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flash lamp

Xenon flash lamp

A xenon flash lamp is an electric glow discharge lamp designed to produce extremely intense, incoherent, full-spectrum white light for very short durations.

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. A charged capacitor is usually used for this purpose 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 ionizing the gas mixture, then sending a very large pulse of current through the ionized gas. Ionization is necessary to decrease the electrical resistance of the gas so that a pulse measuring as much as thousands of amperes can travel through the tube. The initial ionization pulse may be generated by a step-up transformer. A short high voltage peak produces the first ions at the sharp tip of the cathode (the housing is grounded). By applying radio frequency voltage the ions do not need to reach the anode, but couple capacitively to the housing (and the anode). This may be enhanced by putting a metal band onto the glass or a wire that is wrapped around the glass tube or by using water cooling, since water has a high dielectric constant and if ionized also conducts. When this current pulse travels through the tube, it excites electrons surrounding the xenon atoms causing them to jump to higher energy levels. The atoms' electrons immediately drop back to a lower orbit, producing photons in the process. Depending on the size and application of the flashlamp, xenon fill pressures may range from a few kilopascals to tens of kilopascals (0.01–0.1 atmosphere or tens to hundreds of torr). For low electrode wear the electrode needs to be at high temperature for the thermionic emission of electrons.

Output spectrum

As with all ionized gases, xenon flash lamps emit light in various spectral lines. This is the same phenomenon that gives neon signs their characteristic color. However, for xenon, there are enough spectral lines, and they are distributed across the spectrum in such a way, that to the human eye the light appears mostly white. The spectral profile of a xenon arc peaks in the green range, which is well matched to many applications involving visible light. This is the primary motivation for selecting xenon as a filler in spite of its high cost; 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. 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 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. Because they can generate bright, attention-getting flashes with a relatively small continuous input of electrical power, they are also used in warning lights, emergency vehicle lighting, fire alarm annunciator devices (horn lights), aircraft anticollision beacons, and other similar applications.

Due to their high-intensity and relative brightness at short wavelengths (extending into the ultraviolet) and short pulsewidths, flashlamps are also ideally suited as light sources for pumping atoms in a laser to excited states where they can subsequently be stimulated to emit coherent monochromatic light. Proper selection of the filler gas is crucial here, so the maximum of radiated output energy is concentrated in the bands that are the best absorbed by the lasing medium; e.g. krypton flashlamps are more suitable than xenon flashlamps for pumping s, 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.


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