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