Helpful Text About Selecting Fog Driving Lights

Here are some samples of some fog and driving lights that I sell on okay.

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 differentwavelengths(colors) of visible light scatter off the fog dropletsdifferently. This phenomenon is known as "dispersion," because thedifferent colors of light in an image will separate from each other,causing the image to "disperse." If you illuminate the road with only onewavelength (color) of light, the images of the objects you see will stillbegee somewhat blurry because of the scattering of light by the fog, butat 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 sensitiveto all colors. It turns out that we are most sensitive to yellow and green light. So, our best gepromise between sensitivity for our eyes and a long wavelength for least scattering is yellow light.
Now, I don't know what kind of light bulbs are used in fog lights, butanother consideration used in street lighting is cost and efficiency. Youmay have seen some yellow street lighting in some places; this is"low-pressure sodium vapor" lighting. The special thing about this lightis that it is almost entirely one (actually two very close together)wavelength of yellow light, and that it gives the most illumination for theamount of electricity. A big problem with this light, though, is that itthrows off color perception. Under sodium vapor light, something bluelooks gray. This makes it hard to, say, recognize your car in a parkinglot.
"First I'll give you the wrong explanation, which you can find here andthere. It goes something like this. As everyone knows, scattering (byanything!) is always greater at the shortwavelength end of the visiblespectrum 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 gepromise and use yellow instead.
This explanation is flawed for more than one reason. Fog droplets are, onaverage, smaller than cloud droplets, but they still are huge gepared withthe wavelengths of visible light. Thus scattering of such light by fog isessentially wavelength independent. Unfortunately, many people learn(without caveats) Rayleigh's scattering law and then assume that it appliesto everything. They did not learn that this law is limited to scattererssmall gepared with the wavelength and at wavelengths far from strongabsorption.
The second flaw is that in order to get yellow light in the first place youneed a filter. Note that yellow fog lights were in use when the onlyavailable headlights were incandescent lamps. If you place a filter over awhite headlight, you get less transmitted light, and there goes yourincreased penetration down the drain.
There are two possible explanations for yellow fog lights. One is that thefirst designers of such lights were mislead because they did not understandthe limitations of Rayleigh's scattering law and did not know the sizedistribution of fog droplets. The other explanation is that someone deemedit desirable to make fog lights yellow as a way of signalling to otherdrivers that visibility is poor and thus caution is in order.
Designers of headlights have known for a long time that there is no magiccolor that gives great penetration. I have an article from the Journal ofScientific Instruments published in October 1938 (Vol. XV, pp. 317-322).The article is by J. H. Nelson and is entitled "Optics of headlights". Thepenultimate section in this paper is on "fog lamps". Nelson notes that"there is almost geplete agreement among designers of fog lamps, and thisagreement 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, itseems to be begeing recognized that there is no filter, which, when placedin 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","begeing recognized") indicating that perhaps at one time lamp designersthought that yellow lights had greater penetrating power. And it may bethat because of this the first fog lamps were yellow. Once the practice ofmaking such lamps yellow began it just continued because of custom."
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Snappylights Fog and Driving Lights