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.

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 efficiency than incandescent lighting units. The efficiency advantage is near fourfold, with approximately 85–108 lumens per watt of electricity. Unlike regular incandescent halogen lamps where a halide gas is used to regenerate the filament and keep the evaporated tungsten from darkening the glass, the mercury vapour and the metal halides in HMI lamps are what emit the light. The high color rendering index (CRI) and color temperature are due to the specific lamp chemistry.

In the late 1960s German television producers requested lamp developer OSRAM to create a safer and cleaner replacement for carbon arc lighting used by the film industry. Osram developed and began producing HMI lamps at their request.

Philips produced a variation on the HMI, a single-ended version called MSR (medium source rare-earth). It uses a standard two-prong lampbase. To avoid the colour shift during use they added a secondary envelope around the gas-chamber. Several other bulb variations exist, including GEMI (General Electric metal iodide), CID (compact iodide daylight; Thorn EMI, UK, since 1990 GE), CSI (compact source iodine; Thorn EMI, UK), DAYMAX (made by ILC), and BRITE ARC (|[Sylvania Lighting Inc. (SLI)]). All are variations on, and different names for, essentially the same concept.

Much research was invested into making HMI lamps smaller because of their use in moving light fixtures such as those manufactured by Vari-Lite, Martin, Robe, and Highend. Philips' main contribution after this was the invention of a phosphor coating on the weld of the filament to the molybdenum foil that reduces oxidization and early failures at that point, making that area capable of withstanding extreme heat.

Multi-kilowatt HMI lights are used in the film industry and for large-screen slide projection because of their daylight-balanced light output, as well as their efficiency.

Similar to fluorescent lights, HMIs present problems with color temperature when used for film or video lighting applications. Unlike incandescent-lighting units, which are blackbody radiators limited to a theoretical maximum of 3680 K (the melting point of tungsten), HMI lamps, like all gas discharge lighting, emit the emission spectral lines of its constituent elements, specifically chosen so that combined, they resemble the blackbody spectrum of a 6000 K source. This closely matches the color of sunlight (but not skylight), because the Sun's surface is a 6000 K blackbody radiator.

With HMI bulbs, color temperature varies significantly with lamp age. A new bulb generally will output at a color temperature close to 15,000 K during its first few hours. As the bulb ages, the color temperature reaches its nominal value of around 5600 K or 6000 K. With age, the arc length becomes larger as more of the electrodes burn away. This requires greater voltage to sustain the arc, and as voltage increases, color temperature decreases proportionately at a rate of approximately 0.5–1 kelvin for every hour burnt. For this reason, and other safety reasons, HMI bulbs are not recommended to be used past half their lifetime.^{[citation needed]}