Xenon flash lamp

Construction

The lamp comprises a sealed tube, often made of fused quartz, which is filled with a mixture of gases, primarily xenon, and electrodes to carry electrical current to the gas mixture. Additionally, a high voltage power source is necessary to energize the gas mixture; this high voltage is usually stored on a capacitor so as to allow very speedy delivery of very high electrical current when the lamp is triggered.

The glass envelope is most commonly a thin tube, which may be straight, or bent into a number of different shapes, including helical, "U" shape, and circular (to surround a camera lens for shadowless photography - 'ring flashes'). The electrodes protrude into each end of the tube, and are connected to a capacitor that is charged to a relatively high voltage. This is usually between 250 and 2000 volts, depending on the length of the tube, and the specific gas mixture.

Operation

A flash is initiated by first thermionic emission of electrons.

Output spectrum

As with all ionized gases, xenon flash lamps emit light in various krypton is also occasionally used, although it is even more expensive. Krypton has much greater output in the near-IR range, which is better matched to the absorption profile of Nd:YAG laser media than xenon emissions.

During normal operation in most photographic-type systems, the spectral component of a flashlamp's emission is overshadowed by blackbody radiation. The proportion of light produced by spectral action compared to thermal action depends on current density in the arc. Higher current densities favor blackbody radiation over spectral radiation. For this reason, many laser systems intentionally utilize lower current densities than photographic flashes since more narrow spectral lines are usually favorable for pumping lasers, while a broadband output is better for photographic purposes. Production of greenish blue light instead of pure white is a clear indication of low-current density operation.

Intensity and duration of flash

For short pulses the number of emitted electrons from the cathode is the limit. For longer pulses or continuous operation the cooling is the limit. Discharge durations for common flashlamps are in the microsecond to a few milliseconds range and can have repetition rates of hundreds of hertz.

The flash that emanates from a xenon flash lamp may be so intense that it can ignite flammable materials within a short distance of the tube. Carbon nanotubes are particularly susceptible to this spontaneous ignition when exposed to the light from a flashtube.^[1] Similar effects may be exploited for use in aesthetic or medical procedures known as Intense Pulsed Light (IPL) treatments. IPL can be used for treatments such as hair removal and destroying lesions or moles.

Applications

Because the duration of the flash that is emitted by a xenon flash tube can be accurately controlled, and due to the high intensity of the light, xenon flash lamps are most commonly used as photographic strobe lights. Xenon flashlamps are also used in the technique of very high speed or "stop-motion" photography, which was pioneered by Harold Edgerton in the 1930s.

Due to their high-intensity and relative brightness at short wavelengths (extending into the Nd:YAG lasers, as krypton emission in near infrared is better matching to the absorption spectrum of Nd:YAG.

Xenon flash lamps have been used to produce an intense flash of white light, some of which is absorbed by Nd:glass that produces the laser power for inertial confinement fusion. In total about 1 to 1.5% of the electrical power fed into the flash tubes is turned into useful laser light for this application.

Animation

References

1. ^ http://news.rpi.edu/update.do?artcenterkey=383&setappvar=page(1)

• Timing electronics can trigger multiple flashes and then activate a Q-switch
• Emission spectra of different flash lamps

See also

• Strobe light
• Flash (photography)
• List of light sources