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.
The correlation between the process parameters of auxiliary ion source and the SiO2 film stress was systematically studied using dual-ion-sputtering deposition. Spectrophotometer and ellipsometer were used to measure the SiO2 film’s transmittance spectrum and reflection ellipsometry. The refractive index and thickness of SiO2 film were obtained by total spectrum inversion calculation. The laser interferometer measures the substrate surface shape to obtain film stress. The experimental result shows that film stress is related to sputtering energy during deposition, high-energy oxygen ion assisted deposition can significantly reduce it and the assisted ion beam voltage plays an important role in controlling it by data normalization analysis. Ultra-low stress high reflective film was prepared by dual-ion-beam sputtering deposition, where stress was 70% lower than traditional film. At the same time, this film can ensure sufficiently high optical quality to keep it reliable and stable in high-precision laser systems.
SiO2 is a very important low refractive index film material that can operate in the UV to mid-IR regions. The SiO2 films used were deposited on Si substrates by different deposition techniques. We designed and manufactured a high-temperature infrared spectrum measuring equipment connected with a Fourier transform infrared (FTIR) spectrometer; the measuring temperature could range from room-temperature to 600°C. The FTIR transmission spectra of SiO2 films under different working temperatures were measured in the wavenumber region from 4000 to 400 cm − 1. From the measured spectra, it can be seen that IBS-SiO2 and IAD-SiO2 films are more stable and operating temperature can reach 400°C. EB-SiO2 film is worse and operating temperature is only 300°C. Complex dielectric functions of SiO2 films under different temperature conditions were calculated from FTIR transmittance spectra, and the best fitted method was obtained for calculating optical constants of dielectric materials in the high temperature condition. The results show that the structure of the Si-O-Si network in IBS-SiO2 films was the most stable structure. The experimental results were of great significance to practical application.
Yttrium oxide (Y2O3) thin films has been prepared on glass substrates at room temperature by thermal evaporation technique using Y2O3 powders (99% purity) and then are annealed at different temperatures ranging from 150℃ to 450℃ for 24 hours in air. The effects of the annealing temperatures on the structural and optical properties of the Y2O3 thin films were studied. The results show that the refractive index, extinction coefficient and forbidden band width of the Y2O3 thin film change to different degrees with the increase of annealing temperature. In addition, the roughness and stress of the Y2O3 thin film showed a trend of increasing first and then decreasing. The crystal state of the film is improved, indicating that the grain size becomes large. The research indicates that annealing treatment can effectively change the optical properties and structural properties of the Y2O3 thin films which has guiding significance for the selection of optimal heat treatment temperature for Y2O3 film modification.
Hafnium oxide (HfO2) thin films has been prepared on glass substrates at room temperature by thermal evaporation technique using HfO2 powders (99% purity) and then are annealed at different temperatures ranging from 150° to 450° for 24 hours in air. The effects of the annealing temperatures on the structural and optical properties of the HfO2 thin films were studied. Amorphous structures of the HfO2 films were researched by XRD technology. The results of the study show that the state of the HfO2 film changes from amorphous to polycrystalline with the increase of heat treatment temperature and the the stress is released. Besides, the refractive index of the films decreases and the band energy changes significantly. Therefore, the heat treatment can effectively change the film properties and it is necessary to comprehensively select the optimal heat treatment temperature according to the specific application of the HfO2 thin films.
The radiation wavelength of the carbon dioxide is 4.3μm. It is a kind of background noise that affect the image contrast in infrared imaging system seriously. Mirror is an important optical element in infrared optical system. This paper intends to design a 4.3μm wavelength filter base on the mirror. So, it is called reflective filter. It can replace the original mirror in the infrared optical system in order to filter out the radiation wavelength of the carbon dioxide and enhance the image quality. The reflective filter consists of metal and dielectric films. The absorption of the Al film which is the underlayer has been induced by the outer side dielectric multilayer films at 4.3μm. And it will keep high reflectivity in other wavelength at the same time. The design result as follow was obtained after optimization design. The average reflectivity is about 98% in the range of 3.7-4.2μm and 4.4-4.8μm (reflection bands) and the reflectivity is less than 10% at 4.3μm (absorption band) when the incident angle is 45°. The reflection phase of the metal-dielectric films was analyzed. The electric field distribution of reflection bands and absorption band was shown respectively. At last, in order to filter out the peak as much as possible, the design method and result of absorption band widening of the reflective filter was shown. Compare the all dielectric transmission minus filter, the layer number of the reflective filter is fewer and the total thickness of the coating is lower. Therefore, the manufacturing process became easier. The reliability became higher. More important, a better parameter of filter was obtained.
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