PWM flicker: Invisible light that's harming our health?
63 comments
·June 18, 2025hinterlands
cosmic_cheese
I think it’s pretty common for people to be able to perceive PWM flicker in their peripheral vision that they can’t when looking directly at the source. I encounter this fairly regularly myself.
addaon
> There are two ways to dim LEDs: linear regulation and some sort of pulse modulation. Linear regulation is wasteful and you're pretty unlikely to encounter it. Pulse modulation is common.
Within the pulse modulation case, though, there are two important subcases. You can PWM a load that consists basically of just the LED itself, which acts as a resistive load, and will flash on and off at high rate (potentially too fast to be noticeable, as you say). But you can also PWM an LED load with an inductor added, converting the system into a (potentially open loop) buck converter. And this allows you to choose both the brightness ripple and the PWM frequency, not just have 100% ripple. Taking ripple down to 5%, or 1%, or less, is perfectly straightforward… but inductors are expensive, and large, so shortcuts are taken.
phkahler
Higher PWM frequency will let you use smaller inductors.
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mtalantikite
These LED light flickers actually trigger ocular migraines for me. I had tried to put in LEDs when the incandescent ban hit the US, and ended up with a Philips Hue system. I had 4 migraines in 3 days and had to send them back. I purchased as many incandescent bulbs as I could find, but they were somewhat impossible to find at that point.
I've got a couple bulbs from Waveform Lighting and they don't flicker, but I totally can tell the reds are off.
I really hate the LED transition. My building replaced all the outdoor lights with them, and now it's just too bright to sit on my stoop at night like used to be so common here in Brooklyn. My backyard neighbor put in an LED floodlight and now I have to buy blackout curtains. I drive rarely, but the oncoming headlights are blinding when I do. It's pretty depressing if I think about it too much.
mousethatroared
If you're savvy with manufacturing, make yourself a left handed edison thread (I can't find them anywhere). Left handed incandecent lightbulbs are still legal
Also, you can buy high wattage lights, and the three ways have lower wattage settings.
Finally, outdoor and appliance incandescents lamps are very inefficient, but last forever.
oakwhiz
Lights for high speed cameras use really good filtering on their PWM switching, or just linear power supplies. It would be nice to have a premium bulb that has longer life and much less flickering.
igor47
That sucks; I feel your pain. I, too, strongly dislike overly bright lighting.
I wonder if there's room to at least engage with the neighbor to talk about friendlier light options? You might also be able engage with these folks to see if there are efforts to improve the lighting in new York: https://darksky.org/
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PaulHoule
My take is that PWM dimmers are dramatically more energy efficient than the old rheostat dimmers people used to use. If you operate a transistor in a digital mode where it is either on or off it is close to 100% efficient, but if you operate it in a 50% power mode you have to send 50% of the power to a load and the other 50% to a resistor. Thus CMOS logic eradicated bipolar, switching power supplies replaced linear power supplies, a Class D amplifier can be a fraction the size of a Class A amplifier, etc.
You could probably still reduce the flicker by either increasing the switching frequency or putting some kind of filter network between the switch and the load.
mtalantikite
For sure, they're definitely way more efficient. They just unfortunately give me migraines. I'd be open to trying some that have a filter network or some other smoothing on the flicker.
But I've also never lived in a house that has dimmers (they've all been old homes in the north eastern US) and I never use overhead lighting, so it's not something I need or would miss.
card_zero
Apparently fourth-generation LED tube lights are designed not to flicker.
JKCalhoun
I wonder to the degree their effects are much worse than migraines. Perhaps irritability? Mental confusion? Anxiety? I'm spitballing here, but to be sure it seems like our world is somehow a place of more anxiety, irritation .... I would love for it to be something we could take control of.
genewitch
i doubt it's the light bulbs. I posited the other day, by assembling a few different ideas, that Trauma Based Entertainment is to blame for this. something like 2/3rds of all Television programing is law-enforcement adjacent. True Crime is super popular on TV, law and order, NCIS, FBI this-and-that. And what's one of the largest advertising cohorts?
Medicine for depression, anxiety, insomnia...
it's nearly a closed loop; something i intuitively realized shortly after 2001/09/11 - by the end of that year i decided i would no longer have a "Television" attached to CATV/SAT/ANT service.
I'm not sure if i am correct, i haven't really dedicated a lot of time to getting the exact numbers, talking to psychologists and sociologists and the like. But two people i know had "breakdowns" (grippy sock) in the last month and both of them always have true crime on TV in the background or listen to true crime podcasts. Shortly after that happened i was listening to the moe facts podcast where Moe used the term "trauma based entertainment" and something clicked - Moe didn't mention "it's because of pharma ads" - that's my own input after having worked for the largest television "broadcast" company in the world, just long enough to see the advertiser "dinner".
RiverCrochet
The only ones watching traditional OTA TV anymore are elders. That advertising cohort is why OTA TV ads are filled with pharmaceuticals and "you may be entitled to financial compensation" type ads, at least where I'm at. Traditional TV has been dying since Youtube and broadband. MTV plays Ridiculousness constantly because no one is actually watching it.
> it's nearly a closed loop; something i intuitively realized shortly after 2001/09/11 - by the end of that year i decided i would no longer have a "Television" attached to CATV/SAT/ANT service.
Curiously this is about the same time I decided to give up on TV and radio as well.
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edoceo
I have LED in my home office. The "temperature" and this flicker were driving me bonkers. Fortunately no headache. Now I have them all pointed away to reflect off wall or ceiling, or behind diffusers. Much less bothersome,
SLHamlet
I had no idea about this:
"To understand why PWM bulbs have so much flicker, imagine them being controlled by a robot arm flicking the on/off switch thousands of times per second. When you want bright light, the robot varies the time so the switch is in the 'on' mode most of the time, and 'off' only briefly. Whereas when you want to dim the light, the robot arm puts the switch in 'off' most of the time and 'on' only briefly."
cwillu
It's entirely fine if the rate is high enough, but lowering the frequency of the PWM and using smaller inductors (or even no inductor at all) is a prime way to make the bulbs cheaper.
bsder
This the reverse, actually, you can use much smaller inductors the higher the switching frequency. That's why the GaN chargers are so much smaller, for example.
loph
I can tell you that lights strobing exacerbate my migraines. Even 120 hertz from fluorescent lights will affect me. I have mitigated this in the past by adding incandescent lights in my office, or demanding to work near a window. LED lamps are no good, as another commenter posted, even the simplest ones strobe. Incandescent bulbs grow harder to find as time goes on. Progress?
consp
> even the simplest ones strobe
The simplest ones always strobe at line frequency or the double of it (due to cheaping out on the power supply). Those have visible strobe. Simpel is bad with led light.
Find some not too cheap dimmable warm colored bulbs. They won't be cheap but might contain both a high frequency driver and fluorescent afterglow and my guess is you will not notice anything.
orwin
I flagged because this is a submarine ad, but it was still interesting tbh.
JKCalhoun
Suspicious of "DC dimming". If you can just lower the current to an LED to dim it, everyone would. Someone will know better than me, but I believe there is a kind of threshold voltage for the (solid-state) LED.
I am not aware of LED bulbs (and here I am talking about home lighting, not phones or laptops) that dim by shutting down some of the (multiple) LEDs.
Most home lighting bulbs appear to have several LED elements. A circuit could enable dimming by simply shutting some of them off — running the rest full-on. 50% dim would of course shut half the LEDs off. No PWM required.
Kirby64
DC dimming LEDs is relatively easy, and somewhat common. The problem is that it's expensive compared to PWM dimming. It requires more expensive current-adjustable circuitry.
Additionally, for bulbs that are used in regular household fixtures, they basically need a way to convert TRIAC chopped 50/60Hz AC into constant current... which makes things even more expensive. Smart bulbs that are supplied a constant non-chopped AC can do it easier, but it's still expensive to do DC dimming.
PaulHoule
I guess there is some threshold below which the LED turns off so the voltage/current -> light function needs to be set accordingly.
When I was in high school we were messing around with liquid nitrogen and overvolting LEDs and noticed the odd effect that the color of the LED would change if you overvolt it. It was years before I found out why
https://www.reddit.com/r/AskElectronics/comments/v28qbh/why_...
https://spectrum.ieee.org/a-definitive-explanation-for-led-d...
Kirby64
Voltage, yes. Current, no not really. You can drive extremely low currents and still get photon emissions from LEDs. That said, it's highly non-linear, so you basically need to assign set points. Doubling the current won't double the lumen output.
mrob
You can just lower the current. Not everyone does because it generally requires more expensive components, e.g. inductors. There is a threshold voltage ("forward voltage") needed for LEDs to turn on but there's no threshold for minimum radiant flux. LEDs are actually more efficient at low current (although this might be counteracted by greater losses in the power supply).
kjkjadksj
You can in fact dim leds. You can see a lot of controllers that are just that at various parts suppliers.
genewitch
you can dim LED that are running on DC (it requires more than a potentiometer i guess - probably a buck circuit controlled by a pot, though) or AC; i have scant idea how the AC ones work, although variacs have existed for a real long time; but you have to buy special LED bulbs that can handle being on a dimming circuit.
this is different than a bulb like hue etc that have the ability to dim themselves through whatever mechanism.
SAI_Peregrinus
Traditional dimmers used TRIACs. Those don't dim LEDs well, they make very visible flicker. TRIACs turn the AC off for part of the waveform, essentially a very slow version of PWM. With an incandescent filament that flicker isn't as noticeable since it takes some time to cool down & stop glowing, which visibly smooths the flicker. It just stabilizies around a lower temperature. With LEDs, the turn-off is nearly instant. You visibly see the flicker at the AC mains frequency.
There are two ways to dim an LED: supply less current at the same voltage, or PWM dim it with a fast enough switching speed that you don't notice the flicker (this being slower than it needs to be is what the article is about). A current source is pretty easy to build, and doesn't flicker, but it does dissipate all the excess energy as heat. That's not what you want inside the dimmer switch in your wall, it can be quite a lot of heat and would be a fire hazard in such a confined area. It does work for things like photography lamps which can have exterior heat sinking.
Jazgot
There is a strong and widespread tendency to view anything artificial as highly dangerous. I understand this perspective, but on the other hand, we have science and reasoned arguments.
blacksmith_tb
I would certainly agree that finding LED bulbs that you like and/or don't bother you can take some work (especially if you want to put them on a dimmer, in which case you may also need to replace your dimmer). However, I am skeptical that subtle PWM flickering is unavoidable. For the chateau example, it would be better to choose bulbs with fewer lumens and run them at 100%?
baggachipz
I wonder about this too. If I have a dimmer and a LED bulb, does putting the dimmer all the way up still use PWM? I have a hunch that it still does, but would love to be proven wrong.
satiated_grue
"Perceiveved brightness"?
And perceiveved brightness is equal to the peak of the PWM wave?
That image from courtesy Daylight Computer Company is consuming too much of my attention.
normie3000
Do computer screens flicker and release this bad light?
layer8
Yes, Notebookcheck regularly measures PWM in displays: https://www.notebookcheck.net/PWM-Ranking-Notebooks-Smartpho...
For OLED I remember reading that PWM dimming is necessary because DC dimming causes shifts in color/whitepoint.
codethief
Some do, some don't. Sites like notebookcheck.net typically mention in their reviews whether a given laptop screen exhibits PWM.
igor47
In the article they rank some smartphones by how much they flicker for dimming, I assume it's the same when computer screens dim?
null
I think this article is pretty confused.
There are two ways to dim LEDs: linear regulation and some sort of pulse modulation. Linear regulation is wasteful and you're pretty unlikely to encounter it, especially in battery-powered devices such as phones or laptops. Pulse modulation is common.
Human vision has a pretty limited response speed, so it seems pretty unlikely that PWM at a reasonable speed (hundreds of hertz to tens of kilohertz) can be directly perceived. That said, it can produce a stroboscopic effect, which makes motion look weird and may be disorienting in some situations. So I don't have a problem believing that it can cause headaches in predisposed individuals.
You can dim your laptop screen in a darkened room and wave your hand in front. Chances are, you're gonna see some ghost images.
Other than adjusting the frequency, pulse modulation can be "smoothed" in a couple of ways. White LEDs that contain phosphor will have an afterglow effect. Adding capacitors or inductors can help too, although it increases the overall cost. But that doesn't make the display "PWM-free", it just makes it flicker less.