Flash Lamps

A technically simple approach (concerning the required electronics) is external triggering, using an additional electrode, which is usually outside the glass envelope. For example, it can be a nickel wire wound around the glass tube. Using a small external trigger transformer, one can apply a high voltage pulse between the trigger electrode (sparker electrode) and the cathode, which generates a spark streamer that subsequently leads to the start of the main discharge between cathode and anode. The generation of the spark streamer involves capacitive effects, so that it can work despite the insulating glass. Both a sufficiently high trigger voltage and duration of the trigger pulse are necessary for reliable triggering.

In many lamp driver circuits, an inductor in the connection from the main capacitor and the flash lamp limits the peak current. (The cable itself may already introduce a significant inductance.) One usually tries to designed such a circuit for critical damping, avoiding oscillations. The circuit characteristics (including the lamp impedance) determine the electrical pulse length and thus also the optical pulse duration. Are also more sophisticated pulse forming networks, containing multiple capacitors and inductors.

An advantage of external triggering is that the trigger transformer can act independently of the part of the circuit which provides the main electrical pulse. A disadvantage, however, is that the external trigger electrode creates electrical insulation problems e.g. when a flashlamp is used in a pump chamber of a lamp-pumped laser. Therefore, such lamp applications usually need to work with a different triggering technique. A further problem is that the ignition spark pulls the discharge towards the lamp envelope, which may reduce the lamp lifetime.

There are also lamps with a trigger electrode inside the envelope. That can have various advantages, such as lower trigger voltage, mitigation of insulation problems and longer lamp lifetime.

Series triggering can be applied to lamps without an additional trigger electrode. Here, the secondary winding of the trigger transformer is connected in series to the lamp (usually on the cathode side). The voltage spike generated by the trigger transformer adds to the voltage of the main capacitor such that the voltage becomes sufficient for starting the discharge.

A disadvantage of this approach is that the full lamp current must go through the secondary coil of the trigger transformer. Therefore, a substantially larger transformer is required.

Particularly for flash lamps operated at substantial pulse repetition rates, as required e.g. for the pumping of pulsed lasers, one often uses simmer operation for obtaining most reliable flashing and long lamp lifetimes. Here, between the flashes the lamp continues to be operated with a small DC current, far below the peak current for flash generation. The lamp does not have to be ignited for each flash and can thus generate many more pulses during its lifetime. The required laser power supply is then somewhat more complex, but this can be very worthwhile. The obtained pulse energies can be more consistent (lower noise) than without simmering.

High quality flash lamps, which are operated under good conditions (far below their explosion energy), can sometimes reach lifetimes of more than 100 million or even above one billion pulses. For higher energies, the achieved lifetime may be orders of magnitude lower, and simmering does not help much.

For low pulse repetition rates, simmer operation is less suitable, since much of the drive power is then used for simmering. In that regime, one may use a pseudo simmer. Here, a some time (e.g. 100 ms) before generating the actual pulse, one triggers the lamp, but only for establishing a relatively small simmer current before the main discharge is started. With that two-step startup process, the lamp lifetime can be substantially increased, particularly for low pulse energies, and at the same time the pulse stability is improved.

If a capacitor is discharged through a flash lamp with a series inductance, as explained above, the pulse duration is limited by the characteristics of that LC circuit. For reducing the pulse duration, one has to use a capacitor with smaller capacitance and at the same time increase the charging voltage in order to achieve the required electrical energy. The capacitor voltage may then be above the ignition threshold of the lamp. In such cases, one needs to use an additional high-voltage switching device, which allows for fast switching and can handle the very high peak current. For such purposes, a solid state switch may no longer be suitable; instead, some other devices like a spark gap thyratron or an ignitron may be employed. In order to achieve a reasonably long lamp lifetime despite the high peak current, one may in addition realize simmer operation (see above).

For longer and variable pulse durations, there are electronic circuits with a high voltage switching transistor, which can limit the lamp current according to its drive signal. That way, the lamp current can be well controlled even for pulse durations of tens of milliseconds. If an approximately constant current is applied throughout the pulse, this is called square wave operation.

Flash lights are used for high speed imaging, e.g. in industrial settings. Even rapidly moving parts can be accurately imaged when being illuminated with a short light flash.

Particularly xenon flash lamps are used for stroboscopes, applying a periodic sequence of pulses. Here, they usually need to be fired with a substantial repetition rate. They should thus be optimized for that mode of operation, possibly with a limitation of the applied electrical pulse energy.

Some strobe lamps are available with a very short arc (small electrode gap), so that one approaches a point source. Such a lamp format is also referred for various scientific applications.

Various flash lamps are used in dermatology, i.e., for skin treatments, including medical and cosmetic treatments of acne, age spots, and for hair removal and tattoo removal.

Flash lamp systems can be used in spectroscopy. For example, one may use a xenon flash lamp for exciting fluorescence in a sample; spectral analysis of the fluorescence light delivers chemical information. Such methods are applied in clinical chemistry and for air and water pollution analysis, for example.

Flash lamps are still widely used for pumping of pulsed, namely of free-running or Q-switched) solid-state lasers. In particular, krypton and xenon lamps are used for pulsed YAG lasers. While xenon lamps are somewhat more efficient themselves, the emission spectrum of krypton lamps fits somewhat better to the neodymium absorption.

Usually, laser flash lamps have the form of thin cylindrical tubes, which are mounted in parallel to a long laser rod, all within a diffuse reflector in a pump chamber. Particularly in case of Q-switched lasers, such lamps need to emit relatively short pulses with durations of e.g. below 100 μs, since the gain medium of such a laser can usually not store the provided energy over a longer time.

Although diode-pumped lasers can achieve a far higher wall plug efficiency, flash lamps are still widely used in applications with very high pulse energies, essentially because their cost per watt (or per joule of pulse energy) is far lower. Note also that for lasers with low pulse repetition rates and thus low average power (or few operation hours), the poor energy efficiency may not be very relevant. The cooling demands are also moderate for low repetition rates.

For more details, see the article on lamp-pumped lasers.

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