Quote:
Originally Posted by 3dplane
... The factory headlamp was designed to throw light in the correct directions and angles with minimal light spilling based on the EXACT location of the light source in the fixture, which is the filament in the bulb that it was designed with.
If we change this location within the fixture,(different bulb or incorrectly installed bulb)the mirrors behind the bulb will throw the light in all kinds of directions except where it needs to go...
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3dplane is exactly right on this. The headlight has a carefully designed reflector that's based on the glowing filament being in exactly the right spot. Unfortunately an HID glow spot has a different shape and size from the halogen's so the systems focus is not the same. They might get some of the glowing field in the pinpoint where the halogen had it, but the rest of he HID's glowing field is in some other place where it's not supposed to be.
The low beam concentrates light on the nearby road surface and spares oncoming drivers from glare. The high beam is close to a pencil beam so it is effective at a distance, with relatively little spreading. Both need aiming but in practicality, if you aim the low beam right the high will be ok (assuming one bulb is doing both jobs like in our Civics).