Flash tube
The flash tube is a gas discharge lamp in which the gas discharge does not occur continuously but in the form of short pulses. During such a pulse, a previously charged energy storage device ( capacitor ) is discharged through the flash tube in a very short time.
Layout and function
A flash tube consists of a glass bulb filled with the noble gas xenon or (especially with laser pump lamps) with krypton and into which two electrodes are melted. When a sufficiently high voltage is applied, a gas discharge or formation of a spark takes place in it, which becomes visible as a bright flash of light. The necessary electrical energy is provided by a capacitor which, in battery-powered flash units ( photo flash ), is charged to around 400 V using a flyback converter, for example.
Due to impact ionization , the discharge current increases within approx. 0.1 ms to values of several 100 A after ignition and the storage capacitor is discharged. For a short time there is enormous power in the range from a few to over 100 kW. This increases the temperature and pressure in the lamp and the spectrum improves due to the line broadening towards a quality similar to daylight.
With photo flash units, the flash energy can be controlled by short-circuiting the discharge, but in particular by switching off the discharge current. Both methods allow the flash to be switched off after a certain amount of light has been emitted.
application
Flash tubes are used in light flash devices, such as stroboscopes and flash lights in the field of photography, whereby the xenon flash tubes have practically completely replaced the magnesium flash lamps that were used in the past .
Flash tubes are used in signal and warning lights , such as B. navigation lights of the aircraft , the firing of aviation obstructions or light signal transmitters to rescue agents , as well as for the hearing impaired, as an alternative to acoustic signals ( alarm , bell ). Flash tubes with gas reservoirs are used for applications in which high reliability and long service life are required.
Flash tubes are also used as pump sources for pulsed solid-state lasers (see also pulsed lasers , Nd: YAG lasers ) and for brief heating of surfaces ( annealing , recrystallization or heating / healing of crystal defects).
The largest known flash lamp systems are equipped with 3.8 m long, individual lamp tubes (3.72 m arc length) and can be used for the architectural glass, display and photovoltaic industries.
In measurement technology and spectroscopy , flash tubes are used for LIDAR (measurement of cloud height), in fluorescence spectrometers to determine the fluorescence lifetime and in pulsed solar simulators to record the performance curve of solar modules .
In general, flash tubes can also be used for exposure (examples: contact copying , some copiers ).
ignition
An auxiliary electrode is often used to control the timing of the discharge and to enable ignition at low voltages. The auxiliary electrode can be designed as a wire wrapping or as a conductive coating on the outside of the tube and often extends over a longer part of the discharge tube. The auxiliary electrode is connected to an ignition coil which generates a low-energy high-voltage pulse (approx. 1 to 8 kV). This impulse has a capacitive effect through the glass bulb and causes partial ionization of the gas filling, which makes the gas conductive. The ignition coil is fed with a discharge from a small auxiliary capacitor by means of a contact or a thyristor .
Further ignition variants are:
- for a tube: an ignition coil connected in series that can carry the high discharge current
- with two tubes connected in series: application of the ignition pulse to the junction of the two tubes
The spontaneous ignition voltage between the main electrodes depends on the tube length and is between 0.5 and 25 kV. It limits the charging voltage of the storage capacitor.
The operating voltage or the voltage of the storage capacitor must always be lower than the spontaneous ignition voltage and, in the case of mains-fed photo flash units, can e.g. B. with a voltage doubler circuit (Greinacher circuit) can be generated from the mains voltage:
at U mains = 230 V.
Technical specifications
Characteristic sizes of a flash tube are:
- Maximum energy per flash in joules (watt seconds), this generally corresponds to the energy content of the storage capacitor:
With
- … Capacity of the capacitor
- … The end-of-charge voltage of the capacitor
- Level of the required ignition voltage pulse
- Charging voltage range of the storage capacitor
- maximum lightning repetition frequency or maximum average power loss
literature
- The big book of technology. Publishing house for knowledge and education, Gütersloh 1972
- Tobias Pehle: Light effects for parties in the house and garden. Falken Verlag, Niedernhausen 1997, ISBN 3-8068-1798-7
Web links
- XENON FLASH LAMPS - Technical Information ( Hamamatsu Photonics KK 2005, PDF file; 342 kB)