French cars were recognized earlier at night from afar, on the striking yellow headlights. Today, the characteristic yellow lights are prohibited, there are only two exceptions: Older vehicles may continue to use the already pre-installed headlights - and fog lights may still shine in the color "Selective Yellow".
Because yellow lights have advantages, especially in winter, when fog obstructs the view: their dark yellow light fades less strongly than white. This is due to the so-called Rayleigh scattering. In doing so, the electromagnetic waves of the light are deflected by particles smaller than the respective wavelength when they strike atoms or molecules in the atmosphere - but not all parts of the light.
Colors such as yellow and red can penetrate the air more easily thanks to their high wavelength, while the short-wave blue light components are scattered, making the sky blue. Exactly this effect occurs also in the spotlight in the fog. Yellow light components are deflected less strongly and can better penetrate a fog wall, which in turn can provide greater visibility. Not least because of this effect, yellow headlights were mandatory in France until 1993.
Yellow headlights are "light killers"
But why were these headlamps banned if they offered benefits in the darker half of the year? That was due to the functional principle of the yellow lights, explains Steffen Pietzonka, Head of Marketing at headlight manufacturer Hella. Because the cheerful color of the French headlights halogen lights behind a yellow disc - or completely yellow-colored bulbs. However, this design inevitably has a big disadvantage, Pietzonka emphasizes: "Such a construction always takes away light, whether it is yellow, blue or green."
So yellow headlights were de facto light killers because the blue spectral components were taken out through the yellow lens. Although the yellow headlights offered a slightly better visibility in fog, overall, and thus in all other scenarios, a worse light output - and thus not the best possible view for the driver. Today they are therefore illegal as low beam. Only fog lights in the ECE-compliant color "Selective Yellow" are permitted. These are offered by some companies as retrofit sets, but play no role in the market anymore.
Laser light can compensate for lack of brightness - but not other disadvantages
Although modern LED headlights could also shine without washers or similar devices yellow and would therefore be just as bright as headlights without coloring. However, even with yellow LED lights the blue spectral component would be missing. And that would create another problem.
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"Natural color rendition and a good contrast ratio are crucial to distinguish objects in a dark environment." This is especially relevant for the elderly, says Pietzonka. "Fine nuances like a rather gray boar in front of brown tree trunks can only be recognized in the dark by good colors and contrasts." In the yellow light, however, exactly those were lost and everything is a gray mass. "Yellow light", calculates Pietzonka, "offers no advantages, even laser light does not change anything."
Cold daylight offers the greatest security
Even in wintry conditions therefore daylight-like headlight is considered optimal. "A light source in daylight white, in which all color components are contained, the environment is most natural," says Pietzonka. However, such a headlight shines a little bluish, especially in comparison to the halogen headlamps older design.
Their light has a temperature between around 3000 Kelvin - and is thus more in the range of the perceived as comfortable warm white light. With a light color of 6000 to 8000 Kelvin, daylight, whose color temperature only reaches xenon or LED headlights, is more reminiscent of the lighting in an operating room.
The already small advantage of yellow spotlights through the Rayleigh scattering can compensate for modern, daylight white headlights in other ways. To avoid glare effects, they can dimming areas such as reflective surfaces in the wet or reflective fog in the direct field of view, explains Pietzonka. "This more than compensates for the minimal benefits of yellow headlights in fog."