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Hydrargyrum Medium-Arc Iodide, frequently called just HMI is a mercury-halide discharge short arc lamp with a color temperature of approximately 5600K. The name is derived from Hydrargyrum, an archaic term for mercury while Iodide indicates that iodine is the halogen used to form the active compound.

HMIs use ballasts, which can be either electronic or magnetic. The ballast provides the ignition pulse, and then acts as a choke, regulating current in order to maintain the arc.

History

In the late 1960s German television producers sought out lamp developer Osram to create a less expensive replacement for incandescent lights for the film industry. Osram developed and began producing HMI bulbs at their request.

Philips produced a variation on HMI, a single-ended version called MSR. It uses a standard two-prong lampbase. In order to avoid the colour shift over use they added a secondary envelope around the gas-chamber.

The last ten years a lot of research went in making HMI's and MSR's smaller, because of the use in moving light fixtures such as Vari-Lite, Martin and Highend. Philips' main contribution after this was the invention of new ceramic bases, which are capable of withstanding extreme heat. Osram developed a new shape to their bases and added gold-coating to it.

Today's use

Multi-kilowatt HMI lights are used in the film industry because of their optical spectrum, as well as their efficiency. The light they emit is close to that of noontime sunlight. The main use of 575 to 1200 Watt lights is in TV, theatre and entertainment, mainly because of size and the small arc-size, which is ideal for projections. Using proper insulation techniques these lights can be used for underwater lighting.

Efficiency

HMI bulbs are commonly two to four times as efficient in generating light than are their tungsten equivalents.

Color Temperature Loss

For every hour an HMI bulb is turned on, it loses about 1K. For example, if you use a fresh bulb and it starts at 5600K then you use it for four hours, it will actually be 5596K. Gradually HMI bulbs will start to lose color temperature and will appear warmer.

Safety

HMI bulbs are roughly the same color temperature as the sun, and like most other mercury-based high intensity discharge lamps, generate ultra-violet light. Each HMI light has a UV safety glass cover that should be used to protect any people that may be in front of the light. Exposure to an un-protected bulb can cause retina damage and severe skin burns.

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