Xenon gas discharge lamp
Xenon gas discharge lamps are high pressure gas discharge lamps . They use the radiation emission of an electric arc , have particularly high luminance levels and a very high color rendering index because the spectrum generated almost corresponds to that of sunlight. Lamps with an input power of 25 W to 15 kW are common.
construction
Xenon gas discharge lamps are made of a thick-walled, with xenon -filled quartz glass bulb in which two massive tungsten - electrodes are sealed in between which remains a gap of a few millimeters. The cathode is a comparatively small cone that is welded onto a thinner tungsten pin. The anode is a large cylindrical tungsten body, the end face of which is chamfered and directed towards the cathode. In the case of high-performance lamps, the anode has channels for water cooling through it. The polarity must not be reversed, otherwise the cathode cone will melt. The cathode contains up to 3 percent 232 thorium to reduce the work function for electrons and thus the ignition voltage. The gas filling consists either of pure xenon, but can also contain small additions of mercury.
business
Xenon gas discharge lamps are ignited with high-voltage pulses of 10 to 60 kV in rapid succession, which ionizes the gas path between the electrodes. If a high-performance power supply supplies sufficient current, an arc with an arc voltage of around 20 V is formed between the electrodes . This can rise to 60 V as the operating time increases. The operating current of around 10 ... 200 A must be maintained by a direct current source ( constant current source ) because the arc has a negative differential resistance . The better the operating current is smoothed, the longer the service life of the lamp and the more flicker-free the light emitted.
The pressure of the xenon inert gas filling increases during operation from around 8 bar (0.8 M Pa ) in the cold state to up to 70 bar (7 MPa). The high pressure widens the emission lines of the plasma to a spectrum that is almost continuous in the visible range with a color temperature of slightly over 6000 K, which corresponds to the color of daylight. Large heat losses arise from the strong heating of the anode. This becomes red hot due to the incoming electrons during operation. In order to be able to radiate the heat better, it needs a larger outer surface. Hot re-ignition of the lamps is possible with suitable ballasts.
Technical features
The high temperatures and pressures under which xenon short-arc lamps work require cooling, an explosion-proof housing and protective measures against the intensive, ultraviolet-rich radiation.
Depending on the intended use, xenon gas discharge lamps are also made of doped quartz glass in order to hold back ultraviolet radiation and thus reduce ozone formation and the radiation load on the optics. If the UV radiation is disturbing, for example because it leads to the formation of ozone , it can be reduced by using special types of glass.
Xenon gas discharge lamps have a light center that is only a few millimeters wide, with the point of greatest light density lying directly on the cathode . The luminance decreases by a factor of about 10 along the arc to the anode. The short arc and its large luminance, comparable to the surface of the sun, is particularly suitable for collimation .
Around 80% of the electrical power supplied is converted into radiation energy, of which only around 30% is in the visible and 6% in the UV range. The rest is given off as infrared radiation. The luminous efficacy of approx. 30 ... 50 lm / W is higher than that of incandescent lamps , but lower than that of many other gas discharge lamps.
The traditional installation position is vertical, with the anode on top. If you swap the electrodes or the electrical polarity, the lamp is quickly destroyed. In the case of film projectors , however, horizontal installation is preferred because of the higher light output due to elliptical mirrors. So that the arc does not bulge upwards, it can be held on the cathode-anode axis by a correctly dimensioned, heat-resistant magnet.
application
For vehicle headlights, see also xenon light .
Xenon short-arc lamps are used wherever maximum luminance, an almost point-like light source and a spectrum similar to daylight are required, for example:
- large cinema film projectors
- Effect and search lights
- Light sources for scientific applications, for example solar simulators
- Lighthouses
Dangers in handling
Due to the high pressure even when cold, there is a risk of explosion if the lamp is mechanically damaged. Serious injuries can be caused to bystanders if the piston splinters. A hot lamp is correspondingly more dangerous due to the higher pressure and the materials used.
Even the ozone-free lamps emit large amounts of longer-wave UV radiation. Without suitable protective measures, the radiation on uncovered skin quickly leads to burns and increases the risk of cancer in the long term.
The luminance of the arc is so high that an unprotected view of it can cause severe damage to the retina and even blindness within a very short time.
A number of safety measures are therefore necessary when deployed, for example:
- trained operating and maintenance personnel
- Operation only in intended, closed housings in the prescribed operating position
- Operation only with the prescribed electricity and adequate cooling
- Skin and eye protection through clothing and special goggles or blocking the path of light propagation
- Change and transport only in special containers, for example made of polycarbonate
- Gloves and protective clothing when handling
- Removal of the ozone created by the radiation
costs
Xenon short-arc lamp XBOs with outputs from 50 watts to 1000 watts are available in stores at prices from 650 to 1600 euros (as of January 2017)
literature
- August Scherl: The great book of technology. (The large Bertelsmann Lexicon Library, Vol. 17), Bertelsmann Lexikon Verlag, Gütersloh, Vienna 1970