| 摘要 |
PROBLEM TO BE SOLVED: To provide a simple visual inspection method for a plastic substrate, for a recording medium, which is used for a fixed magnetic disk storage device. SOLUTION: A plastic substrate 1 is held in its central part, and incident light from its outer circumferential part is introduced into the substrate. The light which is introduced into the substrate is scattered to the outside from a defect such as a flaw or the like existing on the surface of the substrate or from a defect such as air bubbles or the like existing inside the substrate. The scattered light by the defect specifies a position in the radial direction when a detector 6 and a detector 7 used to detect only light in the vertical direction are made to agree with the radial direction of the plastic substrate 1 so as to face and install two faces of the plastic substrate 1. On which face out of the two faces the defect exists, can be specified by finding the face on which a quantity of strong scattered light exists out of the two faces. The position in the circumferential direction of the defect can be specified on the basis of an angle of rotation from the reference position of the plastic substrate 1.
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