| 摘要 |
A multi-layered optical disc having its recording unit comprised of plural information recording layers, in which two or more information recording layers are layered on a substrate 0.3 to 1.2 mm in thickness, with the interposition of a transparent layer, to form a recording unit, a light transmitting protective layer is formed on the recording unit to a thickness of 10 to 177 mum and in which the light is illuminated from the side of the light transmitting protective layer to effect recording and/or reproduction of information signals. At least one of the information recording layers except the information recording layer formed at a remotest position from the light transmitting protective layer has a phase change material as a recording material. A refractive index nc1 and an extinction coefficient kc1 in the crystallized state and a refractive index na1 and an extinction coefficient ka1 in the amorphous state of the phase change material constituting the first information recording layer as counted from the light transmitting protective layer satisfy the relationship of (nc1/na1)<=12, (kc1/ka1)<=12 and (kc1/ka1)<=5 , (nc1/na1) where the cases of kc1/ka1<=1 and nc1/na1<=1 are excluded.
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