The OPT 27H downlight fitting allows an airtight seal to be maintained within a display case when using fibre optics. These fittings are made from aluminium and the air seal is maintained by a series of three o-rings around the flange, eyeball and ferrule. The ferrule is tightened fully onto the fitting and the focus adjustment is made using the trumpet. The further onto the ferrule the trumpet is screwed, the wider the beam. The tight tolerances attained during manufacture help ensure many years of reliable operation.