> It's obvious that the mundane "white" LED bicycle lights do not light
> up the asphalt roads very well. ?Incandescents, halogens, and sodium
> lights do better. ?There seem to be a lot of oranges and yellows on
> the illuminated asphalt surface.
> My question is why does it appear this way?
> I presume that the yellow-orange-red spectral emissions from the LED's
> are weaker than the blues. ?The blues are voltage induced while the y-
> o-r's are secondary from phosphors in the casing. ?Therefore to help
> refine my question,
> 1. ?Do the stronger blues in the LED cause the weaker yellow/orange/
> reds to be less perceived?
"White" LEDs have a strong peak in the blue and a nearly Gaussian
curve centered in the green, extending from the blue-green to the red.
They're kinda weak in the tails. Do a Google Image search for "white
led spectrum" and see for yourself.
The other lamp types you mention are stronger in the red, which the
eye is more sensitive to, particularly at night.
> 2. ?Are the yellows and oranges on the asphalt caused by sulfur
> compounds or by dust and dirt embedded in the tarry surface?
Asphalt may look black to the casual glance, but reflectance spectra
tell the tale:
Click on the various locations, check out the various surfaces.
Notice for instance "new paving, dark black asphalt, not mixed with
is pretty flat, but "old, dirty asphalt paving":
rises from the short wavelength (blue) to the long wavelength (red)
...dirty asphalt just reflects red better.
Mark L. Fergerson