| 摘要 |
PURPOSE:To determine a mark position at a high speed, by forming an edge- density distribution and computing the mark position from the edge density in a given value or above. CONSTITUTION:when electron beams are scanned, the edge positions,on which intensity of reflective beams crosses a threshold value, are memo. rised in an edge position-detecting circuit 21, to obtain edge distribution data E(I) at a high speed In an edge distribution-generating circuit 22. Edge-density distribution is computed from this edge distribution in the edge density distribution- generating circuit 23. A running average computing device is used in the circuit 23, to memorise the edge density Ed(I). And, positions IS1, IS2, IS3, and IS4, on which the edge density Ed(I) crosses a threshold value LE, are supplied into a control circuit 20. Finally, the mark position is computed from data of the threshold value LE or above of the edge density Ed(I) and the position data IS1-IS4 in a weighted-mean operational circuit 24.
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