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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.
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
Might be a first world problem but that doesn't make any less real.
It will never change unless someone starts the conversation.
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.
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.
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.
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.
also, is this an advert for prad?
I hope not, dass vor Ort saugt wie ein östlich Deutsch im Urlaub.
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.
He probably also has to walk 15 feet to grab a new Cheetos bag.
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?
He probably also has to walk 15 feet to grab a new Cheetos bag.
It's 15 feet to the soda machine. Cheetos are 17 feet away you insensitive clod.
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.
Add a 10pF cap, no more flicker. But seriously, i have had ZERO experience with this. None of the 20+ laptop/desktop screens we have at work, or my screens at home (TV/computer) flicker at all. They are all cheap crap LEDLCD's. A few are the newer LEDIPS. Zero flicker. Even when filmed with 60hz cameras, no flicker at all.
Idiot. You are very likely in the majority that cannot see that flicker. There is a minority that can, and for them it is a very serious problem.
Using higher PWM frequencies is not an issue at all, it just has to be done.
Most ACs are not even worth the keystrokes to insult them. Be generically insulted by this and ignored otherwise.
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
Yeah about as serious as running out of ding dongs.
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.
waaaa, my problem.... its serial
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!
I suspect author is also bothered by wifi signals emanating from his router.
If my eyes are watering after a long session its because my screen is too bright, which is exactly the opposite of what he postulates as the problem (on off cycles of LEDs). Brighter requires longer "on" cycles, which in turn are less perceptible. Yet for most people overly bright screens are the source of complaints.
Sig Battery depleted. Reverting to safe mode.
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.
If you hate 1st world problems so much, *PLEASE* , feel free to go live in the 3rd world. I'm sure you'll be much happier hearing your neighbors complain of 3rd world problems.
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.
You probably couldn't hear me whining about my partial blindness...
I run my monitors a bit brighter to cut that problem. The only time I've noticed flicker is running at minimal brightness.
My laptops are at full bright usually, my desktop lcds are usually around 80%. My rooms typically have >10klux of light. I like bright. But as i said before, never had a problem with flicker on a working monitor.
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.
If my eyes are watering after a long session its because my screen is too bright, which is exactly the opposite of what he postulates as the problem (on off cycles of LEDs). Brighter requires longer "on" cycles, which in turn are less perceptible. Yet for most people overly bright screens are the source of complaints.
That was my experience when I got my first LED-backlit laptop. I found I had to dial the brightness way down (not trivial because Ubuntu isn't offering a backlight control GUI -- had to poke around to find the /dev brightness entry).
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 ?
At 50Hz Ac and 60Hz in the US every lightbulb flickers around 100 or 120Hz. In especially the lightbulb emit a pulsating light which can easily be measured... I'm thinking that in a world full of 00-120Hz light the backlight of your monitor might not be your biggest problem.
Damnit! Now I can't not see it. I was fine untl you pointed it out. Thanks for ruining my life forever.
I have serious doubts that this flicker is a significant issue. I could never watch PAL TV because the extreme flickering caused visual artifacts and migraines, but I've never noticed a flicker on an LCD monitor. Most people have no problem with 50hz, so the number that have a problem with 100hz must be very tiny.
captcha:eyesight
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!
You sound 14.
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.
ManBearPig!!!
I thought my new monitor was awesome and bright until I opened Excel. I could barely look at the screen!
I suspect author is also bothered by wifi signals emanating from his router.
If my eyes are watering after a long session its because my screen is too bright, which is exactly the opposite of what he postulates as the problem (on off cycles of LEDs). Brighter requires longer "on" cycles, which in turn are less perceptible. Yet for most people overly bright screens are the source of complaints.
Ah, the good old "it doesn't happen to me, he's a liar" reasoning.
PWM lighting is annoying if the frequency isn't high enough. Rates that that drive me crazy don't drive everyone else crazy. I perviously didn't know why some displays made me slightly nauseous and others didn't until I started to dabble in electronics and learned what PWM actually is and built a circuit that gave me headaches.
I don't understand how the carrier frequency is chosen in consumer goods, but it seems in times past it was based around whatever clock source was conveniently available, and those sources are generally completely arbitrary. I found is rather funny how one arbitrary number can make me hate your product if it wasn't high enough.
To be fair, things are a lot better for me now than they used to be. Probably because the conveniently available clock sources are faster now, or maybe some switched to adjusting the current directly? Maybe also that VFDs and LED displays have given way to LCD displays. And nothing was worse to me than a CRT with phosphors that decayed faster than the retrace. Yetch.
More Twoson than Cupertino
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.
Agree. LEDs can be driven at remarkably high pulse rates.
I've looked at a couple of laptops here at work, and I've measured 1kHz on one of them,
which is pretty cool.
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.
If my eyes are watering after a long session its because my screen is too bright, which is exactly the opposite of what he postulates as the problem (on off cycles of LEDs). Brighter requires longer "on" cycles, which in turn are less perceptible. Yet for most people overly bright screens are the source of complaints.
You could put a 'privacy filter' on the screen that will reduce the brightness (with an intended side effect of reducing viewing angle)
Every problem that a slashdot reader has is a first world problem. Unless you're dying from or transmitting a disease, starving to death, cannot find clean water, have too many children because you don't about contraception, or are growing up uneducated, your problems are most likely all first world problems.
- Vincit qui patitur.
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
Here is your brightness control. Brightness
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.
Yeah, some people are definitely more sensitive to it than others. I know people who had no problem looking at a 60 Hz CRT, but could never do it for more than a few minutes, as i could see the light flickering. I've never had any problems with LCD/LED displays, but I'm sure there's people with enough sensitivity in their eyes for it to bother them.
Anthropic principle: We see the universe the way it is because if it were different we would not be here to see it.
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.
There is a real component to it, particularly if you're a migraine headache sufferer. Migraineurs tend to be sensitive to certain frequencies of flicker. I find that fluorescent lights are uncomfortable and tiring whereas LED bulbs and incandescent bulbs are fine. Then again, I've never had a problem with LED/LCD, fluorescent LCD and CRT monitors because the flicker rates seem to be at rates that don't bother me. (Staring at highway markings close to the car at highway speed drives me absolutely bonkers, though. Good thing I don't really need to do that. :)
As someone who experiences this issue, I can confirm it exists. I imagine most people are sensitive to it at some frequency but it may not be at frequencies that are ordinarily an issue. Get a strobe light, play with it and chances are you'll find a frequency that bothers you.
Incandescents don't noticeably flicker. They might imperceptibly dim as the voltage changes but they take hundreds of milliseconds to dim completely, so the dimming and brightening is likely imperceptible.
You couldn't watch PAL content on an NTSC display, or PAL content on a PAL display? The latter will look a lot more normal than the former.
...or maybe his problem with eye strain have something to do with staring, wide-eyed, at a single object, in a florescent light, dry, air-conditioned environment for 8 hours a day while on a steady diet of diuretics like sugary caffeinated substances.
I did not see anything in the summary to indicate that jones_supa had positively identified the LED backlight as the source of his problems to the exclusion of all else.
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.
Yeah... until it's /your/ IPod which gets stolen!
CLI paste? paste.pr0.tips!
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.
I don't understand how the carrier frequency is chosen in consumer goods
I do. I write firmware that does backlight PWM for a living (among other things).
Everyone used to do high frequency flicker-free PWM, in the kilohertz range. Then they noticed that you can improve the motion handling capability of an LCD panel but flickering the backlight at the same frequency as the screen refresh. It's kind of like how a CRT's phosphors fade and thus flicker at the refresh rate. Turns out it stops LCDs blurring with motion too.
In practice most monitors do both. They use high frequency PWM to set brightness and then switch that on and off at a low frequency like 60Hz. This is what causes the annoying flicker, but hay, at least the crappy review sites can say motion reproduction is better than the competition.
const int one = 65536; (Silvermoon, Texture.cs)
SJW, n: "Someone I don't like, and by the way I'm a fuckwit" - AC
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.
Incandescents don't noticeably flicker. They might imperceptibly dim as the voltage changes but they take hundreds of milliseconds to dim completely, so the dimming and brightening is likely imperceptible.
Quite right. You can measure the flicker though, but it is really small due to thermal sluggishness.
Most ACs are not even worth the keystrokes to insult them. Be generically insulted by this and ignored otherwise.
PAL flickers at 25Hz (two half-images are used). And traditional CRT TV screens already provide significant dampening. LEDs provide no dampening at all.
Most ACs are not even worth the keystrokes to insult them. Be generically insulted by this and ignored otherwise.
Actually, your run-of-the-mill red LED has no problems going up to 100kHz or so. White is supposedly slower because of color conversion. And IR LEDs used for network connections (in a GBX, for example), do > 1GHz.
Most ACs are not even worth the keystrokes to insult them. Be generically insulted by this and ignored otherwise.
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
Ah, the good old "it doesn't happen to me, he's a liar" reasoning.
There are enough examples of the placebo effect that this reasoning is a fair place to start. If you claim to observe an effect that most people cannot, you need to produce data that shows you can discriminate the effect under blind conditions. Otherwise we have no reason to believe you are any different from EM hypersensitives, etc.
Give me Classic Slashdot or give me death!
Yeah about as serious as running out of ding dongs.
Don't worry, Slashdot has an endless supply...
PWM frequencies are usually chosen as a trade-off: Too low frequencies mean flickering, audible noises, and, depending on the application, larger caps/coils. Too high frequencies mean higher CMOS switching losses, and RF signals emissions which may violate regulations.
The WLAN frequency was chosen as the resonance frequency of water because due to absorption that frequency is basically useless, so nobody really wanted it.
Actually some people might notice things that others don't. Most LED tail-lights drive me absolutely crazy, but my dad doesn't even notice them. Same for sounds, the old TV in our bedroom has a high whine that only I can hear, my girlfriend can't hear it, and thinks I'm crazy since I unplug the TV before going to bed. Same for flourencent lights, some people can see the flicker from crappy ballasts, some are oblivious. People have different sensitivity to frequencies at the edge of perception, some people won't notice it, and some will. Welcome to normal human variation.
I recently went shopping for decent IPS displays, and most of the LED ones do noticeably flicker at low backlight levels. Some, cheaper ones, were tested with noticeable flickers at all levels. I picked one that still used tubes, since I generally work with low brightness levels (for print work), and even decent LED monitors started flickering there (and its hard to get a good, wide gamut, monitor with LEDs and not break the bank).
A patriot must always be ready to defend his country against his government. -edward abbey
PWM lighting will only effect you if it is under the rate at which your eyes can perceive.
100 fps or so is way above the threshold of about 20 frames per second.
If the author got perturbed by 100fps led light, then he would REALLY get bothered by the 24 fps shutter at movie theaters.
Now a lot of gamers (including me) notice if the fps of a game goes under 60, but this isn't the same sort of case. Most of the time under 60 fps only causes some input latency or jerkiness when viewed by an avid pc gamer, but when watching another player (no connection to an input), it is hard to tell if the framerate drops below even 30.
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
Do you mean GBIC? I'm not aware of them using LEDs. For optical GBICs lasers are used (usually VCSELs - or they were when I last cared about the internal workings of fibre stuff, which was about 10 years ago ).
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
have too many children because you don't about contraception
Doesn't matter; had sex
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.
I remember having headaches and eye strain after getting my first LCD monitor. Thinking back I'm wondering if the monitor was passive matrix all recent monitors don't bother me though.
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.
Ah, yes. Has been some time here too. You could argue that a laser diode is just an LED with a bit of extra filters (which admittedly is a gross simplification). The "laser" part is not for speed, but for avoiding different travel times of different light frequencies. In principle, laser diodes should switch slower than LEDs.
Most ACs are not even worth the keystrokes to insult them. Be generically insulted by this and ignored otherwise.
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.
But why do you sound so resentful and disbelieving of the claims? If he is right and it bothers him but doesn't bother you, what negative things are you gonna have to suffer from to accomodate him? I don't get when people shout down stuff like this. It is a known phenomenon and it only affects a small portion of the population. What's in it for you to prove him wrong?
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?
Well you said it was Excel, so....
Try a different spread sheet.
Sig Battery depleted. Reverting to safe mode.
As someone with this problem, yes! Surprisingly setting the backlight to 100% pretty much eliminated my eye/migraine issues from upgrading to 3 BENQ 2250s. A SOLUTION to the brightness that worked very well for me was automotive window tint film (nothing special, $10 from O'Reilly) on plexiglass panels. I thought the image quality would be toast, but everything is perfectly clear and easier on the eyes now.
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.
But if they were blind, how do they seen the flicker? :D
Finally a good reason to be an insensitive clod!
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.
Laptops have fairly wimpy backlights.
My desktop monitors are insane at 100% brightness. Great for reading it from 100 meters I suppose but from 2 feet it can be painful.
Hell even this mostly white slashdot page at 75% brightness is slightly uncomfortable.
Bit odd. I know for a fact KDE has brightness control included.
My Dell also has a hardware brightness key combination. FN + Up or Down arrows.
The awesome thing is KDE knows when I use the hardware keys. Perfect example of not shoving propriety crap in to a laptop.
Its not you are lying. That implies you know you are doing it.
The most sensitive eye can not see anything anywhere near 100Hz.
If you have seen PWM less than about 400Hz I'd be extremely surprised and generally it is in the kHz range which if anyone says they can see flicker is bullshitting badly.
You simply cannot buy PWM controllers which go low enough - pretty much all generate their own clock btw since precision isn't required.
Remember audiophiles say that good expensive cable sounds better when they are listening to coat hangers. Same thing.
there is a whole lot of sand in your vagina it seems
Physics. The human eye cannot see hundreds or up to hundreds of thousands of Hz.
It just isn't possible therefore the guy is an idiot.
Grab an arduino, hook up a bright led, a pot and make it output via serial what Hz the pot has selected.
Then blink the LED at that speed. You can test it yourself.
I'm surprised no one here has mentioned DLP projectors. You know, the ones that use light reflected off a DMD and passed through a rotating colour wheel to produce a red, then a green, then a blue, then a white picture.
Those things drive me crazy. I'm fine so long as I sit sufficiently far back with my eyes motionless, looking at the centre of the screen, but if I move them even slightly, the rainbow effect becomes painfully clear.
This seems to be yet another negative side-effect of those of us with high-frequency light perception.
"Nine times out of ten, starting a fire is not the best way to solve the problem." - my wife
Erm wouldn't the logical conclusion be to use a multiple of the refresh rate, not the refresh rate its self?
E.g. PWM of 3kHz would do the exact same effect you just described while also maintaining that effect for both PAL and NTSC sources.
Most LED tail-lights are fine, but certain auto-makers have apparently decided to make theirs insanely bright and/or use low refresh rates. *cough* Audis, Mazda 3's, mid-2000 Honda Accords, etc.
I'm not Corey Hart and I don't want to wear my sunglasses at night....so I can, so I can...get through my damn commute without getting a migraine in stop and go traffic.
No, pal has a full-frame at 25Hz. But it scans the full size of the screen at 50Hz on alternating scanlines, this has the effect of reducing the flicker to next to nothing due to the persistence of the phosphors
"Then they noticed that you can improve the motion handling capability of an LCD panel but flickering the backlight at the same frequency as the screen refresh"
Does it have to be the same or could it be a multiple of the screen refresh? eg: run the backlight at 240Hz, screen at 60Hz and solve both problems?
YOUR eyes can't see it. Mine can't either. We know that some people can though. To admit that though, would shake the foundations of your beliefs, much like religion "revelations." There is most likely a perfectly valid reason that this phenomenon exists and we just don't know what it is yet. If you accept that some people have problems with certain frequencies, that doesn't mean you have to throw all of physics out the door.
:)
Oh and I did that experiment years ago with a 555 timer. You can take your arduinio with you when you leave my lawn
"Ah, the good old "it doesn't happen to me, he's a liar" reasoning. ...Rates that that drive me crazy don't drive everyone else crazy."
Ah, the good old "I am special" reasoning.
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.
Got a citation of a double blind study showing someone detecting anything over 100Hz?
Cause without that, we know nothing of the sort.
You overestimate the amount of attention anyone pays to you and your goings on.
I can see flicker of LEDs well over 100Hz. I work with them and build LED devices all day long.
Still waiting on Serviscope_minor to wake up to fucking reality and realize that Jessica Price isn't going to fuck him.
You citation needed folks get on my nerves, but in my line of work the saying is "show me the data." I have alot of free time so I went ahead and googled it for you. Now most of the studies are behind paywalls and the wiki articles all allude to the problem. You however went ahead and thought you were being clever by quantifying anything over 100Hz. As I said, most of the wiki document alluding to it were behind paywalls but I did find ONE that has a number in it. It's a PDF and it is on slide 2. Go read that and holler back at me. Once you have your revelation, you can accept it as a fact that doesn't mean the end of all physics, or you can move your goalposts some more.
From the DOE
Also for your perusal Go to the lighting section.
Here is one of the offending paywall studies. The synopsis says there is some negative effects but it didn't have that magical 100Hz mentioned and I'm not paying to find out- I already know.
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.
>100fps or so is way above the threshold of about 20 frames per second
Bullshit. Period, full-stop, boldface, italicized, and surrounded in BLINK tags.
Your corneal surface isn't equally sensitive to flicker. Rod-dominated peripheral vision is several orders of magnitude more sensitive to motion and flicker than cone-dominated foveal vision. In terms of framerate or high-contrast flicker, the magic point at which literally nobody can tell the difference between N and 2N (where "N" = "framerate in hertz") regardless of which part of the eye it strikes, its brightness & contrast, and the amount of frame-to-frame change, is somewhere around 400fps/Hz (slightly more for some, slightly less for others). Now, you don't literally have to go quite THAT high to have a satisfactory viewing experience with most content, but if you really want to split hairs and look for edge cases where people can reliably tell the difference in double-blind tests... that's approximately where it is. WAY above 20fps.
The ONLY reason why 24fps (motion picture film) presents any kind of illusion of motion is because film's analog nature encodes additional higher-order information into it in the form of motion blur and other artifacts. I can guarantee that raw, razor-sharp blur-free high-contrast 24fps video is VERY unpleasant to watch, even if you independently take care of flickering so it appears as a sequence of sequential solid images.
And yes, somewhere between 5-20% of the population (slightly lopsided towards males) ARE more sensitive to flicker. It's been studied, documented, and quantified. The number is well known to monitor manufacturers... they just don't care. The best we can hope for is some future backlight ASIC whose behavior can be tweaked by end users via I2C, even if the manufacturer of the monitor itself doesn't lift a finger to support it. Then some bean counter in management will decide he can shave 1.7 cents from the manufacturing cost by omitting the pullup resistor needed for the I2C bus to work, and we'll be fsck'ed again.
^^^ Argh. replace 'corneal' with 'retinal'. It's late.
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).
No, it has to be the same as refresh rate, or you get PWM artifacts:
http://www.blurbusters.com/faq/lcd-motion-artifacts
The best implementation is called LightBoost -- PWM at one strobe flash per refresh, for the CRT effect
http://www.blurbusters.com/faq/60vs120vslb (60Hz versus 120Hz versus LightBoost).
Also TFTCentral's "Motion Blur Reduction Backlights Including LightBoost" article:
http://www.tftcentral.co.uk/articles/motion_blur.htm
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 be building them very well if you are PWMing around 100Hz.
Try 400Hz - 15kHz and you products might work better.
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,
Simply adding a capacitor will consume more power. You need a switching power supply that generates the correct voltage. (and 10 pF wouldn't be sufficient anyway -- you need to store enough energy to power the backlight during the interval when the PWM is turned off)
Imagine you have a 10 volt power source, and you're trying to down-regulate to 5 volts, by pulsing it on and off into a 1000 uF capacitor. To control the pulsing, you set up a schmitt trigger to turn the voltage source on when the voltage falls to 4.9 volts, and to turn it off again when it rises to 5.1 volts. The wire connecting your voltage source to your capacitor has a resistance of 0.1 ohms.
When the voltage falls to 4.9 volts, your power source is switched on. This feeds current at about 50 amps (5 volts / 0.1 ohms) into the capacitor. This charges your capacitor to 5.1 volts in 4 s. (0.2 volts * 1000 F / 50 amps = 4 s) So we take 50 amps * 5 volts * 4 s and find that charging the capacitor consumed 1 mJ of energy, released as heat in the wiring connecting your power source to your capacitor. Meanwhile, we increased the voltage on the capacitor by 0.2 volts, and so the energy we added to the capacitor was 1/2 * C * V, or 0.5 * 1000 F * (0.2 v) ^ 2 = 20 J.
Thus, by attaching a capacitor to our PWM circuit, we've decreased it from 100% efficiency to 2% efficiency.
Note that the size of the capacitor has no effect, since we maintain the same voltage drop. Lower resistance would help, but 0.1 ohms is already quite low, so there's a limit to how well you're going to do there, e.g. 0.01 ohms would get you up to 17% efficiency. ...or, you can do as the LCD manufacturers do, and leave out the capacitor so that the full voltage drop occurs over the LED, so that all of the energy goes into the LED and none is wasted.
Switching power supplies overcome this problem by using an inductor to bridge the voltage drop to the capacitor. Just like how the current that flows into a capacitor isn't consumed, but is instead stored, the same applies to the inductor, which kindly bridges the entire voltage drop (whatever it may be) such that it is able to charge with no voltage drop at all, and so it consumes all of the energy that would be wasted as the capacitor charges from the power source through the inductor. Then, when the power source is switched off, the inductor creates a negative voltage in order to maintain its present current flow, which a diode then switches to the negative pole of the capacitor, allowing the inductor to release that stored energy into the capacitor, where we wanted it all along. So, in theory, a switched power supply is 100% efficient, but usually we end up with 80% to 90% due to imperfect components and wiring and such.
Anybody heard of a low-pass filter? Smooths PWM out very nicely.
See, i buy LCD's knowing that 800nits is daylight-visible-bright. I also don't buy screens advertising over 10k:1 contrast. While they look great, they hurt your eyes. So my 300nit, 3000:1 20" LCD isn't Apple Cinema picture quality, but, it's perfect for autocad.
As an electrical engineer, i know the formulas. The 10pF cap would not maintain the LEDs, i know. It would, however, smooth the attack of the flicker. So even tho it would still be flickering, it would be far less noticible. Also, yes, a small amount of power would be consumed, but a good tantalum / high quality film cap would minimize this.
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.
As an electronics hobbyist, I know that the fact that you think that the power consumed has anything at all to do with the quality of the capacitor means that you either didn't read or didn't understand what I said.
Also, I doubt it's the slope of the on/off transitions that are bothering anyone, but rather, the fact that the LEDs are turning on and off at all. To prevent that from happening, you'd require a capacitor sufficient to power the LEDs during the off phase of the PWM. The 10 pF you suggested isn't going to power anything. Hell, there's probably that much stray capacitance in the circuit as it exists already.
Charging a 10pF cap will consume less power. That's a fact. And since i deal with PWM every day, i'm going to say, yes, it will help.
They can whine like hell though when used with certain dimmer switches. I had to change the dining room lights from ordinary incandescent bulbs to those GE "natural light" ones (can't recall the branding) just to be able to think while the dimmer was turned low.
And for all those who are about to bitch me out because I'm still using incandescent bulbs, screw-in LEDs are way too expensive, and every CFL I've tried really bothers my eyes. I've used CFLs in low-occupation areas where I just don't spend much time (stairway, garage, etc.), but I won't tolerate them for long periods. When A19-base LED replacements become economical, I'll re-evaluate them, but CFLs are right out.
- T
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.
Very often when people 'hear' strange high pitched whines from electronics especially the old CRT's what they are really experiencing is electro-sensitivity. I know for a while I had it and it could be very unpleasant. The major culprits in my case were mostly transformers - on switch mode PSU's, mains transformers, even the fridge motor. I'm guessing there was an electrical field in the air that the leaky magnetic fields in the transformers put into oscillation. Over a few years it got less bad and then mostly disappeared - LCD's are definitely much better than CRT's though.
Hell, the flicker from LED car tail lights and LED Christmas lights has been driving me nuts for a decade!
You're right about the whine.
And I have no issues with incandescent bulbs. They don't flicker like CFLs do (and they're far more innocuous to discard), and they don't pollute the amateur radio bands like I've recently discovered that LED bulbs do.
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.
Yes, but it will also do less (essentially nothing) to address the problem. It won't maintain the voltage driving the LEDs, that will still turn on and off at regular intervals, and so the problem will remain.
...pollute the amateur radio bands like I've recently discovered that LED bulbs do.
Interesting. I'd suggest complaining to the FCC. Perhaps if they get enough input on that problem, LEDs will become subject to regulation that eliminates (or sufficiently reduces) that interference before they become so popular that new regulation would be considered a burden on the manufacturers.
- T
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.
I build them just fine because I use constant current drivers like a real LED device engineer.
China, OTOH....
Still waiting on Serviscope_minor to wake up to fucking reality and realize that Jessica Price isn't going to fuck him.
It's real to the sufferers. When I started playing games for a long time on my iPod touch, I started getting blurred vision and watery eyes if I stared at the screen for too long. The problem was somewhat less if I looked at the screen for only less than a minute. And I didn't get this problem staring at an older iPod (which was LCD).
When I searched the web for this issue, I found a thread at discussions.apple.com about many Apple fan boys returning their new MacBook Pro because it hurt their eyes. I thought I was crazy until they generally came to the consensus that the eye problems had something to do with LED laptop screen. I'm not sure whether it's only PWM, but one thing's for sure, these LED screens are unpleasant to look at and have worsened my eyesight. LCDs on the other hand have a cool feeling and don't cause any discomfort or harm vision.
I gets worse -- LCD screens are being phased out by manufacturers and are being replaced by LED screens because they have better picture quality. I hope any company incorporating LED screens into their products fully investigate this issue and offer LCD screens as an alternative to LEDs. LCDs have a duller yellowish hue, but it's better than suffering eyestrain, headaches etc.
Just in case anyone is interested, this is the Apple thread discussing LED eye strain
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.
Lots of LED bulbs are no-name Chinese imports. That they accept and operate at 120VAC does not mean that they're compliant with FCC Part 15.
If you have specific examples of LED bulbs that are noisemakers, please complain to the FCC. It's an easy and cheap fix on the manufacturing front (an extra capacitor and/or an inductor), but that doesn't mean that the manufacturers are even aware of the problem.
Kid-proof tablet..
>Can't be building them very well if you are PWMing around 100Hz
>ignores WELL OVER 100Hz claim
sorry, you aren't prepared to deal with this convo.
If you think I sound resentful, that is your own perception. I am stating the fact that there are numerous reasons for eye strain in work environments and this guy made no indication that he had positively identified the source of his eye strain to be his monitor. If you are trying to solve a problem, it makes sense to try to isolate the source of the problem.
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.