| 摘要 |
<p>A luminescent color is controlled easily without changing a manufacturing condition and a film material, by forming films on a radiation transmissive bulb on whose surface a Frost process is performed while varying a film thickness into concave and convex to thereby adjust the film thickness. A depth of a Frost 19 formed by performing a Frost process on a surface of a bulb 12 made of quartz glass, is set to be approximately over 4 µm, or 2µm or more and 4µm or less. As a first layer, a low refractive index film 21 whose major constituent is SiO2 is formed by coating it on the Frost 19 by a dipping method and thereafter baking it, and as a second layer, a high refractive index film 22 whose major constituent is Fe2O3 is formed by coating it by the dipping method and thereafter baking it. The low refractive index films 21 and the high refractive index films 22 of a third and subsequent layers are alternately formed in plural layers.
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