Fog lights are yellow and driving lights are white ... WRONG. There's more to it than that.

When someone is thinking about installing fog lights, I start with questions: First of all, do you know what fog lights are? Secondly, what do you drive, where do you drive, what kinds of roads do you drive on? What kind of weather do you drive in? What do you want the fog lights for -- do you live in a real foggy area? Are they for pea soup fog or light fog, for snow, rain? Or just to look cool when you're cruising?

Fog lights aren't necessarily amber lights, nor are amber lights always fog lights. There are amber driving lights, amber spotlights, and, in France, there are amber headlights. What makes a fog light is the light pattern, regardless of the type of lamp or color of the light. A real fog light -- one that will do you good when you're trying to see in the fog -- has a wide beam pattern (70 degrees to 120 degrees) with a sharp, flat cutoff on top.

So beyond that, not all "fog lights" are created equal. There are lots of lights being marketed as fog lamps -- some very low priced, some hundreds of dollars a pair -- that, if I were driving in extreme weather, I would not turn on. They would make it harder to see.

Fog lights, of course, are just one kind of auxiliary light. There are also driving lights, midbeams (passing lights) and spotlights for other purposes. And fog lights have many other functions besides fog, such as other types of bad weather and increasing side light for cornering. While fog lights are beneficial in fog or any other glare-producing conditions, they can also help you see better when you drive on a clear night. Mainly when you think of fog lights, you think of fog.

Seeing through the fog?
You can't see "through" fog. You see in foggy weather by lighting up the road under the fog, illuminating as little of the fog as possible to avoid producing glare.

Fog is defined as a thick cloud of water droplets, 0.00039 to 0.00156 inches in diameter, suspended in the atmosphere at or near the earth's surface and reducing visibility to below half a mile.

Your high beam headlights produce a wall of glare -- a whiteout -- from the light bouncing off these droplets of water. You're being blinded by the glare of your own lights reflected off the water vapor in front of you. Similar experiences occur in rain, snow, dust, etc.

One solution would be to buy heated blowers like giant hair driers to dry the air in front of your car -- works great, costs a lot and uses a ton of energy. We may never deliver any of these, but you can start sending deposits in now (big ones).

The amount of glare from the airborne moisture is affected by the relationship of angles between your line of sight and the angle of your car's lights. That's why you want a wide, flat beam from your fog lights. What you want your fog lamp to do is light up the road in front of you without lighting up the fog that's higher up, right in front of your eyes.

Lenses and reflectors
It helps to know something about the construction of the light -- specifically its lens and its reflector. The cuts in the lens, called fluting, shape the pattern by how they deflect the light. Ever knocked the lens out of your fog light? And noticed how it turned into a spot light? That's what it would be if it had a totally clear lens, with no fluting.

Since the light passes through the lens, obviously the material of the lens would have an effect on how much light passes through. Lead crystal, being the clearest, allows the most light to pass through. Both ordinary glass and hardened glass are full of dirt and impurities that affect the passage of light. Plastic lenses block even more of the light.

Most of the better lights are made with lead crystal lenses. Hardened glass doesn't break as easily, but is still as impure as ordinary glass. Plastic is basically useless for lenses because of impurities and because it gets scratched up.

Breakage is, of course, an occasion for replacing lenses. And if fog lights are mounted in an air dam and you drive across the desert a lot, they get essentially sand blasted and all the little pits affect the flow of light and the light pattern. Even if you don't drive across the desert, your lenses will still be scuffed up by grit and you likely will want to replace them sooner or later, especially if you're using good quality lights.

You also will care about the quality of the reflector, the shiny part that gathers the light from the bulb and reflects it out at precise angles through the lens. Good reflectors aren't chrome -- chrome appears to be shiny and reflective to the eye, but for reflecting light, it's not efficient!

Some reflectors are silver plated. This is from the olden days. Prior to the invention of sealed beam headlights, American cars had separate bulbs, reflectors and lenses, and the reflectors were silver. Silver is not optimally reflective even when first applied. And through tarnishing, it loses 40 - 50% of its reflectivity after about 48 hours. These reflectors were supposed to be polished monthly.

The preferred and most reflective material is an aluminized vapor coating. This is the neat stuff they use in microscopes and in mirrors for aiming lasers. Most of the higher quality lamps use this.

Some lights also use a bulb shield, like a cup in front of the bulb. It blocks direct light from the bulb to the lens so that only light from the designed shape of the reflector passes through the lens. This reduces stray light above the cut-off.

A word about bulbs
Halogen bulbs have been around for about 30 years now. Most of the replaceable bulb type lamps utilize a halogen bulb. Some use a tungsten bulb -- this does not produce as much light per watt.

Three common types of single-filament halogen type bulbs are H1, H2 and H3. The H1 puts out 29 lumens per watt (lm/w), the H2 is 33 lm/w and the H3 is 26 lm/w -- versus the 13 lm/w of a tungsten bulb. Any of these three could be found in a fog light. The bulbs don't interchange -- a lamp is built to take a certain bulb.

The most common bulb wattage is 55 watts. A brighter option is a 100w bulb, which produces nearly twice as much light at the bulb filament, which wouild be a benefit in a larger lamp. In many of the smaller lamps, the reflector area is too small to efficiently gather the light and very little increase in light output occurs.

Of course, the question of yellow, or amber, versus white needs to be addressed. I prefer white. Regardless of the fog lights' various types of construction, generally speaking amber or yellow fog lights become amber or yellow from coloring the lens or the reflector. This may reduce the light 15% to, in extreme cases, 50%.

I once equipped a car with amber headlights, driving lights, spot lights and fog lights. I could light up reflective signs two miles away. But from not having the full spectrum of light, a lot of darker objects simply were not as visible.

So I use white lights for their benefits the majority of the time. Some people prefer amber lights for more severe weather, and, obviously, for much less glare off of snow. But to me the disadvantages outweigh that benefits.

Bigger lights are, all other things being equal, better. More reflector area reflects more light. Bigger, better lights do, however, cost more.

Round lights are better than square ones with an equal area because the corners in a square lamp are sacrificing area. It isn't for the aerodynamics that rally cars have rows of big round lights across the front.

There are various designs ranging from sealed beams (of which there are still some on the market) to the projector lights. Projector lights have the bulb sitting way in the back with an ellipsoid reflector behind it. A shield is used to perform the cutoff. The convex lens projects the light -- hence the term projector -- like a slide projector light. It's a sophisticated design whose advantages are that you can get the same amount of light with half the size of lamp, the beam is very wide and even, and the stray light above the cutoff is practically nil.

The oldest and still most common types of separate bulb lamps use parabolic reflectors, which are something like a cross between a bowl and a funnel in shape.

Another kind of lamp is called bi-focus or free-form. Instead of a parabolic reflector, it has a multi-planed reflector. This means the reflector itself has many different planes which focus the light before it hits the fluting in the lens, sort of "pre-aiming" the light from the bulb.

Obviously, this begins to get fairly complex and much is done by the lighting manufacturers to research and develop materials and construction to make a light do what it's supposed to do.

Mounting
After you have the right kind of fog light with the right pattern, next is how to mount it to get maximum benefit and this depends on what your goals are for your fog lights. For really severe weather, mounting the lights lower is better.

But the lower the lights are mounted, the more vulnerable they are to impacts from stones and other various objects. The lower mountings also decrease how far forward the light will go (thanks to hills and the like). And the lower they're mounted, the more the shadowy effect from from rises and depressions in the road increases, which I find irritating.

Mounting the lights low on the vehicle is useful only for pea-soup fog, snow squalls, etc. -- 20 mph driving conditions -- which I encounter very infrequently. So for the majority of the time, I feel I am better off with fog lights mounted on top of the bumper -- for greater distance and more visibility overall in less severe conditions. I'm still retaining the option of aiming them lower for extreme conditions (I can get out a wrench and point them at the ground 10 feet in front of the bumper when the weather is really bad).

I also drive with my fog lights on in clear weather as cornering lights and for seeing animals and other things off on the side of the road waiting to attack my vehicle.

Another important consideration when mounting fog lights is that they be mounted rigidly, so the beam stays pointed the way it's aimed, as opposed to dancing over tar strips on the road and the like. Dancing light beams will prove to be very irritating and, if they're dancing up and hitting those droplets of water, you're still going to have glare.

Aiming
Fog lights should be aimed straight ahead and level, or down a few degrees. For really severe weather, aim them down lower and give up the longer distance. In that situation, you don't need them to go 600 feet down the road, since you can only see 60 feet.

A wide pattern with the light aimed straight ahead will give you some light off to the sides for cornering. So some people, whose prime concern is cornering light, will aim their fog lights so they spread apart off to the sides, pointed away from each other, to increase the off-the-road light. This will increase the amount of side light from the outside part of each light. But it will decrease the light in front of the car, and thus the distance they go straight down the road in front of you.

Here's a better idea. On some of the rally cars I've prepared, we aimed the fog lights so the light patterns would cross each other (the left light would light up the right side, and the right light, the left side). This gives the same amount of light off to each side, but in front of the vehicle, it's brighter where the beams overlap.

Keep 'em clean
No matter what the color or pattern of your fog lights, keep them clean. Just a few days' worth of road dirt can cut the light be up to 50%. In winter, mud and snow can reduce light up to 90%!

But now you know the basics -- what fog is, the types of fog lamps available, how to mount and aim fog lights for your intended purpose.

So, how do you chose a fog light?
Buy them all and compare them, or consult an expert. You could read everything there is about a light, but that won't tell you everything. For example, you can't compare reflective ranges from different manufacturers, because they may not all use the same reflector.

Ultimately, you can't know how a light works until you put it in a vehicle and drive down the road with it -- or talk to someone who has.

David Hueppchen is the proprietor of OJ Rallye Automotive and has been racing and rallying for decades. He is the owner of numerous Volvos.