Abstract: An optically immersed infra-red detector assembly comprises a planar sapphire substrate bearing a detector with radially disposed lens contact pads. The detector and contact pads are produced by etching a single portion of cadmium mercury telluride and have mutually coplanar surfaces. A hemispherical silicon optical immersion lens is bonded to the pads by applying dilute adhesive to lens-pad interfaces. Capillary attraction draws the adhesive between the lens and the pads to form very thin layers of adhesive after solvent evaporation. The adhesive layers ensure that an air gap is produced between the lens and detector, the gap being much smaller than the infra-red wavelength of interest as required to ensure optical immersion of the detector by the lens.