Flashtube - WikiHQ

frame|Helical xenon flashtube emitting [[Black body#Transmission, absorption, and reflection|greybody radiation as white light. (Animated version below.)]]

A flashtube (flashlamp) produces an electrostatic discharge with an extremely intense, incoherent, full-spectrum white light for a very short time. A flashtube is a glass tube with an electrode at each end and is filled with a gas that, when triggered, ionizes and conducts a high-voltage pulse to make light. Flashtubes are used most in photography; they also are used in science, medicine, industry, and entertainment.

Construction

thumb|U-shaped xenon flashtube

The lamp consists of a hermetically sealed glass tube which is filled with a noble gas, usually xenon, and electrodes to carry electric current to the gas. Additionally, a high voltage power source is necessary to energize the gas as a trigger event. A charged photoflash capacitor is usually used to supply energy for the flash, so as to allow very speedy delivery of very high electrical current when the lamp is triggered.

Glass envelopes

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'). 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. There are several methods of triggering.

External triggering

thumb|Xenon flashtubes used on [[smartphones and cameras are usually externally triggered.]]

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 either 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 (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 capacitor voltage is greater than the voltage drop between the cathode and the anode, 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 of light. If external triggering is used for extremely short pulses, 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, to keep the current density from rising too high, 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.

Ablative flashtubes

Ablative flashtubes are triggered by under-pressurizing. Ablative flashtubes are typically constructed using quartz tubing and one or both electrodes hollowed out, allowing a vacuum pump to be attached to control the gas pressure. The electrodes of the lamp are connected to a charged capacitor, and then the gas is vacuumed from the lamp. When the gas reaches a low enough pressure (often just a few torr) randomly-ionized particles are able to accelerate to velocities sufficient to begin ejecting electrons from the cathode as they impact its surface, resulting in a Townsend avalanche that causes the lamp to self-flash. At such low pressures, the efficiency of the flash would normally be very low. However, because of the low pressure, the particles have room to accelerate to very high speeds, and the magnetic forces expand the arc so that the bulk of its plasma becomes concentrated at the surface, bombarding the glass. The bombardment ablates (vaporizes) large amounts of quartz from the inner wall. This ablation creates a sudden, violent, localized increase in the internal pressure of the lamp, increasing the efficiency of the flash to very high levels. The ablation, however, causes extensive wear to the lamp, weakening the glass, and they typically need replacement after a very short lifetime.

Ablative flashtubes need to be refilled and vacuumed to the proper pressure for each flash. Therefore, they cannot be used for very high-repetition applications. Also, this usually precludes the use of very expensive gases like krypton or xenon. The most common gas used in an ablative flashtube is air, although sometimes cheap argon is also used. The flash usually must be very short to prevent too much heat from transferring to the glass. However, because nearly all the plasma is concentrated at the surface, the lamps have very low inductance and flashes can often be shorter than a normal lamp of comparative size. The flash from a single ablative flashtube can also be more intense than multiple lamps. For these reasons, the most common use for the lamps is for the pumping of dye lasers.

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. 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 narrow 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.

Due to its high-efficiency, 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. 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. None of xenon's spectral lines match Nd:YAG's absorption lines so, when pumping Nd:YAG with xenon, the continuum radiation must be used.

Krypton and other gases

thumb|Spectral outputs of various gases at the current density where visual output nearly equals IR. Krypton has very few spectral lines in the near-IR, so most energy is directed into two main peaks.

thumb|Argon flashlamp spectral line radiation. The texture of the table diffracts the light, allowing the camera to image the IR lines.

All gases produce spectral lines which are specific to the gas, superimposed on a background of continuum radiation. With all gases, 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.

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.

Within the plasma, positive ions accelerate toward the cathode while electrons accelerate toward the anode. Neutral atoms move toward the anode at a slower rate, filling some localized pressure differential created by the ions. At normal pressures this motion is in very short distances, because the particles interact and bump into each other, and, exchanging electrons, they reverse direction. Thus, during the pulse neutral atoms are constantly ionizing and recombining, emitting a photon each time, relaying electrons from the cathode to the anode. The greater the number of ion transitions for each electron; the better the conversion efficiency will be, so longer tubes or higher pressures both help increase the efficiency of the lamp. During the pulse, skin effect causes free electrons to gather near the inner wall, creating an electron sheath around the plasma. This makes the area electro-negative and helps to keep it cool. The skin effect also increases inductance by inducing eddy currents in the central plasma.

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. Bremsstrahlung radiation increases with increasing energy density, and causes a shift toward the blue and ultraviolet end of the spectrum. 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.

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.

Failure from heat is usually caused by excessively long pulse-durations, high average-power levels, or inadequate electrode-size. The longer the pulse; the more of its intense heat will be transferred to the glass. 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 are not of a sufficient diameter to handle the peak currents they may produce too much resistance, rapidly heating up and thermally expanding. If the electrodes heat much faster than the glass, the lamp may crack or even shatter at the ends.]]

thumb|A flashtube (lower half of image) with a length of , ( arc length), for substrate annealing.

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 lights. Xenon flashtubes are also used in 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 aircraft warning lights, emergency vehicle lighting, fire alarm notification appliances, aircraft anticollision beacons, and other similar applications.

In dentistry it is used in "light box" devices to light-activate the hardening of various restorative and auxiliary light-curing resins (for example: Megaflash mini, Uni XS and other devices).

Due to their high intensity and relative brightness at short wavelengths (extending into the ultraviolet) and short pulse widths, flashtubes are also ideally suited as light sources for pumping atoms in a laser to excited states where they can be stimulated to emit coherent, monochromatic light. Proper selection of both the filler gas and current density is crucial, so that the maximum 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.

Pulsed light (PL) is a technique to decontaminate surfaces by killing microorganisms using pulses of an intense broad spectrum, rich in UV-C light. UV-C is the portion of the electromagnetic spectrum corresponding to the band between 200 and 280 nm. Pulsed light works with xenon lamps that can produce flashes several times per second. Disinfection robots use pulsed UV light.

A recent application of flashlamps is photonic curing.

History

thumb|This [[shadowgraph of a bullet in supersonic flight was taken at the Edgerton Center (Strobe Alley, MIT), using a discharge from a high-speed flashtube]]

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 flashbulbs and flash lamps in mainstream photography.

Because electrical arcs could be made that were much faster than mechanical-shutter speeds, early high-speed photographs were taken with an open-air, electrical-arc discharge, called spark photography, helping to remove blur from moving objects. This was typically done with the shutter locked open while in a dark or dimly lit room, to avoid overexposing the film, and a method of timing the flash to the event to be photographed. The earliest known use of spark photography began with Henry Fox Talbot around 1850. 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 a gunshot.

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.

Popular culture

In the 1969 book The Andromeda Strain 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'.)

Animation

thumb|Helical xenon flashtube being fired

Frame 1: The tube is dark.

Frame 2: The trigger pulse ionizes the gas, glowing with a faint, blue light. Spark streamers form from each electrode, moving toward each other along the inner surface of the glass tube.

Frame 3: Spark streamers connect and move away from the glass, and a plasma tunnel forms allowing current to surge.

Frame 4: Capacitor current begins to run away, heating the surrounding xenon.

Frame 5: As resistance decreases voltage drops and current fills the tube, heating the xenon to a plasma state.

Frame 6: Fully heated, resistance and voltage stabilize into an arc and the full current load rushes through the tube, causing the xenon to emit a burst of light.

References

External links

Emission spectra of different flash lamps