# auto headlights blinking - pwm?



## 5S8Zh5 (May 5, 2021)

While cycling, through my take-a-look mirror, saw a brighter than normal headlight coming up alongside and noticed the daytime headlights were blinking rapidly. Is that pwm?


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## Lynx_Arc (May 5, 2021)

5S8Zh5 said:


> While cycling, through my take-a-look mirror, saw a brighter than normal headlight coming up alongside and noticed the daytime headlights were blinking rapidly. Is that pwm?


It could be, or maybe it has some sort of light sensor electronics that is acting up.


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## flatline (May 5, 2021)

Yes, I've seen lots of newer cars that have PWM on brake lights, turn signals, and (not as often) daytime running lights.

I do not consider this to be a good thing because I find it very distracting.

--flatline


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## -Virgil- (Jun 1, 2021)

Yes, you're seeing the PWM operation of the DRLs to reduce their apparent intensity to provide the front position ("parking") light function. The same thing is done with headlamps to provide a DRL function, and stop ("brake") lights to provide a rear position ("tail") light function. It's called the "bead effect" and I agree, it's distracting. What's even worse is non-legitimate aftermarket "LED bulbs" installed in place of appropriate halogen bulbs in cars that use PWM to provide the DRL function via the headlamps; those not only flicker, but they appear to dance as viewed in a moving mirror (car or bike rear-view or side-view, for example).


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## Ledzilla (Jul 2, 2021)

-Virgil- said:


> Yes, you're seeing the PWM operation of the DRLs to reduce their apparent intensity to provide the front position ("parking") light function. The same thing is done with headlamps to provide a DRL function, and stop ("brake") lights to provide a rear position ("tail") light function. It's called the "bead effect" and I agree, it's distracting. What's even worse is non-legitimate aftermarket "LED bulbs" installed in place of appropriate halogen bulbs in cars that use PWM to provide the DRL function via the headlamps; those not only flicker, but they appear to dance as viewed in a moving mirror (car or bike rear-view or side-view, for example).



What do you mean more specifically by "appear to dance?" To my eyes, all flickering lights (from DRL or tail lights) appear to dance while my eyes are steady on something, and create a phantom array during saccades.

I haven't yet figured out if there is any connection between the dancing percept and frequency (or some other factor). Would love to hear how others see this, as I haven't seen it discussed anywhere (only phantom arrays).


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## alpg88 (Jul 7, 2021)

i see it too, people put led bulbs in high beams, however drl high beams are driven at half the voltage, some led drivers can not deal with voltage so low so leds appear to flicker\strobe. it is annoying, to the point you get tired when someone with such lights is following you, you have to switch lanes away from them, or maybe it is the intended purpose.


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## Alaric Darconville (Jul 8, 2021)

alpg88 said:


> i see it too, people put led bulbs in high beams, however drl high beams are driven at half the voltage, some led drivers can not deal with voltage so low so leds appear to flicker\strobe. it is annoying, to the point you get tired when someone with such lights is following you, you have to switch lanes away from them, or maybe it is the intended purpose.


People are stupid to put LED 'bulbs' in high beams. Obviously undesirable effects are the outcome, whether extreme flicker or nighttime collisions.


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## -Virgil- (Jul 12, 2021)

Ledzilla said:


> What do you mean more specifically by "appear to dance?" To my eyes, all flickering lights (from DRL or tail lights) appear to dance while my eyes are steady on something, and create a phantom array during saccades.



The presence/degree of a "dancing" or wiggling appearance of the lamp is a function of the PWM frequency. If it's too low, this is the result. The beads effect (you see what looks like a string of beads as the light moves within your visual field) is also a function of the PWM frequency, but unlike the "dancing", it doesn't really go away until you get to _very_ high frequencies, which can be impractical for the vehicle lighting functions being produced.


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## JustAnOldFashionedLEDGuy (Jul 14, 2021)

Is there any movement in automotive similar to general lighting to create a specification for acceptable flicker? IEEE1789 for those not aware. When I was doing automotive stuff, I remember quite low PWM frequencies on tail lights, a few 100 Hz with 100% modulation. You don't have the same exposure issues as with general lighting but in terms of distracting effects, I would expect more due to constant differential movement between vehicles.


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## -Virgil- (Jul 15, 2021)

There is some discussion about it in the SAE Lighting Committee. It's being driven mostly by trying to avoid screwing up camera-based "vision" in other cars, with some afterthought given to distraction of humans. That SAE group, like most of them, is populated mostly by people from the various OEMs and suppliers...and so as per usual, there's a big push to make SAE standards as cheap and easy to meet as possible, couched in terms of nice-sounding ideas like "maximum design flexibility".


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## Ledzilla (Jul 18, 2021)

-Virgil- said:


> The presence/degree of a "dancing" or wiggling appearance of the lamp is a function of the PWM frequency. If it's too low, this is the result.



Very interesting. Do you have any studies on this? Perisaccadic artefacts have been researched a lot, but I have never seen anything on this dancing effect. Nor have I heard its proper name really (though the "beads effect" is also called many names - it's "phantom arrays" in my field).


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## -Virgil- (Aug 5, 2021)

There is research on this, and I'm sure I have some of them somewhere around here, but I'm laboring under the best of intentions to organize my library, and it's...not going as easily as I hoped.


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