The duration of a flash discharge defines the ability of the flash system to capture or 'freeze' a moving subject, and there's a long standing debate continuing as to how it should be measured. The problem is that the intensity of light given off by a xenon flash tube isn't constant through the duration of the discharge. When a fully charged capacitor starts to supply energy to the tube, the light output rises to a maximum very quickly indeed. As the capacitors start to discharge, the light output starts to fall, quickly to start off with, then more slowly as the energy is finally exhausted. In theory, the total flash duration should cover the entire period between the flash being triggered and the total extinction of the tube. Because of the shape of the flash curve - rapid rise, rapid fall, then a slow discharge at the end or 'tail' of the curve, it isn't practical to use this total time, especially as the light energy at the tail of the curve has little practical effect on exposure. Instead, manufacturers who follow ISO and DIN standards use the t=0.5 standard, which defines the duration of the flash to be the time during which the flash intensity exceeds 50% of its peak value. This gives a good basis for comparison between different manufacturer's units, and most studio flash units have t=0.5 durations of between 1/250 and 1/2,000 second.
The t=0.1 definition, which defines flash duration as the time during which the intensity exceeds 10% of its peak value. Generally, the t=0.1 flash duration is about three times as long as the t=0.5 value, and this can be used as a reliable guide to the motion stopping ability of a flash unit.
