It makes the light seem whiter by filtering out light that isn't blue-ish. Yes, this means you get less total light output from the same filament. To make up for this, hotter-burning and shorter lived filaments are used.
Now obviously those Twenty20 bulbs aren't nearly as bad as those completely blue-coated bulbs boy racers seem to love, but it's still completely unnecessary and counterproductive. Any kind of coating that changes the glass from 100% transparent to anything less, will degrade the total light output and quality.
You
don't want whiter (actually bluer) light from your headlights, as it makes your pupils contract more, leading to effectively worse vision at night. This makes them seem brighter, but your vision at night will actually be worse. This is closely related to why wankers drive around with their front fog lights on all the time, because they think "more light = better", when in fact the additional light right in front of the car actually worsens the overall vision farther ahead, where you actually need it.
Good old-fashioned non-coated halogen bulbs actually have a pretty much ideal color temperature, HIDs are actually slightly worse (but make up for it with more total light output and sharper defined beams).
Just say no to blue-tinted bulbs, they're 100% marketing horse manure.
http://www.danielsternlighting.com/tech ... d/bad.html
http://www.danielsternlighting.com/tech ... /good.html
http://www.danielsternlighting.com/tech ... white.html
Yes, I am a curmudgeonly fuddy-duddy about these kinds of things, but for good reason :-) Optimal light in front of you without needlessly blinding oncoming motorists is absolutely essential for safety.
Personally, I've found the standard Osram H7 bulbs to be a very good match for the 406's headlights. If you find that you need brighter bulbs to effectively see at night, you might want to check your headlights for hazing or discoloration and/or check that your headlights are actually getting proper power. Alternatively, go see an eye doctor
