Patent Publication Number: US-2023152564-A1

Title: Microscope optical system, microscope device, and image formation lens

Description:
TECHNICAL FIELD 
     The present invention relates to microscope optical systems, microscope devices, and image formation lenses. 
     TECHNICAL BACKGROUND 
     In recent years, there have been proposed various kinds of iz.ge formation lenses for microscopes, adapted to objective lenses having a wide field of view (for example, refer to Patent literature 1). Such image formation lenses are required to have high resolution while keeping a wide field of view. 
     PRIOR ARTS LIST 
     Patent Document 
     Patent literature Japanese Laid-Open Patent Publication No. 2016-75860(A) 
     SUMMARY OF THE INVENTION 
     A microscope optical system according to the present invention comprises: an objective lens that converts light from an object into parallel light; and an image formation lens that forms an image from the light from the objective lens, wherein the image formation lens comprises, in order from the object side, a first lens group including a cemented lens, a second lens group having positive refractive power, and a third lens group having negative refractive power, and the following conditional expression is satisfied: 
       0.1&lt;Φen/f&lt;0.2,
 
     where Φen: the pupil diameter of the objective lens, and 
     f: the focal length of the image formation lens. 
     A microscope device according to the present invention comprises the above the microscope optical system. 
     An image formation lens according to the present invention is an image formation lens for a microscope, the image formation lens forming an image from light from an objective lens, the image formation lens compris.ng, in order from the object side: a first lens group including a cemented lens; a second lens group having positive refractive power; and a third lens group having negative refractive power, wherein the following conditional expression is satisfied: 
       0.1&lt;Φen/f&lt;0.2,
 
     where Φen: the pupil diameter of the objective lens, and 
     f: the focal length of the image formation lens. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic configuration diagram showing a microscope optical system according to the present embodiment; 
         FIG.  2    is a schematic configuration diagram showing a fluorescence microscope which is an example of a microscope device; 
         FIG.  3    is a cross-sectional diagram showing the configuration of an image formation lens according to a first example; 
         FIG.  4    is a diagram showing several kinds of aberration of the image formation lens according to the first example; 
         FIG.  5    is a cross-sectional diagram showing the configuration of an image formation lens according to a second example; 
         FIG.  6    is a diagram showing several kinds of aberration of the image formation lens according to the second example; 
         FIG.  7    is a cross-sectional diagram showing the configuration of an image formation lens according to a third example; 
         FIG.  8    is a diagram showing several kinds of aberration of the image formation lens according to the third example; 
         FIG.  9    is a cross-sectional diagram showing the configuration of an image formation lens according to a fourth example; 
         FIG.  10    is a diagram showing several kinds of aberration of the image formation lens according to the fourth example; 
         FIG.  11    is a cross-sectional diagram showing the configuration of an image formation lens according to a fifth example; 
         FIG.  12    is a diagram showing several kinds of aberration of the image formation lens according to the fifth example; 
         FIG.  13    is a cross-sectional diagram showing the configuration of an image formation lens according to a sixth example; and 
         FIG.  14    is a diagram showing several kinds of aberration of the image formation lens according to the sixth example. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, a microscope optical system, microscope device, and image formation lens of the present embodiment will be described with reference to the figures. The present embodiment describes an image formation lens, microscope optical system, and microscope device having a wide field of view and high resolution. 
     First, a microscope optical system according to the present embodiment will be described. As shown in  FIG.  1   , a microscope optical system MCS according to the present embodiment comprises, in order from the object side, an objective lens OL and an image formation lens IL. The objective lens OL converts light from an object Ob into parallel light. The image formation lens IL collects light from the objective lens OL and forms an image of the object Ob on an image surface Img. The image of this object Ob is observed by the observer&#39; eye Eye through an eyepiece EP. Note that in  FIG.  1   , the object Ob is an object point on the optical axis. The image of the object Ob may be formed not only through the eyepiece EP, but it may be formed again on a second image surface where an image sensor (not shown) is located, for example, by using a relay lens (not shown). 
     Next, the image formation lens of the microscope optical system according to the present embodiment will be described. As an example of the image formation lens IL, an image formation lens IL( 1 ) shown in  FIG.  3    comprises, in order from the object side along the optical axis, a first lens group G 1  including a cemented lens CL 11 , a second lens group G 2  having positive refractive power, and a third lens group G 3  having negative refracture power. Note that the entrance pupil surface Pu of the image formation lens IL( 1 ) corresponds to the exit pupil surface of an infinity-corrected objective lens OL. 
     In the microscope optical system MCS according to the present embodiment, the image formation lens IL satisfies the following conditional expression (1): 
       0.1&lt;Φen/f&lt;0.2   (1),
 
     where Φen: the pupil diameter of the objective lens OL, and 
     f: the focal length of the image formation lens IL. 
     In the present embodiment, by satisfying conditional expression (1), it is possible to provide an image formation lens and microscope optical system having a wide field of view and high resolution. In the microscope optical system MCS according to the present embodiment, the image formation lens IL may be an image formation lens IL( 2 ) shown in  FIG.  5   , an image formation lens IL( 3 ) shown in  FIG.  7   , or an image formation lens IL( 4 ) shown in  FIG.  9   . In the microscope optical system MCS according to the present embodiment, the.mage formation lens IL may also be an image formation lens IL( 5 ) shown in  FIG.  11    or an image formation lens IL( 6 ) shown in  FIG.  13   . In the microscope optical system MCS according to the present embodiment, the configuration of the image formation lens IL may be such that the image formation lens IL consists of, in order from the object side along the optical axis, the first lens group G 1  including the cemented lens CL 11 , the second lens group G 2  having positive refractive power, and the third lens group G 3  having negative refractive power, that the air distance between the first lens group G 1  and the second lens group G 2  is largest in the image formation lens IL, and that the third lens group G 3  consists of one cemented lens CL 31 . 
     Conditional expression (1) defines the relationship between the pupil diameter (exit pupil diameter) of the objective lens OL and the focal length of the image formation lens IL. If the corresponding value of conditional expression (1) is smaller than the lower limit, the components, out of the light from the objective lens OL, having large numerical aperture NA are restricted. Thus, it is difficult to make the numerical aperture NA of the image formation lens IL larger to make it adapted to the objective lens OL having a wide field of view, making it difficult to provide high resolution while keeping a wide field of view. To ensure the effects of the present embodiment, the lower limit of conditional expression (1) may preferably be 0.15. If the corresponding value of conditional expression (1) is larger than the upper limit, the aperture is large, and this increases the amount of aberration, making it difficult to correct aberration to achieve high-resolution images. To ensure the effects of the present embodiment, the upper limit of conditional expression (1) may preferably be 0.18. 
     In the microscope optical system MCS according to the present embodiment, the image formation lens IL may satisfy the following conditional expression (2): 
       35 [mm]&lt;(Φen×Φ1max)/Φim&lt;120 [mm]  (2),
 
     where Φ1max: the effective diameter of the lens having the largest effective diameter in the first lens group G 1 , and 
     Φim: the diameter of the image circle in which the light from the image formation lens IL forms an image. 
     Conditional expression (2) defines the relationship between the pupil diameter of the objective lens OL, the effective diameter of the lens with the largest effective diameter in the first lens group G 1 , and the diameter of the image circle. To provide an image formation lens and microscope optical system having a wide field of view and high resolution, it is necessary to increase the numerical aperture NA of the image formation lens to make it adapted to the objective lens having a wide field of view. satisfying conditional expression (2), it is possible to favorably correct the aberration that occurs as the numerical aperture NA of the image formation lens increases, while keeping a wide field of view. 
     If the corresponding value of conditional expression (2) is smaller than the lower limit, the image formation lens IL forms an image on the image surface Img, using only part of the light from the objective lens OL. Thus, the amount of light in the periphery of the field of view is small, making it difficult to correct the off-axis aberration such as the coma aberration. To ensure the effects of the present embodiment, the lower limit of conditional expression (2) may preferably be 65 [mm]. If the corresponding value of conditional expression (2) is larger than the upper limit, to achieve a wide field of view, the size of the lens needs to be larger, making the production difficult. To ensure the effects of the present embodiment, the upper limit of conditional expression (2) may preferably be 90 [mm]. 
     In the microscope optical system MCS according to the present embodiment, the cemented lens CL 11  of the first lens group G 1  may comprise a first positive lens and a negative lens joined to the first positive lens, the first lens group G 1  or the second lens group G 2  may comprise a second positive lens, and the image formation lens IL may satisfy the following conditional expression (3): 
       vdp2&lt;vdn&lt;vdp1   (3),
 
     where vdp 1 : the Abbe number of the first positive lens, 
     vdp 2 : the Abbe number of the second positive lens, and 
     vdn: the Abbe number of the negative lens. 
     Conditional expression (3) defines the relationship between the dispersion (Abbe number) of the first positive lens and the dispersion (Abbe number) of the negative lens in the cemented lens CL 11  of the first lens group G 1  and the dispersion of the second positive lens located in the first lens group G 1  or the second lens group G 2 . By satisfying conditional expression (3), it is possible to favorably correct the longitudinal chromatic aberration and the second-order chromatic aberration. 
     In the microscope optical system MCS according to the present embodiment, the first positive lens may satisfy the following conditional expression (4): 
       70&lt;vdp1   (4).
 
     Conditional expression (4) defines an appropriate range of the dispersion (Abbe number) of the first positive lens. By satisfying conditional expression (4), it is possible to favorably correct the longitudinal chromatic aberration. If the corresponding value of conditional expression (4) is smaller than the lower limit, it is difficult to correct the longitudinal chromatic aberration. To ensure the effects of the present embodiment, the lower limit of conditional expression (4) may preferably be smaller than 80. 
     In the microscope optical system MCS according to the present embodiment, the second positive lens may satisfy the following conditional expression (5): 
       vdp2&lt;45   (5).
 
     Conditional expression (5) defines an appropriate range of the dispersion (Abbe number) of the second positive lens. By satisfying conditional expression (5), it is possible to favorably correct the second-order chromatic aberration. If the corresponding value of conditional expression (5) is higher than the upper limit, it is difficult to correct the second-order chromatic aberration. To ensure the effects of the present embodiment, the upper limitof conditional expression (5) may preferably be 42. In addition, to ensure the effects of the present embodiment, the lower limit of conditional expression (5) may preferably be larger than 15. 
     In the microscope optical system MCS according to the present embodiment, the image formation lens IL may satisfy the following conditional expression (6): 
       1.0&lt;|f1|/f   (6),
 
     where f 1 : the focal length of the first lens group G 1 . 
     Conditional expression (6) defines the relationship between the focal length of the first lens group G 1  and the focal length of the image formation lens IL, By satisfying conditional expression (6), it is possible to favorably correct the off-axis aberration such as the field curves and the coma aberration. If the corresponding value of conditional expression (6) is smaller than the lower limit, the power of the first lens group G 1  is too high, and this would cause higher-order off-axis aberration. To ensure the effects of the present embodiment, the lower limit of conditional expression (6) may preferably be 1.2. In addition, to ensure the effects of the present embodiment, the upper limit of conditional expression (6) may preferably be smaller than 4.0. 
     In the microscope optical system MCS according to the present embodiment, the image formation lens IL may satisfy the following conditional expression (7): 
       1.0&lt;h2/h1   (7),
 
     where h 1 : the height of a principal ray that enters the first lens group G 1 , and 
     h 2 : the height of the principal ray that enters the second lens group G 2 . 
     Conditional expression (7) defines the relationship between the height of the principal ray that enters the first lens group G 1  and the height of the principal ray that enters the second lens group G 2 . Note that the principal ray is the ray that passes through the center of the entrance pupil of the image formation lens (the exit pupil of the objective lens). By satisfying conditional expression (7), it is possible to favorably correct the chromatic aberration. If the corresponding value of conditional expression (7) is smaller than the lower limit, the effects of correcting chromatic aberration of magnification by the lens on an image side of the second lens group G 2  is small, making it difficult to correct the chromatic aberration. To ensure the effects of the present embodiment, the lower limit of conditional expression (7) may preferably be 1.2. In addition, to ensure the effects of the present embodiment, the upper limit of conditional expression (7) may preferably be smaller than 2.0. 
     In the microscope optical system MCS according to the present embodiment, the image formation lens IL may satisfy the following conditional expression (8): 
       0.7&lt;TL/f   (8),
 
     where TL: the entire length of the image formation lens IL. 
     Conditional expression (8) defines the relationship between the entire length of the image formation lens IL and the focal length of the image formation lens IL. Note that the entire length of the image formation lens IL means the distance on the optical axis from the apex of the lens surface closest to the object in the image formation lens IL to the image surface of the image formation lens IL. By satisfying conditional expression (8), it is possible to favorably correct the off-axis aberration such as the field curves. If the corresponding value of conditional expression (8) is smaller than the lower limit, higher-order off-axis aberration would occur, making it difficult to correct the field curves. To ensure the effects of the present embodiment, the lower limit of conditional expression (8) may preferably be 1.1. In addition, to ensure the effects of the present embodiment, the upper limit of conditional expression (8) may preferably be smaller than 2.0. 
     Next, a microscope device according to the present embodiment will be described. As an example of a microscope device, a fluorescence microscope  100  will be described with reference to  FIG.  2   . The fluorescence microscope  100  comprises a stage  101 , a light source  111 , an illumination optical system  121 , a microscope optical system  131 , an eyepiece  141 , and an imaging device  151 . On the stage  101  is placed, for example, a sample SA held between a microscope slide (not shown) and a cover glass (not shown). The sample SA placed on the stage  101  may be contained together with immersion liquid in a sample container (not shown). The sample SA includes fluorescent substances such as a fluorescent dye. The sample SA is, for example, cells fluorescently stained in advance or the like. 
     The light source  111  generates excitation light in a specified wavelength band. The specified wavelength band is set to a wavelength band that enables excitation of the sample SA including fluorescent substances. The excitation light emitted from the light source  111  enters the illumination optical system  121 . 
     The illumination optical system  121  illuminates the sample SA on the stage  101  with the excitation light emitted from the light source  111 . The illumination optical system  121  comprises a collimator lens  122  and a dichroic mirror  124  in order from the light source  111  side toward the sample SA side. The illumination optical system  121  comprises an objective lens  132  which is also included in the microscope optical system  131 . The collimator lens  122  collimates the excitation light emitted from the light source  111 . 
     The dichroic mirror  124  has characteristics of reflecting the excitation light from the light source  111  and transm..tting the fluorescence from the sample SA. The dichroic mirror  124  reflects the excitation light from the light source  111  toward the sample SA on the stage  101 . The dichroic mirror  124  transmits fluorescence generated at the sample SA toward a mirror  133  of the microscope optical system  131 . Between the dichroic mirror  124  and the collimator lens  122  is arranged an excitation filter  123  that transmits the excitation light from the light source  111 . Between the dichroic mirror  124  and the mirror  133  is arranged a fluorescence filter  125  that transmitsthe fluorescence from the sample SA. 
     The microscope optical system  131  comprises the objective lens  132 , the mirror  133 , a first image formation lens  134 A, and a second image formation lens  134 B. The microscope optical system  131  also comprises the dichroic mirror  124  which is also included in the illumination optical system  121 . The objective lens  132  is located above the stage  101  on which the sample SA is placed so as to face the stage  101 . The objective lens  132  condenses the excitation light from the light source  111  and illuminates the sample SA on the stage  101 . The objective lens  132  receives fluorescence generated on the sample SA and converts it into parallel light. 
     The mirror  133  is, for example, configured using a half mirror having a ratio of transmittance to reflectance set to 1:1. A part of the fluorescence incident on the mirror  133  passes through the mirror  133  and enters the first image formation lens  134 A. The fluorescence having passed through the first image formation lens  134 A forms an image on a first image surface ImgA. The observer can observe an image of the sample SA formed on the first image surface ImgA, using the eyepiece  141 . The other part of the fluorescence incident on the mirror  133  is reflected by the mirror  133  and enters the second image formation lens  134 B. The fluorescence having passed through the second image formation lens  1348  forms an image on a second image surface ImgB. At the second image surface ImgB is located an area sensor  152  of the imaging device  151 . 
     Note that the mirror  133  is not limited to a half mirror but may be configured using an optical-path switching mirror capable of selectively switching the reflection direction of light. In this case, the mirror  133  reflects the fluorescence from the sample SA alternately toward one of the first image formation lens  134 A and the second image formation lens  1348  by switching. 
     The imaging device  151  comprises an image sensor  152 . The image sensor  152  comprises an imaging device such as a CCD or a CMOS. The imaging device  151  is capable of capturing an image of the sample SA formed on the second image surface ImgB by using the image sensor  152 . 
     In the fluorescence microscope  100  thus configured, the excitation light emitted from the light source  111  passes through the collimator lens  122  and becomes parallel light. The excitation light having passed through the collimator lens  122  passes through the excitation filter  123  and becomes incident on the dichroic mirror  124 . The excitation light incident on the dichroic mirror  124  is reflected on the dichroic mirror  124  and passes through the objective lens  132 . The excitation light having passed through the objective lens  132  is projected onto the sample SA on the stage  101 . With this configuration, the illumination optical system  121  illuminates the sample SA on the stage  101  with the excitation light emitted from the light source  111 . 
     The illumination with excitation light excites the fluorescent substances included in the sample SA, and fluorescence is emitted. Fluorescence from the sample SA passes through the objective lens  132  and becomes parallel light. The fluorescence having passed through the objective lens  132  becomes incident on the dichroic mirror  124 . The fluorescence incident on the dichroic mirror  124  passes through the dichroic mirror  124 , passes through the fluorescence filter  125 , and becomes incident on the  33 . 
     Part of the fluorescence incident on the mirror  133  passes through the mirror  133  and enters the first image formation lens  134 A. The fluorescence having passed through the first image formation lens  134 A forms an image on the first image surface ImgA. The other part of the fluorescence incident on the mirror  133  is reflected by the mirror  133  and enters the second image formation lens  134 B. The fluorescence having passed through the second image formation lens  134 B forms an image on the second image surface ImgB. 
     The observer observes an image of the sample SA formed on the first image surface ImgA, using the eyepiece  141 . The imaging device  151  captures an image of the sample SA formed on the second image surface ImgB, using the image sensor  152 . This fluorescence microscope  100  comprises the image formation lens IL of the microscope optical system according to the foregoing embodiment, as the first image formation lens  134 A and the second image formation lens  134 B. This fluorescence microscope  100  also comprises the objective lens OL of the microscope optical system according to the foregoing embodiment, as the objective lens  132 . This makes it possible to provide a microscope device having a wide field of view and high resolution. 
     Note that in the case in which a field of view is wide, and the resolution is high, the amount of information on an image of the sample SA obtained by the imaging device  151  is large. To deal with it, use of a time delay integration (TDI) image sensor for the image sensor  152  makes it possible to obtain an image of the sample SA in a short time. 
     The fluorescence microscope  100  has been described as an example of the microscope device according to the present embodiment, but the present disclosure is not limited to this example. For example, the microscope device according to the present embodiment may be a multiphoton excitation microscope, a light sheet microscope, a phase contrast microscope, a confocal microscope, a super resolution microscope, or the like. The fluorescence microscope  100  is not limited to an upright microscope as shown in  FIG.  2    but may be an inverted microscope. With the present embodiment, as described above, it is possible to build microscope systems having various functions. 
     EXAMPLES 
     Hereinafter, examples of the image formation lens IL in the microscope optical system MOS according to the present embodiment will be described with reference to the drawings. The image formation lens IL according to each example is used in combination with an infinity-corrected objective lens OL to form a magnified image of an object.  FIGS.  3 ,  5 ,  7 ,  9 ,  11 , and  13    are cross-sectional diagrams showing the configurations of the image formation lenses IL (IL( 1 ) to IL( 6 )) according to the first to sixth examples. In these  FIGS.  3 ,  5 ,  7 ,  9 ,  11 , and  13   , each lens group is indicated by a combination of a symbol G and a number (or an alphabet), and each lens is indicated by a combination of a symbol L and a number (or an alphabet). In this case, to avoid cumbersome situations using many kinds of symbols and numbers and using large numbers, combinations of symbols and numbers are used independently in each example to indicate lenses or others. Thus, even if a combination of the same symbol and number are used in some of the examples, it does not mean the same constituent. 
     Below are shown Tables 1 to 6, in which Table 1 shows the specification data on the first example, Table 2 on the second example, Table 3 on the third example, Table 4 on the fourth example, Table 5 on the fifth example, and Table 6 on the sixth example. In each example, to calculate aberration characteristics, d-line (wavelength λ=587.6 nm), g-line (wavelength λ=435.8 nm), C-line (wavelength λ=656.3 nm), and F-line (wavelength λ=486.1 nm) are selected. 
     In the table of [General Data], f represents the focal length of the image formation lens IL. The symbol Φen represents the pupil diameter of the objective lens OL. The symbol Φim represents the diameter of the image circle in which the light from the image formation lens IL forms an image. The symbol Φ1max represents the effective diameter of the lens with the largest effective diameter in the first lens group G 1 . The symbol f 1  represents the focal length of the first lens group G 1 . TL represents the entire length of the image formation lens IL. The symbol h 1  represents the height of the principal ray that enters the first lens group G 1 . The symbol h 2  represents the height of the principal ray that enters the second lens group G 2 . 
     In the table of [Lens Data], the surface number indicates the order of the lens surface from the object side, R indicates the curvature radius corresponding to each surface number (R has a positive value if the lens surface is convex toward the object), D indicates the lens thickness or the air gap on the optical axis, corresponding to each surface number, nd indicates the refractive index of the optical material corresponding to the surface number at &amp;-line (wavelength λ=587.6 nm), and vd indicates the Abbe number of the optical material corresponding to each surface number based on d-line. The symbol “m” in the curvature radius indicates a fat surface or an opening. Mentioning that the refractive index of air nd=1.00000 is omitted. 
     In all the specification values below, the unit of the focal length f, curvature radius R, surface distance D, other lengths, and the like listed is generally “mm” unless otherwise specified. However, the unit is not limited to this one because the same or si lar optical performance can be obtained even if an optical system is proportionally enlarged or proportionally reduced in size. 
     The explanation on the tables up to this point is common in all of the examples, and hence repetitive description will be omitted below. 
     First Example 
     A first example will be described with reference to  FIGS.  3  and  4    and Table 1.  FIG.  3    is a cross-sectional diagram showing the configuration of an image formation lens according to the first example. The image formation lens IL( 1 ) according to the first example comprises, in order from the object side along the optical axis, a first lens group G 1  having positive refractive power, a second lens group G 2  having positive refractive power, and a third lens group G 3  having negative refractive power. 
     The first lens group G 1  comprises a cemented lens CL 11  having, in order from the object side, a biconvex positive lens L 11 , a biconcave negative lens L 12 , and a biconvex positive lens L 13  joined together. The positive lens L 11  of the first lens group G 1  (or the positive lens L 13 ) corresponds to a first positive lens in the present embodiment. The negative lens L 12  of the first lens group G 1  corresponds to a negative lens in the present embodiment. 
     The second lens group G 2  comprises a biconvex positive lens L 21 . The positive lens L 21  of the second lens group G 2  corresponds to a second positive lens in the present embodiment. 
     The third lens group G 3  comprises a cemented lens CL 31  having, in order from the object side, a biconvex positive lens L 31  and a biconcave negative lens L 32  joined together. The image surface Img is located on the image side of the third lens group G 3 . Note that the entrance pupil surface Pu of the image formation lens IL( 1 ) corresponds to the exit pupil surface of the infinity-corrected objective lens OL. 
     The following Table 1 shows the specification values of the image formation lens according to the first example. Note that the surface of surface number  1  indicates the entrance pupil surface of the image formation lens (in other words, the surface corresponding to the exit pupil surface of the objective lens). 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                 [General Data] 
               
               
                   
                   
               
               
                   
                 f = 200.0 
               
               
                   
                 Φen = 35.0 
               
               
                   
                 Φim = 25.0 
               
               
                   
                 Φ1max = 56.5 
               
               
                   
                 f1 = 430.9 
               
               
                   
                 TL = 255.5 
               
               
                   
                 h1 = 10.6 
               
               
                   
                 h2 = 15.4 
               
               
                   
                   
               
            
           
           
               
            
               
                 [Lens Data] 
               
            
           
           
               
               
               
               
               
            
               
                 Surface 
                   
                   
                   
                   
               
               
                 Number 
                 R 
                 D 
                 nd 
                 νd 
               
               
                   
               
               
                 1 
                 ∞ 
                 170.00 
               
               
                 2 
                 143.588 
                 8.80 
                 1.4560 
                 91.36 
               
               
                 3 
                 −94.993 
                 4.00 
                 1.5638 
                 60.71 
               
               
                 4 
                 89.827 
                 7.60 
                 1.4560 
                 91.36 
               
               
                 5 
                 −309.677 
                 114.10 
               
               
                 6 
                 116.697 
                 8.50 
                 1.6477 
                 33.73 
               
               
                 7 
                 −363.426 
                 21.00 
               
               
                 8 
                 56.818 
                 9.30 
                 1.5725 
                 57.30 
               
               
                 9 
                 −208.394 
                 15.20 
                 1.7380 
                 32.33 
               
               
                 10 
                 33.862 
                 67.04 
               
               
                   
               
            
           
         
       
     
       FIG.  4    is a diagram showing several kinds of aberration (spherical aberration, astigmatism, chromatic aberration of magnification, and coma aberration) of an image formation lens according to the first example. In each aberration diagram in  FIG.  4   , represents the pupil diameter (the entrance pupil diameter of the image formation lens, in other words, the exit pupil diameter of the objective lens), and Y represents the image height, and d indicates the aberration at d-line (wavelength λ=587.6 nm), g at g-line (wavelength λ=435.8 nm), C at C-line (wavelength λ=656.3 nm), and F at F-line (wavelength λ=486.1 nm). In the spherical aberration diagram, the vertical axis represents the normalized value with the maximum value of the pupil diameter set to 1, and the horizontal axis represents the aberration value [mm] of each ray. In the astigmatism diagram, a solid line represents the meridional.image surface for each wavelength, and a dashed line represents the sagittal image surface for each wavelength. In the astigmatism diagram, the vertical axis represents the image height [mm], and the horizontal axis represents the aberrat value [mm]. In the diagram of chromatic aberration of magnification, the vertical axis represents the image height [mm], and the horizontal axis represents the aberration value [mm]. The coma aberration diagram shows the aberration value [mm] in the case in which the image height Y is 12.5 mm. Note that the aberration diagrams of each example shown below use the same symbols as those in this example, and hence, repetitive description is omitted. 
     The aberration diagrams show that each aberration is favorably corrected in the image formation lens according to the first example even in the case of a. large pupil diameter (numerical aperture NA), and that thus the image formation lens according to the first example has excellent image-forming performance. 
     Second Example 
     A second example will be described with reference to  FIGS.  5  and  6    and Table 2.  FIG.  5    is a cross-sectional diagram showing the configuration of an image formation lens according to the second example. The image formation lens IL( 2 ) according to the second example comprises, in order from the object side along the optical axis, a first lens group G 1  having positive refractive power, a second lens group G 2  having positive refractive power, and a third lens group G 3  having negative refractive power. 
     The first lens group G 1  comprises, in order from the object side, a cemented lens CL 11  having a biconvex positive lens L 11  and a negative meniscus lens L 12  with the concave surface on the object side, joined together and a biconvex positive lens L 13 . The positive lens L 11  of the first lens group G 1  corresponds to a first positive lens in the present embodiment. The negative meniscus lens L 12  of the first lens group G 1  corresponds to a negative lens in the present embodiment. 
     The second lens group G 2  comprises a biconvex positive lens L 21 . The positive lens L 21  of the second lens group G 2  corresponds to a second positive lens in the present embodiment. 
     The third lens group G 3  comprises a cemented lens CL 31  having, in order from the object side, a biconvex positive lens L 31  and a biconcave negative lens L 32  joined together. The image surface Img is located on the image side of the third lens group G 3 . Note that the entrance pupil surface Pu of the image formation lens IL( 2 ) corresponds to the exit pupil surface of the infinity-corrected objective lens OL. 
     The following Table 2 shows the specification values of the image formation lens according to the second example. Note that the surface of surface number  1  indicates the entrance pupil surface of the image formation lens (in other words, the surface corresponding to the exit pupil surface of the objective lens). 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
            
               
                   
                 [General Data] 
               
               
                   
                   
               
               
                   
                 f = 200.0 
               
               
                   
                 Φen = 35.0 
               
               
                   
                 Φim = 25.0 
               
               
                   
                 Φ1max = 59.4 
               
               
                   
                 f1 = 258.7 
               
               
                   
                 TL = 250.5 
               
               
                   
                 h1 = 8.1 
               
               
                   
                 h2 = 11.5 
               
               
                   
                   
               
            
           
           
               
            
               
                 [Lens Data] 
               
            
           
           
               
               
               
               
               
            
               
                 Surface 
                   
                   
                   
                   
               
               
                 Number 
                 R 
                 D 
                 nd 
                 νd 
               
               
                   
               
               
                 1 
                 ∞ 
                 130.00 
               
               
                 2 
                 1828.100 
                 14.00 
                 1.4978 
                 82.57 
               
               
                 3 
                 −66.700 
                 18.00 
                 1.6700 
                 47.14 
               
               
                 4 
                 −1762.800 
                 14.00 
               
               
                 5 
                 139.800 
                 19.00 
                 1.5691 
                 71.31 
               
               
                 6 
                 −384.900 
                 23.00 
               
               
                 7 
                 219.500 
                 20.00 
                 1.7552 
                 27.57 
               
               
                 8 
                 −438.500 
                 0.50 
               
               
                 9 
                 80.200 
                 19.00 
                 1.4978 
                 82.57 
               
               
                 10 
                 −118.800 
                 20.00 
                 1.6541 
                 39.68 
               
               
                 11 
                 42.700 
                 103.01 
               
               
                   
               
            
           
         
       
     
       FIG.  6    is a diagram showing several kinds of aberration of an image formation lens according to the second example (spherical aberration, astigmatism, chromatic aberration of magnification, and coma aberration). The aberration diagrams show that each aberration is favorably corrected in the image formation lens according to the second example even in the case of a large pupil diameter (numerical aperture NA), and that thus the image for n lens according to the second example has excellent image-forming performance. 
     Third Example 
     A third example will be described with reference to  FIGS.  7  and  8    and Table 3.  FIG.  7    is a cross-sectional diagram showing the configuration of an image formation lens according to the third example. The image formation lens IL( 3 ) according to the third example comprises, in order from the object side along the optical axis, a first lens group G 1  having positive refractive power, a second lens group G 2  having positive refractive power, and a third lens group G 3  having negative refractive power. 
     The first lens group G 1  comprises, in order from the object side, a cemented lens CL 11  having a biconvex positive lens L 11  and a biconcave negative lens L 12  joined together and a biconvex positive lens L 13 . The positive lens L 11  of the first lens group G 1  corresponds to a first positive lens in the present embodiment. The negative lens L 12  of the first lens group G 1  corresponds to a negative lens in the present embodiment. 
     The second lens group G 2  comprises a biconvex positive lens L 21 . The positive lens L 21  of the second lens group G 2  corresponds to a second positive lens in the present embodiment. 
     The third lens group G 3  comprises a cemented lens CL 31  having, in order from the object side, a positive meniscus lens L 31  with the convex surface on the object side and a negative meniscus lens L 32  with the convex surface on the object side, joined together. The.image surface  1 mg is located on the image side of the third lens group G 3 . Note that the entrance pupil surface Pu of the image formation lens IL( 3 ) corresponds to the exit pupil surface of the infinity-corrected objective lens OL. 
     The following Table 3 shows the specification values of the image formation lens according to the third example. Note that the surface of surface number  1  indicates the entrance pupil surface of the image formation lens (in other words, the surface corresponding to the exit pupil surface of the objective lens). 
     
       
         
           
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
            
               
                   
                 [General Data] 
               
               
                   
                   
               
               
                   
                 f = 200.0 
               
               
                   
                 Φen = 35.0 
               
               
                   
                 Φim = 25.0 
               
               
                   
                 Φ1max = 58.6 
               
               
                   
                 f1 = 469.6 
               
               
                   
                 TL = 239.4 
               
               
                   
                 h1 = 10.0 
               
               
                   
                 h2 = 13.5 
               
               
                   
                   
               
            
           
           
               
            
               
                 [Lens Data] 
               
            
           
           
               
               
               
               
               
            
               
                 Surface 
                   
                   
                   
                   
               
               
                 Number 
                 R 
                 D 
                 nd 
                 νd 
               
               
                   
               
               
                 1 
                 ∞ 
                 160.00 
               
               
                 2 
                 119.600 
                 14.00 
                 1.4978 
                 82.57 
               
               
                 3 
                 −87.800 
                 7.00 
                 1.6074 
                 56.74 
               
               
                 4 
                 83.680 
                 9.50 
               
               
                 5 
                 92.190 
                 14.00 
                 1.4978 
                 82.57 
               
               
                 6 
                 −665.190 
                 44.00 
               
               
                 7 
                 227.200 
                 12.00 
                 1.6477 
                 33.73 
               
               
                 8 
                 −235.400 
                 0.50 
               
               
                 9 
                 57.790 
                 19.00 
                 1.4978 
                 82.57 
               
               
                 10 
                 1369.400 
                 18.00 
                 1.6730 
                 38.15 
               
               
                 11 
                 36.760 
                 101.44 
               
               
                   
               
            
           
         
       
     
       FIG.  8    is a diagram showing several kinds of aberration of an image formation lens according to the third example (spherical aberration, astigmatism, chromatic aberration of magnification, and coma aberration). The aberration diagrams show that each aberration is favorably corrected in the image formation lens according to the third example even in the case of a large pupil diameter (numerical aperture NA), and that thus the image formation lens according to the third example has excellent image-forming performance. 
     Fourth Example 
     A fourth example will be described with reference to  FIGS.  9  and  10    and Table 4.  FIG.  9    is a cross-sectional diagram showing the configuration of an image formation lens according to the fourth example. The image formation lens IL( 4 ) according to the fourth example comprises, in order from the object side along the optical axis, a first lens group G 1  having positive refractive power, a second lens group G 2  having positive refractive power, and a third lens group G 3  having negative refractive power. 
     The first lens group G 1  comprises, in order from the object side, a cemented lens CL 11  having a biconvex positive lens L 11  and a biconcave negative lens L 12  joined together and a biconvex positive lens L 13 . The positive lens L 11  of the first lens group G 1  corresponds to a first positive lens in the present embodiment. The negative lens L 12  of the first lens group G 1  corresponds to a negative lens in the present embodiment. 
     The second lens group G 2  comprises a biconvex positive lens L 21 . The positive lens L 21  of the second lens group G 2  corresponds to a second positive lens in the present embodiment. 
     The third lens group G 3  comprises a cemented lens CL 31  having, in order from the object side, a positive meniscus lens L 31  with the convex surface on the object side and a negative meniscus lens L 32  with the convex surface on the object side, joined together. The image surface Img is located on the image side of the third lens group G 3 . Note that the entrance pupil surface Pu of the image formation lens IL( 4 ) corresponds to the exit pupil surface of the infinity-corrected objective lens OL. 
     The following Table 4 shows the specification values of the image formation lens according to the fourth example. Note that the surface of surface number  1  indicates the entrance pupil surface of the image formation lens (in other words, the surface corresponding to the exit pupil surface of the objective lens). 
     
       
         
           
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
             
            
               
                   
                 [General Data] 
               
               
                   
                   
               
               
                   
                 f = 200.0 
               
               
                   
                 Φen = 35.0 
               
               
                   
                 Φim = 25.0 
               
               
                   
                 Φ1max = 60.8 
               
               
                   
                 f1 = 448.0 
               
               
                   
                 TL = 239.4 
               
               
                   
                 h1 = 10.6 
               
               
                   
                 h2 = 14.2 
               
               
                   
                   
               
            
           
           
               
            
               
                 [Lens Data] 
               
            
           
           
               
               
               
               
               
            
               
                 Surface 
                   
                   
                   
                   
               
               
                 Number 
                 R 
                 D 
                 nd 
                 νd 
               
               
                   
               
               
                 1 
                 ∞ 
                 170.00 
               
               
                 2 
                 132.330 
                 10.00 
                 1.4978 
                 82.57 
               
               
                 3 
                 −85.600 
                 8.00 
                 1.6127 
                 58.54 
               
               
                 4 
                 92.390 
                 10.74 
               
               
                 5 
                 101.480 
                 10.00 
                 1.4978 
                 82.57 
               
               
                 6 
                 −402.800 
                 52.20 
               
               
                 7 
                 237.210 
                 7.00 
                 1.6259 
                 35.72 
               
               
                 8 
                 −242.470 
                 0.50 
               
               
                 9 
                 57.060 
                 19.50 
                 1.4978 
                 82.57 
               
               
                 10 
                 554.550 
                 18.00 
                 1.6730 
                 38.15 
               
               
                 11 
                 36.660 
                 103.44 
               
               
                   
               
            
           
         
       
     
       FIG.  10    is a diagram showing several kinds of aberration of an image formation lens according to the fourth example (spherical aberration, astigmatism, chromatic aberration of magnification, and coma aberration).The aberration diagrams show that each aberration is favorably corrected in the image formation lens according to the fourth example even in the case of a large pupil diameter (numerical aperture NA), and that thus the image formation lens according to the fourth example has excellent image-forming performance. 
     Fifth Example 
     A fifth example will be described with reference to  FIGS.  11  and  12    and Table 5.  FIG.  11    is a cross-sectional diagram showing the configuration of an image formation lens according to the fifth example. The image formation lens IL( 5 ) according to the fifth example comprises, in order from the object side along the optical axis, a first lens group G 1  having positive refractive power, a second lens group G 2  having positive refractive power, and a third lens group G 3  having negative refractive power. 
     The first lens group G 1  comprises, in order from the object side, a first cemented lens CL 11  having a biconvex positive lens LII and a biconcave negative lens L 12  joined together and a second cemented lens CL 12  having a negative meniscus lens L 13  with the convex surface on the object side and a biconvex positive lens L 14  joined together. The positive lens L 11  of the first lens group G 1  corresponds to a first positive lens in the present embodiment. The negative lens L 12  of the first lens group G 1  corresponds to a negative lens in the present embodiment. 
     The second lens group G 2  comprises a biconvex positive lens L 21 . The positive lens L 21  of the second lens group G 2  corresponds to a second positive lens in the present embodiment. 
     The third lens group G 3  comprises a cemented lens CL 31  having, in order from the object side, a biconvex positive lens L 31  and a biconcave negative lens L 32  joined together. The image surface Img is located on the image side of the third lens group G 3 . Note that the entrance pupil surface Pu of the image formation lens IL( 5 ) corresponds to the exit pupil surface of the infinity-corrected objective lens OL. 
     The following Table 5 shows the specification values of the image formation lens according to the fifth example. Note that the surface of surface number  1  indicates the entrance pupil surface of the image formation lens (in other words, the surface corresponding to the exit pupil surface of the objective lens). 
     
       
         
           
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
             
            
               
                   
                 [General Data] 
               
               
                   
                   
               
               
                   
                 f = 200.0 
               
               
                   
                 Φen = 35.0 
               
               
                   
                 Φim = 25.0 
               
               
                   
                 Φ1max = 59.3 
               
               
                   
                 f1 = 625.5 
               
               
                   
                 TL = 272.7 
               
               
                   
                 h1 = 10.6 
               
               
                   
                 h2 = 16.1 
               
               
                   
                   
               
            
           
           
               
            
               
                 [Lens Data] 
               
            
           
           
               
               
               
               
               
            
               
                 Surface 
                   
                   
                   
                   
               
               
                 Number 
                 R 
                 D 
                 nd 
                 νd 
               
               
                   
               
               
                 1 
                 ∞ 
                 170.00 
               
               
                 2 
                 231.728 
                 10.00 
                 1.4560 
                 91.36 
               
               
                 3 
                 −87.077 
                 4.00 
                 1.6228 
                 57.10 
               
               
                 4 
                 463.433 
                 4.35 
               
               
                 5 
                 530.860 
                 5.00 
                 1.5168 
                 64.14 
               
               
                 6 
                 72.310 
                 11.00 
                 1.4978 
                 82.57 
               
               
                 7 
                 −231.728 
                 93.00 
               
               
                 8 
                 153.222 
                 10.00 
                 1.6477 
                 33.73 
               
               
                 9 
                 −273.210 
                 20.00 
               
               
                 10 
                 66.741 
                 18.00 
                 1.5891 
                 61.22 
               
               
                 11 
                 −201.935 
                 17.00 
                 1.7380 
                 32.33 
               
               
                 12 
                 38.634 
                 80.39 
               
               
                   
               
            
           
         
       
     
       FIG.  12    is a diagram showing several kinds of aberration of an image formation lens according to the fifth example (spherical aberration, astigmatism, chromatic aberration of magnification, and coma aberration). The aberration diagrams show that each aberration is favorably corrected in the image formation lens according to the fifth example even in the case of a large pupil diameter (numerical aperture NA), and that thus the image formation lens according to the fifth example has excellent image-forming performance. 
     Sixth Example 
     A sixth example will be described with reference to  FIGS.  13  and  14    and Table  6 .  FIG.  13    is a cross-sectional diagram showing the configuration of an image formation lens according to the sixth example. The image formation lens IL( 6 ) according to the sixth example comprises, in order from the object side along the optical axis, a first lens group G 1  having positive refractive power, a second lens group G 2  having positive refractive power, and a third lens group G 3  having negative refractive power. 
     The first lens group G 1  comprises a cemented lens CL 11  having, in order from the object side, a biconvex positive lens L 11 , a biconcave negative lens L 12 , and a biconvex positive lens L 13  joined together. The positive lens L 11  of the first lens group G 1  corresponds to a first positive lens in the present embodiment. The negative lens L 12  of the first lens group G 1  corresponds to a negative lens in the present embodiment. The positive lens L 13  of the first lens group G 1  corresponds to a second positive lens in the present embodiment. 
     The second lens group G 2  comprises a cemented lens CL 21  having, in order from the object side, a positive meniscus lens L 21  with the convex surface on the object side, a negative meniscus lens L 22  with the convex surface on the object side, and a biconvex positive lens L 23 , joined together. 
     The third lens group G 3  comprises a cemented lens CL 31  having, in order from the object side, a negative meniscus lens L 31  with the convex surface on the object side and a negative meniscus lens L 32  with the convex surface on the object side, joined together. The image surface Img is located on the image side of the third lens group G 3 . Note that the entrance pupil surface Pu of the image formation lens IL( 6 ) corresponds to the exit pupil surface of the infinity-corrected objective lens OL. 
     The following Table 6 shows the specification values of the image formation lens according to the sixth example. Note that the surface of surface number  1  indicates the entrance pupil surface of the image formation lens (in other words, the surface corresponding to the exit pupil surface of the objective lens). 
     
       
         
           
               
               
             
               
                   
                 TABLE 6 
               
               
                   
                   
               
             
            
               
                   
                 [General Data] 
               
               
                   
                   
               
               
                   
                 f = 200.0 
               
               
                   
                 Φen = 35.0 
               
               
                   
                 Φim = 25.0 
               
               
                   
                 Φ1max = 57.7 
               
               
                   
                 f1 = 374.0 
               
               
                   
                 TL = 263.9 
               
               
                   
                 h1 = 10.6 
               
               
                   
                 h2 = 15.4 
               
               
                   
                   
               
            
           
           
               
            
               
                 [Lens Data] 
               
            
           
           
               
               
               
               
               
            
               
                 Surface 
                   
                   
                   
                   
               
               
                 Number 
                 R 
                 D 
                 nd 
                 νd 
               
               
                   
               
               
                 1 
                 ∞ 
                 170.00 
               
               
                 2 
                 408.401 
                 7.00 
                 1.4560 
                 91.36 
               
               
                 3 
                 −84.103 
                 2.00 
                 1.6134 
                 44.27 
               
               
                 4 
                 68.057 
                 9.00 
                 1.5750 
                 41.51 
               
               
                 5 
                 −181.483 
                 100.00 
               
               
                 6 
                 98.580 
                 6.00 
                 1.4560 
                 91.36 
               
               
                 7 
                 166.731 
                 5.00 
                 1.6134 
                 44.27 
               
               
                 8 
                 113.691 
                 9.00 
                 1.4560 
                 91.36 
               
               
                 9 
                 −256.750 
                 11.72 
               
               
                 10 
                 64.312 
                 15.00 
                 1.4978 
                 82.57 
               
               
                 11 
                 52.888 
                 15.20 
                 1.7380 
                 32.33 
               
               
                 12 
                 33.548 
                 83.96 
               
               
                   
               
            
           
         
       
     
       FIG.  14    is a diagram showing several kinds of aberration of an image formation lens according to the sixth example (spherical aberration, astigmatism, chromatic aberration of magnification, and coma aberration). The aberration diagrams show that each aberration is favorably corrected in the image formation lens according to the sixth example even in the case of a large pupil diameter (numerical aperture NA), and that thus the image formation lens according to the sixth example has excellent image-forming performance. 
     Next, the table of [Conditional Expression Corresponding Value] is shown below. This table shows the values corresponding to conditional expressions (1) to (8) for all examples (the first to sixth examples) together. 
       0.1&lt;Φen/f&lt;0.2   Conditional Expression (1)
 
       35 [mm]&lt;(Φen×Φ1max)/Φim&lt;120 [mm]  Conditional Expression (2)
 
       vdp2&lt;vdn&lt;vdp1   Conditional Expression (3)
 
       70&lt;vdp1   Conditional Expression (4)
 
       vdp2&lt;45   Conditional Expression (5)
 
       1.0&lt;|f1|/f   Conditional Expression (6)
 
       1.0&lt;h2/h1   Conditional Expression (7)
 
       0.7&lt;TL/f   Conditional Expression (8)
 
     [Conditional Expression Corresponding Value] 
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Conditional 
                 First 
                 Second 
                 Third 
               
               
                   
                 Expression 
                 Example 
                 Example 
                 Example 
               
               
                   
                   
               
               
                   
                 (1) 
                 0.175 
                 0.175 
                 0.175 
               
               
                   
                 (2) 
                 79.10 
                 83.16 
                 82.04 
               
               
                   
                 (3)νdp1 
                 91.36 
                 82.50 
                 82.57 
               
               
                   
                 (3)νdn 
                 60.71 
                 47.14 
                 56.74 
               
               
                   
                 (3)νdp2 
                 33.73 
                 27.57 
                 33.72 
               
               
                   
                 (4) 
                 91.36 
                 82.50 
                 82.57 
               
               
                   
                 (5) 
                 33.73 
                 27.57 
                 33.72 
               
               
                   
                 (6) 
                 2.15 
                 1.29 
                 2.35 
               
               
                   
                 (7) 
                 1.45 
                 1.42 
                 1.35 
               
               
                   
                 (8) 
                 1.28 
                 1.25 
                 1.20 
               
               
                   
                   
               
               
                   
                 Conditional 
                 Fourth 
                 Fifth 
                 Sixth 
               
               
                   
                 Expression 
                 Example 
                 Example 
                 Example 
               
               
                   
                   
               
               
                   
                 (1) 
                 0.175 
                 0.175 
                 0.175 
               
               
                   
                 (2) 
                 85.12 
                 83.02 
                 80.78 
               
               
                   
                 (3)νdp1 
                 82.57 
                 91.36 
                 91.36 
               
               
                   
                 (3)νdn 
                 58.54 
                 57.10 
                 44.27 
               
               
                   
                 (3)νdp2 
                 35.72 
                 33.73 
                 41.51 
               
               
                   
                 (4) 
                 82.57 
                 91.36 
                 91.36 
               
               
                   
                 (5) 
                 35.72 
                 33.73 
                 41.51 
               
               
                   
                 (6) 
                 2.24 
                 3.13 
                 1.87 
               
               
                   
                 (7) 
                 1.34 
                 1.52 
                 1.45 
               
               
                   
                 (8) 
                 1.20 
                 1.36 
                 1.32 
               
               
                   
                   
               
            
           
         
       
     
     With each of the above examples, it is possible to achieve an image formation lens and microscope optical system having a wide field of view and high resolution. 
     Here, the above examples are to show specific examples of the present embodiment, and hence the embodiment is not limited to these examples. 
     Although the first lens group G 1  has positive refractive power in each of the above examples, the configuration is not limited to these examples, but the first lens group G 1  may have negative refractive power. Specifically, since the first lens group G 1  has weaker refractive power than the second lens group G 2  and mainly has a function of correcting chromatic aberration, the first lens group G 1  may have approximately no refractive power or may have weak negative refractive power. 
     EXPLANATION OF NUMERALS AND CHARACTERS 
     G 1  first lens group G 2  second lens group G 3  third lens group