{TAIL ~ The light} detecting {HEAD ~ means} is capable of sending an active signal as a light point is received thereon 
{ TAIL ~ The light } detecting { HEAD ~ means } is capable of sending an active signal as a light point is received thereon
TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH