# Patent

## Title
Four-piece infrared single wavelength lens system

## 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 f1, the focal length of the second lens element and the third lens element combined is f23, and they satisfy the relation: −210<f1/f23<365. When the above relation is satisfied, a wide field of view can be obtained and the resolution can be improved evidently.

## Background
BACKGROUND OF THE INVENTION
Field of the Invention
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.
Description of the Prior Art
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.
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's previous lens groups are all 2D plane games, which cannot meet the 3D game focusing on the deep induction efficacy.
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.
The present invention mitigates and/or obviates the aforementioned disadvantages.
SUMMARY OF THE INVENTION
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.
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.
A focal length of the first lens element is f1, a focal length of the second lens element and the third lens element combined is f23, and they satisfy the relation: −210<f1/f23<365.
When the above relation is satisfied, a wide field of view can be obtained and the resolution can be improved evidently.
Preferably, the focal length of the first lens element is f1, a focal length of the second lens element is f2, and they satisfy the relation: −230<f1/f2<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.
Preferably, a focal length of the third lens element is f3, the focal length of the second lens element is f2, and they satisfy the relation: 3.5<f3/f2<86.
Preferably, the focal length of the third lens element is f3, a focal length of the fourth lens element is f4, and they satisfy the relation: 0.5<f3/f4<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.
Preferably, the focal length of the first lens element is f1, the focal length of the third lens element is f3, and they satisfy the relation: −55<f1/f3<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.
Preferably, the focal length of the second lens element is f2, the focal length of the fourth lens element is f4, and they satisfy the relation: 0.05<f2/f4<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.
Preferably, the focal length of the third lens element is f3, a focal length of the first lens element and the second lens element combined is f12, and they satisfy the relation: 3.5<f3/f12<93, which is favorable to obtain a wide field of view and effectively correct image distortion.
Preferably, the focal length of the first lens element and the second lens element combined is f12, a focal length of the third lens element and the fourth lens element combined is f34, and they satisfy the relation: 0.35<f12/f34<0.65, which is favorable to obtain a wide field of view and effectively correct image distortion.
Preferably, the focal length of the first lens element is f1, a focal length of the second lens element, the third lens element and the fourth lens element combined is f234, and they satisfy the relation: −280<f1/f234<480, which is favorable to obtain a wide field of view and effectively correct image distortion.
Preferably, the four-piece infrared single wavelength lens system has a maximum view angle FOV, and it satisfies the relation: 45<FOV<75, so that the four-piece infrared single wavelength lens system will have an appropiately large field of view.
Preferably, a central thickness of the second lens element along the optical axis is CT2, a distance along the optical axis between the second lens element and the third lens element is T23, and they satisfy the relation: 1.5<CT2/T23<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.
Preferably, the distance along the optical axis between the second lens element and the third lens element is T23, a central thickness of the third lens element along the optical axis is CT3, and they satisfy the relation: 0.6<T23/CT3<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.
Preferably, the central thickness of the third lens element along the optical axis is CT3, a distance along the optical axis between the third lens element and the fourth lens element is T34, and they satisfy the relation: 7<CT3/T34<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.
Preferably, an Abbe number of the second lens element is V2, an Abbe number of the third lens element is V3, and they satisfy the relation: 30<V2−V3<42, which is favorable to correct the chromatic aberration of the lens system.
Preferably, a f-number of the four-piece infrared single wavelength lens system is Fno, and it satisfies the relation: 1.2<Fno<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.
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
FIG. 1A shows a four-piece infrared single wavelength lens system in accordance with a first embodiment of the present invention;
FIG. 1B shows the longitudinal spherical aberration curve, the astigmatic field curve and the distortion curve of the first embodiment of the present invention;
FIG. 2A shows a four-piece infrared single wavelength lens system in accordance with a second embodiment of the present invention;
FIG. 2B shows the longitudinal spherical aberration curve, the astigmatic field curve and the distortion curve of the second embodiment of the present invention;
FIG. 3A shows a four-piece infrared single wavelength lens system in accordance with a third embodiment of the present invention;
FIG. 3B shows the longitudinal spherical aberration curve, the astigmatic field curve and the distortion curve of the third embodiment of the present invention;
FIG. 4A shows a four-piece infrared single wavelength lens system in accordance with a fourth embodiment of the present invention;
FIG. 4B shows the longitudinal spherical aberration curve, the astigmatic field curve and the distortion curve of the fourth embodiment of the present invention;
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 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
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.
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.
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.
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.
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.
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.
The equation for the aspheric surface profiles of the respective lens elements of the first embodiment is expressed as follows:
wherein:




  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 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;
c represents a paraxial curvature equal to 1/R (R: a paraxial radius of curvature);
h represents a vertical distance from the point on the curve of the aspheric surface to the optical axis 190;
k represents the conic constant;
A, B, C, D, E, G, . . . : represent the high-order aspheric coefficients.
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.
In the first embodiment of the present four-piece infrared single wavelength lens system, a focal length of the first lens element 110 is f1, a focal length of the second lens element 120 and the third lens element 130 combined is f23, and they satisfy the relation: f1/f23=−24.071.
In the first embodiment of the present four-piece infrared single wavelength lens system, the focal length of the first lens element 110 is f1, a focal length of the second lens element 120 is f2, and they satisfy the relation: f1/f2=−29.133.
In the first embodiment of the present four-piece infrared single wavelength lens system, a focal length of the third lens element 130 is f3, the focal length of the second lens element 120 is f2, and they satisfy the relation: f3/f2=22.598.
In the first embodiment of the present four-piece infrared single wavelength lens system, the focal length of the third lens element 130 is f3, a focal length of the fourth lens element 140 is f4, and they satisfy the relation: f3/f4=8.144.
In the first embodiment of the present four-piece infrared single wavelength lens system, the focal length of the first lens element 110 is f1, the focal length of the third lens element 130 is f3, and they satisfy the relation: f1/f3=−1.289.
In the first embodiment of the present four-piece infrared single wavelength lens system, the focal length of the second lens element 120 is f2, the focal length of the fourth lens element 140 is f4, and they satisfy the relation: f2/f4=0.360.
In the first embodiment of the present four-piece infrared single wavelength lens system, the focal length of the third lens element 130 is f3, a focal length of the first lens element 110 and the second lens element 120 combined is f12, and they satisfy the relation: f3/f12=23.929.
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 f12, a focal length of the third lens element 130 and the fourth lens element 140 combined is f34, and they satisfy the relation: f12/f34=0.476.
In the first embodiment of the present four-piece infrared single wavelength lens system, the focal length of the first lens element is f1, a focal length of the second lens element 120, the third lens element 130 and the fourth lens element 140 combined is f234, and they satisfy the relation: f1/f234=−36.974.
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 CT2, a distance along the optical axis 190 between the second lens element 120 and the third lens element 130 is T23, and they satisfy the relation: CT2/T23=2.407.
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 T23, a central thickness of the third lens element 130 along the optical axis 190 is CT3, and they satisfy the relation: T23/CT3=0.780.
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 CT3, a distance along the optical axis 190 between the third lens element 130 and the fourth lens element 140 is T34, and they satisfy the relation: CT3/T34=10.635.
In the first embodiment of the present four-piece infrared single wavelength lens system, an Abbe number of the second lens element 120 is V2, an Abbe number of the third lens element 130 is V3, and they satisfy the relation: V2−V3=32.03.
The detailed optical data of the first embodiment is shown in table 1, and the aspheric surface data is shown in table 2.







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
















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









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.
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.
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.
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.
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.
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.
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.
The detailed optical data of the second embodiment is shown in table 3, and the aspheric surface data is shown in table 4.







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
















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









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.
Moreover, these parameters can be calculated from Table 3 and Table 4 as the following values and satisfy the following conditions:










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










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.
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.
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.
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.
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.
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.
The detailed optical data of the third embodiment is shown in table 5, and the aspheric surface data is shown in table 6.







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
















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









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.
Moreover, these parameters can be calculated from Table 5 and Table 6 as the following values and satisfy the following conditions:










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










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.
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.
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.
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.
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.
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.
The detailed optical data of the fourth embodiment is shown in table 7, and the aspheric surface data is shown in table 8.







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
















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









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.
Moreover, these parameters can be calculated from Table 7 and Table 8 as the following values and satisfy the following conditions:










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










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.
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.
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.
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.
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.
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.
The detailed optical data of the fifth embodiment is shown in table 9, and the aspheric surface data is shown in table 10.







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
















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









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.
Moreover, these parameters can be calculated from Table 9 and Table 10 as the following values and satisfy the following conditions:










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










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.
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.
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.
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.

## Claims

1. A four-piece infrared single wavelength lens system, in order from an object side to an image side, comprising:
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; and
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;
wherein a focal length of the first lens element is f1, a focal length of the second lens element and the third lens element combined is f23, and they satisfy the relation: −210<f1/f23<365.

2. The four-piece infrared single wavelength lens system as claimed in claim 1, wherein the focal length of the first lens element is f1, a focal length of the second lens element is f2, and they satisfy the relation: −230<f1/f2<390.

3. The four-piece infrared single wavelength lens system as claimed in claim 1, wherein a focal length of the second lens element is f2, a focal length of the third lens element is f3, and they satisfy the relation: 3.5<f3/f2<86.

4. The four-piece infrared single wavelength lens system as claimed in claim 1, wherein a focal length of the third lens element is f3, a focal length of the fourth lens element is f4, and they satisfy the relation: 0.5<f3/f4<39.

5. The four-piece infrared single wavelength lens system as claimed in claim 1, wherein the focal length of the first lens element is f1, a focal length of the third lens element is f3, and they satisfy the relation: −55<f1/f3<100.

6. The four-piece infrared single wavelength lens system as claimed in claim 1, wherein a focal length of the second lens element is f2, a focal length of the fourth lens element is f4, and they satisfy the relation: 0.05<f2/f4<0.6.

7. The four-piece infrared single wavelength lens system as claimed in claim 1, wherein a focal length of the third lens element is f3, a focal length of the first lens element and the second lens element combined is f12, and they satisfy the relation: 3.5<f3/f12<93.

8. The four-piece infrared single wavelength lens system as claimed in claim 1, wherein a focal length of the first lens element and the second lens element combined is f12, a focal length of the third lens element and the fourth lens element combined is f34, and they satisfy the relation: 0.35<f12/f34<0.65.

9. The four-piece infrared single wavelength lens system as claimed in claim 1, wherein the focal length of the first lens element is f1, a focal length of the second lens element, the third lens element and the fourth lens element combined is f234, and they satisfy the relation: −280<f1/f234<480.

10. The four-piece infrared single wavelength lens system as claimed in claim 1, wherein the four-piece infrared single wavelength lens system has a maximum view angle FOV, and it satisfies the relation: 45<FOV<75.

11. The four-piece infrared single wavelength lens system as claimed in claim 1, wherein a central thickness of the second lens element along the optical axis is CT2, a distance along the optical axis between the second lens element and the third lens element is T23, and they satisfy the relation: 1.5<CT2/T23<2.6.

12. The four-piece infrared single wavelength lens system as claimed in claim 1, wherein a distance along the optical axis between the second lens element and the third lens element is T23, a central thickness of the third lens element along the optical axis is CT3, and they satisfy the relation: 0.6<T23/CT3<1.3.

13. The four-piece infrared single wavelength lens system as claimed in claim 1, wherein a central thickness of the third lens element along the optical axis is CT3, a distance along the optical axis between the third lens element and the fourth lens element is T34, and they satisfy the relation: 7<CT3/T34<12.5.

14. The four-piece infrared single wavelength lens system as claimed in claim 1, wherein an Abbe number of the second lens element is V2, an Abbe number of the third lens element is V3, and they satisfy the relation: 30<V2−V3<42.

15. The four-piece infrared single wavelength lens system as claimed in claim 1, wherein a f-number of the four-piece infrared single wavelength lens system is Fno, and it satisfies the relation: 1.2<Fno<1.8.