Four of {HEAD ~ the optical signals} from the other two of the inspection regions are paired for {TAIL ~ input} into the other of the optical cameras 
are paired for
Four of { HEAD ~ the optical signals } from the other two of the inspection regions are paired for { TAIL ~ input } into the other of the optical cameras
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH REL REL REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH