Thermal Infrared Lenses of High Aperture and Performance

High aperture lenses of a triplet construction containing germanium lens elements and designed for the 8 to 13 micro-meter spectral bandwidth have been known for at least 10 years. Pilkington P.E. have recently been carrying out research into the aperture limitations of the basic triplet design. The following text describes a simple modification that can be made to an all-germanium triplet which raises its aperture from a typical f/0.75 to the very high aperture of f/0.55 without recourse to aspheric surfaces or loss of field-of-view. An achromatised design utilizing three elements of germanium and one of KRS 5 and achieving an aperture of f/0.5 8 is also described.