The outputs of {TAIL ~ 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 the photodiodes 
The outputs of { TAIL ~ 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 the photodiodes
OTH OTH OTH 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 OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH