Abstract: A method of producing a surface having a reduced reflectance to electromagnetic radiation in a predetermined wavelength band which comprises arranging on the surface a regular array of protuberances having a height which is not less than one third of the length of the longest wavelength in the band and at a spacing which is less than the length of the shortest wavelength of the band divided by the refractive index of the material of which the protuberances consist.

Abstract: Apparatus, for storing energy from incident electromagnetic radiation, hag an electromagnetic radiation energy absorbing means; an optical reflection filter having a substantially greater reflectance of electromagnetic radiation of longer than of shorter wavelengths and a substantially greater transmission for electromagnetic radiation of visible and shorter wavelengths than it has for electromagnetic radiation of wavelengths greater than those of the visible spectrum, the filter being positioned relative to the said energy absorbing means so that incident electromagnetic radiation can pass through the filter to reach the absorbing means, but energy re-emitted by the absorbing means in the direction of the filter is substantially returned by the filter to the absorbing means; an output channel for drawing off energy from the energy absorbing means; and means for restricting the outflow of energy otherwise from the energy absorbing means.