Background: I'm planning to build a fog headlight for offroad bike activities in a misty autumn forest. I've found that amber LEDs have their peak at about 580nm wavelength while SOX lamps (known for their good properties in a fog) have their spectral double-line at 589nm. So I decided to give a try.

A friend of mine offered me his two extra 5W Osram Diamond Dragon - amber LEDs plus a 1A driver. He suggested to use both of them with this driver to make them highly underdriven rather than only one of them, slightly underdriven in order to get better light/heat ratio.
I checked the internet and there are articles like this that seem to support what he said. But this article is almost 5 years old.
My question is: Does it really matter? Is there a real advantage to have those LEDs underdriven?

Note: I wonder if cheap retrofit LED bulbs that are known for LED overheating produce more light in time when switched on - that gradually fades out when getting hot? Or, do LED streetlights give more light in winter? (That'd be just the opposite to PL fluorescent streetlights.)

Virtually all LED's are most efficient at low currents. Both the quantum efficiency (ratio between number of emitted photons vs electrons passing through the device; hypothetical maximum is each one electron form one photon) is high, as well as resistive losses low.
Both tend to worsen with higher temperature, but the first one is directly a function of the current (so the temperature does not play that significant role).

But there is one very good reason to not use amber as a head light:
With all road vehicles illumination color is used one rule: White are headlights, red are tail lights and amber are side lights. It became part of the technical specifications for all road vehicles, including bikes. When not following that, the vehicle become illegal on public roads. Police is not enforcing that, but it does not mean it is not important:
When you see red lights, it means the vehicle is moving away from you. When the vehicle is moving towards you, you will see white lights.
But when you see an amber light, it means that vehicle is moving sidewise, not against you. So if you will have amber headlight, someone may misjudge it as a vehicle not crossing his path, so continue driving. But you will be approaching him, so an accident waiting to happen. And guess who would end up worse there...

And by the way the effect of extra visibility with a monochromatic light is working ONLY when that monochromatic light is THE ONLY light source on the scene. ut if there are more light sources of different colors (or white light), the effect is completely gone, the other light sources "make sure" there is enough scattered light to destroy the visibility back...

That's why I wrote for offroad bike activities. And only for them. There are quite large forests in our area where there are barriers preventing cars to enter there. There's hardly anybody there in the evening anyways, just lots of wild boars, fallows etc.
I'm even going to use the white light for that about 1km public road leading to the forest. I know that police have problems even with some types of daylight car LED lights...

The amber light will be the only light in the forest - there's a complete darkness under trees.

In France until 1992, yellow headlights were mandatory on any car, bus, truck, etc. (low beams, high beams, and fog lamps were all yellow; position lamps has been white; bikes were allowed to use both white and yellow headlights). As far as I remember, despite their lower power, they allowed the driver to distinguish the road from the sides, since the original colour contrast between all elements of the road are visible through yellow light.

However I've notice it's another story when hiking in the forest using multicolour leds (I haven't tried yellow/amber, but I used green, red and blue). Trails are not as well delimited as road, so you need a good light -- high CRIs are better, they required less candlepower than standard leds.

I've used both standard "cool white" and "warm" high-CRI LEDs in havy fog situations, and I found that warm leds were satisfying.

If you ride a bike, you need to see the details better (i.e. faster) than a hiker, so I think amber led lighting is not the best of you in the forest. Try it and if you think that it could be better, consider trying 2700K high-CRI leds (e.g. the Cree XP-G 90).

@Cavannus: Thanks for the tips, looks like you've got an experience.
I did a lot of night biking when I was younger with a krypton and later with halogen headlight. Thick fog was really a big problem even with their low colour temperature and perfect CRI. That's why I wanted to try that SOX-like monochromatic light. Actually, I've got my new amber headlight ready so I think I'll test it soon.

My current bike headlight is rather bluish (high colour temperature LEDs have less phosphor and better efficacy) and it's not a good choice for a fog. If the amber light won't work as expected, I'll probably try that warm white LED you suggested. It might have properties that are better in a fog than that krypton/halogen.

As far as I know, the requirement for yellow headlights in France came just before WW2 as a measure to quickly recognize german vehicles.
After the war it served as a way to complicate import of (again mainly german) cars, so in fact support french car makers. The "improved vision" was just a cover story for international trade organizations (as direct support would be against many treaties France signed), the real improvement was really limited to only some specific driving conditions (that occurs really rarely) and it is by far not so dramatic (allow speed increase by just few percent), so it will hadrly justify yellow fog lights, but not at all the main headlights...

I agree when Medved says "The 'improved vision' was just a cover story", I remember that:

- I read on old French automotive books that "yellow light is better for night driving" and my father used to say that contrasts were better;

- in 1990-1992 in France some less-coloured yellow bubs were sold (still yellow but the closer to white): noboby really stated whether they were permitted or not, but everyone agreed that vision were better;

- in 1992 everybody in France was saying that the saw better with clear white bubs, but some people still preferred yellow headlights in some fog etc. conditions;

- when 6500K HID headlights came into the market, I read several times that "cool white is better for night driving"... which is the exact contrary of what my old books said!

Anyway, amber lights for biking may be efficient so I'm looking for Marc's results.

Monochromatic orange light is probably the worst choice for wooded areas as the green foliage will be poorly illuminated, this is why SOX is not such a good idea in areas dominated with greenery such as parks and footpaths etc.. Also remember that colour differences between illuminated objects also add contrast to the scene even in quite misty conditions :-)

I remember fog lights on old British cars being yellow.


BG