IR IMAGING: Thin silicon Fresnel lens enables inexpensive thermal-IR imaging

摘要

A group of French researchers has designed and fabricated a type of small, thin silicon (Si) Fresnel lens that can simplify compact thermal infrared (IR) cameras and make them lower in cost. Although the lens' resolution is not superb, it is good enough to reveal the presence of a person and some general features. Potential applications for the imager include more affordable surveillance, particularly for home use, the researchers say. The aspherical lens has a thickness of 1 mm, an f/# of 1.5, and a diameter of just under 4 mm, with the Fresnel surface on the inner (image-facing) side (see figure). The lens stop is placed a bit less than 1 mm in front of the lens at approximately the telecentric position, with the result that edge-of-field rays with a 65° incidence angle are focused by a region of the lens near its edge that hardly overlaps with the lens region focusing the rays with 0° incidence angle. (And of course the chief rays are all parallel to the optical axis due to telecentricity.)