Metal Halide Lamp
Lamp that produces light as a result of an electrical discharge, generated between two electrodes, in a high-pressure mercury vapor with metal halide additives, that is contained in a transparent bulb.
Metal Halide Gas Discharge Lamps
Metal halide lamps are high-pressure mercury gas discharge lamps that contain metal halides in addition to the mercury [2, 3]. In the heated discharge tube, the metals of the halides take part in the discharge process and radiate their own spectrum. Compared with high-pressure mercury lamps, both color properties and efficacy are considerably improved. Thanks to the fact that no fluorescent powder is needed, the small gas discharge tube itself is the light-emitting surface. This small light-emitting surface makes the lamps extremely suitable for use in reflector and floodlight luminaires. Originally, they were solely produced in extremely high lumen packages with relatively large lamp dimensions (about the size of a 1 litre bottle). These lamps are particularly suited for use in floodlights for the lighting of stadiums. Today, compact metal halide lamps are also available in small lumen packages, which makes them very suitable for use in compact reflector luminaires for accent lighting both indoors and outdoors. These versions have become an energy-efficient alternative for halogen lamps. Special compact metal halide versions are being produced for use in film studios and theaters. The smallest metal halide lamps are used for car headlamps (these are the so-called xenon lamps, whose gas discharge tube is no larger than a match head). Metal halide lamps belong to the group of high-intensity discharge (HID) lamps, because they are available in high-lumen output (and thus high-luminous-intensity) versions.
Suitable metals that vaporize in the hot discharge tube so as to contribute to the discharge process cannot be added directly to the mercury in the discharge tube. This is because metals suitable for this purpose attack the wall of the gas discharge tube at the high temperatures that are needed for the gas discharge to occur. The solution to this problem is to add these metals in the form of their nonaggressive chemical compounds with a halogen (iodine, bromide, or chloride), hence the name metal halide lamps. The solid metal halide starts evaporating once, after switching on the lamp, the mercury discharge has increased the temperature in the discharge tube. When this metal halide vapor enters the area of the mercury discharge in the center of the tube with its very high temperature (around 3,000 °C), it dissociates into its separate elements: metals and halogen. There the metals in their pure, vaporized state take part in the discharge process and determine the efficacy and color characteristics of the radiation. The aggressive vaporized metal cannot reach the tube wall because at the lower wall temperature (some 1,000 °C), they recombine again to form the harmless metal halide compound. Like all gas discharge lamps (with very few exceptions), a metal halide lamp cannot be operated without a ballast to limit the current flowing through it. It also needs, again like most gas discharge lamps, an igniter for starting the lamp.
Materials and Construction
Metal halide additives
Metal Halide Additives
In theory, some 50 different metals can be used for metal halide compounds, and different manufacturers have introduced various combinations of these metals. Examples of some of the metals used in metal halide compounds are sodium, thulium, thallium, indium, scandium, dysprosium, and tin. Most lamps use a mixture of at least three different metal halides. Each different combination results in a different spectrum, but also in different efficacy, lumen maintenance, and lifetime characteristics.
Besides metal halides and mercury, an inert gas is also added to the discharge tube, like in most other discharge lamps.
The electrodes of metal halide lamps are of the type used in normal high-pressure mercury lamps. They consist of a tungsten rod, with a tungsten coil impregnated with emissive material, wound around it. They are of heavier construction than the normal high-pressure mercury lamps because of the higher operating temperature.
Almost 25 % of the input power of metal halide lamps is emitted in the form of visible radiation. Compare this with 15–17 % of normal high-pressure mercury lamps and 30 % of high-pressure sodium lamps.
System Luminous Efficacy
The compact versions of the metal halide lamp have lumen efficacies (depending on the metal halide mixture and gas discharge tube material used) of between 70 and 95 lm/W. When comparing this with normal halogen incandescent lamps with their maximum efficacy of 25 lm/W, it is clear that compact metal halide lamps are often very suitable to replace halogen lamps. The larger versions of metal halide lamps have efficacies from some 75 lm/W to slightly more than 105 lm/W.
Compact versions are available in lumen packages from some 1,500 to 25.000 lm (corresponding wattage range 20–250 W). Larger versions range from some 20,000 lm to more than 200,000 lm (corresponding wattage range 250–2,000 W). Special (compact) types for use in film studios, theaters, and professional photography are produced in lumen packages of up to more than 1,000,000 lm (12 kW).
Compact metal halide lamps are normally produced in the color temperature range of 3,000 K to some 4,500 K in two color rendering varieties: Ra ca. 80 and ca. 90. The larger metal halide lamps are available in the color temperature range from approximately 4,000 to 6,000 K, with color rendering Ra in the range from 65 to more than 90. The special, daylight, metal halide lamp types for film studio and theater lighting have high color temperatures of between 6,000 and 8,000 K with color rendering values Ra between 65 and more than 90.
The lamp life of most metal halide types is somewhat shorter than that of other gas discharge lamps. This is because the electrodes are heated to a higher temperature with a correspondingly higher evaporation rate and are gradually worn out by chemical reactions with the metal halides. It has also been shown that different metal halide lamp types employ widely different construction methods. As a consequence, lamp life varies strongly with type. The economic life of compact versions lies between some 7,000 and 14,000 h (20 % mortality). The high-lumen-package versions vary from 4,000 h (single-envelope types for stadium floodlighting) to some 10,000 h (again 20 % mortality). Those ceramic metal halide lamps specifically developed for use in road lighting, where lights may be used 4,000 h a year, have lifetimes up to 20,000 h (20 % mortality).
Metal halide lamps have a higher lumen depreciation than most other discharge lamps. This is because of the higher degree of blackening from evaporated electrode material. Here too, the type of construction and sort of metal halides used play a role. Lumen depreciation values vary between some 20 % and 30 % (after 10,000 h). Some special types, such as those employed in road lighting, depreciate less rapidly (some 10 % after 10,000 h).
With the larger gas discharge tubes in particular, the burning position may affect the actual location of the various metals in the tube. This means that different burning positions may result in different color shifts. Also, with some types of lamp construction, the burning position can influence the lifetime of the lamp, for example, because of attack of the electrodes by some of the metals of the halides. Many metal halide lamps therefore have restrictions as to their permitted burning position (these are, of course, specified in their accompanying documentation). Compact, single-ended types, with either quartz or ceramic tube, have universal burning positions.
Run-Up and Reignition
The metal halides in the discharge tube need time to heat up, evaporate, and dissociate into metal and halide. During this process, which takes about 2–3 min, the light output and color gradually change until the final stable condition is reached. If there is an interruption in the power supply, medium- and high-wattage lamps will take approximately 10–20 min for the pressure in the lamp to decrease enough for it to reignite. Compact ceramic lamps reignite much faster: after some 3–5 min. Immediate reignition is only possible by applying a very high-voltage pulse (typically 60 kV). For this purpose, special, hot restrike, lamps are produced with special heavily insulated electrical contacts for applying this high-voltage pulse.
The dimming of metal halide lamps is difficult because with the resulting decrease in temperature, some of the metal halides condense, changing the constitution of the metal halides actually participating in the discharge. This in turn changes the color properties of the light. By employing a specially shaped burner, electronically driven lamp versions have been developed that can be dimmed to some 50 % without suffering from this problem.
Mains-voltage variations affect lumen output, lifetime, and light color. A mains voltage that deviates 10 % from its nominal value will result in perceivable color shifts. Thanks to the higher operating temperature of ceramic lamps, the effect on color change here is considerably smaller. Both up and down variations in the mains shorten lamp life.
Different metal halide lamp types
Single ended/double ended
Outer bulb/coated outer bulb/no outer bulb
Color temperature Tk
3,000–4,000 K/4,000–7,000 K
Color rendering index Ra
Ra >90 (900 series)/80< Ra <90 (800 series)/Ra <80
Standard (7,000–14,000 h)/long (up to 20,000 h)
No reflector/built-in reflector
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