這種google就有的東西就別沾沾自喜了

The effective flash duration t 0.5 is defined as being the period during which the flash intensity exceeds 50% of its maximum.
50% of the flash power is produced after t 0.5 has been reached.  

The total flash duration t 0.1 is defined as being the period during which the flash intensity exceeds 10% of its maximum.