As you can see, neither type of fog lamps by themselves provided adequate light, and the ratings for low beams with fog lamps are only very slightly elevated from the ratings for low beams without fog lamps. The new ("J2510") fog lamp specification in this comparison is a very up-to-date, state-of-the-art technical standard requiring substantially better performance than the old ("J583") specification. The important thing to understand here is that even fog lamps conforming to the newest, most stringent performance requirements give only very minimal improvement in actual seeing ability—and the rating differences, if subjected to rigorous analysis, would likely not be statistically significant.

Of course, it must also be understood that all lamp performance standards specify a minimum acceptable performance. Extremely high-performing fog lamps are quite rare, but they do exist. And under abnormal driving conditions (very thick fog, very heavy snow) they can be of some help. That's the key point: fog lamps are meant to be used in heavy fog, rain, or snow to help the driver see the edges of the road close to the car so s/he can safely make progress through foul weather at very low speeds. That is all these lamps are designed, intended, and able to do, and most of the ones available as factory or optional equipment or in the aftermarket aren't even capable of doing that. And as explicated in dense scientific detail in this study(pdf), it just doesn't make anything better—though there is the potential for a real safety improvement by using a red rear fog lamp.

Fog lamps of any type should not be used in dry weather. Leaving the fog lamps on at all times does not actually improve the lighting safety performance or the driver's ability to see, though many people do so in the mistaken belief that they can see better this way at normal road speeds in dry weather. In fact, a systematic study done by one of North America's preëminent traffic safety research institutes shows that in the United States more people inappropriately use their front fog lamps in dry weather than use them properly in poor weather.

Why? Because we human beings generally can't accurately tell how well or how poorly we see. We have subjective impressions, reactions, and feelings about how "good" or "bad" our headlamps are, and they feel very real to us, but they're very far out of line with the objective, measurable, real lighting performance and seeing ability. It's not that we're fooling ourselves, it's that our visual systems just aren't equipped to correctly assess how well or how poorly we can see. The primary driver for a subjective impression of "good" headlighting is foreground light—and remember, that's what fog lamps produce—but foreground light is very far down the list of factors that go into the actual, real safety performance of the car's lighting system; that is, how well it actually lets the driver see what must be seen to avoid a crash. In clear conditions, though it makes us feel (falsely) more secure, more foreground light is not a good thing, it's a bad thing. Of course, some foreground light is necessary so you can use your peripheral vision to see where you are relative to the road edges, the lane markings and that pothole 20 feet in front of your left wheels.

But foreground light is far less safety-critical than light cast well down the road into the distance, because at any significant speed (much above 25 mph), what's in the foreground is too close for you to do much about. That is, at normal road speeds, whatever is close enough to be within the foreground light is too close for you to avoid hitting. If you increase the foreground light (such as by turning on the fog lamps), your pupils react to the brighter pool of foreground light by constricting, which in turn substantially reduces your distance vision—especially since there's no increase in down-the-road distance light to go along with the increased foreground light. This is also the reason why it is not appropriate to have fog lamps lit with the high beam headlamps: if you're going fast enough to need high beams, you definitely don't want to spoil your distance vision by overly lighting the foreground.

So all in all, when we use front fog lamps inappropriately, we feel like our seeing is better than it really is and we unconsciously adjust our driving to match how safe we feel. That, in turn, makes us less safe!

In the past, many US-specification low beam headlamps tended to provide relatively low, arguably inadequate levels of light in the foreground and to the sides. They created the impression of a "black hole" in front of the car, with essentially the entire beam concentrated in a narrow band or ball of light thrown into the distance. With headlamps like these, a decent argument can be made for the use of fog lamps to fill the "black hole", that is, to add-back the missing foreground and lateral-spread light when driving at moderate speeds on dark and/or twisty roads. Of course, lamps to rectify inadequate foreground light must be thoughtfully and carefully selected, correctly aimed and properly used. Otherwise, they're useless at best and dangerous at worst.

So, what is a good fog lamp? A good fog lamp produces a wide, bar-shaped beam of light with a sharp horizontal cutoff (dark above, bright below) at the top of the beam, and minimal upward light above the cutoff. Almost all factory-installed or dealer-optional fog lamps, and a great many aftermarket units, are essentially useless for any purpose, especially for extremely demanding poor-weather driving. Many of them are too small to produce enough light to make a difference, produce beam patterns too narrow to help, lack a sufficiently-sharp cutoff, and throw too much glare light into the eyes of other drivers, no matter how they're aimed.

Good (and legal) fog lamps may produce white or Selective Yellow light—it is the beam pattern, not the light colour, that defines a fog lamp—and most of them use tungsten-halogen bulbs though there are some legitimate (and a lot of illegitimate) LED fog lamps beginning to appear. Xenon or HID bulbs are inherently unsuitable for use in fog lamps, and blue or other-colored lights are also the wrong choice.

The fog lamps' job is to show you the edges of the road, the lane markings, and the immediate foreground. When used in combination with the headlamps, good fog lamps weight the overall beam pattern towards the foreground so that even though there may be a relatively high level of upward stray light from the headlamps causing glareback from the fog or falling rain or snow, there will be more foreground light than usual without a corresponding increase in upward stray light, giving back some of the vision you lose to precipitation.

When used without headlamps in conditions of extremely poor visibility due to snow, fog or heavy rain, good fog lamps light the foreground and the road edges only, so you can see your way safely at reduced speeds.

In some places, the law prohibits the use of fog lamps without the low beam headlamps also being on. Whether or not this is the case where you drive, it's vital to realize that fog lamp beams, by definition, have a much shorter reach than headlamp beams. If you drive in conditions foul enough to call for the use of fog lamps without headlamps, it's essential to have good fog lamps that are up to the task and are properly aimed, and it's even more imperative that you slow down because even with high-performance fog lamps, you can't see as far with fog lamps and in poor weather as you can with headlamps and in clear weather.

If the road is wet or slick with ice, but there's no falling precipitation or fog, fog lamps should be used with discretion. Their extra downward light can help compensate for the tendency of water to "soak up" the light on the road from your headlamps. But, this extra downward light hitting a road surface shiny with water or ice will also create high levels of reflected glare for other drivers. Since we're all "other drivers" to everybody else on the road, it's well to think of roadway safety as a cooperative effort. In most driving situations, fog lamps are neither useful nor necessary, but—back to that study linked above—more people use their fog lamps when the prevailing conditions don't call for their use, than use them when the conditions do call for their use. Fact is, it's not helping you, and nobody thinks your car is cool because it has fog lamps, and glare is dangerous, so do yourself and everyone a favor: choose them carefully, aim them properly, use them thoughtfully and sparingly, and leave them off except when they're genuinely necessary.

Daniel Stern Lighting (Daniel J. Stern, Proprietor)

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Copyright ©2013 Daniel J. Stern. Latest revisions 10/13. No part of this text may be reproduced in any form without express permission of author. Permission to quote is granted for the purposes of communication with the author. Processing code 09:F9:11:02:9D:74:E3:5B:D8:41:56:C5:63:56:88:C0