Metal halide lamps are a type of high pressure discharge lamps, whose color temperature is close to sunlight (generally between 4500°K and 6000°K), with a luminous emission of approximately 100 lumen/watt, which makes them a good choice for lighting buildings, parks or stadiums. 
Such lamps produce a very good luminous emission for their size, being a compact and efficient light source. They are particularly used in outdoor lighting, particularly of roads parks and gardens.

A widespread use of these lamps is inside professional lighting fixtures, where they are available in the range of 150W 250W 575W 1200W and 2500W. Being discharge lamps, they need an auxiliary electronic circuit for regulating the electric flow inside the bulb and for passing the correct voltage to the electrodes. The discharge tube is made of quartz, in order to withstand the high pressure and temperature, and is inserted in an empty bulb of glass. The color temperature is rather cold.

According to the manufacturer, metal halide lamps are known by several acronyms:
•    HMI lamps (Hydrargyrum medium-arc iodide), trademark by OSRAM, contain a mixture of mercury vapours and metal iodide in a quartz glass tube, with two tungsten-coated electrodes at the extremities which produce a voltaic arc that excites mercury vapors under pressure, producing a significant light emission, with greater efficiency than the traditional light bulbs. Since HMI lamps contain mercury vapours, they emit a significant amount of UV rays mixed with light.
•    Philips produces the MSD and MSR variants (medium source rare earth) that contain the rare earths chemical elements under gaseous form. Besides mercury vapors, the discharge tube contain also thallium iodine and rare earths like holmium, dysprosium…. Such substances participate actively with mercury vapors to the discharge, with a significant increase of luminous efficiency.
•    from the mid-80s, a new type of metal halide lamp has been developed (ceramic metal halide lamps CMH or ceramic discharge metal halide lamps CDM) that, instead of a quartz glass bulb, are equipped with a ceramic bulb that reduces the ion spillage from the bulb. Before that, the UV radiations caused by the mercury vapors and the gases’ ionization used to migrate in the quartz bulb, depleting the bulb’s content, which in turn reduced the light emission. The ceramic bulb does not allow the ions’ leaking, maintaining a constant light color along the lamp’s life.