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.
this is different than a bulb like hue etc that have the ability to dim themselves through whatever mechanism.
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.
No. That's only true for a linear regulator, which is just one, very terrible, implementation of a current source that's only used for very low power applications. Linear regulators are never used for things like room illumination.
The alternative, and what's used for all commercially available DC LED drivers (plentiful and cheap), is to just use a regular AC->DC switching supply in current mode (current for feedback rather than voltage feedback). The only flicker is the ripple left in the filtered output.
Why aren't these used? Because most dimmer switches use tech from incandescent age, and just chop off parts of the AC sine wave, so the bulbs are designed around the switches you can buy in the store. Why do dimmer switches chop? Because that's what the bulbs you can buy at the store expect, sometimes damaging them if not dimmed as they expect.
You can buy in wall DC dimmer switches from any LED supply store, but they require DC lighting, also only found at LED supply stores. It's entirely a very recent momentum problem, that's slowly going away.
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.
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...
For OLED I remember reading that PWM dimming is necessary because DC dimming causes shifts in color/whitepoint.
It identifies a "health risk", describes the mechanism in terms that sound very convincing, assigns numbers to its cause and effects, provides a table grading health risks of various products, all without linking to a single scientific study demonstrating that the effect is anything other than nocebo. The closest they come is a image of a table that refers to a few institutions that apparently did a study related to PWM (leaving it an exercise to the reader to find the studies they're supposedly referencing) and a link to a Wikipedia page which links to a Scientific American article which says:
> In 1989, my colleagues and I compared fluorescent lighting that flickered 100 times a second with lights that appeared the same but didn’t flicker. We found that office workers were half as likely on average to experience headaches under the non-flickering lights. No similar study has yet been performed for LED lights. But because LED flickering is even more pronounced, with the light dimming by 100% rather than the roughly 35% of fluorescent lamps, there’s a chance that LEDs could be even more likely to cause headaches.
I'm willing to entertain the idea that LED flicker is actually problematic, but I wish essays like this would be honest about the degree of confidence we have given the current state of the evidence. This piece instead takes it as a given that there's a problem, to the point where they confidently label devices on a scale of Low to Extremely High health risks.
There is nothing anecdote about flickering in LED light causing health risks.
What I'm asking for is for articles like this that cite numbers and provide tables purporting to quantify the degree of harm caused by various devices to point to where they're getting their numbers from or, if they can't do that, stop making up numbers and assigning things to "harm" scales that they invented themselves based on vibes.
Either there's a study showing that 246 Hz flickering poses "Extremely High" health risks or there isn't.
> Either there's a study showing that 246 Hz flickering poses "Extremely High" health risks or there isn't.
They calculated it using the definition from the standard.
They list the 'Xiaomi 15 Ultra' as having a 'Moderately High' health risk, and cite it as having a 2.16 kHz PWM frequency at 30-75% modulation depth.
The IEEE article has recommended practices that state:
8.1.2.3 Example 3: PWM dimming Using Figure 20, the recommended practice for PWM dimming at 100% modulation depth is that the frequency satisfies f > 1.25 kHz. This can also be derived using Recommended Practice 1 and solving 100% = 0.08×fFlicker. This level of flicker could help minimize the visual distractions such as the phantom array effects.
Seems like even at 100% mod depth, >1.25 kHz is just fine.
Also, the article does not seem to distinguish between modulation at reduced brightness, which the IEEE article calls out specifically as something that is unlikely to cause issues. E.g., movie theaters using film all flicker at 48 Hz and nobody complains about that.
You can see on page 27 how this is meant to be used: it should produce a per-hazard matrix.
You might be thinking of Figure 18 on page 29, which does identify Low-risk and No-effect regions by Modulation % and Frequency, but that also does not claim to identify high-risk regions, it just identifies the regions we can be highly confident are safe. And importantly, as a sibling comment notes, TFA's table actually contradicts the line on Figure 18, labeling several devices as higher than Low even when they're squarely within the Low-Risk and No-Effect zones.
Sure, PWM light can cause health risks for some people, in some contexts. But taking research out of context is bad science.
Do you genuinely believe the Pixel 7 and 8 Pro have an "extremely high health risk", in the context of what a lay person would understand?
Edit: I specify 'lay-person' because clearly this is an introductory blog post (or advertisement for Daylight Computer). If they want to use a more specific definition of health risk, then they better define it.
The standard also linked to the researches during their discussion.
Please read it, instead of just randomly throw out things hoping that they supported your argument.
Cite the exact page number and quote that you claim justifies the assertion that 246 Hz PWM carries an "extremely high" health risk. Then we can talk.
If you want to redo the numbers and check if they fit the definition, please feel free to do so, but you will need to put some works in (since the flicker hz -> risk showing in the article is a computed value, you need to find the modulation value and plug it in too)
I understand your fight and your idea, I am just saying that in this specific instance, this is not a fight to be fought. The article is generally correct, and if you want to complain about the writing style or it being an ads, it’s up to you. But this is not the same situation with GMO stuffs
No, they said that IEEE 1789 also uses Modulation % (which they've renamed Flicker %) to calculate risks. That is pointedly not the same thing as claiming that they used IEEE 1789's formulas.
You're reading their copy generously, but that doesn't usually pay with marketing copy. Articles like this always like to wave in the general direction of official-sounding sources while carefully refraining from actually claiming that they got their numbers from anywhere in particular.
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.
The simplest LED sources running from AC mains power strobe at mains frequency, which is very visible and very annoying.
Fancy LED sources don't strobe at all. I'm using an LED panel intended for videography as a room light; any flickering could show up as scanlines in video, so most lights intended for that purpose are flicker-free.
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.
"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."
Give me a nice candle.
Why do we use anonymity for that? What's gained and lost by that?
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.
Eh, they use what they can get away with. Nobody is out there policing flicker rates. Especially when you add a dimmer into the mix, there's a lot of room between good and bad, and when you're at the hardware store buying bulbs, there's not much to indicate which bulbs are terrible.
Lots of people don't seem to notice, so the terrible ones don't get returned often enough to get unstocked, and anyway, when you come back for more in 6 months, everything is different even if it has the same sku.
Not only flickering, but lots of other information about internationally available brands, including cheap Chinese stuff: CRI, real power use, etc.
Use your favorite online translator.
> You can't perceive that
I very easily can. I had to get rid of an otherwise good monitor a few years ago before I knew it used PWM to control the backlight (and before I even knew PWM was used at all for this functionality — I only had experience with CCFL backlight before that).
It was really annoying to look at, like looking directly at cheap fluorescent lighting. Miraculously, setting brightness to 100% fixed the issue.
By googling around, I found that it used PWM with a modulation frequency of 240 Hz, with a duty cycle of 100% at full brightness, which explained everything.
I can also easily perceive flickering of one of my flashlights, the only one that uses PWM at a frequency of a few hundred hertz. Other flashlights either run at multiple KHz, or don't use PWM at all, and either one is much easier on the eyes.
Some of us really do perceive this stuff, which can be hard to believe for some reason.
Next time you see a high refresh screen, move the cursor around rapidly. It's very easy to tell.
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.
There's a third way: a switched-mode power supply with regulated output current. This is used in most better-designed flashlights (which doesn't always correlate to price) and can be used by anything else that needs to drive an LED as well.
The article doesn't discuss what technique should be used for "constant current reduction"; it probably shouldn't be a linear regulator where efficiency is a priority.
PWM is less annoying if the frequency is very high (several kHz), though I'll leave it to people who research the topic to speak to health effects.
I wrote more here: https://news.ycombinator.com/item?id=44312224
That effectively lowers the frequency of the lcd.
The other reason is LED brightness and color is quite non-linear with current, so PWM gives a much more straightforward dim than adjusting a current smoothly.
just throws me right off the argument in an article when the fine print notes that a cited study is confounding the thing the author cares about ("sensitivty to flicker") with a much simpler and better-understood explanation (CO₂ poisoning)
If you see the strobe effect, return the bulb and buy another one.
Initially I thought it might be related to the alternator.
I still don't know why I perceive these headlights as having an annoying flicker or why. I'd love it if some (informed) commenter could clear it up for me. Am I imagining it?
I also just hate hate hate seeing the flicker in my peripheral vision.
mtalantikite•5h ago
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.
igor47•5h ago
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/
PaulHoule•5h ago
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•4h ago
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•4h ago
https://en.wikipedia.org/wiki/LED_tube#History
JKCalhoun•5h ago
genewitch•5h ago
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•1h ago
> 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.
ToDougie•1h ago
oakwhiz•1h ago
mousethatroared•39m ago
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.
Zak•15m ago
How do the reds look to you?
I looked at the photometric reports from a couple Waveform models on their website and the R9 (saturated red rendering) was in the 90s for both with tint almost exactly on the blackbody line. The 2700K did have a bit worse R9 than the 4000K so I could imagine it doesn't look exactly like an incandescent.