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Transmission spectra were measured for anodic alumina films prepared in several acid solutions. The films anodized in a phosphoric acid solution are the most appropriate for use in the infrared range, since impurity absorptions are weak in those films. The refractive indices of the phosphoric-acid films were evaluated by the measurements of transmittance and reflectance. The refractive index decreases either by extending anodization time or by immersing a film in a sulfuric acid solution. This phenomenon is elucidated well by considering pore widening due to the dissolution of alumina. The dependence of refractive index on porosity agreed well with theoretical evaluations.
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Nakamura et al. (1992) studied this question.
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