Naked Science Forum
General Science => General Science => Topic started by: AllenG on 08/08/2012 22:29:09
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The family and I were riding home the other night and I was complaining about how LED lights strobe when I look away from them. The rest of my family couldn't see it.
What's going on here?
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.awcg.com%2FImages%2FTNS%2Flexus-is-tail-lights-1440.jpg&hash=6b9bb13b66af4f7acbb4d1251e685b85)
^^^
What my family sees.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.awcg.com%2FImages%2FTNS%2Flexus-is-tail-lights-trail.jpg&hash=0b1a7e827d9dce8b93ff0ce0df29b864)
^^^
What I see.
It's just LED lights that strobe too. Incandescent don't. Some makes of cars are worse than others as well. Cadillac's SUVs are especially bad.
The trails are strobes as well, not streaks. Do LEDs power on and off like fluorescent lights?
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It does sound as if they are turning on and off. The clue is that you notice it with your peripheral vision which has a faster response than your central vision.
My hunch is that they are deliberately maximizing the apparent brightness by driving the leds with short duration, high current, pulses. The eye is more of a peak detector than an integrator, so, for a particular average power, the LEDs will appear brighter when pulsed than when driven with DC. You can't do the same trick with incandescent bulbs because they have a lot of thermal inertia. LEDs can turn off and on relatively quickly.
If that is what they are doing, it's a bit naughty. Some people are very sensitive to strobed light. In my case it can even trigger a migraine attack.
I have noticed that the LED lights on Cadillac SUVs are particularly annoying. I thought it was just because they turn on so quickly, but maybe I'm detecting the strobe without realizing it.
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I would think that LED or incandescent automotive lights would be running on DC, and thus not strobing. Although, perhaps the alternator would cause variable battery voltage, and give somewhat of a strobe effect.
Geezer's point is that they could potentially save power by strobing the lights. But, the saving would be minimal. Perhaps it helps extend the bulb life at high power output, but again with well constructed LEDs, the savings might be minimal. Keep in mind that tail lights are supposed to be relatively dim, with brake lights much brighter (in the USA, most tail lights, brake lights, and rear turn signals are red).
I have noticed sometimes after driving at night, when I enter my house, I will get an after-effect as if my eyeballs are strobing. So, perhaps it is all the other strobing lights, such as headlights coming and going, passing signs and street lights, and etc. I'm trying to remember, it may have also happened with walking in the city. Anyway, perhaps it is not as much the taillights, as everything else around you.
BTW, do you wear glasses?
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I do wear glasses.
However it occurs with specific tail lights and not others. One car can have strobing lights while the one next to it will not. It only occurs when I look away from the lights. But given how our vision works, only the very center of our field of view is in focus and one constantly scans around, the strobing can be very annoying.
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I think you ought to do an experiment with your family. See if you can find an LED bicycle light so that you can move the light quickly instead of your family having to do the work by moving their eyes. If they still can't see the effect, it's a biological difference limiting their ability to see the action, but I doubt that'll happen. I suspect they just aren't moving their eyes the way that you do or that they're actually closing them while moving them, thereby missing the thing you're trying to get them to see.
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Of course, it could simply be the mechanism they are using to control the power. LEDs are diodes (doh!) and they have a low forward voltage (around a couple of volts) that only varies slightly with current, so they require some sort of current limiting device. The simplest way to do that is with a series resistor, but that's very inefficient if they are being driven from 12 volts.
They might be using some sort of switching regulator to drop the voltage and limit the current while operating in tail light mode. The cheapest way to control the power efficiently is simply to turn the LEDs off and on with a particular duty cycle. It may be that they need to up the frequency a bit.
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Ah ha!
"The IC delivers a 200Hz drive signal to the LEDs, with duty cycle adjusted according to the voltage level at DIM"
http://www.maxim-ic.com/app-notes/index.mvp/id/4316 (http://www.maxim-ic.com/app-notes/index.mvp/id/4316)
Some people can probably detect 200Hz with their peripheral vision.
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My hunch is that they are deliberately maximizing the apparent brightness by driving the leds with short duration, high current, pulses.
Tail lights have maximum permitted brightness in different geographical regions ...
Outside North America, the range of acceptable intensity for a stop lamp is 60 to 185 candela.
In North America, the acceptable range for a single-compartment stop lamp is 80 to 300 candela.
https://en.wikipedia.org/wiki/Automotive_lighting#Stop_lamps_.28brake_lights.29
To control the brightness of an LED the most efficient way is to pulse it ...
Some other types of light sources such as light-emitting diodes (LEDs), however, turn on and off extremely rapidly and would perceivably flicker if supplied with low frequency drive voltages. Perceivable flicker effects from such rapid response light sources can be reduced by increasing the PWM frequency. If the light fluctuations are sufficiently rapid, the human visual system can no longer resolve them and the eye perceives the time average intensity without flicker (see flicker fusion threshold).
https://en.wikipedia.org/wiki/Pulse-width_modulation#Power_delivery
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To control the brightness of an LED the most efficient way is to pulse it ...
Gosh! Is there an echo in here?
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I notice flicker with some LED tail lights - and increasingly on truck lights.
I think some people are just more sensitive to flicker than other people.
It is fairly easy to increase the switching frequency of LEDs - I just think that on some vehicles, the frequency has been set too low.
There are some new standards in development that would use the upcoming household LED lighting as a data communications point for portable electronic devices within the house. The challenge here is to transmit varying information streams without visibly changing the brightness of the LED lights. One of the draft standards even allows the LED lights to be dimmed, while still transmitting a useful amount of data.
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In my estimation, it is intended that these lights show up solid if viewed directly, and flickering if viewed either as your eyes dither, re-focus side to side, or if you look essentially ahead, and the Cadillac moves in your field of vision side to side (such as would happen if a vehicle were to try to run a red light from one side).
It is my understanding that humans see such intensity changes as a solid on, if viewed directly ahead and the speed is above around 24 Hz. This permits our vision to sense movies and TV as continuous motion. This must be intended as a safety feature. Bravo Cadillac. I am truly sorry that this irritates you.
Cadillac are an early adopter of technology and probably understand this phenomenon. They want to be just above your speed to sense the LED's as solid unless a vehicle moves side to side in your field of view. If your kids don't sense it, they might be able to if they look at one side of the vehicle and then the other, without letting their focus be on the vehicle itself. I understand that animals have a much higher threshold maybe around 90 Hz. That may be why animals seem to be frustrated with TV.
btw, at higher speeds than this it is possible to transfer messages, that aren't irritating or sensed by humans, are described in this patent, that I wrote a few years ago. I look forward to comments or suggestions.
http://www.freepatentsonline.com/7961086.pdf
James Bradley
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In my estimation, it is intended that these lights show up solid if viewed directly, and flickering if viewed either as your eyes dither...
A competent driver will have more saccadic (https://en.wikipedia.org/wiki/Saccade) movements than a passenger, which could explain why the strobing is more apparent to the driver than the passengers.
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Certainly plenty of LED taillights do strobe. Nothing like as badly as some rarely-seen LED-illuminated cats-eyes at a few motorway exits though (which IHMO shouldn't be legal as they totally hijack my brain).
I seem to be very sensitive to flicker compared to my peers - I think it does vary quite a bit between people.
I was distracted by the flickering of the discharge lamps in one gym where we sat exams at university, and seem to get distracted by -and report- failing lamps at work and in shops before anyone else. I can't stand the plasma TV screen/monitor (projector-screen-substitute) in one of the meeting-rooms at work as it flickers (pulsates to me) so awfully, but no-one else seems to be bothered...
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I am new to this forum, and found it by a google search. I have exact same problem AllenG has, when driving at night behind LED tail lights.
It started last December. I talked to my eye doc about it, and he dismissed it, so I am glad to find someone else had same experience.
I wondered if light frequency for LED lights differed and this thread answers my questions!
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[attachment=post_tmp_37600_0][/attachment]
My hunch is that they are ... driving the leds with short duration, high current, pulses.
I have noticed that the LED lights on Cadillac SUVs are particularly annoying. I thought it was just because they turn on so quickly, but maybe I'm detecting the strobe without realizing it.
Geezer is right: here's a photo of a Cadillac SUV I took today, panning the camera rapidly as the shot was taken,
in order to map time on to the horizontal axis.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fcadillac_flicker_50pc.jpg&hash=d5d0a200410aed1df9c68f889aae9a60)
It shows that the lights are emitting sharp pulses that are no longer in duration
than about a fifth of the interval between them. For comparison, smooth trails of incandescent lights are elsewhere
in the picture.
This flickering of Cadillac tail lights is distracting to me, though no one else in my family can see it.
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[ Invalid Attachment ]
There's a forum for that kind of photography ... http://www.flickr.com/groups/cameratoss/ [:0]
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I can barely view (no pun intended) these LED tail lights as a "safety feature". Here's my observation. I had a 2002 Cadillac DTS. If I'm not mistaken, they were one of the first cars with the LED tail lamps (in the U.S. anyway). I had no problem on the highway at night behind another DTS. Cadillac then introduced that smaller STS with the similar vertical LED tail lamps. I had to look away if behind one of those cars at night until I could change lanes and get far enough behind it. Yes, they do "pulse" and "trail". It's similar on the SUVs and that wagon type vehicle they introduced too. They changed something. Just for the record, I do have lighter blue eyes and have a very hard time with fluorescent lighting and those newer swirly CFS light bulbs. They blind me and I'll see spots and floaters for hours.
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I think blue lights would be the best option for the tail lights.. One of my colleague told me about it.
[Spam Removed]
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I think blue lights would be the best option for the tail lights..
Blue lights would not be legal for taillights in the USA, or much of the world. There are some "blue-dot" taillights that were used on hot-rods once, but even so the legality is questioned.
Here one must have red taillights, and either red or amber turn signals.
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I'm new here and was also brought here by doing a search on PWMed LED tail lights. I don't have this problem but my wife does. When she is driving at night and encounters a vehicle with these lights she says it's very difficult for her to determine the source of the lights. That she sees them all over the place. One time driving at night past the university, one of those blue LED lights atop an emergency phones was going off and she started to pull over thinking it could an emergency vehicle behind her. It's the lower frequency PWMed LEDs, like on the Cadillac, that are the worst. She says the Chevy Volt is even worse and is now getting the same sensation from the "angle eyes" style LED headlight trimmings becoming more popular on cars. I have sent messages to both GM and the DOT about this but never heard back. Increasing the PWM frequency to something above 300Hz would probably solve the problem. Some day I'll build a circuit to see where the problem goes away for my wife.
I'ver not run across any other people who see this but now there are three.
The family and I were riding home the other night and I was complaining about how LED lights strobe when I look away from them. The rest of my family couldn't see it.
What's going on here?
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.awcg.com%2FImages%2FTNS%2Flexus-is-tail-lights-1440.jpg&hash=6b9bb13b66af4f7acbb4d1251e685b85)
^^^
What my family sees.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.awcg.com%2FImages%2FTNS%2Flexus-is-tail-lights-trail.jpg&hash=0b1a7e827d9dce8b93ff0ce0df29b864)
^^^
What I see.
It's just LED lights that strobe too. Incandescent don't. Some makes of cars are worse than others as well. Cadillac's SUVs are especially bad.
The trails are strobes as well, not streaks. Do LEDs power on and off like fluorescent lights?
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When she is driving at night and encounters a vehicle with these lights she says it's very difficult for her to determine the source of the lights. That she sees them all over the place.
What about bicycle lights? Fast vs slow strobe on the bike lights? Some people have suggested that a bike should have both steady and strobe lights to help people judge where it is, but my opinion is that just seeing it is important enough.
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Is there a technical or scientific name for this effect?
I also suffer really badly from it, it's not just tail light of cars but a lot a bars and cinemas now use such LEDs in strips. This echo effect is then multiplied tens of times as each LED in the row strobes in my side vision. It drives me crazy to the stage I feel I want to throw up and often have to leave. The only hope I had so far was that this does not happen with all LED lights, but now I read others suffer from this too I feel better.
I do have an history of migraines and my eyes are light blue, but there seems to be very little information out there. I would hope that someone could suggest a temporary solution which would prevent it like wearing some kind of filtered glasses.
I believe as mentioned this is cause by low pulsing rates and/or cheap circuits. Unfortunately I guess these these kind of lights will continue to be used until maybe in 3-5 years new technologies comes into play as standard and these old lights fade out. (no joke intended :)
Please feel to contact me offline if you can help or advise.
many thanks!
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Is there a technical or scientific name for this effect?
It's called persistence of vision.
- If the pulse rate is high enough, the LED appears to be on, continuously
- If the pulse rate falls below a critical flicker fusion threshold (https://en.wikipedia.org/wiki/Flicker_fusion_threshold#Lighting), the LED appears to flash
- I guess this threshold could be different for different people.
- This is especially a problem if your eyes are moving rapidly, as it turns the LED into a dotty line
- The time when your eyes move most rapidly is during a saccade. The sensitivity of vision is supposed to be reduced during saccades (saccadic masking (https://en.wikipedia.org/wiki/Saccade#Saccadic_masking)), but I guess if your brain doesn't turn the sensitivity down enough, the flicker will be annoying.