Hydrargyrum medium-arc iodide, or HMI, is a Osram brand metal-halide gas discharge medium arc-length lamp manufactured for the film and entertainment industry. Hydrargyrum is Latin for mercury (Hg). The term HMI has become a genericized trademark for all similar high-quality metal-halide lamps made for film and entertainment, regardless of manufacturer.
An HMI lamp uses mercury vapour mixed with metal halides in a quartz-glass envelope, with two tungsten electrodes of medium arc separation. Unlike traditional lighting units using incandescent light bulbs, HMIs need electrical ballasts, which are separated from the head via a header cable, to limit current and supply the proper voltage. The lamp operates by creating an electrical arc between two electrodes within the bulb that excites the pressurized mercury vapour and metal halides, and provides very high light output with greater efficacy than incandescent lighting units. The efficiency advantage is near fourfold, with approximately 85–108 lumens per watt of electricity. Unlike tungsten-halogen lamps where the halide gas is used to regenerate the filament and keep the evaporated tungsten from darkening, the mercury vapour and the metal halides in HMI lamps are what emit the light. The high CRI and color temperature are due to the specific lamp chemistries.