{TAIL ~ A modulated laser beam} is scanned across the object to create a plurality of spots which are viewed by {HEAD ~ a pair} of well matched receivers 
{ TAIL ~ A modulated laser beam } is scanned across the object to create a plurality of spots which are viewed by { HEAD ~ a pair } of well matched receivers
TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH