The outputs of each operational amplifier is input into a subtracter circuit to subtract common dark or photocurrent components so {HEAD ~ the output} of the subtracter circuit is proportional to {TAIL ~ the difference} in light incident on the photodiodes 
The outputs of each operational amplifier is input into a subtracter circuit to subtract common dark or photocurrent components so { HEAD ~ the output } of the subtracter circuit is proportional to { TAIL ~ the difference } in light incident on the photodiodes
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH