The best HID bulbs for visibility are around 4300k. As the temperature goes up on HID bulbs, the visibility declines, but the color changes to a bluish white. This color is highly sought after by people using HID lights, and they are willing to give up a bit of performance for the color. If you are interested in strictly illuminating the road in front of you, stick with a 4300k bulb. This will put out a basic white light, but provides the best performance. HID stands for high intensity discharge lights. These high performance bulbs come standard on high end vehicles, and are quite popular as aftermarket parts.
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