You've seen them in the clubs and on your Mom's camera. The Xenon flashtube can create a brilliant white incoherent light.

A Xenon bulb starts with a hollow glass container. Two electrodes are implanted in the tube, and the air inside the tube is removed. The vacuum is then filled with inert Xenon gas.

To create light, the electrodes are connected to a high voltage source. When the electrodes are charged, they become extremely hot in less than a second. The thermal energy of the electrodes cause the Xenon gas to ionize, and the bulb is lit. The Xenon gas gives off mostly blue and red light, which combined gives the familiar bluish-white light. The blue component of the spectrum is centered at about 500 nanometers in wavelength, and the red portion is centered around 800 nanometers in wavelength.

To produce the strobing effect, the high voltage is pulsed, which causes the Xenon gas to flash at the same rate. Xenon bulbs may be used as a normal light source for situations where a bright light is needed, but it is not very efficient.

Normally, it would take an extremely high voltage to cause the xenon gas inside a flash tube to ionize. Once the tube is conducting, though, it will maintain discharge until the storage capacitor is discharged. To get the tube started, a third trigger electrode is used. On most xenon tubes, this is a wire wrapped around the glass, or a conductive trace silvered onto it. Cheap disposable cameras just have a piece of copper foil stuck next to the tube. This electrode is wired to a small trigger coil. The trigger coil is a small high-voltage autotransformer that will step up a pulse of voltage off of the strobe or flashgun's storage capacitor to about 1,500 volts. A large storage capacitor connected in parallel with the tube's electrodes is charged to 200-500 volts by a small converter. The trigger pulse is carried across the glass by capacitive coupling, and ionizes the gas enough to start the discharge. The tube will continue to conduct until the capacitor is mostly discharged.

The trigger coil is fired by a set of contacts attached to the shutter (on a camera flash), or by a small mercury vapor filled bulb which flashes over and becomes conductive once suitable voltage has built up on the storage capacitor to fire the tube.

The schematic for a camera flash trigger circuit is shown below. C1 is the main storage capacitor, and C2 is a small capacitor used to prevent the trigger coil primary from discharging C1. S1 is the shutter switch, which would be replaced by the mercury vapor bulb in a strobe. T1 is the trigger coil.

from power |             | T1      | |
supply    --- C1         $||       | |flash
          ---         +--$||       | |tube
           |          |  $||       | |
           |        ---  $||      || |
           |     C2 ---  $||      || |
           |          |  $--------|| |
           |         / S1  trigger | |
Note that I said it mostly discharges the capacitor - if you plan to mess with the innards of a flash gun or strobe light, discharge it first with a test lead or screwdriver. Otherwise, you may get a nasty little surprise.

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