Yellow (Amber) Headlights.For Crazy Frenchmen Only?
Yellow Headlights: Purely Cosmetic or Functional?
Have you ever wondered if yellow (technically selective yellow that filters out blue, indigo and violet) headlights are purely cosmetic, if they actually have a function, or if they’re just a product of wacky Frenchmen? My uninformed impression is that they were mostly cosmetic while sacrificing some illumination. The answer, as it just about always is when you dig a bit, is more complex.
They possibly improve nighttime visibility under some conditions without sacrificing much brightness. There is an approximately 12% loss of light which many would find affordable given the brightness of today’s headlights, especially the high performance H4 bulbs and LED’s. On the plus side, there is a gain in visibility in inclement, hazy or foggy weather. The technical reason is that the selective yellow removes blue to violet colors from the wavelength spectrum that can cause difficulty for human eyes to process, which reduces what we think of as reflection.
On the color spectrum yellow light has a longer wavelength than blue to violet colors. Shorter wavelengths are harder for the human eye to see and tend to focus in front of the retina. In inclement weather, the yellow headlight filters out filtering out blue and the like, so there is less of that light produced to be reflected back from rain, fog or snow. So there is less perception of glare.
 Color is measured on a Kelvin Scale:
Most LED headlights measure in the 5500-6500 Kelvins area while yellow headlights are in the 3000 or a bit less area. Interestingly, LED headlights with yellow lenses emit a light fairly close to the warm light we are used to seeing from older headlights.
It all depends on your environment and most common prevailing conditions. If you live in an area with frequent fog, rain, snow, dust or smog, it might be worth considering yellow headlights.
If you would *really* like to dig down into this Daniel Stern has an amazing article about selective yellow lighting.