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Metal halide bulb.
An example of the element Scandium

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Scandium Metal halide bulb
Metal halide bulb.
The most efficient currently available light sources are arc-discharge lights. The most efficient of all are low-pressure sodium vapor bulbs, but they give a very strongly yellow light that most people don't much like, perhaps because it makes everyone look dead. High pressure sodium lights and mercury vapor lights are better and only slightly less efficient.
The brilliant idea behind metal halide lights is to start with a mercury vapor light, which is very bright and very efficient, and mix in a pinch of this element and dab of that one, each with its own characteristic spectral lines, in order to build up out of individual emission lines a reasonably facsimile of the color of natural daylight. The result is a bulb that is only slightly less efficient than the most efficient low pressure sodium lights, but that gives a very pleasant natural daylight. And of course, the most important of those elements is scandium
Why don't you have any in your house? Generally they come only in very high wattages (400W, which gives an amount of light equivalent to a 1500W incandescent bulb, or higher). And worse, they take several minutes to warm up before you get much light at all. At the farm I have about ten 400W metal halide fixtures in the shop and four 1000W ones as yard lights: Producing that much high-quality light by any other technology would be impractical and hugely expensive in electricity, which is why this type of bulb is widely used in commercial and industrial settings (look up at most warehouse-style mega stores and you'll probably see an array of metal halide lamps).
There is one company, however, that sells a home-use, scandium-containing lamp: www.microsun.com, a division of the parent company of my favorite scandium supplier. I love the way their marketing department tries to make the slow-start sound like a feature: "Unlike other artificial lighting, the Microsun Metal Halide Light Source reaches full illumination gradually--resembling natural sunlight." Um, right. But aside from the slow start, metal halide light really is very attractive in many ways and it's ridiculously efficient, 4 or 5 times as efficient as incandescent light.
This 400W metal halide bulb is displayed in my Bulb Stand.
Source: Hardware Store
Contributor: Theodore Gray
Acquired: 28 November, 2003
Text Updated: 11 August, 2007
Price: $40
Size: 9"
Purity: <1%
Sample Group: Light Bulbs
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