| 摘要 |
PROBLEM TO BE SOLVED: To efficiently measure a focal position by simple constitution. SOLUTION: A CPU calculates a contrast ratio C1 in a left end L of an optical head, a contrast ratio C2 in the central part C of the optical head, and a contrast ratio C3 in a right end R of the optical head respectively (step ST1 to step ST3). The maximum value out of the calculated contrast ratios is determined as MAX (step ST4). The CPU determines whether (MAX=C2)is satisfied or not, and conducts determination as a non-defective when affirmed (step ST6).
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