Air-gap flash

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.

An air-gap flash is a photographic light source capable of producing sub-microsecond light flashes, allowing for (ultra) high-speed photography. This is achieved by a high-voltage (20 kV typically) electric discharge between two electrodes over the surface of a quartz (or glass) tube. The distance between the electrodes is such that a spontaneous discharge does not occur. To start the discharge a high-voltage pulse (70 kV for example) is applied on an electrode inside the quartz tube.

The flash can be triggered electronically by being synchronised with an electronic detection device such as a microphone or an interrupted laser beam in order to illuminate a fast event. A sub-microsecond flash is fast enough to photographically capture a supersonic bullet in flight without noticeable motion blur.