The transducer produces an output electrical signal proportional to the extent to which {TAIL ~ the third laser beam energy} is absorbed by {HEAD ~ the propane gas} 
The transducer produces an output electrical signal proportional to the extent to which { TAIL ~ the third laser beam energy } is absorbed by { HEAD ~ the propane gas }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD