A diamond window was used in the Pioneer Venus I.R. radiometer. The paper suggests that diamond should be considered more widely as a possible optical component for windows and small lenses in space-optics and other optical applications. The cost is not high in relation to other costs involved in space research or for some land-based instruments. The optical properties of type II diamond are (i) transparency from 230 nm to far infra-red (with only weak absorption in the near infra-red), and (ii) high refractive index (2·43). Mechanically, diamond is the strongest of all solids and strong vacuum-tight joints to metal can be made. The thermal conductivity is better than copper. The main difficulty and cost is in working surfaces (other than flats) to optical tolerances.