To the moron who said K doesnt stand for Kelvin.... takefn from the STICKIED HID Ref. thread.... props to EK9 for this one
EK9 (From HID Ref. thread) said:Bulbs
The bulbs. The common mistake some people here is that all these high kelvin rated bulbs are the shizzle. Well, they couldn't be more wrong. The higher you go in kelvin, the less light and lumens you'll have. Pratically anything over 6k is really a waste if your at all concerned with your safety and brightness of lighting. So what is the best bulb out there then you ask? 4100-4300k. It has the most lumens out of all the HID bulbs produced. Thats why car manifacturers still use them today. Below is a graph showing you the variances of the light spectrum. As you can see, 4100k would be right where the "sweet spot" is on that chart. It produces near to the suns same kelvin thus giving you daylight-like output. Think of it like this, high kelvin bulbs would be like being out in the sun with sunglasses on vs a 4100k being in the sun w/o glasses on.
Also here is another good thing to know taken from the FAQ:
Yellow:
1500 k Candlelight
2700-2900 k Yellow painted fog halogen bulbs
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Yellowish white:
3200 k Sunrise/sunset
3200 k Premium H7 non painted halogen bulb
3400 k 1 hour from dusk/dawn
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White:
4100 k Philips/Osram OEM HID D2S
5500 k Bright sunny daylight around noon
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Blueish white
5500-5600 k Electronic photo flash
6000 k Philips Ultinon HID D2S
6500-7500 k Overcast sky
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Blue:
9000-12000 k Blue sky
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Purple:
28000 Northern sky
12000-30000 k Ultra Violet light (black light) .