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For those unfamiliar with HMI instruments, the letters stand for Hydrargyrum Medium-arc Iodide. That explains everything, right?
An HMI lamp uses mercury vapor mixed with metal halides in a quartz-glass envelope with two tungsten electrodes of medium arc separation. Still not sure what an HMI actually is? Yeah. I wouldn't be either. Especially, if I'd never worked with HMIs. (Which I have, many times, but mostly for video production.)
In a nutshell, an HMI is an arc light, one that requires a ballast to ignite it and keep it lit. (HMIs need extra voltage to operate, hence the ballast.) HMIs throw bright, intense, powerful light with the color temperature of sunlight. (About 6000K.) HMIs are often used for daylight film and video production since the light they produce successfully competes with daylight in terms of brightness and color temp.
Most HMI instruments come with a selection of lenses to mount in front of their lamps, you know, like a Fresnel lens as an example. Often, the lenses are used to control the spread of the light. Besides their use in daylight, HMIs are also used for interior lighting along with other, more common, continuous lighting instruments like tungsten lamps. When they are mixed in with tungsten, they often need to be converted to a much lower color temp via a filter placed in front of the lamp. (Unless, of course, daylight color temp is what you want, even in an interior location.) HMIs would likely be used more often by photographers if they weren't so expensive. A good HMI can cost quite a few thousands of dollars.
The photo at the top is one I captured with an HMI. There was also plenty of sunshine coming through the large bank of windows. (Which the HMI had no problem being equal to or brighter than.) I also employed a couple of reflectors to exploit the natural light. The image below features the HMI as well as the reflectors. While many of you may never work with an HMI, and it's certainly true that, for my photography, I do so only rarely, I still think it's always good to learn about other ways to light even if the gear likely won't end up part of our production work-flows.
FYI: If the reflector to the right of the model is one you're unfamiliar with, in gaffer parlance it's called a "shiny board." It's an efficient reflector (in terms of the amount and intensity of light it reflects) producing fairly hard specular light.
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