Measurement of Transmittance Variation Due to the Water Vapor Adsorption on Optical Glass Surfaces

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  • 水分子吸着による人工衛星光学硝材の透過率変動計測
  • ミズブンシ キュウチャク ニ ヨル ジンコウ エイセイ コウガク ショウザイ ノ トウカリツ ヘンドウ ケイソク

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Gas adsorption onto optical surfaces equipped in satellites is one of the causes of signal degradation occurred in orbit and water is one of the most potent ad-molecules. To estimate the degradation caused by water adsorption onto optical glass surface, the transmittance measurements were carried out in UV-VIS (200--800nm) and IR (1.7--20μm) wavelength ranges. Five kinds of glasses, SiO2, BK7, Al2O3, CaF2, and ZnSe, which are typically used for satellite optics, were selected as glass samples. In IR wavelength range, a local absorption feature was appeared at λ=3.1μm when a glass temperature was below 200K. The phase change of adsorbed water from vapor to solid may account for the newly appeared absorption. In UV-VIS region, there was no local absorption feature but broad transmittance decrease. BK7 and ZnSe, which are hydrophobic, showed little transmittance decrease and it was hard to find the wavelength dependence for their transmittance spectra. On the other hand, the transmittance spectra of hydrophilic glasses, SiO2, Al2O3, and CaF2, showed wavelength dependence and had the local minimal value. From our computation, it is found that the growth of ice grains on a glass surface makes it possible to reproduct the spectral transmittance degradation detected for hydrophilic glasses.

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