| 摘要 |
PURPOSE:To enable the stable mark position to be detected constantly regardless of the level of mark signals by a method wherein, when the changing ratio in amplification degree of an amplifier is decreased exceeding the specified lower limit, the position signals detected and held so far are erased. CONSTITUTION:A little changes in signal levels are increased in the scannings 11, 12 as shown in the section A while the threshold value 17 is set up as, e.g., 1/2 of the maximum value of signals so that a mark detecting circuit may detect any mistaken mark positions to be held as they are in the section A. However, in the scannings 13, 14 containing a true mark 10 on the way of scanning, the signal levels are rapidly increased decreasing the amplification degree Y of an amplifier to make the true mark signals coincide with the target value 16 as shown in the section B. On the other hand, the mark detecting circuit is actuated to erase the so far held mistaken mark position data holding the true mark positions to be successively detected.
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