# Fog Lights



## cyberspyder (Jul 12, 2006)

haven't posted here in a LOOOOOONG time. do yellow coloured 'fog lights' on many cars IMPROVE lighting conditions in the fog compared to regular white beams?

Brendan


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## Orbit (Jul 12, 2006)

Please do a search. 

There are a few threads that might be useful, but I think this is the info you want.


Question: Why are fog lights yellow? Does it help that they are? Does it matter if you are driving in city or countryside? 


Answer: My understanding is that it is important for fog lights to be one color 
(rather than white, which is all colors) because the different 
wavelengths(colors) of visible light scatter off the fog droplets 
differently. This phenomenon is known as "dispersion," because the 
different colors of light in an image will separate from each other, 
causing the image to "disperse." If you illuminate the road with only one 
wavelength (color) of light, the images of the objects you see will still 
become somewhat blurry because of the scattering of light by the fog, but 
at least you won't have extra problems from dispersion. So, if we want to 
use just one wavelength of light, which wavelength should we use? It turns 
out that light with short wavelengths scatters more than light with long 
wavelengths (short to long: violet, indigo, blue, green, yellow, orange, 
red). So, a long wavelength light will be best. There's another thing to 
consider, too: our eyes are not equally sensitive to all colors. It turns 
out that we are most sensitive to yellow and green light. So, our best 
compromise between sensitivity for our eyes and a long wavelength for least 
scattering is yellow light. 
A consideration used in street lighting is cost and efficiency. You 
may have seen some yellow street lighting in some places; this is 
"low-pressure sodium vapor" lighting. The special thing about this light 
is that it is almost entirely one (actually two very close together) 
wavelength of yellow light, and that it gives the most illumination for the 
amount of electricity. A big problem with this light, though, is that it 
throws off color perception. Under sodium vapor light, something blue 
looks gray. This makes it hard to, say, recognize your car in a parking 
lot. 

There was the theory, now here is the reality. 

There is no good reason why fog lights are yellow 

"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." 


Also, take a look at the following web site: 
http://www.gi.alaska.edu/ScienceForum/ASF5/593.html


there are a few other opinions on this now, relating to the human eye and contrast, differentiation between colours in a closed environment(ie inside fog) etc etc. but This should get you started. as i said PM me if you want more details.

ORB


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## Pila_Power (Jul 12, 2006)

Wow, very interesting read orb.

I had a set of yellows and the only thing they seemed good for was lighting up the orange cat-eyes for miles down the road while leaving everything else relatively dark.

I liked em tho.


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## smokinbasser (Jul 12, 2006)

the color of the beam is less important than the correct location for lamps to be used for fog, the closer to the pavement the less reflection you receive from the fog droplets as opposed to having the lights elevated like on the roll bar. low mounted lights illuminate the road with less dispersion from the droplets reflecting light back towards your eyes. A long time ago when two round sealed beams were your entire light source they made yellow lens adapters with suction cups to keep them on the headlights and all they did was turn the fog yellow a few paces in front of your car, as someone else pointed out the most valuable benefit was it alerted oncoming vehicles to marginal visibility coming up.


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## Orbit (Jul 12, 2006)

Correct.
Also,
Ideal height is >300mm of the ground.
wattage between 30 and 50.
The warning to others is an important point also.


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## lildave (Jul 13, 2006)

yellow has no blue and blue destroys night vision, white has blue. yellow has no blue. red and orange is too dim. amber a nice compromise. the Eye is a magical thing.


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## Orbit (Jul 13, 2006)

i agree in part....but evidence?

Dimness is not a good meausre, could you elaborate on what you mean?

And please provide a example of it's application.

Intrested in your thoughs.
ORB>


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## smokinbasser (Jul 13, 2006)

fog lamps are not to be confused with driving lamps, DLs have a long focus point in order to see objects farther away than the "normal" headlights reach, while fog lamps are to increase close up visibility in less than ideal climatic situations. High wattage lamps can create more "dazzle" during poor visibility conditions while lower wattage lamps iluminate w/o blinding the driver with reflected light particles. Fog lamps could be said to have flood, while driving lamps have reach, the lens design is partially responsible along with wattage.


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## cyberspyder (Jul 14, 2006)

sorry for not replying, but i was away on a camping trip. anyways, now that this is cleared up, i have yet another questions. as mony of you guys own BMW's, are the 'angeleye' driving lamps better than regular lamps, let's say from PIAA? i think the ring around the lamp is it's low beam (correct me if i am wrong).

Brendan


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## Orbit (Jul 14, 2006)

The ring is the parkers.

The lamps are about equal in colimation.


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## cyberspyder (Jul 15, 2006)

ahh okay, thanks for the reply orbit.

Brendan


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## PhillyRube (Jul 15, 2006)

I have a nice set of Hella 550s on one of my cars. The heck with this yellow stuff, I just burn a hole through the fog...heheheheheh


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## Diesel_Bomber (Jul 16, 2006)

I can't add anything to the comments made by Orbit and others, other than one bit which seems to be lost on a lot of people:

When there is no fog, keep the fog-lights *off*.


Cheers and good luck. :buddies:


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## Orbit (Jul 16, 2006)

Diesel_Bomber said:


> I can't add anything to the comments made by Orbit and others, other than one bit which seems to be lost on a lot of people:
> 
> When there is no fog, keep the fog-lights *off*.
> 
> ...


 
I couldn't agree more it's one of my altime hates!!
If the condidtions warrant teh extra light i have no problems, but you have to drive to the condidtions and the means selecting hte appropraite lighing to use!


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## GreySave (Jul 17, 2006)

I have no scientific basis to back this up, but for my personal opinion is that colored fog lamps seem to provide for better contrast. I have used them since the late '70s (Back when they were not a fad). It also makes it much easier to see the actual beam, and that can be very important because in addiiton to proper mounting, proper aim is critical if you want the lights to work properly and avoid blinding oncoming drivers.

If you are considering purchasing a vehicle with factory fog lamps, try them at night before you make the purchase. Some of them are more decorative than functional, and you may be very disappointed if their use is important to you. Ditto with vehicle headlamps in general. They all must meet a minimum standard, but there are wide variances in performance beyond that set standard. There are vehicles out there with outstanding headlamps and some with lamps that barely meet the requirements. I hyave owned and driven both, and the difference is amazing.

Alan
25+ Year Dealership Employee


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## Orbit (Jul 17, 2006)

There is a scientific basis for what you are refering too (PM me and i'll have a crack at explaining it if you like). BUT it is rather compliacated and HIGHLY environment dependant (terrain, altitude, specific weather, eyesight etc etc). Generally speaking, a well designed, adjusted and appropriatley positioned fog lamp with no filter will be more then adequate. The fundemental floor that most people make (including car manufactures) is the wattage of the globe utalised.
A halogen globe of between 30-50watts is ideal.
It must be remembered that the colour of a halogen, especially a lower wattage halogen is yellow (3000-3500k) and by adding extra filtration to the beam (ie yellow filter) you not only move the colour spectrum more towards yellow (by removing the blue) but more importantly you reduce the lumens emitted!
Most (keyword most) who sware by yellow filters are utilising a bulb that is too bright for the job.

Some food for thought.

ORB>


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