Bike lights in the rain

[mark a.]

How come my bike light that's perfectly decent normally is pretty useless in the rain?

I recently changed my route home in the evenings and it now includes a stretch on unlit country roads. (Previously I used lit, but busy, roads.) My B&M Ixon IQ is a good light, especially on full power.

When it's raining, though, it's barely adequate. I've only had it properly rain twice so far so I haven't analysed it, but is it down to physics or psychology?

[tatanab]

Same in fog. I think it is because of the colour of the light. In these circumstances I have found old halogen lights to be more useful. I don't keep such lights any more so just have to put up with LEDs on those days they may not be the best. Fortunately those days are quite few.

[gentlegreen]

The B&Ms are very nice lights - amazing what the German optics do with only 1 watt - but I've never used so little power on unlit roads - albeit my two triple 2 watt domestic lamps are a bit wasteful.
Even my emergency lamp is now a 3 watt torch ...

For years I had a 21 watt car lamp front and back, so having 12 watts available to chuck on the road seems perfectly reasonable to me.

[thirdcrank]

If you mean that the light does not illuminate the road surface so well, I think that is because a wet road surface acts rather like a mirror and reflects light. When that light arrives at a shallow angle it is reflected at the same angle further down the road, which is why oncoming headlights cause so much dazzle in the wet. I think that if you were at the edge of a large enough real mirror, and shone your lights on it, the surface of the mirror would be invisible for all practical purposes, but approaching traffic would get the light full on.

[andrew_s]

Like thirdcrank said, it's because wet road reflects light differently.

Dry road scatters light in all directions, so you see returned light from the whole road surface.
The water on a wet road smooths out all the microscopic roughness on the road, so each bit of road reflects all of the light on it in one direction. This is rarely back towards your eyes, so what you see is widely scattered bright spots with blackness between. If there's enough water to form a puddle, all the light hitting it goes forwards to dazzle oncoming road users.

B+M style LED lights with the sharp cut-off at the top of the beam are very good in fog, especially when mounted fairly low (low rider bosses or fork crown). It's all the LED lights with circular torch-style beams that are poor in fog, with head torches being worst of all.

[CREPELLO]

The light reflection thing is something I'd not really taken into account before. But I do think that the colour (temperature) of the light also has a big effect as tatanab suggested above. It's almost like being colour blind. The human eye may respond better to warmer colours in a wet environment. It certainly feels cosier to cycle in a warm coloured environment.

I don't understand how it may work, but a I had (very briefly) a pink-white Cree LED torch. I had one chance to compare it with a blue-white LED torch in the wet on a country lane.
The pink LED was definitely dimmer than the blue-white one, so not as good in the dry. In the wet though, I recall the whole scene seemed better defined than under blue-white light.

I'd speculate that a lot of the tones of the various surfaces had more blue to them than red or yellow. Vegetation is an obvious example. The wet road surface was somehow just rendered less black by having the blue in the spectrum reduced.

I somehow lost the torch a few weeks later - I hadn't even used it properly, being early autumn

[gilesjuk]

I find the same is true in the car sometimes. You are forced to rely on the reflection of the white lines rather than see them.

Some shades of LED are better than others for bike lights. Supernova lights are great.

[mark a.]

This all makes sense. I think another factor is that my light is mounted on the fork crown so is low enough to normally show up bumps and contours in the road nicely, but when it's wet it's just one flat surface.

Also, when a car comes the other way, then the same reason why my light is no longer so good means I get an eyeful of car headlight which means my night vision is impaired more than on a dry road.

This sounds like a good excuse to get another bright light. I should get a MTB light for my off-road excursions which I know won't be great in the fog and won't be ideal on the road since it will blind oncoming traffic, but as things stand the light from my B&M isn't quite adequate.

[pete75]

If you mean that the light does not illuminate the road surface so well, I think that is because a wet road surface acts rather like a mirror and reflects light. When that light arrives at a shallow angle it is reflected at the same angle further down the road, which is why oncoming headlights cause so much dazzle in the wet. I think that if you were at the edge of a large enough real mirror, and shone your lights on it, the surface of the mirror would be invisible for all practical purposes, but approaching traffic would get the light full on.

Yep. The blacker the road the better the front light is. The darker the night the better the front light is.

[vjosullivan]

The blacker the road the better the front light is.

I would have expected it to be the other way around. A white road would surely show up better (all things being equal).

[tyred]

I really struggle to see the road on a wet night with any LED light I've tried. It's why I've decided to be unfashionable and continue to use the Lumotec halogen.

[mark a.]

I'm surprised that light colour has any major effect for general use where the world has a full spectrum of colours to reflect back at you. I remember the yellow headlights that French cars used to have but I thought the theory behind that was discounted years ago (and of course they no longer use them).

[largeallan]

I believe yellow light penetrates through fog better than white

[Mick F]

Fog and rain "attenuate" the returning light. All the power pushed out has to come back to your eyes after bouncing off what you're illuminating. The returning light is at a low power and has difficulty penetrating the water droplets.

Just try telling the Officer of the Watch on the bridge of a ship that his radar isn't performing well because of the weather! I've had many an argument/discussion. High frequencies are worst than low frequencies, so navigation radar is more affected than the long range stuff. Light is a very very very high frequency, so is affected quite a bit, so the laser guided weapon systems perform poorly in fog.

It's the same with car headlights and bike lights too.

[pete75]

The blacker the road the better the front light is.

I would have expected it to be the other way around. A white road would surely show up better (all things being equal).

Don't know I've never ridden on a white road. I do know that my front light illuminates a black surface tarmac road better than the grey bridport gravel surfaces because there's more contrast between the light and the road surface colour.