Dear rtech:
The bright orange used for survival gear is reflective pigment which is optically subtractive, not emitted light which is optically additive!
To understand this, one has to come to terms with the concept of additive colors (the case of light) as opposed to subtractive colors (the case of paint).
For example, mixing all reflective pigments of the colors of the rainbow will give you black pigment as the result, whereas mixing all emissive light colors in the rainbow will give you white light as the result.
So, the argument that alternating yellow/orange and black paint stripes are used on survival gear, does not lead by implication, to the fallacious premise that orange light would be somehow be better than white light.
Yellow light is easier to spot through the fog than blue light.
If that is what you meant, you are right!
It is true that the spectral response of the human eye is higher for yellow light than for blue light.
So, if we were to use the yellow-orange photon emissions
from a 5890 Angstrom sodium-vapor lamp as a fog-lamp
then we could enjoy the advantage of lower electrical energy consumption for better visibility.
Now let's dwell a little on the "so-called" dispersive scattering effect.
Yellow light does suffers less backscatter than blue.
But this is discernible only for light scattered off the very tiny aerosol particles (dust, smoke and in hundreds of miles of fog -- blue light is scattered out of the path from the sun to the viewer of a sunset, leaving white-minus-blue light to reach us... so we sees a yellow, then orange, then red sunset.
Mind you, it takes sunlight penetrating through miles and miles of fog, to get this color effect!
The size of water drops in fog on the road is not small enough for producing the kind of color scattering mentioned above. If it did, then in a fog, white headlights would appear very red to oncoming traffic. We know of course, that they don’t, don’t we?
We must appreciate that, filtration of light is an intensity-destructive process, not an intensity-enhancing process!
So it does not follow that yellow or amber light is any easier to spot through the fog than the unfiltered white light that yielded that yellow component, after filtration! If you wanted to be seen better, a flashing aircraft strobe (such as used on airport emergency vehicles) would be more effective, but we don't see much of those as car fog lights.
Conventional automotive yellow fog-lights produce full-spectrum white light via an incandescent filament or halogen source. Then they filter off the blue end of the spectrum, leaving yellow light behind.
This trashes both:
the visibility objective
as well as
the efficiency objective.
Filtering off and discarding the blue component, does not make the resulting light any brighter. If anything it will make it dimmer!
It would be more visible to just leave the white light unfiltered.
The oncoming driver would see you better if you used all the white light.
Accordingly, modern European cars no longer have amber fog lights!
Most likely, the real reason that people have used yellow fog lamps is for their cool, bold, aggressive image.
Last edited by Ram : 12th July 2006 at 18:14.
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