By comparing a digital image signal obtained by inverting a digital image obtained from one of the scan lines by 180 degrees with a digital image signal obtained from the other of the scan lines, therefore, the tail 175 appears on both sides of the pattern 171 as {HEAD ~ noncoincidence} in {TAIL ~ the case} where charge up has occurred in the pattern 171 
By comparing a digital image signal obtained by inverting a digital image obtained from one of the scan lines by 180 degrees with a digital image signal obtained from the other of the scan lines , therefore , the tail 175 appears on both sides of the pattern 171 as { HEAD ~ noncoincidence } in { TAIL ~ the case } where charge up has occurred in the pattern 171
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