Abstract: Reflectors of infra red radiation are used in infra red (thermal) imager systems to scan and reflect thermal radiation onto detectors. In this invention the highly reflecting surfaces of the reflectors, e.g. mirrors and rotating prisms, are coated with a thin infra red transparent layer of glassy diamond-like carbon. The carbon layer may be formed directly on the surface or an initial thin bonding layer, e.g. of silicon or germanium, may be deposited on the reflecting surface followed by the carbon layer. Both the bonding layer and carbon layer may be deposited by a glow discharge technique.