Arsenic sulphide athermal singlets for 3-5 mu m imaging

An arsenic trisulphide refractive + diffractive hybrid element is both achromatic and athermal across the 3-5 mu m waveband when set in an aluminium mount. We present two possible ways of manufacturing this element: using a photodarkening process to realise 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 mu m waveband.