Metal halide lamp
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. Originally created in the late 1960's 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.  They are also considered a "point" light source, so reflective luminaires are often required to concentrate the light for purposes of the lighting application.

# Uses
Metal-halide lamps are used both for general industrial purposes, and for very specific applications which require specific UV or blue-frequency light. They are used for indoor growing applications, because they can provide the spectrum and temperature of light which encourage general plant growth. They are most often used in athletic facilities. Metal Halide lights are also quite popular with reef aquarists, who need a high intensity light source for their corals.

# Operation
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 2000 F (1090 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.

# Components
Metal halide lamps consist of the following main components.  They have a metal base (in some cases they are double-ended) that allows an electrical connection. They are covered with an outer glass shield (or glass bulb) to protect the inner components and provide a shield to UV light generated by the mercury vapor. Inside the glass shield, a series of support and lead wires hold the inner fused quartz arc tube and its embedded tungsten electrodes. 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 and noble gas. The composition of the metals used defines the color of the lamp.
Instead of the quartz tube used in mercury vapour lamps, many metal halide types have an alumina arc tube similar to the high pressure sodium lamp. They are usually referred as ceramic metal halide or CMH.
Some bulbs have a phosphor coating on the inner side of the outer bulb to improve the spectrum and diffuse the light.

# Ballasts
Metal halide lamps require electrical ballasts to regulate the arc current flow and deliver the proper voltage to the arc. Probe start metal halide bulbs contain a special 'starting' electrode within the lamp 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 do not require a starting electrode, and instead use a special starting circuit referred to as an ignitor to generate a high-voltage pulse to the operating electrodes.  American National Standards Institute (ANSI) lamp-ballast system standards establish parameters for all metal halide components (with the exception of some newer products).
A few electronic ballasts are now available for metal halide lamps.  The benefit of these ballasts is more precise management of the lamp's wattage, which provides more consistent color and longer lamp life.  In some cases, electronic ballasts are reported to increase efficiency (i.e. reduce electrical usage).  However with few exceptions, high-frequency operation does not increase lamp efficiency as in the case of high-output (HO) or very high-output (VHO) fluorescent bulbs.  High frequency electronic operation does however allow for specially designed dimming metal halide ballast systems.

# Color temperature
Metal halide lamps were initially preferred to mercury vapor lamps in instances where natural light was desired because of the whiter light generated (mercury vapor lamps generating light that was much bluer).  However the distinction today is not as great.  Some metal halide lamps can deliver very clean "white" light that has a color-rendering index (CRI) in the 80's.  With the introduction of specialized metal halide mixtures, metal halide lamps are now available that can have a correlated color temperature as low as 3000 K (very yellow) to 20,000 K (very blue).  Some specialized lamps have been created specifically for the spectral absorption needs of plants (indoor gardening) or animals (indoor aquariums).  Perhaps the most important point to keep in mind is that, due to tolerances in the manufacturing process, color temperature can vary slightly from lamp to lamp, and the color properties of metal halide bulbs cannot be predicted with 100% accuracy.  Moreover, per ANSI standards the color specifications of metal halide bulbs are measured after the bulb has been burned for 100 hours (seasoned).  The color characteristics of a metal halide lamp will not conform to specifications until the bulb has been properly seasoned.  Color temperature variance is seen greatest in "probe start" technology lamps (300 kelvins).  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 it will have a lower physical temperature and its light output will be 'cooler' (more blue, or very similar to that of a mercury vapor lamp). This is because the lower arc temperature will not completely vaporize and ionize the halide salts which are primarily responsible for the warmer colors (reds, yellows), thus the more-readily ionized mercury will dominate the light output. This phenomenon is also 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 rupture due to overheating and overpressure.  Moreover, the color properties of metal halide lamps often change over the lifetime of the bulb. Often, in large installations of MH lamps, particularly of the quartz arc-tube variety, it will be seen that no two are exactly alike in color.

# 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 (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; a cool-down period of 5-10 minutes will be required before the lamp can be re-started. This is a major concern in some lighting applications where prolonged lighting interruption could create manufacturing shut-down or a safety issue. A few metal halide lamps are made with "instant restrike" capabilities that use a ballast with very high operating voltages (30,000 volts) to restart a hot lamp.

# End of life
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.
More-sophisticated 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.