LWIR imaging lens, image capturing system having the same, and associated methods

Imaging lens (250) for use with an operational waveband over any subset of 7.5-13.5 microns (LWIR) includes a first optical element (210) of a first high-index material and a second optical element (220) of a second high-index material. At least two surfaces (B, E) of the first (210) and the second optical element (220) are optically powered surfaces. The largest clear aperture (E) of all optically powered surfaces does not exceed by more than 30% the image diameter corresponding to a field of view of at least 55 degrees. The first and second high-index materials (e.g. Si, Ge, ZnSe, ZnS or a chalcogenide glass) have a refractive index greater than 2.2 and an absorption per mm smaller than 75% in the operational waveband and an absorption per mm greater than 75% in the 400-650nm waveband.