h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Er, sorry this has nothing to do with driving on the left or right or climbing spiral staircases but there are obviously some transport history experts frequenting this discussion and such opportunities must be grasped.

So, why did/do the French often have yellow headlights? Actually, it's not so common now but it used to be.

I've read two explanations. One is that they're easier to see in fog (???) and the other is that during the Normandy Landings and aftermath the Allies told French Resistance types to paint their headlamps yellow as an identifier (this is also the case with the black and white stripes on Allied vehicles and aircraft) and this tradition carried on. And on. Anyone got a definitive answer?

-Dan-

Just that yellow is more bright - something to do with a higher reflective index or some hogwash like that ...

OK. Sort of. But are headlights meant to illuminate the road ahead or to make the vehicle more visible to oncoming traffic? Do yellow painted lamps throw as much light onto the road? I'm sure I remember going on a family holiday to France in about 1980 where we actually had to buy some yellow varnish stuff to paint the lights with, as a legal requirement for driving there. But that sounds a bit odd really.

I've also often wondered why we all drive around brightly lit city centers with blinding headlamps on instead of sidelights (or, even better, those flashing green LEDs sensible cyclists use).

-Dan-

Yes! I remember that yellow goo as well. I suspect the use of it was more to do with keeping the peace than anything else, although, naturally, any coloured transparent material will affect the light frequency of the source of any light that passes through it. No doubt the benefit of improved visibility was fundamentally for the bloke whizzing around the Ring Parifique (is that how 'North Circular' is spelt in French? ... I somehow doubt it...) at 220 Km/h, although the half dozen or so pedestrians and oncoming scooters that he narrowly missed en-route were no doubt grateful of the benefit. You will no doubt remember that the chevron signs at road bends also tended to be yellow on black in France. Some of those do now include flashing LEDs of course

Yes! I remember that yellow goo as well. I suspect the use of it was more to do with keeping the peace than anything else, although, naturally, any coloured transparent material will affect the light frequency of the source of any light that passes through it. No doubt the benefit of improved visibility was fundamentally for the bloke whizzing around the Ring Parifique (is that how 'North Circular' is spelt in French? ... I somehow doubt it...) at 220 Km/h, although the half dozen or so pedestrians and oncoming scooters that he narrowly missed en-route were no doubt grateful of the benefit. You will no doubt remember that the chevron signs at road bends also tended to be yellow on black in France. Some of those do now include flashing LEDs of course

Yes! I remember that yellow goo as well. I suspect the use of it was more to do with keeping the peace than anything else, although, naturally, any coloured transparent material will affect the light frequency of the source of any light that passes through it. No doubt the benefit of improved visibility was fundamentally for the bloke whizzing around the Ring Parifique (is that how 'North Circular' is spelt in French? ... I somehow doubt it...) at 220 Km/h, although the half dozen or so pedestrians and oncoming scooters that he narrowly missed en-route were no doubt grateful of the benefit. You will no doubt remember that the chevron signs at road bends also tended to be yellow on black in France. Some of those do now include flashing LEDs of course

Yes! I remember that yellow goo as well. I suspect the use of it was more to do with keeping the peace than anything else, although, naturally, any coloured transparent material will affect the light frequency of the source of any light that passes through it. No doubt the benefit of improved visibility was fundamentally for the bloke whizzing around the Ring Parifique (is that how 'North Circular' is spelt in French? ... I somehow doubt it...) at 220 Km/h, although the half dozen or so pedestrians and oncoming scooters that he narrowly missed en-route were no doubt grateful of the benefit. You will no doubt remember that the chevron signs at road bends also tended to be yellow on black in France. Some of those do now include flashing LEDs of course

Erm... sorry for the multiple response ... I kept getting silly connection failure messages

It used to be believed that yellow lights were better in fog. Actually, they're no better than white ones. It's just that the yellow light doesn't reflect off the fog quite as much as white light does, so the yellow lights *seem* to be better.

Doesn't yellow improve contrast?
Isn't that why wearing yellw tinted glasses helps in the same way.

As I heard it; someone told Charles de Gaulle that yellow light "travels further" into fog than white light. Something to do with refractive indices, I guess.

What no-one told the Great Man, was that this was based on light of the same intensity. If you take a white light-source and paint it with yellow goo, you get less light, because you cut out all the light that is not yellow, and therefore the yellow headlights do not "travel further" into fog. So the whole thing was based on erroneous thinking.

I cannot verify either the physics or the history though.

I thought that was why English streetlights were yellow, because they showed up through fog better.
Also, AFAIK you are not allowed to paint your headlights yellow to drive abroad anymore (which was done so that your headlights didn't beam straight into oncoming traffic), you have to put those sticker things on to bend the beam.

Most English streetlights are sodium. Is anyone a chemist? Is sodium lighting "naturally" yellow? Ie - not white light being cut back with yellow glass in the bulb?

In the Cotswolds, you can see the individual lights on Blunsden Hill in Swindon, 20+ miles away....

Aargh! Light pollution!
Astronomers beware.

Yellow Streetlights are generally low-pressure sodium (LPS) discharge lamps.
They are used because of their longevity and efficiency - there are a lot of 'em, they're a bit of a pain in the arse to replace. Watt for Watt, they produce about 95% more light (measured in lumens) than your average incandescent light bulb. Typically, a LPS lamp will achieve an average life of 16000 hours usage, compared to the rather pathetic 1000 hours that the bulb in your bedroom manages.
There is a trade off naturally, and that is in the colour temperature that the lamp produces (measured in ° Kelvin). Tungsten Halogen lamps tend to produce a colour temperature of around 3200 °K which is kind of milky, slightly yellowish, but mostly white. Daylight is getting up to something around the 5600°K mark, but the sodium's belt out a cracking ... 1800°K. Daylight is to boiling water what LPS is to a nice cool bath, basically.
Lamp manufacturer General Electric (set up by the chap responsible for the first incandescent lamp - Thomas Edison) claim that the LPS lamps that they make "produce light at wavelengths close to the peak sensitivity of the human eye and so produce one of the most efficient types of artificial light". So there you go...