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 time that is less than {HEAD ~ a predetermined period} of {TAIL ~ 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 time that is less than { HEAD ~ a predetermined period } of { TAIL ~ 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 OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL