What is Metal Halide Lamp?
Metal halide lamp is special type of arc discharge lamp that works on the arc stream via some iodide salts along with argon gas and mercury vapor pressure at several millimeters with the arc tube temperature of 1000 K.
Who Invented Metal Halide Lamp?
Dr. Reiling had discovered Metal Halide Lamp in 1960.
What is the Constructional Feature of a Metal Halide Lamp?
Metal halide lamp consists of
- Glass bulb
- Arc tube
- Auxiliary electrode with high resistance
- Glass stem
- Molybdenum wire
- Argon gas
- Mercury vapor
- Indium, thallium and sodium iodides
Schematic Diagram of Metal Halide Lamp
Schematic diagram of a metal halide lamp is shown below.
How does a Metal Halide Lamp Work?
- When full voltage is applied across the main electrodes, no arc is produced at the time of switching.
- The auxiliary electrode or starter electrode near the main electrodes attached to the glass stem creates initial discharge between them.
- A bimetal switch is there to short the starter electrode to the main electrode just at the time of starting.
- Starter electrode is used to create initial arc between main and auxiliary electrode that heats up the metal halide salts.
- Starter electrode or auxiliary electrode is of high resistance to limit the current at initial arc.
- Again discharge is first in argon and then in mercury.
- Small amount of mercury vapor helps to establish main arc formation between main electrodes through metal halides vapor one by one.
- To reach up to full light output this lamp takes 5 minutes.
How arc Inside the Metal Halide Arc Tube is Established?
In the OFF condition of the lamp, metal iodides i.e. indium, thallium and sodium iodides used inside the lamp are present on the bulb wall.
Due to rising of arc temperature, the metal iodides vaporize and diffuse from wall into the arc stream. Then they dissociate and yield free metal and iodine atoms.
Almost like this fashion, the mercury atoms inside the bulb are excited and ionized.
Generally all iodide salts do not get vaporized at same time in metal halide lamp. So step wise, the process of vaporization is
- First argon gas and then mercury get vaporized to form arc.
- Only indium gets vaporized first to form a blue sheath around the mercury arc.
- Then thallium gets vaporized and it forms yellow sheath around the thallium.
- Finally sodium iodides get vaporized and it makes the lamp very sensitive to changes in the lamp wattage.
Lamp will be deficient in yellow and red if the lamp watts are lower than rated value. It is because of very small amount of sodium gets vaporized.
Again in case of sodium-scandium metal halide lamps, scandium vaporizes first and then sodium.
What are the Basic Requirements of Metal Halides Inside the Arc Tube?
The basic requirements of the metal additive are mentioned below:
- The iodide vapor pressure must be relatively high enough.
- The energy level configuration of the metal must be robust to encourage a high percentage of the visible radiation.
- At the bulb wall operating temperature of the iodide of the metal must be stable.
- The excitation level of the metal atoms must be lower than the average of the excitation level of mercury (about 7.8 eV).
Thallium has a strong spectral line at 535 nm and it requires only 3.3 eV for its arc generation. Its iodide salt creates vapor pressure at about 10 mm at 800 K on the arc tube.
Why Metal Halide Lamp is with Frameless Construction?
This lamp has frameless construction. Molybdenum is used as outer electric wire and it is non magnetic. The reason is ac current would flow through the frame and create pulsating magnetic field. So this would cause the electron to leave the frame and would attract to combine with the sodium ions which would migrate through the arc tube wall. It would cause serious depletion of the sodium from the arc stream.
Why is there Reflective Coating at both ends of Metal Halide Arc Tube?
The metal halide lamp has an arc tube whose ends are taken to be treated with a reflective white coating that is used to redirect the energy back into the tube. A uniform temperature can be maintained over entire arc tube length due to short length.