The output of the operational amplifier is proportional to {HEAD ~ the incident light} provided {TAIL ~ the intensity} on each photodiode are proportional to each other 
provided
The output of the operational amplifier is proportional to { HEAD ~ the incident light } provided { TAIL ~ the intensity } on each photodiode are proportional to each other
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH