Thermal radiation from high-temperature application conditions of infrared windows can cause serious interference with infrared imaging. To suppress the thermal radiation from the window, the modulation effects of the optical properties and curvature characteristics of the two surfaces on the thermal radiation distribution from the window are investigated, respectively. The film–substrate–film radiation system is modeled. And the backward emissivity in the different cases of surface optical properties is investigated by the application of zinc sulfide (ZnS) windows and Y2O3 thin films as an example. It is shown that the backward emissivity is suppressed when the outer surface reflectivity is lower and the inner surface reflectivity is higher. A new definition of spectral directional locational emissivity is proposed to characterize the nonplate window emissivity. A model of thermal radiation propagation from the spherical substrate is developed. The emissivity distribution of the window with different curvature characteristics is investigated. It is shown that the curvature radius of the two surfaces can be matched to change the spatial distribution of the emissivity to reduce the effect of thermal radiation on the imaging of the backward optical system.

We investigate niobium-silicon mixed films prepared by plasma-assisted reactive magnetron sputtering. Multilayer films composed of niobium-silicon oxide with various Nb fractions were fabricated. The Nb fractions were calculated using the Brüggemann model and measured by x-ray photoelectron spectroscopy. The morphology of the samples shows that the stress in mixed monolayers depends on the Nb fraction and annealing temperature. A high-reflectivity (HR) multilayer film was fabricated from two mixed-oxide materials with Nb fractions of 20% and 95%, which are optimal for stress self-compensation and a maximum difference in refractive index to facilitate film design. The residual stress of this mixed-oxide multilayer HR film is completely self-compensated through annealing. Although annealing this film redshifts the transmission spectra and increases the surface roughness of the film, the results of cavity ring-down tests indicate that the reflectance remains high in the wavelength band of interest (1064 nm) and spatially uniform over the substrate surface. This detailed characterization of these films shows that the mixed-oxide multilayer HR mirror with self-compensated stress would be appropriate in numerous applications.