Ask Slashdot: Does LED Backlight PWM Drive You Crazy? 532
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.
Bitch, please (Score:4, Funny)
Your comment gave me a headache. Also, are you allergic to Wi-FI?
Sigh (Score:4, Insightful)
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).
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Re: (Score:3, Interesting)
Re:Sigh (Score:5, Interesting)
I'm positive it's placebo here.
LED PWM frequencies are FAR higher than the old CRT refresh rates.
Also, while the OP talks about phosphor persistence, remember, the duty cycle of CRTs was VERY short. A pixel would only be "energized" for a tiny fraction of each display cycle. Even with phosphor persistence, I would not be surprised if even at very low brightness levels, PWMed LED backlights are still at a higher duty cycle than CRTs.
I have a friend who is extremely photosensitive - the flicker of fluorescent lights without high frequency ballasts make him begin feeling sick almost immediately, and before he was on seizure medications, would cause seizures. To use a PC monitor, he had to always have ultra-high-refresh rate CRTs - until LCDs became common. He has NEVER had ANY issues with any LCD monitor, regardless of whether the backlight was LED or CCFL. They have been a godsend for him.
Re:Sigh (Score:4, Interesting)
There is a possibility that the OP did run into some badly filtered backlights that actually use mains-frequency. That would be visible to some people but not to most.
Re:Sigh (Score:5, Interesting)
I'm highly sensitive to this - I can see the flicker of the old fluorescent lights. It's not a placebo. Most cheap LED PWM frequencies (up to about 250 Hz) are blindingly obvious. Above about 250 Hz I have to look for the effect to see it, though I can detect it up to a bit over 1 kHz.
The old CRT refresh rates were mitigated by having phosphors, so they slowly dimmed in between refreshes, never turning off (when you turned the CRT off, the length of time it took for the screen to go completely black was how long the phosphors stayed lit). So if you scanned your vision side-to-side, even though the CRT scan image might not remain constant in brightness, it was still a continuously scrolling image.
By contrast, LED PWM is almost binary - totally on to totally off. If you scan side-to-side while viewing an LED PWM screen, you see multiple individual images instead of one continuously scrolling one. It's like watching a poorly animated cartoon from the 1970s - easy to lose track of which parts are supposed to be static and which are supposed to be moving. (Well, I assume those of you with normal vision can tell 1970s cartoons were more poorly animated.)
In static applications like a computer screen it doesn't make me sick. In fact, for me at least, it's pretty easy to ignore since I rarely have to scan side to side. Most of the scanning I do is just slightly side to side or slightly up and down. I'm just aware it's flickering. Then again I rarely get seasick so perhaps I'm not as sensitive to contradictory signals from my eyes and other senses.
Where it kills me is in mobile applications. Certain cars are using LEDs with low refresh rate PWM (I'd estimate around 50 Hz) on their tail lights. When I'm driving at night, I'm not staring straight ahead. I scan side-to-side every few seconds to maintain situational awareness. If one of these cars is ahead of me, the act of scanning turns my field of vision into a sea of individual sets of lights [imageshack.us] making it difficult to pick apart separate cars. With the old continuous lighting, I could count the light trails and tell you how many cars there were. But if there are multiple cars ahead of me with the PWM lights, it's nearly impossible for me to tell how many cars there are while I'm scanning. I have to wait a couple tenths of a second to finish scanning, regain a static image, and see individual car lights. The lower the frequency of the PWM, the further the individual images of the lights are, and the harder it is to "connect the dots" and rationalize that they all represent one car.
I never had much problem with CCFL - either they didn't use PWM or used it at such a high frequency it didn't bother me. Most LED screens however use PWM to decrease brightness. If you use the monitor at or near max brightness, you're unlikely to notice the PWM. But if you lower the brightness a lot like a laptop screen used indoors, the PWM becomes pretty obvious. I've learned to slow down how quickly I scan my eyes across the screen to compensate. Also, your peripheral vision is more sensitive to the flickering than your central vision, so avoid brightly-colored or cluttered desktop backgrounds.
Re:Sigh (Score:4, Informative)
No, it's real. Some monitors flicker their backlights at 60Hz, the same as the screen refresh rate, to improve motion reproduction.
It turns out that even if the transition time of LCD pixels was zero motion would still look a bit blurred compared to CRT. The slight flicker that CRTs have actually makes objects in motion look sharper and easier for the eye to pick out. Some LCD monitors flicker their backlights to mimic this effect.
IIRC the first manufacturer to do it was Benq with a CCFL backlight. In fact I have the non-flicker version of that monitor, the FPW-24Z or something like that.
how about DC backlight? (Score:2)
square wave energy clogs radio bandwidth. plasma TV is just awful at putting background noise out there, and the undercabinet light power packs are also pure evil.
Re:Sigh (Score:5, Insightful)
I can see car brake LEDs flicker; my wife can't. Doesn't mean my eyes are better, or worse, just different. And that the car light systems could probably work better.
Maybe it's not a tiny minority, and maybe it's not enough to make people ill. Maybe a tenth or a quarter of the world feels a little less well than they might all the time, but it's "just" a few percent so not enough to notice and each person thinks they just need more sleep. Or maybe a tenth or a quarter of the world is "just" a few IQ points lower than they might have been.
Re:MOD PARENT DOWN (Score:4, Informative)
Certain high-end displays flicker on purpose (Score:3, Informative)
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.
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Max brightness will burn your eyes out of sheer brightness. I've prefered CRT monitors as long as they are set to 85 or 100Hz to overbright LCD ones.
No (Score:4, Insightful)
No. You're imagining things.
But, that being said, you're not alone. I heard somebody walk into the retail establishment that I work at and said, "I'm disappointed that you guys installed automatic doors that emit so much radiation, but I'm glad that at least you don't have horrible fluorescent lights that would make me unable to shop here." Of course, she was saying that standing under about 500 CFL's that she assumed weren't fluorescent because of their size, shape, and color.
Re:No (Score:4, Interesting)
No. You're imagining things.
Happened to me once that I got upgraded to a larger monitor, and when I turned it on it was like being physically smacked in the face. It's a long time ago so I can't remember exactly what happened then, but I didn't use that monitor.
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When we first got LCDs in high school (circa 2004), I still had all CRTs at home. I found the LCDs hard to focus on for more than fifteen minutes at a time. However, once I switched to using LCDs everywhere (home, school and work), the problem went away. Now the only time LCDs give me an issue is if the backlight is just too dim and even that I get used to if I use the monitor long enough.
I find it to be much more of a problem using someone else's keyboard because the key spacing is never the same.
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No, he's not. That whining noise is well known, objectively measurable, and explainable by actual physics.
Unless IBM was imagining things. [ip.com]. Or you deny the existence of eddy currents [raftabtronics.com] in what is essentially a high frequency electromagnet (see material under "stray losses").
But, that being said, once you'v
Re:No (Score:4, Informative)
I can actually see the flicker effect he talks about - on some rare models. I can only see it near the edge of my vision (where your eyes are primarily motion/change sensitive). 70hz seems to be about the point where this stops, but anything lower than that is perceivable to me.
Just like I can hear that irritating whine he talks about (though likewise, not all of them do that)
The inverter on my LCD at home actually makes all sorts of horrible whines if it's in standby mode - so I either leave it on or unplug the thing.
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There's a huge difference in the amount of power a standard WAP emits versus how much you'd need to span 1.5 km at 5 GHz.
Is it only the monitor? (Score:5, Insightful)
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?
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Ditching the old-school iron ballasts might not be a bad idea, either. They are seriously inefficient, and suffer from hum and flicker. Contemporary electronic ballasts perform considerably better.
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Maybe even ditch the whole assembly and go with LED bulbs that replace the fluorescent tubes?
People argue about LED lighting and if 120 Hz is a headache inducer compared to 150 Hz. However, I'll take either over the 60Hz ballasts of old.
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Re:Is it only the monitor? (Score:5, Informative)
NEWER ballasts run in the 10-40kHz range.
Older fixtures still use magnetic ballasts. No solid state switching circuitry. Big, heavy, tar-filled ballsts that hum and drive the lamps at a good ol' 60Hz.
Sure, they last 10-15 years or so, and the drop-in replacements can be solid-state... But the magnetic ballasts are pervasive, manufactured by the millions, and are still sitting new-in-box in supply closets all over the world.
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No fluorescent bulbs refresh at 60Hz.
Contemporary fluorescent ballasts were just high voltage transformers running at AC line frequency. All of this fancy high frequency PWM stuff is very very new.
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No fluorescent bulbs refresh at 60Hz. The ballast on most fluorescent bulbs tends to run in the 10 to 40kHz range. The only bulbs that refresh at line frequency are incandescent, but the resistive load of the filaments tends to level that flicker out a bit. But lets not let facts get involved with a good story.
Not to mention, 60hz AC has two peaks per cycle, one positive and one negative. So a bulb (incandescent or old fluorescent non-electronic ballasts which are still in use in a lot of buildings) using this as a power source directly (and not cutting off half of the wave for some reason) will flash twice per cycle, leading to 120hz operation.
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Wrong. Nearly all tube-style fluorescents refresh at 60 (or 120 - one per half cycle) Hz.
High frequency electronic ballasts are RARE for tube-style fluorescents. CFLs are a different story - nearly all of these have high frequency electronic ballasts these days - but the "tube" style fixtures found in offices rarely have high frequency ballasts and use passive reactive ballasts instead.
No (Score:2)
No. Used plenty of LED displays without issues.
THE NOISE (Score:2)
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.
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How much time do you spend next to CRTs? And why?
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I guess it's not a lot thankfully, but now they have become rare enough that when I'm near one it's jarringly loud. To think my entire CRT-enjoying childhood I used to enjoy "knowing" when a CRT was turned on in a room.
I still won't miss it when it's gone!
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But what about that one CRT you keep around for the pre-hd consoles?
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nope (Score:2)
fluorescent lighting (Score:2)
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People can see 40,000Hz on a properly working tube bulb?
This is about the frequency range that LED PWM drivers operate at.
No problem (Score:3)
Car Braklights!!! (Score:3)
I thought I was the only one - but perhaps I still am - but car LED brakelights have been driving me ***CRAZY** for years!!!!
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...car LED brakelights have been driving me ***CRAZY** for years!!!!
I see what you did there, and it gave me a headache.
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Nah, the taillights bug the shit out of me, too. I have no idea why they don't just increase the pulse frequency.
It's the saccades (Score:3, Interesting)
http://en.wikipedia.org/wiki/Saccades [wikipedia.org]
Nice troll... (Score:4, Interesting)
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?
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The average CRT was crap. It flickered and whined like hell. The geometry on most of them was a total joke. Often the image bounced around too.
I have a very nice one I still use sometimes that does higher res than any LCD you can buy and uses a shadow mask. This cost multiples of the average POS crt.
Hard to say... (Score:2)
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.
Doesn't seem likely (Score:2)
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.
Psychosomatic (Score:2)
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.
Single data point (Score:2)
So do LEDs bother your eyes?
No. Can't say I've ever seen or heard of anyone having trouble with LEDs specifically. I honestly cannot even see a flicker in most LED screens whereas I was pretty sensitive to it on CRT screens. I find LEDs to be much easier on my eyes than even the best CRTs. I've seen light sensitivities that are due to interactions with poor quality or old fluorescent bulbs. My last office was next to a window which made for some glare problems and excessive brightness problems at times. I've also seen issues wit
What kind of display are you using? (Score:2)
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?
Re: (Score:3)
He's probably using a El Cheapo monitor with a brand name like AOC or something. And it's probably defective.
I've never heard of anyone being bothered by LED flickering. I would suggest that he buy a quality brand like Samsung or LG so that he can be sure he as a good working unit. His current one has something wrong with it.
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White LEDs actually do have a nonzero rise and fall time(because if it says 'white' on the label, that means 'glob of phosphor being pumped by a blue or UV die, since we don't have wideband LEDs').
For me it's the blue LEDs that drive me crazy, so perhaps the OP has a problem with the spectrum rather than flicker.
some as low as 90Hz (Score:5, Informative)
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.
OLED (Score:2)
I notice a 'walking pixel' effect on my laptop once, so this could the effect in action.
what the heck? (Score:2)
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There is another cause, white LEDS do not turn on and off very quickly. This is because they are really UV LEDs with phosphor painted on them.
Different types of backlights (Score:3)
LCD displays with CCFL backlights are less susceptible to the problem than ones with LED backlights. And some monitors use non-pulsed backlights or at least smooth it out with a filter circuit or something.
It does happen (Score:4, Interesting)
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We have a user here who got a new laptop last summer, it had a LED backlit LCD. Within 20 minutes she was calling saying it was making her feel sick/headache. We tried adjusting refresh rate, brightness, no help. Put a CFL backlit LED laptop in front of her and she was fine. Tried LED standalone monitor, it also bugged her though not as much.
So, we had to find a laptop that had a CFL backlit screen, wasn't junk,and met our other requirements (docking connector mostly). Ended up getting a previous year model Toshiba Tecra with a Core2Duo.All the rest of the laptops we bought had i5's in them by that point.
What was the DPI on the new laptop vs the old one? Dollars to donuts the new one was higher, in the 220 range, and the older one was under 200 (the point at which you can still make out pixels at typical use distance). She was getting eyestrain from trying in vain to focus on the screen, high DPI monitors take a lot of "user calibration" before they are comfortable to use.
"watering eyes and headache" (Score:2)
Hey, jones_supa: It's not yer monitor, stoopid, it's yer cellphone... stop holding it next to yer head!
Go see an expert, you dope (Score:5, Interesting)
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.
You are suffering from brain cloud (Score:2)
CRT was worst (Score:3)
Old CRT were giving me pretty severe headackes. This has all disapeared as soon as I used TFT panels, even with the early models that were not as good as recent ones. I definitely could see flicker on CRTs, I cannot anymore on TFTs.
Maybe you need to turn off "true motion" option on your TFT TV ?
yes (Score:4, Interesting)
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.
Re:first world problems (Score:4, Interesting)
Might be a first world problem but that doesn't make any less real.
It will never change unless someone starts the conversation.
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Re:first world problems (Score:4, Funny)
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.
Re:first world problems (Score:5, Interesting)
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.
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Re:first world problems (Score:4, Interesting)
...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.
Re:first world problems (Score:5, Informative)
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.
Re:first world problems (Score:4, Interesting)
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.
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But if they were blind, how do they seen the flicker? :D
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"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.
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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)
Re:first world problems (Score:5, Interesting)
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).
Re:first world problems (Score:4, Insightful)
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.
Seizure disorder (Score:3)
Waa my computer is too flickery, someone call the waambulance.
For people with photosensitive epilepsy, it might more more like "someone call the ambulance."
Re:Seizure disorder (Score:5, Informative)
Waa my computer is too flickery, someone call the waambulance.
For people with photosensitive epilepsy, it might more more like "someone call the ambulance."
Generally triggers between 3-30Hz with some rare cases up to 60Hz (who can't do much under indoor lighting). 100Hz for a backlight is not an issue and if it is, make a few thousand bucks selling yourself to science.
Re:Seizure disorder (Score:4, Interesting)
Many of the backlight control boards are only 400Hz carrier for efficiency. If you are dimming to 10% that is 20Hz pulses, which is the range for problems. Changing the current changes the color temperature of the backlight, distorting colors. Adjustable backlight color temperature therefore further complicates matters, since they need part of the dimming range to compensate for the current change.
It can be improved, but it is a cost/efficiency/performance trade off
Re:Seizure disorder (Score:5, Informative)
I'm pretty sure you're confused about this. PWM is 'pulse width modulation', not 'pulse removal modulation'. If you are dimming to ten percent, you'd expect the pulses to be ten percent as wide as at 100%, with the pulse rate unchanged.
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eink display ftw? since it's not flickering it cant cause issues. it's cheap enough, but color versions are still in infancy.
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The cycle rate for triggering seizures isn't typically that high. Most LED backlights cycle at 120hz (they should be driven ). Older LCD flatscreens had crappy tubes and had a very visible flicker.
http://en.wikipedia.org/wiki/Photosensitive_epilepsy [wikipedia.org]
When functioning correctly, mains-powered fluorescent lighting has a flicker rate sufficiently high (twice the mains frequency, typically 100 Hz or 120 Hz) to reduce the occurrence of problems. However, a faulty fluorescent lamp can flicker at a much lower rate a
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People who suffer migraines especially photosensitive trigger types have similar problems. My old CRT at anything under 70Hz would cause serious problems including trips to the hospital. Loss of vision from a migraine is *not* fun. CFL's cause the same problem with me, took awhile before I figured that one out. A good quality monitor though(using a samsung syncmaster e2320 for the last few years), and I don't have problems with flatscreens now.
first world problems: Part II - The Irony (Score:3, Insightful)
Re:first world problems (Score:5, Insightful)
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.
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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.
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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.
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also, is this an advert for prad?
I hope not, dass vor Ort saugt wie ein östlich Deutsch im Urlaub.
max brightness is way too bright (Score:3)
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.
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If you wanted to use the phrase "literally" to mean "figuratively", couldn't you have just said "figuratively"?
Re:Whiny little bitch (Score:5, Insightful)
You are just another asshole saying hurtful things without even understanding the problem. There is a minority in the population that can see lower-frequency flicker. You are very likely not one of them, or you would not say such incredible stupid things.
Re:Whiny little bitch (Score:5, Interesting)
For the WIFI people there are strong indications it is psychological only. For the flicker-sensitives, it has been known to exist as effect since movies exist. The original cinematic 24 pictures/second was selected because most people cannot see that flicker or are not bothered, but it is known that some can and may even get headaches, etc. The problem is really not new, just the place it turns up in is. So, we can say scientifically that some LED backlights may have that effect on some people. Of course, this would require rather low PWM frequencies, for example because an old CFL design was just adapted, where the slow CFL PWM is used for LEDs. CFL inverters run somewhere in the 50-100Hz range and their PWMs are synchronized to that. If you use the same PWM for LEDs, some people will see flicker. CFLs are pretty sluggish, leaving only minimal flicker, while LEDs are fast.
Of course, if you design for LEDs, you can run the PWM at > 1kHz, and there nobody should see any flicker. Ideally, you can run it at > 50kHz, then nobody can hear anything either.
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I think you literally do not understand what the word "literally" means.
The difference between an xray tube and CRT tube is disturbingly subtle.
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The difference between an xray tube and CRT tube is disturbingly subtle.
Fortunately most of the EM radiation from a CRT monitor goes out the back, not through the screen. You're frying the guy sitting in front of you, and you need to worry about the monitor behind you. However, it is most not all. You still get a dose from your own monitor too.
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That's like saying herbicides and insecticides are almost the same because they both contain water.
The differences between a CRT and an x-ray tube are not at all subtle. OK, they both have filaments to boil off electrons and both use a high voltage to accelerate them, but so do certain types of amplifier tubes and some types of particle accelerators.
If you do a quick web search you can easily find out just how different they are.
Re:Whiny little bitch (Score:5, Interesting)
Am I the only one who has an issue with this definition? I realize that English is an evolving language, but it seems like this became a new definition because too many people were too fucking stupid to understand the actual meaning of the word. Similar to how the word "epic" no longer has the same impact it once did. Perhaps we can redefine "figuratively" to have the classic meaning of "literally".
Re:Whiny little bitch (Score:5, Funny)
Am I the only one who has an issue with this definition?
Yes, you are literally the only one.
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I usually couldn't see any flicker at 60Hz and never above 72Hz. Most of the ones with apparent flicker were older monitors that usually had a perpetual burnt capacitor smell lingering around them.
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Idiot. Try finding out whether something is valid or not before posting nonsense. Here is a hint: A small part of the population can see flicker in lower frequency ranges, while most cannot.