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