This is done by controlling {TAIL ~ an adjustable optical filter} having an adjustable optical element for each of {HEAD ~ the sampling positions} 
This is done by controlling { TAIL ~ an adjustable optical filter } having an adjustable optical element for each of { HEAD ~ the sampling positions }
OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD