Application of chalcogenide glass in designing a long-wave infrared athermalized continuous zoom wide-angle lens

Current commercial wide-angle infrared lenses usually adopt simple fixed-focus design structures and lack proper athermalized design. To improve the performance of current wide-angle infrared lenses, the design principle of zoom optical systems and the theorem of passive athermalization were utilized to develop an athermalized continuous zoom wide-angle infrared lens. The effective focal length of the proposed system was 10-24 mm(zoom ratio:2.4:1), the field of view(FOV) was 34-90, the operating wavelength was 8-12m, and the F number was 2.8. Considering the fact that infrared lenses were widely used in environments with large temperature variations, the proposed system was designed as a combination of six lens elements, including three fabricated with the chalcogenide glass of NBU-IR2(Ge20Sb15Se65) and three fabricated with conventional infrared materials such as germanium(Ge) and zinc sulfide(ZnS). By carefully allocating optical powers among the lens elements and properly controlling their air-spacing thicknesses, athermalization was realized by the proposed system design, in addition to the performance of continuous optical zoom. Simulation results show that the proposed system can produce thermal images with a close-to-diffraction-limit performance for the temperature range of -40 to 60℃. The Modulation Transfer Functions(MTFs) of the system are higher than 0.25 for the entire FOV. The system also has a compact/light-weight structure that only includes one aspheric surface on a chalcogenide glass lens element. Modern precision molding technique can be used to fabricate aspheric surfaces on chalcogenide glasses so that the fabrication costs of the proposed system can be controlled. The overall system design is suitable for the application of vehicle night vision.