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I was reading a discussion about different HID lamps and I was wondering why would you use HID lamps in the first place?
Is it for fuel/energy efficiency?
Is it for their higher brightness?
Color of light?

They cost quit a bit!

EDIT: To make it clear, I don't see why one would spend that much money to replace their headlights with HID lights. I want to know what people usually want to achieve when they do this. Do they want to have brighter more focused lights? or they want to conserve fuel/energy and improve their MPG? or they just want to be cool?
In my opinion this is not an open ended discussion.

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Welcome to the site. Do you have a specific question about a particular application? At present, this sounds like an open-ended discussion topic that fits squarely under this FAQ topic: Perhaps some additional content to keep this from sounding so open-ended would be in order? – Bob Cross May 16 '12 at 17:50

From Wikipedia:

High-intensity discharge (HID) lamps make more visible light per unit of electric power consumed than fluorescent and incandescent lamps since a greater proportion of their radiation is visible light in contrast to heat.

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They also tend to have a much sharper cutoff, with less light wasted on place you don't need it. – Brian Knoblauch May 16 '12 at 19:33
@Brian we could make this whole post a wiki to capture this sort of info. – Bob Cross May 16 '12 at 21:15
Theoretically they should last longer as well. On my car they are expected to never need replacing during the car's lifespan. – Rory Alsop May 17 '12 at 19:26

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