http://www.newton.dep.anl.gov/askasc...9/phy99xx4.htm

http://www.gi.alaska.edu/ScienceForum/ASF5/593.html

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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."


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

Dr. Lawrence D. Woolf
General Atomics
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Interesting read.

I don't know about other cities, but San Diego uses the yellow low pressure sodium lights, not to save on electricity, but to reduce light pollution because of the nearby observatory.

Actually it's because human retina is less sensative to scattered orange/yellow light and thus makes it easier to see in fog and rain conditions. Yellow/orange lights are perceived as less intense to the human eyes thoughs making them less irritating when reflected by fog particles.

Scattered blue/white lights are irritating to human eyes especially in low light conditions such as the cockpit of a vehicle. That's why military and smart consumer vehicles utilizes orange/yellow spectrum lights. Likes of the 'hip crowd,' Honda and Jetta designers like to impress their younger consumers with bright blue lights which causes eye fatigue in the long run.

Same idea with the fog light. When there is fog, what you need is not brightness but rather the contrast~ You want reflection from road objects but not the fog itself. That's why you would also find most fog lamps are mount away from the line of sight from the driver and further away from the passenger cabin.

So there~

Dang, this fog topic has now become a forum epidmeic...

IMHO, fog lights are not to help you SEE better, but are to help you be SEEN better by other drivers.

The best fog lights I have ever used are made by PerLux (tm) and are for large trucks. They have louvers inside to keep light from the bulbs from being reflected UP and BACK at the driver. They are about the same size as the KC HighLighters (tm) you see in most stores so using them on a G is out of the question unless you are building a ralley car.

I think you bring up a good point about being seen. I hate it when it's a hazy or misty day and there's one guy who decides not to turn on eve his parking lights since he can see everyone else but doesn't realize that barely anyone can see him.

I'd have to disagree here. Try driving an older car that uses Sealed beam headlamps in the fog. You will quickly realize how poor these lamps are for fog as much light goes up. Fog lights have sharp cut-offs to prevent light from going upward as this upward light will hit the water droplets directly in front of you and make it harder to see the road. They have no provisions to actually aid other drivers to see you better. In fact, anything that does so would likely mean this light is directed towards another driver's eyes, which would reduce their visibilty. This could raise the potential for a head-on collision. Generally, if you are in fog, you speed should be adjusted as such. Any headlamp of any color will likely be easy to spot in fog if you are driving a safe speed for conditions. Fog lights use cutoffs and are generally mounted lower than headlamps to help minimize the amount of fog that is in the light between you and the road. This helps a driver to see more of the road better.

Im going to be an ******* and say

Interesting read

Interesting read.

I always tell this to friends and Family when they Dont put their lights on in inclimate weather. And they always say, Ohh I didn't think of it that way.

The fog lights that manufacturers use is mostly for asthetics. They really don't alluminate any better.

glad HI doesn't have fog...'cuz i got yellow fog lights