a trisection detector for detecting the amount of {HEAD ~ the first portion} of the focused laser beam passing through {TAIL ~ each} of the pinholes; and 
a trisection detector for detecting the amount of { HEAD ~ the first portion } of the focused laser beam passing through { TAIL ~ each } of the pinholes ; and
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH