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