Is that a UFO or just some guy in a BMW?
March 3, 2008 2:02 AM   Subscribe

I've noticed that many cars seem to have headlights that change colour depending on angle. Is this deliberate or unintentional, and how do I avoid buying bulbs that do this?

What I've observed is that these colour changes occur as the road rises and falls, or as I round a bend. This is not a subtle effect - I see all kinds of vivid colours flashing in my rear view mirror. I find this very distracting. At just the right angle these lights can briefly mimic the lights of an emergency vehicle, which is obviously not a good thing.

My initial guess is this is due to chromatic aberration or something, rather than being intentional. But is it? Can anyone enlighten me further? Are these things legal (specifically in the UK)? What should I look for in a new set of headlight bulbs so that other drivers won't curse me at night?
posted by le morte de bea arthur to Travel & Transportation (6 answers total) 1 user marked this as a favorite
I think what you are seeing are cheap aftermarket Xenon headlights. Very annoying indeed.
posted by swordfishtrombones at 2:12 AM on March 3, 2008

Best answer: You are spot-on morte, the color shift is caused by chromatic aberration. Typically, this effect is most pronounced with projector-style high intensity gas discharge headlamps, and the magnitude of the effect varies greatly between manufacturers of headlamps.

I disagree with what swordfishtrombones has commented - cheap "xenon" headlight replacements typically do not replace a reflector housing with a projector, which means that the aberration does not occur. However, such retrofits have issues of their own, which include increased glare at eye level.

If you would like to minimize the distraction you cause to other drivers, then simply do not purchase a car with a high intensity gas discharge projector-style headlamp. However, in doing so you will simultaneously be reducing the effectiveness of your own lamp-assisted vision.

If you already have a car, and you're talking about replacement bulbs, then I'm going to guess that you have halogen lights, which do not suffer from the same effect - in which case you are not causing the problem you describe.
posted by merkuron at 2:30 AM on March 3, 2008

I've noticed this too, and was always wondering if the lights flickered, or they were flashing blue lights at me.
posted by lain at 2:54 AM on March 3, 2008

I've asked the same question at my local garage. They said the same - that it's gas discharge lamps.

The flickering effect is due to the bumps in the road making the lights' beam move up and down in relation to your point of view. The chroma of the beam varies according to where in the beam you 'are'.

I hate them, too. It seems that something that improves one driver's vision, but buggers up everyone else's, isn't a good thing for on the road. Now I sound like my Dad.
posted by dowcrag at 3:23 AM on March 3, 2008

My old 98 Lexus LS400 had the self-leveling lights. I'm not sure, maybe they self level as you drive so on bumps and hills they are going up and down? I don't recall, it seems like they should self level when you turn them on, yet what if you were on a big hill when you turned them on?
posted by thilmony at 5:33 AM on March 3, 2008

Self-leveling is there to eliminate issues with respect to potential load imbalances in the car. In a simple implementation, the car measures the ride height difference between front and rear and tilts the headlights up or down to compensate. However, on most, if not all cars, the transient response of this system is very slow - enough to catch gradual changes in pitch during acceleration/braking, but not enough to slam the headlights down when you hit a speed bump. Not that self-leveling headlights have the range of motion to be able to compensate for a speed bump anyway...
posted by merkuron at 8:46 AM on March 3, 2008

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