Metal-halide lamp should not be confused with Halogen lamp
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. It is a type
of high-intensity discharge 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.
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 atms, 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. Uses
Metal-halide lamps are used both for general lighting purposes both indoors
and outdoors, automotive and specialty applications. Because of 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. Metal-halide lamps are used in
automobile headlights, where they are commonly known as "xenon headlamps" due
to the use of xenon gas in the bulb instead of the argon typically used in
other halide lamps. They produce a more intense light than incandescent
headlights. Another widespread use for such lamps is
in photographic lighting and stage 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.
Operation Like other gas-discharge lamps such as
the very-similar mercury-vapor lamps, metal-halide lamps produce light by
making an electric arc in a mixture of gases. In a metal-halide lamp, the
compact arc tube contains a high-pressure mixture of argon or xenon,
mercury, and a variety of metal halides, such as sodium iodide and scandium
iodide,. The particular mixture of halides influences the correlated color
temperature and intensity. The argon gas in the lamp is easily ionized, which
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 5–50 atm or more and 1000–3000 °C. Like all other
gas-discharge lamps, metal-halide lamps have negative resistance, and with the
rare exception of self-ballasted lamps with a filament, require a ballast 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, making them substantially more efficient than incandescent bulbs, which
typically have efficiencies in the range 2–4%.
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
high colour 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,, 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 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 of a mercury
arc discharge to create the first metal-halide lamp can be traced to
patent US1025932 in 1912 by Charles 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 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 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: 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. 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 pulse to start the arc. Electronic ballasts
include the igniter circuit in one package. American National Standards
Institute lamp-ballast system standards establish parameters for all
metal-halide components. Self-ballasted lamps
As of 2012 several companies started to offer self-ballasted metal-halide lamps
as a direct replacement for incandescent and self-ballasted mercury-vapor lamps.
These lamps include an arc tube with a starting electrode as well as a tubular
halogen lamp which is connected in series and used to regulate the current
in the arc tube. A resistor provides the current limiting for the starting
electrode. Like self-ballasted mercury-vapor lamps, self-ballasted
metal-halide lamps are connected directly to mains power and do not
require an external ballast. In contrast to the former, these lamps usually have
a clear outer bulb without a coating, making the arc tube and the halogen lamp
tube clearly visible from the outside. 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 according to ANSI standards. Newer
metal-halide technology, referred to as "pulse start," has improved color
rendering and a more controlled kelvin variance.
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 lower operating 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" 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. 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= 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.
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. 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 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" 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.
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 standards. Lamps that require an enclosed fixture
are rated "/E". Lamps that do not require an enclosed fixture are rated
"/O". 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. Other safety concerns
= Eyes= Broken and unshielded high-intensity
metal-halide bulbs are known to cause eye and skin injuries, particularly in
school gymnasiums. ANSI ballast codes
See also Hydrargyrum medium-arc iodide lamp, high
power MH lamps as used in cinematography High-intensity discharge lamp
Arc lamp Sodium-vapor lamp
Mercury-vapor lamp Sulfur lamp
Neon lamp History of street lighting in the United
States References
Waymouth, John. Electric Discharge Lamps. Cambridge, MA: The M.I.T. Press.
ISBN 0-262-23048-8. Further reading
Raymond Kane, Heinz Sell Revolution in lamps: a chronicle of 50 years of
progress, The Fairmont Press, Inc. 2001 ISBN 0-88173-378-4