (3) f4/F = 7.9  
               
               
                 (4) f5/F = 5.5  
               
               
                 (5) (W × h)/(I × F) = 0.06  
               
               
                 (6) n = 1.47460  
               
               
                 (7) v = 81.6  
               
             
          
         
       
     
     FIGS. 4A to  4 D illustrates the various aberrations of the microscope objective according to the second embodiment. As is clear from the drawings, the various aberrations are satisfactorily corrected. 
     In addition, the microscope objective according to another embodiment is an infinity-corrected lens, and is used in combination with the imaging lens described by the specifications in Table 3 below, with a spacing of approximately 150 mm. In addition, FIG. 5 shows the configuration of the imaging lens. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Surface No. 
                 r 
                 d 
                 vd 
                 nd 
               
               
                   
               
             
             
               
                 1 
                 −30.63 
                 2.0 
                 67.8 
                 1.4585 
               
               
                 2 
                 2406.20 
                 5.0 
                 95.2 
                 1.4339 
               
               
                 3 
                 −39.06 
                 1.0 
               
               
                 4 
                 −417.39 
                 5.0 
                 95.2 
                 1.4339 
               
               
                 5 
                 −51.92 
               
               
                   
               
             
          
         
       
     
     As explained above, the microscope objective of the present invention has a magnification on the order of 50×, is sufficiently corrected for chromatic aberration with respect to visible light and light in the vicinity of a wavelength of 266 nm and has a sufficiently long working distance W.D. with respect to numerical aperture N.A.; more particularly, a semiachromatic microscope objective can be provided. 
     Glossary of Symbols 
     G 1 : First lens group 
     G 2 : Second lens group 
     G 3 : Third lens group 
     G 4 : Fourth lens group 
     G 5 : Fifth lens group 
     L 31 : Second lens element of the cemented lens of third lens group G 3   
     L 41 : Second lens element of the cemented lens of fourth lens group G 4