| http://www.w3.org/ns/prov#value | - The grating according to claim 12, wherein the distance d satisfies the following formula: |D??sin ??1|???d???|D??sin ??2| where D is an effective diameter of the light beam, ??1 is a track angle at an innermost circumference and ??2 is a track angle at an outermost circumference of the optical disc.
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