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.
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.
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.
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?
Re:Sigh (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.
It does happen (Score:4, Interesting)
Re:first world problems (Score:3, Interesting)
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.
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: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: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.
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: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: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: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.
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: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.
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).