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