I wish there was more awareness and concern about LED headlight PWM flicker.
PWM (Pulse Width Modulation) is a technique where the power to the output (bulb) is turned on and off at high frequencies in order to increase efficiency, power handling, and control of the output.
If this on/off switching is done fast enough, your eyes/brain will interpret it as a normal always on light source.
PWM is significantly cheaper and easier to implement vs current source LEDs, so it of course has become dominant in the industry.
When the PWM frequency drops below 120-240hz (cycles a second), some people can start to pick up a strobe like effect from the bulb. This flicker is known to cause additional eye fatigue and can trigger headaches in many.
What's worse is that many aftermarket LED headlights are now using PWM at 60hz. These cheap headlights are often bright blue in order to look modern and impressive. When a car with these installed is behind me at night I have to move my mirror away because the flicker is incredibly distracting and fatiguing.
Even at fairly high PWM frequencies, strobing can be visible when either the eyes or the source are moving. Which, in a car, they are! You don't need to have exceptional vision. Just glancing from left to right can be enough to leave a confusing and distracting trail of multiple images on your retina.
Motion must explain why I find these LCDs in my home office so offensive. Normally they're not a bother, but when my ceiling fan is on and I look at the lights (below the fan), I swear there is some strobing.
Ford's marker lights are an example of this. The huge consistently illuminated panels are simply amazing. The huge light source is perfect for maximum visibility without presenting a blinding spot source like the majority of automotive lights.
But that practically singular investment in automotive optical engineering and innovation is let down by the low frequency PWM driver baling wired into the final design to cut cost by perhaps a dollar or two.
I may be putting too fine a point on it, but I think that even something as perhaps seemingly trivial as being blinded by intense car headlights (these days in the U.S.) is another one of those things that fester in our subconscious building to a creeping sense of dread that the world is not how it should be — that somehow we're living in a modern society that no one would have ever designed, voted for, wished for, desired.
> somehow we're living in a modern society that no one would have ever designed, voted for, wished for, desired.
As mentioned elsewhere in this thread, the majority of the population doesn't seem to notice this crap, at least not in a way that affects their day-to-day happiness.
Why not vote for something that sounds good at surface level if you've never perceived that annoying sound or flicker? Why would you even think about it?
It took me awhile to figure out why the LED tail lights made me have a specific weird feeling.
If you're old enough, you may have seen the science fiction children's show "Captain Power." The LED tail lights remind me of the target strobe meant to trigger the accompanying toys.
Of course if you're old like me, you probably only watched an episode to see what Lightwave3d was capable of, but ..
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Wait, what? They are economizing on headlight LED power draw? LEDs are ridiculously efficient to begin with, and a rounding error against a car’s total energy usage. (And in gas cars, get the energy from the battery, which can be recharged off of otherwise-wasted energy from the engine idling.)
All on the basis that “lol humans don’t notice the flicker”? Uh, okay, but animals? The 99th percentile humans? Subconscious effects on fatigue (that you mention)?
Yes, this is the real answer. Most LEDs do not operate well at high temperature, unlike incandescent bulbs or HID bulbs (which can both operate inside ovens, if you need them to). A headlight designed for incandescent or HID will not provide enough cooling for LED lamps unless the LED lamps are efficient enough.
> Wait, what? They are economizing on headlight LED power draw?
Mass produced cars have reached the point that a $0.50 saving per vehicle is worth designing and building for. When you reduce the current draw from something that is always-on (like headlights) you increase the life of the LED, you can reduce the gauge (size) of the wires to the lights, you can reduce the size of the alternator, you can load the alternator less, which means increase the service life of the alternator belt and idlers, and (in theory, at least) decrease fuel consumption.
All of these are tiny, tiny efficiencies, but they do add up.
While it might seem penny-wise and pound-foolish, imagine following this same logic for all systems in a modern vehicle. This is why even such a "rugged offroad" vehicle as a Jeep Wrangler has a plastic clutch slave cylinder (weight and cost savings) and why the headlights will dim in virtually all vehicles if you turn on every single electrical device that draws current (the wires are not sufficient gauge for everything to draw max current at the same time).
They're saving actual money on every vehicle built, which is what they care about.
> which can be recharged off of otherwise-wasted energy from the engine idling
That’s not how it works. When charging, the alternator will put more load on the engine and that will increase fuel input to keep rpm up enough to keep the engine from stalling.
Overall, every vehicle designer will seek to minimize any and all loads (even small ones).
Worst is when they remove the spare tire, grrrrrr.
A lot of modern cars only have a single LED bulb that provides normal headlights and high beams. In the past with halogen headlights they'd either have two separate bulbs or a single complex bulb with two different filaments for normal and high beams. I would assume LED + PWM lets them have a single bulb whose brightness can be controlled by software, so the choice is probably more about cost savings and reducing complexity than energy usage.
I don't have a car with LED bulb so I can't check, but I wonder how would one bulb works when normal and high beams have different beam pattern. One might be able to accomplish that with one LED bulb if it switches on/off certain LEDs in the module or the bulb is movable, but then no PWM is required.
A typical pre-LED solution is a projector and a mechanical plate that moves (very quickly) to block or unblock the part of the light that makes up the high beam coverage.
LEDs can be done similarly, but an LED matrix headlight just controls which rows of LEDs are lit up (e.g. the Samsung PixCell light that Tesla uses on the Model 3/Y). This is where we'll maybe see adaptive headlights, too, though in the case of Tesla I don't believe they've ever been used anywhere in the world, legal or not. Just to write "TESLA" on the wall when you do the Light Show.
It does seem entirely overkill to not just install a second LED. That's how bulbs have worked for years. So instead of just adding _one_ additional LED we're going to oversize that LED and then use a high frequency driver to turn it on and off really fast?
All of the builder LED lights in my home had this issue. Very expensive to replace these.
Newer construction doesn't install proper fixtures anymore. No more user-replaceable bulbs. Everything is screwed directly into the box and you have to do a minor electrical job on every unit you want to replace.
A local business had installed some expensive LED lights and were having flickering issues. At the time I remembered seeing something about power supply design and how having no resistive load on the circuit would cause issues with the power supplies. I had them replace one bulb on one string of lights with an incandescent and they stopped flickering. Another one for the circuit on the other side of the room and the LED flickering was gone. The fluorescent bulb flicker was an entirely different issue, but they weren't nearly as obvious from a distance as the LEDs were.
I too strongly dislike the lack of replaceable bulbs in newer LED fixtures. It's outrageous how many things in modern construction shave a few cents here and there to get the initial cost down, and the long term costs are enormous. Once the house is sold it's someone else's problem and that mentality is disgusting.
That's interesting; I've been going the other direction. One by one I've been retrofitting my recessed can lights with fixed LED fixtures made by Cree. Because the LED floodlight bulbs fail relatively quickly, no matter which brand I buy, but I've never had a single one of the Cree fixed-LED fixtures go bad (and the oldest are pushing 10 years now). I figure the heat management is better on the purpose-built fixture, which explains why it lasts.
Agreed. I actually considered retrofitting the existing units with parts from digikey. I am 99% sure the issue is due to total absence of any electrolytic capacitors whatsoever on the PCB.
LEDs are a lemon market where the only people making profit are those who ship lemons, everyone else gets pushed out of the market.
The luxury days of installing an incandescent bulb that lasts for ten years are gone, lucky to get six months out of "contractor grade" LED products and consumer devices usually don't make it much over a year.
Sure, burns a little less coal, but its not like LED manufacture and shipping is as environmentally friendly as beewax candle production LOL. Overall we're probably worse off as a civilization with LED lighting.
> The luxury days of installing an incandescent bulb that lasts for ten years are gone, lucky to get six months out of "contractor grade" LED products and consumer devices usually don't make it much over a year.
I'm always surprised to read such comments, incandescent bulbs definitely burned out regularly, and the LEDs we have now are more or less as durable in my experience.
I still have a few CFL bulbs that are more than 10 years old (that I keep because their slow warm-up is actually desirable in some cases) and my Ikea LED bulbs are all between five and three years old. One of them (out of 12 I think?) did fail last year, it was four years old.
The bedroom, bathroom, toilets for example because it's not as jarring as when the light turns on at once at full power. And it's infinitely cheaper and easier than setting up dimming.
The terrible PWM drivers add another downside - At the frequencies in question the peaks are long enough to increase the retina-stabbing factor much higher than the average brightness, even if they're not long enough to cause noticable strobing. Smoothing capacitors as an aftermarket QOL option is a market that needs to exist.
However, poor (or nonexistent) light source shrouding is also a common factor. Bicycle headlights make a great lower-powered example. Cheap bicycle headlights throw much less light than a car headlight but are painfully blinding due to the exposed light source point. Bicycle headlights that meet StVZO regulations shroud the light source, have decent optical design, are far more effective as headlights, and don't make anyones eyes bleed even when they're flashing.
Early composite car headlights (post sealed beam) received significant design investment involving established headlight manufacturers and worked extremely well. Auto manufacturers brought headlight design in-house by the time xenon and HID headlights showed up in general use; inexperience and cost avoidance gave those lights an undeserved reputation. A lack of regulatory outcomes rewarded that strategy, leading even established composite headlight quality to decline. Light source shrouds disappeared or became ineffective due to poor reflector design.
Headlights on new cars in the US market have declined to the point that they're effectively overpowered cheap bicycle headlights plus extra PWM induced stabbiness, while costing orders of magnitude more as a result of cost cutting. The worst have returned to sealed beam levels of utility, but at least there were aftermarket options to improve sealed beams. Modern aftermarket lights are so bad that it's normal for drivers to install them for malice against other drivers with blinding stock headlights. There really aren't any optically well designed headlights on the US market any more, except for bicycles.
That term comes from EPA regulations about emissions, and led the entire aftermarket industry to figure if they put that on their parts, all was good. (The application here, applied to DOT lighting regulation, is likely bleed over)
A few years ago the EPA clarified that when they said “off road use only”, they meant in machines that were never intended to be on a road. So, aftermarket shit designed for a lawnmower, cool. But if you’re designing for a vehicle that was originally designed for the road, you have to stick to EPA regs. This killed a lot of those “off-road use only” disclaimers, but it also panicked the amateur racing community because they tend to use modified road cars. I’m not sure how they worked it out with the EPA, but I don’t hear them complaining any more, so I assume they did.
This happens even with cheap non-dimmable lights. If you skimp on filtering in the power supply the LEDs will flicker, but not in a way that's visible to the eye, but very noticeable with a rolling shutter.
Commercial LED PWM drivers for automotive applications are in the hundreds of kilohertz. If there are aftermarket modules that are only 100 Hz that’s a matter of regulation. It should just be outlawed.
Well I don't know the frequency, but it is easily noticeable to me on most modern cars when moving my eyes. It's definitely not a question of aftermarket modules.
The PWM I use at home on an ESP32 is supposed to be in the 1-10 kHz range and I don't see flicker when moving my eyes.
Yeah, you'd be superman if you thought you could see a khz flicker.
I associate flickering headlights with aftermarket because cars with halogen headlights use low-frequency PWM to regulate down high system voltages to 12V. So if you install aftermarket LED kits on those cars you are going to see a PWM of ~90Hz which is awful.
Actually I forgot to mention that is it mostly the case for taillights (the most noticeable since you're going to follow cars quite often) and accent lights (like the vertical strips on the front of recent Peugeot cars).
Not really on headlights I guess so I might be off-topic. And although I understand how some people can find it annoying, I don't really mind them. But I can definitely see the flicker and can compare the duty cycle between different cars.
System voltage when the engine is running are higher otherwise the 12V battery would never charge. Can be as high as 18V. So cars with halogen lights with just chop it down to 12V.
Interesting, so they're using PWM to hold the headlights at constant brightness when the alternator runs. Wiring a capacitor in parallel with the headlights should fix that.
The usual ~200kHz PWM is cheaper, more reliable, smaller, and usually more powerful than anything you can build on the ~100Hz. Besides everybody sells chips for ~200kHz, but finding anything that can operate at ~100Hz is a challenge.
I can understand cheap household ones using the mains frequency to avoid an AC/DC conversion. But AFAIK, using them on a car is stupid from every conceivable point of view.
There's an emergent interaction between PWM headlights and rear view mirror cameras(RVMC)[1]. The PWM headlight frequency and RVMCs frame rates are slightly different and phase in and out of each other resulting in a strobing effect. If RVMCs become more popular, eventually an accident will result in the way these two technologies interact. I found it an amusing interaction, but it also has serious safety implications that should be addressed.
1. I'm not sure if there's a better name, but the system where the rear view mirror is a display linked to a camera at the rear of the vehicle in some higher end cars and as an aftermarket addon.
As I remember PWM is to control the brightness of the LED. LEDs can only shine at one brightness, they're either on or off, so to dim them to half-brightness you need to have them on half the time, off half the time. That's how you get all the different colours from three LEDs.
But they can switch on and off very fast, so if 50Hz is flickery, you can use 500Hz, or 5000Hz. Most microcontrollers have circuitry for this built in, so you just literally write a different constant into the controller.
I once made an LED dashboard for use in police cars. After it was rolled out, it was discovered that the flicker frequency was interfering with the radio (the only bug ever found in my code!)
My client called me back in to fix it, so I added a zero in the relevant place, and recompiled the code.
> LEDs can only shine at one brightness, they're either on or off,
This is factually incorrect.
LEDs shine at a range of brightnesses. The amount of light emitted is related to the amount of current flowing through the LED, which you are free to control however you want. The cheapest way to do this is to use something like PWM and monitor the average current flowing through the LEDs. However, you can also just use a constant current source, and adjust it to provide the amount of current that you want.
I don't know where the idea comes from that LEDs are only on or off. There's no basis in fact here, as far as I can tell.
PWM (Pulse Width Modulation) is a technique where the power to the output (bulb) is turned on and off at high frequencies in order to increase efficiency, power handling, and control of the output.
If this on/off switching is done fast enough, your eyes/brain will interpret it as a normal always on light source.
PWM is significantly cheaper and easier to implement vs current source LEDs, so it of course has become dominant in the industry.
When the PWM frequency drops below 120-240hz (cycles a second), some people can start to pick up a strobe like effect from the bulb. This flicker is known to cause additional eye fatigue and can trigger headaches in many.
What's worse is that many aftermarket LED headlights are now using PWM at 60hz. These cheap headlights are often bright blue in order to look modern and impressive. When a car with these installed is behind me at night I have to move my mirror away because the flicker is incredibly distracting and fatiguing.