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Ask Slashdot: Does LED Backlight PWM Drive You Crazy?
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.
Waa my computer is too flickery, someone call the waambulance.
Would you rather have some 300 pound CRT monitor literally frying your eyeballs out of the back of your skull? Take and aspirin and get back to work, hipster-slacker-dweeb.
Your comment gave me a headache. Also, are you allergic to Wi-FI?
Gimme a break. Whats next, your neighbours WiFi is making you sick?
http://en.wikipedia.org/wiki/Betteridge's_law_of_headlines
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).
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.
So I never had problems with either type of monitor.
You live with what we have, NOT whine about your own deficiencies... I am partially deaf. Have you ever seen me to whine about that?
Well, you can at minimum reduce the CRT whine by casting the deflection coils in resin. Clumsy, but it works.
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.
I don't respond to AC's.
perhaps you should buy some nasal clearing spray. it's summer and the pollen makes my eyes water and my breathing harder and that gives a headache.
also, is this an advert for prad?
another also, this is the first time I heard anyone bitch about this except people using cameras where it's visible. in fact I've bumped into many people who thought the usual way was to vary the voltage...
ALSO TURN THE FUCKING BRIGHTNESS TO MAXIMUM and your problem is solved.
world was created 5 seconds before this post as it is.
I've not noticed it in LCD's yet...but it could easily be I've not run across it yet. Is this just something newer LCD monitors are starting to do?
Back in the CRT days, a 60Hz refresh rate would drive me nuts. A 54Hz refresh rate on a problematic computer once actually gave me a headache. 72Hz was passable, and 75Hz the minimum at which I no longer noticed the flicker. 85Hz was even better. I remember seeing a Viewsonic monitor of some sort once that would handle 120Hz refresh rate at 640x480...and it looked amazing. What I always found interesting was how many people didn't notice the flicker at 60Hz at all...they just couldn't comprehend what I was talking about. I guess biologically, we are all a little different in that regard.
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?
No. Used plenty of LED displays without issues.
-- Using the preview button since 2005
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.
http://www.india4movie.com/
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 recently switched to an LED monitor. It can only refresh at 60 Hz (or 59Hz but who does that?). I haven't noticed any flickering or had any headaches. I have noticed less power consumption and less heat coming from my display so I think you're just gonna have to deal with it. The benefits of LED displays will outweigh stuff like this forever. You won't be changing the industry.
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 the flickering from the taillights in newer cars, gadgets, LED equivalent bulbs that dim, etc.
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.
I thought I was the only one - but perhaps I still am - but car LED brakelights have been driving me ***CRAZY** for years!!!!
It maked your screen flicker as well, no matter how fast your backlight is flickering.
http://en.wikipedia.org/wiki/Saccades
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?
Site & blog: http://www.mayaposch.com
Like you I get headaches from looking at CRT screens. I could never stand refresh rates below 80 Hz. The problem is I need a lot of screen real estate and usually the lower end monitors can only do higher resolution at lower refresh rates. Interlace mode is even worse.
So when LED came out I rejoiced. It is a God send for me. I never needed glasses until I was diagnosed with astigmatism. I work exclusively with computers and blame CRT monitors until now.
So while I am bothered by the flicker on LED monitors, I am also thankful for it being a huge improvement over its predecessor.
Take off every 'sig'!
All your 'sig' are belong to us!
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.
I've found that neurotypical people tend not to be bothered by things like fluorescent lights, CRTs and LEDs/LCDs that use PWM. However, I myself can easily notice 60Hz flicker in fluorescent lights and CRTs that aren't running at a 75Hz+ refresh. LEDs/LCDs don't bother me, but I can see how someone with greater photosensitivity than I could find PWM'd LEDs/LCDs to be extremely annoying.
I highly suggest reading up on the visual effects that often happen to people on the autism spectrum. The UC Davis MIND Institute, for example, uses custom fluorescent fixtures that run at 300Hz instead of the standard 60Hz line frequency in order to reduce or eliminate the horrible flickering that photosensitive people notice. Temple Grandin has a really great talk, "My Experience with Autism", where she goes over the symptoms and root causes of certain symptoms, you can look it up on YouTube.
Photosensitivity isn't coupled hand-in-hand with autism, granted, but if you're working in a technical field already and you have photosensitivity, you might consider reading up on it.
This is about as real as wifi giving people headaches.
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 with brightness due to external stimuli or just a larger brightly lit screen. My wife is actually rather light sensitive so I had to dial the brightness down on a 24" monitor at home.
I'd be curious it the original poster's issue is with the LEDs or with the backlighting or with something else unrelated to the LED screen.
I'd give anything to be able to go back to the easy-eyes days of big CRTs. You could set the resolution to anything you wanted. With a really good one (not those cheap noisy ones), and a good video card, it was like gazing at a calm blue sea.
But it's not going to happen, and I don't miss the heat.
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?
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.
if this is supposed to be a new economy, how come they still want my old fashioned money?
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.
Why OpalCalc is the best Windows calc
I bet you complain about more than just monitors. I'm sure your friends and friend's wives just love to have you over as company.
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. The diffusion of the screen blocks the blatant flicker that's likely occurring. I think the author of this article merely has too much caffeine or is eating crunchy granola while using his monitor or something because if anyone would have a problem, it would be me, and I don't.
I would be surprised if frequencies of two or three hundred Hertz cause a problem in anyone. I have noticed that sometimes my MacBook Air has started to flicker at a visible rate when first used, but quickly settles down. The samples on PRAD are too quick to show flicker at a problematic rate. The most problematic rate is around 6-10 Hz, but varies wildly between people.
It appears similar technology is being introduced for office lighting, with disappointing results.
Waa my computer is too flickery, someone call the waambulance.
For people with photosensitive epilepsy, it might more more like "someone call the ambulance."
PWM brake lights and tail lights are bad for me though.
Bright enough to leave a brief afterimage so when you move your eyes there is a blinking tracer.
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.
You do realize that right? The refresh rate is how often new data is sent to the pixels on the screen. The backlight is the led's that surround the edges of the screen. They are unrelated.
Most PWM controllers use a few 10's of khz upto a a few using low 100's of khz. If you look up the ramp up current of led's you'll see they arent instant on/off either, so your point is really moot on two cases.
UDL
Hey, jones_supa: It's not yer monitor, stoopid, it's yer cellphone... stop holding it next to yer head!
Waa I heard someone make a complaint that doesn't affect most people in third world countries, someone call the waambulance.
Seriously, just look up the numerous studies that have been done on this. This flicker is *extremely* annoying to a lot of people. Ever seen the LED tail lights on some Cadillacs that look like red strobe lights? C'mon, people!
By your own admission, she said "at least you don't have horrible fluorescent lights."
In that context, it is unclear if she believes all fluorescent lights are horrible, or if she's referring to the subset of fluorescent lights that she believes are horrible. I choose to believe that you're beating a strawman by ignoring the fact that she was probably referring to the tube-type fluorescents with traditional / old-style ballast that NOBODY likes because they flicker at 1-2Hz and hum like they're singing you the song of their people.
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.
Proverbs 21:19
There was a documentary on that in the early 90's http://en.wikipedia.org/wiki/Joe_Versus_the_Volcano starring Tom Hanks and Meg Ryan.
Jesus was a compassionate social conservative who called individuals to sin no more.
The screens I've seen haven't bothered me to the point of having headaches, they are just aesthetically displeasing when I notice the pulses due to persistence of vision.
It's not a dealbreaker for me, but I think it would be pretty cool if everyone used a variable current source and didn't chop up the light output with pwm.
The screen refresh rate of the LCD pixels is not the same as the LED backlight refresh rate. The LCD refresh rate is more analagous to the refresh rate of an analogue monitor, and like phosphors, it does take some picoseconds for an LCD cell to fade once it's power is ceased, so you get the "smoothing" of the images in the same fashion.
The LED refresh rate, on the other hand, has to do entirely with the light behind the LCDs. Whether those shut off immediately or not I don't know, but I've certainly never had a problem and I'm very sensitive to refresh rates (they trigger migraines when too low.)
Switching to an older style monitor that doesn't use LED backlighting would resolve the problem for the original poster. Cranking up the refresh rates on the LEDs would induce more flickering, not less, because they don't have a decay/shutdown period according to them.
I do not fail; I succeed at finding out what does not work.
More and more autos, trucks, and buses are using LED's for taillights/brakelights, and the PWM frequency they use is slow enough that I can notice the strobelight-like effect when the vehicle is moving or I move my eyes. It's very distracting, and I really don't see the reason why they can't raise the frequency enough so it isn't noticable anymore.
Are YOU using the TOOL, or is the TOOL using YOU? Think about it!
Another cause of "walking pixel" is the dithering needed to get smooth gray ramps on a 6-bit TN LCD. Most panels will perform "temporal dithering", changing the dither pattern from frame to frame to make it less visible for still images.
I guess I'm in the minority, but I've found flicker to be a problem on lots of tech:
- LED monitors, I considered picking up the samsung S27A850D a while back, but no. I can easily see the backlight flicker. Interestingly, the dell LED's aren't so bad unless you turn down the brightness, and then flicker is a problem there too.
- Plasma TV's: I can't stand plasma due to phosphor lag. I can see that different colours of phosphor seem to stay turned on longer, making the images appear to ghost.
- CFL's: I don't see flicker, but I see colour separation in some models. I suspect it's due to the mix of phosphors having different decay times for different colours, simialr to plasma panels. Old-school fluorescent lights don't seem to ever have this problem though.
- DLP projectors: That old rainbow effect, I can see the sequential colours as the little colour wheel inside spins
Obviously many others can't see these problems, otherwise these items wouldn't be as successful as they are, but It would be nice to not be able to notice these.
I've had eye strain issues with some of the low end Dell LCDs. Out of 50 in our office with the same screen I was the only one to complain. So I gambled, spent $150, and bought a Samsung LCD. Problem solved. They later bought me a 24" Dell LCD and that was fine too. TLDR; If your screen bothers you get a different make/model.
I run my monitors a bit brighter to cut that problem. The only time I've noticed flicker is running at minimal brightness.
Seriously, LED-backit monitors a big improvement on the cold cathode monitors, where the cold cathode would fail after a while, or backlit unevenly when first switched on, until fully heated up.
The old CRTs had low refresh rates, and even the nice Trinitron ones had those annoying mask wires crossing the face of the monitor.
I think the OP is whingeing, and should get this tin foil hate professionally fitted next time.
When I slam 3-4 red bulls and have a Pot of 2X strength coffee being consumed fora marathon session. But at that point I can see the overhead lights flickering at me.
Do not look at laser with remaining good eye.
No. Why was this survey question posted as a story? Surveys are supposed to be in a slashbox, not the main section of the page.
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 ?
Damnit! Now I can't not see it. I was fine untl you pointed it out. Thanks for ruining my life forever.
I remember an old DOS app called Flicker Free, where the screen would scroll smoother and you could change colors... maybe it needs an update!
I've had similar problems as I have gotten older, but I don't think that it is the light frequency (for me). Instead, I think that I just can't punish my body like I used to do.
I'd try the following things before getting too worried:
1. Take work breaks every 30 minutes;
2. Update my eyeglass prescription;
3. Make sure that no other bright light sources (like windows) are in your field of view when looking at your monitor (It should be the brightest thing in the room);
4. Sleep 8 hours a night;
5. Don't use the computer when you are hung over;
6. Take allergy medication if you have seasonal allergies.
When I do all of the above, I don't get headaches or watering eyes. Generally, I do not do all of the above, but happily accept the headache or watery eyes because I know that I can make it go away if I sleep more.
Just because you've never had a problem doesn't make it non existent. No one is going to take away your new shiny monitor.
Most companies run infrastructure to failure. That means fluorescent lights with magnetic ballast that flicker at 60 Hz x 2. Previously, an old CRT with worn out phosphors running at a close frequency would have very noticeable flicker. The beat frequency (difference or lack of sync between two rapidly flickering sources) is what matters. Personally I could see some flicker with a monitor under fluorescent lights up to 70 Hz. Some coworkers always left their CRTs at 60 (where flickering was most noticeable) and we're never bothered. LCDs with CFL backlights generally don't have Mich of an issue (high frequency driver due to size).
I can see how an LED backlight with faulty circuitry could become an issue, especially if it has a flicker close to powerlines frequency. Again it would not be noticeable to everyone. That doesn't make it psychosomatic. Just means the guy who sees it has a slightly different visual cortex in hos brain from you (faster clock speed)? Stop throwing around this nonsense and accusing the OP of being allergic to WiFi too.
By the way, if you ever work in a large company or government, you often will be stuck with degrading equipment for quite some time. Especially outside of IT.... I feel sorry for the OP if this is the case as if his boss can't see it, he's stuck with it until it breaks or he quits.
Also someone mentioned LED lights on cats. Cars run off of DC, but voltage regulators and alternator can add a pulsation to any lighting.
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.
In all seriousness, I think it is just that the OP has a poorly-constructed monitor. I have used an LED backlit laptop for over a year now (an Asus EEE, so it's a pretty chap computer) and it has not bothered me in the least. I saw a comment that suggested wiring in a capacitor, but I wonder if there is a missing or ineffective capacitor in there somewhere that is supposed to mitigate this, and that it was either omitted to save $.02 on the manufacturing costs or if it was from a bad batch.
As for the rest of you, you're acting like a bunch of assholes. Fuck you all. If you aren't in his seat, you don't know if the problem is real or not.
www.wavefront-av.com
Go check out the HardOCP display forums. There are lots of people sensitive to PWM flicker, especially when you turn the brightness right down (which maximizes the amount of time each LED spends in an "off" state.
There are monitors out there with DC backlights...check the more technical reviews like the one mentioned in the summary.
These are also present on some high-end cars with LED tail lights. Some Cadillac Infiniti, Saab and the latest VW cars all have them.
It's not just you. I have many LED lights in my house, and can clearly see the flicker on the walls. What I have done to compensate is to mix CEF with LED in three and four socket configurations (SMD5050 based bulbs are the best).
With monitors, I can see 70hz and below through peripheral vision. Also, fast moving objects such as ice skaters spinning look like they are in a strobe light to me. I can see the refresh line moving up and creating a diagonal split while watching a DVD. Interlaced video shakes in general. My DLP projector is fine unless I move my eyes quickly. Then I see the rainbow effect.
All of this is fine by me - no headaches. Would I trade in my Visio LED/ LCD TV for a plasma? No.
It bothers my wife though, so she keeps one 100 watt incandescent in her office ceiling fan. If it bothers you too, then you have my sympathy.
I have a Dell 2407 and I run at 25% brightness. It also doesn't use PWM since it's old enough to use CCFL. At 100% brightness it is uncomfortable to look at for any length of time.
Drives me nuts. I'm so happy when I see the occasional one using DC or high-frequency PWM.
With CCFL backlights flicker was less of an issue because the backlight itself smoothed out the flickering. With LED backlights the on/off cycle is sharp and immediate and therefore more noticeable.
Also, some LED backlights drive the PWM as low as 90Hz, which can be visible when the *whole screen* is flickering at that rate.
Older LCD displays used CCFL backlight, while many newer ones use LED. The LED ones show backlight flicker due to PWM much more clearly.
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.
I'm able to see the flicker of 60Hz and perhaps a little higher than that. I'll get some massive eyestrain too if I sit at a CRT monitor with a 60Hz refresh. At 85Hz I can rarely see the flicker if I concentrate and it's relatively comfortable. Higher than 85Hz and it's much better.
I'm sitting in front of three LED backlit LCDs and they're not flickering at all. The LEDs stay on. All of my smartphones with LED backlighting haven't flickered either. The wife just got a new phone with an AMOLED display and I do see some flicker on occasion. I do see it with LED brake lights on most cars; a few of them only flicker with parking light brightness and then don't flicker at the brake light intensity.
I think jones_supa needs to change monitor manufactures.
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.
Turning up the duty cycle will reduce both the time that they're on and the time that they're off, but the ratio will remain the same.
Bullshit. I remember people asking for LCD's in my old job for the same reason. "Oh boo hoo the flickering CRT". I told them immediately they were full of shit and ensured they would never receive an LCD, even if I was overstocked in the office.
Neuroscientist weighing in.
"Flicker fusion" is the rate at which multiple flashes appear to be a constant light. When your eyes are still, flicker fusion is in the few tens of Hz for your central vision. In your peripheral vision, it can be roughly 100 Hz (which is why old fluorescent lights bother some people, me included). The killer is when you're moving your eyes around a lot. With large eye movements, flicker fusion can be in the 100s of Hz -- high enough for the backlight to bother a small number of people. I bet DLP projectors drive you crazy, too -- on many you can get a nauseating rainbow effect when you flick your eyes from one edge to the other because of how the color wheel works.
As a side note, CRTs were way worse. When I measured one with a phototransistor and a 'scope, I was really surprised that their duty cycle is ~10%. You probably just didn't use them as much per day, or age is making you more sensitive.
Might've been mentioned before and I missed it buried in all of the posts, but I can tell you why CRTs are whining all the time.
It's because LCDs have replaced them. :)
Many CRTs and fluorescent lights (yes, even CFLs) flicker like crazy. I've swapped out a few in my house because of it. It is a known problem with video cameras, which run at similar frequencies to the human eye. Why would so many of you deny that this is possible for some people?
One maddening thing that people seem to miss is that the flickering from PWM is NOT at the same frequency as the PWM.
Say you are running PWM at 100 hz. at 50% brightness, your light is flickering at 50 hz. At 10% brightness, your light is pulsing at 10 hz.
This easily explains why LED flicker can be visible (to those sensitive to flicker) even when PWM is hundreds of hertz.
Speaking personally, I suffer from migraines and am exceptionally sensitive to flickering light sources as a trigger. This happens even at frequencies much higher than those that trigger photosensitive epilepsy. Flame away if you think this is psychosomatic. I wish.
That said, I have noticed very few LED-backlit monitors that suffer from flicker issues at normal brightness. And I *can* detect flicker from many LED sources such as brake lights, traffic lights, and flashlights that use PWM, and especially things run off mains power at 60 hz (intolerable) or rectified "flicker free"120 hz (still pretty intolerable, since the lights still cut out at the crossover point).
I'd also point out that migraines are pretty distinguishable from regular headaches, and (for me and many) also have a significant nausea-related comment, so while the OP may be sensitive to flicker, his symptoms sound more like normal eyestrain.
It's a migraine trigger for me. Within 5 minutes I start to see an aura (loss of vision) and it begins to feel like someone is trying to remove my eyeball with a pair of pliers by twisting it out. It's not with all LEDs just some. But it doesn't occur with non LED displays. High contrast such as trying to look at a display in bright sunlight will also do it.
I have a variety of sensitivities and what I've always said is that I'm the canary in the coal mine. If it bothers me today, it will cause others real problems down the road. Some of this has been born out with chemical sensitivities.
Just my USD 0.02 worth.
Why not paint the back with glow in the dark paint. it will continue to glow for a bit while the backlight LEDs are off, simulating a phosphor.
I see the blinking of LED car brake lights (ironically most are high-budget cars, somewhat counter intuitive), just about any 7-segment display, and most dimmable indicator lights in vehicles or devices. Especially when moving through my FOV i see the a trail of flashes, and when looking at overly bright LED walls i see the overlapping negatives for minutes. (note i work with LED walls daily, no getting used to yet) My clock results in a marqueue of the current time.
But LED backlights in LCD monitors i rarely encounter, so no experience there.. although i've seen some cheap models in stores that just flashed like Fluorescent lights. Oh i hate those fluorescent lamps .. i see them flashing and can predict which ones on that floor are going to fail within two months before anyone sees them blink.
I also get a headacke from 60Hz CRT monitors, but 85Hz is fine. And i can play games at 10fps without it bothering me .. perhaps because i am used to seeing the flicker even in the best games?!
Reviews on tftcentral.co.uk also measure PWM frequency. This link describes how they measure it just using just a digital camera:
http://www.tftcentral.co.uk/articles/pulse_width_modulation.htm
Flicker on a CRT was typically at 25/50/75/100Hz, 30/60/90/120Hz. At or above 50Hz, flicker was usually not noticeable or a problem except for sitting very close to very large screens (because flicker gets noticed faster in our peripheral view). If you get serious headaches from 50/60Hz flicker, you should get headaches from ANY non-solid-state artificial lighting (incandescent, fluorescent, halogen, ...) because the net operates at 50/60Hz and thus these lights go dimmer/brighter at that frequency (not on/off because there is an afterglow (heat)).
LED PWM generally sits at a carrier of ~1kHz and can easily be as high as 1MHz for consumer grade (there is actually an increased cost for lower frequencies as the capacitors and coils are larger), this is well outside the range that our brain can notice or process. You should have more issues with displays that have non-LED backlight because they could be at a low multiple of the net's frequency.
Custom electronics and digital signage for your business: www.evcircuits.com
Does reviews that include assessments of PWM in monitors. Very recommended.
http://www.tftcentral.co.uk/
Benq is in the process of introducing non-PWM flicker free backlight LED monitors.
http://www.tftcentral.co.uk/news_archive/28.htm#benq_bl2710pt
The OP probably wrote this article while he was riding BART from the anti-Smart Meter protest he just attended in San Jose to the Electromagetism Allergy focus group in Berkeley.
I'm always amazed at how many people think that there is a lot of variance in physiology. like they have super-fast rods and cones. is this just a generally harmless form of psychosis? I'm not arguing that mutants don't exist (http://en.wikipedia.org/wiki/Tetrachromacy, or the myostatin), or that training can make you very sensitive to, say, perfect pitch. just that this topic (and related, such as wifi allergy) seems to attract remarkable delusions (imo) of grandeur.
When I claim to be sensitive to X, people don't believe me because they don't notice it. I conclude that I am simply more sensitive than they are.
When this guy claims to be sensitive to X, I don't believe him because I don't notice it. I conclude that this guy is a freak whose experience is worthless.
Video tearing happens when the software updates the video frame buffer on the graphics card with a new image while the previous image is still being transmitted. Tearing artifacts do not depend on display technology.
For example, with 1080p60 video mode, a new image is transmitted 60 times a second. The video signal (VGA or DVI/HDMI) has 1125 horizontal scanlines (1080 visible and 45 invisible sync lines), so it takes (1000/60) * (1080/1125) = 16 milliseconds to transmit each image, and there's 667 microsecond "vertical blanking interval" gap (VBLANK) betewen each image.
The software (game, video player, whatever) should only update the framebuffer with a new image during the VBLANK, because otherwise multiple images become visible on the display. If the image contents differ, the borders between the multiple images is visible as a horizontal "tear", because the video signal scans horizontal lines from top to bottom.
Big deal. Grab an LDR or photodiode, plug it into a CRO, point it at a white part of the screen. Instant measurable square wave. Next!
You'll find the same problem mentioned by flashlight modders (candlepowerforums.com, budgetlightforums.com) -- some multi-level lights have quite annoying PWM.
GLOSSY bothers me more. Too much reflection, not even talking about fingerprints. In a way, a glossy screen is going back to a cheap CRT.
I've got better things to do tonight than die.
I can eliminate PWM on some of my monitors by setting the brightness to maximum and lowering contrast and the color levels. When I watch those monitors for a full day, my eyes are OK. When I watch my laptop (240 Hz PWM) where I can't eliminate the flicker, my eyes are tired by the end of the day.
When I first learned about how LCDs are made, I was furious that after all the problems with CRT monitors, LCD makers resorted to such a poor choice. The world my eyes see is flicker-free. Why should monitors be different?
The technology for flicker-free LCDs exists and it's called calibrated color level compensation. It should be the standard.
I kinda need both my eyes until the end of my years, so I'd appreciate it if manufacturers stopped wearing them out with PWM...
Physically hack the monitor get some light blocking film or use sunglasses.
Techno hack the monitor to do current dimming rather than PWM dimming. This of course will screw the colors to hell and gone.
Techno hack the monitor's LED backlight array so you can turn off some of the LED or replace the LED array with something not as bright or with more LEDs and the ability to turn them off individually.
Techno hack the monitor's LCD backlight by placing an LCD shutter over the LED and use that to dim it.
Or you can go whine about it on slashdot.
*Techno hack, sorry about that but I felt the real term was too rich and nuanced for the author.
I'd go on a Vegan diet but the delivery time from Vega is too long. --brownkitty
I see the same issue with plasma monitors. The phosphors used to have a long enough decay, so a 60Hz refresh used to be fine. But manufacturers have been moving to phosphors with a much shorter decay time to avoid ghosting and other issues when you have a high contrast object moving across the screen. So now most late model plasma monitors remind me of some of the first SVGA monitors that came to market.
Can I have a 120Hz screen refresh, please? Europe solved a similar issue with PAL CRTs decades ago by boosting them to 100Hz.
I think the problem is that the Flicker Fusion Threshold might be poorly understood by manufacturers.
I think lots of factors affect our perception, young people can perceive flicker more than older people, and things like caffeine can make flicker more noticable.
Who the hell modded this insightful? It's just Belia being a douchetard. It is not insightful and adds nothing to the topic at hand. Is the new Karma Whoring secret to put 1st world problems in your post? Can I get extra points if I also figure out how to put swag, yolo, & brah in my post?
I've been using CRT monitors since 1993. I can stare at them all day long as the refresh rate is 75 Hz or higher. At 60 Hz, my eyes get tired out pretty quickly.
As for LCD monitors, I bought one years ago for home use and had to sell it off on Ebay months later when I realized it was giving me eye strain (the effect is noticeable after a few minutes... strain at the temples). For the next 7 years, I avoided LCD monitors when possible, eventually becoming the last person in a 100+ person office to use still CRTs (since I didn't want to give them up at work or home). Even kept using CRT TVs. My theory at the time was fluorescent back lighting was the culprit, since fluorescent lights gave me similar eye strain. To my horror, I got a LED backlit LCD monitor at work and still got the same headaches/eyestrain.
Then one day I realized I was looking at my cell phone screen all the time with zero eyestrain (HTC Incredible, original version as when first released). I was puzzled, until I read that this particular model uses AMOLED display, which is different than normal TFT displays, since the pixels themselves provide the brightness.
Next, I searched to find a test which could stimulate my eye strain pretty quickly. Behold, the test: http://www.lagom.nl/lcd-test/inversion.php
This test creates eye straining flicker for me on regular LCD displays, but not on CRT and AMOLED displays. Hooray. Given this test, I've found some LCD laptop displays to be more bearable than others. For example, Apple Macbook Air (circa 2012) does pretty well on most of the Inversion Walk tests, but not all. My Dell Latitude E6430 (2013) does even better. So good, that I can use that laptop most of the day with its built in LCD screen. Hopefully I'll eventually find a LCD desktop monitor that does well with the Inversion Walk tests. In the meantime, I dream of color e-ink with fast response times, or an affordable AMOLED monitor.
On a side note: older compact fluorescent bulbs gave me similar eye strain. Seems the newest generation of bulbs are bearable, so hopefully I don't have to hoard 60 W incandescents if/when they are banned.
I can easily see the flashing of PWM driven LEDs as used on cars, to the extent that I can identify the brand of a distant car at night purely from the speed of the flicker of the taillights. I can also easily see the flicker of my Galaxy S3 screen, especially at night. During the day I can easily see the flicker of LED DRLs. :(
Whatever way my vision works, I often end up gaining detail from a quick scan of my eye's focal point around what I am concentrating on. When a PWM-driven light source is in my field of view I see it as a trail of distinct dots, where a continuously driven light would appear as an evenly illuminated streak. Maybe it's just because I've got a lot of practice of looking at things from 30 years of observational astronomy so I am used to getting the best from my mk1 eyeballs
Cinemas are often painful as I can very easily see individual frames in panning shots, even with the motion blur on the individual frames. Motion blur annoys me as my vision system can't figure out if it is out of focus or blurred, so my eyes get very tires from certain films
Back in the CRT days I could easily differentiate between 85hz and 100hz monitors, and I could identify whether a game was operating at 100hz or 120hz if the monitor framerate was higher than that. Mostly by being able to see each frame when the POV was changing quickly, and I could gauge whether there were 8 or 10 frames used for a rocketjump and that kind of thing. This led me to having to spend real money on decent monitors that I didn't see the flicker on.
Newer flat panels have light sources that are a lot more steady, and that never truly drop in brightness as there is no longer an active scan in progress. These no longer flicker as much as older monitors did.
Ophthalmologists didn't find anything odd with my eyes at that stage, it was just my having a particularly discerning vision system. Lucky that I have, as I've located drusen on my retina that I've spotted early enough to possibly be useful in any treatment of this precursor to macular degeneration. Sucks, but I'm still in my 30s so worst case I have more time than most to get used to using peripheral vision for daily tasks.
- This sig deliberately left blank. Nothing to see, move along.
there is a whole lot of sand in your vagina it seems
My guess is your eyes hurt because you suppress blinking while looking at the computer screen. When people watch things interesting they do not blink as much. Your eyes dry out and they hurt. Your eyes cannot physically respond to fast blinking. Its called persistence of vision. Once the blink rate exceeds that there is no physiological response. If you think there is you are fooling yourself. I once had a guy who said he could hear a 20 khz signal in a drive motor circuit. I would turn on the power and He would grab his ears. Problem was is that the power supply was unplugged. Either he was trying to fool me or he was fooling himself. What you should do is set up an experiment. Set up a pair of computer controlled LEDs to flicker at different rates then guess which flickers faster. Have the computer randomly change the rate and which one is faster. Then see at what flicker rate the error rate goes to zero. That is when you can actually see the flicker.
My response sounded pretty dickish. I have bad communication skills. It's a problem I know. I'm not trying to be that way and I'll tell ya why.
The citation needed shit get on my nerves because, like I said at the end of the post, I already know. Being skeptical seems to be one of those new hipster douche fads. Skepticism is good but these days you can't claim the sky is blue without someone screaming for a citation. One could respond that I don't know shit but I have already studied this subject years ago. That person is the one that doesn't know, they need to educate themselves- ya know be all bootstrappy and stuff. So say someone makes a claim that GWB was caught having sex with a goat and it's on youtube. When I hear something preposterous, I too get skeptical but you know what I do? I go look on youtube. If I can't find that info, then I can come back to the discussion and start asking for citations. Citation needed has been relegated to a synonym for "shut up" and I don't like it.
Nexus 7 suffers from random flickering if the backlight is set "too dim." (No it's not caused by automatic brightness setting as some claim).
Totally unknown is if it is a HW design flaw or something related to LEDs in general (possibility to see the flicker in low light).
You ideally need PWM >10000 Hz.
- http://opensiuc.lib.siu.edu/cgi/viewcontent.cgi?article=1538&context=tpr (500 Hz detected)
- http://www.lrc.rpi.edu/programs/solidstate/assist/flicker.asp (300 Hz detected)
- http://www.lrc.rpi.edu/programs/solidstate/assist/pdf/AR-Flicker.pdf (10,000 Hz detected)
The last one has a rather interesting diagram where PWM, in certain cases, up to 10,000Hz, is detected via a stroboscopic / phantom array effect (not too different from wagon wheel effect).
This is true, 10Khz is detectabable, agreed by this paper
- http://www.lrc.rpi.edu/programs/solidstate/assist/pdf/AR-Flicker.pdf (10,000 Hz detected)
The last one has a rather interesting diagram where PWM, in certain cases, up to 10,000Hz, is detected via a stroboscopic / phantom array effect (not too different from wagon wheel effect).
For those people who don't mind CRT's, but get eyestrain from motion blur instead, there's a new technology called LightBoost (google "LightBoost") which is essentially PWM at one strobe per refresh, with 92% less motion blur than regular 60Hz LCD (full order of magnitude less motion blur).
Competition gamers have been purchasing 120Hz computer monitors as of late, and enabling the LightBoost strobe backlight, to regain CRT like clarity; covered at the Blur Busters Blog - http://www.blurbusters.com/ -- which also has 60Hz versus 120Hz versus LightBoost comparisions available.
And TFTCentral's "Motion Blur Reduction Backlights Including LightBoost":
http://www.tftcentral.co.uk/articles/motion_blur.htm
They found that LightBoost greatly outperformed scanning backlights.
Obviously, this technology is not for flicker sensitive people, but it can be turned on/off, and it's another option on the market.
Can't stand the strobing effect wen you move your head around, having one following you and looking in the rear view mirror at a car with them is headache inducing, every bump of the road is Magnified into a discoesque strobe and smudged akin a slightly too long hand held photo exposed at night.
Oh and a lot of newer power indicator LED's do this too,
Anybody heard of a low-pass filter? Smooths PWM out very nicely.
I am one that suffers badly from this. LCD TV LCD monitor , I have to be very picky in getting the right one. Led is worse for me. Some are OK some are bad and can not view for a few minutes. Not headache but feeling cloudy and unable to focus. I would love a solution to this and find which TV, monitor, laptop that do not affect me. I have taken items back so many time cause of this. Drives me mad.
Were you aware that extra strength placebos are more effective than regular strength placebos?
Competition Good, Monopoly Bad.
See the last sentence: http://en.wikipedia.org/wiki/Flicker_fusion_threshold "Display refresh rate Computer CRT displays usually operate at a vertical scan rate well over 60 Hz (modern ones are around 100 Hz), and can thus be considered flicker-free. Most people do not detect flicker above 75 Hz. Other display technologies do not flicker noticeably so the frame rate is less important. LCD flat panels do not seem to flicker at all as the backlight of the screen operates at a very high frequency of nearly 200 Hz, and each pixel is changed on a scan rather than briefly turning on and then off as in CRT displays. However, the nature of the back-lighting used can induce flicker - LEDs cannot be easily dimmed, and therefore use pulse-width modulation to create the illusion of dimming, and the frequency used can be perceived as flicker by sensitive users.[6][7]"
LED Christmas tree lights are the worst. Not only do you get the 60Hz wave, but you only get one half the wave (that's 60 half-wave rectified pulses, not 120). I cannot tell by just staring at the lights but any eye movement makes the effect apparent. Not all LED Christmas tree lights do this for some reason (perhaps some are smoothed out to DC), but passing through the department stores at Christmas time, I can tell a lot of trees just look out of whack.
Hell, the flicker from LED car tail lights and LED Christmas lights has been driving me nuts for a decade!
Some LCD monitors with low colour-depth panels flicker because they use temporal dithering to display the full colour range (this means that pixels are rapidly flashed between two colours such that the eye will hopefully blend them together).
The one I use at work has what I think is temporal dithering. I can see it if I move my gaze, but if I am sitting still it doesn't stand out much. I wonder if this might be a more common experience than seeing backlight flicker (though I don't doubt that backlight flicker is sometimes seen too).
The biggest offender for flickering led tail lights is Cadillac.
It's not PWM. However, I did experience initial discomfort when I first switch from a high-end CRT to LCD monitor. The reason was easy to identify:
* The pixel resolution of the new, cheap, low-end LCD was far lower than my old high-end CRT
* The pixel light was more diffused on my old CRT so pixels has software outlines as well as been finer pitched
This difference *will* hurt your eyes. It's like the difference between a computer monitor (72dpi) vs. printed book (600dpi-2000dpi, depending).
Of course, now I use a rMBP and it's comparable to what old-school CRTs were like.
If the backlight uses 'white' LEDs, these are actually phosphor-coated blue or near-UV LEDs, and have persistence. That said, just like a an old fluorescent tube, a driven phosphor light source has very perceivable flicker at 60Hz. 85 Hz is higher than most of the population can detect, although there are a few people who can perceive 100 Hz flicker.
Mission: To provide products that consume time and energy as entertainingly as permitted by the laws of thermodynamics.
You could build a small tester using one of the ultra small arduino boards.
Maybe a coin cell, switch and phototransistor plus some leds for output.
Code it to show on the LEDs what the pulsing light frequency is.
Under 100Hz, 200Hz, 1000Hz etc.
Then if you see a product you like you can easily check what the PWM frequency is before you buy it.
Remember to do the check with the brightness set to min. Max brightness is often 100% duty and will have zero flicker.
How arrogant! The range of experiences posted by non- AC users here should have convinced you of the silliness of your post by now. Next time, be less sure of yourself.