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Old 1st March 2008, 12:12   #1486
Ram
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Default Yellow fog lights? urban legend

Here is more scientific support for my earlier post.

There is no good reason why fog lights are yellow.

an excellent explanation by
Prof. 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.

Defect no. 1:
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 (therefore color) independent.

Unfortunately, many people learn Rayleigh's scattering law (without caveats) and then assume that it applies to everything.

They have ignored the fact that this law is limited to scatterers which are small compared with the wavelength
and only works at wavelengths far from strong absorption.

Defect no. 2:
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.

Then why did people try yellow fog lights at all?

One:
the first designers of such lights were misled.
They did not understand the limitations of Rayleigh's scattering law
and did not know the size distribution of fog droplets.

Two: Someone thought it, useful to make fog lights yellow to signal to other drivers that visibility is poor and to please be careful.

Headlight designers have known for a long time
that there is no magic color that gives great penetration!!!

"Optics of headlights" is an article from the Journal of Scientific Instruments, published in October 1938 (Vol. XV, pp. 317-322).

Author: Prof. J. H. Nelson

Nelson notes that

"there is almost complete agreement among designers of fog lamps, and this agreement is in most cases extended to the color 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 in the 1930s, lamp designers imagined that yellow lights had greater penetrating power.

And it may be that because of this
that the first fog lamps were yellow.

Once the practice of making such lamps yellow began
it just continued because of custom."

I rest my case.

Ram
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Old 1st March 2008, 12:18   #1487
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yellow fog lamps are against law in kerela.You get fined if you have yellow fogs here.One person who was fined went to consumer court as his santro came default with yellow fogs,he got an exeption as the yellow ones were OE in his car.
Ram
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Old 1st March 2008, 13:28   #1488
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Very nice post Ram it explains a lot. I too had a doubt about placing yellow filters in fog lamps. The filter doesn't 'add' colour to the light produced. It just blocks out other wavelengths of the light produced by the bulb in effect reducing the lighting intensity.

Only LEDs produce a particular colour (or wavelength of light) without the help of a filter.
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Old 1st March 2008, 13:49   #1489
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Great Post Ram. Thanks for sharing! Shatters some of our pre-concieved notions and prejudices. brilliant.
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Old 1st March 2008, 14:05   #1490
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Quote:
Originally Posted by binoyb22 View Post
How much will the xenon's(head lights and fog) cost?? iam planning to change it for my Getz.
Between 7-12k per HID Kit.

Please dont run HID's in the headlamp as well as fogs without projectors, your going to annoy a lot of people.

Quote:
Originally Posted by ram View Post
The original idea to have yellow fog lamps was to benefit from color scatter.It makes no difference whether your fog lamp is yellow or white.

In fact, yellow light is achieved by having a yellow bulb or a yellow lens on the fog lamp. If anything, these attenuate, i.e., reduce the light coming through.

So yellow fog lamps serve no purpose. In fact modern fog lamps are no longer yellow.
Nice info Ram, thanks.

Just to be clear when I said yellow I dont mean the local yellow fogs or the yellow filters.

I stated 4300k HID Headlight and 3000k HID for the fogs as these would offer the best combination of visibility. AFAIK it is not COLOUR but the colour TEMPERATURE that affects visibility, illumination and output.
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Old 1st March 2008, 14:32   #1491
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Thanks guys.. this is very informative.. Thanks to Ram for this very scientific inputs..

Can any one recommend a brand for H11 fog light (white).. Philips doest not offer H11 white fog lamps..

If i can find this then i'll go for the blue vision 90/100 for the headlights along with that..

As ram+hyundai suggested, I have to stay with yellow turn indicators..

I might also go for the HID.. headlight+fog , total cost =12K,, but before that i'll do a little research in the forum..
But i agree that without projectors it will be a 'hell' beam for others.. Dont know the cost of puttin projectors.. will check that out..
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Old 1st March 2008, 15:00   #1492
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Can anybody throw some light on the function on projectors? Why is it required for HID's?
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Old 1st March 2008, 18:05   #1493
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Quote:
Originally Posted by tsk1979 View Post
Depends on Vehicles.
According to the MV Act, SUVs are allowed to fit Auxillary lights as long as they are not higher than a certain level, and confirm to the certain standards.
What is the level and what are the standards is a matter of national security and you have to get clearance from the President of India to know that.
This statement rocks

wonder how many of us have got the clearance?
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Old 1st March 2008, 18:42   #1494
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Glad to read Ram's post, which confirms what I thought. I just had the sense that yellow foglamps were something old-fashioned I hadn't seen in UK for a long time, coupled with the sailing knowledge that, when it comes to coloured lamps, it is the white ones (unfiltered) that are visible from the greatest distance.
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Old 3rd March 2008, 13:54   #1495
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I want to fit Hella rallye lights to my Palio.
My workshop guys havent done such work yet to a Palio and asking me back for some ideas..
Just need to know how the fitting/welding to be done..
A pic or detail of the fitting in palio would be really nice to have a clear idea.
Also, will there be any concern with battery charging/drain etc..palio stock lighting itself runs 4 bulbs on long beam i suppose..adding 2 Hella lights above that would be an issue??
may be i would prefer running Hellas *alone* while touring..and stock lights alone inside city..can that be done?

thanks in advance.
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Old 4th March 2008, 13:44   #1496
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Quote:
Originally Posted by Daddy View Post
I want to fit Hella rallye lights to my Palio.
I assume the "Hella rallye lights" are auxilliary headlights.

Daddy how much do they cost in Bangalore? I think if you add a bullbar, then it will be easy to fit.
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Old 4th March 2008, 15:01   #1497
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Quote:
Originally Posted by Lord Lucan View Post
I think if you add a bullbar, then it will be easy to fit.
I can do that..but the bullbar is a direct fit to Palio?
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Old 4th March 2008, 17:30   #1498
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Quote:
Originally Posted by Daddy View Post
I can do that..but the bullbar is a direct fit to Palio?
I do not think OE bullbars are available for Palio but decent ones should be available in accessory shops.

Check out Hella USA and Canada, mounting instructions are present for some of the products.

Will really appreciate, if you can disclose the (Bangalore) price and the Hella Rallye model (3000, 4000, Compact ??) that you are considering
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Old 4th March 2008, 17:46   #1499
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Quote:
Originally Posted by ram View Post
Here is more scientific support for my earlier post.

There is no good reason why fog lights are yellow.

an excellent explanation by
Prof. 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.

Defect no. 1:
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 (therefore color) independent.

Unfortunately, many people learn Rayleigh's scattering law (without caveats) and then assume that it applies to everything.

They have ignored the fact that this law is limited to scatterers which are small compared with the wavelength
and only works at wavelengths far from strong absorption.

Defect no. 2:
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.

Then why did people try yellow fog lights at all?

One:
the first designers of such lights were misled.
They did not understand the limitations of Rayleigh's scattering law
and did not know the size distribution of fog droplets.

Two: Someone thought it, useful to make fog lights yellow to signal to other drivers that visibility is poor and to please be careful.

Headlight designers have known for a long time
that there is no magic color that gives great penetration!!!

"Optics of headlights" is an article from the Journal of Scientific Instruments, published in October 1938 (Vol. XV, pp. 317-322).

Author: Prof. J. H. Nelson

Nelson notes that

"there is almost complete agreement among designers of fog lamps, and this agreement is in most cases extended to the color 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 in the 1930s, lamp designers imagined that yellow lights had greater penetrating power.

And it may be that because of this
that the first fog lamps were yellow.

Once the practice of making such lamps yellow began
it just continued because of custom."

I rest my case.

Ram
Great article Ram, but the question is what about reflection. What I could gather from various sources is
If you have Yellow lights instead of white, you get lesser backscatter. This is very different from "Penetration".
Thats why Skiers wear Yellow glasses, so that they can filter out the blue wavelengths which scatter more.
With a standard white light you will see more backscatter.
Also there are bulbs with 3000K or lower color temperature available, which do not require any yellow coating.

Also I have noticed that at a traffic signal(LED types where brightness is same for both green and red), in Fog the Red is more visible than the green, and visible from the greater distance.

Last edited by tsk1979 : 4th March 2008 at 17:48.
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Old 4th March 2008, 20:33   #1500
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hi guys, planning to go for a HID kit for my honda civic. i want to know which are the best brands, i dont want the cheap ones. what is the correct size, color intensity...etc, i mean the technical aspects to look for before buying. also what is the price like for good brands. please reply ASAP. i need to get it soon, as not satisfied with stock lights. also from where in delhi i can get genuine HID's at good price.
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