| 摘要 |
PURPOSE:To prevent occurrence of a crack by forming a vacuum evaporated glass layer composed of SiO2, Na2O, Al2O3, B2O3, etc., on the surface of a glass blank. CONSTITUTION:A core glass 101 is set to a dome 203 for film formation and placed in a vacuum tank 201. The vacuum tank is evacuated through an evacuation opening 205 and the core glass 101 is heated by a heater 202 to a prescribed temperature. After evacuation to a prescribed degree of vacuum, an electric power is applied to an antenna 204 for application of a high-frequency wave while introducing Ar gas, etc., through a gas inlet 206 and a high-frequency discharge is carried out so that the core glass 101 may be subjected to plasma cleaning. After stopping the above-mentioned introduction of Ar gas, etc., and recovering the degree of vacuum, a glass 207 for evaporation as an evaporation source is evaporated using an electronic gun 208, thus giving the objective glass blank for an optical unit containing an evaporated glass surface layer composed of 70-90wt.% SiO2, <=3wt.% Na2O, <=3wt.% Al2O3, B2O3 and the remainder, formed on the core glass 101 and having 50-500Angstrom thickness.
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