Gas discharge tube
The gas discharge tube - sometimes just called a discharge tube - is an arrangement of cathode and anode within a gas-filled glass tube, in which a gas discharge with emission of light occurs when a design-specific minimum voltage is applied . The physical mechanism of action of the light emission after external stimulation is called luminescence , the technical advancement to light sources is generally called gas discharge lamp .
In the 19th century Heinrich Geißler developed and built the Geißler tubes named after him . Depending on the gas filling, they show different spectral compositions of the emitted light. With them, the pressure-dependent diameter and the structure of the gas discharge can be observed well.
Philipp Lenard experimented with discharge tubes and thus contributed significantly to the further development of atomic physics . In his experiments, he connected a vacuum pump to the bulb-shaped discharge tube , which greatly reduced the air pressure in the tube. On the side of the tube was a so-called Lenard window , which consisted of an aluminum foil (around 0.5 micrometers thick). This is thick enough to withstand the pressure difference, but still allows cathode rays (electrons) to pass, which are therefore accessible for examination outside the tube. For this purpose, there was, for example, a fluorescent screen next to the window .
Gas discharge tubes consist of an approximately tubular discharge vessel made of glass (low pressure lamps) , quartz glass (high and ultra high pressure lamps) or aluminum oxide ceramic (high pressure lamps). There are two electrodes in the housing , between which an electric field is built up and a gas discharge takes place. The electrical connections are led to the outside in a gastight manner through a pinch foot . The electrodes can be “cold” or glowing.
All gas discharge lamps with the exception of flash lamps require a current limitation for operation, otherwise the charge carrier density and the current rise rapidly due to the impact ionization , which is desirable for surge arresters and null electrodes , but leads to their destruction in lamps (see characteristic curve). The current limitation is through a resistor (glow lamps), a choke or an electronic ballast (EVG, engl. Ballast ) is reached.
Flash lamps, on the other hand, often work from a storage capacitor without current limitation. The average power consumption is nevertheless limited by the amount of energy stored in the capacitor and the lightning frequency. Flash lamps are usually filled with xenon (for pumping solid-state lasers also with krypton ) and generate very high light outputs of a quality similar to daylight within about 0.1 to 5 milliseconds.
There are designs with heated or with unheated (cold) electrodes . There are also gas discharge tubes without electrodes, such as nullodes and microwave lamps (such as the sulfur lamp ). Lately there are also electrodeless energy-saving lamps that work with high frequency .
At room temperature , these gases have a low pressure in the flask, which favors the ignition of the discharge by impact ionization and thus the generation of the plasma. The filling with gas mixtures determines certain properties of the discharge or serves the sole purpose of providing sufficient heat for the evaporation of the substance intended for the actual plasma. Due to the resulting temperatures, a supply of the plasma-forming material located in the piston is evaporated and thus increases the pressure in the discharge space.
The plasma-forming substances are metals or their vapors ( sodium , mercury , see also metal vapor lamp ), whereby noble gases are always included for ignition , or pure noble gases ( xenon , krypton , neon ) or mixtures of halogens and metals ( Metal halide lamp ). Spectral lamps use other gases as well.
High-pressure gas discharge lamps are also referred to as HID lamps designated (from English High Intensity Discharge ); Current and luminance are significantly higher here than with low-pressure plasmas, the discharge works in the area of an arc or an arc discharge.
An essential differentiating criterion for gas discharge tubes is the pressure in the discharge vessel or the burner . One differentiates:
- Low-pressure discharge lamps (low-pressure plasma, glow discharge, e.g. fluorescent tubes , fluorescent lamps , neon tubes ),
- High-pressure discharge lamps (pressures up to about 1 MPa, e.g. mercury vapor lamps , krypton arc lamps for laser excitation),
- Ultra-high pressure lamps (pressures of up to about 10 MPa, for example high-pressure mercury vapor lamps in photolithography , xenon short-arc lamps for projection purposes).
In the case of mercury vapor lamps, a distinction is also made between so-called medium pressure lamps; These are manufactured in great lengths with great performance and are used for UV curing, for example, of paints and resins.
The typical current - voltage - characteristic of a high-pressure gas discharge (right) is closely related with the luminous phenomena. A small current flows in the area of the dependent discharge (1) (ignition; fractions of a second). At (2) the glow discharge begins (start of discharge; fractions of a second). The operating voltage required to maintain the glow discharge is lower than the ignition voltage. An increase in the current leads to an arc discharge in (3) . It is characterized by low operating voltage and high current or high current density; the electrodes start to glow.
In the vicinity of the dashed transitions in the adjacent picture, all gas discharge lamps act like negative differential resistances . When the voltage rises, the current intensity decreases and the current intensity increases when the voltage decreases. To limit the current, gas discharge lamps must always be operated with a series resistor. With high performance and power with alternating current, are usually employed Vorschaltdrosseln to keep the heat generation low.
Low pressure discharge lamps
The low-pressure discharge lamp is characterized by the fact that the electron and gas temperatures are hardly coupled. There is no thermal equilibrium . The form of discharge is also known as a glow discharge . A typical application is the fluorescent lamp , which is a special form of the mercury vapor lamp.
In a partially evacuated glass tube, a glow discharge forms on opposing electrodes if the voltage is high enough (burning voltage about one hundred to several 100 V). The negative glow light occurs near the cathode (-) and the so-called positive column in the middle up to the anode (+) .
Low pressure lamps work
- with directly heated incandescent cathodes (in fluorescent lamps and low-pressure sodium vapor lamps ). The hot cathodes are usually heated before they are ignited and then retain their temperature by themselves through reheating.
Gas-filled surge arresters insulate as long as the voltage remains below their ignition voltage and do not interfere because of their low capacitance of only around 2 pF. If the ignition voltage is exceeded, the resistance drops to very low values within microseconds, whereby current peaks of up to 20,000 A can be diverted. If the current continues, they will be thermally overloaded. For this reason, an overcurrent protection device (e.g. fuse ) is usually provided, which responds to the diverted current.
The glow lamp uses the glow discharge and is mostly used for signaling purposes in various applications. The glow light is generated at the cathode ; when operating with AC voltage , both electrodes light up alternately. Since glow lamps are usually filled with the noble gas neon, they are classified as neon tubes in this design.
The glow lamp is inexpensive to manufacture, but its function as a signaling display is increasingly being replaced by light-emitting diodes (LEDs).
In fluorescent tubes as cold cathode tubes and neon tubes (then with red emitting neon filling), the electrodes are located far apart from each other in a glass tube. Here the positive column lights up when there is a voltage of several 100 volts. Other colors are achieved through other fillings and phosphors. Fluorescent tubes, neon tubes and cold cathode tubes have cold cathodes and require higher voltages than glow lamps or fluorescent lamps.
Sometimes fluorescent tubes are also referred to as fluorescent lamps, because they often contain a fluorescent substance just like fluorescent lamps . Real fluorescent lamps (also called fluorescent tubes) as well as compact fluorescent lamps and so-called energy-saving lamps have hot cathodes (directly heated oxide cathodes). Due to glow emissions, the required operating voltage drops to values that allow operation via a series reactor directly on the mains voltage .
The point in the circuit symbol of the tube symbolizes the filling with a gas or steam. Without the dot, it would be a vacuum tube.
Low pressure sodium lamps
These are also known as LS, NA or SOX lamps. They are among the most efficient light sources, but with their almost monochromatic orange they only allow very poor color rendering. They are therefore mainly used in street lighting.
High pressure discharge lamps (HID lamps)
These are usually metal halide lamps, which can be further subdivided into mercury, sodium and metal halide lamps.
The current density is so high that the low-pressure discharge immediately changes into an arc discharge when it starts, so that the internal pressure increases sharply as the temperature rises and the filling components evaporate. Depending on the design, the electrodes reach temperatures of around 1000 degrees Celsius to several thousand degrees Celsius and are not preheated. Mercury vapor lamps have ignition electrodes in addition to the main electrodes, so that an ignition device, as is required with other high-pressure gas discharge lamps, can be omitted.
Due to the higher density and the resulting smaller free path of the particles, the electron and gas temperature are almost in equilibrium in the high pressure discharge lamp (p> 0.1 bar). In contrast to fluorescent tubes , the voltages are low (50… 200 V) and the discharge currents (typically 1… 10 amps) are significantly higher.
High-pressure gas discharge lamps are also called HID lamps (from high intensity discharge ).
In the case of high-pressure gas discharge lamps, line broadening occurs due to the thermal movement, which is why these lamps, even without fluorescent material, already have a somewhat better color rendering than low-pressure discharge lamps without fluorescent material.
High-pressure gas discharge lamps often have an additional protective glass bulb, which is also used for thermal insulation and partially carries a fluorescent material.
- Mercury vapor high pressure lamp , often also called HM lamp (industrial and street lighting); For a picture of a discharge vessel see under construction
- High pressure sodium vapor lamp , often also called HS lamp (street lighting)
- Metal halide lamp , often also called HI lamp (C-HI: metal halide lamp with ceramic discharge vessel) (shop displays, stadium lighting, architecture, projectors )
High pressure gas discharge lamps
In these lamps, the mercury-noble gas mixture (argon, xenon) already has overpressure at room temperature. During operation, the pressure rises to 100 to 400 bar. The lamps have a compact, thick-walled quartz glass vessel and massive tungsten electrodes. Lamps with an input power of 50 watts to 24 kilowatts are common.
These lamps are produced as a projection light source or in sunlight simulators with a xenon filling and, for photolithography purposes, with a mercury vapor filling.
These lamps must be ignited before they can continue to operate because there is a non-conductive gas path between the two electrodes. This is done by applying a high voltage pulse (up to 50 kV) that exceeds the breakdown voltage of the gas line. The resulting flashover ionizes the gas path between the electrodes and makes them conductive. So that the ionized path does not break off again immediately, a smoothed direct current must now be supplied in continuous operation at a voltage of 20 V to 30 V. The better the operating voltage is smoothed, the longer the life of the lamp and the higher the quality of the light emitted.
Ultra-high pressure gas discharge lamps have a light center that is only a few millimeters wide, with the point of highest light density (hot spot) being directly on the cathode ; they are therefore also referred to as short-arc lamps. The small size of the light source and its high intensity allow effective focusing or collimation in lighthouses, spotlights and projectors .
Extremely high pressure gas discharge lamps have a strong thermal line broadening of their emission spectrum and in the case of xenon high pressure lamps emit an almost continuous spectrum similar to daylight.
Due to the risk of explosion, high pressure lamps must be handled and operated with care (gloves, protective goggles) (no free assembly possible, operating position is often prescribed).
Flash tubes have cold cathodes and only work in pulse mode. A storage capacitor is used as the power source and can deliver a peak electrical power of up to megawatts ( photo flash ) or even gigawatts (pumping of pulse lasers ). The gas filling is xenon for a color temperature similar to daylight, but also krypton, especially for the effective excitation of pulse solid-state lasers . Further applications are stroboscopes and the brief heating of surfaces.
Flash tubes such as (approximately 4 to 20 kV) is ignited and high-pressure lamps with a high voltage ignition pulse ge or triggers . They have an electrode attached to the outside of the glass bulb for this purpose. This is possible because the operating temperature of the glass bulb is low.
Areas of application
In addition to demonstration purposes , gas discharge tubes are nowadays primarily used as gas discharge lamps , known as fluorescent lamps, fluorescent tubes . Glow lamps serve as signal lamps; Numeric displays based on this principle ( Nixie tubes ) are out of date, however.
Sodium vapor lamps are often used for street and industrial lighting. Sodium has a double line at 589.0 and 589.6 nanometers ( sodium D line ), which cause the dominant yellow-orange color rendering. The lower level of these lines is the ground state, so that the radiation density of these resonance lines is very high.
High-pressure gas discharge lamps filled with mercury, metal halide or sodium vapor are used in floodlight systems. Xenon lamps have the best color rendering. Metal halide gas discharge lamps (including metal halide lamps, not incandescent halogen lamps !), Which also contain mercury vapor, have relatively good color rendering . They are often used in shop displays.
Manufacturers such as Osram offer HMI lamps up to 24 kW, which are also used in cinema projectors or daylight spotlights. These lamps have a luminous efficacy of 95 lm / W and achieve a luminous flux of up to 2,300,000 lm.
So-called xenon light has been used in the headlights of motor vehicles since 1991 . These are metal halide lamps with xenon as the filling and starting gas. The exact designation of the lamps is difficult, however, since mainly xenon as a gas and, in addition, to a small extent mercury and metal halides are present. Compared to incandescent lamps they have a higher luminous flux with lower power consumption (halogen lamp H7 with 1500 lumens compared to HID lamps with around 3000 lumens).
In the field of metrology , the radiation emitted by gas discharge lamps (spectral lamps ) is used as normal lengths . For this purpose, precautions are taken so that the spectral lines are as narrow-banded as possible . An example is the Krypton-86 lamp , which was used between 1960 and 1983 to define the meter .
- Peter Flesch: Light and light sources. High-intensity discharge lamps. Springer, Berlin a. a. 2006, ISBN 3-540-32684-7 .
- Wilhelm Gerster: Modern lighting systems for inside and outside. The practical reference work for do-it-yourselfers. Compact, Munich 1997, ISBN 3-8174-2395-0 .
- Hans R. Ris: Lighting technology for practitioners. Basics - lamps - luminaires - planning - measurement. 2nd, expanded edition. VDE-Verlag u. a., Berlin a. a. 1997, ISBN 3-8007-2163-5 .
- Adolf Senner: Electrical engineering. 4th edition. Publishing house Europa-Lehrmittel, Wuppertal-Barmen 1965.
- Günter Springer (editing): Electrical engineering. 18th, completely revised and expanded edition. Verlag Europa-Lehrmittel, Wuppertal 1989, ISBN 3-8085-3018-9 .
- High pressure discharge lamps (PDF file)
- Glow lamps and glow tubes, UV lamps, laser, flash and stroboscope tubes
- Electronics history: cold cathode tubes (the cold cathode tube works on the principle of the gas discharge lamp)
- Lamp museum with registration
- Construction of UHP lamps, University of Osnabrück (PDF file; 961 kB)