Flashtube
From Wikipedia, the free encyclopedia
A flashtube, also called a flashlamp, is an electric glow discharge lamp designed to produce extremely intense, incoherent, full-spectrum white light for very short durations.
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[edit] Construction
The lamp comprises a hermetically sealed glass tube, 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, often made of fused quartz, borosilicate or pyrex, 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 sealed to the glass using a few different methods. "Ribbon seals" use thin strips of molybdenum foil bonded directly to the glass, which are very durable, but are limited in the amount of current that can pass through. "Solder seals" bond the glass to the electrode with a solder for a very strong mechanical seal, but are limited to low temperature operation. Most common in laser applications is the "rod seal", where the rod of the electrode is wetted with another type of glass and then bonded directly to a quartz tube. This seal is very durable and capable of withstanding very high temperature and currents.[1]
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. Flashtubes usually have a cathode with a flattened radius, to decrease sputter caused by peak currents, which may be in excess of 1000 amperes.[1]
Depending on the size, type, and application of the flashtube, gas fill pressures may range from a few kilopascals to hundreds of kilopascals (0.01–4.0 atmospheres or tens to thousands of torr).[1] 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.[1]
[edit] Operation
The electrodes of the lamp are usually connected to a capacitor, which is charged with a relatively high direct current voltage, (generally between 250 and 5000 volts), using a step-up transformer and a rectifier. 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.[2] There are several methods of triggering.
[edit] External 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 below the lamp's self-flash threshold. An extremely high voltage pulse, (usually between 2000 and 150,000 volts), the "trigger pulse", is applied directly to, or very near, the glass envelope. (Water cooled flashtubes 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 electrostatic field, which ionizes the gas inside the tube. The capacitance of the glass couples the trigger pulse into the envelope, where it exceeds the breakdown voltage of the gas surrounding one or both of the electrodes, forming spark streamers. The streamers propagate via capacitance along the glass at a speed of 1 centimeter in 60 nanoseconds. (A trigger pulse must 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 applying the trigger pulse to a "reference plane", which may be in the form of a metal band or reflector affixed to the glass, a conductive paint, or a thin wire wrapped around the length of the lamp. When the internal spark streamers bridge the electrodes, 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.[1]
[edit] Series triggering
Series triggering is more common in high powered, water cooled flashtubes, such as those found in lasers. The high voltage leads of the trigger-transformer are connected to the flashtube in series, (one lead to an electrode and the other to the capacitor). The 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 develope well away from the glass, but at a much higher cost.[1]
[edit] Simmer voltage triggering
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.[3] If external triggering is used, the spark streamers may 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 flashtubes 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.[1]
[edit] Variable pulse width control
In addition, an Insulated-Gate Bipolar Transistor can be connected in series with both the trigger transformer and the lamp, making adjustable flash durations possible.[1][4][5] An IGBT used for this purpose must be rated for a high pulsed current, so as to avoid over-current damage to the semiconductor junction.[4] This type of system is used frequently in high average power laser systems, and can produce pulses ranging from 500 microseconds to over 20 milliseconds. It can be used with any of the triggering techniques, like external and series, and can produce square wave pulses. It can even be used with simmer voltage to produce a "modulated" continuous wave output, with repetition rates over 300 hertz. With the proper large bore, water cooled flashtube, several kilowatts of average power output can be obtained.[1]
[edit] Electrical requirements
The electrical requirements for a flashtube can vary, depending on the desired results. The usual method, once maximum power and the safe amount of operating energy is determined, is to pick a current density that will emit the desired spectrum, and let the lamp's resistance determine the necessary combination of voltage and capacitance to produce it. The resistance in flashtubes 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 Ko, which is expressed as ohms(amps0.5).[1][6]
[edit] Output spectrum
[edit] Xenon
As with all ionized gases, xenon flashtubes 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 densities when compared to those seen in flashtubes, which favors spectral lines of longer wavelengths . Higher current densities tend to favor shorter wavelengths.[7] The light from xenon, in a neon sign, likewise is rather violet.
The spectrum emitted by flashtubes is far more dependent on current density than on the fill pressure or gas type. Low current densities produce spectral line emission, against a faint background of continuous radiation. 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. At low current densities, most of xenon's output will be directed into the invisible IR spectral lines around 820, 900, and 1000 nm.[8] Low current densites for flashtubes are generally less than 1000 A/cm2.
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 in the visual range 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 near green, and produces the right combination for white light.[4][6] Greybody radiation is produced at densities above 2400 A/cm2.
Current densities that are very high, approaching 4000 A/cm2, tend to favor blackbody radiation. As current densities become even higher, xenon's output spectrum will begin to settle on that of a blackbody radiator with a color temperature of 9800 kelvins (a rather sky-blue shade of white).[1] Blackbody radiation is usually not desired, because much of the radiation from within the arc can be absorbed before reaching the surface, imparing output efficiency.[6][8][9]
Due to its high efficient 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. At low current densities, krypton's spectral line output in the near-IR range is better matched to the absorption profile of neodymium based laser media than xenon emission, and very closely matches the narrow absorption profile of Nd:YAG.[10][11]
[edit] Krypton and other gases
An extensive study was done in the 1960s on the characteristics of other gases when operated in flashtubes.[8] All gases produce spectral lines which are specific to the gas, superimposed on a background of continuum radiation. Like xenon, low current densities produce mostly spectral lines, with the highest output being concentrated in the near-IR between 650 and 1000 nm. Krypton's strongest peaks are around 760 and 810 nm. Argon has many strong peaks at 670, 710, 760, 820, 860, and 920 nm. Neon has peaks around 650, 700, 850, and 880 nm.[8] As current densities become higher, the output of continuum radiation will increase more than the spectral line radiation at a rate 20% greater, and output center will shift toward the visual spectrum. At greybody current densities there is only a slight difference in the spectrum emitted by various gases. At very high current densities, all gases will begin to operate as blackbody radiators, with spectral outputs centered in the near-UV.[8]
Heavier gases exhibit higher resistance, and therefore, have a higher value for Ko. Impedence, being defined as the resistance required to change energy into work, is higher for heavier gases, and as such, the heavier gases are much more efficient than the lighter ones. Helium and neon are far too light to produce an efficient flash. Krypton can be as good as 40% efficient, but requires up to a 70% increase in pressure to achieve this. Argon can be up to 30% efficient, but requires an even greater pressure increase. At such high pressures, the voltage drop between the electrodes, formed by the spark streamer, may be greater than the capacitor voltage. These lamps often need a "boost voltage" during the trigger phase, to overcome the extremely high trigger impedence.[8]
Nitrogen, in the form of air, has been used in flashtubes in home made dye lasers, but the nitrogen and oxygen present form chemical reactions with the electrodes, and themselves, causing premature wear and the need to adjust the pressure for each flash.[12]
Some research has been done on mixing gases to alter the spectral output. The effect on the output spectrum is negligible, but the effect on efficiency is great. Adding a lighter gas will only reduce the efficiency of the heavier one.[8]
[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. Other glasses have a much lower threshold. Extremely fast systems, with inductance below .8 microhenries, usually require a shunt diode across the capacitor, to prevent current reversal from destroying the lamp.
The limits to long pulse durations are the number of transferred electrons to the anode, sputter caused by ion bombardment at the cathode, and the temperature gradients of the glass. For continuous operation the cooling is the limit. Discharge durations for common flashtubes 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.[1][6]
The flash that emanates from a xenon flashtube 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.[13] 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 flashtube can be accurately controlled, and due to the high intensity of the light, xenon flashtubes are commonly used as photographic strobe lights. Xenon flashtubes 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, flashtubes 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 flashtubes are more suitable than xenon flashtubes for pumping Nd:YAG lasers, as krypton emission in near infrared is better matched to the absorption spectrum of Nd:YAG.
Xenon flashtubes 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 flashtubes is turned into useful laser light for this application.
[edit] Safety
Flashtubes operate at high voltages, with currents high enough to be deadly. Shocks as low as 1 joule have been reported to be lethal. The energy stored in a capacitor can remain long after power has been disconnected. A flashtube will usually shut down before the capacitor has fully drained, and it may regain part of its charge through a process called "dielectric absorption". In addition, the charging system can be equally deadly. The trigger voltage can deliver a painful shock, usually not enough to kill, but which can often startle a person into bumping something more dangerous. At high voltages a spark can jump, delivering the high capacitor current without even touching anything.
Flashtubes operate at high pressures and are known to explode, producing violent shockwaves. The "explosion energy" of a flashtube, (the amount of energy that will destroy it in just a few flashes), is well defined, and to avoid catastrophic failure, it is recommended that no more than 30% of the explosion energy be used.[6] Flashtubes should be shielded behind glass or in a reflector cavity. If not, eye and ear protection should be worn.
Flashtubes produce very intense flashes, often faster than the eye can register, and may not appear as bright as they are. Quartz glass will transmit nearly all of the long and short wave UV, including the germicidal wavelengths, and can be a serious hazard to eyes and skin. This ultraviolet radiation can also produce large amounts of ozone, which can be harmful to people, animals, and equipment.[14]
[edit] Animation
Frame 1: The trigger pulse ionizes the gas. Spark streamers form.
Frame 2: Spark streamers connect and move away from the glass, as amperes surge.
Frame 3: Capacitor current begins to flow, heating the surrounding xenon.
Frame 4: As resistance decreases current fills the tube, heating the xenon to a plasma state.
Frame 5: Fully heated, the full current load rushes through the tube and the xenon emits a burst of light.
[edit] Popular culture
In both the 1971 motion picture and the 1969 book The Andromeda Strain, specialized exposure to a xenon flash apparatus was used to burn off the outer epethelial layers of human skin as an antiseptic measure to eliminate all possible bacterial access for persons working in an extreme ultraclean environment. (The book used the term 'ultraflash'; the movie identified the apparatus as a 'xenon flash'.)
[edit] See also
[edit] References
- ^ a b c d e f g h i j k l "High Performance Flash and Arc Lamps" (pdf). PerkinElmer. http://optoelectronics.perkinelmer.com/content/RelatedLinks/CAT_flash.pdf. Retrieved on 03 Feb 2009.
- ^ Edgerton, Harold E.. Electronic Flash Strobe. MIT Press.
- ^ Holzrichter, J. F.; Schawlow, A. L.. "Design and analysis of flashlamp systems for pumping organic dye lasers". Annals of the New York Academy of Sciences.
- ^ a b c "Interupting xenon flash current?". http://sci.tech-archive.net/pdf/Archive/sci.electronics.design/2008-03/msg01208.pdf. Retrieved on 03 Feb 2009.
- ^ "Application Notes – Discharge Circuits" (pdf). www.lightingassociates.org. http://www.lightingassociates.org/i/u/2127806/f/tech_sheets/Application_Notes__Discharge_Circuits.pdf. Retrieved on 03 Feb 2009.
- ^ a b c d e Klipstein, Don. "General Xenon Flash and Strobe Design Guidelines". http://members.misty.com/don/xeguide.html. Retrieved on 03 Feb 2009.
- ^ Gebel, Radames K. H.; Mestwerdt, Hermann R. ; and Hayslett, Roy R. (November 1971). "Near-infrared sensitized photocathodes and film sensitivities for typical xenon-lamp radiation and related subjects" (pdf). Ohio Journal of Science 71 (6): 343. https://kb.osu.edu/dspace/bitstream/1811/5654/1/V71N06_343.pdf.
- ^ a b c d e f g Oliver, J. R.; Barnes, F. S. (May 1969). "A Comparison of Rare-Gas Flashlamps". I.E.E.E. Journal of Quantum Electronics 5 (5): 232–7. doi:. ISSN 0018-9197.
- ^ Emmett, J. L.; Schawlow, A. L.; Weinberg, E. H. (September 1964 1964). "Direct measurement of xenon flashtube opacity". J. Appl. Phys 35: 2601. doi:.
- ^ Dishington, R. H.; Hook, W. R.; Hilberg, R. P. (1974). "Flashlamp discharge and laser efficiency". Applied Optics 13 (10): 2300–2312. doi:.
- ^ "Lamp-pumped Lasers". Encyclopedia of Laser Physics and Technology. RP Photonics. http://www.rp-photonics.com/lamp_pumped_lasers.html. Retrieved on 03 Feb 2009.
- ^ Goldwasser, Samuel M. (2008). "Sam's Laser FAQ". http://www.repairfaq.org/sam/lasercdy.htm. Retrieved on 03 Feb 2009.
- ^ RPI: News & Events – We Have Ignition! Carbon Nanotubes Ignite When Exposed to Flash
- ^ Klipstein, Don. "Xenon Strobe and Flash Safety Hints". http://members.misty.com/don/xesafe.html. Retrieved on 03 Feb 2009.
