Hydrargyrum medium-arc iodide lamp
Hydrargyrum medium-arc iodide (HMI) is the trademark name of Osram's brand of metal-halide gas discharge medium arc-length lamp, made specifically for film and entertainment applications. Hydrargyrum comes from the Greek name for the element mercury.
Source: Wikipedia — Hydrargyrum medium-arc iodide lamp (CC BY-SA 4.0)