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Actually, Mark, if the bulb burned at 10,000 degrees K, you would actually not see very much light coming from it at all. Humans don't see too well at that range. My understanding of HID lamps is that they (as well as the Xenon HMI lamps) are designed to burn at or around 5,000 to 5,500 K - which puts them right around daylight with a blue sky. The HMI's are used extensively in filmmaking - they put out huge amounts of light that is balanced for daylight, so you can use them to fill a scene when shooting outdoors. The biggest one I've ever used is a 12k HMI (12,000 Watts). We were shooting Tom Selleck on a beach in Hawaii in bright sunlight - the 12k was placed 75 feet AWAY from him, and managed to fill ALL the shadows caused by direct sunlight. These suckers are MEAN. As for the "blue" allowing you to see farther in a cave - well, Geo is right about blue light traveling farther, and HID lights DO emit more radiation from the Blue and Violet spectrum (also a fair amount of UV, so you don't want to stare at one for very long, if at all) - since the light is so white, it also gives the impression of greater penetration (meaning it emits broad amounts of radiation in the green spectrum). Tungsten bulbs emit most of their radiation in the near red to infrared spectrum, so they consequently put out less "blue" light. Also, by placing a dichroic filter in front of a lightsource, all you are doing is filtering out certain parts of the spectrum emitted by the bulb (hence the term applied to dichroic filters as "subtractive" filters). But if you want to REALLY attract a human's attention, you need to use Green light. The human eye is most sensitive in the green part of the spectrum. Interesting point here about light and human sensitivity: We see "visible" light because that is where our Sun emits the majority of its radiation. In order to see the feeble amount of Radio Waves emitted by the Sun, we would require eyes the size of giant radio telescopes. Same for Microwaves, UV, X-Rays, etc. Other solar systems out there have suns that emit radiation in completely different spectrums - this was the background premise behind the vision of the "Predator" in those movies. The "Predator" came from a planet where most of it's sun's radiation was emitted in the form of Infrared - so it had to wear "sunglasses" to protect it from the damaging effects of "visible light". But I digress... I think the main benefit of HID lights are the "whiteness" of the illumination, and the sheer Lumen for Wattage that they offer. Combine that with their small size and you've got a pretty neat package. I intend to build some of these suckers myself pretty soon - but mine are going to be really big to light a whole scene for filming. How cool would it be to have a 12k HMI floating above a reef at night? Probably ruin the circadian rhythm of the reef-dwellers, eh? Take care, Kevin. -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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