Air-gap flash

This is achieved by a high-voltage (20 kV typically) electric discharge between two electrodes over the surface of a quartz (or glass) tube.

A sub-microsecond flash is fast enough to photographically capture a supersonic bullet in flight without noticeable motion blur.

The person credited with popularising the flash is Harold Eugene Edgerton, though the earlier scientist Ernst Mach also used a spark gap as a fast photographic lighting system.

William Henry Fox Talbot is said to have created the first spark-based flash photo, using a Leyden jar, the original form of the capacitor.

However, since a large capacitance would have a relatively long discharge time that would make the flash slow, the only practical solution is to use a very high voltage on a relatively small capacitor, with a very low inductance.

Typical values are 0.05 µF capacitance, 0.02 µH inductance, 10 J energy, 0.5 µs duration and about 20 MW power.

The spark is guided over a quartz surface to improve the light output and benefit from the cooling capacity, making the flash faster.

Spark between anode and cathode, triggered by the third electrode inside the inner quartz tube. The inner tube serves as a guide for the spark and to cool it even faster. The outer tube muffles the explosive sound that the spark produces.
A photo of a Smith & Wesson Model 686 firing, taken with the flash above. The photo was taken in a darkened room, with the camera's shutter open, and the flash was triggered by the sound of the shot using a microphone.
Ultra-high-speed photo of a bullet travelling at about 870 metres per second (2,850 ft/s).
Air-gap flash spectrum generated with a grating .
Upper half shows the air-gap in daylight. Lower half shows the phosphorescence of the quartz ignition tube in blue in a darkened environment after a flash has occurred.