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