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.

Comments

Bitch, please

[Anonymous Coward]

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

Re:first world problems

[Urban Nightmare]

Might be a first world problem but that doesn't make any less real.

It will never change unless someone starts the conversation.

Sigh

[ledow]

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).

Re:Sigh

[Andy Dodd]

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

[DutchUncle]

No. It's not necessarily just one person. Like allergies and other chemical sensitivities, each person who has a problem thinks they're weird, because nobody else seems to have the problem; but just like ANY OTHER aspect of humanity there is a wide range of reactions.

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

Maybe it's not a tiny minority, and maybe it's not enough to make people ill. Maybe a tenth or a quarter of the world feels a little less well than they might all the time, but it's "just" a few percent so not enough to notice and each person thinks they just need more sleep. Or maybe a tenth or a quarter of the world is "just" a few IQ points lower than they might have been.

Re:Sigh

[Solandri]

I'm positive it's placebo here.

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

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

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

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

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

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

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

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

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

Is it only the monitor?

[barc0001]

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?

Re:Is it only the monitor?

[ZorinLynx]

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.

Re:first world problems

[gweihir]

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.

Re:Whiny little bitch

[gweihir]

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.

Go see an expert, you dope

[wcrowe]

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.

Re:Seizure disorder

[KitFox]

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:Whiny little bitch

[The Grim Reefer]

2. Used to acknowledge that something is not literally true but is used for emphasis or to express strong feeling.

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

[rokstar]

Am I the only one who has an issue with this definition?

Yes, you are literally the only one.

Re:MOD PARENT DOWN

[Palmateer]

I will agree with the Cadillac taillight issue. It doesn't give me a headache, but it does have a Persistence of Vision effect which disorients me. As the vehicle moves either subtly in it's lane, or changes lanes, it leaves a trail of ghost images which occupy a significant part of my field of vision. It's a good thing these are (for now) in the minority. I don't understand how these could have been approved from a safety perspective.

Re:first world problems

[Applekid]

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.

some as low as 90Hz

[Chirs]

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.

Re:Whiny little bitch

[gweihir]

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.

Re:Seizure disorder

[dentin]

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.

Re:first world problems

[AmiMoJo]

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

[Omestes]

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).