wherein said defect detection circuit continues to maintain said substantially constant output voltage to said drive circuit when said amount of irradiation beam detection signal exceeds said reference voltage for a period of {TAIL ~ time} that is less than a predetermined period of {HEAD ~ time} 
wherein said defect detection circuit continues to maintain said substantially constant output voltage to said drive circuit when said amount of irradiation beam detection signal exceeds said reference voltage for a period of { TAIL ~ time } that is less than a predetermined period of { HEAD ~ time }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD