HMI (Hydrargyrum Medium-Arc Iodide) is a type of light which uses an arc lamp instead of an incandescent bulb to produce light.
HMI lights are high-quality and correspondingly expensive. They are popular with film and television production companies but their price puts them out of reach of those with modest budgets.
HMI lights require a ballast, an electronic (or magnetic) device which provides the ignition pulse and regulates the arc.
Advantages of HMI lights include:
- Efficiency: 2 to 5 times as efficient as incandescents. This means they use less power and run cooler.
- Colour temperature: HMIs run at around 5600K, daylight temperature. This makes daylight shooting easier, as well as eliminating loss of light from gels (which are necessary with incandescents).
- Light Quality: Directors of photography speak highly of the light produced by HMIs.