Fog Lights or Lamps: Who Needs Them?

The Science and Technology Behind Fog Lights and Lamps

yellow fog lights
Fog lights can be yellow, but that isn't their defining feature. picturegarden / Photographer's Choice / Getty

Front fog lamps aren’t standard equipment by any means, and there’s actually a lot of confusion out there on the subject of how and when to use them. Unlike high and low beam headlights, which both see regular use, fog lights are only really useful in a small handful of very specific situations. The fact is that fog lights are specifically designed for use in poor weather and other situations where visibility is severely reduced by mist, fog, or even sand and dust in the air.

The basic argument in favor of fog lamps is that regular headlights, and especially high beam headlights, tend to reflect back in a driver’s eyes. This type of dangerous situation can be avoided by aiming the lights at a steeper angle in a bar shape, which is what fog lamps are designed to do.

Further adding to the confusion is a misconception that all fog lights are yellow, and the fact that a lot of aftermarket suppliers use the terms “fog lamps” and “driving lamps” to refer to the exact same product or offer combined “fog and driving lamp” assemblies. The term “driving lamp” is actually a nebulous phrase that sometimes refers to main beam headlights, sometimes refers to auxiliary headlights used primarily for off-roading, and can even refer to products marketed for use in the fog.

What Are Fog Lights or Fog Lamps?

Front-facing fog lights and fog lamps are a type of automotive headlight that are designed to emit light in a bar-shaped beam.

The beam is typically designed to have a sharp cutoff on top, and the actual lights are usually installed low and aimed toward the ground at a sharp angle.

The position and orientation of fog lights can be compared and contrasted with high beam and low beam headlights to reveal exactly how different these seemingly similar devices are.

High beam and low beam headlights are both aimed at a relatively shallow angle, which allows them to illuminate the road surface a great distance in front of a vehicle. In contrast, the sharp angle used by fog lights means that they only illuminate the ground immediately in front of a vehicle.

Some fog lights produce selective yellow light, and there is a relatively widespread misconception that all fog lights have yellow bulbs, yellow lenses, or both. In fact, selective yellow has actually been used for both fog lights and regular main beam headlights at various points throughout the history of the automobile. So while some fog lights do produce selective yellow light, many produce white light.

It’s actually the bar-shaped beam of light, and the way that the beam is aimed, that makes a fog lamp a fog lamp, rather than the color.

What is Selective Yellow Light?

The idea behind selective yellow headlights and fog lamps is that shorter blue and violet wavelengths of light tend to produce glare and dazzle effects during nighttime driving. This is especially true in poor driving conditions, where blue light will tend to create a glare effect when it reflects off ​fog, snowflakes, or even rain.

Since selective yellow light is less likely to produce dangerous glare during night driving in poor conditions, some vehicles have used selective yellow lights. This same advantage has seen selective yellow used in fog lamps. However, filtering out blue light does have a consequence in terms of total light output, which is not desirable for night driving in good weather conditions.

When to Use Fog Lights

Since fog lights are aimed low, and many of them use selective yellow light, they are relatively useless when the driving conditions are good. That means there is no reason to ever turn on your fog lights unless you experience a poor visibility situation when driving.

Some situations where fog lights can be useful include poor visibility conditions caused by rain, fog, snow, or even an excessive amount of dust in the air. If find yourself driving in a condition of poor visibility, and your high beams reflect back at you, causing a glare or dazzle effect, you should avoid using your high beams. If your low beams also create excessive glare, to the point where all you can see is snow, fog, rain, or dust, then a set of good fog lamps may allow you to actually see the road.

The catch is that fog lights, unlike main beam headlights, only illuminate the ground immediately in front of your vehicle. This makes it incredibly dangerous to drive at any kind of a high rate of speed when using only your fog lights. In fact, it’s actually illegal in some areas to drive with just your fog lights on, even if your main beam headlights are producing glare.

In most cases where fog lights are actually necessary, their primary function should be to allow you to proceed slowly, and carefully, until you reach either your destination or another location where you can wait out the bad weather.

What are Rear Fog Lights?

While front-facing fog lamps are designed to allow you to slowly make your way through extremely poor visibility conditions, rear fog lamps are designed to prevent anyone from hitting you under those same conditions. The issue is that in very poor visibility conditions, your tail lights may not alert other drivers to your presence until it is too late. This is especially true if the person behind you is driving at an unsafe speed for the prevailing conditions.

In most cases, rear fog lights are red, which makes them superficially similar to brake lights and running lights. In fact, rear fog lights and brake lights produce a similar intensity of light. So even if a vehicle doesn’t have rear fog lights, applying the brakes has a similar effect in terms of visibility.

The main issue with rear fog lights is that since they are the same color, and just as bright, as brake lights, there exists some potential for a driver to mistake the two. To combat this, regulations specify that rear fog lights have to be located a specific distance from the brake lights. Some vehicles also only use a single rear fog lamp instead of two.

Who Needs Fog Lights?

Since fog lights illuminate the ground directly in front of your vehicle, they really have two uses. The first is the intended use, which is to cut down on glare in very poor visibility and allow you to slowly proceed to your destination. The other is to see what is on the ground immediately in front of your vehicle under normal visibility conditions since main beam headlights typically leave a large void space between the front of a vehicle and the place where the beam actually hits the road surface.

While it may be tempting to use fog lights all the time to fill in this void space, there’s actually a good reason to turn them off. The issue is that having the road surface lit up right in front of you can tend to dilate your eyes, which actually reduces your ability to adequately see the darker road far in front of your vehicle. So while using your fog lights to see right in front of your car while driving very slowly is potentially useful, leaving them on at normal driving speeds, and in normal driving conditions, can be very bad news.

The fact is that, while fog lights do have their uses, most people don’t actually need them. Since they are only useful in a very narrow range of circumstances, you only need them if you actually find yourself driving under those specific circumstances a lot. And even if you do drive in poor visibility a lot, fog lights still won’t allow you to drive through snow or fog at a high rate of speed with anything even approaching a reasonable degree of safety.