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