High speed photographic techniques can be used to capture or freeze events that take place in a very short period of time, and which are consequently of particular interest because they reveal detail normally unseen by the human eye. Examples are events such as a bullet striking an egg, an object falling in to liquid, or a fast-moving projectile exploding an inflated balloon.
Such events are in most cases too fast to be captured even by the fastest shutter speed available on a general purpose camera. Flash is therefore used in a darkened room to illuminate the key moment of the action. The output of a flash unit is controlled by limiting the duration of the flash rather than its intensity. Consequently, if a flash gun is limited to its minimum output, perhaps 1/128 power, the duration of the flash may be as short as 1/25,000 or 1/30,000 of a second.
High speeds such as these are useful only if the camera's shutter is open and the flash is fired at precisely the right moment, so an event trigger must be used to synchronize the flash with the action. various types of trigger mechanism can be used, including infrared beams, laser, optical and sound triggers. These are typically used in conjunction with a programmable delay mechanism to control the delay between the trigger event and the desired exposure being made. Some experimentation is inevitably involved.