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.

History

The person credited with popularising the flash is Harold Eugene Edgerton, though the earlier scientist

Ernst Mach Ernst Waldfried Josef Wenzel Mach ( , ; 18 February 1838 – 19 February 1916) was a Moravian-born Austrian physicist and philosopher, who contributed to the physics of shock waves. The ratio of one's speed to that of sound is named the Mach n ...

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 A Leyden jar (or Leiden jar, or archaically, sometimes Kleistian jar) is an electrical component that stores a high-voltage electric charge (from an external source) between electrical conductors on the inside and outside of a glass jar. It ty ...

, 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_ \propto

, in which L is the

inductance Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The flow of electric current creates a magnetic field around the conductor. The field strength depends on the magnitude of th ...

and C the

capacitance Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are ...

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 =

, where V is the

voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...

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.Edgerton, H. E. K, K. Cooper and J. Tredwell, Submicrosecond Flash Source, J. SMTPE, vol. 70, p. 117, March, 1961 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 A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to ident ...

, mainly of

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

since air is 79% nitrogen. The spectrum is rich in UV but covers the entire visible range down to

infra-red Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

. When a

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical f ...

tube is used as ignition tube, it shows a clear

phosphorescence Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike fluo ...

in blue after the flash, induced by the UV.

References

External links

Amateur air-gap flash for ultra-high-speed photography by Niels NoordhoekMIT Edgerton centerScientific American article on air-gap flash
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