I know what you're thinking. LED and fluorescent, okay, I can see them as cool light sources. But HMI? Get real! Have you ever stepped close to a 24,000W HMI? Granted, a light of that magnitude does generate considerable heat, but we're looking at "cool" here in terms of bang for the watts—energy and operating cost-efficiency relative to light output, as well as heat projection onto a set, not actual surface temperatures. And watt for watt (or, more precisely, in terms of lumens per watt, or luminous efficacy), an HMI is more efficient than tungsten, which is certainly true of LED and fluorescent. In fact, these light sources generate roughly three or more times the amount of lumens per watt compared with a tungsten bulb of the same wattage, hence the moniker "cool lighting." Any way you look at it, these light sources increasingly come into play today in both still digital capture and HD DSLR full-motion video. In the future, we'll likely see LEP (light-emitting plasma) and OLED (organic light-emitting diode) as serious contenders.
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One thing is certain—the good and bad points, as well as applications, surrounding each lamp type are open to argument. So don't be surprised when you read contradictory statements. In fact, they may not be contradictory at all, but a reflection of the different technologies employed and methodologies implemented by various manufacturers.
Let's begin our exploration of cool lighting, starting with HMI, which may be a well-traveled path for some and an uncharted road for others.
HMI, HID, CDM: The Same, Yet Different
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HMI (technically, a registered trademark owned by Osram, though widely used generically) is an acronym for Hydrargyrum Medium arc Iodide; it's subsumed under the broader heading of HID, or high-intensity discharge lamp technology. Many mistakenly refer to HID as a distinct form of discharge lighting, albeit related. Admittedly, lines get blurred. The Philips analog to the Osram HMI is MSR (Medium Source Rare-Earth). CDM, or Ceramic Discharge Metal halide (CMH/Ceramic Metal Halide), is another technology that also falls under the HID umbrella, but is distinct from HMI.
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Compared to CDM, HMI is available in much higher wattages. The higher-wattage HMIs are often rented and come from the rental house with a host of paraphernalia, not least of which is a matched electronic ballast and an assistant highly trained to connect and manage it all. But even more modest and more easily manageable HMIs under 1,000W can be pricey and, therefore, remain in the rental category. The other thing is that brighter lights have greater electrical demands. Many relatively low-wattage HMI (and CDM) fixtures can run on 15-amp circuits, but higher-wattage units may require as much as 20 amps or more. Depending on the instrument, even modest HMIs may require an experienced hand.
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