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