Large-aperture three-lens objective with aspherical-surfaces
TECHNICAL FIELD 
     The invention relates to a three-lens objective comprising three lenses and having a relatively long front focal length and a relatively short back focal length, each of said lenses having a front surface facing an object and a rear surface facing an image of said object. 
     Preferably, the invention relates to a three-lens objective for infrared spectral ranges, the objective having lenses of infrared-transparent materials. 
     The latter type of objectives is used, when infrared images having very small image points are to be formed all over an extended image area. A possible use of such objectives is in an infrared seeker wherein the image of a field of view if formed on a two-dimensional, image-detecting sensor and the picture elements (pixels) detected by the sensor are subjected to image processing. The following specifications of the objective are required for such uses: 
     The spectral range should lie between the wavelengths of 3 μm and 5 μm, for other applications between 8 μm and 12 μm. 
     The aperture should be large (f-number between 2.0 and 1.0) 
     The angle of field of view should be between +-2° and +-30°. 
     The image points should be nearly diffraction-limited. 
     The objective should be nearly temperature-compensated in a temperature interval from -40° C. to +60° C. 
     The number of lenses should be as small as possible. 
     The objective should have minimum overall length, the &#34;overall length&#34; of the objective being the spacing between the front surface of the objective and the image plane 
     BACKGROUND ART 
     The DE-A-1,472,189 (German laid-open patent application) describes a three-lens objective for infrared having two positive power lenses made of silicon and a negative power lens made of germanium therebetween. The two positive power lenses are menisci. The concave sides of these menisci face the shorter back or front focal length, respectively. 
     In this prior art objective, the spacings between the front lens and the median lens and between the median lens and the rear lens are approximately equal. All lenses are spherical. Thereby, the overall length of the objective becomes undesirably large. 
     The DE-A-2,519,961 also describes a three-lens objective for infrared. The three-lens objective comprises a single, positive power front lens in the form of a meniscus and made of germanium, and a rear lens element composed of two adjacent meniscus lenses convex towards the front. At least one of these rear meniscus lenses consists of germanium, as well. The rear surface of the front lens is aspherical. The remaining surfaces are spherical. 
     In this prior art three-lens objective, the median and rear lenses are closely adjacent. 
     The DE-A-3,726,355 relates to a bifocal optical infrared system in the form of a three-lens objective. The three-lens objective comprises a positive power front lens, a negative power medial lens and a rear lens having, again, positive power. The lenses consist of germanium. The rear surface of the front lens and the front surface of the rear lens are aspherical. 
     Also in this three-lens objective, the median and rear lenses are closely adjacent. Such closely adjacent median and rear lenses in three-lens objectives result in an increase of the overall length. 
     The DE-A-2,743,175 describes an infrared objective which consists of three mutually adjustable optical elements. The median and the rear optical elements, in turn, consist of two lenses each. The spacing of the median and rear optical elements is smaller than the spacing of the median optical element from the front one. 
     The DD-A-299,930 (German Democratic Republic patent) describes a four-lens infrared objective of the Petzval type. 
     A paper by J. Kross and R. Schuhmann &#34;Zur Korrektion optischer Systeme mit aspharischen Flachen&#34; in &#34;Optik&#34; Vol.70 (1985), 76-85, describes the correction of different lens systems by means of aspherical surfaces. 
     DISCLOSURE OF THE INVENTION 
     It is an object of the invention to provide a high-resolution objective with a minimum of lenses and with small overall length. 
     A more specific object of the invention is to provide a high-resolution objective for infrared ranges. 
     According to the invention, these and other objects are achieved with a three-lens objective of the type mentioned in the beginning, wherein 
     a front one of said lenses consists of a first material of relatively low dispersion, a median one of said lenses consists of a second material of relatively larger dispersion, and a rear one of said lenses consists of a material of relatively low dispersion, 
     said front lens has positive power, said median lens has negative power, and said rear lens has positive power, 
     the spacing between said front lens and said median lens is substantially smaller than the distance between said median lens and said rear lens, and 
     each of said lenses has at least one aspherical surface. 
     As can be gathered from the prior art discussed hereinbefore, the spacing of the three lenses in three-lens objectives from the respective neighboring lens are normally substantially equal. The smaller the spacing between the front and median lenses is, the poorer are the image points, if the lenses are spherical. The reason is that, if the median lens is placed close to the front lens, aberrations, in particular astigmatism and coma, are corrected by this median lens to a lesser degree. The median lens, virtually, corrects chromatic aberrations only. The invention is based on the discovery, that placing the median lens closer to the front lens, whereby the spacing between the median lens and the front lens is noticeably smaller than the spacing of the rear lens from the median lens, is feasible, if at least one of the surfaces of each lens is aspherical. By this arrangement of the lenses, in turn, reduced overall length as compared with prior art three-lens objectives is achieved. 
     Modifications of the invention are subject matter of the dependent claims. 
     Embodiments of the invention will be described in greater detail hereinbelow with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a first embodiment of a three-lens objective for use in a spectral range between the wavelengths of 3.4 μm to 5.2 μm. 
     FIG. 2 shows a second embodiment of a three-lens objective for use in a spectral range between the wavelengths of 3.8 μm to 5.5 μm. 
     FIG. 3 shows a third embodiment of a three-lens objective for use in a spectral range between the wavelengths of 8 μm to 12 μm. 
     FIG. 4 illustrates, for the various fractional object heights the piercing points of various parallelly incident rays through the focal plane of the three-lens objective illustrated in FIG. 1. 
     FIG. 5 shows the modulation transfer function obtained with the three-lens objective of FIG. 1. 
     FIG. 6 shows the image point function for various fractional object heights obtained with a three-lens objective of FIG. 1. 
     FIG. 7 is similar to FIG. 4 and illustrates, for the various fractional object heights the piercing points of various parallelly incident rays through the focal plane of the three-lens objective illustrated in FIG. 2. 
     FIG. 8 is similar to FIG. 5 and shows the modulation transfer function for tangential and sagittal rays obtained with the three-lens objective of FIG. 1. 
     FIG. 9 is similar to FIG. 6 and shows the image point function for various fractional object heights obtained with a three-lens objective of FIG. 2. 
     FIG. 10 illustrates a further embodiment of a three-lens objective designed in accordance with the basic principle explained above, this three-lens objective being appropriate for long-wavelength infrared and, in addition, permitting the provision stop behind the rear lens. 
     FIG. 11 is similar to FIG. 4 and illustrates, for the various fractional object heights the piercing points of various parallelly incident rays through the focal plane of the three-lens objective illustrated in FIG. 10. 
     FIG. 12 is similar to FIG. 5 and shows the modulation transfer function for tangential and sagittal rays obtained with the three-lens objective of FIG. 10. 
     FIG. 13 shows a modified embodiment of a three-lens objective, which is composed of germanium and zinc selenide lenses and, therefore, appropriate for long-wavelength infrared, and which permits an aperture stop to be placed behind the rear lens. 
     FIG. 14 shows a three-lens objective, which is composed of germanium and zinc sulfide lenses and, therefore, is appropriate for long-wavelength infrared, and wherein the aperture stop is positioned behind the rear lens. 
     FIG. 15 shows a three-lens objective, which is composed of germanium and zinc selenide lenses and, therefore, is appropriate for long-wavelength infrared, and wherein the aperture stop is to be arranged between the median and rear lenses, this three-lens objective permitting a small f-number to be achieved. 
     FIG. 16 is similar to FIG. 4 and illustrates, for the various fractional object heights the piercing points of various parallelly incident rays through the focal plane of the three-lens objective illustrated in FIG. 15. 
     FIG. 17 is similar to FIG. 5 and shows the modulation transfer function for tangential and sagittal rays obtained with the three-lens objective of FIG. 15. 
     FIG. 18 shows a further embodiment of a three-lens objective, which is composed of germanium and zinc sulfide lenses, an aperture having to be arranged, in this three-lens objective, closely behind the median lens. 
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION 
     Referring to FIG. 1, a three-lens objective consists of three lenses, namely a front lens 10, a median lens 12 and a rear lens 14. 
     The front lens 10 consists of a first material which exhibits a relatively low dispersion in the spectral range of interest. In the present case, the first material is silicon. This material is transparent to infrared radiation in the spectral range of wavelengths between 3.4 μm and 5.2 μm. The front lens 10 has positive power. The front lens has a front surface 16 and a rear surface 18. The front surface 16 has a positive radius of curvature, i.e the surface 16 is convex towards the front. The front surface 16 is aspherical. The rear surface has also a positive radius of curvature, i.e. is dished towards the front. The radius of curvature of the front surface 16 of the front lens 10 is substantially smaller than the radius of curvature of the rear surface 18 of the front lens 10. 
     The median lens 12 consists of a second material, which exhibits a relatively large dispersion as compared with the material (silicon) of the front lens 10. In the present case, the second material is germanium. Also germanium is transparent in the spectral range from 3.4 μm to 5.2 μm. The median lens 12 has negative power. The median lens 12 has a front surface 20 and a rear surface 22. The front surface 20 has a negative radius of curvature, i.e. the surface 20 is dished towards the rear or concave. Also the front surface 20 of the median lens 12 is aspherical. Also the rear surface 22 of the median lens 12 has a negative radius of curvature, i.e. the rear surface 22 is convex as viewed from the rear. The amount of the radius of curvature of the front surface 20 is smaller than the amount of the radius of curvature of the rear surface 22. 
     The lenses 10 and 12 are spaced and define an air space 24 therebetween. 
     The rear lens 14 again consists of the first, relatively low dispersion material, thus of silicon. The rear lens 14 has positive power. The rear lens 14 has a front surface 26 and a rear surface 28. The front surface 26 has a positive radius of curvature. Also the rear surface 28 has a positive radius of curvature. The front surface 26 is aspherical. The radius of curvature of the front surface 26 of the rear lens 14 is smaller than the radius of curvature of the rear surface 28. 
     An air space 30 is defined between the median lens 12 and the rear lens 14. The distance between the front lens 10 and the median lens 12 is substantially smaller than the distance between the median lens 12 and the rear lens 14. 
     In detail, the data of the first embodiment of the three-lens objective are as follows: 
     
                       TABLE I                                                     
______________________________________                                    
Lens Data                                                                 
                              Aperture                                    
Surface Radius     Thickness  Radius Material                             
______________________________________                                    
1       --         --         25.576769                                   
                                     Air                                  
2         95.775000                                                       
                   5.700000   25.576769                                   
                                     Si                                   
3        362.560000                                                       
                   9.800000   25.039522                                   
                                     Air                                  
4       -115.480000                                                       
                   4.300000   21.969655                                   
                                     Ge                                   
5       -296.400000                                                       
                   40.000000  22.070983                                   
                                     Air                                  
6         39.525000                                                       
                   5.000000   15.410790                                   
                                     Si                                   
7         40.973000                                                       
                   41.479150  13.972384                                   
                                     Air                                  
8       --         --          4.519556                                   
                                     Air                                  
______________________________________                                    
Aspheric Surface Data                                                     
2   CC      -1.260288                                                     
                     AD   -2.10940E-08                                    
                                   AE   -4.82038E-11                      
4   CC     -12.095072                                                     
                     AD    2.05607E-07                                    
                                   AE   -3.62017E-11                      
6   CC      -0.984138                                                     
                     AD    8.95639E 07                                    
                                   AE   -2.08363E-11                      
______________________________________                                    
Refractive Indices                                                        
                                       Abbe-                              
                                       Dispersion                         
Surface                                                                   
       Material N1       N2     N3     Number                             
______________________________________                                    
1      Air      --       --     --     --                                 
2      Si       3.424332 3.429186                                         
                                3.421818                                  
                                       329.041382                         
3      Air      --       --     --     --                                 
4      Ge       4.021400 4.034114                                         
                                4.014581                                  
                                       154.677323                         
5      Air      --       --     --     --                                 
6      Si       3.424332 3.429186                                         
                                3.421818                                  
                                       329.041382                         
7      Air      --       --     --     --                                 
8      Air      --       --     --     --                                 
______________________________________                                    
 
    
     The first paragraph of Table 1 provides the lens data. The surfaces are characterized by consecutive numbers. Surface &#34;2&#34; is the front surface 16 of the front lens 10 in FIG. 1. Surface &#34;3&#34; is the rear surface 18 of the front lens 10. Surface &#34;4&#34; is the front surface 20 of the median lens 12. Surface &#34;5&#34; is the rear surface 22 of the median lens 12. Surface &#34;6&#34; is the front surface 26 of the rear lens 14. Surface &#34;7&#34; is the rear surface 28 of the rear lens 14. The radius of curvature is given for each surface, the radius of curvature being positive, when the surface is curved towards the front, and the radius of curvature being negative, when the surface is curved towards the rear. The &#34;thickness&#34; given is the distance of the respective surface from the next-following one, thus, for example, of the surface &#34;2&#34;(16) from the surface &#34;3&#34;(18). 
     The materials recited in the table refer to the material behind the respective surface. 
     A further paragraph states the aspheric surface data. The last paragraph states the refractive indices. Also here, the statements refer to the region behind the recited surfaces. 
     The tables given hereinbelow for other embodiments are structured correspondingly. 
     A three-lens objective having the above recited data exhibits the following characteristics: 
     
                       TABLE 1A                                                    
______________________________________                                    
Optical System Data                                                       
Field of View Angle (+/-Degree) =                                         
                      2.828                                               
F-Number (--) =       1.783                                               
Focal Length (mm) =  91.196                                               
Wavelengths [μm) =                                                     
                      4.300   3.400  5.200                                
______________________________________                                    
Image Point Data                                                          
                         Piercing                                         
                                Optical                                   
                         Points Path                                      
Wave-          Airy      Diameter                                         
                                Difference                                
                                         Strehl                           
length         Diameter  95%    95%      Def.                             
(μm) FOB    (μm)   (μm)                                          
                                (μm)  Ratio                            
______________________________________                                    
4.300   --     18.880    9.434  0.167    0.985                            
3.400   --     14.926    10.538 0.121    0.988                            
5.200   --     22.831    9.221  0.158    0.991                            
Polychr --     18.879    9.657  0.153    0.987                            
4.300   0.700  18.914    8.675  0.101    0.995                            
3.400   0.700  14.953    11.876 0.229    0.956                            
5.200   0.700  22.873    8.569  0.098    0.996                            
Polychr 0.700  18.914    9.449  0.133    0.985                            
4.300   1.000  18.957    8.992  0.131    0.991                            
3.400   1.000  14.987    13.298 0.319    0.917                            
5.200   1.000  22.925    9.009  0.139    0.993                            
Polychr 1.000  18.957    10.072 0.180    0.973                            
______________________________________                                    
 
    
     the first paragraph of table 1A recites the characteristics of the whole three-lens objective. These characteristics are the angle of field of view, the f-number and the focal length. 
     The second paragraph of table 1A states the characteristics of the image points. It states, for different wavelengths of 4.300 μm, 3.400 μm and 5.200 μm and for polychromatic light, and at different fractional object heights, the Airy diameter, the diameter within which 95% of the piercing points of parallelly incident rays through the focal plane are located, the distance within which the optical path differences of 95% of the parallely incident rays lie, and the Strehl definition ratio. 
     Also the corresponding tables relating to other embodiments hereinbelow are structured in the same way. 
     The front lens 10 and the median lens 12 with different dispersions and opposite powers cause correction of chromatic aberrations. The rear lens substantially serves to correct astigmatism. Further correction of aberrations is achieved by the aspherical shape of the surfaces 16, 20 and 26. 
     All of the aspherical surfaces 16, 20 and 26 exhibit symmetry of revolution with respect to the optical axis 32 of the three-lens objective. The generatrix of the surface of revolution can be represented by a function ##EQU1## wherein Z is the coordinate in the direction of the optical axis, Y is a coordinate in a direction orthogonal to the optical axis, R is the radius of curvature in the vertix, and AD and AE are the aspheric coefficients of fourth and sixth order, respectively. The values of R, CC, AD and AE are given in the table hereinbefore. 
     FIG. 4 shows for different fractional object heights FOB=0; 0.7 and 1 the piercing points of various incident parallel rays through the focal plane with a three-lens objective of FIG. 1. The picture point-height ratio is indicative of the deviation of the image point from the axis 32 referenced to the angle of the field of view. It will be noted, that for all fractional object heights 95% of all piercing points are located within a circle of 5 μm diameter. All piercing points are located within a circle of 15 μm diameter. The piercing points are represented by squares, crosses (+) and triangles to indicate long, medium and short wavelengths, respectively. 
     FIG. 5 illustrates the modulation transfer function (MTF) for the three-lens objective. This modulation transfer function provides a measure of the quality of the image. If a sequence of parallel, sinusoidal, dark bars is imaged into the image plane, then the density of the bars (in bars per inch image plane) provides a &#34;spatial frequency&#34;. This spatial frequency is the abscissa of the graphs of FIG. 5. The ordinate is the &#34;modulation&#34;. This is the difference between the brightest and darkest locations in the image plane. With a coarse pattern, i.e. low &#34;spatial frequency&#34;, the modulation has the value of &#34;one&#34;. With finer patterns, i.e. higher spatial frequencies, the modulation descends, linearly even with ideal imaging. This is due to the diffraction limitation of the imaging. With real imaging, usually a &#34;belly&#34; sags relative to the dropping straight line. In FIG. 5, the descending straight line, which would correspond to the ideal imaging and diffraction limitation, is dotted. The solid and the dashed lines represent the modulation transfer function of the three-lens objective of FIG. 1 for tangential and sagittal sections. The modulation transfer function has been computed polychromatically with a weight 1 associated with the medium and a weight .5 associated with the boundary wavelengths. 
     FIG. 6 illustrates, for the three-lens objective of FIG. 1, the image point function at different fractional object heights FOB. The image point function shows, for an imaged point light source, the light intensity as a function of position. The square illustrated in perspective in FIG. 6 has an edge length of 40 pm. Also here, the narrow limitation of the image point can be recognized. FIG. 6 also shows that the image point function remains virtually unchanged, if the fractional object height is changed. 
     The three-lens objective of FIG. 2 has a front lens 34, a median lens 36 and a rear lens 38. Furthermore, an aperture stop 40 is arranged in the path of rays of this three-lens objective closely behind the median lens 36. A plane-parallel plate 42 is arranged in the converging path of rays on the image side of the rear lens 38. The surface normal of plate 42 forms a very small angle with the optical axis 44 of the three-lens objective. This plane-parallel plate 42 is arranged to be driven such that it rotates continuously about the optical axis and thus makes a wobbling movement. This causes a continuous gyrating movement of the image generated by the three-lens objective relative to the two-dimensional array of detector elements of the image detecting sensor. This permits, in well-known manner, to bridge the gaps between the detector elements. Thereby the resolution of a seeker constructed with such a three-lens objective is improved. As the plane-parallel plate is located in the converging path of rays, it has to be taken into account when computing the three-lens objective. 
     This is also true for a window 45 and a filter 46. The window seals the cooled sensor environment from atmosphere. The filter 46 keeps undesired radiation away from the detector. Numeral 48 designates a cooled light stop. The cooled light stop 48 keeps infrared radiation emitted by the environment of the detector itself away from the detector. 
     The front lens 34 consists of silicon. The front lens 34 has positive power. The front lens 34 has a front surface 50 and a rear surface 52. The front surface 50 has a positive radius of curvature. The front surface 50 is aspherical. Also the rear surface 52 has a positive radius of curvature. The radius of curvature of the front surface 50 of the front lens 34 is substantially smaller than the radius of curvature of the rear surface 52 of the front lens 34. 
     The median lens 36 consists of germanium. The medium lens 36 has negative power. The median lens 36 has a front surface 54 and a rear surface 56. The front surface 54 has a negative radius of curvature. The front surface 54 is aspherical. Also the rear surface has a negative radius of curvature. The front surface 54 of the median lens 36 has--as far as the amount is concerned--a smaller radius of curvature than the rear surface 56. 
     The lenses 34 and 36 are spaced and define an air space 58 therebetween. 
     The rear lens 38, again, consists of silicon. The rear lens 38 has positive power. The rear lens 38 has a front surface 60 and a rear surface 62. The front surface 60 has a positive radius of curvature. Also the rear surface 62 has a positive radius of curvature. The front surface 60 is aspheric. The radius of curvature of the front surface 60 of the rear lens 38 is smaller than the radius of curvature of the rear surface 62. 
     An air space 64 is defined between the median lens 36 and the rear lens 38. The distance between the front lens 34 and the median lens 36 is substantially smaller than the distance between the median lens 36 and the rear lens 38. An air space 66 is defined between the rear lens 38 and the plate 42. An air space is defined between the plate 42 and the window 45. The cooled light stop 48 is arranged at a distance behind the window 45. The filter 46 is arranged at a further distance behind the cooled light stop 48. The image of the field of view is generated in an image plane or focal plane 70. The plate 42 consists of silicon. The window 45 consists of germanium. The filter, again, consists of silicon. 
     The detailed data of the three-lens objective illustrated in FIG. 2 are as follows: 
     
                       TABLE 2                                                     
______________________________________                                    
Lens Data                                                                 
                            Aperture                                      
Srf  Radius     Thickness   Radius   Material                             
______________________________________                                    
 1   --         -13.003369  13.847083                                     
                                     Air                                  
 2     40.973000                                                          
                4.000000    15.493000                                     
                                     Si                                   
 3    139.240000                                                          
                6.450000    14.923000                                     
                                     Air                                  
 4    -34.974000                                                          
                2.000000    10.586000                                     
                                     Gel                                  
 5   -102.180000                                                          
                --          10.609000                                     
                                     Air                                  
 6   --         --          10.444000A                                    
 7   --         13.300000   10.444000                                     
                                     Air                                  
 8     23.544000                                                          
                3.000000    10.324000                                     
                                     Si                                   
 9     35.481000                                                          
                3.500000     9.695000                                     
                                     Air                                  
10   --         3.000000     8.999000                                     
                                     Si Scanner                           
11   --         2.197175     8.728000                                     
                                     Air                                  
12   --         2.000000     8.005000                                     
                                     Ge                                   
                                     Window                               
13   --         2.000000     7.851000                                     
                                     Air                                  
14   --         --           7.204000                                     
15   --         6.000000     7.204000                                     
                                     Air                                  
16   --         --           5.263000                                     
17   --         --           5.263000                                     
                                     Air                                  
18   --         1.000000     5.263000                                     
                                     Si                                   
19   --         2.000000     5.173000                                     
                                     Air                                  
20   --         --           4.526000                                     
                                     Air                                  
______________________________________                                    
Aspheric Surface Data                                                     
2    CC     -0.974975 AD   -3.32409E-07                                   
                                     AE  2.23708E-11                      
4    CC     -10.283235                                                    
                      AD   -7.33080E-07                                   
                                    AE   2.38799E-09                      
8    CC     -0.989312 AD   -3.82908E-06                                   
                                    AE   6.37996E-10                      
______________________________________                                    
Refractive Indices                                                        
                                     Abbe-Disperson                       
Srf  Material N1       N2     N3     Number                               
______________________________________                                    
 1   Air      --       --     --     --                                   
 2   Si       3.423380 3.426478                                           
                              3.421153                                    
                                     455.143820                           
 3   Air      --       --     --     --                                   
 4   Ge       4.018807 4.027057                                           
                              4.012839                                    
                                     212.321165                           
 5   Air      --       --     --     --                                   
 6            1.000000 1.000000                                           
                              1.000000                                    
                                     --                                   
 7   Air      --       --     --                                          
 8   Si       3.423380 3.426478                                           
                              3.421153                                    
                                     455.143820                           
 9   Air      --       --     --     --                                   
10   Si       3.423380 3.426478                                           
                              3.421153                                    
                                     455.143820                           
11   Air      --       --     --     --                                   
12   Ge       4.018807 4.027057                                           
                              4.012839                                    
                                     212.321165                           
13   Air      --       --     --     --                                   
14            1.000000 1.000000                                           
                              1.000000                                    
                                     --                                   
15   Air      --       --     --     --                                   
16            1.000000 1.000000                                           
                              1.000000                                    
                                     --                                   
17   Air      --       --     --     --                                   
18   Si       3.423380 3.426478                                           
                              3.421153                                    
                                     455.143820                           
19   Air      --       --     --     --                                   
20   Air      --       --     --     --                                   
______________________________________                                    
 
    
     A three-lens objective having the above recited data exhibits the following characteristics: 
     
                       TABLE 2A                                                    
______________________________________                                    
Optical System Data                                                       
Field of View Angle (+/-Degree) =                                         
                      7.755                                               
F-Number (--) =       1.201                                               
Focal Length (mm) =  33.266                                               
Wavelengths [μm) =                                                     
                      4.600   3.800  5.500                                
______________________________________                                    
Image Point Data                                                          
                         Piercing                                         
                                Optical                                   
                         Points Path                                      
Wave-          Airy      Diameter                                         
                                Difference                                
                                         Strehl-                          
length         Diameter  95%    95%      Def                              
(μm) FOB    (μm)   (μm)                                          
                                (μm)  Ratio                            
______________________________________                                    
4.600   --     13.579    17.954 0.143    0.991                            
3.800   --     11.218    16.881 0.139    0.987                            
5.500   --     16.236    18.956 0.141    0.994                            
Polychr --     13.653    17.936 0.141    0.990                            
4.600   0.700  13.701    24.786 0.407    0.926                            
3.800   0.700  11.317    23.784 0.401    0.896                            
5.500   0.700  16.382    25.790 0.422    0.944                            
Polychr 0.700  13.775    24.787 0.409    0.923                            
4.600   1.000  13.863    36.624 0.862    0.707                            
3.800   1.000  11.451    36.368 0.861    0.603                            
5.500   1.000  16.577    36.899 0.858    0.787                            
Polychr 1.000  13.938    36.628 0.861    0.701                            
______________________________________                                    
 
    
     The three lenses 34, 36 and 38 of the second embodiment of the three-lens objective described above with reference to FIG. 2 cooperate in the same way as the three lenses in the first embodiment of FIG. 1. The values of cc have the same meaning as has been explained above by means of the formula with reference to the embodiment of FIG. 1. 
     FIGS. 7 to 9 illustrate in a similar way as FIGS. 4 to 6 the quality of the image on the basis of the &#34;ray piercing points&#34;, the modulation transfer functions and the image functions. 
     It will be recognized from a comparison of FIG. 4 and FIG. 7, that for the fractional object heights FOB=0 and FOB=0.7 the three-lens objective of FIG. 2 is similarly good, as far as the piercing points are concerned, as the three-lens objective of FIG. 1. It is, however, to be noted that the square fields in FIG. 4 have an edge length of 20 μm, while in FIG. 7 the edge length is 40 μm. Near the edge of the field of view, the ray piercing points are more widely spread. 
     The image function shows a similar waveform. With an fractional object height of FOB=0 and FOB=.7, the image function exhibits high and rather sharp peaks. With an fractional object height of one, i.e. for the marginal areas of the field of view, the peak of the image function is lower and wider. 
     FIG. 8 shows the modulation transfer function of the three-lens objective of FIG. 2 for tangential and sagittal rays and for different image point-height ratios FOB. The dotted, descending, straight line shows the ideal shape, as resulting merely from the diffraction limitation of the image. The solid line 74 is obtained for the fractional object height FOB=0. This graph nearly coincides with the ideal shape of the modulation transfer function. The dashed graph 76 is obtained for sagittal rays and an image point height ratio of FOB=0.7. This graph 76 deviates slightly, though not heavily, from the ideal shape of the graph. The solid graph 78 is obtained for tangential rays and an fractional object height of FOB=0.7. This graph 78 coincides to a large extent with the graph 76. The dashed graph 80 is obtained for sagittal rays and an fractional object height of FOB=1. The solid graph 82 is obtained for tangential rays and an fractional object height of FOB=1. As compared with the ideal shape, the graphs 80 and 82, in particular graph 82, exhibit a heavily sagging &#34;belly&#34;. 
     It results therefrom, that, with the three-lens objective of FIG. 2, the image quality deteriorates towards the edge of the field of view or field of image, respectively. Comparison of the data of Table 1 and Table 2 shows, however, that the angle of the field of view of the three-lens objective of FIG. 1 is 2.828°, while the angle of the field of view of the three-lens objective of FIG. 2 is 7.755°. Thus the three-lens objective of FIG. 2 is a wide angle objective. 
     A further three-lens objective, which is designed in accordance with the same principle as the three-lens objectives of FIG. 1 and FIG. 2, is illustrated in FIG. 3. 
     The three-lens objective of FIG. 3 has a front lens 84, a median lens 86 and a rear lens 88. 
     The front lens 84 consists of a first material which, in the spectral range of interest, exhibits a relatively low dispersion (&#34;relative&#34; as compared with the material of the median lens 86).In the present case, this first material is germanium. This material is transparent to infrared radiation in the spectral range of interest from wavelengths of 8 μm to 12 μm. The front lens 84 has positive power. The front lens has a front surface 90 and a rear surface 92. The front surface has a positive radius of curvature. The front surface 90 is aspherical. Also the rear surface 92 has a positive radius of curvature. The radius of curvature of the front surface 90 of the front lens 84 is substantially smaller than the radius of curvature of the rear surface 92 of the front lens 84. 
     The median lens 86 consists of a second material, which exhibits a relatively large dispersion as compared with the first material germanium of which the front lens 84 consists. In the present case, the second material is zinc selenide. Also zinc selenide is transparent in the spectral range of wavelengths from 8 μm to 12 μm. The median lens has a front surface 94 and a rear surface 96. The front surface has a negative radius of curvature. The front surface 94 of the median lens 86 is aspherical. Also the rear surface 96 of the median lens 86 has a negative radius of curvature. The front surface 94 of the median lens 84--as far as the amount is concerned--has a smaller radius of curvature than the rear surface 96. 
     The lenses 84 and 86 are spaced and define an air space 98 therebetween. 
     The rear lens 88, again, consists of the first material of relatively low dispersion. This is germanium. The rear lens 88 has positive power. The rear lens has a front surface 100 and a rear surface 102. The front surface 100 has a positive radius of curvature. The rear surface 102 has also a positive radius of curvature. The front surface 100 is aspherical. The radius of curvature of the front surface 100 of the rear lens 88 is smaller than the radius of curvature of the rear surface 102. 
     An air space 104 is defined between the median lens 86 and the rear lens 88. The distance between the front lens 84 and the median lens 86 is substantially smaller than the distance between the median lens 86 and the rear lens 88. 
     When considering the three three-lens objectives which are illustrated in FIG. 1, FIG. 2 and FIG. 3, it will be recognized that these three three-lens objectives have a unitary basic structure. This can also be gathered from the description hereinbefore. Also a comparison of Table 1 and Table 2 reveals a plurality of conformities between the two three-lens objectives of FIG. 1 and FIG. 2. 
     In all cases, the front surface of the front lens has a substantially smaller radius of curvature than the rear surface of the front lens. The radii of both surfaces are positive. According to table 1 and table 2, the radius of curvature of the front surface of the front lens is about one fourth to one third of the radius of curvature of the rear surface. With respect to the surfaces 16 and 18 in FIG. 1, the ratio is 95.8:362.6=0.264. With respect to the surfaces 50 and 52 in FIG. 2, the ratio is 40.97:139.24=294. 
     In all cases, the front surface of the median lens has a smaller radius of curvature--as far as the amount is concerned- than the rear surface. The radii of curvature of both surfaces are negative. The radius of curvature of the front surface of the median lens is about one third of the rear surface. With respect to the surfaces 20 and 22 in FIG. 1, the ratio is -115.5:-296.4=0.39. With respect to the surfaces 54 and 56 in FIG. 2, the ratio is 35:102=0.343. 
     The front surface of the rear lens has a smaller radius of curvature than the rear surface. The radii of curvature of both surfaces are positive. The radii of curvature of both surfaces are in the same order of magnitude. At lens 14 in FIG. 1, the two radii of curvature are 39.525 for surface 26 and 40.973 for surface 28. These radii of curvature are substantially equal. At the lens 38 in FIG. 2, the two radii of curvature are 23.544 for surface 60 and 35.481 for surface 62. The ratio of these two radii of curvature is about 2:3. 
     Also the amounts of the radii of curvature of the front surfaces of the front and median lenses are in the same order of magnitude: In table 1 and FIG. 1, the radius of curvature of surface 16 is 95.7. The radius of curvature of surface 20 is--as far as the amount is concerned--115.5. The ratio is about 1.2. In table 2 and FIG. 2, the radius of curvature of the surface 50 is 40.97. The radius of curvature of the surface 54 is--as far as the amount is concerned--34.97. The ratio is 1.17. 
     The thicknesses of the air spaces 30 and 64 are a multiple of the respective thicknesses of the associated air spaces 24 and 58, respectively, namely about four or two times, respectively, of these thicknesses. 
     In the embodiments described hereinbefore,.the front surfaces of the lenses are aspherical. However also the rear surfaces may be aspherical, or the front surface of one lens or some lenses and the rear surface of the remaining lens or lenses. Providing a lens with two aspherical surface presents manufacturing problems. 
     The three-lens objective of FIG. 10 consists of a front lens 110 of germanium, a median lens 112 of zinc selenide, and a rear lens 114 again of germanium. The front lens 110 has positive power. The median lens 112 has negative power. The rear lens 114, again, has positive power. An air space 116 is defined between the front lens 110 and the median lens 112. An air space 118 is defined between the median lens 112 and the rear lens 114. The distance of the median lens 112 from the front lens 110 is substantially smaller than the distance of the rear lens 114 from the median lens 112. The front surface 120 of the front lens 110, the front surface of the median lens 112 and the front surface 124 of the rear lens 114 are aspherical. 
     The three-lens objective of FIG. 10 is appropriate for long-wavelength infrared radiation with a wavelength range between 8 μm and 12 μm. The path of rays is such that an aperture stop 126 can be arranged behind the rear lens 114. This aperture stop 126 can then be a &#34;cold light stop&#34;, if the three-lens objective is used in a seeker having a cooled sensor, the light stop being cooled down to low temperatures. Such a cold light stop 126 ensures that the sensor is exposed substantially to pure useful radiation. 
     In detail, the three-lens objective of FIG. 10 has the following data: 
     
                       TABLE 3                                                     
______________________________________                                    
Lens Data                                                                 
                           Aperture                                       
Srf  Radius    Thickness   Radius  Glass Notes                            
______________________________________                                    
1    --        -177.23730  41.66667                                       
                                   Air                                    
2      70.01694                                                           
                7.47307    55.00000                                       
                                   Ge    1. Lens                          
3      71.50233                                                           
               27.00000    52.43139                                       
                                   Air                                    
4    -517.42425                                                           
                5.63758    52.00000                                       
                                   ZNSE  2. Lens                          
5    -743.63318                                                           
               52.44256    52.00000                                       
                                   Air                                    
6      85.02567                                                           
                6.55532    32.00000                                       
                                   Ge    3. Lens                          
7     112,74740                                                           
                6.74923    31.00000                                       
                                   Air                                    
8    --        50.38182    22.27019A                                      
                                   Air                                    
9    --        --          10.98252                                       
                                   Air   Focal                            
                                         Plane                            
______________________________________                                    
Special and Aspheric Data                                                 
2             CC     -0.06772                                             
4             CC     -16.82474                                            
6             CC     -0.28703                                             
______________________________________                                    
Refractive Indices                                                        
Srf  Glass   N1         N2      N3    V                                   
______________________________________                                    
1    Air     --         --      --    --                                  
2    Ge      4.00420    4.00560 4.00190                                   
                                      811.81345                           
3    Air     --         --      --    --                                  
4    ZnSe    2.41220    2.41726 2.39296                                   
                                       58.12443                           
5    Air     --         --      --    --                                  
6    Ge      4.00420    4.00560 4.00190                                   
                                      811.81345                           
7    Air     --         --      --    --                                  
8    Air     --         --      --    --                                  
9    Air     --         --      --    --                                  
______________________________________                                    
 
    
     A three-lens objective having the above recited data exhibits the following characteristics: 
     
                       TABLE 3A                                                    
______________________________________                                    
Optical System Data                                                       
Field of View Angle (+/-Degree) =                                         
                      5.000                                               
F-Number (--) =       1.200                                               
Foca Length (mm) =   100.000                                              
Wavelengths (μm) =                                                     
                      9.000  8.000   12.000                               
______________________________________                                    
Image Point Data                                                          
                         Piercing                                         
                                Optical                                   
                         Points Path                                      
Wave-          Airy      Diameter                                         
                                Difference                                
                                         Strehl-                          
length         Diameter  95%    95%      Def.                             
(μm) FOB    (μm)   (μm)                                          
                                (μm)  Ratio                            
______________________________________                                    
9.000   --     26.578    35.066 1.511    0.757                            
8.000   --     23.619    13.587 0.498    0.962                            
12.000  --     35.447    19.531 0.836    0.953                            
Polychr --     28.055    25.812 1.089    0.858                            
9.000   0.700  26.589    34.011 1.277    0.820                            
8.000   0.700  23.627    26.280 0.841    0.897                            
12.000  0.700  35.461    24.080 0.787    0.958                            
Polychr 0.700  28.066    29.596 1.046    0.874                            
9.000   1.000  26.575    33.940 1.393    0.789                            
8.000   1.000  23.615    51.801 2.153    0.489                            
12.000  1.000  35.443    49.551 2.008    0.759                            
Polychr 1.000  28.052    42.308 1.736    0.707                            
______________________________________                                    
 
    
     FIG. 11 is a diagram similar to FIG. 4 and illustrates for a number of fractional object heights FOB=0; FOB=0.6 and FOB=1  the piercing points of the various parallelly incident rays through the focal plane in a three-lens objective of FIG. 10. 
     FIG. 12 is a diagram similar to FIG. 5 and illustrates for three different fractional object heights 0; 0.7 and 1 the modulation transfer function for tangential and sagittal rays in a three-lens objective of FIG. 10. The graphs pertaining to the fractional object height F=0 are marked by a cross (+), the graphs pertaining to FOB=0.7 are marked by a square, and the graphs pertaining to FOB=1 are marked by a triangle. The graphs for tangential rays are drawn with solid lines. The graphs for sagittal rays are drawn with dashed lines. The ideal graphs are drawn with dotted lines. 
     FIG. 13 shows a three-lens objective having a front lens 130, a median lens 132 and a rear lens 134. The front lens 130 consists of germanium, the median lens 132 consists of zinc selenide, and the rear lens consists of germanium again. The front lens 130 has positive power, the median lens 132 has negative power, and the rear lens 134 has positive power again. An air space 136 is defined between the front lens 130 and the median lens 132. An air space 138 is defined between the median lens 132 and the rear lens 134. The distance of the median lens 132 from the front lens 130 is substantially smaller than the distance of the rear lens 134 from the median lens 132. The front surface 140 of the front lens 132, the front surface 142 of the median lens 132 and the front surface 144 of the rear lens are aspherical. 
     Also the three-lens objective of FIG. 13 is appropriate for long-wavelength infrared in a wavelength range between 8 μm and 12 μm. Also here, the path of rays is such that an aperture stop can be arranged behind the rear lens 134. 
     In detail, the data of the three-lens objective of FIG. 13 are as follows: 
     
                       TABLE 4                                                     
______________________________________                                    
Lens Data                                                                 
                           Aperture                                       
Srf Radius     Thickness   Radius  Glass Notes                            
______________________________________                                    
1   --         -156.29093  33.33333                                       
                                   Air                                    
2     64.24548  7.47307    45.50000                                       
                                   Ge    1. Lens                          
3     65.75754 17.00000    42.74139 S                                     
                                   Air                                    
4    -670.00482                                                           
                5.63758    42.50000                                       
                                   ZnSe  2. Lens                          
5   -1105.21495                                                           
               52.44256    42.50000                                       
                                   Air                                    
6     74.24877  6.55532    23.50000                                       
                                   Ge    3. Lens                          
7     89.57195  4.00000    22.00000                                       
                                   Air                                    
8   --         50.38182    17.81615A                                      
                                   Air                                    
9   --         --           8.78602                                       
                                   Air   Focal                            
                                         Plane                            
______________________________________                                    
Special and Aspheric Data                                                 
2             CC     -0.06463                                             
4             CC     -91.21630                                            
6             CC     -0.37256                                             
______________________________________                                    
Refractive Indices                                                        
Srf  Glass      N1      N2      N3    V                                   
______________________________________                                    
1    Air        --      --      --                                        
2    Ge         4.00420 4.00560 4.00190                                   
                                      811.81345                           
3    Air        --      --      --    --                                  
4    ZnSe       2.41220 2.41726 2.39296                                   
                                       58.12443                           
5    Air        --      --      --    --                                  
6    Ge         4.00420 4.00560 4.00190                                   
                                      811.81345                           
7    Air        --      --      --    --                                  
8    Air        --      --      --    --                                  
9    Air        --      --      --    --                                  
______________________________________                                    
 
    
     A three-lens objective having the above recited data exhibits the following characteristics: 
     
                       TABLE 4A                                                    
______________________________________                                    
Optical System Data                                                       
Field of View Angle (+/-Degree) =                                         
                      5.000                                               
F-Number (--) =       1.500                                               
Focal Length (mm) =  100.000                                              
Wavelengths [μm) =                                                     
                      9.000  8.000   12.000                               
______________________________________                                    
Image Point Data                                                          
                         Piercing                                         
                                Optical                                   
                         Points Rath                                      
Wave-           Airy     Diameter                                         
                                Difference                                
                                        Strehl-                           
length          Diameter 95%    95%     Def.                              
(μm) FOB     (μm)  (μm)                                          
                                (μm) Ratio                             
______________________________________                                    
9.000   --      33.233   16.754 0.604   0.957                             
8.000   --      29.532   10.379 0.129   0.997                             
12.000  --      44.321    8.568 0.201   0.997                             
Polychr --      35.080   13.114 0.384   0.977                             
9.000   0.700   33.260   22.380 0.665   0.948                             
8.000   0.700   29.555   20.141 0.520   0.959                             
12.000  0.700   44.358   18.886 0.482   0.984                             
Polychr 0.700   35.108   20.947 0.583   0.960                             
9.000   1.000   33.253   23.685 0.782   0.928                             
8.000   1.000   29.549   38.726 1.296   0.772                             
12.000  1.000   44.350   31.811 1.088   0.922                             
polychr 1.000   35.102   29.477 0.987   0.887                             
______________________________________                                    
 
    
     FIG. 14 shows a three-lens objective having a front lens 150, a median lens 152 and a rear lens 154. The front lens 150 consists of germanium, the median lens 152 consists of zinc sulfide, and the rear lens 154 consists of germanium again. The front lens 150 has positive power, the median lens 152 has negative power, and the rear lens 154 has positive power again. An air space 156 is defined between the front lens 150 and the median lens 152. An air space 158 is defined between the median lens 152 and the rear lens 154. The distance of the median lens 152 from the front lens 150 is substantially smaller than the distance of the rear lens 154 from the median lens 152. The front surface 160 of the front lens 152, the front surface 162 of the median lens 152 and the front surface 164 of the rear lens 154 are aspherical. 
     Also the three-lens objective of FIG. 14 is appropriate for long-wavelength infrared in a wavelength range between 8 μm and 12 μm. Also here, the path of rays is such that an aperture stop can be arranged behind the rear lens 154. 
     In detail, the data of the three-lens objective of FIG. 14 are as follows: 
     
                       TABLE 5                                                     
______________________________________                                    
Lens Data                                                                 
                          Aperture                                        
Srf Radius     Thickness  Radius   Glass Notes                            
______________________________________                                    
l   --         -139.39410 33.08307 Air                                    
2   66.70624     7.47378  44.00000 Ge    1. Lens                          
3   68.69355     15.00000 41.00000 Air                                    
4   -2421.54311                                                           
                 5.63812  41.00000 ZnS   2. Lens                          
5   -4863.26768                                                           
                 52.44758 40.00000 Air                                    
6   101.51108    6.55595  22.00000 Ge    3. Lens                          
7   137.03175    54.38703 18.69937A                                       
                                   Air                                    
8   --         --          8.75690 Air   Focal                            
                                         Plane                            
______________________________________                                    
Special and Aspheric Data                                                 
2        CC               -0.06888                                        
4        CC               -489.14951                                      
6        CC               -0.78014                                        
______________________________________                                    
Refractive Indices                                                        
Srf    Glass    N1      N2      N3    V                                   
______________________________________                                    
1      Air      --      --      --    --                                  
2      Ge       4.00420 4.00560 4.00190                                   
                                      811.81345                           
3      Air      --      --      --    --                                  
4      ZnS      2.2l230 2.22280 2.17000                                   
                                       22.96014                           
5      Air      --      --      --    --                                  
6      Ge       4.00420 4.00560 4.00190                                   
                                      811.81345                           
______________________________________                                    
 
    
     A three-lens objective having the above recited data exhibits the following characteristics: 
     
                       TABLE 5A                                                    
______________________________________                                    
Optical System Data                                                       
Field of View                                                             
          (+/- Degree) =  5.000                                           
Angle                                                                     
F-Number  (--)         =  1.511                                           
Focal Length                                                              
          (mm)         = 100.000                                          
Wavelengths                                                               
          [μm)      =  9.000  8.000 12.000                             
______________________________________                                    
Image Point Data                                                          
                        Piercing                                          
                                Optical                                   
                        Points  Path                                      
Wave-   Airy            Diameter                                          
                                Difference                                
                                        Strehl-                           
length         Diameter 95%     95%     Def.                              
(μm) FOB    (μm)  (μm) (μm) Ratio                             
______________________________________                                    
9.000   --     33.506   18.321  0.596   0.958                             
8.000   --     29.774    3.332  0.102   0.998                             
12.000  --     44.690    5.459  0.181   0.998                             
Polychr --     35.369   11.358  0.369   0.978                             
9.000   0.700  33.580   17.885  0.536   0.966                             
8.000   0.700  29.839   21.443  0.653   0.936                             
12.000  0.700  44.791   17.572  0.516   0.982                             
Polychr 0.700  35.448   18.696  0.561   0.962                             
9.000   1.000  33.623   34.295  1.126   0.857                             
8.000   1.000  29.877   49.122  1.614   0.669                             
12.000  1.000  44.849   46.047  1.470   0.862                             
Polychr 1.000  35.493   44.940  1.334   0.811                             
______________________________________                                    
 
    
     The three-lens objective of FIG. 15 consists of a front lens 170 of germanium, a median lens 172 of zinc selenide, and a rear lens 174 of germanium again. The front lens 170 has positive power, the median lens 172 has negative power, and the rear lens 174 has positive power again. An air space 176 is defined between the front lens 170 and the median lens 172. An air space 178 is defined between the median lens 172 and the rear lens 174. The distance of the median lens 172 from the front lens 170 is substantially smaller than the distance of the rear lens 174 from the median lens 172. The front surface 180 of the front lens 172, the front surface 182 of the median lens 172 and the front surface 184 of the rear lens 174 are aspherical. 
     Also the three-lens objective of FIG. 15 is appropriate for long-wavelength infrared in a wavelength range between 8 μm and 12 μm. In contrast, for example, to the three-lens objective of FIG. 14, however, the path of rays is such that an aperture stop can be arranged behind the median lens 172 but in front of the rear lens 174. 
     In detail, the data of the three-lens objective of FIG. 15 are as follows: 
     
                       TABLE 6                                                     
______________________________________                                    
Lens Data                                                                 
                          Aperture                                        
Srf Radius     Thickness  Radius   Glass Notes                            
______________________________________                                    
l   --         -42.32215  41.66667 Air                                    
2   69.08962     7.47307  45.50000 Ge    1. Lens                          
3   70.49711    27.00000  41.52428 Air                                    
4   -467.15591   5.63758  38.00000 ZnSe  2. Lens                          
5   -703.91399  52.44256  33.23295 A                                      
                                   Air                                    
6   80.92787     6.55532  31.33321 Ge    3. Lens                          
7   105.26772    6.74923  29.36970 Air                                    
8   --          50.38182  22.27019 Air                                    
9   --         --         10.98252 Air   Focal                            
                                         Plane                            
______________________________________                                    
Special and Aspheric Data                                                 
______________________________________                                    
2        CC               -0.08201                                        
4        CC               -52.88740                                       
6        CC               -0.32580                                        
______________________________________                                    
Refractive Indices                                                        
Srf    Glass    Nl      N2      N3    V                                   
______________________________________                                    
l      Air      --      --      --    --                                  
2      Ge       4.00420 4.00560 4.00l90                                   
                                      811.81345                           
3      Air      --      --      --    --                                  
4      ZnSe     2.41220 2.41726 2.39296                                   
                                       58.12443                           
5      Air      --      --      --    --                                  
6      Ge       4.00420 4.00560 4.00190                                   
                                      811.81345                           
7      Air      --      --      --    --                                  
8      Air      --      --      --    --                                  
9      Air      --      --      --    --                                  
______________________________________                                    
 
    
     A three-lens objective having the above recited data exhibits the following characteristics: 
     
                       TABLE 6A                                                    
______________________________________                                    
Optical System Data                                                       
Field of View                                                             
          (+/- Degree) =  5.000                                           
Angle                                                                     
F-Number  (--)         =  1.200                                           
Focal Lenght                                                              
          (mm)         = 100.000                                          
Wavelengths                                                               
          [μm)      =  9.000  8.000 12.000                             
______________________________________                                    
Image Point Data                                                          
                        Piercing                                          
                                Optical                                   
                        Points  Path                                      
Wave-          Airy     Diameter                                          
                                Difference                                
                                        Strehl-                           
length         Diameter 95%     95%     Def.                              
(μm) FOB    (μm)  (μm) (μm) Ratio                             
______________________________________                                    
9.000   --     26.615   65.173  1.194   0.841                             
8.000   --     23.651   47.256  0.897   0.883                             
12.000  --     35.497   58.274  1.031   0.930                             
Polychr --     28.094   58.969  1.079   0.874                             
9.000   0.700  26.587   35.342  0.924   0.901                             
8.000   0.700  23.626   31.538  1.172   0.809                             
12.000  0.700  35.461   34.200  1.133   0.916                             
Polychr 0.700  28.065   34.106  1.038   0.882                             
9.000   1.000  26.580   53.625  1.935   0.634                             
8.000   1.000  23.619   65.091  2.250   0.458                             
12.000  1.000  35.451   59.131  2.166   0.725                             
Polychr 1.000  28.057   57.868  2.072   0.613                             
______________________________________                                    
 
    
     FIG. 16 is, again, a diagram similar to FIG. 4 and illustrates for a number of fractional object heights FOB=0; FOB=0.7 and FOB=1 the piercing points of the various parallelly incident rays through the focal plane in a three-lens objective of FIG. 15. 
     FIG. 17 is a diagram similar to FIGS. 5 and 12 and illustrates 0 three different fractional object heights 0; 0.7 and 1 the for modulation transfer function for tangential and sagittal rays in a three-lens objective of FIG. 15. The graphs pertaining to the fractional object height F=0 are marked by a cross (+), the graphs pertaining to FOB=0.7, are marked by a square, and the graphs pertaining to FOB=1 are marked by a triangle. The graphs for tangential rays are drawn with solid lines. The graphs for sagittal rays are drawn with dashed lines. The ideal graphs are drawn with dotted lines. 
     The three-lens objective of FIG. 18 consists of a front lens 190 of germanium, a median lens 192 of zinc sulfide, and a rear lens 194 of germanium again. The front lens 190 has positive power, the median lens 192 has negative power, and the rear lens 194 has positive power again. An air space 196 is defined between the front lens 190 and the median lens 192. An air space 198 is defined between the median lens 192 and the rear lens 194. The distance of the median lens 192 from the front lens 190 is substantially smaller than the distance of the rear lens 194 from the median lens 192. The front surface 200 of the front lens 192, the front surface 202 of the median lens 192 and the front surface 204 of the rear lens 194 are aspherical. 
     In detail, the data of the three-lens objective are as follows: 
     
                       TABLE 7                                                     
______________________________________                                    
Lens Data                                                                 
                          Aperture                                        
Srf Radius     Thickness  Radius   Glass Notes                            
______________________________________                                    
1   --         -24.76890  33.33333 Air                                    
2   75.26509   7.47306    34.33407 Ge    1. Lens                          
3   80.17272   15.00457V  32.03975 Air                                    
4   7827.23829 5.63758    29.96496 ZnS   2. Lens                          
5   2903.51135 52.44256   29.29838A                                       
                                   Air                                    
6   152.32088  6.55532    24.63244 Ge    3. Lens                          
7   260.72381  54.38182   23.76313 Air                                    
8   --         --          8.40445 Air   Focal                            
                                         Plane                            
______________________________________                                    
Special and Aspheric Data                                                 
______________________________________                                    
2        CC               -0.08439                                        
4        CC               -1.8823E + 04                                   
6        CC               -2.47048                                        
______________________________________                                    
Refractive Indices                                                        
Srf  Glass   RN1/RN4   RN2/RN5 RN3/RN6 VNBR                               
______________________________________                                    
1    Air     --        --      --      --                                 
2    Ge      4.00420   4.00560 4.00190 811.81345                          
3    Air     --        --      --      --                                 
4    ZnS     2.21230   2.22280 2.17000  22.96014                          
5    Air     --        --      --      --                                 
6    Ge      4.00420   4.00560 4.00190 811.81345                          
7    Air     --        --      --      --                                 
8    Air     --        --      --      --                                 
______________________________________                                    
 
    
     A three-lens objective having the above recited data exhibits the following characteristics: 
     
                       TABLE 7A                                                    
______________________________________                                    
Optical System Data                                                       
Field of View                                                             
          (+/- Degree) =  5.000                                           
Angle                                                                     
F-Number  (--)         =  1.500                                           
Focal Lenght                                                              
          (mm)         = 100.000                                          
Wavelengths                                                               
          [μm)      =  9.000  8.000 12.000                             
______________________________________                                    
Image Point Data                                                          
                        Piercing                                          
                                Optical                                   
                        Points  Path                                      
Wave-          AIRY     Diameter                                          
                                Difference                                
                                        Strehl-                           
length         Diameter 95%     95%     Def.                              
(μm) FOB    (μm)  (μm) (μm) Ratio                             
______________________________________                                    
9.000   --     33.297   47.830  0.839   0.918                             
8.000   --     29.590   34.826  0.478   0.965                             
12.000  --     44.408   29.271  0.405   0.989                             
Polychr --     35.148   39.939  0.641   0.947                             
9.000   0.700  33.348   21.025  0.673   0.946                             
8.000   0.700  29.634   18.465  0.777   0.911                             
12.000  0.700  44.475   18.705  0.794   0.958                             
Polychr 0.700  35.201   19.805  0.729   0.940                             
9.000   1.000  33.412   35.305  1.384   0.792                             
8.000   1.000  29.691   45.260  1.610   0.670                             
12.000  1.000  44.560   49.096  1.645   0.831                             
Polychr 1.000  35.269   41.242  1.506   0.771                             
______________________________________                                    
 
    
     The various embodiments have in common, that the two positive power lenses have a bending towards the longer one of the back or front focal lengths, respectively. 
     Furthermore, it has been found, that the aspherical surfaces can be pure conic section surfaces of revolution, thus ellipsoids of revolution, hyperboloids of revolution or paraboloids of revolution.