Four-piece infrared single wavelength lens system
A four-piece infrared single wavelength lens system includes, in order from the object side to the image side: a stop, a first lens element with a positive refractive power, a second lens element with a positive refractive power, a third lens element with a negative refractive power, a fourth lens element with a positive refractive power. Such arrangements can provide a four-piece infrared single wavelength lens system which has a wide field of view, high resolution, short length and less distortion.
1. A four-piece infrared single wavelength lens system, in order from an object side to an image side, comprising:
a stop;
a first lens element with a positive refractive power, having an object-side surface being convex near an 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 first lens element being aspheric;
a second 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 second lens element being aspheric;
a third lens element with a negative 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 second lens element is f 2 , a focal length of the third lens element is f 3 , and they satisfy the relation: −42<f 2 /f 3 <−3.2.
2. The four-piece infrared single wavelength lens system as claimed in claim 1 , wherein a focal length of the first lens element is f 1 , the focal length of the second lens element is f 2 , and they satisfy the relation: 0.005<f 1 /f 2 <0.09.
3. The four-piece infrared single wavelength lens system as claimed in claim 1 , wherein 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: −2.1<f 3 /f 4 <−1.3.
4. The four-piece infrared single wavelength lens system as claimed in claim 1 , wherein a 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: −0.55<f 1 /f 3 <−0.15.
5. The four-piece infrared single wavelength lens system as claimed in claim 1 , wherein a focal length of the first lens element is f 1 , a focal length of the fourth lens element is f 4 , and they satisfy the relation: 0.35<f 1 /f 4 <0.77.
6. The four-piece infrared single wavelength lens system as claimed in claim 1 , wherein the focal length of the second lens element is f 2 , a focal length of the fourth lens element is f 4 , and they satisfy the relation: 6.0<f 2 /f 4 <68.5.
7. The four-piece infrared single wavelength lens system as claimed in claim 1 , wherein 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: −0.55<f 1 /f 23 <−0.1.
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 f 12 , a focal length of the third lens element and the fourth lens element combined is f 34 , and they satisfy the relation: 0.1<f 12 /f 34 <0.45.
9. The four-piece infrared single wavelength lens system as claimed in claim 1 , wherein the focal length of the second lens element is f 2 , a focal length of the third lens element and the fourth lens element combined is f 34 , and they satisfy the relation: 2.7<f 2 /f 34 <30.
10. The four-piece infrared single wavelength lens system as claimed in claim 1 , wherein a focal length of the second lens element and the third lens element combined is f 23 , a focal length of the fourth lens element is f 4 , and they satisfy the relation: −2.5<f 23 /f 4 <1.2.
11. The four-piece infrared single wavelength lens system as claimed in claim 1 , wherein a 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: 0.13<f 1 /f 234 <0.5.
12. The four-piece infrared single wavelength lens system as claimed in claim 1 , wherein a focal length of the first lens element, the second lens element and the third lens element combined is f 123 , a focal length of the fourth lens element is f 4 , and they satisfy the relation: 0.55<f 123 /f 4 <1.25.
13. The four-piece infrared single wavelength lens system as claimed in claim 1 , wherein a refractive index of the fourth lens element is N4, an Abbe number of the fourth lens element is V4, and they satisfy the relations: 1.61<N4; V4<25.
14. The four-piece infrared single wavelength lens system as claimed in claim 1 , wherein a focal length of the four-piece infrared single wavelength lens system is f, a distance from the object-side surface of the first lens element to an image plane along the optical axis is TL, and they satisfy the relation: 0.5<f/TL<0.9.