Metal halide lamp
Metal halide lamps are light sources from the group of gas discharge lamps . They are a further development of the mercury vapor lamps . By adding halogen compounds and rare earths , color rendering and luminous efficiency can be increased. It also contains xenon or neon as starter gas .
General
The halogen metal halide lamp reaches over other high-pressure gas discharge lamps a very high color rendering index of up to 90 (with Osram HMI lamps for the film range up to 96), very high luminous efficacy of up to 117 lumens per watt (lm / W) with good and up 110 lm / W with very good color rendering and, along with sodium vapor lamps and LED lamps, is one of the most efficient lamps of all. It can convert up to 38% of the electrical power it consumes into visible light and has a service life of up to 30,000 hours on electronic ballasts .
Like almost all high-pressure lamps, it requires a few minutes to achieve full light output and is available in various color temperatures from 2700 K to over 20,000 K with color rendering indices from 60 to 96. Colored lamps for effect lighting are also produced.
They were invented in 1964/1965 by Gilber Reiling at General Electric , who also developed the first Ceramic Metal Halide Lamps (CMH) in 1992 ; In 1993 Philips followed with their Ceramic Discharge Metal (CDM). At Osram it is called HCI and is produced in Berlin-Spandau . At Sylvania it's called HSI.
The main areas of application are the lighting of displays, the lighting for film sets and television, theater lighting , the lighting of exhibition halls / warehouses, industrial halls and stadiums, architectural and aquarium lighting, traffic and outdoor lighting. This type of lamp is increasingly replacing high-pressure mercury lamps, which it is superior to with far better color rendering and more than twice the light output .
Metal halide lamps must be operated with a ballast . They are available with outputs from 20 W to 24 kW, as well as in different designs (single and double-sided, different base sizes and color temperatures). Despite the similarity of names, their light generation and structure differ in principle from the halogen light bulbs with which they are sometimes confused.
Types and types
The heart of the metal halide lamp is the discharge tube (short: burner, burner tube; English discharge tube ) with the two opposite electrodes. It is often inserted into an evacuated envelope bulb , which is used for protection and heat insulation and which leads the two electrode connections to the outside of the base. There are single-ended and double-ended types that differ primarily in terms of performance.
Common bases:
- G8.5 (crimp base or pin base) with 20 to 70 W.
- G12 (ceramic base) with 20 to 150 W.
- G22 (ceramic base) with 250 and 400 W.
- RX7S (ceramic base on both sides) with 70 to 250 W.
- FC2 (ceramic base on both sides) with 250 to 400 W.
- E27 (screw base) with 35 to 150 W.
- E40 (screw base) with 250 to 3500 W.
In the professional field (film, TV, theater, etc.) the following base forms and services have established themselves:
- GX9.5 or GY9.5 (ceramic base) with 125 to 1200 W.
- G22 (ceramic base) with 575 to 1200 W.
- PG47 (ceramic base) with 250 to 1500 W.
- G (X) 38 (ceramic base) with 1200 to 12000 W.
- G (X) 52 (ceramic base) with 18000 W as a new development; however, lamps with a double cap are usually used in this power class
- G (X) 38 (ceramic base) with 24000 W base on both sides.
A distinction is also made between quartz and ceramic technology, manufacturer-specific burner types as well as color rendering index and color temperature . Quartz glass discharge vessels are usually formed from a piece of quartz glass tubing, i.e. have a tubular or spherical shape with squeezed ends with a lateral pump nozzle through which the vessel is evacuated. The power feedthroughs are made of molybdenum sheet. The position of the pump nozzle (tip) is often prescribed in order to avoid thermal stress. As early as 1952, however, a technology was known to melt the burners in the area of the current feedthroughs. Tipless shaped ellipsoidal quartz glass burners (from Venture and Sylvania) achieve higher volume constancy and thus color and luminous flux constancy and achieve either very high luminous efficacy (up to 110 lm / W with color rendering 65) or high color rendering indices of 85 to 90 with 80 lm / W. Ceramic discharge vessels such as those used in sodium vapor lamps (tube pieces made of high-purity aluminum oxide ceramic) have been increasingly popular since 1993. They allow significantly higher internal temperatures and pressures and thus a more continuous spectrum with high color rendering indices of over 90 and luminous efficacies of over 100 lm / W. Variants with 35, 70, 150, 250 and 400 W are produced. They are called CMH at General Electric , CDM at Philips, HCI-Power Ball at Osram, CeraArc at Iwasaki, C-HIT at Ushio-BLV. The burner or the discharge vessel of the CDM lamps is cylindrical, as can be seen in the adjacent figure ( see picture ), the ends of the CMH are double-walled on the inside, the HCI Power Ball , also shown in the figure, is almost spherical and elliptical, and the CeraArc is elliptical around the arc (for example the Powerball from Osram or the CDM-T 250 W from Philips ).
Double-sided lamps are usually hot-ignitable ( see below ), as are many with G22 and other large (widely spaced) sockets. All other lamps, i.e. with (Edison) screw bases and the small lamps with a base on one side, cannot be ignited hot or immediately restarted due to the insufficient spacing between the connections (voltage flashover at the ignition voltage of up to 60 kV).
For a particularly high color rendering index (up to 95, for film and television), Osram introduced the so-called HMI lamp (other manufacturers: MSI or RSI). HMI stands for Hydrargyrum medium-arc iodide , i.e. for mercury + medium arc length + iodine, and often as a synonym for daylight-like headlights.
Most metal halide lamps are designed for use in closed luminaires: Since the discharge vessel is under high pressure, it must be prevented that fragments fly around in the event of bursting. There are increasingly metal halide lamps with anti-burst cylinders (shroud) made of quartz glass around the discharge vessel for use in open lights; they save a protective screen. Metal halide lamps with a quartz burner without a bulb or with a quartz glass bulb emit high levels of UV radiation . This is why an additional, ultraviolet-absorbing glass pane is required, especially with HQI-TS from 400 W and all types without a protective glass bulb.
The different designs and types of power are usually characterized by the ILCOS lamp designation system and are described in more detail there.
Quartz glass or ceramic
From the user's point of view, ceramic technology (type designations dependent on the manufacturer and not standardized; e.g. HCI, CDM) has the advantage of color stability over the service life, while with the older quartz technology (HQI) the color spectrum usually tends towards green over the course of the operating time shifts.
The difference is in the material used for the burner tube. Due to its inexpensive manufacturing technology, quartz glass is well suited as a material for the highly stressed, central area, but for the reasons mentioned above it is being replaced by translucent ceramics (high-purity aluminum oxide ceramics with as few scattering grain boundaries as possible). This ceramic can also withstand higher maximum temperatures than quartz glass. Another advantage of the newer ceramic burners is the higher light output of up to over 100 lm / W ( lumens per watt), compared to around 80 lm / W with quartz burners.
functionality
Like all gas discharge lamps, metal halide lamps must be operated with a ballast that limits the current to a constant value after ignition when an arc has formed. A lamp voltage of usually 100 to 150 V is established. This depends, among other things, on the lamp power used. Choke ballasts, the advantages of which are the robust, simple construction and the relatively low acquisition costs, as well as electronic ballasts (EVG) are common: the latter have fewer losses and can start up the lamps faster and more gently (40 seconds start-up time, up to 90% of the luminous flux is reached) , which extends the life of the lamps and reduces the decrease in luminous flux with the burning time. Electronic ballasts regulate mains voltage fluctuations and often provide flicker-free and flicker-free light, which is necessary for the use of metal halide lamps in offices and above certain workplaces. Many metal halide lamps, especially CDM-TC, all 20 W and many high-performance types may only be operated on one electronic ballast. Most electronic ballasts operate the lamps with square-wave AC voltage from 80 to 400 Hz, suitable for all types. Most electronic ballasts over 250 W are high-frequency devices. High frequency is not suitable for the thick, cylindrical CDM discharge vessels; acoustic resonances and, under certain circumstances, an explosion occur as soon as they start up. There are also dimmable ECGs: The new generation of metal halide lamps in particular are becoming increasingly dimmable, with suitable dimmable ECGs up to 50%. Since certain operating conditions such as electrode temperature and partial vapor pressures of the metals in the arc should be met during continuous operation, metal halide lamps cannot be dimmed as required.
An ignition device is required for ignition, which is located in the current path (superimposed ignition) and provides ignition pulses of around 5 to 80 kV (depending on the lamp power) until the lamp ignites. Double-capped lamps are often hot-ignitable (English hot restrike ): They can be ignited again with very high voltages even if they have not yet cooled down. If metal halide lamps are switched off without this capability or without a suitable ignition device, they must first cool down before they can be ignited again. Cooling down takes about 10 to 15 minutes, depending on the rated output and the structure of the luminaire .
In the first few minutes after the ignition process, the mixture of metals, halogens and rare earths must first heat up in order to melt and evaporate the solid components. During start-up ( run up ) the luminous flux increases to reach its normal value after 40 seconds to 5 minutes.
The xenon headlights in the car are not pure metal halide lamps, since they contain only a small amount of metal salts and mercury in addition to xenon (a few milligrams). They mainly serve to lower the color temperature and are intended to shift the light spectrum of the rather purple xenon in the direction of daylight (5000 to 6000 Kelvin ).
Operational phases
Ignition phase
The burner contains a mixture of mercury , halogens , sodium , thallium , indium and usually also scandium , in lamps with very good red rendering also calcium , rubidium or strontium , in daylight types there are also rare earth metals (e.g. dysprosium (III)) iodide , holmium (III) iodide , thulium (III) iodide ) and a noble gas (e.g. argon ). This mixture, which is partially solid, liquid and gaseous at room temperature, is initially not ionized and therefore has a high resistance. The high voltage from the ignition device first ignites an arc. The vast majority of lamps are designed for ignition voltages of up to a maximum of 5 kV, since at higher voltages the base as well as the ignition devices and the supply lines would be too costly to implement due to the required insulation.
To ensure reproducible ignition below 5 kV, it is necessary to slightly pre-ionise the gas. For this purpose, the burner electrodes of the Osram HCI or Philips CDM, for example, were alloyed with thorium Th232 / Th228 until 2012 , and krypton Kr85 is also included in the burner filling. Due to the radioactive decay, the starting gas in the burner is slightly pre-ionized, which reduces the required ignition voltage by a factor of 5. The load caused by the emitters is extremely low and is less than 0.01 millisievert. With current burners, technical progress enables ionization via ultraviolet, which is generated by corona discharge via an auxiliary electrode on the outside of the burner or a small discharge vessel near the burner . These designs are easy to see when looking at the burner. Correspondingly designed light sources are free of radioactive emitters, so that the legal requirements for the production of the lamps do not apply and the state of the art unnecessary exposure to ionizing radiation is completely eliminated.
After ignition, the resistance is greatly reduced due to impact ionization . In addition, electrodes and thereby reduce heat their work function , whereby the lamp voltage further decreases. Since it is mainly the mercury ions (the mercury serves to better ignite the lamp) that contribute to the lighting and the gas pressure is low, the lamp initially emits little light with a high proportion of blue and ultraviolet.
Burning up
The gas discharge heats the burner, melts and vaporizes the solid filling components it contains. Due to the different melting and boiling points, this process does not take place simultaneously. First the mercury reaches its boiling point of 356 ° C and thus contributes to the light emission at an early stage. The mercury supply is relatively plentiful in order to ensure a partial pressure sufficient for ignition over the service life of the lamp. For this reason, the spectrum of the emitted light initially moves through a blue-green area, which later becomes more intense and covers a large part of the visible spectrum.
As the temperature increases, the other metals also boil and increasingly contribute to the generation of light. In this phase, a rapid color change from green to white and a strong increase in brightness can be observed - the lamp has reached its operating parameters.
Aging
The aging of the lamps is based on various processes. The most important component, because it is unavoidable, is the wear and tear on the electrodes, which are correspondingly stressed by the arc. Tungsten atoms evaporate from the entry / exit points of the arc and are deposited elsewhere in the torch.
Another process is the corrosion of the burner vessel from the various fillers. In addition, the service life can be significantly reduced by diffusing gases from the burner environment. The aging of a metal halide lamp (burner blackening due to the transport of tungsten from the hot electrodes to the burner wall) can be reduced in two ways.
- By reducing the convection in the burner: The maximum temperature difference in the discharge vessel is reduced when the angular burner changes into a more ellipsoidal shape (Osram Powerball products, Philips CDM-T 250 W, GE CMH ultra, Iwasaki CeraArc). The lower temperature difference leads to lower convection (transport flow).
- By optimizing the halogen cycle: Similar to halogen lamps, the anionic halogen components iodide and bromide cause the evaporated or sputtered tungsten to be transported back (electrode material; Philips CDM Elite and CMH ultra products)
Electronic ballasts
The electronic ballasts typically used generate a square-wave voltage of the order of magnitude of 100 V to operate the burner. In contrast to a sinusoidal voltage, the gas in the burner remains permanently ionized, which ensures stable operation, especially with aged lamps. During the ignition phase, needle-shaped pulses with a voltage of typically 4… 5 kV are superimposed on the square-wave signal.
The detailed recording shows that the ignition pulse consists of a high-frequency oscillation. This allows the pulse to be coupled into the lamp circuit using a small transformer connected in series , which is usually constructed with a small ferrite core.
During operation, the ballast switches off the ignition pulses, so that the regular square-wave voltage and a square-wave current overlaid with the 50 Hz ripple voltage of the power network remain for operating the burner. In principle, electronic ballasts are inverters : First, the mains input voltage is rectified and fed to an active power factor correction (PFC). A DC voltage of approx. 400 V is present at the output of this PFC stage, which is reduced to the operating voltage of the lamp by a current - regulated buck converter . This is followed by an inverter , typically designed as an IGBT full bridge, which uses it to generate the square wave voltage with a frequency of up to a few 100 Hz to operate the lamp. The low frequency prevents damage to the burner through acoustic resonance: If the gas pressure rises slowly but steadily to up to 40 bar during the start-up phase, the burner would otherwise come into resonance at some point when operating at higher frequencies, which would result in high mechanical stress would have. The very low working frequency does not produce any flickering of twice the frequency, since the square-wave voltage passes through the zero crossing very steeply. As a disadvantage, however, some ballasts produce an audible noise when their coils are excited with the frequency.
properties
Metal halide lamps have a luminous efficacy of around 95 lm / W ( lumens per watt) and the average service life is between 750 and 30,000 hours. Special types sometimes only have a service life of 500 to 2000 hours. Operation is only possible with a ballast.
The light color and the color rendering are comparable to fluorescent tubes , but are determined by the mixture of the constituents of the burner or the gas discharge and their operating pressure. The color temperature is typically between 3000 and 7000 K, so that both incandescent and daylight-like lighting can be created. The color rendering level is 1 B to 1 A. By far the majority of metal halide lamps have the following light colors:
Type Color temperature Colour reproduction Remarks 640 or 740 4000 K 65… 75 MH 200 to 450 W. 830 3000 K 81 ... 88 CDM, CMH, HCI / 830 930 ... 932 3000 K 90… 93 150 W and new generation of ceramic metal halide lamps 941 ... 943 4100 ... 4300 K 90 ... 96 neutral white ceramic metal halide lamps 950 ... 960 5200 ... 6000 K 89… 96 daylight colored lamps, mostly still quartz technology
For special applications such as swimming pools or aquariums, there are metal halide lamps with significantly higher color temperatures of 10,000 to 20,000 K, as well as color emitters. The surface temperature of the envelope bulb is approx. 500 ° C.
Areas of application
Metal halide lamps are mainly used for daylight- like lighting with directional light or in headlights with long switch-on times and high luminous intensities .
Typical areas of application are shops and exhibitions , where particularly long operating times occur and where low heat loss , combined with high luminous efficiency, justify the higher purchase costs compared to halogen lamps . Metal halide lamps can also be found in floodlight systems for illuminating buildings and architecture, workshops and sports halls, stadiums and, in some cases, streets and squares.
Another area of application is the film industry . A defined color temperature (daylight, approx. 6,500 K) and stable color rendering are particularly important here. System powers of up to 18 kW are used to brighten up outdoor shots. Muscolights are sometimes used for night shots showing entire streets : These consist of 15 6 kW spotlights on the movable arm of a self-propelled generator .
Metal halide lamps are also used as a light source in color-changing headlights and moving heads . Their luminous flux and the desired color are set with motorized adjustable color filters . Movable mirrors , lenses or prisms are used for beam deflection .
In aquaristics , these lamps are used because of their good color rendering, but also because of their point light source character and the resulting play of light and shadow through waves on the water surface to illuminate medium-sized and large aquariums . These lamps are also being used more and more frequently in terrariums because they can provide natural light intensities (depending on the reflector type up to well over 100,000 lux ) with a low connected load.
HQI lamps are the preferred choice for emergency services, as the compact design and the high luminous flux per unit enable large areas to be illuminated with just a few lights.
Metal halide lamps are also increasingly being used for street lighting because they are more efficient than the high-pressure mercury-vapor lamps that were previously used when better color rendering was required.
The lamps generally used up to now for low color rendering requirements have been high pressure sodium lamps. A comparison of two systems, each with a power consumption of 70 watts, shows the advantages of the metal halide lamp:
- Metal halide lamp
- Type Ceramic Metal Halide , luminous flux 7400 lumens (more than 8000 lm at twilight vision), color rendering index 90 ... 96
- Sodium vapor lamp
- Types NAV-E, SON-E, LU-E , luminous flux 5600 lumens (less than 5000 lm with twilight vision, because yellow), color rendering index 20… 25
Disadvantages of street lighting with blue-whitish light are its attractiveness for insects, which pollute the lamps and attract spiders and bats, the light pollution , which is more relevant with shorter-wave light, as well as the disadvantageous properties in fog due to the higher scattering of shorter-wave light on small fog droplets .
Hazards and design of the lights
The high pressure in the discharge vessel harbors the risk of hot glass splinters if it bursts. The high ultraviolet component of the unfiltered light from the burner (both quartz glass and aluminum oxide ceramic are ultraviolet-permeable) usually requires a special lamp construction or a filtering envelope. The requirements for the luminaire often differ from those for incandescent or halogen light bulbs. Basic requirements are, for example, the limitation of UV emissions and luminaire parts made of UV-resistant material. Furthermore, the protection of the environment against broken glass from a destroyed light source must be guaranteed by the luminaire - the melting of an underlying plastic cover by hot fragments must also be prevented. For this reason, metal halide lamps can generally only be operated safely in closed, specially designed luminaires. Commercial lights usually meet these requirements. In the case of self-made constructions, there is often a considerable risk to the environment, especially when lamps are loosely suspended. Only a few lamp types offer adequate protection without a surrounding light. Such protection of the lamp is achieved by a sufficiently strong and sufficiently large ellipsoidal envelope bulb.
A corresponding note ("only for operation in closed luminaires") is given on the lamp packaging or in the data sheet.
costs
Metal halide lamps with an output of 35 to 150 W and a luminous efficacy of approx. 100 lm / W are available for approx. 20 euros net each, whereby the shorter-life quartz glass types are significantly cheaper. Lamps with up to 400 W are available at prices around 40 euros net.
A metal halide lamp for an 18 kW film spotlight costs around 1500 to 3000 EUR and has a lifespan of just 300 hours, as stated by the manufacturer.
Electronic ballasts and ignition devices cost around EUR 35 to 200 net at the beginning of 2015 for connected loads of 20 to 250 watts.
Specialist literature
- Hans R. Ris: Lighting technology for the practitioner. Basics, lamps, lights, planning, measurement. 2., ext. Edition VDE-Verlag et al., Berlin et al. 1997, ISBN 3-8007-2163-5 .
- Günter Springer: Expertise in electrical engineering. 18th, completely revised and expanded edition. Europa – Lehrmittel, Haan-Gruiten 1989, ISBN 3-8085-3018-9 .
Web links
Individual evidence
- ↑ Patent US2664517 A
- ↑ HMI lamps. Retrieved April 24, 2020 .
- ↑ Examples can be found in the technical report ( Memento of February 22, 2012 in the Internet Archive ) on IEC 1231 of the ZVEI
- ^ Peter G. Flesch: Light and Light Sources: High-Intensity Discharge Lamps . Springer, 2007, ISBN 3-540-32685-5 , pp. 45 ( limited preview in Google Book search).
- ↑ according to POWERTRONIC - intelligent lighting control for metal halide lamps ( memento from April 26, 2016 in the Internet Archive ) 160 ... 180 Hz
- ↑ Osram HQI-TS / HIT-DE ( Memento from January 18, 2015 in the web archive archive.today ), accessed at the beginning of 2015
- ^ Metal Halide & HMI Lamps , B&H Photo Video, New York dealer, accessed October 17, 2014
- ↑ http://www.licht-versand.de Accessed in early 2015