PURPOSE:To provide excellent optical characteristics on a multi-layered thin optical film for IR formed to the substrate shape consisting of zinc sulfide, etc., having a relatively small refractive index and to contrive an improvement in the adhesiveness to a substrate, mechanical strength, etc., by using a diamond- like carbon film for the outermost layer. CONSTITUTION:This thin optical film for IR, for example, multi-layered antireflection film consisting of a material such as zinc sulfide, zinc selenide, etc., having the relatively small refractive index ns is formed by using the diamond-like carbon film 5 for the outermost layer thereof on noticing that the diamond-like carbon film 5 has relatively outstanding mechanical strength, chemical resistance and water resistance. The diamond-like carbon film 5 is used for the outermost layer and single or multi-layered films 3, 4 contg. at least one layer of germanium (Ge), lead telluride (PbTe), silicon (Si) etc., having the refractive index higher than the refractive index of the substrate material are provided under said layer. The sufficient antireflection characteristic is thereby obtd. In addition, the diamond-like carbon film 5 is used for the outermost layer and, therefore, the mechanical strength, chemical resistance, durability etc., are improved.
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