| 摘要 |
This invention decreases deformation of a plastic substrate caused by thermal expansion and contraction in an evaporation process of forming a multi-layered film of an ND filter, thus arising of wrinkles is prevented. The plastic substrate serving as the base of the ND filter is made of a polyimide-based plastic material having a glass transition temperature of 200° C. or more, and has a total transmittance of light of 90% or more and has a haze factor, which represents the haze of 0.5% or less. As the multi-layered film, Al<SUB>2</SUB>O<SUB>3 </SUB>films for reduction of the reflectance and TiO<SUB>x </SUB>films for reduction of the transmittance are alternately formed on the substrate. An MgF<SUB>2 </SUB>film made of a low-refraction material is formed as the uppermost layer to further reduce the reflection. In the evaporation process, even when the temperature of the plastic substrate reaches almost 200° C., the sufficiently high glass transition temperature of the plastic material can suppress arising of wrinkles on that portion of the plastic substrate where the multi-layered film is formed.
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