Foglights and why they are yellow...

[Mike Painter]

The subject of foglights and lights used in fog comes up often. I could not see any reason why a particular color would be any better in such a situation so looked around a bit.

It turns out there is no reason. (If you don't read the whole thing, this has been known for over 65 years.)

Note that the following information does not deal with the position of the light. In fog or any turbulance (such as diving) holding it as far away as possible will give you the best illumination. This is because you are not faced with the light reflecting from the particles.

Begin article. (Bold print added by me.)

There is no good reason why fog lights are yellow. Here is an
excellent explanation provided by Professor Craig Bohren of Penn State
University:


"First I'll give you the wrong explanation, which you can find here and
there. It goes something like this. As everyone knows, scattering (by
anything!) is always greater at the shortwavelength end of the visible
spectrum than at the longwavelength end. Lord Rayleigh showed this, didn't
he? Thus to obtain the greatest penentration of light through fog, you
should use the longest wavelength possible. Red is obviously unsuitable
because it is used for stop lights. So you compromise and use yellow
instead.


This explanation is flawed for more than one reason. Fog droplets are, on
average, smaller than cloud droplets, but they still are huge compared with
the wavelengths of visible light. Thus scattering of such light by fog is
essentially wavelength independent. Unfortunately, many people learn
(without caveats) Rayleigh's scattering law and then assume that it applies
to everything. They did not learn that this law is limited to scatterers
small compared with the wavelength and at wavelengths far from strong
absorption.

The second flaw is that in order to get yellow light in the first place you
need a filter. Note that yellow fog lights were in use when the only
available headlights were incandescent lamps. If you place a filter over a
white headlight, you get less transmitted light, and there goes your
increased penetration down the drain.

There are two possible explanations for yellow fog lights. One is that the
first designers of such lights were mislead because they did not understand
the limitations of Rayleigh's scattering law and did not know the size
distribution of fog droplets. The other explanation is that someone deemed
it desirable to make fog lights yellow as a way of signalling to other
drivers that visibility is poor and thus caution is in order.

Designers of headlights have known for a long time that there is no magic
color that gives great penetration. I have an article from the Journal of
Scientific Instruments published in October 1938 (Vol. XV, pp. 317-322).
The article is by J. H. Nelson and is entitled "Optics of headlights". The
penultimate section in this paper is on "fog lamps". Nelson notes that
"there is almost complete agreement among designers of fog lamps, and this
agreement is in most cases extended to the colour of the light to be used.
Although there are still many lamps on the road using yellow light, it
seems to be becoming recognized that there is no filter, which, when placed
in front of a lamp, will improve the penetration power of that lamp."

This was written 61 years ago. Its author uses a few words ("seem",
"becoming recognized") indicating that perhaps at one time lamp designers
thought that yellow lights had greater penetrating power. And it may be
that because of this the first fog lamps were yellow. Once the practice of
making such lamps yellow began it just continued because of custom."


Dr. Lawrence D. Woolf
General Atomics

[senna94]

You know, I really want to believe you. However, I have no idea what you are talking about.

Paul

[trivergata]

What about in smoke? Is a smoke particle small enough to make a difference between wavelengths?

[Roy]

I agree that the color of fog lights is not critical.......but as I understand it, the shape (geometry) of the light IS. True fog lights have a lens that allows very little light to be beamed upward. Backscatter is the real problem...light being reflected back into your eyes. Mounting the fog lights as low as posible also helps reduce backscatter. With flashlights, a tight beam should work better than a flood type pattern. Also holding the flashlight as far away from your eyes as possible should also help reduse backscatter.

[beezaur]

I've read that before.

Then I went outside with my red-orange and cyan Aleph IIIs. The guy is wrong.

I am in grad school, and have spent my share of time around PhDs. They are all pretty inelligent, but they are not immune to being wrong. There are PhDs who have literally been booed off the podium presenting their ideas. Sometimes, which I think is the case here, they are pressed into service answering questions outside their areas of expertise. Their answers in such cases are not real reliable. If Woolf was a particle physicist (General Atomics), then his specialty would not have been optics. The person you need to consult in a case like that would be an experimental chemist. They use scattering all the time to measure the properties of aerosols.

I am not a physicist, nor do I pretend to fully understand the physics of scattering in fog. However, I can tell you that I have looked into it to the tune of graduate-level physics texts plus my own experimentation and can say with certainty that there are a whole lot of complicated things going on with fog. For one thing, there are several different kinds of fog that form under different conditions. Some fogs have really small particles; some fog droplets are so big they barely stay in suspension. Different fogs do not behave the same optically.

All the same, it is pretty easy to prove the effect of wavelength with the LEDs available today. Get a red one (a bright one, around 50+ lumens) and one that is white. Compare these two to an incan of comparable brightness, like a SureFire G2. Go out in the fog and tell me what you see. What I see when I do that (as I have done many times) is that the red is best at penetrating fog, followed by the incan. White LEDs and any color shorter in wavelength than green is terrible.

I know from firsthand experience that wavelength makes a difference. I cannot give you a full explanation as to why, but I would be perfectly happy to demonstrate it to anyone in person. It is perfectly repeatable.

Scott

[VWTim]

Well, nice explaination, but I think the truth comes somewhere in the middle. As I live in the PNW, lots of rain/weather. I've been using Hella Yellow Star bulbs in E-code housings. Going between clear to the amber I can see thru fog slightly better, but what is nice is when I'm driving in snow or driving rain, I'm not blinded by the reflection of the water droplets in the air coming towards my windshield as I am with normal clear halogens. This makes my vision a little better in inclimate weather.

[vandrecken]

I agree with the scattering thing but note that many shooters wear yellow tinted glasses because it's perceived to improve contrast and in France cars are fitted with yellow headlights ...

Is there another side to the argument based on perception ?

[Yooper]

The argument presented misses the point. The physiology of vision isn't even addressed. Blue light, because of the Rayleigh phenomenon, is more glare inducing to human eyes, so filtering it out increases contrast sensitivity, and contrast is what our visual system is geared to pick up. This is how blue-blocker sunglasses work, as well as amber shooting glasses, skiing goggles, etc.

I use 85/100W Hella Yellowstars in E-code headlamps as my low beams as well. It's like having really bright fog lamps on all the time. They rock here in winter-land. My vehicle (FJ62 Land Cruiser) has a 4 headlight system, and my additional high beams are 130W H1 halogens, regular white, so I can choose what I want based on conditions.

[SilverFox]

Hello Mike,

You may find this article interesting...

Tom

[Yooper]

Originally Posted by SilverFox

Hello Mike,

You may find this article interesting...

Tom

That was interesting. Crappy experiment, but interesting.

[pathalogical]

I read somewhere that yellow fogs are more visible from a greater distance when coming toward you. In a snowstorm/blizzard, white fogs (I guess headlights too) tend to blend in with the snowy background. Most new cars now have built in fogs, usually white. Remember years ago those older style 'amber' fogs that mounted on your front bumper ? Remeber how much more noticeable they were when coming toward you from a distance ?

[Mike Painter]

Originally Posted by trivergata

What about in smoke? Is a smoke particle small enough to make a difference between wavelengths?

No.

[Mike Painter]

Originally Posted by Yooper

The argument presented misses the point. The physiology of vision isn't even addressed. Blue light, because of the Rayleigh phenomenon, is more glare inducing to human eyes, so filtering it out increases contrast sensitivity, and contrast is what our visual system is geared to pick up. This is how blue-blocker sunglasses work, as well as amber shooting glasses, skiing goggles, etc.

I use 85/100W Hella Yellowstars in E-code headlamps as my low beams as well. It's like having really bright fog lamps on all the time. They rock here in winter-land. My vehicle (FJ62 Land Cruiser) has a 4 headlight system, and my additional high beams are 130W H1 halogens, regular white, so I can choose what I want based on conditions.

Filtering is specificaally mentioned, both at the light which reduces over all out put as well as at the eye which increases contrast.

[jtr1962]

One thing not discussed yet is that using yellow light basically kills your peripheral vision. This so-called tunnel effect has been noted when driving under sodium vapor lamps, and is one reason the next generation of street lamps will likely be white or even blue-white. Another drawback is that at low light levels yellow light is less effective than the photopic lumens would indicate (hence the complaints about how dim the streets looked after the switch from mercury vapor to sodium vapor). Put these two things together, and they more than negate any possible penetration advantage yellow light might have. Note that I said "possible" because I have observed metal halide and sodium vapor lighting under fog conditions, and the sodium vapor didn't seem to penetrate any better than the metal halide.

[jtr1962]

Originally Posted by pathalogical

I read somewhere that yellow fogs are more visible from a greater distance when coming toward you. In a snowstorm/blizzard, white fogs (I guess headlights too) tend to blend in with the snowy background. Most new cars now have built in fogs, usually white. Remember years ago those older style 'amber' fogs that mounted on your front bumper ? Remeber how much more noticeable they were when coming toward you from a distance ?

This has nothing to do with penetrating ability and more to do with being different in color from the general background light (scattered moonlight/starlight in this case). I'll also add that in a city they would have no advantage, probably even a disadvantage, because the general background color during a snowstorm in most cities is yellow due to the scattering of the yellow sodium vapor light. I've actually noticed that HID is far more noticeable here during poor visibility conditions since it differs from the general yellowish background color.

[LEDMaster2003]

Originally Posted by trivergata

What about in smoke? Is a smoke particle small enough to make a difference between wavelengths?

Jameson smoke detectors used to have yellow lenses on their lights...

http://img.photobucket.com/albums/v6.../codealarm.jpg

[eluminator]

Originally Posted by beezaur

I've read that before.

Then I went outside with my red-orange and cyan Aleph IIIs. The guy is wrong.

Scott

Checking it out for yourself? That's heresy!

I agree with you though. In my experience, a red beam of light in the fog is invisible because there is no back scatter. But the beam of a green light is visible. It's this back scatter that makes driving in fog so difficult. Talking about "penetration" is barking up the wrong tree.

At night a ship shows a red light on the left side and a green light on the right. The beam is about 90 degrees with a fairly sharp cutoff. Like all light on a ship, they are designed not to shine on the ship itself to preserve the night vision of those standing watch. On a clear night the only way to see if these lights are working is to walk to the rail and lean over. But when it's foggy, the beam of the green light is obvious. No need to lean over the rail to see if it's working. But the red light has no visible beam. You'd swear it must not be working until you lean over and check.

Don't get me started on college perfessors. We judge colleges by how good their football team is, not by how stupid the professors are.

[asdalton]

Originally Posted by trivergata

What about in smoke? Is a smoke particle small enough to make a difference between wavelengths?

Smoke strongly absorbs light, while fog mainly scatters it away. So there are some qualitative differences, and the behavior of a beam of light in one won't necessarily extrapolate to the other.

[asdalton]

I think I've found some data that can resolve this debate. Strangely, it turns out that nearly everyone is right in some way.

1. Glenn’s Treatise on a Preferred Color, or the Lack Thereof, of Fog Lights

2. Forward Vehicular Lighting and Inclement Weather Conditions (PDF)
This second reference is particularly valuable, because it reports human responses to different lighting conditions.

The data are consistent with the following being true:

a) Wavelength has little or no effect on the ability of light to penetrate fog.

b) Wavelength has little or no effect on the amount of backscattering in fog.

c) Wavelength does have an effect on the way that backscattered light "dazzles" the human eye, with blue being worst and red being best. This is a real effect, but it is a physiological effect not caused by any wavelength-dependent interaction of light and fog. Interestingly, the data do not show a significant difference between white and yellow headlights, which is the original source of this debate.

d) Visibility is best when lights are mounted away from the viewer's line of sight.

e) Visibility is best when beam spread is low ("all throw").

f) The effects of altering (d) and (e) can easily make (c) irrelevant.

[woodfluter]

Very interesting thread, but not resolved by any means in my mind.

I have done a lot of B&W photography over the years. Often employing colored filters. I am one of the few who actually possesses and uses a very dark blue filter to emphasize atmospheric effects (rather than reduce them), and have very often taken duplicate exposures with and without various filters. So have some direct experience.

Using panchomatic film (approximating the eye's sensitivity to colors, but not perfectly duplicating it): without question, blue emphasizes haze, yellow and green reduces it, ornage and red markedly reduces that further. In photos taken out west where the air is very dry, as in Utah and New Mexico, the colored filters often have little or no perceived effect on the contrast and clarity of distant parts of the scenery apart from altering colors of sky and vegetation. In the east, even far from sources of pollution but where there is more moisture (water droplets) in the air, the differences are very dramatic. In the presence of fog, very very dramatic. Although my personal experience is limited to panchromatic films, I understand that orthochomatic films (more red-sensitive) do not show the same response to filters.

As a scientist, I understand the arguments about Rayleigh scattering and can accept that. Plus the plots of scattering vs wavelength in the most recent post.

But as a scientist, I also cannot dismiss the empirical evidence. The bumblebee flies, folks! Subjective factors or the relative role of scotopic vs photopic vision might play a role in vision, but they can't be fooling B&W film. If not light scattering, please suggest a mechanism that accounts for the enhancement or clearing of effects related to atmospheric moisture depending on filter color.

- Bill

[xochi]

This is slightly catty cornered to the topic but I'll ante up my two cents .

My dad tried out some of those blue white headlights and I though they were pretty cool. Several weeks later he took'em out and put a warmer bulb in. He said that the warmer bulbs gave him better visibility and that the blue white bulbs had alot of oncoming drivers flashing there high beams at him. He's 62 and at the time I thought he was nuts about the visibility thing, of course I was in the thrall of the luxeon and confident that whiter was better. After the buzz of the luxeon wore off and I began to notice the greater depth I got from incandescent light I started paying attention to car headlights and noticed the same effect my dad pointed out. When I drive at night, I notice the blue white headlights on account of there difference but they really wipe out my nitevision when I pass a car using them. The effect of those lights seems to be like a staticky radio signal , lots of sound but an unclear message whereas the more yellow lights seem to provide a better signal to noise ratio.

[asdalton]

Originally Posted by woodfluter

In photos taken out west where the air is very dry, as in Utah and New Mexico, the colored filters often have little or no perceived effect on the contrast and clarity of distant parts of the scenery apart from altering colors of sky and vegetation.

Over large distances, Rayleigh scattering is definitely important. It's just not a measurable factor over distances of a few meters.

I suspect that the difference that you see between the eastern and western U.S. is due to sulfur haze rather than humidity. (Humidity is not like fog; it is water in the gas phase and has no refractive properties, although it will contribute to Rayleigh scattering just as the other molecules in air do.) In doing some quick research, I couldn't find anything definite about a wavelength dependence of scattering from sulfur aerosols. These particles can be smaller than 1 micron, which is much smaller than fog droplets and approaching the wavelength of visible light. So it's possible that there could be a weak Rayleigh scattering effect from this kind of haze.

This page attributes blue haze in the Appalachian mountains to hydrocarbon haze.

By the way, the bumblebee thing is an urban legend.

[woodfluter]

Thank you Andrew for the helpful explanation!

In reverse order (sort of):



Yes, I know the bumblebee thing is a legend. It seemed useful because I couldn't think of a well-know real illustration that empirical observations have to take precedent over theory. Just a feeble stab at humor...



Yep, haze in eastern US mountains is largely natural hydrocarbon emissions, now augmented by power plants. But you point out that gaseous H2O can contribute to Rayleigh scattering - over substantial distances - so I have to wonder, what with the great difference between the atmospheric moisture in the east and dry western desert, if that is not also a factor?



I agree of course that fog droplets should not scatter light via the Rayleigh mechanism. I do wonder if there are other mechanisms at play, possibly internal refraction within droplets that affects how much of different wavelengths is reflected from the far side of the droplet. But that might have been laid to rest in the cited articles I so briefly skimmed.



In any case, the filtration issue I raised is probably not relevant - sorry! - case of apples and oranges, or perhaps lemons.



- Bill

[a99raptors]

so, at the end of the day, when it comes to penetrating power, LEDs fare just as well as incandescents, except if the colour is white, just point at something from another angle? Woo Hoo! That would really go a long way in seeing more LED use. A lot of people are using incandescent sources because of the belief that it is easier to see it in the outdoors through fog.

[Daniel Ramsey]

Where I live (Alaska) we often have frozen fog, blowing ice crystals, sometimes heavy snow but not really thick stuff like when I lived at Lake Tahoe, CA.

I've experimented with different colors of halogen and Xenon/HID bulbs.

Found the 3000K Xenon HID bulbs monted low to the ground with driving lamp cut lenses works very good for fog while the 12000K deep violet works very well against blowing snow, I use 9000K HID 9005 bulbs (high beams) in my low beam projector headlights in my 2002 Silverado, they have a prismatic band that flouresces road markers quite well. Truckers that routinely drive between Anchorage and Prudhoe Bay have huge driving lights with blue filters that they use in blowing snow conditions,not totally efficient but thats the real difference....what you need is pure UV either from a high K number like 30000K or so HID to get a distance thrown and a tight beam.
I've seriously considered building a 400watt UV lamp (120 volt and using an invertor) designed for stage work in a specific parabolic reflecter with a servo motor to move the bulb where to get a wide-tight beam, like a Maglight. Maybe have a movable fresnel lense to stipple the beam and remove thany hot spots. Maybe. Realistically the ideal headlight system of the future can change the K numer or color of the emitted light upon demand, plus offer some IR or FLIR with a HUD to see in total obscurity.

There is a use in that respect for super luxeons, having different emmiters clustered for different weather conditions, use amber for fog, use green or violet for snow, normal conditions gan use the three major colors to create white, its possible to have a photo sensor of reflective light (fog, smoke, snow etc. ) that automatically changes the headlight or aux lights.


I've had a similar project of using large scale RC control rods to move my projector headlights insync with the direction of my steering off the tie rod.

Now i need to make them blink....

[asdalton]

One of the complications of comparing HID versus incandescent headlights is that people usually don't know the important characteristics of the headlight systems that they are comparing--total output (lumens), beam spread, beam angle (compared to the road), and illuminance (lux) on the target at a given distance. Manufacturers' advertised specs probably can't be taken at face value.

HID lamps are becoming more popular, but I personally wouldn't put them in my car unless I knew that they were going to put equal or better lux on a target many meters ahead. If you have sets of incandescent and HID headlamps that are matched in that characteristic, then it makes sense to compare them directly to see if the HID performs more poorly due to its tint. Otherwise, you might end up merely confirming the not-so-surprising fact that a brighter light is better than a dimmer one.