BIRNS HMI/MSR-- Frequently Asked Questions
    What is “HID”?
    What is “HMI”?
    What is a “Metal Halide” lamp, and why is it better?
    How does a Metal Halide lamp work?
    What are the metallic halides used in a Metal Halide lamp?
    What function does the ballast serve?


  1. What is “HID”?

    “HID” stands for “High Intensity Discharge”. HID lamps do not have an incandescent filament; instead, they have a gas-filled arc tube, in which light is produced by the passage of an electric current through the gas. HID lamps include the groups of lamps commonly known as Mercury-Vapor, Metal Halide, and High-Pressure Sodium Vapor lamps.


  2. What is “HMI”?

    “HMI” is a registered trademark of Osram/Sylvania. “HMI” is Osram/Sylvania’s brand of Metal Halide lamp.
  3. What is a “Metal Halide” lamp, and why is it better?

    A Metal Halide lamp is essentially a type of mercury-vapor lamp, but also contains various metallic halides in addition to mercury and argon in the arc tube. This produces better color rendition and higher lamp efficacy. In some cases, color rendition can be functionally equivalent to “daylight”, and lamp efficacy can be between 75 and 100 lumens per watt (excluding ballast losses).
  4. How does a Metal Halide lamp work?

    When the lamp has warmed up and attained its full operating temperature, the metal halides in the arc tube are partially vaporized. When the halide vapors approach the high-temperature core of the discharge, they are dissociated into the halogen and the metals, and the metals radiate their characteristic spectrally selective wavelengths (colors). Then, as the halogen and metal atoms move near the cooler arc tube wall, they chemically recombine, and the cycle begins again.

    By careful selection blending of the types and quantities of metallic halides used, the color of the light output can be optimized for the application.
  5. What are the metallic halides used in a Metal Halide lamp?

    Two typical “blends” of metallic halides used in Metal Halide lamps are:

    1. Scandium (Sc) and sodium (Na) iodides

    2. Dysprosium, Holmium and Thulium Rare-Earth (RE) iodides (better color dopants which improve performance but which are more costly)

  6. What function does the ballast serve?

    The ballast incorporates an electronic starter circuit working in conjunction with a magnetic component; together, they serve a starting function for the lamp. The starter supplies a high-voltage, high-frequency pulse on each cycle or half cycle of the supply voltage to ionize the starting gas and initiate the starting sequence of the lamp.