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 {HEAD ~ drive circuit} during {TAIL ~ a first period} of time, 
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 { HEAD ~ drive circuit } during { TAIL ~ a first period } of 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 HEAD HEAD HEAD HEAD HEAD HEAD OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH