Amber light

Also called yellow light, yellow fog lights, amber fog lights, selective yellow, amber lens

Amber light is off-road lighting in the yellow part of the spectrum, and what the manufacturers who sell it claim for it is less glare back from dust and fog, not better penetration through them.

What is amber light supposed to do?

Two separate claims travel under the same word, and almost every argument about yellow lights is really two people defending different ones. The first claim is penetration: that yellow light gets through dust and fog with less scattering on the way, so more of it reaches the thing you are looking at. The second is glare: that whatever does scatter back bothers your eyes less. They are not the same mechanism and they do not stand or fall together.

What the evidence supports

The glare claim is the one the industry makes. Baja Designs recommend amber on their dust and fog zone and on their cornering zone, and both times the stated benefit is reducing glare and minimising reflection. Neither page says amber penetrates anything.

What they build says more than what they write. Their forward emitters are exclusively 5000K daylight white, on the stated grounds that it reduces driver fatigue and improves terrain recognition, and amber is a lens you fit over the top of it. A company that defined a dedicated dust and fog lighting zone does not emit amber light in that zone.

The penetration side has been answered for a long time, and not by lamp makers. NCHRP Report 95, the Highway Research Board's 1970 review of driving in fog, works through the scattering physics and concludes that in most well-developed natural fogs, light transmission through the fog is essentially independent of wavelength for wavelengths in the visible spectrum. Elsewhere in the same report: theory predicts that the inherently poor signal to noise ratio of active systems cannot be affected by changing the wavelength of radiation, at least over the visible and near infrared spectrum.

The reason is particle size. Fog droplets and trail dust are one to two orders of magnitude larger than a wavelength of visible light, which puts them in what the International Lighting Vocabulary calls non-selective scattering: the scattering properties do not vary with wavelength. It is the same reason fog looks white rather than blue.

The measurement that splits the two claims

A 2023 study in Scientific Reports put halogen, xenon and LED headlamps through a fog chamber and measured both halves of the argument separately, which is exactly what makes it useful.

On transmission, the bluest source won: the transmittance of the xenon headlights was two to three percent higher than the others. On detection, at equal illuminance, the warmest source won: participants found the target fastest under halogen. The authors' explanation for that second result is glare, in their words the white wall effect in the eyes of the participants as a result of the white light reflection in a foggy environment.

Warmer light did not get through the fog better. It came back at the driver less.

That is both halves of the argument, measured separately, coming out in the direction the manufacturers already claim: nothing for penetration, something for glare. Treat it with the caution one study deserves. It compares three lamp technologies rather than isolating colour, so beam pattern, colour rendering and source type all move together, and it was fog rather than dust.

Is yellow legal?

Under the UNECE regulations that most of the world outside North America follows, yes, for one lamp only. Regulation 48 lists the permitted colour of a front fog lamp as white or selective yellow, at the applicant's choice, and Regulation 19 says the yellow may come from the light source, from the lens, or by any other suitable means. Headlamps get no such option: main beam and dipped beam are white, full stop.

In the United States the answer is more awkward, because the federal standard does not reach the question. FMVSS 108 requires headlamps to be white, but it does not regulate fog lamps at all: they appear in none of its required-lamp tables, and its colour rule binds only the lamps the standard applies to. The one federal constraint on anything you add is that it must not impair the effectiveness of the lighting the vehicle is required to have.

So fog lamp colour in the US is a matter of state vehicle code, and those vary. SAE J578 does define a selective yellow chromaticity region, and the appendix to its 1995 revision says plainly that selective yellow is used on a limited basis primarily for fog lights. Worth knowing before you assume either that yellow is illegal or that it is fine.

Amber, yellow, or selective yellow?

Three words for overlapping things. Amber is the automotive lighting term and the one Dirtbeam uses. Yellow is what almost everybody searching for it types, by roughly two to one. Selective yellow is the narrower European term for a defined chromaticity range that filters out the blue end while passing most of the rest, and it is the colour permitted for front fog lamps and headlamps under the UNECE regulations that most of the world outside North America follows.

One thing that is not in dispute: an amber lens over a white emitter throws away output, because it works by absorbing the light it does not pass. Whether a given lamp does that or uses amber emitters directly is a question worth asking the manufacturer, and one we have not been able to answer for the lamps we have looked at.