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 revolver firing, taken with the flash above. The photo was taken in a darkened room, with 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 2,850 feet per second (870 m/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 is applied on an electrode inside the quartz tube.

The discharge can be triggered electronically using a microphone or an interrupted laser beam in order 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.

History

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. There are several commercial flashes available today.

Design parameters

The aim of a high-speed flash is to be very fast and yet bright enough for adequate exposure. An air-gap 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

$t_{discharge} \propto {\sqrt{LC}}$,

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:

$E = {{CV^2} \over 2}$,

where V is the voltage across the capacitor. This shows that high brightness calls for a large capacitance and a high voltage. 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.

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.

The spark is guided over a quartz surface to improve the light output and benefit from the cooling capacity, making the flash faster. This has a negative effect in the form of quartz erosion because of high energy discharge.

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.

External links

• Amateur air-gap flash for ultra-high-speed photography by Niels Noordhoek
• MIT Edgerton center
• Scientific American article on air-gap flash