Do fog lights work?

Dear Car Talk

Dear Car Talk | Feb 01, 2002

Dear Tom and Ray:

Do fog lights work? Do they help you see through the fog, or do they just light up the fog? Are yellow fog lights better than clear fog lights? Should they be mounted low on the vehicle to search forth underneath the fog cloud? I am so confused -- you might even say I am "foggy" on the fog-light issue. Therefore, I ask you to search through the fog of your collective brain cells and enlighten the world to the truth about fog lights. -- Pat

RAY: Great question, Pat. How I wish this were the only subject we were foggy on!

TOM: There are many different kinds of lights that people put on the front of their cars these days. There are driving lights, fog lights, Velux motorized sky lights ... we've seen 'em all. Most of them are purely decorative. And many are used and/or aimed incorrectly, and they simply blind oncoming drivers.

RAY: But fog lights, when used and aimed correctly, might be useful to some drivers. As you probably know, if you project light directly into the fog, it bounces off the fog droplets and reflects in all directions, making it even harder to see. That's why you use your low beams in fog rather than your high beams.

TOM: Fog lights are low-mounted lights (bumper level or below) that project light that's cut off at the top. So the light pattern on a good-quality fog light goes straight out at bumper level and down, but not higher than that.

RAY: The reason for this is that fog tends to hover about 12 to 18 inches off the ground. So by projecting light in that fog-free pocket, you can illuminate the road a greater distance from your car and therefore see farther ahead. Some people swear by fog lights. Others claim that they don't really make much difference.

TOM: So the question becomes, to yellow or not to yellow? There's a lot of debate about this, but the research says that yellow lights are no better than white lights at penetrating fog. The theory bandied about was that yellow light has a longer wavelength and is therefore less likely to be reflected by the fog particles. Turns out, this is complete poppycock.

RAY: Apparently, the fog particles themselves are so big that they reflect all colors of light. Basically, all light bounces off of them, so using yellow light instead of white light gives you no advantage.

TOM: Plus, in order to get yellow light, what fog-light manufacturers do is put a yellow lens over a white light. That cuts your light output by 20 percent to 30 percent, which is counterproductive.

RAY: So if you live in a coastal area where fog is a real problem and you want to give fog lights a try, we'd suggest a set of high-quality, white fog lights that are professionally mounted to be sure they're aimed correctly. And don't forget to check their aim periodically. Since they're mounted low, they can be knocked out of alignment when you run over things like snow banks and stalled Toyotas.

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