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