Xenon arc lamp
Xenon gas discharge lamps are high -pressure gas discharge lamps. They use the radiation emission of an arc, have particularly high luminance and a very high color rendering index, because the spectrum produced almost equal to the sunlight. Usual, lamps with an input power of 50 W to 15 kW.
Construction
Xenon gas discharge lamps made of a thick-walled quartz glass bulb filled with xenon, in the two solid tungsten electrodes are sealed, and between which is a distance of a few millimeters. The cathode is a relatively small cone, which is welded to a thinner tungsten pin. The anode is a large cylindrical tungsten body, the end face is provided with a chamfer and which is facing towards the cathode. In high performance lamps, the anode of channels is traversed for water cooling. The polarity can not be reversed, otherwise melts the cathode cone. The cathode contains up to 3 percent 232Thorium to reduce the work function for electrons, and thus, the ignition voltage. The gas filling is made of either pure xenon, but can also contain small amounts of mercury.
Operation
Xenon gas discharge lamps are ignited shortly consecutive high voltage pulses of up to 50 kV, to ionize some gas atoms in the non-conductive path between the electrodes. Provides a high-performance power supply enough power, an ionized arc with an arc voltage of about 20 V. forms between the electrodes with increasing operating time, this can rise to 60 V. The operating current of 25 A, for example, must be maintained by a special power supply, because the arc has a negative differential resistance, and therefore can become unstable. The more the operating current is smoothed, the larger the life of the lamp and the quality of the emitted light.
The pressure of the xenon noble gas filling rises during operation of about 8 bar ( 0.8 MPa) in the cold state up to 70 bar ( 7 MPa ) to. The large pressure broadening the emission line of the plasma to an almost continuous spectrum in the visible range with a color temperature of about 6000 K, which corresponds to the color of light. Large heat losses caused by the strong heating of the anode. This is red hot by the incident electrons in operation. In order to radiate the heat better, it requires a larger outer surface. A hot re-ignition of the lamp is possible with suitable ballasts.
Technical Features
The high temperature and the pressure under which a commercially available xenon gas discharge lamp is, cause the operation of such a luminous body is connected to a relatively large expense. Most types of lamps needed for the smooth operation of cooling devices and an explosion-proof housing. Depending on the intended use xenon gas discharge lamps are manufactured or coated with a doped quartz glass, in order to retain the ultraviolet radiation spectrum of the radiation produced, and thus to reduce the formation of ozone, and the radiation load on the optical system.
Xenon gas discharge lamps have a luminous center of only a few millimeters expansion, the point of greatest luminance is located directly at the cathode. As can be seen in the picture, the light intensity decreases until the anode by about a factor 10 They are therefore also referred to as short-arc lamps. The limited size of the light source and their high intensity allows effective grouping.
About 80 % of the supplied electrical power is converted into radiant energy, of which only about 30% to the visible and 6% to the UV region. If the UV radiation interferes because, for example, increased ozone pollution can be reduced by special types of glass. The rest is found mainly as infrared radiation. The light output is much higher than, for example, the incandescent lamps. However, it is about 50 lm / W is lower than that of many other gas discharge lamps.
The optimum mounting position is vertical, the anode must be on top. Reverses to the electrodes or the electrical polarity, the lamp is rapidly destroyed. In film projectors, however, the horizontal mounting is preferred because of the higher light yield by elliptical mirror. So that the arc does not become unstable, it must be supported by a correctly dimensioned hitzfesten bar magnet on the cathode-anode -axis.
Application
Because of their high price and the elaborate and quite dangerous operation, the xenon gas discharge lamp is used only for special purposes. These factors fall primarily in cinema projectors, solid-state lasers, effects and search lights, light sources for scientific applications and lighthouses significant.
A special variant of the xenon gas discharge lamp contains a small amount of mercury to produce a particularly high proportion of ultraviolet radiation. This is required in the production of circuit boards, for biological purification of water, for curing of certain adhesives, or for ozone generation. Lamp types for these application areas are made of undoped glass with high UV transmittance.
Dangers when handling
The high pressure even when cold, there is a risk of explosion in mechanical damage to the lamp. By shattering the piston serious injury to bystanders may result. A hot lamp is due to the higher pressure and the claimed materials in accordance dangerous.
Even the ozone-free lamps emit large quantities of longer-wavelength UV light. Without appropriate precautions, the radiation on uncovered skin soon leads to burns and increases long-term cancer risk.
The luminance of the arc is so high that the unprotected look at this in a very short time can cause serious damage to the retina and blindness.
Therefore, in the professional field ( cinema projection, medical, instrumentation and material testing ) a number of security measures are common, such as
- Detailed training of operating and maintenance personnel
- Operating only in suitable sealed enclosures
- Eye protection with special glasses or shutting off of light propagation paths
- Exchange and transport only in special Lexanbehältern
- Use of special protective clothing when handling
Costs
Xenon short arc lamp XBO with outputs of 50 watts to 1000 watts are commercially available at prices from 250 to 600 euros available (as of January 2010)