[tsk1979]

ram, I disagree with you here.
In case of dense fog, if you have an equal luminous intensity Red light and Blue light, the red light will be visible from a greater distance.
This is because red will scatter less than blue. Infact high scatter rate is what makes sky blue.
Coming back to fog lamps. what you said is partially right
Putting a yellow glass will cut the light output. So if you had white light originally, it would penetrate the same, as it contains lights from all parts of the spectrum to an extent. So its either the fog scattering the non red part or the glass blocking it.
But the problem with white light will not be penetration related, it will be scatter related. In dense fog no matter how intense your light, you will only make matters work as the fog scatters it all to blind you.
So whats the solution?
Infrared. All these fancy cars with IR vision screens are great in fog because IR is able to penetrate real deeply into the fog and on your screen you can still see a lot.

[Boom Shiva]

So then are foglights meant for the driver's visibility or for oncoming traffic?

I know red is really easy to spot at a distance, but I'm not sure if I'd get the best on-road visibility from red lights.

In any case, most OE foglights on our Indian cars are just for show. Optra and Santro ones are total crap at least.

[tsk1979]

The fog lights are meant for increasing visibility slightly. Since they are placed low the refleced like has less chance of blinding. Similar effect can be achieved by drving on low beam, but I have noticed that vehicles on lowbeam are hard to spot in dense fog. So having fog lamps will also ensure no idiot slams into you.
Many people just rev up even during fog and drive like crazy. By the time an oncoming headlight on low beam becomes visible its too late.

[Ram]

Originally Posted by tsk1979

ram, I disagree with you here.
In case of dense fog, if you have an equal luminous intensity Red light and Blue light, the red light will be visible from a greater distance.

Sunlight starts out being white. Atmospheric haze scatters the blue components off, particularly at the acute angles common at dawn and dusk. And that is why sunsets are yellow, then orange, then red.
tsk1979, you actually agree, not disagree, with me!

Originally Posted by tsk1979

Coming back to fog lamps. what you said is partially right
Putting a yellow glass will cut the light output. So if you had white light originally, it would penetrate the same, as it contains lights from all parts of the spectrum to an extent. So its either the fog scattering the non red part or the glass blocking it.

But the problem with white light will not be penetration related, it will be scatter related. In dense fog no matter how intense your light, you will only make matters work as the fog scatters it all to blind you.

Modern fog lights are white, not yellow. Even HID Xenon fogs are not yellow.

That's because the water droplet size in fogs is too huge to result in meaningful color-scatter.

Cloud, fog and mist droplets have a mean diameter of 10-15 microns.
However, in any one cloud individual drops can range from 1 to 100 micron dia.
And precipitated raindrops are 200 to 15000 microns in size.

By way of comparison, Visible light wavelengths vary from 400 to 700 nm, i.e. 0.4 to 0.7 microns.
There go any expectations of serious color-backscatter effects!

With such huge droplets, both yellow and blue light are scattered equally.
Old wives' tales have a long time dying.

Another thing springs back to memory: the "bird-beak" Renault R16 when it was first introduced, back in 1965. Today it's known as the world's first hatchback and defined that popular style, for future generations from all manufacturers.
I distinctly remember wondering why its headlights were yellow.

Then I saw that all Citroens, Renaults, Peugeots and Simcas had yellow headlights.
The French had yellow headlights in the sixties.

The yellow scattered in fog just as bad as white light, and when the French realized that yellow lights didn't do any good in fog!! They stopped using yellow lamps!


A fog lamp works because it's low mounted and angled downwards and fans out 45-degrees. Not because it's yellow!

The only reason a 55W yellow lensed lamp gives you less kickback from the fog than a 55W white lens lamp is that 18% of the output has been absorbed by the filter!

A fog lamp is not defined as "yellow", but as a lamp that produces a very wide bar of light with minimal-to-no uplight and a sharp horizontal cutoff. What determines a good fog lamp is amount of uplight (less is better) and sharpness of cutoff (sharper is better), not beam color!!

[sandeepm]

Folks.. I was planning to mount extra lights on the roof of my Forester, using either a cross bar or a roof rack to mount them in a line of 4.
But I hear that mounting lights above the roof line is actually illegal as per RTO rules? Is this true?

Sandeep

[itwasntme]

I believe any additional lights installed above the line of OEM headlights are illegal. Especially in Bangalore, the cops/RTO will stop you one day or the other.

But are you planning to use them onroad or offroad? If the latter, it should be ok and you could have a quick-fit & remove set of lights.

[tsk1979]

Note from mod, Threads merged

[sandeepm]

Hmm.. that is an issue then... on roof is illegal.. and i dont want to mount in front of grill as it affects airflow. (Have seen its adverse effects on my Gypsy :(

will have to think of some other tactic..

Sandeep

Originally Posted by itwasntme

I believe any additional lights installed above the line of OEM headlights are illegal. Especially in Bangalore, the cops/RTO will stop you one day or the other.

But are you planning to use them onroad or offroad? If the latter, it should be ok and you could have a quick-fit & remove set of lights.

[tsk1979]

Get a nudge guard and mount them on top of nudgeguard so that there is sufficient clearance between grill and the lights, that way no issues with airflow

[RJFM]

Hi guys,

In general what color fog lamps are good for foggy and rainy conditions in the mountains (ghats) and in plains or coastal areas. Any suggestions please. Many thanks. My car has Hella 500 series white fog lamps.

[rahuldeodhar]

Hi Ram - thanks for all the information. You have really clarified the issue.

One question about jeeps with lamps on overhead bar:

Is it for protection in the jungle (against animals and overhead clearances) or is it something to do with fogs?

I am guess it will be the former - based on your clarifications. So in fact people in Mumbai who I saw with lights on overhead bars are basically style freaks - but functionally dodo - right?

Rahul

[ac 427]

Originally Posted by Technocrat

Whoa Thanks Ram I am sure most of us didnt knew this & after reading it surely makes lot of sense

Man we can a learn a lot from your experiences.

Originally Posted by Ram

The theory that yellow light has a longer wavelength and is therefore less likely to be reflected by the fog particles. is complete poppycock.
Why?

Yeah Technocrat you are right, if nothing i learnt a new word for bullcrap.
'Poppycock'

You rock Ram

Cheers,
AC

[anantnehru]

Originally Posted by sandeepm

Folks.. I was planning to mount extra lights on the roof of my Forester, using either a cross bar or a roof rack to mount them in a line of 4.
But I hear that mounting lights above the roof line is actually illegal as per RTO rules? Is this true?

Sandeep

No dont do that. Brother had them on his Scorpio. Got fined and then he had to take them off. Bumper of below would make more sense.

[sandeepm]

Tks guys.. I have basically given up the idea.. will just stick to handheld spots that can run off the AC adapter, I guess.

PS - on the suggestion of mounting lights on a nudge guard... thats a whole different thread iI guess... with a nudge guard, the airbag sensors dont work properly

Sandeep

Originally Posted by anantnehru

No dont do that. Brother had them on his Scorpio. Got fined and then he had to take them off. Bumper of below would make more sense.

[jat]

That's a very good research by Ram. Hats off to you. Loved reading it (better late than never hehehe - do you agree?).

Regarding the scatter part, the scatter takes place due to refraction. The larger wavelength, lesser they bend and hence lesser they deviate from the original path. So for the same power output, the red will be visible furthest (and not yellow).

The best lamp is the natural day light as that is what human eye is used to and can see best.

As rightly pointed out by Ram (and others), the position is mainly to improve the visibility of the road and reduce the reflected light from the fog (which makes the fog appear as a white wall) High lamps will relect most of the light back to the driver and reduce visibility and colour doesn't matter because the water particles size is much much higher than largest wavelength.

Rahul> You are right. The high lamps blinds and sort of hypnotises the animals in jungles.