How does Metal Halide lamp works?

A metal-halide lamp is an electric lamp that produces light by an electric arc through a gaseous mixture of vaporized mercury and metal halides (compounds of metals with bromine or iodine). It is a type of high-intensity discharge (HID) gas discharge lamp. Developed in the 1960s, they are similar to mercury vapor lamps, but contain additional metal halide compounds in the quartz arc tube, which improve the efficiency and color rendition of the light. The most common metal halide compound used is sodium iodide. Once the arc tube reaches its running temperature, the sodium dissociates from the iodine, adding orange and reds to the lamp's spectrum from the sodium D line as the metal ionizes. As a result, metal-halide lamps have high luminous efficiency of around 75 - 100 lumens per watt, which is about twice that of mercury vapor lights and 3 to 5 times that of incandescent lights and produce an intense white light. Lamp life is 6,000 to 15,000 hours. As one of the most efficient sources of high CRI white light, metal halides as of 2005 were the fastest growing segment of the lighting industry. They are used for wide area overhead lighting of commercial, industrial, and public spaces, such as parking lots, sports arenas, factories, and retail stores, as well as residential security lighting and automotive headlamps (xenon headlights).

The lamps consist of a small fused quartz or ceramic arc tube which contains the gases and the arc, enclosed inside a larger glass bulb which has a coating to filter out the ultraviolet light produced. They operate at a pressure between 4 to 20 atmospheres, and require special fixtures to operate safely, as well as an electrical ballast. Metal atoms produce most of the light output. They require a warm-up period of several minutes to reach full light output.

 

Components

Metal-halide lamps consist of an arc tube with electrodes, an outer bulb, and a base.

Arc tube

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.

Besides the mercury vapor, the lamp contains iodides or sometimes bromides of different metals. Scandium and sodium are used in some types, thallium, indium and sodium in European Tri-Salt models, and more recent types use dysprosium for highcolour temperature, tin for lower colour temperature. Holmium and thulium are used in some very high power movie lighting models. Gallium or lead is used in special high UV-A models for printing purposes. The mixture of the metals used defines the color of the lamp. Some types for festive or theatrical effect use almost pure iodides of thallium, for green lamps, and indium, for blue lamps. An alkali metal, (sodium or potassium), is almost always added to reduce the arc impedance, allowing the arc tube to be made sufficiently long and simple electrical 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. Argon filled lamps are typically quite slow to start up, taking several minutes to reach full light intensity; xenon fill as used in automotive headlamps has a much better start up time.

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 those used in the high-pressure sodium lamp. This development reduces the effects of ion creep that plagues fused silica arc tubes. During their life, sodium and other elements tends to migrate into the quartz tube, because of high UV radiation and gas ionization, resulting in depletion of light emitting material that causes 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.

The concept of adding metallic iodides for spectral modification (specifically: sodium - yellow, lithium - red, indium - blue, potassium and rubidium - deep red, and thallium - green) of a mercury arc discharge to create the first metal-halide lamp can be traced to patent US1025932 in 1912 byCharles Proteus Steinmetz, the "Wizard of General Electric".

The amount of mercury used has lessened over years of progress.

 

Outer bulb

Most types are fitted with an outer glass bulb to protect the inner components and prevent heat loss. The outer bulb can also be used to block some or all of the UV light generated by the mercury vapor discharge, and can be composed of specially doped "UV stop" fused silica. Ultraviolet protection is commonly employed in single ended (single base) models and double ended models that provide illumination for nearby human use. Some high powered models, particularly the lead-gallium UV printing models and models used for some types of sports stadium lighting do not have an outer bulb. The use of a bare arc tube can allow transmission of UV or precise positioning within the optical system of a luminaire. The cover glass of the luminaire can be used to block the UV, and can also protect people or equipment if the lamp should fail by exploding.

 

Base

Some types have an Edison screw metal base, for various power ratings between 10 to 18,000 watts. Other types are double-ended, as depicted above, with R7s-24 bases composed of ceramic, along with metal connections between the interior of the arc tube and the exterior. These are made of various alloys (such as iron-cobalt-nickel) that have a thermal coefficient of expansion that matches that of the arc tube.

 

Ballasts

The electric arc in metal-halide lamps, as in all gas discharge lamps has a negative resistance property; meaning that as the current through the bulb increases, the voltage across it decreases. If the bulb is powered from a constant voltage source such as directly from the AC wiring, the current will increase until the bulb destroys itself; therefore, halide bulbs require electrical ballasts to limit the arc's current. There are two types:

1. Inductive ballast - Many fixtures use an inductive ballast similar to those used with fluorescent lamps. This consists of an iron-core inductor. The inductor presents an impedance to AC current. If the current through the lamp increases, the inductor reduces the voltage to keep the current limited.
2. Electronic ballast - These are lighter and more compact. They consist of an electronic oscillator which generates a high frequency current to drive the lamp. Because they have lower resistive losses than an inductive ballast, they are more energy efficient. However, high-frequency operation does not increase lamp efficacy as for fluorescent lamps.

Pulse-start metal-halide bulbs don't contain a starting electrode which strikes the arc, and require an ignitor to generate a high-voltage (1–5 kV on cold strike, over 30 kV on hot restrike) pulse to start the arc. Electronic ballasts include the igniter circuit in one package. American National Standards Institute (ANSI) lamp-ballast system standards establish parameters for all metal-halide components (with the exception of some newer products).

 

Color temperature

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 in 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, because of the loweroperating temperature, its light output will be bluish because of 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. It is also very apparent with dimming ballasts. The inverse is true for an overpowered bulb, but this condition can be hazardous, leading possibly to arc-tube explosion because of overheating and overpressure.

 

Starting and warm up

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 restriking the arc; with a normal ignitor a cool-down period of 5–10 minutes will be required before the lamp can be restarted, but with special ignitors and specially designed lamps, the arc can be immediately re-established. On fixtures without instant restrike capability, a momentary loss of power can mean no light for several minutes. For safety reasons, many metal-halide fixtures have a backup tungsten-halogen incandescent lamp that operates during cool-down and restrike. Once the metal halide restrikes and warms up, the incandescent safety light is switched off. A warm lamp also tends to take more time to reach its full brightness than a lamp that 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, because of power consumption and heating of the passively cooled lamp ballast that is attempting to relight the lamp.

 

End of life behaviour

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 once again causes the arc to strike. This causes the lamp to glow for a while and then goes out, repeatedly. In rare occurrences 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

Although such failure is associated with end of life, an arc tube can fail at any time even when new, because of unseen manufacturing faults such as microscopic cracks. However, this is quite rare. Manufacturers typically "season" new lamps to check for such defects before the lamps leave the manufacturer's premises.All HID arc tubes deteriorate in strength over their lifetime because of various factors, such as chemical attack, thermal stress and mechanical vibration. As the lamp ages the arc tube becomes discoloured, absorbing light and getting hotter. The tube will continue to become weaker until it eventually fails, causing the breakup of the tube.

Since a metal-halide lamp contains gases at a significant high pressure (up to 50 psi), 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 (such as 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 "nonpassive failure" (explosion) of an arc tube is very small. According to information gathered by the National Electrical Manufacturers Association, there are approximately 40 million metal-halide systems in North America alone, and only a very few instances of nonpassive failures have occurred. Although it is impossible to predict or eliminate the risk of a metal-halide lamp exploding, there are several precautions that can 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 seven days of continuous operation.
• Relamping fixtures as a group. Spot relamping is not recommended.

Also, there are measures that can be taken to reduce the damage caused by a lamp failure violently:

• Ensuring that the fixture includes a piece of strengthened glass or polymeric materials between the lamp and the area it is illuminating. This can be incorporated into the bowl or lens assembly of the fixture.
• Using lamps that 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.

Lamps that require an enclosed fixture are rated "/E". Lamps that do not require an enclosed fixture are rated "/O" (for open). Sockets for "/O" rated fixtures are deeper. "/E" rated bulbs flare at the base, preventing them from fully screwing into a "/O" socket. "/O" bulbs are narrow at the base allowing them to fully screw in. "/O" bulbs will also fit in an "/E" fixture.