{HEAD ~ The sensor} includes a light source and {TAIL ~ a receiver} which is sensitive to its radiation 
{ HEAD ~ The sensor } includes a light source and { TAIL ~ a receiver } which is sensitive to its radiation
HEAD HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH