| 摘要 |
PROBLEM TO BE SOLVED: To obtain a magneto-optic disc substrate which is excellent in signal characteristics by specifying a glass transition temperature of a polycarbonate resin of an optical disc substrate, and a difference in a vertical refractive index of the substrate. SOLUTION: A difference in a vertical refractive index can be remarkably decreased by selecting a polycarbonate resin having a lower glass transition temperature than that of an ordinary polycarbonate resin for an optical disc. That is, in order to form a magneto-optic disc of a high recording density, the polycarbonate resin is required to have 100 to 140 deg.C as the glass transition temperature. For the resin exceeding 140 deg.C in the glass transition temperature, reduction of the difference in the vertical refractive index becomes very difficult to be achieved. Further, for the difference in the vertical refractive index, 300×10 or under is required. By satisfying this condition, a use value to the magneto-optic disc substrate which is excellent in signal characteristics, i.e., high in recording density, becomes extremely high.
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