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