a defect detection circuit configured to compare sad amount of irradiation beam detection signal with a reference voltage and maintain a substantially constant output voltage to said drive circuit during {HEAD ~ a first period} of {TAIL ~ time}, 
of
a defect detection circuit configured to compare sad amount of irradiation beam detection signal with a reference voltage and maintain a substantially constant output voltage to said drive circuit during { HEAD ~ a first 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 HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL OTH