{HEAD ~ The first and second photodiodes} are operably coupled to {TAIL ~ means} for subtracting the total currents flowing through each photodiode to produce an output electrical signal substantially independent of dark current contributions, and proportional to the level of illumination incident on the photodiodes 
are operably coupled to
{ HEAD ~ The first and second photodiodes } are operably coupled to { TAIL ~ means } for subtracting the total currents flowing through each photodiode to produce an output electrical signal substantially independent of dark current contributions , and proportional to the level of illumination incident on the photodiodes
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH