Abstract:
An imaging lens having three lens groups is designed so that image aberrations of an object at infinity (i.e, a collimated beam) and of an object at a finite distance are very well-corrected so that the imaging lens is particularly suitable for use imaging interference patterns formed on a screen by a grazing incidence interferometer. The imaging lens is composed of the following lens components, in successive order from the most object side: a first lens component having an overall meniscus shape with its concave surface on the object side, a second lens component having negative or positive refractive power, and a third lens component of positive or negative refractive power. The refractive power of the second lens component and the third lens component are always of opposite sign, and specified conditions are satisfied in order to assure high quality imaging. Each lens component may be formed of a single lens element or of multiple lens elements.

Description:
BACKGROUND OF THE INVENTION 
     Grazing incidence interferometric devices which can measure the surface contours of a surface of interest by conveying a coherent light beam at grazing incidence to the surface are well-known. FIG. 15 illustrates a typical configuration of these prior art grazing incidence interferometric devices. Such a device conveys collimated, coherent light to a diffractive beam splitter  102  that divides the wave front into two light beams. One of the two light beams, termed the “object beam”, is then conveyed at grazing incidence to a surface  2   a  and the light reflected therefrom is combined with the collimated light (termed the reference beam) which has not been reflected from the surface  2   a . Diffractive beam combiner  104  redirects the reference beam so as to be combined with the object beam and travel along a common axis. Imaging lens  106  (in this case the lens of a television camera  108 ) then forms an interference pattern image of the surface  2   a  that is recorded by the television camera  108 . The surface contours of the surface  2   a  can then be measured based on the recorded interference pattern image. 
     However, the lengths of the optical paths from each location on the surface  2   a  to the interference pattern image formed by imaging lens  106  differ. Thus, there is a problem in that distortions are formed in the interference pattern image recorded by the television camera  108 . Hence, the surface contours of surface  2   a  cannot be accurately measured to as high a precision as would otherwise be possible. 
     As a partial solution to this problem, a grazing incidence interferometric device as shown in FIG. 16 is known that avoids distortions from being formed in the interference pattern image by positioning an interference pattern observation screen  110  so that its surface lies at the conjugate image of surface  2   a . As is apparent from the spacing of the components in FIG. 16, the lens  106  and  112  form an optical system that relays the image of object  2   a  at unit magnification to interference pattern observation screen  110 . As a result of the orientation of the interference pattern observation screen  110  now being conjugate to the object (i.e., the surface  2   a  to be measured), an interference pattern image that more accurately represents the surface  2   a  is formed on interference pattern observation screen  110 . 
     The conjugate image arrangement with unit magnification is achieved as shown in FIG. 16, wherein imaging lens  106  is arranged with its focal point at a mid-point of the surface  2   a , collimator lens  112  is arranged with its focal point at the image of this mid-point as formed by the imaging lens  106 , and the interference pattern observation screen  110  is provided with its mid-point at the focal point of lens  112 . Further, the focal distance of the lens  112  is made equal to the focal distance of lens  106 , and the interference pattern observation screen is oriented so that its surface is aligned with the conjugate points of the surface  21  as imaged by lens  106  and lens  112 . 
     An alternative prior art arrangement is shown in prior art FIG. 17, wherein a reflecting mirror  114  is arranged at the second focal point of the imaging lens  106  for conveying a bundle of rays of the interference pattern in the reverse direction. A beam splitter  116  is provided between the imaging lens  106  and the diffractive beam combiner  104  to reflect the rays from the reflecting mirror  114  to an interference pattern observation screen  110  that is, once again, provided to have its surface coincide with the conjugate image of unit magnification of the surface  2   a . Once again, interference pattern images formed on interference pattern observation screen  110  are viewed by the television camera  108 . In this manner the total length of the interferometric device may be prevented from becoming too long. 
     In the above-described grazing incidence interferometric devices shown in FIGS. 16 and 17, the surface  2   a  and the observation screen  110  are provided at the conjugate positions where the magnification is 1. In FIG. 16, lens  106  and lens  112  form a lens system, and object  2   a  and observation screen  110  are positioned with their mid-points a focal length away from this lens system so that unit magnification is achieved. Similarly, in FIG. 17, lens  106  and mirror  114  form an optical system, with object  2   a  and observation screen  110  again positioned in respective paths with their mid-points positioned a focal length away from the lens system so that unit magnification is achieved. In this way, the television camera  108  records the interference pattern formed by the image of object (surface  2   a ) and the collimated light from the reference beam, assuming the difference in path length of the object and reference beams does not exceed the coherence length of the light. Thus, instead of using collimator lens  112  as in FIG. 16 to form a unit magnification image of the surface  2   a  onto observation screen  110 , in FIG. 17 the mirror  114  is positioned at the focus of lens  106  to redirect the light backwards through lens  106  to beam splitter  116 . The unit magnification image of the surface  2   a  is thus formed on surface  110 , and recorded by television camera  108 . 
     The imaging requirements of such an imaging lens  106  are unique and two-fold. First, the lens  106  must generate only very small aberrations when imaging an object at infinity (i.e., the collimated light of the reference beam). Second, the lens  106  must generate only very small aberrations when imaging surface  2   a  at unit magnification onto surface  110 . Only if both imaging requirements of the lens  106  occur with very small aberrations will the interference pattern observed by television camera  110  accurately enable the surface contours of the surface  2   a  to be measured accurately. When the collimator lens  112  is used as in FIG. 16, the first requirement mentioned above is nearly satisfied; however, the second requirement mentioned above is not satisfied. As a result, each single point on the surface  2   a  will not be imaged to a corresponding single point on the screen  110 , thus causing problems in that the location of lines in the interference pattern will be imprecise, and the periphery portions of the surface  2   a  will appear as being out of focus. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an imaging lens for an interferometric device that can simultaneously image an object at unit magnification (i.e., light in the object beam) and can relay collimated light (i.e., light in the reference beam) while generating very low aberrations in both so as to provide a high quality interference pattern image, thereby enabling precise measurements of the object surface contours to be obtained with high accuracy. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given below and the accompanying drawings, which are given by way of illustration only and thus are not limitative of the present invention, wherein: 
     FIG. 1 shows the lens element configuration of Embodiment 1 of the present invention, 
     FIG. 2 shows the spherical aberration of the lens of Embodiment 1 when imaging collimated light, 
     FIGS. 3A-3C show the spherical aberration, astigmatism and distortion, respectively, of the lens of Embodiment 1 when imaging an object at unit magnification, 
     FIG. 4 shows the lens element configuration of Embodiment 2 of the present invention, 
     FIG. 5 shows the spherical aberration of the lens of Embodiment 2 when imaging collimated light, 
     FIGS. 6A-6C show the spherical aberration, astigmatism and distortion, respectively, of the lens of Embodiment 2 when imaging an object at unit magnification, 
     FIG. 7 shows the lens element configuration of Embodiment 3 of the present invention, 
     FIG. 8 shows the spherical aberration of the lens of Embodiment 3 when imaging collimated light, 
     FIGS. 9A-9C show the spherical aberration, astigmatism and distortion, respectively, of the lens of Embodiment 3 when imaging an object at unit magnification, 
     FIG. 10 shows the lens element configuration of Embodiment 4 of the present invention, 
     FIG. 11 shows the spherical aberration of the lens of Embodiment 4 when imaging collimated light, 
     FIGS. 12A-12C show the spherical aberration, astigmatism and distortion, respectively, of the lens of Embodiment 4, when imaging an object at unit magnification, 
     FIG. 13 shows the lens of the present invention imaging collimated light onto a planar surface normal to the optical axis (as occurs in relaying the collimated reference beam via mirror  14  in FIG.  17 ), and illustrates the arrangement by which the spherical aberrations shown in FIGS. 2,  5 ,  8  and  11  for the imaging lens of Embodiments 1-4, respectively, are generated. 
     FIG. 14 is a diagram illustrating unfolded ray paths of the imaging lens of the present invention when combined with a mirror  114  (as illustrated in FIG. 17) a focal distance away from the lens, and is the arrangement by which the aberrations shown in FIGS. 3,  6 ,  9  and  12  for Embodiments 1-4, respectively, are generated, 
     FIG. 15 is a schematic diagram for explaining a prior art interferometric device, 
     FIG. 16 is a schematic diagram for explaining another prior art interferometric device, and 
     FIG. 17 is a schematic diagram for explaining still another prior art interferometric device. 
    
    
     DETAILED DESCRIPTION 
     The present invention relates to an imaging lens for an interferometric device and, more particularly, relates to a triplet-type imaging lens arranged between a second diffraction grating of a grazing incidence interferometric device and a screen. 
     An imaging lens for an interferometric device of the present invention is formed of, in successive order from the most object side of the imaging lens, a first lens component which has an overall meniscus shape with its concave surface on the object side, a second lens component, and a third lens component. 
     The second and third lens components have opposite refractive power, and the following Conditions (1) and (2) are satisfied: 
     
       
         −0.1&lt; f/f   1 &lt;0.6  Condition (1) 
       
     
     
       
         0.3 &lt;d   f   /d &lt;0.6  Condition (2) 
       
     
     where 
     f is the focal length of the imaging lens, 
     f 1  is the focal length of the first lens component, 
     d F  is the distance between the surfaces of the first lens component that are nearest the object side and the image side, respectively, and 
     d is the overall length of the imaging lens. 
     Moreover, when the second lens component is negative and the third lens component is positive, it is preferable that the following Conditions (3) and (4) are satisfied: 
     
       
         −3.0&lt; f   2   /f &lt;−1.0  Condition (3) 
       
     
      0.5 &lt;f   3   /f &lt;1.2  Condition (4) 
     where 
     f 2  is the focal length of the second lens component, and 
     f 3  is the focal length of the third lens component. 
     Further, when the second lens component is positive and the third lens component is negative, it is preferable that the following Conditions (5) and (6) are satisfied. 
     
       
         0.5 &lt;f   2   /f &lt;1.2  Condition (5) 
       
     
     
       
         −3.0 &lt;f   3   /f &lt;−1.0  Condition (6) 
       
     
     The imaging lens of the present invention is intended for use with a grazing incidence interferometric device and, when so used, is preferably arranged between a wave front combining means of the grazing incidence interferometric device and a screen. 
     If Condition (1) is not satisfied, the image quality of a surface (such as surface  2   a ) that is imaged onto another surface will deteriorate, particularly as a result of field curvature. In other words, satisfying Conditional (1) provides favorable imaging. 
     Condition (2) specifies the ratio of the thickness of the first lens component L 1  relative to the overall length of the imaging lens. If the lower limit is not satisfied, the properties of forming the image of surface  21  onto another surface will deteriorate, particularly as a result of field curvature; on the other hand, if the upper limit is not satisfied, the first lens component L 1  will be too thick and the cost will be disadvantageous. In other words, in order to obtain favorable curvature of field in consideration of processing costs, Condition (2) needs to be satisfied. 
     Conditions (3) and (5) specify the ratio of the focal length of the second lens component relative to that of the imaging lens. If the lower limit is not satisfied, the refractive power of the second lens component will be too small and the spherical aberration when imaging an object at infinity cannot be corrected. On the other hand, if the upper limit is not satisfied, the spherical aberration when imaging an object at infinity will be over-corrected. Thus, in order to provide favorable correction of spherical aberration for a distant object, it is necessary to satisfy Condition (3) when the second lens component is negative and Condition (5) when the second lens component is positive. Furthermore, since spherical aberration will degrade the interference pattern itself, resulting in the interference pattern no longer accurately indicating the actual surface contours of surface  2   a , measurement errors of surface contour will arise if the applicable Condition (3) or (5) is not satisfied. 
     Conditions (4) and (6) specify the ratio of the focal length of the third lens component L 3  relative to the focal length of the imaging lens. If the lower limit of Condition (4) or Condition (6) is not satisfied, the power of the third lens component will be too weak and the spherical aberration for a distant object will not be sufficiently corrected. On the other hand, if the upper limit of Condition (4) or Condition (6) is exceeded, the spherical aberration when imaging a distant object will be over-corrected. In other words, in order to provide favorable correction of aberration for a distant object, it is necessary to satisfy Condition (4) when the second lens component is negative and Condition (6) when the second lens component is positive. 
     Various embodiments of the present invention will now be explained in detail. The imaging lens of the present invention is intended to replace the imaging lens  106  as illustrated in the prior art interferometric arrangement shown in FIG.  17 . 
     Embodiment 1 
     FIG. 1 shows the basic lens component configuration of the imaging lens of Embodiment 1. As shown in FIG. 1, there is arranged, in successive order from the most object side, a first lens component L 1  which has a negative meniscus shape with its concave surface on the object side, a second lens component L 2  having a negative meniscus shape with its convex surface on the object side, and a third lens component L 3  having a biconvex shape with surfaces of different curvature and with the surface of smaller radius of curvature on the object side. Thus, luminous flux incident from the object side along optical axis X is formed on an image surface to the right of the imaging lens. 
     Table 1, below, lists surface number # in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis spacing D (in mm) between surfaces, as well as the refractive index N d  and Abbe number ν d  (at the sodium d line) of each lens component of Embodiment 1. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 # 
                 R 
                 D 
                 N d   
                 ν d   
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 −21.8298 
                 6.66613 
                 1.62005 
                 36.3 
               
               
                 2 
                 −25.0617 
                 0.26664 
               
               
                 3 
                 62.0820 
                 3.85218 
                 1.62005 
                 36.3 
               
               
                 4 
                 44.8530 
                 2.10089 
               
               
                 5 
                 62.2369 
                 6.35492 
                 1.51633 
                 64.1 
               
               
                 6 
                 −99.7917 
                 108.34581 
               
               
                   
               
             
          
         
       
     
     Table 2 below lists the focal length f of the imaging lens, the focal length f 1  of the first lens component L 1 , the focal length f 2  of the second lens component L 2 , the focal length f 3  of the third lens component L 3 , the overall length d of the imaging lens, the thickness d F  of the first lens component L 1 , as well as the values of the f/f 1 , f 2 /f, f 3 /f and d F /d. Thus, the above-noted Conditions (1)-(4) are each satisfied for this embodiment. 
     
       
         
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
             
             
               
                 f = 100.00 
                 Condition (1) value: f/f 1  = −0.08 
               
               
                 f 1  = −1287.00 
                 Condition (2) value: d F /d = 0.35 
               
               
                 f 2  = −286.57 
                 Condition (3) value: f 2 /f = −2.87 
               
               
                 f 3  = 75.49 
                 Condition (4) value: f 3 /f = 0.75 
               
               
                 d = 19.24 
               
               
                 d F  = 6.67 
               
               
                   
               
             
          
         
       
     
     As is apparent from Table 2, this embodiment satisfies Conditions (1)-(4). 
     FIG. 2 shows the spherical aberration of the lens of Embodiment 1 when imaging collimated light. The spherical aberration when imaging collimated light is the wave front aberration of the lens for an object at infinity imaged onto a flat surface  20 , as shown in FIG.  13 . 
     FIGS. 3A-3C show the spherical aberration, astigmatism and distortion, respectively, of the lens of Embodiment 1 when imaging an object at unit magnification. FIG. 14 illustrates unfolded ray paths of the arrangement by which the aberrations shown in FIGS. 3A-3C are generated. 
     Moreover, as is clear from FIGS. 2 and 3, according to the present embodiment, both the spherical aberration of the lens when imaging collimated light, as required in relaying the reference beam via mirror  114  in FIG. 17, and the various aberrations of the lens when imaging an object at unit magnification, as required for the object beam in FIG. 17, are favorably corrected by the present invention. Embodiment 2 
     FIG. 4 shows the basic lens component configuration of the imaging lens of Embodiment 2. As shown in FIG. 4, there is arranged, in successive order from the most object side: a first lens component L 1  having a meniscus shape with its concave surface on the object side; a second lens component L 2  having a biconvex shape with surfaces of different curvature, with the surface of larger radius of curvature on the object side; and, a third lens component L 3  having a negative meniscus shape with its concave surface on the object side. 
     As with Embodiment 1, the lens of this embodiment is configured to satisfy the Conditions (1) and (2) mentioned above. Further, the imaging lens of this embodiment is configured to satisfy the above Conditions (5) and (6) instead of Conditions (3) and (4). 
     Table 3, below, lists surface number # in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis spacing D (in mm) between surfaces, as well as the refractive index N d  and Abbe number ν d  (at the sodium d line) of each lens component of Embodiment 2. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 # 
                 R 
                 D 
                 N d   
                 ν d   
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 −23.1144 
                 6.66622 
                 1.62005 
                 36.3 
               
               
                 2 
                 −25.4246 
                 2.66645 
               
               
                 3 
                 102.9098 
                 4.09814 
                 1.51633 
                 64.1 
               
               
                 4 
                 −64.8976 
                 2.29848 
               
               
                 5 
                 −41.8814 
                 3.49608 
                 1.62005 
                 36.3 
               
               
                 6 
                 −58.7888 
                 106.29159 
               
               
                   
               
             
          
         
       
     
     Table 4 below lists the focal length f of the imaging lens, the focal length f 1  of the first lens component L 1 , the focal length f 2  of the second lens component L 2 , the focal length f 3  of the third lens component L 3 , the overall length d of the imaging lens, the thickness d F  of the first lens component L 1 -, as well as the values of the f/f 1 , f 2 /f, f 3 /f and d f /d. Thus, the above-noted Conditions (1), (2), (5) and (6) are each satisfied for this embodiment. 
     
       
         
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
             
             
               
                 f = 100.00 
                 Condition (1) value: f/f 1  = 0.024 
               
               
                 f 1  = 4103.09 
                 Condition (2) value: d F /d = 0.35 
               
               
                 f 2  = 77.98 
                 Condition (3) value: f 2 /f = 0.78 
               
               
                 f 3  = −256.41 
                 Condition (4) value: f 3 /f = −2.56 
               
               
                 d = 19.23 
               
               
                 d F  = 6.67 
               
               
                   
               
             
          
         
       
     
     As is apparent from Table 4, this embodiment satisfies Conditions (1),(2), (5) and (6). 
     FIG. 5 shows the spherical aberration of the lens of Embodiment 2 when imaging collimated light. The spherical aberration when imaging collimated light is the wave front aberration of the lens for an object at infinity imaged onto a flat surface 20, as shown in FIG.  13 . 
     FIGS. 6A-6C show the spherical aberration, astigmatism and distortion, respectively, of the lens of Embodiment 2 when imaging an object at unit magnification. FIG. 14 illustrates unfolded ray paths of the arrangement by which the aberrations shown in FIGS. 6A-6C are generated. 
     Moreover, as is clear from FIGS. 5 and 6, according to the present embodiment, both the spherical aberration of the lens when imaging collimated light, as required in relaying the reference beam via mirror  114  in FIG. 17, and the various aberrations of the lens when imaging an object at unit magnification, as required for the object beam in FIG. 17, are favorably corrected by the present invention. 
     Embodiment 3 
     FIG. 7 shows the basic lens component configuration of the imaging lens of Embodiment 3. As shown in FIG. 7, there is arranged, in successive order from the most object side: a first lens component L 1  having a meniscus shape with its concave surface on the object side; a second lens component L 2  having a negative meniscus shape with its concave surface on the object side; and a third lens component L 3  of biconvex shape having surfaces of different curvature with the surface of larger radius of curvature on the object side. 
     Table 5, below, lists surface number # in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis spacing D (in mm) between surfaces, as well as the refractive index N d  and Abbe number ν d  (at the sodium d line) of each lens component of Embodiment 3. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 # 
                 R 
                 D 
                 N d   
                 ν d   
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 −25.334 
                 10.751 
                 1.80518 
                 25.4 
               
               
                 2 
                 −25.681 
                 3.574 
               
               
                 3 
                 −24.724 
                 3.844 
                 1.51633 
                 64.1 
               
               
                 4 
                 −37.542 
                 0.107 
               
               
                 5 
                 218.676 
                 3.806 
                 1.80518 
                 25.4 
               
               
                 6 
                 −124.687 
                 121.248 
               
               
                   
               
             
          
         
       
     
     Table 6 below lists the focal length f of the imaging lens, the focal length f 1  of the first lens component L 1 , the focal length f 2  of the second lens component L 2 , the focal length f 3  of the third lens component L 3 , the overall length d of the imaging lens, the thickness d F  of the first lens component L 1 , as well as the values of the f/f 1 , f 2 /f, f 3 /f and d F /d. Thus, the above-noted Conditions (1)-(4) are each satisfied for this embodiment. 
     
       
         
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
             
             
               
                 f = 100.00 
                 Condition (1) value: f/f 1  = 0.54 
               
               
                 f 1  = 183.97 
                 Condition (2) value: d F /d = 0.49 
               
               
                 f 2  = −156.68 
                 Condition (3) value: f 2 /f = −1.57 
               
               
                 f 3  = 99.90 
                 Condition (4) value: f 3 /f = −1.00 
               
               
                 d = 22.08 
               
               
                 d F  = 10.75 
               
               
                   
               
             
          
         
       
     
     As is apparent from Table 6, this embodiment satisfies Conditions (1)-(4). 
     FIG. 8 shows the spherical aberration of the lens of Embodiment 3 when imaging collimated light. The spherical aberration when imaging collimated light is the wave front aberration of the lens for an object at infinity imaged onto a flat surface  20 , as shown in FIG.  13 . 
     FIGS. 9A-9C show the spherical aberration, astigmatism and distortion, respectively, of the lens of Embodiment 3 when imaging an object at unit magnification. FIG. 14 illustrates unfolded ray paths of the arrangement by which the aberrations shown in FIGS. 9A-9C are generated. 
     Moreover, as is clear from FIGS. 8 and 9, according to the present embodiment, both the spherical aberration of the lens when imaging collimated light, as required in relaying the reference beam via mirror  114  in FIG. 17, and the various aberrations of the lens when imaging an object at unit magnification, as required for the object beam in FIG. 17, are favorably corrected by the present invention. 
     Embodiment 4 
     FIG. 10 shows the basic lens component configuration of the imaging lens of Embodiment 4. As shown in FIG. 10, there is arranged, in successive order from the most object side: a first lens component of negative meniscus shape with its concave surface on the object side, the first lens component being formed of a first lens element L 1  of negative meniscus shape joined to a second lens element L 2  of positive meniscus shape; a second lens component formed of a third lens element L 3  of negative meniscus shape with its concave surface on the object side; and a third lens component formed of a fourth lens element L 4  that is biconvex having surfaces of different curvature, with the surface of larger radius of curvature on the object side. 
     Table 7, below, lists surface number # in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis spacing D (in mm) between surfaces, as well as the refractive index N d  and Abbe number ν d  (at the sodium d line) of each lens component of Embodiment 4. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 7 
               
               
                   
                   
               
               
                   
                 # 
                 R 
                 D 
                 N d   
                 ν d   
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 −23.842 
                 3.854 
                 1.75520 
                 25.4 
               
               
                 2 
                 −1983.238 
                 7.133 
                 1.77250 
                 49.6 
               
               
                 3 
                 −26.155 
                 6.332 
               
               
                 4 
                 −25.246 
                 1.973 
                 1.67003 
                 47.2 
               
               
                 5 
                 −31.543 
                 0.133 
               
               
                 6 
                 188.500 
                 4.003 
                 1.72342 
                 38.0 
               
               
                 7 
                 −120.121 
                 119.822 
               
               
                   
               
             
          
         
       
     
     Table 8 below lists the focal length f of the imaging lens, the focal length f 1  of the first lens component formed of lens elements L 1  and L 2  that are joined, the focal length f 2  of the second lens component L 3 , the focal length f 3  of the third lens component L 4 , the overall length d of the imaging lens, the thickness d f  of the first lens component (the combined thickness of L 1  and L 2 ), as well as the values of the f/f 1 , f 2 /f, f 3 /f and d F /d. Thus, the above-noted Conditions (1)-(4) are each satisfied for this embodiment. 
     
       
         
               
               
             
           
               
                 TABLE 8 
               
               
                   
               
             
             
               
                 f = 100.00 
                 Condition (1) value: f/f 1  = 0.37 
               
               
                 f 1  = 268.34 
                 Condition (2) value: d F /d = 0.47 
               
               
                 f 2  = −216.80 
                 Condition (3) value: f 2 /f = −2.17 
               
               
                 f 3  = 102.55 
                 Condition (4) value: f 3 /f = 1.03 
               
               
                 d = 23.43 
               
               
                 d F  = 10.99 
               
               
                   
               
             
          
         
       
     
     As is apparent from Table 8, this embodiment satisfies Conditions (1)-(4). 
     FIG. 11 shows the spherical aberration of the lens of Embodiment 4 when imaging collimated light. The spherical aberration when imaging collimated light is the wave front aberration of the lens for an object at infinity imaged onto a flat surface  20 , as shown in FIG.  13 . 
     FIGS. 12A-12C show the spherical aberration, astigmatism and distortion, respectively, of the lens of Embodiment 4 when imaging an object at unit magnification. FIG. 14 illustrates unfolded ray paths of the arrangement by which the aberrations shown in FIGS. 12A-12C are generated. 
     Moreover, as is clear from FIGS. 11 and 12, according to the present embodiment, both the spherical aberration of the lens when imaging collimated light, as required in relaying the reference beam via mirror  114  in FIG. 17, and the various aberrations of the lens when imaging an object at unit magnification, as required for the object beam in FIG. 17, are favorably corrected by the present invention. 
     FIG. 13 illustrates the lens of the present invention having lens components L 1 , L 2 , and L 3 , in order from the object side, imaging collimated light (i.e., light as contained in the object beam) onto a flat surface  20 . 
     FIG. 14 illustrates the unfolded ray paths for the lens of the present invention in an arrangement with a reflecting mirror similar to the arrangement illustrated by prior art lens  106  and mirror  114  of FIG. 17, but with the object  2   a  oriented normal to the optical axis of the lens. 
     FIGS. 15 is a schematic diagram for explaining a prior art interferometric device, 
     FIG. 16 is a schematic diagram for explaining another prior art interferometric device, and 
     FIG. 17 is a schematic diagram for explaining still another prior art interferometric device. 
     As explained above, the imaging lens of the present invention is configured to specify the shapes of its three lens components and to satisfy specified conditions, so that the spherical aberration when relaying a collimated beam of light, and the spherical aberration, astigmatism, and distortion of the lens in imaging object  2   a  onto observation screen  110  in the arrangement shown in FIG. 17 are made favorable. As a result, even when the imaging lens of the present invention is arranged between a wave front combining means and a screen of a grazing incidence interferometric device, a point on a surface (such as surface  2   a ) will be imaged by the lens into a single point on the screen, and problems of the interference pattern being imprecise or distorted, as well as problems of there being blurriness of the interference pattern image at the periphery thereof will be minimized. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. For example the number of lens elements in each lens group may be increased from those described. Also, a cover glass along with a low pass filter or an infrared cut-off filter may be inserted between the last surface of the imaging lens and the and an image surface. Such variations are-not to be regarded as a departure from the spirit and scope of the invention. Rather the scope of the invention shall defined as set forth in the following claims and their legal equivalents. All such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.