The exposure period is then set to a period which is a predetermined length of {HEAD ~ time} less than {TAIL ~ the browning threshold} 
less than
The exposure period is then set to a period which is a predetermined length of { HEAD ~ time } less than { TAIL ~ the browning threshold }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL