Abstract:
A four-piece infrared single wavelength lens system includes, in order from the object side to the image side: a first lens element with a refractive power, a stop, a second lens element with a positive refractive power, a third lens element with a positive refractive power, and a fourth lens element with a positive refractive power. The focal length of the first lens element is f 1 , the focal length of the second lens element and the third lens element combined is f 23 , and they satisfy the relation: −210&lt;f 1 /f 23 &lt;365. When the above relation is satisfied, a wide field of view can be obtained and the resolution can be improved evidently.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Field of the Invention 
         [0002]    The present invention relates to a lens system, and more particularly to a miniaturized four-piece infrared single wavelength lens system applicable to electronic products. 
         [0003]    Description of the Prior Art 
         [0004]    Nowadays digital imaging technology is constantly innovating and changing, in particular, digital carriers, such as, digital camera and mobile phone and so on, have become smaller in size, so CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor) sensor is also required to be more compact. In addition to be used in the field of photography, in recent years, infrared focusing lens has also be used in infrared receiving and sensing field of the game machine, and in order to make the scope of game machine induction user more broader, wide-angle lens group has become the mainstream for receiving infrared wavelength at present. 
         [0005]    The applicant has also put forward a number of lens groups related to infrared wavelength reception, such as the single focus wide-angle lens modules disclosed in TW Appl. Nos. 098100552, 098125378 and U.S. Pat. Nos. 8,031,413, 8,369,031, however, at present, the game machine is based on a more three-dimensional, real and immediate 3D game, the current or the applicant&#39;s previous lens groups are all 2D plane games, which cannot meet the 3D game focusing on the deep induction efficacy. 
         [0006]    Special infrared receiving and induction lens groups for game machines are made of plastic for the pursuit of low cost, however, poor material transparency is one of the key factors that affect the depth detection accuracy of the game machine, and plastic lenses are easy to overheat or too cold in ambient temperature, so that the focal length of the lens group will be changed and cannot focus accurately. Therefore, the current infrared receiving and induction lens groups cannot meet the 3D game depth precise induction requirement. 
         [0007]    The present invention mitigates and/or obviates the aforementioned disadvantages. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is aimed at providing a four-piece infrared single wavelength lens system which has a wide field of view, high resolution, short length and less distortion. 
         [0009]    Therefore, a four-piece infrared single wavelength lens system in accordance with the present invention comprises, in order from an object side to an image side: a first lens element with a refractive power, having an object-side surface being concave near an optical axis and an image-side surface being convex near the optical axis, at least one of the object-side surface and the image-side surface of the first lens element being aspheric; a stop; a second lens element with a positive refractive power, having an object-side surface being convex near the optical axis and an image-side surface being convex near the optical axis, at least one of the object-side surface and the image-side surface of the second lens element being aspheric; a third lens element with a positive refractive power having an object-side surface being concave near the optical axis and an image-side surface being convex near the optical axis, at least one of the object-side surface and the image-side surface of the third lens element being aspheric; and a fourth lens element with a positive refractive power having an object-side surface being convex near the optical axis and an image-side surface being concave near the optical axis, at least one of the object-side surface and the image-side surface of the fourth lens element being aspheric. 
         [0010]    A focal length of the first lens element is f 1 , a focal length of the second lens element and the third lens element combined is f 23 , and they satisfy the relation: −210&lt;f 1 /f 23 &lt;365. 
         [0011]    When the above relation is satisfied, a wide field of view can be obtained and the resolution can be improved evidently. 
         [0012]    Preferably, the focal length of the first lens element is f 1 , a focal length of the second lens element is f 2 , and they satisfy the relation: −230&lt;f 1 /f 2 &lt;390, so that the refractive power of the first lens element and the second lens element are more suitable, it will be favorable to obtain a wide field of view and avoid the excessive increase of aberration of the system. 
         [0013]    Preferably, a focal length of the third lens element is f 3 , the focal length of the second lens element is f 2 , and they satisfy the relation: 3.5&lt;f 3 /f 2 &lt;86. 
         [0014]    Preferably, the focal length of the third lens element is f 3 , a focal length of the fourth lens element is f 4 , and they satisfy the relation: 0.5&lt;f 3 /f 4 &lt;39, so that the refractive power of the system can be balanced effectively, it will be favorable to reduce the sensitivity of the system, improving the yield of production. 
         [0015]    Preferably, the focal length of the first lens element is f 1 , the focal length of the third lens element is f 3 , and they satisfy the relation: −55&lt;f 1 /f 3 &lt;100, so that the positive refractive power of the first lens element can be distributed effectively, so as to reduce the sensitivity of the four-piece infrared single wavelength lens system. 
         [0016]    Preferably, the focal length of the second lens element is f 2 , the focal length of the fourth lens element is f 4 , and they satisfy the relation: 0.05&lt;f 2 /f 4 &lt;0.6, so that the positive refractive power of the system is more suitable, it will be favorable to correct the aberration of the system and improve the image quality. 
         [0017]    Preferably, the focal length of the third lens element is f 3 , a focal length of the first lens element and the second lens element combined is f 12 , and they satisfy the relation: 3.5&lt;f 3 /f 12 &lt;93, which is favorable to obtain a wide field of view and effectively correct image distortion. 
         [0018]    Preferably, the focal length of the first lens element and the second lens element combined is f 12 , a focal length of the third lens element and the fourth lens element combined is f 34 , and they satisfy the relation: 0.35&lt;f 12 /f 34 &lt;0.65, which is favorable to obtain a wide field of view and effectively correct image distortion. 
         [0019]    Preferably, the focal length of the first lens element is f 1 , a focal length of the second lens element, the third lens element and the fourth lens element combined is f 234 , and they satisfy the relation: −280&lt;f 1 /f 234 &lt;480, which is favorable to obtain a wide field of view and effectively correct image distortion. 
         [0020]    Preferably, the four-piece infrared single wavelength lens system has a maximum view angle FOV, and it satisfies the relation: 45&lt;FOV&lt;75, so that the four-piece infrared single wavelength lens system will have an appropiately large field of view. 
         [0021]    Preferably, a central thickness of the second lens element along the optical axis is CT 2 , a distance along the optical axis between the second lens element and the third lens element is T 23 , and they satisfy the relation: 1.5&lt;CT 2 /T 23 &lt;2.6, so that the thickness of the second lens element and the distance between the lens elements are more suitable, which can effectively reduce the total length of the lens system. 
         [0022]    Preferably, the distance along the optical axis between the second lens element and the third lens element is T 23 , a central thickness of the third lens element along the optical axis is CT 3 , and they satisfy the relation: 0.6&lt;T 23 /CT 3 &lt;1.3, so that the height of the off-axis incident light passing through the second and third lens elements is relatively large, and the third lens element has sufficient capacity to correct the field curve, distortion and coma aberration of the four-piece infrared single wavelength lens system, which is favorable to correct the image quality. 
         [0023]    Preferably, the central thickness of the third lens element along the optical axis is CT 3 , a distance along the optical axis between the third lens element and the fourth lens element is T 34 , and they satisfy the relation: 7&lt;CT 3 /T 34 &lt;12.5, so that the thickness of the third lens element and the distance between the lens elements are more suitable, which can effectively reduce the total length of the lens system. 
         [0024]    Preferably, an Abbe number of the second lens element is V 2 , an Abbe number of the third lens element is V 3 , and they satisfy the relation: 30&lt;V 2 -V 3 &lt; 42 , which is favorable to correct the chromatic aberration of the lens system. 
         [0025]    Preferably, a f-number of the four-piece infrared single wavelength lens system is Fno, and it satisfies the relation: 1.2&lt;Fno&lt;1.8, so that the size of the stop of the four-piece infrared single wavelength lens system can be adjusted properly, making the four-piece infrared single wavelength lens system has a big stop. 
         [0026]    The present invention will be presented in further details from the following descriptions with the accompanying drawings, which show, for purpose of illustrations only, the preferred embodiments in accordance with the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1A  shows a four-piece infrared single wavelength lens system in accordance with a first embodiment of the present invention; 
           [0028]      FIG. 1B  shows the longitudinal spherical aberration curve, the astigmatic field curve and the distortion curve of the first embodiment of the present invention; 
           [0029]      FIG. 2A  shows a four-piece infrared single wavelength lens system in accordance with a second embodiment of the present invention; 
           [0030]      FIG. 2B  shows the longitudinal spherical aberration curve, the astigmatic field curve and the distortion curve of the second embodiment of the present invention; 
           [0031]      FIG. 3A  shows a four-piece infrared single wavelength lens system in accordance with a third embodiment of the present invention; 
           [0032]      FIG. 3B  shows the longitudinal spherical aberration curve, the astigmatic field curve and the distortion curve of the third embodiment of the present invention; 
           [0033]      FIG. 4A  shows a four-piece infrared single wavelength lens system in accordance with a fourth embodiment of the present invention; 
           [0034]      FIG. 4B  shows the longitudinal spherical aberration curve, the astigmatic field curve and the distortion curve of the fourth embodiment of the present invention; 
           [0035]      FIG. 5A  shows a four-piece infrared single wavelength lens system in accordance with a fifth embodiment of the present invention; and 
           [0036]      FIG. 5B  shows the longitudinal spherical aberration curve, the astigmatic field curve and the distortion curve of the fifth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0037]    Referring to  FIGS. 1A and 1B ,  FIG. 1A  shows a four-piece infrared single wavelength lens system in accordance with a first embodiment of the present invention, and  FIG. 1B  shows, in order from left to right, the longitudinal spherical aberration curves, the astigmatic field curves, and the distortion curve of the first embodiment of the present invention. A four-piece infrared single wavelength lens system in accordance with the first embodiment of the present invention comprises a stop  100  and a lens group. The lens group comprises, in order from an object side to an image side: a first lens element  110 , a second lens element  120 , a third lens element  130 , a fourth lens element  140 , an IR cut filter  170 , and an image plane  180 , wherein the four-piece infrared single wavelength lens system has a total of four lens elements with refractive power. The stop  100  is disposed between an image-side surface  112  of the first lens element  110  and an image-side surface  122  of the second lens element  120 . 
         [0038]    The first lens element  110  with a negative refractive power has an object-side surface  111  being concave near an optical axis  190  and the image-side surface  112  being convex near the optical axis  190 , the object-side surface  111  and the image-side surface  112  are aspheric, and the first lens element  110  is made of plastic material. 
         [0039]    The second lens element  120  with a positive refractive power has an object-side surface  121  being convex near the optical axis  190  and the image-side surface  122  being convex near the optical axis  190 , the object-side surface  121  and the image-side surface  122  are aspheric, and the second lens element  120  is made of plastic material. 
         [0040]    The third lens element  130  with a positive refractive power has an object-side surface  131  being concave near the optical axis  190  and an image-side surface  132  being convex near the optical axis  190 , the object-side surface  131  and the image-side surface  132  are aspheric, and the third lens element  130  is made of plastic material. 
         [0041]    The fourth lens element  140  with a positive refractive power has an object-side surface  141  being convex near the optical axis  190  and an image-side surface  142  being concave near the optical axis  190 , the object-side surface  141  and the image-side surface  142  are aspheric, and the fourth lens element  140  is made of plastic material. 
         [0042]    The IR cut filter  170  made of glass is located between the fourth lens element  140  and the image plane  180  and has no influence on the focal length of the four-piece infrared single wavelength lens system. 
         [0043]    The equation for the aspheric surface profiles of the respective lens elements of the first embodiment is expressed as follows: 
         [0044]    wherein: 
         [0000]    
       
         
           
             z 
             = 
             
               
                 
                   ch 
                   2 
                 
                 
                   1 
                   + 
                   
                     
                       [ 
                       
                         1 
                         - 
                         
                           
                             ( 
                             
                               k 
                               + 
                               1 
                             
                             ) 
                           
                            
                           
                             c 
                             2 
                           
                            
                           
                             h 
                             2 
                           
                         
                       
                       ] 
                     
                     0.5 
                   
                 
               
               + 
               
                 Ah 
                 4 
               
               + 
               
                 Bh 
                 6 
               
               + 
               
                 Ch 
                 8 
               
               + 
               
                 Dh 
                 10 
               
               + 
               
                 Eh 
                 12 
               
               + 
               
                 Gh 
                 14 
               
               + 
               
                 … 
                  
                 
                     
                 
                  
                 z 
               
             
           
         
       
     
         [0045]    represents the value of a reference position with respect to a vertex of the surface of a lens and a position with a height h along the optical axis  190 ; 
         [0046]    c represents a paraxial curvature equal to 1/R (R: a paraxial radius of curvature); 
         [0047]    h represents a vertical distance from the point on the curve of the aspheric surface to the optical axis  190 ; 
         [0048]    k represents the conic constant; 
         [0049]    A, B, C, D, E, G, . . . : represent the high-order aspheric coefficients. 
         [0050]    In the first embodiment of the present four-piece infrared single wavelength lens system, a focal length of the four-piece infrared single wavelength lens system is f, a f-number of the four-piece infrared single wavelength lens system is Fno, the four-piece infrared single wavelength lens system has a maximum view angle (field of view) FOV, and they satisfy the relations: f=1.241 mm; Fno=1.4; and FOV=60 degrees. 
         [0051]    In the first embodiment of the present four-piece infrared single wavelength lens system, a focal length of the first lens element  110  is f 1 , a focal length of the second lens element  120  and the third lens element  130  combined is f 23 , and they satisfy the relation: f 1 /f 23 =−24.071. 
         [0052]    In the first embodiment of the present four-piece infrared single wavelength lens system, the focal length of the first lens element  110  is f 1 , a focal length of the second lens element  120  is f 2 , and they satisfy the relation: f 1 /f 2 =−29.133. 
         [0053]    In the first embodiment of the present four-piece infrared single wavelength lens system, a focal length of the third lens element  130  is f 3 , the focal length of the second lens element  120  is f 2 , and they satisfy the relation: f 3 /f 2 =22.598. 
         [0054]    In the first embodiment of the present four-piece infrared single wavelength lens system, the focal length of the third lens element  130  is f 3 , a focal length of the fourth lens element  140  is f 4 , and they satisfy the relation: f 3 /f 4 =8.144. 
         [0055]    In the first embodiment of the present four-piece infrared single wavelength lens system, the focal length of the first lens element  110  is f 1 , the focal length of the third lens element  130  is f 3 , and they satisfy the relation: f 1 /f 3 =−1.289. 
         [0056]    In the first embodiment of the present four-piece infrared single wavelength lens system, the focal length of the second lens element  120  is f 2 , the focal length of the fourth lens element  140  is f 4 , and they satisfy the relation: f 2 /f 4 =0.360. 
         [0057]    In the first embodiment of the present four-piece infrared single wavelength lens system, the focal length of the third lens element  130  is f 3 , a focal length of the first lens element  110  and the second lens element  120  combined is f 12 , and they satisfy the relation: f 3 /f 12 =23.929. 
         [0058]    In the first embodiment of the present four-piece infrared single wavelength lens system, the focal length of the first lens element  110  and the second lens element  120  combined is f 12 , a focal length of the third lens element  130  and the fourth lens element  140  combined is f 34 , and they satisfy the relation: f 12 /f 34 =0.476. 
         [0059]    In the first embodiment of the present four-piece infrared single wavelength lens system, the focal length of the first lens element is f 1 , a focal length of the second lens element  120 , the third lens element  130  and the fourth lens element  140  combined is f 234 , and they satisfy the relation: f 1 /f 234 =−36.974. 
         [0060]    In the first embodiment of the present four-piece infrared single wavelength lens system, a central thickness of the second lens element  120  along the optical axis  190  is CT 2 , a distance along the optical axis  190  between the second lens element  120  and the third lens element  130  is T 23 , and they satisfy the relation: CT 2 /T 23 =2.407. 
         [0061]    In the first embodiment of the present four-piece infrared single wavelength lens system, the distance along the optical axis  190  between the second lens element  120  and the third lens element  130  is T 23 , a central thickness of the third lens element  130  along the optical axis  190  is CT 3 , and they satisfy the relation: T 23 /CT 3 =0.780. 
         [0062]    In the first embodiment of the present four-piece infrared single wavelength lens system, the central thickness of the third lens element  130  along the optical axis  190  is CT 3 , a distance along the optical axis  190  between the third lens element  130  and the fourth lens element  140  is T 34 ,and they satisfy the relation: CT 3 /T 34 =10.635. 
         [0063]    In the first embodiment of the present four-piece infrared single wavelength lens system, an Abbe number of the second lens element  120  is V 2 , an Abbe number of the third lens element  130  is V 3 , and they satisfy the relation: V 2 −V 3 =32.03. 
         [0064]    The detailed optical data of the first embodiment is shown in table 1, and the aspheric surface data is shown in table 2. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Embodiment 1 
               
               
                 f(focal length) = 1.241 mm, Fno = 1.4, FOV = 60 deg. 
               
             
          
           
               
                 surface 
                   
                 Curvature Radius 
                 Thickness 
                 Material 
                 Index 
                 Abbe # 
                 Focal length 
               
               
                   
               
             
          
           
               
                 0 
                 object 
                 infinity 
                 600.000 
                   
                   
                   
                   
               
               
                 1 
                   
                 infinity 
                 0.400 
               
             
          
           
               
                 2 
                 Lens 1 
                 −1.191 
                 (ASP) 
                 0.283 
                 plastic 
                 1.535 
                 56.000 
                 −49.209 
               
               
                 3 
                   
                 −1.351 
                 (ASP) 
                 0.324 
               
             
          
           
               
                 4 
                 stop 
                 infinity 
                 −0.029 
                   
                   
                   
                   
               
             
          
           
               
                 5 
                 Lens 2 
                 2.447 
                 (ASP) 
                 0.590 
                 plastic 
                 1.535 
                 56.000 
                 1.689 
               
               
                 6 
                   
                 −1.275 
                 (ASP) 
                 0.245 
               
               
                 7 
                 Lens 3 
                 −0.466 
                 (ASP) 
                 0.314 
                 plastic 
                 1.636 
                 23.970 
                 38.171 
               
               
                 8 
                   
                 −0.574 
                 (ASP) 
                 0.030 
               
               
                 9 
                 Lens 4 
                 0.716 
                 (ASP) 
                 0.402 
                 plastic 
                 1.535 
                 56.000 
                 4.687 
               
               
                 10 
                   
                 0.814 
                 (ASP) 
                 0.523 
               
             
          
           
               
                 11 
                 IR-filter 
                 infinity 
                 0.300 
                 glass 
                 1.510 
                 64.167 
                 — 
               
               
                 12 
                   
                 infinity 
                 0.045 
               
               
                 13 
                 Image plane 
                 infinity 
                 0.000 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
             
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Aspheric Coefficients 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 surface      
               
             
          
           
               
                   
                 2 
                 3 
                 5 
                 6 
               
               
                   
               
               
                 K: 
                 −9.5130E+00  
                 −1.1551E+01  
                 1.9994E+01 
                 −1.7940E+00 
               
               
                 A: 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
                  0.0000E+00 
               
               
                 B: 
                 3.4675E−01 
                 7.3313E−01 
                 3.5677E−01 
                 −1.2603E−01 
               
               
                 C: 
                 −1.6700E−01  
                 1.3668E−01 
                 −5.4430E+00  
                 −5.3692E−01 
               
               
                 D: 
                 1.0020E+00 
                 −4.1742E+00  
                 4.6055E+01 
                 −9.2919E+00 
               
               
                 E: 
                 −4.7244E+00  
                 2.3873E+01 
                 −3.2278E+02  
                  6.7894E+01 
               
               
                 F: 
                 7.1116E+00 
                 −7.6178E+01  
                 1.0930E+03 
                 −1.8645E+02 
               
               
                 G 
                 −3.3428E+00  
                 8.8096E+01 
                 −1.5526E+03  
                  1.8268E+02 
               
               
                 H 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
                  0.0000E+00 
               
               
                   
               
             
          
           
               
                   
                 surface      
               
             
          
           
               
                   
                 7 
                 8 
                 9 
                 10 
               
               
                   
               
               
                 K: 
                 −5.6473E−01  
                 −2.0111E+00  
                 −6.5629E+00  
                 −4.9315E+00 
               
               
                 A: 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
                  0.0000E+00 
               
               
                 B: 
                 2.4348E+00 
                 −4.6574E−01  
                 3.7808E−01 
                 −4.0044E−01 
               
               
                 C: 
                 −1.4208E+01  
                 −9.1468E−01  
                 −3.9923E+00  
                 −1.3309E−01 
               
               
                 D: 
                 7.4220E+01 
                 9.2925E+00 
                 2.1204E+01 
                  3.8091E+00 
               
               
                 E: 
                 −1.7154E+02  
                 −1.6590E+01  
                 −6.6285E+01  
                 −1.3698E+01 
               
               
                 F: 
                 1.8039E+02 
                 9.3225E+00 
                 1.0542E+02 
                  1.6729E+01 
               
               
                 G 
                 −3.8878E+01  
                 1.1400E+00 
                 −7.2418E+01  
                 −7.1563E+00 
               
               
                 H 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
                  0.0000E+00 
               
               
                   
               
             
          
         
       
     
         [0065]    The units of the radius of curvature, the thickness and the focal length in table 1 are expressed in mm, the surface numbers 0-13 represent the surfaces sequentially arranged from the object-side to the image-side along the optical axis. In table 2, k represents the conic coefficient of the equation of the aspheric surface profiles, and A, B, C, D, E, F, G, H . . . : represent the high-order aspheric coefficients. The tables presented below for each embodiment are the corresponding schematic parameter and aberration curves, and the definitions of the tables are the same as Table 1 and Table 2 of the first embodiment. Therefore, an explanation in this regard will not be provided again. 
         [0066]    Referring to  FIGS. 2A and 2B ,  FIG. 2A  shows a four-piece infrared single wavelength lens system in accordance with a second embodiment of the present invention, and  FIG. 2B  shows, in order from left to right, the longitudinal spherical aberration curves, the astigmatic field curves, and the distortion curve of the second embodiment of the present invention. A four-piece infrared single wavelength lens system in accordance with the second embodiment of the present invention comprises a stop  200  and a lens group. The lens group comprises, in order from an object side to an image side: a first lens element  210 , a second lens element  220 , a third lens element  230 , a fourth lens element  240 , an IR cut filter  270 , and an image plane  280 , wherein the four-piece infrared single wavelength lens system has a total of three lens elements with refractive power. The stop  200  is disposed between an image-side surface  212  of the first lens element  210  and an image-side surface  222  of the second lens element  220 . 
         [0067]    The first lens element  210  with a positive refractive power has an object-side surface  211  being concave near an optical axis  290  and the image-side surface  212  being convex near the optical axis  290 , the object-side surface  211  and the image-side surface  212  are aspheric, and the first lens element  210  is made of plastic material. 
         [0068]    The second lens element  220  with a positive refractive power has an object-side surface  221  being convex near the optical axis  290  and the image-side surface  222  being convex near the optical axis  290 , the object-side surface  221  and the image-side surface  222  are aspheric, and the second lens element  220  is made of plastic material. 
         [0069]    The third lens element  230  with a positive refractive power has an object-side surface  231  being concave near the optical axis  290  and an image-side surface  232  being convex near the optical axis  290 , the object-side surface  231  and the image-side surface  232  are aspheric, and the third lens element  230  is made of plastic material. 
         [0070]    The fourth lens element  240  with a positive refractive power has an object-side surface  241  being convex near the optical axis  290  and an image-side surface  242  being concave near the optical axis  290 , the object-side surface  241  and the image-side surface  242  are aspheric, and the fourth lens element  240  is made of plastic material. 
         [0071]    The IR cut filter  270  made of glass is located between the fourth lens element  240  and the image plane  280  and has no influence on the focal length of the four-piece infrared single wavelength lens system. 
         [0072]    The detailed optical data of the second embodiment is shown in table 3, and the aspheric surface data is shown in table 4. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Embodiment 2 
               
               
                 f(focal length) = 1.24 mm, Fno = 1.4, FOV = 60 deg. 
               
             
          
           
               
                 surface 
                   
                 Curvature Radius 
                 Thickness 
                 Material 
                 Index 
                 Abbe # 
                 Focal length 
               
               
                   
               
             
          
           
               
                 0 
                 object 
                 infinity 
                 400.000 
                   
                   
                   
                   
               
               
                 1 
                   
                 infinity 
                 0.400 
               
             
          
           
               
                 2 
                 Lens 1 
                 −2.076 
                 (ASP) 
                 0.270 
                 plastic 
                 1.535 
                 56.000 
                 603.538 
               
               
                 3 
                   
                 −2.155 
                 (ASP) 
                 0.164 
               
             
          
           
               
                 4 
                 stop 
                 infinity 
                 −0.043 
                   
                   
                   
                   
               
             
          
           
               
                 5 
                 Lens 2 
                 2.343 
                 (ASP) 
                 0.525 
                 plastic 
                 1.535 
                 56.000 
                 1.575 
               
               
                 6 
                   
                 −1.187 
                 (ASP) 
                 0.278 
               
               
                 7 
                 Lens 3 
                 −0.438 
                 (ASP) 
                 0.339 
                 plastic 
                 1.636 
                 23.970 
                 6.176 
               
               
                 8 
                   
                 −0.509 
                 (ASP) 
                 0.032 
               
               
                 9 
                 Lens 4 
                 0.765 
                 (ASP) 
                 0.279 
                 plastic 
                 1.636 
                 23.970 
                 8.599 
               
               
                 10 
                   
                 0.770 
                 (ASP) 
                 0.540 
               
             
          
           
               
                 11 
                 IR-filter 
                 infinity 
                 0.300 
                 glass 
                 1.510 
                 64.167 
                 — 
               
               
                 12 
                   
                 infinity 
                 0.045 
               
               
                 13 
                 Image plane 
                 infinity 
                 0.000 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
             
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Aspheric Coefficients 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 surface      
               
             
          
           
               
                   
                 2 
                 3 
                 5 
                 6 
               
               
                   
               
               
                 K: 
                 −1.9344E+01 
                 −4.1274E+01  
                 1.9796E+01 
                 −1.3866E+00  
               
               
                 A: 
                  0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
               
               
                 B: 
                  6.3906E−01 
                 1.7297E+00 
                 1.0624E+00 
                 1.0126E−01 
               
               
                 C: 
                 −6.4418E−01 
                 −4.1896E+00  
                 −6.3821E+00  
                 −2.5618E+00  
               
               
                 D: 
                 −3.3596E−01 
                 1.8910E+01 
                 1.9827E+01 
                 9.5996E+00 
               
               
                 E: 
                  1.0656E+00 
                 −6.8506E+01  
                 −1.0523E+02  
                 −1.2610E+00  
               
               
                 F: 
                 −1.0869E+00 
                 9.7292E+01 
                 4.6618E+02 
                 −9.3848E+01  
               
               
                 G 
                 −1.3562E+00 
                 −5.5061E+01  
                 −1.1116E+03  
                 1.4480E+02 
               
               
                 H 
                  0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
               
               
                   
               
             
          
           
               
                   
                 surface      
               
             
          
           
               
                   
                 7 
                 8 
                 9 
                 10 
               
               
                   
               
               
                 K: 
                 −5.3218E−01 
                 −1.9814E+00  
                 −7.4578E+00  
                 −5.5620E+00  
               
               
                 A: 
                  0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
               
               
                 B: 
                  1.9722E+00 
                 −1.2325E+00  
                 −4.8679E−01  
                 −1.3429E+00  
               
               
                 C: 
                 −1.2814E+00 
                 9.2894E+00 
                 6.8900E+00 
                 9.4017E+00 
               
               
                 D: 
                 −2.4069E+01 
                 −4.8854E+01  
                 −3.9822E+01  
                 −3.8079E+01  
               
               
                 E: 
                  2.3699E+02 
                 1.4440E+02 
                 1.1264E+02 
                 8.6181E+01 
               
               
                 F: 
                 −7.0542E+02 
                 −2.1489E+02  
                 −1.6479E+02  
                 −1.0539E+02  
               
               
                 G 
                  7.9489E+02 
                 1.3380E+02 
                 9.6151E+01 
                 5.3559E+01 
               
               
                 H 
                  0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
               
               
                   
               
             
          
         
       
     
         [0073]    In the second embodiment, the equation of the aspheric surface profiles of the aforementioned lens elements is the same as the equation of the first embodiment. Also, the definitions of these parameters shown in the following table are the same as those stated in the first embodiment with corresponding values for the second embodiment, so an explanation in this regard will not be provided again. 
         [0074]    Moreover, these parameters can be calculated from Table 3 and Table 4 as the following values and satisfy the following conditions: 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 Embodiment 2 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 f 
                 1.24 
                 f3/f12 
                 4.106 
               
               
                   
                 Fno 
                 1.4 
                 f12/f34 
                 0.503 
               
               
                   
                 FOV 
                 60 
                 f1/f23 
                 358.864 
               
               
                   
                 f1/f2 
                 383.199 
                 f1/f234 
                 465.453 
               
               
                   
                 f3/f2 
                 3.922 
                 CT2/T23 
                 1.884 
               
               
                   
                 f3/f4 
                 0.718 
                 T23/CT3 
                 0.822 
               
               
                   
                 f1/f3 
                 97.717 
                 CT3/T34 
                 10.699 
               
               
                   
                 f2/f4 
                 0.183 
                 V2 − V3 
                 32.030 
               
               
                   
                   
               
             
          
         
       
     
         [0075]    Referring to  FIGS. 3A and 3B ,  FIG. 3A  shows a four-piece infrared single wavelength lens system in accordance with a third embodiment of the present invention, and  FIG. 3B  shows, in order from left to right, the longitudinal spherical aberration curves, the astigmatic field curves, and the distortion curve of the third embodiment of the present invention. A four-piece infrared single wavelength lens system in accordance with the third embodiment of the present invention comprises a stop  300  and a lens group. The lens group comprises, in order from an object side to an image side: a first lens element  310 , a second lens element  320 , a third lens element  330 , a fourth lens element  340 , an IR cut filter  370 , and an image plane  380 , wherein the four-piece infrared single wavelength lens system has a total of four lens elements with refractive power. The stop  300  is disposed between an image-side surface  312  of the first lens element  310  and an image-side surface  322  of the second lens element  320 . 
         [0076]    The first lens element  310  with a negative refractive power has an object-side surface  311  being concave near an optical axis  390  and the image-side surface  312  being convex near the optical axis  390 , the object-side surface  311  and the image-side surface  312  are aspheric, and the first lens element  310  is made of plastic material. 
         [0077]    The second lens element  320  with a positive refractive power has an object-side surface  321  being convex near the optical axis  390  and the image-side surface  322  being convex near the optical axis  390 , the object-side surface  321  and the image-side surface  322  are aspheric, and the second lens element  320  is made of plastic material. 
         [0078]    The third lens element  330  with a positive refractive power has an object-side surface  331  being concave near the optical axis  390  and an image-side surface  332  being convex near the optical axis  390 , the object-side surface  331  and the image-side surface  332  are aspheric, and the third lens element  330  is made of plastic material. 
         [0079]    The fourth lens element  340  with a positive refractive power has an object-side surface  341  being convex near the optical axis  390  and an image-side surface  342  being concave near the optical axis  390 , the object-side surface  341  and the image-side surface  342  are aspheric, and the fourth lens element  340  is made of plastic material. 
         [0080]    The IR cut filter  370  made of glass is located between the fourth lens element  340  and the image plane  380  and has no influence on the focal length of the four-piece infrared single wavelength lens system. 
         [0081]    The detailed optical data of the third embodiment is shown in table 5, and the aspheric surface data is shown in table 6. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Embodiment 3 
               
               
                 f(focal length) = 1.137 mm, Fno = 1.6, FOV = 65 deg. 
               
             
          
           
               
                 surface 
                   
                 Curvature Radius 
                 Thickness 
                 Material 
                 Index 
                 Abbe # 
                 Focal length 
               
               
                   
               
             
          
           
               
                 0 
                 object 
                 infinity 
                 600.000 
                   
                   
                   
                   
               
               
                 1 
                   
                 infinity 
                 0.400 
               
             
          
           
               
                 2 
                 Lens 1 
                 −0.761 
                 (ASP) 
                 0.311 
                 plastic 
                 1.535 
                 56.000 
                 −17.269 
               
               
                 3 
                   
                 −0.947 
                 (ASP) 
                 0.308 
               
             
          
           
               
                 4 
                 stop 
                 infinity 
                 −0.013 
                   
                   
                   
                   
               
             
          
           
               
                 5 
                 Lens 2 
                 2.438 
                 (ASP) 
                 0.488 
                 plastic 
                 1.535 
                 56.000 
                 1.647 
               
               
                 6 
                   
                 −1.255 
                 (ASP) 
                 0.286 
               
               
                 7 
                 Lens 3 
                 −0.467 
                 (ASP) 
                 0.267 
                 plastic 
                 1.636 
                 23.970 
                 139.114 
               
               
                 8 
                   
                 −0.565 
                 (ASP) 
                 0.030 
               
               
                 9 
                 Lens 4 
                 0.610 
                 (ASP) 
                 0.296 
                 plastic 
                 1.535 
                 56.000 
                 3.759 
               
               
                 10 
                   
                 0.734 
                 (ASP) 
                 0.569 
               
             
          
           
               
                 11 
                 IR-filter 
                 infinity 
                 0.300 
                 glass 
                 1.510 
                 64.167 
                 — 
               
               
                 12 
                   
                 infinity 
                 0.045 
               
               
                 13 
                 Image plane 
                 infinity 
                 0.000 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
             
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                 Aspheric Coefficients 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 surface      
               
             
          
           
               
                   
                 2 
                 3 
                 5 
                 6 
               
               
                   
               
               
                 K: 
                 −4.2894E+00  
                 −8.3994E+00  
                 1.9519E+01 
                 −2.6858E−01  
               
               
                 A: 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
               
               
                 B: 
                 4.5540E−01 
                 6.3321E−01 
                 4.0053E−01 
                 −2.2477E−01  
               
               
                 C: 
                 −2.4186E−01  
                 8.0617E−01 
                 −6.2522E+00  
                 −4.0891E−01  
               
               
                 D: 
                 1.0748E+00 
                 −3.0908E+00  
                 4.6866E+01 
                 −9.4895E+00  
               
               
                 E: 
                 −4.5161E+00  
                 2.1199E+01 
                 −3.1816E+02  
                 6.7333E+01 
               
               
                 F: 
                 6.9923E+00 
                 −8.1978E+01  
                 1.0791E+03 
                 −1.8672E+02  
               
               
                 G 
                 −3.6134E+00  
                 1.1571E+02 
                 −1.5526E+03  
                 1.7504E+02 
               
               
                 H 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
               
               
                   
               
             
          
           
               
                   
                 surface      
               
             
          
           
               
                   
                 7 
                 8 
                 9 
                 10 
               
               
                   
               
               
                 K: 
                 −5.5700E−01  
                 −2.3678E+00  
                 −4.0000E+00  
                 −3.8422E+00  
               
               
                 A: 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
               
               
                 B: 
                 2.4706E+00 
                 −5.0278E−01  
                 7.2251E−01 
                 6.5888E−02 
               
               
                 C: 
                 −1.4495E+01  
                 −1.0381E+00  
                 −4.0427E+00  
                 4.1413E−01 
               
               
                 D: 
                 7.3612E+01 
                 9.1363E+00 
                 2.1215E+01 
                 3.2140E+00 
               
               
                 E: 
                 −1.7195E+02  
                 −1.6956E+01  
                 −6.6139E+01  
                 −1.4959E+01  
               
               
                 F: 
                 1.8086E+02 
                 8.6218E+00 
                 1.0460E+02 
                 1.7413E+01 
               
               
                 G 
                 −4.0306E+01  
                 7.2118E−01 
                 −7.2057E+01  
                 −7.0507E+00  
               
               
                 H 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
               
               
                   
               
             
          
         
       
     
         [0082]    In the third embodiment, the equation of the aspheric surface profiles of the aforementioned lens elements is the same as the equation of the first embodiment. Also, the definitions of these parameters shown in the following table are the same as those stated in the first embodiment with corresponding values for the third embodiment, so an explanation in this regard will not be provided again. 
         [0083]    Moreover, these parameters can be calculated from Table 5 and Table 6 as the following values and satisfy the following conditions: 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 Embodiment 3 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 f 
                 1.137 
                 f3/f12 
                 90.829 
               
               
                   
                 Fno 
                 1.6 
                 f12/f34 
                 0.509 
               
               
                   
                 FOV 
                 65 
                 f1/f23 
                 −8.692 
               
               
                   
                 f1/f2 
                 −10.483 
                 f1/f234 
                 −13.608 
               
               
                   
                 f3/f2 
                 84.445 
                 CT2/T23 
                 1.704 
               
               
                   
                 f3/f4 
                 37.008 
                 T23/CT3 
                 1.073 
               
               
                   
                 f1/f3 
                 −0.124 
                 CT3/T34 
                 8.892 
               
               
                   
                 f2/f4 
                 0.438 
                 V2 − V3 
                 32.030 
               
               
                   
                   
               
             
          
         
       
     
         [0084]    Referring to  FIGS. 4A and 4B ,  FIG. 4A  shows a four-piece infrared single wavelength lens system in accordance with a fourth embodiment of the present invention, and  FIG. 4B  shows, in order from left to right, the longitudinal spherical aberration curves, the astigmatic field curves, and the distortion curve of the fourth embodiment of the present invention. A four-piece infrared single wavelength lens system in accordance with the fourth embodiment of the present invention comprises a stop  400  and a lens group. The lens group comprises, in order from an object side to an image side: a first lens element  410 , a second lens element  420 , a third lens element  430 , a fourth lens element  440 , an IR cut filter  470 , and an image plane  480 , wherein the four-piece infrared single wavelength lens system has a total of four lens elements with refractive power. The stop  400  is disposed between an image-side surface  412  of the first lens element  410  and an image-side surface  422  of the second lens element  420 . 
         [0085]    The first lens element  410  with a negative refractive power has an object-side surface  411  being concave near an optical axis  490  and the image-side surface  412  being convex near the optical axis  490 , the object-side surface  411  and the image-side surface  412  are aspheric, and the first lens element  410  is made of plastic material. 
         [0086]    The second lens element  420  with a positive refractive power has an object-side surface  421  being convex near the optical axis  490  and the image-side surface  422  being convex near the optical axis  490 , the object-side surface  421  and the image-side surface  422  are aspheric, and the second lens element  420  is made of plastic material. 
         [0087]    The third lens element  430  with a positive refractive power has an object-side surface  431  being concave near the optical axis  490  and an image-side surface  432  being convex near the optical axis  490 , the object-side surface  431  and the image-side surface  432  are aspheric, and the third lens element  430  is made of plastic material. 
         [0088]    The fourth lens element  440  with a positive refractive power has an object-side surface  441  being convex near the optical axis  490  and an image-side surface  442  being concave near the optical axis  490 , the object-side surface  441  and the image-side surface  442  are aspheric, and the fourth lens element  440  is made of plastic material. 
         [0089]    The IR cut filter  470  made of glass is located between the fourth lens element  440  and the image plane  480  and has no influence on the focal length of the four-piece infrared single wavelength lens system. 
         [0090]    The detailed optical data of the fourth embodiment is shown in table 7, and the aspheric surface data is shown in table 8. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 Embodiment 4 
               
               
                 f(focal length) = 1.24 mm, Fno = 1.6, FOV = 60 deg. 
               
             
          
           
               
                 surface 
                   
                 Curvature Radius 
                 Thickness 
                 Material 
                 Index 
                 Abbe # 
                 Focal length 
               
               
                   
               
             
          
           
               
                 0 
                 object 
                 infinity 
                 600.000 
                   
                   
                   
                   
               
               
                 1 
                   
                 infinity 
                 0.400 
               
             
          
           
               
                 2 
                 Lens 1 
                 −1.168 
                 (ASP) 
                 0.264 
                 plastic 
                 1.535 
                 56.000 
                 −54.667 
               
               
                 3 
                   
                 −1.312 
                 (ASP) 
                 0.352 
               
             
          
           
               
                 4 
                 stop 
                 infinity 
                 −0.029 
                   
                   
                   
                   
               
             
          
           
               
                 5 
                 Lens 2 
                 2.446 
                 (ASP) 
                 0.584 
                 plastic 
                 1.535 
                 56.000 
                 1.690 
               
               
                 6 
                   
                 −1.277 
                 (ASP) 
                 0.258 
               
               
                 7 
                 Lens 3 
                 −0.465 
                 (ASP) 
                 0.308 
                 plastic 
                 1.636 
                 23.970 
                 122.960 
               
               
                 8 
                   
                 −0.579 
                 (ASP) 
                 0.035 
               
               
                 9 
                 Lens 4 
                 0.705 
                 (ASP) 
                 0.400 
                 plastic 
                 1.535 
                 56.000 
                 4.228 
               
               
                 10 
                   
                 0.831 
                 (ASP) 
                 0.507 
               
             
          
           
               
                 11 
                 IR-filter 
                 infinity 
                 0.300 
                 glass 
                 1.510 
                 64.167 
                 — 
               
               
                 12 
                   
                 infinity 
                 0.045 
               
               
                 13 
                 Image plane 
                 infinity 
                 0.000 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
             
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 8 
               
               
                   
               
               
                 Aspheric coefficients 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 surface      
               
             
          
           
               
                   
                 2 
                 3 
                 5 
                 6 
               
               
                   
               
               
                 K: 
                 −9.9647E+00  
                 −1.2054E+01  
                 2.0599E+01 
                 −1.7930E+00 
               
               
                 A: 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
                  0.0000E+00 
               
               
                 B: 
                 3.9072E−01 
                 7.4385E−01 
                 3.5054E−01 
                 −1.2680E−01 
               
               
                 C: 
                 −1.1987E−01  
                 2.7860E−01 
                 −5.3357E+00  
                 −5.5462E−01 
               
               
                 D: 
                 9.6614E−01 
                 −3.6634E+00  
                 4.6172E+01 
                 −9.6077E+00 
               
               
                 E: 
                 −4.7391E+00  
                 2.3758E+01 
                 −3.2381E+02  
                  6.6698E+01 
               
               
                 F: 
                 7.0416E+00 
                 −8.0173E+01  
                 1.0869E+03 
                 −1.8747E+02 
               
               
                 G 
                 −3.2699E+00  
                 9.2492E+01 
                 −1.5629E+03  
                  1.9073E+02 
               
               
                 H 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
                  0.0000E+00 
               
               
                   
               
             
          
           
               
                   
                 surface      
               
             
          
           
               
                   
                 7 
                 8 
                 9 
                 10 
               
               
                   
               
               
                 K: 
                 −5.4846E−01  
                 −2.0148E+00  
                 −6.6486E+00  
                 −4.9356E+00 
               
               
                 A: 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
                  0.0000E+00 
               
               
                 B: 
                 2.4425E+00 
                 −4.6566E−01  
                 3.7894E−01 
                 −4.7215E−01 
               
               
                 C: 
                 −1.4141E+01  
                 −8.8766E−01  
                 −4.0054E+00  
                 −1.4922E−01 
               
               
                 D: 
                 7.4097E+01 
                 9.5108E+00 
                 2.1171E+01 
                  4.0462E+00 
               
               
                 E: 
                 −1.7201E+02  
                 −1.6301E+01  
                 −6.6049E+01  
                 −1.3684E+01 
               
               
                 F: 
                 1.8064E+02 
                 9.3283E+00 
                 1.0616E+02 
                  1.6101E+01 
               
               
                 G 
                 −3.1841E+01  
                 −8.9060E−01  
                 −7.3791E+01  
                 −6.7724E+00 
               
               
                 H 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
                  0.0000E+00 
               
               
                   
               
             
          
         
       
     
         [0091]    In the fourth embodiment, the equation of the aspheric surface profiles of the aforementioned lens elements is the same as the equation of the first embodiment. Also, the definitions of these parameters shown in the following table are the same as those stated in the first embodiment with corresponding values for the fourth embodiment, so an explanation in this regard will not be provided again. 
         [0092]    Moreover, these parameters can be calculated from Table 7 and Table 8 as the following values and satisfy the following conditions: 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 Embodiment 4 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 f 
                 1.24 
                 f3/f12 
                 76.975 
               
               
                   
                 Fno 
                 1.6 
                 f12/f34 
                 0.488 
               
               
                   
                 FOV 
                 60 
                 f1/f23 
                 −26.153 
               
               
                   
                 f1/f2 
                 −32.351 
                 f1/f234 
                 −41.184 
               
               
                   
                 f3/f2 
                 72.766 
                 CT2/T23 
                 2.261 
               
               
                   
                 f3/f4 
                 29.081 
                 T23/CT3 
                 0.839 
               
               
                   
                 f1/f3 
                 −0.445 
                 CT3/T34 
                 8.880 
               
               
                   
                 f2/f4 
                 0.400 
                 V2 − V3 
                 32.030 
               
               
                   
                   
               
             
          
         
       
     
         [0093]    Referring to  FIGS. 5A and 5B ,  FIG. 5A  shows a four-piece infrared single wavelength lens system in accordance with a fifth embodiment of the present invention, and  FIG. 5B  shows, in order from left to right, the longitudinal spherical aberration curves, the astigmatic field curves, and the distortion curve of the fifth embodiment of the present invention. A four-piece infrared single wavelength lens system in accordance with the fifth embodiment of the present invention comprises a stop  500  and a lens group. The lens group comprises, in order from an object side to an image side: a first lens element  510 , a second lens element  520 , a third lens element  530 , a fourth lens element  540 , an IR cut filter  570 , and an image plane  580 , wherein the four-piece infrared single wavelength lens system has a total of four lens elements with refractive power. The stop  500  is disposed between an image-side surface  512  of the first lens element  510  and an image-side surface  522  of the second lens element  520 . 
         [0094]    The first lens element  510  with a negative refractive power has an object-side surface  511  being concave near an optical axis  590  and the image-side surface  512  being convex near the optical axis  590 , the object-side surface  511  and the image-side surface  512  are aspheric, and the first lens element  510  is made of plastic material. 
         [0095]    The second lens element  520  with a positive refractive power has an object-side surface  521  being convex near the optical axis  590  and the image-side surface  522  being convex near the optical axis  590 , the object-side surface  521  and the image-side surface  522  are aspheric, and the second lens element  520  is made of plastic material. 
         [0096]    The third lens element  530  with a positive refractive power has an object-side surface  531  being concave near the optical axis  590  and an image-side surface  532  being convex near the optical axis  590 , the object-side surface  531  and the image-side surface  532  are aspheric, and the third lens element  530  is made of plastic material. 
         [0097]    The fourth lens element  540  with a positive refractive power has an object-side surface  541  being convex near the optical axis  590  and an image-side surface  542  being concave near the optical axis  590 , the object-side surface  541  and the image-side surface  542  are aspheric, and the fourth lens element  540  is made of plastic material. 
         [0098]    The IR cut filter  570  made of glass is located between the fourth lens element  540  and the image plane  580  and has no influence on the focal length of the four-piece infrared single wavelength lens system. 
         [0099]    The detailed optical data of the fifth embodiment is shown in table 9, and the aspheric surface data is shown in table 10. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 9 
               
             
             
               
                   
               
               
                 Embodiment 5 
               
               
                 f(focal length) = 1.235 mm, Fno = 1.4, FOV = 60 deg. 
               
             
          
           
               
                 surface 
                   
                 Curvature Radius 
                 Thickness 
                 Material 
                 Index 
                 Abbe # 
                 Focal length 
               
               
                   
               
             
          
           
               
                 0 
                 object 
                 infinity 
                 400.000 
                   
                   
                   
                   
               
               
                 1 
                   
                 infinity 
                 0.400 
               
             
          
           
               
                 2 
                 Lens 1 
                 −2.034 
                 (ASP) 
                 0.267 
                 plastic 
                 1.535 
                 56.000 
                 −350.935 
               
               
                 3 
                   
                 −2.150 
                 (ASP) 
                 0.160 
               
             
          
           
               
                 4 
                 stop 
                 infinity 
                 −0.051 
                   
                   
                   
                   
               
             
          
           
               
                 5 
                 Lens 2 
                 2.329 
                 (ASP) 
                 0.564 
                 plastic 
                 1.535 
                 56.000 
                 1.573 
               
               
                 6 
                   
                 −1.180 
                 (ASP) 
                 0.281 
               
               
                 7 
                 Lens 3 
                 −0.441 
                 (ASP) 
                 0.340 
                 plastic 
                 1.636 
                 23.970 
                 6.651 
               
               
                 8 
                   
                 −0.515 
                 (ASP) 
                 0.031 
               
               
                 9 
                 Lens 4 
                 0.671 
                 (ASP) 
                 0.278 
                 plastic 
                 1.535 
                 56.000 
                 7.472 
               
               
                 10 
                   
                 0.693 
                 (ASP) 
                 0.539 
               
             
          
           
               
                 11 
                 IR-filter 
                 infinity 
                 0.300 
                 glass 
                 1.510 
                 64.167 
                 — 
               
               
                 12 
                   
                 infinity 
                 0.045 
               
               
                 13 
                 Image plane 
                 infinity 
                 0.000 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
             
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 10 
               
               
                   
               
               
                 Aspheric coefficients 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 surface      
               
             
          
           
               
                   
                 2 
                 3 
                 5 
                 6 
               
               
                   
               
               
                 K: 
                 −1.8043E+01 
                 −5.6553E+01  
                 2.0226E+01 
                 −1.4223E+00  
               
               
                 A: 
                  0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
               
               
                 B: 
                  6.6669E−01 
                 1.7321E+00 
                 1.0525E+00 
                 1.0442E−01 
               
               
                 C: 
                 −6.2719E−01 
                 −4.1744E+00  
                 −6.4427E+00  
                 −2.4740E+00  
               
               
                 D: 
                 −5.0887E−01 
                 1.8928E+01 
                 1.9822E+01 
                 9.8575E+00 
               
               
                 E: 
                  9.1321E−01 
                 −7.1236E+01  
                 −1.0535E+02  
                 −1.7606E+00  
               
               
                 F: 
                 −6.6663E−01 
                 8.5793E+01 
                 4.6270E+02 
                 −9.6635E+01  
               
               
                 G 
                 −9.2318E−01 
                 −1.3735E+01  
                 −1.1647E+03  
                 1.4383E+02 
               
               
                 H 
                  0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
               
               
                   
               
             
          
           
               
                   
                 surface      
               
             
          
           
               
                   
                 7 
                 8 
                 9 
                 10 
               
               
                   
               
               
                 K: 
                 −5.4258E−01 
                 −1.9370E+00  
                 −6.4064E+00  
                 −4.8447E+00  
               
               
                 A: 
                  0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
               
               
                 B: 
                  1.9915E+00 
                 −1.2388E+00  
                 −4.5053E−01  
                 −1.3636E+00  
               
               
                 C: 
                 −1.3391E+00 
                 9.2976E+00 
                 6.7284E+00 
                 9.3323E+00 
               
               
                 D: 
                 −2.4476E+01 
                 −4.8885E+01  
                 −4.0043E+01  
                 −3.8227E+01  
               
               
                 E: 
                  2.3534E+02 
                 1.4410E+02 
                 1.1277E+02 
                 8.6191E+01 
               
               
                 F: 
                 −7.1022E+02 
                 −2.1573E+02  
                 −1.6405E+02  
                 −1.0487E+02  
               
               
                 G 
                  7.9108E+02 
                 1.3298E+02 
                 9.4888E+01 
                 5.3155E+01 
               
               
                 H 
                  0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
                 0.0000E+00 
               
               
                   
               
             
          
         
       
     
         [0100]    In the fifth embodiment, the equation of the aspheric surface profiles of the aforementioned lens elements is the same as the equation of the first embodiment. Also, the definitions of these parameters shown in the following table are the same as those stated in the first embodiment with corresponding values for the fifth embodiment, so an explanation in this regard will not be provided again. 
         [0101]    Moreover, these parameters can be calculated from Table 9 and Table 10 as the following values and satisfy the following conditions: 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 Embodiment 5 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 f 
                 1.235 
                 f3/f12 
                 4.406 
               
               
                   
                 Fno 
                 1.4 
                 f12/f34 
                 0.519 
               
               
                   
                 FOV 
                 60 
                 f1/f23 
                 −205.972 
               
               
                   
                 f1/f2 
                 −223.142 
                 f1/f234 
                 −272.167 
               
               
                   
                 f3/f2 
                 4.229 
                 CT2/T23 
                 2.002 
               
               
                   
                 f3/f4 
                 0.890 
                 T23/CT3 
                 0.829 
               
               
                   
                 f1/f3 
                 −52.765 
                 CT3/T34 
                 10.915 
               
               
                   
                 f2/f4 
                 0.210 
                 V2 − V3 
                 32.030 
               
               
                   
                   
               
             
          
         
       
     
         [0102]    In the present four-piece infrared single wavelength lens system, the lens elements can be made of plastic or glass. If the lens elements are made of plastic, the cost will be effectively reduced. If the lens elements are made of glass, there is more freedom in distributing the refractive power of the four-piece infrared single wavelength lens system. Plastic lens elements can have aspheric surfaces, which allow more design parameter freedom (than spherical surfaces), so as to reduce the aberration and the number of the lens elements, as well as the total track length of the four-piece infrared single wavelength lens system. 
         [0103]    In the present four-piece infrared single wavelength lens system, if the object-side or the image-side surface of the lens elements with refractive power is convex and the location of the convex surface is not defined, the object-side or the image-side surface of the lens elements near the optical axis is convex. If the object-side or the image-side surface of the lens elements is concave and the location of the concave surface is not defined, the object-side or the image-side surface of the lens elements near the optical axis is concave. 
         [0104]    The four-piece infrared single wavelength lens system of the present invention can be used in focusing optical systems and can obtain better image quality. The four-piece infrared single wavelength lens system of the present invention can also be used in electronic imaging systems, such as, 3D image capturing, digital camera, mobile device, digital flat panel or vehicle camera. 
         [0105]    While we have shown and described various embodiments in accordance with the present invention, it should be clear to those skilled in the art that further embodiments may be made without departing from the scope of the present invention.