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