Put out the lights on the age of reason

[venta]

Help me, LJ, you (and your knowledge of physics) are my only hope.

Why do LEDs bounce up and down in car mirrors?

I noticed ages ago that, when driving down the motorway, the LED variable-speed-limit signs bounce around crazily when seen in the rear-view mirror. Obviously in a mirror you're looking at the other carriageway's signs, and the road, cars, streetlights, gantry etc are all more or less stationary but the LED signs? Well, they're joggling around like wild things.

Trundling back on Monday from a trip to Scotland, I spotted that some cars (mostly newish Mercs, I think) now have a strip of white LEDs under their normal headlights. When viewed through the rear mirror the car and the headlights look perfectly sensible, while the LEDs are - you'll have guessed - madly dancing around. I don't know if our car has an unusually vibration-prone mirror, but in general the image in the mirror seems stable.

I haven't noticed this driving at non-motorway speeds - though I'm also not sure if I've had the requisite LEDs-behind-me in any other situation.

I just asked Physics!colleague about it, and his first theory ran thusly: LEDs have quite a narrow field of view (compared to other lights), so if the object in the mirror is offset (as a sign on the other carriageway would be) you might be on the edge of the field of view, and the effect is that the light pops in and out of vision. This feels inherently wrong to me: I'd expect the sign to look more flickery than it does. Also, a Merc driving in the lane behind me is not offset.

His second theory (bolstered by a quick google) was that there is no such effect, I am insane, and ChrisC (who claims also to have observed it) is just humouring me.

So... has anyone else seen this happen? Anyone know why it happens?

Comments

[venta.livejournal.com]

I've only started noticing it fairly recently, so I'm guessing it might be that LEDs are just becoming a lot more prevalent than they were a few years ago.

I suspect hunting wouldn't be my forte. At least, not unless I was allowed to claim deer, boar etc that died laughing as I crashed noisily through the undergrowth towards them.

[shermarama]

Oh, yeah. LEDs have a lot to recommend them as a light source but it's only recently they've been usable for actual lighting applications. The difference between what was called an ultrabright LED ten years ago compared to now is almost shocking.