How a metal halide lamp works. Osram HQI-E 150 W/NDL CL
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2 How a metal halide lamp works
Similar to high-pressure mercury lamps or high-pressure sodium lamps, metal halide lamps also belong to the group of discharge lamps. Low pressure discharge lamps include fluorescent lamps and compact fluorescent lamps.
Contact plate UV
-Filter
Quter bulb
(Quartz) c ge ar c tube
Molybdenum foil
Ar
(Quartz)
Dischar Heat r eflector
Lead-in wir e
In discharge lamps, light is generated by a gas discharge of particles created between two hermetically sealed electrodes in an arc tube. After ignition, the particles in the arc are partially ionized, making them electrically conductive, and a “plasma” is created. In high intensity discharge lamps, the arc tube is usually enclosed in an evacuated outer bulb which isolates the hot arc tube thermally from the surroundings, similar to the principle of a thermos flask. But there are also some discharge lamps without outer bulbs, as well as lamps with gas-filled outer bulbs. In contrast to lowpressure discharge, there is high pressure and a high temperature in a discharge tube.
In an arc tube, gas discharge works through excitation of the luminous additives (metal halide salts) and the mercury is excited by the current flow. Visible radiation characteristic for the respective elements is emitted. The mixture of the visible radiation of the different elements results in the designed colour temperature
Getter
Metal halides
Mer cury
Electr ode
Fig. 1: An example of how a metal halide lamp works based on a double-ended lamp with a quartz arc tube.
and colour rendering for a particular lamp. In the operating state, the mercury evaporates completely. The other elements involved are present in saturated form at the given temperatures, i.e. they only evaporate in part; the rest is in liquid form at the coolest point in the arc tube. The fraction of the filling that has evaporated depends on the temperature of the coolest point on the arc tube wall and also varies for the different filling components. Changes to the temperature of the arc tube wall can change the composition of the metal halides in the discharge, thus also changing the colour properties of the lamp.
Hg = Mercury
Me = Metals
Hal = Halids
Outer bulb
Electrodes
Arc tube Mo-Foils ignitor ballast
Alternative: ECG supply voltage
Fig. 2: Tasks of the metals [sodium (Na), thallium (TI), indium (In), tin (Sn), lithium (Li), rare earths; dysprosium (Dy), holmium (Ho), thulium (Tm)]
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