Three-piece infrared single wavelength projection lens system

A three-piece infrared projection lens system, in order from an image side to an image source side, comprising:

• a stop;
• a first lens element with a positive refractive power, having an image-side surface being convex near an optical axis and an image source-side surface being concave near the optical axis, at least one of the image-side surface and the image source-side surface of the first lens element being aspheric;
• a second lens element with a negative refractive power, having an image source-side surface being concave near the optical axis, at least one of an image-side surface and the image source-side surface of the second lens element being aspheric;
• and
• a third lens element with a positive refractive power, having an image-side surface being concave near the optical axis and an image source-side surface being convex near the optical axis, at least one of the image-side surface and the image source-side surface of the third lens element being aspheric;
• wherein a focal length of the second lens element is f2, a focal length of the third lens element is f3, a central thickness of the first lens element along the optical axis is CT1, a central thickness of the second lens element along the optical axis is CT2, and they satisfy the relations:
• −1.3<f2/f3≤−0.49, and 3.15≤CT1/CT2<4.0.