The 'Yellow Foglights are better in rain' myth

[spikespiegal]

I run into this 'urban-myth' quite frequently given I build energy efficient lamps as a hobby. Wouldn't bother me so much if the proponents of it weren't so emotional about it. I'm either missing something obvious, or somebody lacks a fundamental understanding of physics.

Basically, we've all heard that 'warmer colors are better for penetrating fog/smoke/rain/whatever' because "longer wavelengths scatter more than shorter wavelengths". Pop culture proof and example of this has been the 'sky is blue' rebuttal. Another one I hear is some firefighters refusing to use higher tech lighting like HID or LED and preferring incandescent instead for the same reason. "Warmer light goes through smoke better". It's also another supportive reason that high pressure sodium lamps are still common for street lighting because their 'warmer spectra does better under foul weather conditions'.

Ok, I understand how Rayleigh scattering works, but I have a hard time finding the relataionship between sunlight scattering off of hundreds of cubic miles of atmosphere cumulating in a dull blue sky and a head lamp going a few hundred feet.

My counter arguement and proof that Rayleigh scattering in the atmosphere has *nothing* to do with artifical light sources in rain/fog/smoke is quite simple. Take a white light source, headlamp, high powered flashlight, etc. (anything mixed spectra), and whilst in rain/fog/smoke view the beam from several hundred feet away. If "longer wavelengths scatter less than shorter wavelengths in fog/rain/smoke", then shouldn't the light source show a blue halo or ring around it? Even halogen car headlights should show a blue scattering when viewed head on in the rain because they are essentially a full spectrum light source if we assume the 'wavelength scattering scenario' is true. Never seen a halogen light produce a blue ring in rain/fog/smoke, so this should debunk it.

I also blame some of the support for this myth based on the retina's non linear sensitivity to different colors. Blue-green for instance is going to seem brighter than yellow-red under high contrast night conditions.

Does my response have merit, or are there holes in it.

 

[Colt45]

You should probably drive with some french yellows for a while first. The thing I liked (before the cops made me get rid of them), was that they were easier on the eyes, especially in winter ( with all the snow reflection).

When your eyes aren't strained, I guess you can say you can see better... no?
Even more so in situations that impart more strain (rain, snow, fog);

I think the higher frequency light buggers your night vision more, too.

 

[ScottMac]

Off the top of my head, Id add that it likely has to do somewhat with the color sensitivity of the average eyeball too. Green light registers higher than the other colors on either side ... so perhaps using the longer wavelength component (yellow) rather then the shorter wavelength component (blue) plays into it somehow.

I know there is a green light used in aviation, in the cockpit; I believe that in addition to maintaining reasonable night vision, it provides contrast on aviation charts that red does not (some of the figures on the aviation charts are red-tones).

 

[CycloWizard]

Green is the best color for dashboard/cockpit displays because a relatively low light intensity is required for you to see well (due to the eye's natural sensitivity to green wavelengths, as ScottMac mentioned). Thus, since fewer photons are required for you to see your display, less light adaptation occurs, allowing you to still see in the dark. This is also why dashboard indicators dim when you turn on your headlights (at least in more recent cars).

As for the reason yellow could be better than white, it could be due to chromatic aberration. If your light emits only a single wavelength (or at least a narrow distribution), then less chromatic aberration would occur. I'm not really familiar with the hot debate you're talking about, but this is just something you might want to read up on for more information.

 

[Paperdoc]

Use of yellow light is supposed to be based on light scattering, yes, and for two reasons. I will say right up front, however, that I do not know what actual studies have been done to check the arguments. Facts are always better than hypotheses, and the latter is all we have here so far.

First, your argument about observing a white light source from a distance through a fog as a light scatterer. You say you would expect a blue halo effect because the shorter-wavelength blue light will be scattered more. But supposing the "scattering more" part it is true, remember that the scattering will be in ALL directions at every scattering point. This will NOT produce a preference for blue light to be scattered radially out from the beam center, and hence there should NOT be a blue ring or fringe created. Instead what should happen is that less of the blue light actually reaches you as an observer, because more of it was scattered off of the straight light path from source to your eye. Thus the hypothetical prediction is not a blue halo; rather, the prediction is that you, the observer, will see the light as having reduced blue-end color in it - it will look somewhat creamy yellow instead of true balanced white. I believe that is what actually happens.

For you to see something with headlights, the light has to travel from the source to a reflective object at a distance, then return to your eye in such a way that the exact pattern of reflected rays is relatively unimpeded in space, and your brain can interpret the light to discern the shape details of the object and differentiate it from other light patterns from surrounding objects. Now consider what happens if there is fog in the light path - and fog is microscopic water droplets, each of them a potential light-bending object that causes scattering. The first effect is that the light that arrives at the distant object is less intense than in the non-fog case, and its color balance will have changed. Moreover, some of it is not arriving at the right angle - that is, it is not coming straight from the source. Now the light is reflected and must travel back to your eye. It experiences the same sorts of alteration - color balance change, reduction in intensity, and distortion of light path. All of these effects are dependent on the color (wavelength) of the light source. So, hypothetically, a light source with less blue content will offer less opportunity for distortion due to scattering and what does actually return to your eye will be a better-quality image for the brain to analyze. BUT there are two other things to consider. One is that the usual way yellow light is made available in auto headlight systems is to use a yellow glass lens. In reality this is a selective ABSORBER of both blue and red wavelengths, so the net result is that what light is emitted is of significantly lower intensity than the original white light, even in the yellow and green colors the lens is supposed to pass. Another consideration is what filter to use. If the theory says longer wavelengths will scatter less, we should use a filter system that passes only red light. But our eyes as detectors have much lower sensitivity to red light than to yellow or green. So using only red light would require we use a LOT more light intensity to make up for the reduced sensitivity. The balance that has been used traditionally (and may or may not be best) between light source color and eye sensitivity is to use yellow light that has some green and red content, but little blue and violet.

I would question the suggestion that high-pressure sodium lamps are preferred because their color fits the human eye's sensitivity well. It happens in nature that the output of yellow-orange light at two closely-spaced wavelengths from hot sodium is very high, so the efficiency of converting electrical energy to light is very good if sodium is involved. This makes for lower operating costs for a given light output target. It's also relatively easy (and low cost) to buy and use sodium as a material in the lamps. So I'm betting lower cost, not eye sensitivity, is a very significant driver in the prevalence of these yellowish street lamps.

 

[TuxDave]

In regards to smoke and fog, have you considered the effects due to dispersion and not Rayleigh scattering? Fog and smoke will consist mostly of water vapor which will refract the light considerably and have a wavelength dependent nature.

Rayleigh scattering is due to absorption of energy into nitrogen in our atmosphere. In this case with smoke/fog you will probably see a lot more refraction/dispersion as the water beds through the water vapor in the air. For bending in water, I believe red will bend the least while blues (higher frequency) will bend more.

 

[deputc26]

Originally posted by: ScottMac
Off the top of my head, Id add that it likely has to ockpitdo somewhat with the color sensitivity of the average eyeball too. Green light registers higher than the other colors on either side ... so perhaps using the longer wavelength component (yellow) rather then the shorter wavelength component (blue) plays into it somehow.

I know there is a green light used in aviation, in the cockpit; I believe that in addition to maintaining reasonable night vision, it provides contrast on aviation charts that red does not (some of the figures on the aviation charts are red-tones).

As a commercial pilot, I can guarantee that this is not true. First off a little physiology:

There are 2 types of light sensitive cells in your eye. "Cones" see sharp detail and color, in bright light. Rods do not see color but are exceptionally sensitive to light (up to 100,000 as sensitive as cones) and also detect motion. Generally we use cones in the day and rods at night (though really we are always using both types to a degree).

Red is the most commonly used color at night in the cockpit. The reason for this is that red is outside the detection range of rhodopsin. Rhodopsin is a pigment that builds up in your "rod-type" light sensitive cells, the more rhodopsin, the more sensitive to light your rod cells will be. It takes ~30 min of no exposure to bright lights for your eyes to achieve maximum rhodopsin (and thus maximum sensitivity to light). Any light with a longer wavelength than ~620nm does not "bleach" the rhodopsin out of your eyes (620nm corresponds to "red") and is invisible to your rods. Thus in red light your night vision is maintained but you are still able to see (with your cones) objects illuminated in the red light.

This is an excellent site on the subject.

 

[Gunbuster]

Red is the most commonly used color at night in the cockpit. The reason for this is that red is outside the detection range of rhodopsin. Rhodopsin is a pigment that builds up in your "rod-type" light sensitive cells, the more rhodopsin, the more sensitive to light your rod cells will be. It takes ~30 min of no exposure to bright lights for your eyes to achieve maximum rhodopsin (and thus maximum sensitivity to light). Any light with a longer wavelength than ~620nm does not "bleach" the rhodopsin out of your eyes (620nm corresponds to "red") and is invisible to your rods. Thus in red light your night vision is maintained but you are still able to see (with your cones) objects illuminated in the red light.

Right on dude! I cant stand the bright blue instrument lighting fad going on with cars at the moment. Red/Orange/Green in that order for me!

 

[gsellis]

Hmmm... the myth must be born from the statement that the human eye can see better in yellow light (and then someone said because of our yellow sun). No proof that I have ever seen. One of those "everyone knows", but no one knew who They were that said it.

Foglights in the rain are not intuitive. The light is closer to the ground and as it rains, the scatter is even worse because of the proximity to all the bouncing rain drops. That would lead to even worse scattered light in the falling rain and would reduce visiblity!

The best thing in the rain would be clear polarizing lens on the operator or next, polarized sunglasses. But remember that polarized glasses in an airplane cockpit are something different (they increase the difficulty of reading instruments because of the protective covers on each instrument.)

 

[bruceb]

I actually like to use fog lights in the rain. Seems to give the roads here in NJ better visibility. As to color, I have had cars with both white fogs and also with yellow. If they are a good brand, like CIBIE, then either one does an excellent job. But in my view, yellow works a little better in
heavy fog or snow.

 

[spikespiegal]

I cant stand the bright blue instrument lighting fad going on with cars at the moment. Red/Orange/Green in that order for me!

I have a Mazda M3 with the annoying 'red/orange' instrument lighting.

I strongly prefer Honda's white on black style.

Thus in red light your night vision is maintained but you are still able to see (with your cones) objects illuminated in the red light

It's also common knowledge in amatuer astronomy. If you want to read a star chart at 2am you either use a red LED or a heavy red filter on your flashlight, unless oyu want to wait 10minutes for your pupils to dialate fully again.

 

[RiverRicer]

I have a 140mi. roundtrip commute on a rural, mountain road. This means that in the winter I leave and return in the dark. I heard that bluish lighting had better contrast, so I bought a pair of 7000K bulbs to try. The first substantial rainstorm proved this was based on BS - couldn't see a thing through the rain. The 7000Ks now reside somewhere in a landfill, having been replaced with good old 4300Ks.