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Arsenic sulphide athermal singlets for 3- to 5-μm imaging

An arsenic trisulphide refractive plus diffractive hybrid element is both achromatic and athermal across the 3 - 5 micrometer waveband when set in an aluminum mount. We present two possible ways of manufacturing this element: using a photodarkening process to realize the diffractive structures, and by diamond turning, initially not thought to be viable. We demonstrate that the required depths in the diffractive surface are obtainable by both methods, and consider the advantages of each. Above all, however this lens is fabricated it offers, a very low cost, light weight solution to athermal singlet imaging in the 3 - 5 micrometer waveband.