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Ask Slashdot: Does LED Backlight PWM Drive You Crazy? 532

Posted by timothy
from the it's-how-they-send-messages dept.
jones_supa writes "I would like to raise some discussion about a hardware issue that has increasingly started to bug me: backlight flicker, from which many LED-backlit monitors suffer. As you might know, the backlight and its dimming is driven by a pulse width modulated square wave, essentially flicking the LEDs on and off rapidly. Back in the CRT days a 100Hz picture was deluxe, due to the long afterglow of the display phosphor. LEDs, however, shut off immediately and my watering eyes and headache tell that we should be using frequencies in multiple kHz there. Unfortunately we too often fall behind that. As one spark of hope, the display review site PRAD has already started to include backlight signal captures to help assessing the problem. However with laptops and various mobile gadgets, finding this kind of information is practically impossible. This issue sort of lingers in the background but likely impacts the well-being of many, and certainly deserves more attention." So do LEDs bother your eyes? I think CRTs gave me headaches far more often than has any form of flat panel display, at least partly because of the whining noise that CRTs emit.
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Ask Slashdot: Does LED Backlight PWM Drive You Crazy?

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  • by Anonymous Coward on Thursday June 20, 2013 @01:42PM (#44062425)

    Your comment gave me a headache. Also, are you allergic to Wi-FI?

  • Sigh (Score:4, Insightful)

    by ledow (319597) on Thursday June 20, 2013 @01:43PM (#44062453) Homepage

    It's just you.

    If you're sensitive to them, don't buy them.

    Please don't make every LED / LCD on the planet more expensive because of a tiny minority of people who blame things like PWM for their symptoms (correctly or not).

    Like with flourescent lamps, and people who can't be in an air-conditioned room, and people who have to play games with altered FOV's because it makes them sick. You're a tiny minority, or else half the world would feel ill all the time. Please find another way to cope with it (i.e. glasses, double-blind tests to convince yourself it's placebo, or whatever).

    • Yeah, sounds like he's having an acid flashback or something......
    • Re: (Score:3, Interesting)

      by thule (9041)
      I think it is likely placebo. This is the first I've ever heard that people complain about LED lights. On the other hand I *still* hear people complain about fluorescent lights despite the fact that it is pretty rare to find ones driven by magnetic ballasts anymore.
      • Re:Sigh (Score:5, Interesting)

        by Andy Dodd (701) <atd7@corne[ ]edu ['ll.' in gap]> on Thursday June 20, 2013 @02:05PM (#44062763) Homepage

        I'm positive it's placebo here.

        LED PWM frequencies are FAR higher than the old CRT refresh rates.

        Also, while the OP talks about phosphor persistence, remember, the duty cycle of CRTs was VERY short. A pixel would only be "energized" for a tiny fraction of each display cycle. Even with phosphor persistence, I would not be surprised if even at very low brightness levels, PWMed LED backlights are still at a higher duty cycle than CRTs.

        I have a friend who is extremely photosensitive - the flicker of fluorescent lights without high frequency ballasts make him begin feeling sick almost immediately, and before he was on seizure medications, would cause seizures. To use a PC monitor, he had to always have ultra-high-refresh rate CRTs - until LCDs became common. He has NEVER had ANY issues with any LCD monitor, regardless of whether the backlight was LED or CCFL. They have been a godsend for him.

        • Re:Sigh (Score:4, Interesting)

          by gweihir (88907) on Thursday June 20, 2013 @02:19PM (#44062979)

          There is a possibility that the OP did run into some badly filtered backlights that actually use mains-frequency. That would be visible to some people but not to most.

        • Re:Sigh (Score:5, Interesting)

          by Solandri (704621) on Thursday June 20, 2013 @04:14PM (#44064335)

          I'm positive it's placebo here.

          LED PWM frequencies are FAR higher than the old CRT refresh rates.

          I'm highly sensitive to this - I can see the flicker of the old fluorescent lights. It's not a placebo. Most cheap LED PWM frequencies (up to about 250 Hz) are blindingly obvious. Above about 250 Hz I have to look for the effect to see it, though I can detect it up to a bit over 1 kHz.

          The old CRT refresh rates were mitigated by having phosphors, so they slowly dimmed in between refreshes, never turning off (when you turned the CRT off, the length of time it took for the screen to go completely black was how long the phosphors stayed lit). So if you scanned your vision side-to-side, even though the CRT scan image might not remain constant in brightness, it was still a continuously scrolling image.

          By contrast, LED PWM is almost binary - totally on to totally off. If you scan side-to-side while viewing an LED PWM screen, you see multiple individual images instead of one continuously scrolling one. It's like watching a poorly animated cartoon from the 1970s - easy to lose track of which parts are supposed to be static and which are supposed to be moving. (Well, I assume those of you with normal vision can tell 1970s cartoons were more poorly animated.)

          I have a friend who is extremely photosensitive - the flicker of fluorescent lights without high frequency ballasts make him begin feeling sick almost immediately, and before he was on seizure medications, would cause seizures.

          In static applications like a computer screen it doesn't make me sick. In fact, for me at least, it's pretty easy to ignore since I rarely have to scan side to side. Most of the scanning I do is just slightly side to side or slightly up and down. I'm just aware it's flickering. Then again I rarely get seasick so perhaps I'm not as sensitive to contradictory signals from my eyes and other senses.

          Where it kills me is in mobile applications. Certain cars are using LEDs with low refresh rate PWM (I'd estimate around 50 Hz) on their tail lights. When I'm driving at night, I'm not staring straight ahead. I scan side-to-side every few seconds to maintain situational awareness. If one of these cars is ahead of me, the act of scanning turns my field of vision into a sea of individual sets of lights [imageshack.us] making it difficult to pick apart separate cars. With the old continuous lighting, I could count the light trails and tell you how many cars there were. But if there are multiple cars ahead of me with the PWM lights, it's nearly impossible for me to tell how many cars there are while I'm scanning. I have to wait a couple tenths of a second to finish scanning, regain a static image, and see individual car lights. The lower the frequency of the PWM, the further the individual images of the lights are, and the harder it is to "connect the dots" and rationalize that they all represent one car.

          To use a PC monitor, he had to always have ultra-high-refresh rate CRTs - until LCDs became common. He has NEVER had ANY issues with any LCD monitor, regardless of whether the backlight was LED or CCFL. They have been a godsend for him.

          I never had much problem with CCFL - either they didn't use PWM or used it at such a high frequency it didn't bother me. Most LED screens however use PWM to decrease brightness. If you use the monitor at or near max brightness, you're unlikely to notice the PWM. But if you lower the brightness a lot like a laptop screen used indoors, the PWM becomes pretty obvious. I've learned to slow down how quickly I scan my eyes across the screen to compensate. Also, your peripheral vision is more sensitive to the flickering than your central vision, so avoid brightly-colored or cluttered desktop backgrounds.

        • Re:Sigh (Score:4, Informative)

          by AmiMoJo (196126) * <mojo @ w orld3.net> on Thursday June 20, 2013 @04:22PM (#44064437) Homepage

          No, it's real. Some monitors flicker their backlights at 60Hz, the same as the screen refresh rate, to improve motion reproduction.

          It turns out that even if the transition time of LCD pixels was zero motion would still look a bit blurred compared to CRT. The slight flicker that CRTs have actually makes objects in motion look sharper and easier for the eye to pick out. Some LCD monitors flicker their backlights to mimic this effect.

          IIRC the first manufacturer to do it was Benq with a CCFL backlight. In fact I have the non-flicker version of that monitor, the FPW-24Z or something like that.

    • square wave energy clogs radio bandwidth. plasma TV is just awful at putting background noise out there, and the undercabinet light power packs are also pure evil.

    • Re:Sigh (Score:5, Insightful)

      by DutchUncle (826473) on Thursday June 20, 2013 @02:36PM (#44063239)
      No. It's not necessarily just one person. Like allergies and other chemical sensitivities, each person who has a problem thinks they're weird, because nobody else seems to have the problem; but just like ANY OTHER aspect of humanity there is a wide range of reactions.

      I can see car brake LEDs flicker; my wife can't. Doesn't mean my eyes are better, or worse, just different. And that the car light systems could probably work better.

      Maybe it's not a tiny minority, and maybe it's not enough to make people ill. Maybe a tenth or a quarter of the world feels a little less well than they might all the time, but it's "just" a few percent so not enough to notice and each person thinks they just need more sleep. Or maybe a tenth or a quarter of the world is "just" a few IQ points lower than they might have been.
  • by Anonymous Coward on Thursday June 20, 2013 @01:43PM (#44062455)

    Some high-end displays flicker like a movie projector, only turning on LEDs during the refresh interval when the entire image is cached by the TFT so there's no tearing at all, not even as much as a CRT. If this is true you might find those expensive displays especially annoying.

    If it's PWM annoying you, shouldn't setting the display to max brightness entirely fix the problem? I wonder if you're picking a scapegoat for your headaches.

    • Max brightness will burn your eyes out of sheer brightness. I've prefered CRT monitors as long as they are set to 85 or 100Hz to overbright LCD ones.

  • No (Score:4, Insightful)

    by DogDude (805747) on Thursday June 20, 2013 @01:46PM (#44062479) Homepage
    So do LEDs bother your eyes? I think CRTs gave me headaches far more often than has any form of flat panel display, at least partly because of the whining noise that CRTs emit.

    No. You're imagining things.

    But, that being said, you're not alone. I heard somebody walk into the retail establishment that I work at and said, "I'm disappointed that you guys installed automatic doors that emit so much radiation, but I'm glad that at least you don't have horrible fluorescent lights that would make me unable to shop here." Of course, she was saying that standing under about 500 CFL's that she assumed weren't fluorescent because of their size, shape, and color.
    • Re:No (Score:4, Interesting)

      by gnasher719 (869701) on Thursday June 20, 2013 @01:52PM (#44062559)

      No. You're imagining things.

      Happened to me once that I got upgraded to a larger monitor, and when I turned it on it was like being physically smacked in the face. It's a long time ago so I can't remember exactly what happened then, but I didn't use that monitor.

    • When we first got LCDs in high school (circa 2004), I still had all CRTs at home. I found the LCDs hard to focus on for more than fifteen minutes at a time. However, once I switched to using LCDs everywhere (home, school and work), the problem went away. Now the only time LCDs give me an issue is if the backlight is just too dim and even that I get used to if I use the monitor long enough.

      I find it to be much more of a problem using someone else's keyboard because the key spacing is never the same.

    • Re: (Score:2, Informative)

      by DRJlaw (946416)

      So do LEDs bother your eyes? I think CRTs gave me headaches far more often than has any form of flat panel display, at least partly because of the whining noise that CRTs emit.

      No. You're imagining things.

      No, he's not. That whining noise is well known, objectively measurable, and explainable by actual physics.

      Unless IBM was imagining things. [ip.com]. Or you deny the existence of eddy currents [raftabtronics.com] in what is essentially a high frequency electromagnet (see material under "stray losses").

      But, that being said, once you'v

    • Re:No (Score:4, Informative)

      by X0563511 (793323) on Thursday June 20, 2013 @02:30PM (#44063161) Homepage Journal

      I can actually see the flicker effect he talks about - on some rare models. I can only see it near the edge of my vision (where your eyes are primarily motion/change sensitive). 70hz seems to be about the point where this stops, but anything lower than that is perceivable to me.

      Just like I can hear that irritating whine he talks about (though likewise, not all of them do that)

      The inverter on my LCD at home actually makes all sorts of horrible whines if it's in standby mode - so I either leave it on or unplug the thing.

  • by barc0001 (173002) on Thursday June 20, 2013 @01:48PM (#44062519)

    One thing I do know was/is a problem with monitors and eyestrain has to do with fluorescent lighting in the room. We can't see it with the naked eye, but the fluorescents are also flickering at 60hz and I've had it happen in the past that if the CRTs I was using were out of sync, (running at 75hz or similar) after a while I'd get weird eye strain from something we can't consciously perceive but our eyes still try to correct for. I usually solved the problem by either setting the CRT sync rate as high as it would go or syncing it to 60hz, or preferably getting rid of the fluorescent lighting completely in my workspace when possible. Maybe a similar effect is at work here?

    • Ditching the old-school iron ballasts might not be a bad idea, either. They are seriously inefficient, and suffer from hum and flicker. Contemporary electronic ballasts perform considerably better.

      • by mlts (1038732) *

        Maybe even ditch the whole assembly and go with LED bulbs that replace the fluorescent tubes?

        People argue about LED lighting and if 120 Hz is a headache inducer compared to 150 Hz. However, I'll take either over the 60Hz ballasts of old.

    • by WillgasM (1646719)
      I rarely turn the lights on. Unless there's a storm rolling through, I get plenty of glow through the closed window blinds. OSHA seems to think offices need to be saturated, but I can't stand florescents. Between the hum, flicker, and overall brightness, they drive me crazy. Employers often find it strange, but I think they get used to the idea of the IT guy in his dark cave. I even convinced my last boss to give it a try, and we just quit turning on the lights at all. Saves energy, reduces noise (both soni
  • No. Used plenty of LED displays without issues.

  • I cannot wait until I get old enough that I cannot hear the whine of CRT. I don't care what that means for my hearing, that noise alone is just... gross feeling in my head.

    • How much time do you spend next to CRTs? And why?

      • by DrGamez (1134281)

        I guess it's not a lot thankfully, but now they have become rare enough that when I'm near one it's jarringly loud. To think my entire CRT-enjoying childhood I used to enjoy "knowing" when a CRT was turned on in a room.

        I still won't miss it when it's gone!

    • I can't hear the whine of a CRT any more, because they no longer exist in any room that I spend time in.
      • by DrGamez (1134281)

        But what about that one CRT you keep around for the pre-hd consoles?

  • I stare at various LCDs for 16+ hours a day without any problem. I'd even go so far as to say I have sensitive eyes. I can't attest to newer LEDs. I've installed a few for video conference units, but I don't spend my day looking at them. They're definitely not any worse than a CRT. I used to have two giant CRTs on my desk and they'd damn near give you a tan.
  • I still hear people complaining about fluorescent lighting despite the fact that CLF's have electronic ballasts that use extremely high frequencies. I could understand the old, old lights that used magnetic ballasts, but CLF's? Really? Seriously? People can see 40,000Hz on a properly working tube bulb? It is not like a monitor with tiny phosphors where I could see the scanning. LED's flicker way more than I ever noticed fluorescent lights. To make matters worse, LEDs are used in many more places! I noticed
    • by ttucker (2884057)

      People can see 40,000Hz on a properly working tube bulb?

      This is about the frequency range that LED PWM drivers operate at.

  • by Zontar_Thing_From_Ve (949321) on Thursday June 20, 2013 @01:54PM (#44062591)
    Never had any issues with LED displays of any kind. My TVs and PC screens have been LED for years now. I don't have a problem with the question, but I think this is getting into an issue where the person asking the question is in the minority and would like believe that almost everybody else is in the same boat so maybe they get something going to "fix" the problem. For example, based on personal observation, I'd say that about 10% of the population has some kind of vision issue where they cannot see 3D videos at all. Trying to watch those gives them headaches or makes them ill. Those people always complain the loudest about how 3D "sucks" and insist that it's going to fail because they think that everybody on earth has the exact same problem. I'm willing to admit that the original poster may have a very real problem, but is it common enough to attract attention from the manufacturers? Probably not.
  • by bradgoodman (964302) on Thursday June 20, 2013 @01:54PM (#44062593) Homepage
    YESSS!!!!!!

    I thought I was the only one - but perhaps I still am - but car LED brakelights have been driving me ***CRAZY** for years!!!!

    • ...car LED brakelights have been driving me ***CRAZY** for years!!!!

      I see what you did there, and it gave me a headache.

    • by Carnivore (103106)

      Nah, the taillights bug the shit out of me, too. I have no idea why they don't just increase the pulse frequency.

  • It's the saccades (Score:3, Interesting)

    by Anonymous Coward on Thursday June 20, 2013 @01:54PM (#44062603)
    Your eyes are always moving as well, these movements are called "saccades". I think that there is a "beat frequency" between your saccades and the PWM drive that probably triggers headaches. I wonder if it's possible to change the PWM frequency of the chipset just to experiment?

    http://en.wikipedia.org/wiki/Saccades [wikipedia.org]

  • Nice troll... (Score:4, Interesting)

    by Elledan (582730) on Thursday June 20, 2013 @01:55PM (#44062613) Homepage
    The average (quality) CRT is perfectly fine for most people. They do not emit any high-frequency noises, nor do they have major flickering or geometry issues. To suggest that all CRTs are crappy is doing them a total disservice.

    That said, there are plenty of CCFL-using LCDs which have given me dry eyes and a funky feeling after staring at them for a while, possibly due to the polarized light. Or perhaps just because they were low-quality pieces of junk.

    If you want to check if there's any significant flickering that'd annoy you, check the display from the corner of your eyes. The peripheral vision of the eye is far more sensitive to motion than the central part you generally focus on. If you can't see flickering with your peripheral vision, it's just not there for you.

    Thanks for the whine story, though. Would you care for some cheese with that? :)
    • by h4rr4r (612664)

      The average CRT was crap. It flickered and whined like hell. The geometry on most of them was a total joke. Often the image bounced around too.

      I have a very nice one I still use sometimes that does higher res than any LCD you can buy and uses a shadow mask. This cost multiples of the average POS crt.

  • Back when I was using CRTs I had to have 80Hz minimum, and 85Hz was the point at which my eyes no longer felt "weird" for lack of a better word. I currently have a Dell S2230MX for my main screen and it's LED backlit and I have no issues at all. For what it's worth, unrectified LED Christmas light strings drive me bonkers with their strobing, so not really sure what to say on this one.

  • Neither I, nor anyone I know has ever complained about this. However, it's not the first time I've heard about this complaint. And my mind is starting to play tricks on me: I just "noticed" some flicker on an LCD monitor (fluorescent backlight) I've had for years.

    I can't conclude whether or not the issue has any merit, but my preliminary conclusion is that discussing the issue tends to cause it.

  • So do LEDs bother your eyes?

    You need to do double-blind testing to see whether you are really bothered by the LED flicker, or you just think you are bothered by the flicker.

    ...has increasingly started to bug me: backlight flicker...

    Perhaps it has increasingly started to bug you because you are becoming increasingly aware of it, and not vice versa.

    .
    It is a common marketing ploy to create a perceived problem, then magically have a product available for sale that just happens to assuage that newly perceived problem.

  • So do LEDs bother your eyes?

    No. Can't say I've ever seen or heard of anyone having trouble with LEDs specifically. I honestly cannot even see a flicker in most LED screens whereas I was pretty sensitive to it on CRT screens. I find LEDs to be much easier on my eyes than even the best CRTs. I've seen light sensitivities that are due to interactions with poor quality or old fluorescent bulbs. My last office was next to a window which made for some glare problems and excessive brightness problems at times. I've also seen issues wit

  • White LEDs actually do have a nonzero rise and fall time(because if it says 'white' on the label, that means 'glob of phosphor being pumped by a blue or UV die, since we don't have wideband LEDs'). Also, a quick look through the datasheets shows advertised PWM frequencies in the 200KHz-1MHz+ range. Are the cheap seats substantially slower?

    • He's probably using a El Cheapo monitor with a brand name like AOC or something. And it's probably defective.

      I've never heard of anyone being bothered by LED flickering. I would suggest that he buy a quality brand like Samsung or LG so that he can be sure he as a good working unit. His current one has something wrong with it.

    • by TeknoHog (164938)

      White LEDs actually do have a nonzero rise and fall time(because if it says 'white' on the label, that means 'glob of phosphor being pumped by a blue or UV die, since we don't have wideband LEDs').

      For me it's the blue LEDs that drive me crazy, so perhaps the OP has a problem with the spectrum rather than flicker.

    • some as low as 90Hz (Score:5, Informative)

      by Chirs (87576) on Thursday June 20, 2013 @03:27PM (#44063869)

      According to this article (http://www.tftcentral.co.uk/articles/pulse_width_modulation.htm) LED backlights generally pulse between 90-420Hz, not in the KHz or MHz range.

  • by Twinbee (767046)
    Will OLED's have the potential to flicker like this?

    I notice a 'walking pixel' effect on my laptop once, so this could the effect in action.
  • I have absurdly fast eyes. I cannot stand any CRT below 80Hz. I can see red, green, and blue in separately in any DLP projector with any color wheel frequency. LEDs in taillights in modern cars like Lexuses drive me insane because it looks like a trail of blinking LEDs to me. Even LED glowsticks bug me and people don't believe me until I wave them back and forth to prove they're flickering. And yet I've used a dozen different LED monitors, maybe even hundreds, and never had a single problem with them.
    • by h4rr4r (612664)

      There is another cause, white LEDS do not turn on and off very quickly. This is because they are really UV LEDs with phosphor painted on them.

    • LCD displays with CCFL backlights are less susceptible to the problem than ones with LED backlights. And some monitors use non-pulsed backlights or at least smooth it out with a filter circuit or something.

  • It does happen (Score:4, Interesting)

    by GreenEnvy22 (1046790) on Thursday June 20, 2013 @02:06PM (#44062773)
    We have a user here who got a new laptop last summer, it had a LED backlit LCD. Within 20 minutes she was calling saying it was making her feel sick/headache. We tried adjusting refresh rate, brightness, no help. Put a CFL backlit LED laptop in front of her and she was fine. Tried LED standalone monitor, it also bugged her though not as much. So, we had to find a laptop that had a CFL backlit screen, wasn't junk,and met our other requirements (docking connector mostly). Ended up getting a previous year model Toshiba Tecra with a Core2Duo.All the rest of the laptops we bought had i5's in them by that point.
    • by jeffmeden (135043)

      We have a user here who got a new laptop last summer, it had a LED backlit LCD. Within 20 minutes she was calling saying it was making her feel sick/headache. We tried adjusting refresh rate, brightness, no help. Put a CFL backlit LED laptop in front of her and she was fine. Tried LED standalone monitor, it also bugged her though not as much.

      So, we had to find a laptop that had a CFL backlit screen, wasn't junk,and met our other requirements (docking connector mostly). Ended up getting a previous year model Toshiba Tecra with a Core2Duo.All the rest of the laptops we bought had i5's in them by that point.

      What was the DPI on the new laptop vs the old one? Dollars to donuts the new one was higher, in the 220 range, and the older one was under 200 (the point at which you can still make out pixels at typical use distance). She was getting eyestrain from trying in vain to focus on the screen, high DPI monitors take a lot of "user calibration" before they are comfortable to use.

  • Hey, jones_supa: It's not yer monitor, stoopid, it's yer cellphone... stop holding it next to yer head!

  • by wcrowe (94389) on Thursday June 20, 2013 @02:13PM (#44062881)

    For over thirty years now I've been working with various display devices of a wide variety of design, manufacture, size and refresh rates. About sixteen years ago I started having the symptoms you describe -- headaches, watering eyes, etc. The internet back then isn't what it is now, so my first reaction was NOT to post something on a tech forum and open myself up to a lot of ridicule and abuse. Instead, I made an appointment with an ophthalmologist. After a thorough examination and some tests he advised me to take occasional breaks from the monitor throughout the day and rest my eyes. He also gave me some techniques to use for this. I took his advice and my symptoms went away virtually overnight. I have not had any problems since.

    You should go see an ophthalmologist -- not an optometrist -- but a real eye expert. You might be surprised to learn that your problem has nothing to do with refresh rates or anything of the sort.

  • There was a documentary on that in the early 90's http://en.wikipedia.org/wiki/Joe_Versus_the_Volcano [wikipedia.org] starring Tom Hanks and Meg Ryan.
  • by Meeni (1815694) on Thursday June 20, 2013 @02:40PM (#44063275)

    Old CRT were giving me pretty severe headackes. This has all disapeared as soon as I used TFT panels, even with the early models that were not as good as recent ones. I definitely could see flicker on CRTs, I cannot anymore on TFTs.

    Maybe you need to turn off "true motion" option on your TFT TV ?

  • yes (Score:4, Interesting)

    by cnaumann (466328) on Thursday June 20, 2013 @03:02PM (#44063573)

    This goes for LED brake lights, LED Christmas lights, and LED traffic light, and roadside LED signage.

    I find the PWM flicker of LED brake lights _VERY_ disorienting.

    Monitors I can aviod.

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