It may be true that there's an overabundance of
acronyms in
this modern world, and the culture of filmmaking is (second to the military... maybe third to corporate
bureaucracy) responsible for a large share.
However! This is an instance in which I'm inclined to be sympathetic. Imagine the following exchange:
Gaffer Hey, Sal! I need two 5K Hydrargyrum Medium Arc-Length Iodide lights pounded through some opal over that window, there.
Electric You got it, Kauffman! (turns to a junior electric) Rick! Go wrangle me two 5K Hydrargyrum Medium Arc-length Iodides from the truck and a couple a' stingers!
Yes. The HMI is a lighting instrument for film and video production, with HMI standing for Hydrargyrum Medium Arc-length Iodide. Hydrargyrum, for those of you deficient in the classics, is Greek for the element Mercury. So what we're talking about here is a metal-halide discharge lamp; a bulb in which an electrical arc is passed through a high-pressure atmosphere of mercury/iodine vapor.
The HMI is used chiefly to simulate daylight, as the light produced has approximately the same color temperature (approx. 5600-6000K) as sunlight from a blue sky. This is useful for filling shadows on sunny days, adding extra light to interiors that are theoretically being lit by daylight, or shooting day for night (exterior or interior).
This type of light operates at a very high voltage (in the tens of thousands of volts), and to make things more interesting the voltage required to maintain the electrical arc (which creates the light) changes over time due to bulb age and/or operating temperature. A ballast is required to provide power to the instrument. The ballast is essentially a step-up transformer and a power conditioner which receives power from a generator (or other source) and feeds it to the lamp.
Starting up the light is a two stage process. The ballast must first be turned on, and then the light itself. An HMI light will emit a horrifying electrical grinding sound as it is being fired, and then the bulb will begin to glow. Intensity builds over time, and the light will be at full output in a few minutes.
HMIs are fantastically more power efficient than incandescent lights of the same output power, which can be between 120 and 12,000 watts (the range of bulbs made by OSRAM).