The first and second photodiodes are operably coupled to {TAIL ~ means} for subtracting {HEAD ~ 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 
The first and second photodiodes are operably coupled to { TAIL ~ means } for subtracting { HEAD ~ 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
OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD 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