The suitability of zinc sulfide versus germanium for the middle negative lens of the Cooke triplet is studied. For this purpose two designs based on the Cooke triplet configuration are developed with a focal length of 100 mm and a relative aperture of f/2 to cover a total field of 14 deg for use in the 3–5-μm region of the spectrum. For the outer positive lenses both designs use silicon. For the middle negative lens one design uses zinc sulfide and the other uses germanium. The performances of the two designs are compared. It is found that the design with the zinc sulfide negative lens performs better in monochromatic applications, and the design with a germanium negative lens performs better in polychromatic applications.
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