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2 general headlight questions

1.) What is with the headlights that I see that have a reflective coating on the FRONT surface, leaving only a thin circle around the edges free of the coating? Does projecting an unfilled circle of light make for better visibility than a filled circle of projected light?

2.) As a television engineer, I’m probably more sensitive to this than lots of other people, but lots more cars these days sport headlights that change their color temperature as they change their angle toward you. As I see a car either ahead or behind me grow closer, or even bounce over a rut, or hill or valley in the road, they change from bluish to greenish, to yellowish, etc. Are these OEM bulbs, or a cheaper aftermarket attempt to imitate a high-end headlamp?

  1. Can you tell us what type of car you see these lights on? I can’t really picture what you’re describing. Some owner’s put colored covers over their headlights, to customize their cars, but I don’t know if that’s what you are seeing.

  2. There are aftermarket bulbs with different color temperatures, or with a tint on the bulb to filter out some colors, and they can appear different from different angles. These bulbs are said to provide light similar to HID, but they don’t.

HID headlights sometimes appear to have a bluish tint, as I’m sure you’ve noticed.

An automotive headlamp is a point source emission. Light is emitted in all directions from the point source. Reflectors are used to focus the light where it is most useful. Historical designs use a rear hemispherical type reflector and a series of lenses to do this job. I’ve seen newer BMWs where a circle of light is emitted from the lamp. I haven’t had the opportunity to examine one up close but having some design experience in lighting, I suspect the design has two purposes; to better control the light emitted in the forward hemisphere of the lamp and styling.

The light being emitted from the “forward” hemisphere of the lamp radiates out in all directions from the point of emission. Therefore, some of this light is wasted as it is not directed toward the target area for illumination. The forward reflector captures this light, bounces it back to the rear reflector and lenses to be sent out the lamp focused on the target area.

And, from a spectators standpoint, it looks cool too.

Back in the '60s, people used to put chrome ‘eyelids’ on their headlights for some reason. I never knew why, but it never really caught on. I think you can still buy those things.

I have not seen a reflective coating in the middle, but as someone else noted, some BMWs come with what are known as “Angel Eyes” which are circular lights around the perimeter of their headlight. Angel Eyes are intended to be daytime driving lights, but you can buy versions that are brighter than the stock headlight. Angel eyes are a popular aftermarket add-on for BMWs, but most brands of aftermarket units are not very reliable.

1.) On some cars, the headlight lamp is positioned behind an object because only reflected light is desired. Light directly from the lamp would be too intense. However, I don’t think that is what you are seeing. I think what you are seeing is a lens. Old headlights had a type of Fresnel (pronounced Fernel) lens. Because lenses were made of glass, traditional lenses could crack and warp from heat. The Fresnel lens was the solution. I have drawn some crude cross sections of these lenses below.

The Fresnel lens was good at projecting a wide beam of light, but it also illuminated the lens itself. It also didn’t do such a good job projecting the light evenly. There would be highlights and lowlights in the area illuminated by these lenses, so they solved the problem of cracking warping, but they were not the best substitute for traditional lenses.

As those of us who wear glasses know, new materials have been developed for lenses, so now they can make the same traditional lens thinner and less likely to crack or warp, making the Fresnel lens obsolete. This means more light is getting through the lens rather than illuminating the lens itself. So when you look at the headlight, it is hard to see the lens and it looks as though the lamp is behind a solid object.

2.) I can’t help you with this one.

I have aftermarket Projector headlights on my dodge charger. It covers the bulb in a dome of glass and the light coming out of them will be different colors at different angles. The more to the side of the car you go the more you can notice the color change. And about the angle eyes that someone else wrote. They are just decoration and were never meant to be daytime running lamps. They also dont change colors at different angles. They are standard on most BMW’s and available as aftermarked for many many other cars and trucks.

See picture below for an example of a projector headlight with angle eyes/halo’s

  1. I’m unsure what you’re decribing.

  2. I think you’re seeing the effects of dispersion. As the beam is bent more toward the edges of what with todays small focal lens is prismatic, the light is being seperated into it’s different wavelenghts much as a prism. The further away from the center of the lens, the greater the seperation. As your viewing angle changes relative to the lens, you see the different colors.

The old large glass sealed beams were, as someone else suggested, patterned to give a bit of a fresnel effect, much as the light from a lighthouse. Apologies to those from the midwest who have noever seen the lens of a lighthouse. A fresnel lens does not produce the edge dispersion the way a traditional lens does.

Photographers are very familiar with this effect and compensate for it on their craft.

I have an '09 Hyundai Sonata, while driving on a dark country road for the first time. I noticed a distinct horizontal line of light. i posted this on Hyundai web site and was told this horizon is an adjustable feature of the lights. i wonder if it will help when driving into a heavy snow storm and I am sure it would make it easier for on coming drivers at night. Could these be possible reasons for the partially coated lights that I remembered seeing also.