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