[arunkumar_rec]

I have seen the fog lamps fitted in the sports/rally cars(big round ones) emitting yellow light. Whereas the fog lamps factory fitted in the likes of Swift and Getz throw white light only(that's what i feel). Is there a difference using a yellow light in terms of better visibility compared to white light?

Arun

[arunkumar_rec]

Did some research as I didnt find any answer from the experts.
There are two reasons cited as the rationale behind the use of
yellow fog lamps:
1. yellow lamps suffer less backscatter into the eyes of the car's driver than blue or green, allowing them to see better in the fog
2. yellow lamps are more readily seen by an oncoming driver, so that a car with fog lights on will be seen more readily than white lights would be.

The first point is true for light scattered off very tiny aerosol particles in
fog -- blue light is scattered out of the path from the sun to the viewer of a sunset, leaving white light minus blue to reach the observer... so he/she sees yellow/red -- but is pretty irrelevant for the kind of yellow lights used for fog lamps. Those lamps were generally made by putting yellow filters over a hot tungsten filament incandescent lamp, so that a lot of light is lost in trying to make the yellow light -- reducing the brightness available for the driver to see, which is not such a good idea. Using a halogen gas fill increases the brightness of the bulb, but the same general principle of reduced brightness still holds. Additionally, the size of water drops in fog is generally not appropriate for producing the kind of scattering mentioned above. if so, headlights would appear very red to oncoming traffic, which they don't.

The second point, that the yellow color is more visible to oncoming traffic is also true, but irrelevent as well. The point of fog lights is to allow the
car's driver to see better, not to be seen better . if you wanted to be seen better, a flashing strobe would be more effective, but you don't see much of those in the fog light business.

Can experts throw some yellow or white/blue light onto this topic??

Arun

[BaCkSeAtDrIVeR]

http://www.team-bhp.com/forum/indian...ht-banned.html

[rr_zen]

I have white fog lamps on my Esteem (MGP that were fixed at the MASS). I had the option of going in for yellow ones which was MGP too but on comparing the duo I could see that the white one had a clear and definite edge over the yellow one in terms of brightness and in terms of the distance covered by the light rays dispersed. Also, IMO the white ones look better on the car than the yellow ones.

[hellspawn]

Actually fog lights are "yellow" lights.Its just that we have started calling the white ones,fog lights as well.the actual term for the lights fit in the bumper below the headlights or in place of fog lights are called "driving lights".

[eclectix 1st]

Originally Posted by hellspawn

Actually fog lights are "yellow" lights.Its just that we have started calling the white ones,fog lights as well.the actual term for the lights fit in the bumper below the headlights or in place of fog lights are called "driving lights".

Right, I've also experienced that. Few years back while hiring a car in Oz, the rental lady asked us if we wanted fog lights fitted? but the car already had the lights in the front lower bumper. When (foolishly) asked she said those lights are city lights.

cheers

[anupmathur]

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, please read this: Do Fog Lights Really Work?, Alaska Science Forum

[sgiitk]

May have something to do with the French, who at one time ordained Yellow Headlamps only to try and preserve their car industry. Did not work though.

More seriously, the origin of Yellow Fog Lamps is in the past when doing the Le Mans 24 hours drivers in a desperate attempt to see through heavy fog pasted newspaper on the headlamps, to try and diffuse the beam. It did improve the visibility. The papers were yellow.

[alpha1]

Originally Posted by anupmathur

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




Also, please read this: Do Fog Lights Really Work?, Alaska Science Forum

Penn State Univ prof? hah!
He is wrong on few counts.

First - when you put fog lamps. It is not for greater penetration into the fog or haze.
It is for a clearer illumination in the vicinity of your vehicle in a haze.

Second - when you place a filter on white light to get red or orange. You ARE cutting down the light. AGREED.
But the thing is - when you throw white light at fog, the white comprises of blue as well as red. The blue part of the white light scatters the most. And this scattered light forms an impenetrable wall of haze for your eye sight.

Lets have a simple real life example:
I dont how many ppl have observed this but you get those UV lights (the ones that are purple-voilet; and white stuff glows, when this light falls). If you observe, in the vicinity of those UV tubes, you will see haze. This haze is only because of scattering. Scattering by dust particles present in air.
You would never observe that happening in red lamps.

Now even if you somehow combine these red and UV lamps. Even then you would see the haze, because the UV-Blue part scatters and "created" the haze.

HOWEVER, if suppose you had only put the red lamp. You would've not seen the haze. Even though the luminosity of the red light is low, you can atleast SEE around.


So what this means is that: when you put yellow fog lamps, the yellow light scatters less, and creates less haze around you. Allowing you to look further than what was possible with white light.



As regarding the basis hypothesis that water particles don't scatter light be it blue or red or yellow. I don't remember my physics in that much detail. But the essence is that light bends and scatters around any particle it comes across. The amount of bending and scattering is inversely proportional to wavelength. And what is important is not the size of particle, but the gap between the particles.

[anupmathur]

Originally Posted by alpha1

Penn State Univ prof? hah!
He is wrong on few counts.

alpha1, why don't you publish a paper in a respectable Technical Journal?
We'll kind of take it up from there.

[drpullockaran]

Originally Posted by alpha1

Penn State Univ prof? hah!
I don't remember my physics in that much detail. But the essence is that light bends and scatters around any particle it comes across. The amount of bending and scattering is inversely proportional to wavelength. And what is important is not the size of particle, but the gap between the particles.

If you could make light bend in the same medium you would be a multi- trillionare. Just pulling your leg I do know that you know that Light can only be bent using black holes and immense gravitational fields. The scientists are at it and have had some success.
New trick of light bends beams - LiveScience - MSNBC.com

[alpha1]

^ The "bending" I was talking about is pretty simple straightforward wave theory -called diffraction. Relativity or gravity is not involved in it man.

This diffraction is the source of scattering.
Diffraction - Wikipedia, the free encyclopedia

LOL anup, there is no need for papers on what I said. It is commonly observed stuff, and no one needs to come up with mathematical figures.

[Sankar]

Originally Posted by alpha1

Second - when you place a filter on white light to get red or orange. You ARE cutting down the light. AGREED.

Agreed.

But the thing is - when you throw white light at fog, the white comprises of blue as well as red. The blue part of the white light scatters the most. And this scattered light forms an impenetrable wall of haze for your eye sight.

Agreed. Same thing happens with bluevision headlamp bulbs in the rain and in dusty areas.

So what this means is that: when you put yellow fog lamps, the yellow light scatters less, and creates less haze around you. Allowing you to look further than what was possible with white light.

I have found normal bulbs better in the rains and on the highway than the bluevision bulbs. Allweather bulbs should be even better.


So basically what he's saying is normal lamps do produce the yellow wavelength, but it is also letting out easily diffracted wavelength of light like blue since the lamp doesn't have a filter. This blue wavelength would diffract hitting the dust particles suspended in the air and create the haze. This haze would in turn block your visibility provided by the yellow light even though the yellow light beam is reaching much farther out.

Since the yellow foglamp is only letting out the yellow wavelength of light and the blue wavelength is blocked, and thus there is no difraction of blue light happening creating a haze to block your view. So you can see farther ahead without straining your eyes, but the light intensity would be less.

[normally_crazy]

I agree with Sankar here - the yellow lamps are more effective in the rains/fog/mist than the white/blue ones.

I have used both on the diesel octy and ultimately reverted to yellow OE lamps.

Fog lamps - I had a Santro DX (1999) and it had those yellow fog lamps. And boy were they effective or what ! During a trip to Nashik, I got stuck in dense fog on the ghats - those yellow fogs were the only source that helped me see some part of the road. The headlamps were redundant at that point.

[jango]

Normally people in the high range use yellow fog lights, where mist and fog is very common. The white lights or the so called city light are normally used in cities when it rains cats and dogs . Atleast that is what i have seen and understood.