The dicharge bulbs or HMI, compared to the Incandescent bulbs, the light is generated through a dicharge arch. Special gas fill the cruet joining a color temperature between 5600K and 6000K. The HMI bulbs contains the right combination of haloids of different metals and rare earths like dysprosium, Omio and Thulium, in addition to mercury vapors. From these components was born its name: H is the abbreviation for mercury (Hg), that is mainly used to obtain bulb power that has to be high enough. M is for the metal presence that give the typical effect of the daylight. I is for the addition of a mixed halogenated components (ioduri and bromides) necessary for different functions. The known halogen cyclic process - tungsten in the incandescent and halogens bulbs prevents the tungsten particles, blowing away from the electrodes, will deposit on the inner wall of the bulb. Other cyclic processes make sure that in the hot zone of the arc a high concentration of metals vapors and rare earths will be reached, to produce light.

The structure
The HMI lamp is made in high temperature resistant quartz. Two tungsten electrodes are positioned axially inside a discharge tube that has a cylindrical or ellipsoidal shape. The electricity flow happens through molybdenum sheets very thin, dipped in quartz inside the bulb to keep the gas and they are the electrical link between the base and the discharge tube electrodes.

Light beam and light efficiency
The HMI bulbs offer a light efficiency 3 - 4 times superior then Halogen bulbs of the same power. They require an UV absorbing glass, that come from the lens of Fresnel projectors or from the protection lens of some bulbs included in parabolic systems.

Technical features
To start the discharge process in the gas of these bulbs, a high power ignition is necessary. In this phase the start up will require one second maximum and will be sufficient high power impulses but low Kv. In case of hot restart on high power impulse lighters are necessary to ionize the zone between the electrodes: from 20 to 70Kv are required depending on the bulb power type. Once the cold restart is on, the filling materials (mercury, haloids, rare earth) evaporate in 1 - 3 minutes. At the same time gradually increase the bulb power, the electrical power and the luminous flow until the nominal value will be reached. Color temperature and bulb wattage are higher at the beginning compared to the time while the bulb joined it's light stability. The color temperature changes, are very relevant at the beginning until the bulb will join the correct one, and they may cause a trick for uncareful users. The use of a Thermocolorimeter can be useful to measure possible differences between lamps new and just used, if mounted on the same projectors.