Xenon flash lamp
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A xenon flash lamp is an electric glow discharge lamp designed to produce extremely intense, incoherent, full-spectrum white light for very short durations.
Contents
[edit] Construction
The lamp comprises a hermetically sealed tube, often made of fused quartz, which is filled with a noble gas, usually xenon, and electrodes to carry electrical current to the gas. Additionally, a high voltage power source is necessary to energize the gas. 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. 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). Generally, the higher the pressure, the greater the output efficiency. Xenon is used mostly because of its good efficiency, converting nearly 50% of electrical energy into light. Krypton, on the other hand, is only about 40% efficient, but at low currents is a better match to the absorption spectrum of Nd:YAG lasers. A major factor affecting efficiency is the amount of gas behind the electrodes, or the "dead volume". A higher dead volume leads to a lower pressure increase during operation.
For low electrode wear the electrodes are usually made of tungsten, which has the highest melting point of any metal, to handle the thermionic emission of electrons. Cathodes are often made from porous tungsten filled with a barium compound which gives low work function. Anodes are usually made from pure tungsten, and are often machined to provide extra surface area to cope with power loading. DC Arc lamps often have a cathode with a sharp tip, to help keep the arc away from the glass, and to control temperature. Flash Lamps usually have a cathode with a flattened radius, to decrease sputter caused by peak currents, which may be in excess of 1000 amperes.
[edit] Operation
The electrodes of the lamp are usually connected to a capacitor, which is charged with a relatively high voltage, (generally between 250 to 5000 volts), using a step-up transformer. The gas, however, exhibits extremely high resistance, and the lamp will not conduct electricity until the gas is ionized. Once ionized, or triggered, a spark will form between the electrodes, allowing the capacitor voltage to conduct. The sudden surge of amperes quickly heats the gas to a plasma state, where electrical resistance becomes very low. [1] There are several methods of triggering.
"External Triggering" is the most common method of operation, especially for photographic use. The electrodes are charged to a voltage high enough to respond to triggering, but no higher than the lamp's self-flash threshold. An extremely high voltage pulse, (usually between 6000 to 150,000 volts), the trigger pulse, is applied directly to, or very near, the glass envelope. Water cooled flash lamps sometimes apply this pulse directly to the cooling water, and often to the housing of the unit as well, so care must be taken with this type of system. The short, high voltage pulse creates a rising magnetic field, which ionizes the gas inside the tube. Magnetic attraction causes ions to stack up on one or both of the electrodes, forming spark streamers, which propagate via capacitance along the glass at a speed of 1 centimeter in 60 nanoseconds. (A trigger pulse must be have a long enough duration to allow one streamer to reach the opposite electrode, or erratic triggering will result.) The triggering can be enhanced by putting a metal band or reflector onto the glass, or by wrapping a thin wire around the lamp from electrode tip to electrode tip. When the spark streamers connect, the capacitor discharges through the ionized gas, heating the xenon to a high enough temperature for the emission light. When this current pulse travels through the tube, it ionizes the atoms, causing them to jump to higher energy levels. Within the arc plasma three types of particles exist; electrons, positively ionized atoms, and neutral atoms. The ionized atoms number less than 1%, and account for all the emitted light. As they recombine with their lost electrons they immediately drop back to a lower energy state, releasing photons in the process.
"Series Triggering" is more common in high powered, water cooled flash lamps, such as those found in lasers. The high voltage leads of the trigger-transformer are connected to the flash lamp in series, (one lead to an electrode and the other to the capacitor). The high voltage trigger pulse forms a spark inside the lamp, without exposing the trigger voltage to the outside of the lamp. The advantages are better insulation, more reliable triggering, and an arc that tends to develop well away from the glass, but at a much higher cost.
"Simmer Voltage Triggering" is the least common method. In this technique, the capacitor voltage is not initially applied to the electrodes, but instead, a high voltage spark streamer is maintained between the electrodes. The high current from the capacitor is delivered to the electrodes using a thyristor or a spark gap. This type of triggering is used mainly in very fast rise time systems, typically those that discharge in the microsecond regime, such as used in high speed stop-motion photography, or dye lasers[2]. If external triggering is used, the spark streamers will still be in contact with the glass when the full current load passes through the tube, causing wall ablation, or in extreme cases, cracking or even explosion of the lamp. Some microsecond flashlamps are triggered by simply "over-volting", that is, by applying a voltage to the electrodes which is much higher than the lamp's self-flash threshold, using a spark gap.
In addition, an Insulated-Gate Bipolar Transistor can be connected in series with both the trigger transformer and the lamp, making it possible to turn off the lamp halfway through the pulse. [3][4]
The electrical requirements for a flash lamp can vary, depending on the desired results. The usual method, once maximum power and the safe operating energy is determined, is to pick a current density that will produce the desired spectrum, and let the lamp's resistance determine the necessary combination of voltage and capacitance to produce it. The resistance in flash lamps varies greatly, depending on pressure, shape, dead volume, current density, time, and flash duration, and therefore, is usually referred to as impedence. The most common symbol used for lamp impedence is K, which is expressed as ohms (amps^0.5). [5][6]
[edit] 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, neon signs emit red light because of extremely low current density, compared to flash lamps, which favors those spectral lines that predominate. The light from xenon, in a neon sign, likewise is rather purple. The spectrum emitted by flash lamps is far more dependent on current density than on the fill pressure or gas type. Low current densities produce wide spectral line emission. Xenon has many spectral lines in the UV, blue, green, red, and IR portions of the spectrum. Low current densities produce a greenish-blue flash, indicating the absence of significant yellow or orange lines.
Higher current densities begin to produce continuum emission. Spectral lines are less dominant as light is produced across the spectrum, usually peaking, or centered, on a certain wavelength. Optimum output efficiency is obtained at a density that favors "greybody radiation", (an arc that produces mostly continuum emission, but is still mostly transparent to its own light). For xenon, greybody radiation is centered on green, and produces the right combiation for white light.
Current densities that are very high tend to favor blackbody radiation. As current densities become higher, xenon's output spectrum will begin to settle on that of a blackbody radiator with a color temperature of 9800 degrees Kelvin, (a rather sky-blue shade of white). Due to its white output, xenon is used extensively for photographic applications, despite its great expense. In lasers, spectral line emission is usually favored, as these lines tend to better match absorption lines of the lasing media. Krypton is also occasionally used, although it is even more expensive. Krypton has much greater spectral line output in the near-IR range, which is better matched to the absorption profile of Nd:YAG laser media than xenon emission.[7]
[edit] Intensity and duration of flash
For short pulses the only real electrical limit is the total system inductance, including that of the capacitor. Short pulse flashes require that all inductance be minimized. The amount of power loading the glass can handle is the major mechanical limit. Although the amount of energy, or joules, that is used remains constant, electrical power, or wattage, increases in inverse proportion to a decrease in discharge time. Quartz glass, 1 millimeter thick, can usually withstand a maximum of 160 watts per square centimeter of internal surface area. For long pulse durations, the number of transfered electrons to the anode, sputter caused by ion bombardment at the cathode, and the temperture gradients of the glass are the limits. For continuous operation the cooling is the limit. Discharge durations for common flashlamps range from 1 microsecond to tens of milliseconds, and can have repetition rates of hundreds of hertz. Flash duration can be carefully controlled with the use of an inductor.
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.[8] 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.
[edit] 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 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.
[edit] Animation
Frame 1: The trigger pulse ionizes the gas.
Frame 2: Spark streamers form.
Frame 3: The spark streamers connect.
Frame 4: Capacitor current begins to flow.
Frame 5: Current begins to fill the tube, heating the xenon.
Frame 6: Fully heated, the xenon emits a burst of light.
