Flashtube
Encyclopedia
A flashtube, also called a flashlamp, is an electric arc
lamp designed to produce extremely intense, incoherent
, full-spectrum white light for very short durations. Flashtubes are made of a length of glass tubing with electrodes at either end and are filled with a gas that, when triggered, ionizes and conducts a high voltage pulse to produce the light. Flashtubes are used mostly for photographic purposes but are also employed in scientific, medical and industrial applications.
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 flash
es'). In some applications the emission of ultraviolet light is undesired, whether due to production of ozone
, damage to laser rods, degradation of plastics, or other detrimental effects. In these cases a doped fused silica is used. Doping with titanium dioxide
can provide different cutoff wavelengths on the ultraviolet side, but the material suffers from solarization; it is often used in medical and sun-ray lamps and some non-laser lamps. A better alternative is a cerium
-doped quartz; it does not suffer from solarization and has higher efficiency, as part of the absorbed ultraviolet is reradiated as visible via fluorescence
. Its cutoff is at about 380 nm. Conversely, when ultraviolet is called for, a synthetic quartz
is used as the envelope; it is the most expensive of the materials, but it is not susceptible to solarization and its cutoff is at 160 nm.
The electrode
s 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 pumping
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.
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. Cathode
s are often made from porous tungsten filled with a barium
compound, which gives low work function
; the structure of cathode has to be tailored for the application. Anode
s are usually made from pure tungsten, or, when good machinability is required, lanthanum
-alloyed 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 reduce the incidence of hot spots and decrease sputter caused by peak currents, which may be in excess of 1000 amperes. Electrode design is also influenced by the average power. At high levels of average power, care has to be taken to achieve sufficient cooling of the electrodes. While anode temperature is of lower importance, overheating the cathode can greatly reduce the lamp's lifetime.
The power level of the lamps is rated in watts/area, total output power divided by the lamp's surface. Cooling of the electrodes and the lamp envelope is of high importance at high power levels. Air cooling is sufficient for lower average power levels. High power lamps are cooled with a liquid, typically by flowing demineralized water through a tube in which the lamp is encased. The cooling medium should flow also over the ends of the lamps, seals and electrodes. Above 15 W/cm2 forced air cooling is required, liquid cooling if in a confined space. Liquid cooling is generally necessary above 30 W/cm2. Thinner walls can survive higher power loads due to lower mechanical strain across the thickness of the material; e.g. 1 mm thick doped quartz has limit of 160 W/cm2, 0.5 mm thick one has limit of 320 W/cm2. The material of the envelope provides another limit for the output power; 1 mm thick fused quartz has a limit of 200 W/cm2, synthetic quartz of same thickness can run up to 240 W/cm2. Aging lamps require some derating, due to increased energy absorption in the glass due to solarization and sputtered deposits.
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
). 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 laser
s. 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.
, which is charged to a relatively high 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 to discharge. The sudden surge of electric current
quickly heats the gas to a plasma
state, where electrical resistance becomes very low. There are several methods of triggering.
along the glass at a speed of 1 centimeter in 60 nanoseconds (170 km/s). (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. If the voltage drop
between the cathode and the anode is lower than the capacitor voltage, when the internal spark streamers bridge the electrodes the capacitor will discharge through the ionized gas, heating the xenon to a high enough temperature for the emission light.
. This helps to control the flash duration, but prevents the circuit from being used in very fast discharge applications. The triggering can generally take place with a lower voltage at the capacitor than is required for external triggering. However, because the trigger transformer has very low impedance, the transformer and triggering circuit, including the silicon controlled rectifier (SCR), must be able to handle very high peak currents, often in excess of 1500 amps.
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. The simmering spark streamer causes the arc to develop in the exact center of the lamp, increasing the lifetime dramatically. 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. However, because very short pulses often call for very high voltage and low capacitance, 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. Often, a combination of simmer voltage and over-volting is used.
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 impedance
. The most common symbol used for lamp impedance is Ko, which is expressed as ohms(amps0.5).
Ko is determined by the internal diameter, arc length, and gas type of the lamp and, to a lesser extent, by fill pressure. The resistance in flashtubes is not constant, but quickly drops as current density increases. In 1965, Goncz showed that the plasma resistivity
in flashtubes is inversely proportional to the square root of current density. As the arc develops, the lamp experiences a period of negative resistance
, causing the resistance to decrease as the current increases. This occurs until the plasma comes into contact with the inner wall. When this happens, the voltage becomes proportional to the square root of current, and the resistance in the plasma becomes stable for the remainder of the flash. It is this value which is defined as Ko. However, as the arc develops the gas expands, and calculations for Ko do not take into account the dead volume, which leads to a lower pressure increase. Therefore, any calculation of Ko is merely an approximation of lamp impedance.
As with all ionized gases, xenon flashtubes emit light in various spectral line
s. This is the same phenomenon that gives neon sign
s 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. 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. Low current densities 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. 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). Blackbody radiation is usually not desired, because much of the radiation from within the arc can be absorbed before reaching the surface, impairing output efficiency.
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.
Heavier gases exhibit higher resistance, and therefore, have a higher value for Ko. Impedance, 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 impedance.
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.
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.
s, positively ionized atoms, and neutral atom
s. At any given time, the ionized atoms make up less than 1% of the plasma and produce all of the emitted light. As they recombine with their lost electrons they immediately drop back to a lower energy state, releasing photons in the process. The methods of transferring energy occur in three separate ways, called "bound-bound," "free-bound," and "free-free" transitions.
Within the plasma, positive ions move toward the cathode while electrons and neutral atoms move toward the anode. Bound-bound transitions occur when the ions and neutral atoms collide, transferring an electron from the atom to the ion. This method predominates at low current densities, and is responsible for producing the spectral line emission. Free-bound transitions happen when an ion captures a free electron. This method produces the continuum emission, and is more prominent at higher current densities. Some of the continuum is also produced when an electron accelerates toward an ion, called free-free transitions, producing bremsstrahlung
radiation.
, 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 0.8 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 gradient
s of the glass. For continuous operation the cooling
is the limit. Discharge durations for common flashtubes range from 1 microsecond
to tens of millisecond
s, 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 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. 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 lesion
s or mole
s.
can occur from two separate mechanisms; energy
and heat
. When too much energy is used for the pulse duration, structural failure
of the glass envelope can occur. Flashtubes produce an electrical arc contained in a glass tube. As the arc develops a supersonic
shock wave
forms, traveling radially from the center of the arc and impacting the inner wall of the tube. If the energy level used equals the "explosion energy" rating of the lamp, the impacting shock wave will fracture the glass, rupturing the tube. The resulting explosion creates a loud sonic shock wave, and may throw shattered glass several feet. The explosion energy is calculated by multiplying the internal surface area of the lamp with the power loading capacity of the glass. Power loading is determined by the type and thickness of the glass, and the cooling method that is used. Power loading is measured in watts per centimeter squared. However, since the pulsed power
level increases as the flash duration decreases, the explosion energy must then be decreased in direct proportion to the square root of discharge time.
Failure from heat is usually caused by excessively long pulse durations or high average power levels. When the inner wall of the tube gets too hot while the outer wall is still cold, this temperature gradient
can cause the lamp to crack. Similarly, if the electrodes heat much faster than the glass, the lamp may crack or even shatter at the ends.
of the inner wall.
Sputter occurs when the energy level is very low, below 15% of the explosion energy, or when the pulse duration is very long. Sputter is the vaporization of metal from the cathode, which is redeposited on the walls of the lamp, blocking the light output. Since the cathode
is more emissive than the anode
, the flashtube is polarized, and connecting the lamp to the power source incorrectly will quickly ruin it. It is impossible to predict the lifetime accurately at low energy levels.
At higher energy levels, wall ablation becomes the main process of wear. The electrical arc slowly erodes the inner wall of the tube, forming microscopic cracks that give the glass a frosted appearance. The ablation releases oxygen from the glass, increasing the pressure beyond an operable level. This causes triggering problems, known as "jitter
". However, at higher energy levels the lifetime can be calculated with a fair degree of accuracy.
When operated below 30% of the explosion energy, flashtube lifetime is generally between a few million to tens of millions of flashes.
As 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 light
s. 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 beacon
s, and other similar applications.
Due to their high-intensity and relative brightness at short wavelength
s (extending into the ultraviolet
) and short pulse widths, flashtubes are also ideally suited as light sources for pumping
atoms in a laser
to excited state
s 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 laser
s, 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.
Early high-speed photographs were taken with an open-air electrical arc discharge, called spark photography. The earliest known use of spark photography began with Henry Fox Talbot around 1850. In 1886, Ernst Mach
used an open air spark to photograph a speeding bullet, revealing the shockwaves it produced at supersonic speeds. Open air spark systems were fairly easy to build, but were bulky, very limited in light output, and produced loud noises comparable to that of the gunshot.
In 1927, Harold Edgerton built his first flash unit while at MIT. Wanting to photograph the motion of a motor in vivid detail, without blur, Edgerton decided to improve the process of spark photography by using a mercury-arc rectifier
instead of an open air discharge to produce the light. He was able to achieve a flash duration of 10 microseconds, and was able to photograph the moving motor as if "frozen in time."
Interest in the new flash apparatus soon provoked Edgerton to improve upon the design. The mercury lamps were only as efficient as the warmest part of the lamp, causing them to perform better when very hot but poorly when cold. Edgerton decided that a noble gas
would not be as temperature dependent and, in 1930, he employed the General Electric
company to construct some lamps using argon
instead. The argon lamps were much more efficient, compact, and could be mounted near a reflector, concentrating their output. Slowly, camera designers began to take notice of the new technology and began to accept it. Edgerton received his first major order for the strobes from the Kodak company in 1940. Afterward, he discovered that xenon
was the most efficient of the noble gases, producing a spectrum very close to that of daylight, and xenon flashtubes became standard in most large photography sets. It was not until the 1970s that strobe units became portable enough to use in common cameras.
In 1960, after Theodore Maiman invented the ruby laser
, a new demand for flashtubes began for use in lasers, and new interest was taken in the study of the lamps.
s, 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 surprisingly 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 itself 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 or touching 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. 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.
Many compact cameras charge the flash capacitor immediately after power-up, and some even just by inserting the batteries. Merely inserting the battery into the camera can prime the capacitor to become dangerous or at least unpleasant for up to several days. The energy involved is also fairly significant; a 330 microfarad capacitor charged to 300 volts (common ballpark values found in cameras) stores almost 15 joules of energy.
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.
and the 1971 motion picture
, specialized exposure to a xenon flash apparatus was used to burn off the outer epithelial 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'.)
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...
lamp designed to produce extremely intense, incoherent
Coherence (physics)
In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....
, full-spectrum white light for very short durations. Flashtubes are made of a length of glass tubing with electrodes at either end and are filled with a gas that, when triggered, ionizes and conducts a high voltage pulse to produce the light. Flashtubes are used mostly for photographic purposes but are also employed in scientific, medical and industrial applications.
Construction
The lamp comprises a hermetically sealed glassGlass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
tube, which is filled with a noble gas
Noble gas
The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...
, usually xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
, and electrodes to carry electrical current to the gas. Additionally, a high voltage power source is necessary to energize the gas. A charged capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
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
Fused quartz
Fused quartz and fused silica are types of glass containing primarily silica in amorphous form. They are manufactured using several different processes...
, borosilicate or Pyrex
Pyrex
Pyrex is a brand name for glassware, introduced by Corning Incorporated in 1915.Originally, Pyrex was made from borosilicate glass. In the 1940s the composition was changed for some products to tempered soda-lime glass, which is the most common form of glass used in glass bakeware in the US and has...
, 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
Photography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
—'ring flash
Ring flash
A ring flash, invented by Lester A. Dine in 1952, originally for use in dental photography, is a circular photographic flash that fits around the lens, especially for use in macro photography...
es'). In some applications the emission of ultraviolet light is undesired, whether due to production of ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
, damage to laser rods, degradation of plastics, or other detrimental effects. In these cases a doped fused silica is used. Doping with titanium dioxide
Titanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
can provide different cutoff wavelengths on the ultraviolet side, but the material suffers from solarization; it is often used in medical and sun-ray lamps and some non-laser lamps. A better alternative is a cerium
Cerium
Cerium is a chemical element with the symbol Ce and atomic number 58. It is a soft, silvery, ductile metal which easily oxidizes in air. Cerium was named after the dwarf planet . Cerium is the most abundant of the rare earth elements, making up about 0.0046% of the Earth's crust by weight...
-doped quartz; it does not suffer from solarization and has higher efficiency, as part of the absorbed ultraviolet is reradiated as visible via fluorescence
Fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
. Its cutoff is at about 380 nm. Conversely, when ultraviolet is called for, a synthetic quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
is used as the envelope; it is the most expensive of the materials, but it is not susceptible to solarization and its cutoff is at 160 nm.
The electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s 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
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
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 pumping
Laser pumping
Laser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited...
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.
For low electrode wear the electrodes are usually made of tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
, which has the highest melting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
of any metal, to handle the thermionic emission
Thermionic emission
Thermionic emission is the heat-induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the thermal energy given to the carrier overcomes the binding potential, also known as work function of the metal. The charge carriers can be electrons or ions, and...
of electrons. Cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
s are often made from porous tungsten filled with a barium
Barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...
compound, which gives low work function
Work function
In solid-state physics, the work function is the minimum energy needed to remove an electron from a solid to a point immediately outside the solid surface...
; the structure of cathode has to be tailored for the application. Anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
s are usually made from pure tungsten, or, when good machinability is required, lanthanum
Lanthanum
Lanthanum is a chemical element with the symbol La and atomic number 57.Lanthanum is a silvery white metallic element that belongs to group 3 of the periodic table and is the first element of the lanthanide series. It is found in some rare-earth minerals, usually in combination with cerium and...
-alloyed 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
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...
away from the glass and to control temperature. Flashtubes usually have a cathode with a flattened radius, to reduce the incidence of hot spots and decrease sputter caused by peak currents, which may be in excess of 1000 amperes. Electrode design is also influenced by the average power. At high levels of average power, care has to be taken to achieve sufficient cooling of the electrodes. While anode temperature is of lower importance, overheating the cathode can greatly reduce the lamp's lifetime.
The power level of the lamps is rated in watts/area, total output power divided by the lamp's surface. Cooling of the electrodes and the lamp envelope is of high importance at high power levels. Air cooling is sufficient for lower average power levels. High power lamps are cooled with a liquid, typically by flowing demineralized water through a tube in which the lamp is encased. The cooling medium should flow also over the ends of the lamps, seals and electrodes. Above 15 W/cm2 forced air cooling is required, liquid cooling if in a confined space. Liquid cooling is generally necessary above 30 W/cm2. Thinner walls can survive higher power loads due to lower mechanical strain across the thickness of the material; e.g. 1 mm thick doped quartz has limit of 160 W/cm2, 0.5 mm thick one has limit of 320 W/cm2. The material of the envelope provides another limit for the output power; 1 mm thick fused quartz has a limit of 200 W/cm2, synthetic quartz of same thickness can run up to 240 W/cm2. Aging lamps require some derating, due to increased energy absorption in the glass due to solarization and sputtered deposits.
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
Atmosphere (unit)
The standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...
or tens to thousands of torr
Torr
The torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...
). 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 laser
Nd:YAG laser
Nd:YAG is a crystal that is used as a lasing medium for solid-state lasers. The dopant, triply ionized neodymium, typically replaces yttrium in the crystal structure of the yttrium aluminium garnet , since they are of similar size...
s. 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.
Operation
The electrodes of the lamp are usually connected to a capacitorCapacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
, which is charged to a relatively high voltage
High voltage
The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...
(generally between 250 and 5000 volts), using a step up transformer and a rectifier
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
. The gas, however, exhibits extremely high resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
, and the lamp will not conduct electricity until the gas is ionized. Once ionized, or "triggered", a spark
Electric spark
An electric spark is a type of electrostatic discharge that occurs when an electric field creates an ionized electrically conductive channel in air producing a brief emission of light and sound. A spark is formed when the electric field strength exceeds the dielectric field strength of air...
will form between the electrodes, allowing the capacitor to discharge. The sudden surge of electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
quickly heats the gas to a plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
state, where electrical resistance becomes very low. There are several methods of triggering.
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 capacitanceCapacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
along the glass at a speed of 1 centimeter in 60 nanoseconds (170 km/s). (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. If the voltage drop
Voltage drop
Voltage drop is the reduction in voltage in the passive elements of an electrical circuit. Voltage drops across conductors, contacts, connectors and source internal resistances are undesired as they reduce the supplied voltage while voltage drops across loads and other electrical and electronic...
between the cathode and the anode is lower than the capacitor voltage, when the internal spark streamers bridge the electrodes the capacitor will discharge through the ionized gas, heating the xenon to a high enough temperature for the emission light.
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 develop well away from the glass, but at a much higher cost. The series triggering transformer also acts as an inductorInductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
. This helps to control the flash duration, but prevents the circuit from being used in very fast discharge applications. The triggering can generally take place with a lower voltage at the capacitor than is required for external triggering. However, because the trigger transformer has very low impedance, the transformer and triggering circuit, including the silicon controlled rectifier (SCR), must be able to handle very high peak currents, often in excess of 1500 amps.
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 thyristorThyristor
A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current trigger, and continue to conduct while they are forward biased .Some sources define silicon controlled rectifiers and...
or a spark gap. This type of triggering is used mainly in very fast rise time
Rise time
In electronics, when describing a voltage or current step function, rise time refers to the time required for a signal to change from a specified low value to a specified high value...
systems, typically those that discharge in the microsecond regime, such as used in high speed stop-motion photography or dye lasers. The simmering spark streamer causes the arc to develop in the exact center of the lamp, increasing the lifetime dramatically. 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
Ablation
Ablation is removal of material from the surface of an object by vaporization, chipping, or other erosive processes. This occurs in spaceflight during ascent and atmospheric reentry, glaciology, medicine, and passive fire protection.-Spaceflight:...
, or in extreme cases, cracking or even explosion of the lamp. However, because very short pulses often call for very high voltage and low capacitance, 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. Often, a combination of simmer voltage and over-volting is used.
Variable pulse width control
In addition, an insulated-gate bipolar transistor (IGBT) can be connected in series with both the trigger transformer and the lamp, making adjustable flash durations possible. 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. 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.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 densityCurrent density
Current density is a measure of the density of flow of a conserved charge. Usually the charge is the electric charge, in which case the associated current density is the electric current per unit area of cross section, but the term current density can also be applied to other conserved...
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 impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
. The most common symbol used for lamp impedance is Ko, which is expressed as ohms(amps0.5).
Ko is determined by the internal diameter, arc length, and gas type of the lamp and, to a lesser extent, by fill pressure. The resistance in flashtubes is not constant, but quickly drops as current density increases. In 1965, Goncz showed that the plasma resistivity
Resistivity
Electrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre...
in flashtubes is inversely proportional to the square root of current density. As the arc develops, the lamp experiences a period of negative resistance
Negative resistance
Negative resistance is a property of some electric circuits where an increase in the current entering a port results in a decreased voltage across the same port. This is in contrast to a simple ohmic resistor, which exhibits an increase in voltage under the same conditions. Negative resistors are...
, causing the resistance to decrease as the current increases. This occurs until the plasma comes into contact with the inner wall. When this happens, the voltage becomes proportional to the square root of current, and the resistance in the plasma becomes stable for the remainder of the flash. It is this value which is defined as Ko. However, as the arc develops the gas expands, and calculations for Ko do not take into account the dead volume, which leads to a lower pressure increase. Therefore, any calculation of Ko is merely an approximation of lamp impedance.
Xenon
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
s. This is the same phenomenon that gives neon sign
Neon sign
Neon signs are made using electrified, luminous tube lights that contain rarefied neon or other gases. They are the most common use for neon lighting, which was first demonstrated in a modern form in December, 1910 by Georges Claude at the Paris Motor Show. While they are used worldwide, neon signs...
s 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. 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. Low current densities for flashtubes are generally less than 1000 A/cm2.
Higher current densities begin to produce continuum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
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
White
White is a color, the perception of which is evoked by light that stimulates all three types of color sensitive cone cells in the human eye in nearly equal amounts and with high brightness compared to the surroundings. A white visual stimulation will be void of hue and grayness.White light can be...
light. 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
Color temperature
Color temperature is a characteristic of visible light that has important applications in lighting, photography, videography, publishing, manufacturing, astrophysics, and other fields. The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of...
of 9800 kelvins (a rather sky-blue shade of white). Blackbody radiation is usually not desired, because much of the radiation from within the arc can be absorbed before reaching the surface, impairing output efficiency.
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
Neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite...
based laser media than xenon emission, and very closely matches the narrow absorption profile of Nd:YAG.
Krypton and other gases
An extensive study was done in the 1960s on the characteristics of other gases when operated in flashtubes. 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. 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.Heavier gases exhibit higher resistance, and therefore, have a higher value for Ko. Impedance, 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
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
and neon
Neon
Neon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or...
are far too light to produce an efficient flash. Krypton
Krypton
Krypton is a chemical element with the symbol Kr and atomic number 36. It is a member of Group 18 and Period 4 elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other...
can be as good as 40% efficient, but requires up to a 70% increase in pressure to achieve this. Argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
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 impedance.
Nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, in the form of air, has been used in flashtubes in home made dye lasers, but the nitrogen and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
present form chemical reactions with the electrodes, and themselves, causing premature wear and the need to adjust the pressure for each flash.
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.
Light production
As the current pulse travels through the tube, it ionizes the atoms, causing them to jump to higher energy levels. Three types of particles are found within the arc plasma, consisting of electronElectron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s, positively ionized atoms, and neutral atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s. At any given time, the ionized atoms make up less than 1% of the plasma and produce all of the emitted light. As they recombine with their lost electrons they immediately drop back to a lower energy state, releasing photons in the process. The methods of transferring energy occur in three separate ways, called "bound-bound," "free-bound," and "free-free" transitions.
Within the plasma, positive ions move toward the cathode while electrons and neutral atoms move toward the anode. Bound-bound transitions occur when the ions and neutral atoms collide, transferring an electron from the atom to the ion. This method predominates at low current densities, and is responsible for producing the spectral line emission. Free-bound transitions happen when an ion captures a free electron. This method produces the continuum emission, and is more prominent at higher current densities. Some of the continuum is also produced when an electron accelerates toward an ion, called free-free transitions, producing bremsstrahlung
Bremsstrahlung
Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is...
radiation.
Intensity and duration of flash
For short pulses the only real electrical limit is the total system inductanceInductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
, 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 0.8 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 gradient
Temperature gradient
A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature gradient is a dimensional quantity expressed in units of degrees per unit length...
s of the glass. For continuous operation the cooling
Cooling
Cooling is the transfer of thermal energy via thermal radiation, heat conduction or convection. It may also refer to:-Techniques:* Air conditioning* Air cooling* Computer cooling* Cryogenics* Conduction * Infrared solar cells* Laser cooling...
is the limit. Discharge durations for common flashtubes range from 1 microsecond
Microsecond
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...
to tens of millisecond
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
s, and can have repetition rates of hundreds of hertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
. Flash duration can be carefully controlled with the use of an inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
.
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. Similar effects may be exploited for use in aesthetic or medical procedures known as intense pulsed light
Intense pulsed light
Intense pulsed light , is a technology aimed at producing light of high intensity during a very short period of time. It involves specific lamps together with capacitors whose rapid discharge provides the high energy required....
(IPL) treatments. IPL can be used for treatments such as hair removal and destroying lesion
Lesion
A lesion is any abnormality in the tissue of an organism , usually caused by disease or trauma. Lesion is derived from the Latin word laesio which means injury.- Types :...
s or mole
Mole (skin marking)
A melanocytic nevus is a type of lesion that contains nevus cells .Some sources equate the term mole with "melanocytic nevus". Other sources reserve the term "mole" for other purposes....
s.
Lifetime
The lifetime of a flashtube depends on both the energy level used for the lamp in proportion to its explosion energy, and on the pulse duration of the lamp. Failures can be catastrophic, causing the lamp to shatter, or they can be gradual, reducing the performance of the lamp below a usable rating.Catastrophic failure
Catastrophic failureCatastrophic failure
A catastrophic failure is a sudden and total failure of some system from which recovery is impossible. Catastrophic failures often lead to cascading systems failure....
can occur from two separate mechanisms; energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
and heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
. When too much energy is used for the pulse duration, structural failure
Structural failure
Structural failure refers to loss of the load-carrying capacity of a component or member within a structure or of the structure itself. Structural failure is initiated when the material is stressed to its strength limit, thus causing fracture or excessive deformations...
of the glass envelope can occur. Flashtubes produce an electrical arc contained in a glass tube. As the arc develops a supersonic
Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...
shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
forms, traveling radially from the center of the arc and impacting the inner wall of the tube. If the energy level used equals the "explosion energy" rating of the lamp, the impacting shock wave will fracture the glass, rupturing the tube. The resulting explosion creates a loud sonic shock wave, and may throw shattered glass several feet. The explosion energy is calculated by multiplying the internal surface area of the lamp with the power loading capacity of the glass. Power loading is determined by the type and thickness of the glass, and the cooling method that is used. Power loading is measured in watts per centimeter squared. However, since the pulsed power
Pulsed power
Pulsed power is the term used to describe the science and technology of accumulating energy over a relatively long period of time and releasing it very quickly thus increasing the instantaneous power.-Overview:...
level increases as the flash duration decreases, the explosion energy must then be decreased in direct proportion to the square root of discharge time.
Failure from heat is usually caused by excessively long pulse durations or high average power levels. When the inner wall of the tube gets too hot while the outer wall is still cold, this temperature gradient
Temperature gradient
A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature gradient is a dimensional quantity expressed in units of degrees per unit length...
can cause the lamp to crack. Similarly, if the electrodes heat much faster than the glass, the lamp may crack or even shatter at the ends.
Gradual failure
The closer a flashtube operates to its explosion energy, the greater the risk becomes for catastrophic failure. At 50% of the explosion energy, the lamp may produce several thousand flashes before exploding. At 60% of the explosion energy, the lamp will usually fail in less than a hundred. If the lamp is operated below 30% of the explosion energy the risk of catastrophic failure becomes very low. The methods of failure then become those that reduce the output efficiency and affect the ability to trigger the lamp. The processes affecting these are sputter and ablationAblation
Ablation is removal of material from the surface of an object by vaporization, chipping, or other erosive processes. This occurs in spaceflight during ascent and atmospheric reentry, glaciology, medicine, and passive fire protection.-Spaceflight:...
of the inner wall.
Sputter occurs when the energy level is very low, below 15% of the explosion energy, or when the pulse duration is very long. Sputter is the vaporization of metal from the cathode, which is redeposited on the walls of the lamp, blocking the light output. Since the cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
is more emissive than the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
, the flashtube is polarized, and connecting the lamp to the power source incorrectly will quickly ruin it. It is impossible to predict the lifetime accurately at low energy levels.
At higher energy levels, wall ablation becomes the main process of wear. The electrical arc slowly erodes the inner wall of the tube, forming microscopic cracks that give the glass a frosted appearance. The ablation releases oxygen from the glass, increasing the pressure beyond an operable level. This causes triggering problems, known as "jitter
Jitter
Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...
". However, at higher energy levels the lifetime can be calculated with a fair degree of accuracy.
When operated below 30% of the explosion energy, flashtube lifetime is generally between a few million to tens of millions of flashes.
Applications
Photography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
strobe light
Strobe light
A strobe light or stroboscopic lamp, commonly called a strobe, is a device used to produce regular flashes of light. It is one of a number of devices that can be used as a stroboscope...
s. Xenon flashtubes are also used in the technique of very high speed or "stop-motion" photography
High speed photography
High speed photography is the science of taking pictures of very fast phenomena. In 1948, the Society of Motion Picture and Television Engineers defined high-speed photography as any set of photographs captured by a camera capable of 128 frames per second or greater, and of at least three...
, 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
Aircraft warning lights
Aircraft warning lights are high-intensity lighting devices that are attached to tall structures and are used as collision avoidance measures. Such devices make structures more visible to passing aircraft and are usually used at night, although they may be used during the day as well...
, emergency vehicle lighting, fire alarm annunciator devices
Fire alarm system
An automatic fire alarm system is designed to detect the unwanted presence of fire by monitoring environmental changes associated with combustion. In general, a fire alarm system is classified as either automatically actuated, manually actuated, or both...
(horn lights), aircraft anticollision beacon
Navigation light
A navigation light is a colored source of illumination on an aircraft, spacecraft, or waterborne vessel, used to signal a craft's position, heading, and status...
s, and other similar applications.
Due to their high-intensity and relative brightness at short wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s (extending into the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
) and short pulse widths, flashtubes are also ideally suited as light sources for pumping
Laser pumping
Laser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited...
atoms in a laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
to excited state
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
s where they can subsequently be stimulated to emit coherent
Coherence (physics)
In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....
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 laser
Nd:YAG laser
Nd:YAG is a crystal that is used as a lasing medium for solid-state lasers. The dopant, triply ionized neodymium, typically replaces yttrium in the crystal structure of the yttrium aluminium garnet , since they are of similar size...
s, 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
Inertial confinement fusion
Inertial confinement fusion is a process where nuclear fusion reactions are initiated by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of deuterium and tritium....
. In total about 1 to 1.5% of the electrical power fed into the flashtubes is turned into useful laser light for this application.
History
The flashtube was invented by Harold Edgerton in the 1930s as a means to take sharp photographs of moving objects. Flashtubes were mainly used for strobe lights in scientific studies, but eventually began to take the place of chemical and powder flash lamps in mainstream photography.Early high-speed photographs were taken with an open-air electrical arc discharge, called spark photography. The earliest known use of spark photography began with Henry Fox Talbot around 1850. In 1886, Ernst Mach
Ernst Mach
Ernst Mach was an Austrian physicist and philosopher, noted for his contributions to physics such as the Mach number and the study of shock waves...
used an open air spark to photograph a speeding bullet, revealing the shockwaves it produced at supersonic speeds. Open air spark systems were fairly easy to build, but were bulky, very limited in light output, and produced loud noises comparable to that of the gunshot.
In 1927, Harold Edgerton built his first flash unit while at MIT. Wanting to photograph the motion of a motor in vivid detail, without blur, Edgerton decided to improve the process of spark photography by using a mercury-arc rectifier
Mercury arc valve
A mercury-arc valve is a type of electrical rectifier used for converting high-voltage or high-current alternating current into direct current . Rectifiers of this type were used to provide power for industrial motors, electric railways, streetcars, and electric locomotives, as well as for...
instead of an open air discharge to produce the light. He was able to achieve a flash duration of 10 microseconds, and was able to photograph the moving motor as if "frozen in time."
Interest in the new flash apparatus soon provoked Edgerton to improve upon the design. The mercury lamps were only as efficient as the warmest part of the lamp, causing them to perform better when very hot but poorly when cold. Edgerton decided that a noble gas
Noble gas
The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...
would not be as temperature dependent and, in 1930, he employed the General Electric
General Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...
company to construct some lamps using argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
instead. The argon lamps were much more efficient, compact, and could be mounted near a reflector, concentrating their output. Slowly, camera designers began to take notice of the new technology and began to accept it. Edgerton received his first major order for the strobes from the Kodak company in 1940. Afterward, he discovered that xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
was the most efficient of the noble gases, producing a spectrum very close to that of daylight, and xenon flashtubes became standard in most large photography sets. It was not until the 1970s that strobe units became portable enough to use in common cameras.
In 1960, after Theodore Maiman invented the ruby laser
Ruby laser
A ruby laser is a solid-state laser that uses a synthetic ruby crystal as its gain medium. The first working laser was a ruby laser made by Theodore H. "Ted" Maiman at Hughes Research Laboratories on May 16, 1960....
, a new demand for flashtubes began for use in lasers, and new interest was taken in the study of the lamps.
Safety
Flashtubes operate at high voltageHigh voltage
The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...
s, with currents high enough to be deadly. Shocks as low as 1 joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...
have been reported to be lethal. The energy stored in a capacitor can remain surprisingly 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
Permittivity
In electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how...
". In addition, the charging system itself 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 or touching something more dangerous. At high voltages a spark can jump
Electrical breakdown
The term electrical breakdown or electric breakdown has several similar but distinctly different meanings. For example, the term can apply to the failure of an electric circuit....
, delivering the high capacitor current
Pulsed power
Pulsed power is the term used to describe the science and technology of accumulating energy over a relatively long period of time and releasing it very quickly thus increasing the instantaneous power.-Overview:...
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. 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
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
, which can be harmful to people, animals, and equipment.
Many compact cameras charge the flash capacitor immediately after power-up, and some even just by inserting the batteries. Merely inserting the battery into the camera can prime the capacitor to become dangerous or at least unpleasant for up to several days. The energy involved is also fairly significant; a 330 microfarad capacitor charged to 300 volts (common ballpark values found in cameras) stores almost 15 joules of energy.
Animation
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.
Popular culture
In the 1969 book The Andromeda StrainThe Andromeda Strain
The Andromeda Strain , by Michael Crichton, is a techno-thriller novel documenting the efforts of a team of scientists investigating a deadly extraterrestrial microorganism that rapidly and fatally clots human blood, while in other people inducing insanity...
and the 1971 motion picture
The Andromeda Strain (film)
The Andromeda Strain is a 1971 American science-fiction film, based on the novel published in 1969 by Michael Crichton. The film is about a team of scientists who investigate a deadly organism of extraterrestrial origin that causes rapid, fatal blood clotting. Directed by Robert Wise, the film...
, specialized exposure to a xenon flash apparatus was used to burn off the outer epithelial 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'.)
See also
- Flash (photography)Flash (photography)A flash is a device used in photography producing a flash of artificial light at a color temperature of about 5500 K to help illuminate a scene. A major purpose of a flash is to illuminate a dark scene. Other uses are capturing quickly moving objects or changing the quality of light...
- List of light sources
- Strobe beaconStrobe beaconA strobe beacon is a flashing electric lamp used in a variety of industries as an attention-getting device, either to warn of possible hazards, or to attract potential customers...
- Strobe lightStrobe lightA strobe light or stroboscopic lamp, commonly called a strobe, is a device used to produce regular flashes of light. It is one of a number of devices that can be used as a stroboscope...