Plastic substrate for optical disk and process for producing the same

摘要

A recording medium for optical high-density data storage and retrieval, comprising a flat transparent plastic substrate characterized by the following relationship between the refractive index (nz) in the direction perpendicular to the flat surfaces of the transparent substrate, and the refractive indexes (nx) and (ny) in the direction parallel to the flat surfaces of the transparent substrate: ¦nz-nx¦ smaller than 4x10-4; and ¦nz-ny¦ smaller than 4x10-4. The recording medium is produced by an injection molding process wherein the pressure on the molten resin filled into the molding cavity of a mold is decreased or increased, as appropriate, in order to maintain the desired differences between the refractive index nz in the direction perpendicular to the flat surfaces of the transparent substrate, and the refractives indexes (nx) and (ny) in the direction parallel to the flat surfaces of the transparent substrate.