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| [[Image:Triggered spark gap.jpg|thumb|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.]]
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| [[Image:Bullet coming from S&W.jpg|thumb|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.]]
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| [[File:.308 Winchester bullet (150 grain FMJ).JPG|thumb|Ultra-high-speed photo of a bullet travelling at about {{convert|2850|ft/s|m/s}}.]]
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| [[Image:Microflash-spectrum.jpg|thumb|Air-gap flash spectrum generated with a [[Diffraction grating|grating]].]]
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| [[Image:Microflash-quartz-phosphorescence.jpg|thumb| 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.]]
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| 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.
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| 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.
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| 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. [[Henry Fox Talbot|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.<ref>http://people.rit.edu/andpph/text-microflash-549-manual.pdf</ref> There are several commercial flashes available today.
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| == Design parameters ==
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| 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
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| <math>t_{discharge} \propto {\sqrt{LC}}</math>,
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| in which L is the inductance and C the capacitance of the system. To be fast, both L and C must be kept small.
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| The brightness of the flash is proportional to the energy stored in the capacitor:
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| <math>E = {{CV^2} \over 2}</math>,
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| 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.
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| Typical values are 0.05 µF capacity, 0.02 µH inductance, 10 J energy, 0.5 µs duration and about 20 MW power.<ref>Edgerton, Harold E. (19706). ''Electronic flash, strobe'', Chapter 7, Mc Graw Hill, New-York. ISBN 007018965x / 0-07-018965-x.</ref>
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| 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.
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| The spark is guided over a quartz surface to improve the light output and benefit from the cooling capacity, making the flash faster.<ref>Topler, M, Ann Physik, vol. 4, no. 27, pp 1043-1050, 1908</ref><ref>Edgerton, H.E.K, K. Cooper and J. tredwell, Submicrosecond Flash Source, J. SMTPE, vol. 70,p. 117, March, 1961</ref> This has a negative effect in form of quartz erosion because of high energy discharge.
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| == Spectral properties ==
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| Since the spark gap discharges in air generating a [[Plasma (physics)|plasma]], the spectrum shows both a continuum and [[spectral lines]], mainly of [[nitrogen]] since air is 79% nitrogen.
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| The spectrum is rich in [[UV]] but covers the entire visible range down to [[infra-red]].
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| When a [[quartz]] tube is used as ignition tube, it shows a clear [[phosphorescence]] in blue after the flash, induced by the UV.
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| ==References==
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| {{reflist}}
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| ==External links==
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| {{commons category}}
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| *[http://www.njnoordhoek.com/?p=735 Amateur air-gap flash for ultra-high-speed photography by Niels Noordhoek]
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| *[http://web.mit.edu/Edgerton/ MIT Edgerton center]
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| *[http://web.mit.edu/Edgerton/www/pdf/Article.pdf Scientific American article on air-gap flash]
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| {{Photography}}
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| [[Category:Flash photography| ]]
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| [[Category:Photographic lighting]]
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| [[Category:Light sources]]
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| [[Category:Photography equipment]]
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