means for extracting the low frequency and d.c. components from the photo detector output signal, said {HEAD ~ extracted components} being proportional to {TAIL ~ the average light intensity incident} on the photo detector, 
proportional to
means for extracting the low frequency and d.c . components from the photo detector output signal , said { HEAD ~ extracted components } being proportional to { TAIL ~ the average light intensity incident } on the photo detector ,
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 REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH