Since this gets brought up OVER AND OVER AND OVER.
Might as well make its own thread for a discussion.
First off, I should mention that I have SAE certifications on gaseous discharge lighting, and headlight design.
So, I do know something on the subject.
Stock halogen housings, especially free form housings found on newer vehicles, are designed around very strict tolerances, in having the focal point directly on the filament of a standard bulb. Retrofitting a HID bulb, even one thats re-based to fit the housing, creates a large amount of glare, because the gas chamber that the arc is created in, is larger in size then the filament, therefore, some of the light is outside the focal point, and is reflected outside the desired beam.
I however, am a LOUSY photoshop artist, so bear with my drawing here:
On the left you have a standard bulb in a reflector housing designed for it.
The light pattern is exactly whats expected, because the focal point is, especially in newer cars with computer designed free form reflectors, designed specifically for where the filiment is located in the standard bulb.
On the right, the HID bulb's gas chamber, between the two electrodes, where the arc takes place, is longer lengthwise, and significantly wider then the regular bulb's filiment, therefore the light is bounced off the reflectors outside of the focal area, causing glare outside the intended beam pattern.
On a older car with a fluted lens, its even worse, because the reflector only focuses the beam to the general direction and pattern, and the flutes spread it out and distribute the light to the beam pattern that you want.
In the case of a hid bulb behind a fluted lens, since the beam is already spread wider then it should be, the flutes make it worse by spreading the light out of focus and out of pattern.
Might as well make its own thread for a discussion.
First off, I should mention that I have SAE certifications on gaseous discharge lighting, and headlight design.
So, I do know something on the subject.
Stock halogen housings, especially free form housings found on newer vehicles, are designed around very strict tolerances, in having the focal point directly on the filament of a standard bulb. Retrofitting a HID bulb, even one thats re-based to fit the housing, creates a large amount of glare, because the gas chamber that the arc is created in, is larger in size then the filament, therefore, some of the light is outside the focal point, and is reflected outside the desired beam.
I however, am a LOUSY photoshop artist, so bear with my drawing here:
On the left you have a standard bulb in a reflector housing designed for it.
The light pattern is exactly whats expected, because the focal point is, especially in newer cars with computer designed free form reflectors, designed specifically for where the filiment is located in the standard bulb.
On the right, the HID bulb's gas chamber, between the two electrodes, where the arc takes place, is longer lengthwise, and significantly wider then the regular bulb's filiment, therefore the light is bounced off the reflectors outside of the focal area, causing glare outside the intended beam pattern.
On a older car with a fluted lens, its even worse, because the reflector only focuses the beam to the general direction and pattern, and the flutes spread it out and distribute the light to the beam pattern that you want.
In the case of a hid bulb behind a fluted lens, since the beam is already spread wider then it should be, the flutes make it worse by spreading the light out of focus and out of pattern.