{HEAD ~ Circuit} 50 comprises {TAIL ~ a photodiode} 52 with anode 54 and cathode 56 and photodiode 58 with anode 60 and cathode 62 
{ HEAD ~ Circuit } 50 comprises { TAIL ~ a photodiode } 52 with anode 54 and cathode 56 and photodiode 58 with anode 60 and cathode 62
HEAD HEAD HEAD HEAD HEAD OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH