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 a quartz (or glass) tube's surface. The distance between the electrodes is such that a spontaneous discharge does not occur. To start the discharge a high-voltage pulse is applied on an electrode inside the quartz tube.
The discharge can be triggered electronically using a microphone or an interrupted laser beam to illuminate a fast event. A sub-microsecond flash is fast enough to photographically stop even a supersonic bullet in flight without noticeable motion blur.
The person credited with popularising the flash is Harold Eugene Edgerton, though earlier scientists such as 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. Edgerton was one of the founders of EG&G company who sold an air-gap flash under the name Microflash 549.
Design parameters 
The aim of a high-speed flash is to be fast and bright enough. A flash system typically consists of a capacitor that is discharged through a gas (air in this case). The speed of a flash is mainly determined by the time it takes to discharge the capacitor through the gas. This time is proportional to
in which L is the inductance and C the capacitance of the system. To be fast, both L and C must be kept small. The brightness of the flash is proportional to the energy stored in the capacitor:
where V is the voltage over the capacitor. This calls for a large capacitor and a high voltage. However, a large C makes the flash slow, so the only solution to make a sufficiently fast and bright flash is to use a very high voltage on a relatively small capacitor, with a very low inductance. Typical values are 0.05 µF capacity, 0.02 µH inductance, 10 J energy, 0.5 µs duration and about 20 MW power.
Air (mainly nitrogen) is preferred as a gas because it is fast. Xenon has a much higher efficiency in turning energy into light, but is limited in speed to about 10 microseconds, caused by its own afterglow.
Spectral properties 
Since the spark gap discharges in air generating a plasma, the spectrum shows both a continuum and spectral lines, mainly of nitrogen since air is 79% nitrogen. The spectrum is rich in UV but covers the entire visible range down to infra-red. When a quartz tube is used as ignition tube, it shows a clear phosphorescence in blue after the flash, induced by the UV.
- Edgerton, Harold E. (19706). Electronic flash, strobe, Chapter 7, Mc Graw Hill, New-York. ISBN 007018965x / 0-07-018965-x.
- Topler, M, Ann Physik, vol. 4, no. 27, pp 1043-1050, 1908
- Edgerton, H.E.K, K. Cooper and J. tredwell, Submicrosecond Flash Source, J. SMTPE, vol. 70,p. 117, March, 1961
|Wikimedia Commons has media related to: Air-gap flash|
- Amateur air-gap flash for ultra-high-speed photography by Niels Noordhoek
- MIT Edgerton center
- Scientific American article on air-gap flash