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