wherein said {HEAD ~ 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 {TAIL ~ a predetermined period} of time 
wherein said { HEAD ~ 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 { TAIL ~ a predetermined period } of time
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD 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 TAIL TAIL OTH OTH