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