Abstract: A thin film reflecting interference filter (RIF) is designed to suppress unwanted harmonics thereby improving the monochromaticity of the radiation. An interference layer of material which has a well-defined plasma oscillation is deposited on a substrate and a mismatch layer is formed thereon. This interference layer exploits the interference between wavefronts reflected from the layer-substrate and the vacuum-layer interfaces to suppress higher order harmonics, while allowing good reflectance at the fundamental wavelength. This is achieved by positioning the RIF in the radiation at an angle of incidence which is greater than the critical angle of the desired fundamental wavelength, but less than critical angles of the harmonics to be suppressed. The mismatch layer increases the reflectance of the unwanted harmonics at the vacuum-layer interface, thus allowing more complete destructive interference of the unwanted harmonics.