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

[zotz]

There is surely no room for doubt concerning your sanity.

It sounds like it should be related to the illusory movement described here, which even mentions car mirrors, but annoyingly doesn't give it a name. All the examples I can think of do involve vibration, but I don't know why it would be more obvious with some images, such as LEDs, than with others.

[venta.livejournal.com]

That's interesting. Although, as you say, it doesn't really explain LEDs vs. everything else, I didn't know you could produce stroboscopic conditions just by vibration. I may develop an interest in eating carrots, now...

[zotz]

I don't see how that would work if they weren't flickering, but maybe they are? While the engine's running there could be a significant flicker from the alternator . . . unless it's properly smoothed, of course.

[venta.livejournal.com]

I was briefly confused there, and wondered why carrots would flicker.

I have just realised that I have remarkably little idea of how LEDs actually work, and whether they do emit continuous light or flicker very quickly. The Wikipedia page seems quite long, so my education may have to wait while I finish what I'm actually meant to be doing this morning.

I think the most noticeable effect is from overhead LED signs, where presumably alternators aren't relevant. Though I guess they may be AC powered, if that's in the least relevant. Good grief, I have absolutely no idea how all this stuff works :(

[zotz]

I can't really vouch for the publication - maybe someone else knows more about it - but this article says:

All of these LED driver solutions, including some of the more sophisticated LED drivers exhibit some degree of this line frequency sine wave artifact. Many LED drivers introduce low-frequency flicker at two times the line frequency: 100Hz flicker for a 50Hz line frequency or 120Hz for a 60Hz line frequency.

This little Q&A agrees. Hardly conclusive, though.

[shermarama]

LEDs emit continuous light, as far as I know (they're just converting electrons to photons), are usually run from DC because they need a minimum voltage in a particular direction to work, and there'd probably be a fairly well-smoothed supply going to high-power ones in a road sign. The more powerful they are, the fussier they are about their power supply, or rather it's worth being fussy about the power supply to get the most out of your expensive high-intensity LEDs.

My totally off-the-cuff theory would be something to do with point light sources, and the fact that you can see much more clearly where all the individual points are in the LED strip. They might be moving no greater distance than the headlight above them, but perceived movement of a few cm in a thing that's clearly a small point might trigger different 'moving thing!' brain reactions to the same perceived movement in the relatively much bigger headlight.

[shermarama]

(Never noticed it myself, though, and can't keep an eye out for it because I don't have a car right now. Perhaps in another era you would have made a particularly sharp-eyed hunter?)

[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.