caught in thick fog......

[Steve K]

Xenon strobes illuminate the entire visual field, and have no beam that can be easily traced back to the source. They'll know you're there, but don't know exactly where. This is a recipe for hurting.


...<snip>....

A very bright, stead-burning red lamp might be best for the left-rear of the bicycle, like a rear fog lamp for a car.

well, I did say that I use the white strobe in addition to my red taillights, so the white strobe is just an attempt to get motorists to know that something was ahead, and the "something" is not typical.

the local school buses have white strobes on top of them, and my recollection is that I've noticed the strobes well before I saw their taillights.

 

[swhs]

A beam with a good vertical cutoff is to your advantage. This is why low beams in a car are already better than high beams when in heavy fog.

I tested it. And it didn't make any difference for on the bike at least (angles are slightly bigger than in cars) but as I looked at fairly small angles similar to a car (with my flashlight tests), I think it will not matter there much either. I tested that because some people claim that in fog a cutoff beam is better because with a non cutoff beam you are looking through a fog. This sounds reasonable at first, but I found in my tests that this effect was not noticeable. And when you think more about it, it is not surprising, because what happens in fog is scattering of light. If you have a cutoff beam, light will get scattered to not-very-cutoff-at-all. It doesn't look to me as being retro-reflection, although that effect is present, the tests I did show that at big angles the "wall of light" effect is still big, as long as the light source is close to your eyes, there will a cloud of light emitters (the water particles that reflect or refract which can give a bounce back to the source in multiple stages and those you can consider new light sources. Have those close to you means you get a lot of that light directly into your eyes). Note that the reflective index of water is only 1.33 times that of air, which means the mirror effect at the rear of a water droplet (which makes retro-reflectors of them) will be fairly weak. Also, looking through the haze at small angles (behind the lamp) doesn't give a "wow, I'm getting blinded" effect, so this seems to indicate that scattering of light which gives a 'light emitting cloud' which you need to get reasonably far from you so as not to get too much of it directly into your eyes, is the issue.

 

[smokinbasser]

Encountering heavy fog on a lake is bad enough but on a major river like say the Mississippi or the Ohio on a boat without GPS you might end up at a lock and dam you have no idea of where it is or which way to go to get back to the starting point.

 

[Marcturus]

My observations of beams riding in thick fog, at night, would lead me to offer similar recommendations as Norm and Alaric. The cutoff simply saves you the upper part of a "flashlight" beam that will mostly light up fog (add noise) atop of the part of the beam that is also lighting up the fog, but is actually containing a few photons that will eventually make it to the ground almost where intended, and back to your eyes, too. I was not testing visibility to oncoming traffic, however.

 

[swhs]

My observations of beams riding in thick fog, at night, would lead me to offer similar recommendations as Norm and Alaric. The cutoff simply saves you the upper part of a "flashlight" beam that will mostly light up fog (add noise) atop of the part of the beam that is also lighting up the fog, but is actually containing a few photons that will eventually make it to the ground almost where intended, and back to your eyes, too. I was not testing visibility to oncoming traffic, however.

I tested with cutoff beams and circular beams for what I could see (ahead and on the ground), and the cutoff will mean a little less 'noise' but it was insignificant and as I said, the entire beam mostly lights up the fog because of the scattering, this is the whole reason why it doesn't make much difference as the cutoff isn't that big of a cutoff any more... You can test it by aiming a cutoff beam (I tried e.g. with the Edelux) fairly high (my test was such that most of the light went into the air and under normal circumstances, no fog, this would mean a very dim illumination of the road) and I noticed that there was no difference in visibility of the road: The light going up into the air, comes down onto the road from the scattering. And also, the light going up into the air did NOT diminish what I could on the road or ahead.

 

[swhs]

So, I was thinking about it a bit more, and perhaps the people who do notice a difference between cutoff and non-cutoff, have less sensitive vision? (and it gets under a certain contrast limit for them?) Like some people claim they can only see 35m with the LBL on high, whereas I see 70m with it. But when next there is a fog I will repeat my experiments, though I tested all these things extensively long ago, and for me, there was just about zilch difference in what I could see of the road surface, fences etc. I'll ask a friend of mine to test when he can too, of whom I know he has worse vision than me

But those who really think it makes a difference: What tests did you do? Which lamps?

I tested the Edelux normal and pointing upwards, compared with various flashlights (of about same lightoutput as the Edelux), tested the Betty 1850lm at 10% and at 100% and compared it with a cutoff dynamo lamps (Edelux, Saferide 60), and all of it showed just about no difference in what I could see...

Or perhaps the issue is very wide beam angle symmetric lamps? (the flashlights and Betty that I used have fairly tight beams). The scattering experiment with the Edelux still makes me think it just doesn't matter, but I will check again, when there's fog.