| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for heat treating a fluoride single crystal which comprises lowering the temperature of a fluoride single crystal kept in a heated state and by which the fluoride single crystal having highly uniform refractive index and reduced double refractivity can be obtained in a short time. SOLUTION: The method for heat treating the fluoride single crystal comprises lowering the temperature of the fluoride single crystal, heated and kept at a first temperature lower than the melting point, to a second temperature within a range of 10-100°C from the first temperature. Before reaching the second temperature after starting the cooling-down from the first temperature, the temperature of the fluoride single crystal is kept constant at least once, and the total time consumed to keep the temperature constant is not shorter than 1% of the time elapsed before reaching the second temperature from the first temperature. COPYRIGHT: (C)2007,JPO&INPIT
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