Four of {HEAD ~ the optical signals} from two of the inspection regions are paired for input into one of the optical cameras and recorded on {TAIL ~ a singular frame} 
Four of { HEAD ~ the optical signals } from two of the inspection regions are paired for input into one of the optical cameras and recorded on { TAIL ~ a singular frame }
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL