The lamps date from the 1930s to 1970s, representing the evolution of the state of the art from single- and then two-filament tungsten bulbs through the early-1960s introduction of the first single-filament halogen bulbs, then a decade later the release of the still-familiar H4 two-filament halogen bulb.
Many of the lamps are equipped with cadmium sulphide glass filters to produce the selective-yellow light colour required from all vehicles' road-illumination lamps from 1936 until the requirement was quashed by European decree in 1993.
A great deal of myth surrounds the French yellow-light rule, which is variously said to have been enacted
- because of unique reflection characteristics of French road pavement materials,
- because of a faulty understanding of Rayleigh Scattering and glare or of the effect of light colour on visual acuity,
- because of a military need to identify the nationality of cars at night in the days before pan-European standardisation of lighting equipment, as a technical barrier to competition from lighting manufacturers outside France, etc.
In fact, scholarly research identified glare-reduction benefits to filtering the blue components out of headlamp light (resulting in the selective-yellow colour).
While debate on the matter over the years sometimes strayed from the technical to the political realm, more recent research done at America's two foremost traffic-safety research centers supports the idea that with any given level of intensity, less blue content in the headlamp light can translate to less glare and an easier time seeing in snow and fog.