Metal halide lamps, a member of the high-intensity discharge (HID) family of lamps, produce high light output for their size, making them a compact, powerful, and efficient light source. By adding rare earth metal salts to the mercury vapor lamp, improved luminous efficacy and light color is obtained. Originally created in the late 1960s for industrial use, metal halide lamps are now available in numerous sizes and configurations for commercial and residential applications. Like most HID lamps, metal halide lamps operate under high pressure and temperature, and require special fixtures to operate safely.
Since the lamp is small compared to a fluorescent or incandescent lamp of the same light level, relatively small reflective luminaires can be used to direct the light for different applications (flood lighting outdoors, or lighting for warehouses or industrial buildings).
Uses Metal halide lamps are used both for general lighting purposes, and for very specific applications which require specific UV or blue-frequency light.
Due to their wide spectrum, they are used for indoor growing applications, in athletic facilities and are quite popular with reef aquarists, who need a high intensity light source for their corals.
Another widespread use for such lamps is in professional lighting fixtures, where they are commonly known as MSD lamps and are generally used in 150, 250, 400, 575 and 1,200 watt ratings, especially intelligent lighting.
Most LCD, DLP, and film projectors use metal halide lamps as their light source.
Like other gas-discharge lamps such as the very-similar mercury-vapor lamps, metal halide lamps produce light by passing an electric arc through a mixture of gases. In a metal halide lamp, the compact arc tube contains a high-pressure mixture of argon, mercury, and a variety of metal halides. The mixture of halides will affect the nature of light produced, influencing the correlated color temperature and intensity (making the light bluer, or redder, for example). The argon gas in the lamp is easily ionized, and facilitates striking the arc across the two electrodes when voltage is first applied to the lamp. The heat generated by the arc then vaporizes the mercury and metal halides, which produce light as the temperature and pressure increases.
Common operating conditions inside the arc tube are 70-90 psi (480-620 kPa) and 2,000 degrees F (1,090 degrees C). Like all other gas discharge lamps, metal halide lamps require auxiliary equipment to provide proper starting and operating voltages and regulate the current flow in the lamp. About 24% of the energy used by metal halide lamps produces light (65-115 lm/W), making them generally more efficient than fluorescent lamps, and substantially more efficient than incandescent bulbs.
-Metal halide lamps consist of an arc tube with electrodes, an outer bulb, and a base.
-Arc tube -Besides the mercury vapor, the lamp contains iodides or sometimes bromides of different metals, (scandium and sodium in some types, thallium, indium and sodium in European Tri-Salt models, and more recent types using dysprosium for high colour temperature, tin for lower color temperature, holmium and thulium in some very high power rated movie lighting models and gallium and/or lead in special high U.V.-A models for printing purposes). The mixture of the metals used defines the color of the lamp, and some types for festive or theatrical effect use almost pure iodides of thallium, for green lamps, and indium, for blue lamps. An alkali metal, usually sodium and sometimes potassium, is almost always added to reduce the arc impedance, allowing the arc tube to be made sufficiently long and simple control gear, (Ballasts), to be used.
A noble gas, usually argon, is cold filled into the arc tube at a pressure of about 2 kPa to facilitate starting of the discharge. The ends of the arc tube are often externally coated with white infrared reflective zirconium silicate or zirconium oxide to reflect heat back onto the electrodes to keep them hot and thermionically emitting. Some bulbs have a phosphor coating on the inner side of the outer bulb to improve the spectrum and diffuse the light.
In the mid-1980s a new type of metal halide lamp was developed, which, instead of a quartz (fused silica) arc tube as used in mercury vapor lamps and previous metal halide lamp designs, use a sintered alumina arc tube similar to what has been used in the high pressure sodium lamp. This development reduces the effects of ion creep which plagues fused silica arc tubes. During their life, due to high UV radiation and gas ionization, sodium and other elements tends to migrate into the quartz tube, resulting in depletion of light emitting material and so, cycling. The sintered alumina arc tube does not allow the ions to creep through, maintaining a more constant colour over the life of the lamp. These are usually referred as ceramic metal halide lamps or CMH lamps.
Most types are fitted with an outer glass bulb to protect the inner components, support frame and arc tube from oxidation, heat loss and provide a shield to prevent the short wavelength UV light generated by the mercury vapor discharge, which is transmitted by the fused silica inner bulb or arc tube from escaping as it is blocked by the soda or borosilicate glass bulbs used on older single ended, (single base) models or by specially doped “U.V. stop” fused silica outer bulbs of more contemporary single ended and most double ended models. Inside the fused quartz arc tube two tungsten electrodes doped with thorium, are sealed into each end and current is passed to them by molybdenum foil seals in the fused silica. It is within the arc tube that the light is actually created.
Some high powered models, particularly the Lead-Gallium U.V. printing models and models used for some types of sports stadia lighting do not possess an outer bulb and consist of just the bare arc tube, this allows passage of U.V. or precise positioning within the optical system of a luminaire, the cover glass of which blocks the U.V. and protects people below, should the lamp explosively fail.
Some types have an Edison screw metal base, for various power ratings between 10 and 18,000 watts. Other types are double-ended with R7s-24 bases composed of ceramic and various FerNiCo iron-cobalt-nickel alloys that allow an electrical connection.
Metal halide lamps require electrical ballasts to regulate the arc current and deliver the proper voltage to the arc. Like high-pressure mercury vapor lamps, some metal halide bulbs contain a third electrode to initiate the arc when the lamp is first lit (which generates a slight flicker when the lamp is first turned on). Pulse-start metal halide lamps don’t contain a starting electrode, but they require an ignitor to generate a high-voltage (1-5 kV on cold strike, over 30 kV on hot re-strike) pulse to start the arc. American National Standards Institute (ANSI) lamp-ballast system standards establish parameters for all metal halide components (with the exception of some newer products).
Electronic ballasts include ignitor and ballast into a single package. These ballasts use high-frequency to drive the lamps. Because they have less loss than a line-frequency “iron” ballast, they are more energy efficient. High-frequency operation does not increase lamp efficacy as for fluorescent lamps.
Output spectrum of a typical Metal Halide lamp.
Because of the whiter and more natural light generated, metal halide lamps were initially preferred to the bluish mercury vapor lamps. With the introduction of specialized metal halide mixtures, metal halide lamps are now available with a correlated color temperature from 3,000 K to over 20,000 K. Color temperature can vary slightly from lamp to lamp, and this effect is noticeable places where many lamps are used. Because the lamp’s color characteristics tend to change during lamp’s life, color is measured after the bulb has been burned for 100 hours (seasoned) according to ANSI standards. Newer metal halide technology, referred to as “pulse start,” has improved color rendering and a more controlled kelvin variance (+/-100 to 200 kelvins).
The color temperature of a metal halide lamp can also be affected by the electrical characteristics of the electrical system powering the bulb and manufacturing variances in the bulb itself. If a metal halide bulb is underpowered, due to the lower operating temperature, its light output will be bluish due the evaporation of mercury alone. This phenomenon can be seen during warmup, when the arc tube has not yet reached full operating temperature and the halides have not fully vaporized. The inverse is true for an overpowered bulb, but this condition can be hazardous, leading possibly to arc-tube explosion due to overheating and overpressure.
A “cold” (below operating temperature) metal halide lamp cannot immediately begin producing its full light capacity because the temperature and pressure in the inner arc chamber require time to reach full operating levels. Starting the initial argon arc sometimes takes a few seconds, and the warm up period can be as long as five minutes (depending upon lamp type). During this time the lamp exhibits different colors as the various metal halides vaporize in the arc chamber.
If power is interrupted, even briefly, the lamp’s arc will extinguish, and the high pressure that exists in the hot arc tube will prevent re-striking the arc; with a normal ignitor a cool-down period of 5-10 minutes will be required before the lamp can be re-started, but with special ignitors with specially designed lamps, the arc can be immediately re-established. A warm lamp also tends to take more time to reach its full brightness than a lamp which is started completely cold.
Most hanging ceiling lamps tend to be passively cooled, with a combined ballast and lamp fixture; immediately restoring power to a hot lamp before it has re-struck can make it take even longer to relight, due to power consumption and heating of the passively cooled lamp ballast that is attempting to relight the lamp.
End of life behavior
At the end of life, metal halide lamps exhibit a phenomenon known as cycling. These lamps can be started at a relatively low voltage but as they heat up during operation, the internal gas pressure within the arc tube rises and more and more voltage is required to maintain the arc discharge. As a lamp gets older, the maintaining voltage for the arc eventually rises to exceed the voltage provided by the electrical ballast. As the lamp heats to this point, the arc fails and the lamp goes out. Eventually, with the arc extinguished, the lamp cools down again, the gas pressure in the arc tube is reduced, and the ballast can once again cause the arc to strike. The effect of this is that the lamp glows for a while and then goes out, repeatedly. In rare occurrence the lamp explodes at the end of its useful life.
Modern electronic ballast designs detect cycling and give up attempting to start the lamp after a few cycles. If power is removed and reapplied, the ballast will make a new series of startup attempts.
Risk of lamp explosion
All HID arc tubes deteriorate in strength over their lifetime due to various factors, such as chemical attack, thermal stress and mechanical vibration. As the lamp ages the arc tube becomes discolored, absorbing light and getting hotter. The tube will continue to become weaker until it eventually fails, causing the break up of the tube.
Although such failure is associated with end of life, an arc tube can fail at any time even when new, due to unseen manufacturing faults such as microscopic cracks. However, this is quite rare. Manufacturers typically “season” new lamps to check for manufacturing defects before the lamps leave the manufacturer’s premises.
Since a metal halide lamp contains gases at a significant high pressure, failure of the arc tube, is inevitably a violent event. Fragments of arc tube are launched, at high velocity, in all directions, striking the outer bulb of the lamp with enough force to cause it to break. If the fixture has no secondary containment (e.g. a lens, bowl or shield) then the extremely hot pieces of debris will fall down onto people and property below the light, likely resulting in serious injury, damage, and possibly causing a major building fire if flammable material is present.
The risk of a “non-passive failure” of an arc tube is very small. According to information gathered by the National Electrical Manufacturers Association (www.nema.org), there are approximately 40 million metal halide systems in North American alone, and only a very few instances of non-passive failures have occurred. Although it is not possible to predict, or eliminate the risk, of a metal halide lamp exploding, there are several precautions which can be taken to reduce the risk:
-Using only well designed lamps from reputable manufacturers and avoiding lamps of unknown origin.
-Inspecting lamps before installing to check for any faults such as cracks in the tube or outer bulb.
-Replacing lamps before they reach their end of life (i.e. when they have been burning for the number of hours that the manufacturer has stated as the lamp’s rated life).
-For continuously operating lamps, allowing a 15 minute shutdown for every 7 days of continuous operation.
-Re-lamp fixtures as a group. Spot re-lamping is not recommended.
Also, there are measures that can be taken to reduce the damage caused should a lamp fail violently:
-Ensuring that the fixture includes a piece of strengthened glass or polymeric materials between the lamp and the area it is illuminating. This could be incorporated into the bowl or lens assembly of the fixture.
-Using lamps which have a reinforced glass shield around the arc tube to absorb the impact of flying arc tube debris, preventing it from shattering the outer bulb. Such lamps are safe to use in ‘open’ fixtures. These lamps carry an “O” designation on the packaging reflective of American National Standards Institute (ANSI) standards.