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LED Backlight Flicker in LCD Displays

Updated on October 16, 2014

What Causes Flicker in LCD Monitors?

Computer monitors have come a long way from the bulky, heavy CRTs to the sleek, modern LCDs of today. One of the most significants improvements in display technology as of late is the LED backlight, which results in a brighter, sharper picture while reducing the power consumption. Manufacturers tout the benefits of LEDs over the older CCFL (fluorescent) backlights. However, some people experience issues like unusual eye strain, pain, or headaches which seem to be caused by their new monitor. And the most likely culprit is the LED backlight flicker.

"But wait," you might say, "I thought LCD displays were supposed to be flicker-free!". Indeed, while CRT monitors redraw the whole screen many times a second (determined by their refresh rate), LCD monitors provide a constant image, simply changing the pixel colors from one to another. The speed of this process depends on the monitor's response time, which is different from refresh rate. So, the source of flicker in modern liquid crystal displays isn't the LCD matrix itself, but rather the backlight.

In LED monitors, this flickering occurs when the brightness is reduced from 100% due to a backlight dimming method called PWM (Pulse-Width Modulation). In many cases, when a laptop is running on battery power, PWM will kick in even at maximum brightness to save energy. CCFL monitors also use PWM, but the resulting flicker might actually be less noticeable than with LEDs - more about that below.

An example of PWM dimming in LCD monitors.
An example of PWM dimming in LCD monitors.

How Does PWM Work In LED LCDs?

More information on pulse-width modulation dimming in LCD screens

Pulse-width modulation, or PWM, is a technique that is used for dimming LCD backlights, among other things. It involves cycling the backlight on and off faster than the human eye can see. PWM dimming kicks in as soon as you reduce the brightness setting on your monitor from its maximum value. It has many advantages over analog dimming, which provides continuous lighting: PWM is cheaper to implement, offers a wider range of adjustment, and prevents color shifting at lower brightness settings.

So if PWM-dimmed LEDs are so great, can they really cause problems like extra eye strain or headaches? Humans cannot see the LCD backlight flicker with a naked eye, after all. Certainly, provided a sufficiently high PWM switching frequency is used - such as 500 Hz - even the most sensitive people shouldn't be affected. However, it seems that many LCD monitors currently on the market use backlight frequencies between 100 Hz and 200 Hz. Humans still can't perceive the backlight cycling on and off at that frequency, but it doesn't mean that this LED PWM flicker won't hurt their eyes or affect their nervous system. Consider the following information:

  • According to studies about 1 in 4,000 people are highly susceptible to flashing lights cycling in the 3 to 70 Hz range [...] Less well known is the fact that long-term exposure to higher frequency flickering (in the 70 to 160 Hz range) can also cause malaise, headaches, and visual impairment.

    http://www.digikey.com/us/en/techzone/lighting/resources/articles/characterizing-and-minimizing-led-flicker.html

  • There is good evidence that fluctuations in the light signal are detected by the nervous system up to perhaps 200 Hz.

    http://grouper.ieee.org/groups/1789/public.html

  • Physiological evidence in humans and monkeys shows that flicker rates above the perceptual critical flicker frequency threshold can nevertheless generate cortical and subcortical visual responses.

    Martinez-Conde, S., Macknik, S.L., and Hubel, D.H. (2002). Proc Natl Acad Sci, USA 99, 13920-13925.

  • Different points in the visual system have very different critical flicker fusion rate (CFF) sensitivities. Each cell type integrates signals differently. [...] some retinal ganglion cells can maintain firing rates up to 250 Hz.

    http://www.bio.net/bionet/mm/neur-sci/2007-July/061925.html

More links on the subject can be found further down the page.

LCD LED Flicker Video

See what LED PWM flickering actually looks like

This video shows the the LED backlight of a MacBook Pro flickering 40 times slower, so you can actually see what's happening when you dim the brightness on your LCD screen. This video was taken with high speed camera by TFTcentral.co.uk crew.

CCFL vs LED Backlights in LCD Screens

Why PWM on CCFL backlights might be easier on the eyes

You might wonder why people who never had a problem with CCFL-backlit screens are now complaining about their new LED monitors or TVs hurting their eyes. CCFLs in LCD panels also use PWM dimming, so why don't they cause the same issues? There are two main reasons for this. First of all, a LED can be turned on and off almost instantly, but a fluorescent lamp will glow briefly even after the power is cut, providing a smoother transition between "on" and "off" states. The second reason is that WLED backlights tend to be quite a lot brighter than their CCFL counterparts. This means not only that more people will use lower brightness settings (thus increasing the flicker), but also that the difference between "on" and "off" states is more pronounced. After all, your backlight is blasting full brightness pulses between brief periods of darkness.

"Strobe Effect" Due to Low Backlight Frequency

You can check a monitor for backlight flicker yourself

There is a way to see the effects of low frequency PWM in LED backlights with a naked eye. Simply set your monitor to the lowest brightness possible and wave your hand, finger, or a pencil in front of a white background. If the object has sharply-defined edges, the backlight is operating at a fairly low frequency. If the edges are blurry, then either your monitor uses constant current dimming, or high frequency PWM undetectable by the human eye. This is demonstrated in the video below, which also compares CCFL and LED backlights:

LED Backlight LCD Discussion

Share your experience with LED backlights in modern monitors and TVs. Do you experience any extra eye strain when using LED-backlit LCDs?

Yes, LED monitors cause eye strain or other problems for me.

Yes, LED monitors cause eye strain or other problems for me.

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    • Pete 2 years ago

      I definitely have issues with LED monitors, TV's, but worse than these? LED Lighting. I would get very occasional headaches under fluorescent lighting, but under LED lighting, it's guaranteed. Not only is it too bright and headache-inducing, but LED lights are popping up everywhere due to the energy-savings. Even tail lights on cars (and more recently headlights) have a ridiculous flicker that I have trouble looking at. I flip my visor down when waiting in traffic behind someone with LED tail lights to block them.

    • Unusual 2 years ago

      I waited this kind of article since 2011 :( I had maybe 3-4 headaches in 30 years, now i had it on weekly schedule after a few hours looking at LED monitor and what can i do ? This new healthier technology affected all aspects of my life, in very bad way. . I love computers, i had spectrum 48k..., i love the knowledge which internet offers about everything, i like taking photos\editing, i like to write music, I enjoy "playing" with computers, servers on my job.. all in all after 3 years looking true a wonderful window called LED monitor, im tired of life... Even writing this article so it have any sense is very hard while using led.To make reality even more worse and like all that flickering, flowing, glowing, blinking, switching wasn't enough they had to put a little glue on that wonderful window and stick turbo aggressive coating to add "little" of blur to final touch, a signature.. I work in IT and i have no problem to think, focus 8hrs/day at work in front of old LCD/CCFL ... Soon as I look at any LED i feel like zombie, no thoughts, nothing, strange feeling and emptiness in my mind ...

    • Victoria 2 years ago

      Yes! a million times yes! these LED screens hurt my eyes. It took me months to figure out what was going on. where did this strange pain behind my eyes come from? It felt like the area right above each eyeball was on fire. It turned out it was my new LED backlighted laptop (that had replaced a CCFL lighted laptop) and my new LG LED TV (that replaced a big box TV.

      There is definitely a problem with this LED technology, but it seems to only affect about 10% of the population. I've had to go back to using my 10-year-old laptop and I've had to replace my LED TV with a plasma screen TV. Problem solved. No more burning eyes and headaches.

      I don't necessarily think it has to do with the flicker rate. I've tried turning my work LED monitor brightness up to 100% to stop any flicker, and even though I lower the contrast to about 20 to keep the screen from being so bright, it still hurts my eyes.

      However, if I turn brightness down to 0% and contrast up to 85%, that setting helps a lot.

      Sigh. It's hard being sensitive to LED backlighting now that the whole industry has gone to it for everything from smart phones, to laptops to desktop monitors to TVs. And it's even harder because those who do not have this problem accuse those of us who do with being full of "nonsense."

    • anonymous 3 years ago

      And it is really hard to find a laptop now because all use LED flickering backlights and on top have this bluish coulor. I don' t know what to buy.....

    • anonymous 3 years ago

      I've begun experiencing headaches again. I tried resetting the controls using the test charts, but without any luck. The headaches have continued to persist. They are basically tension headaches/migraines that inflict both sides of the head, can cause extreme dizziness and nausea, and in some instances can affect my sinuses. Furthermore, I seem to experience a flicker in my vision, which is no doubt connected to the PWM (Pulse Width Modulation) of the display.

      The headaches usually begin around 5-10 minutes after logging onto my computer in the morning, and then steadily increase in pain throughout the day. Even after logging off and going home, the symptoms persist well into evening. The next morning, I seem to experience a hangover of the previous day's headache which is only exacerbated when I get into work and the cycle begins again.

    • anonymous 3 years ago

      Now i get it. I have been visiting doctors and undergo many medical tests to discover what was causing a continuous headache. It was my led back light 5 inch smartphone. As soon as stopped looking at the screen, after two days the headache stopped!!!

    • anonymous 3 years ago

      I though I was alone! I get Vertigo with these new monitors - I have changed quite a few of them but to no help. The old LCD Viewsonic I have no problems with. In fact two years back, when I didn't know what was causing this, I had actually landed in emergency room.

    • anonymous 3 years ago

      I discovered your article while researching the same issue. I did some quick tests with my iphone 5s slow-mo feature. Also, I experienced eyestrain with LED light bulbs, and my iphone indicate they flicker around 50hz too.

    • anonymous 3 years ago

      100% веÑно - flicker bad

    • anonymous 3 years ago

      Yes, terrible eye strain with LED screens. It would get worse the longer I used it (my daughter's Acer laptop with LED screen), but I'd often feel dizzy and nausea within minutes. My non-LED screen at work and my desktop at home did not affect me, only the LED screen on my daughter's laptop. At the time I knew nothing of the difference in the screen or that it was an LED screen or there was a "flicker", only that it made me sick. I only figured out what was causing it after researching for a long time online. It was THE most important consideration in buying a new laptop, that I didn't get the same type of LED screen.

    • anonymous 4 years ago

      Terrible eye strain!!! Especially with battery powered devices like tablets and laptops the effect is even worse even if keeping the brightness 100%. This is getting serious as it affects my job. Almost all my work is performed with a laptop. I hear that there are desktop monitors that deal with the issue. But what about laptops?

    • anonymous 4 years ago

      Yes! All times!

    • anonymous 4 years ago

      I'm glad this is starting to get more press. I don't know why manufacturers haven't fixed this easily fixable problem. I won't buy a new monitor without knowing exactly how the backlight system works now. Just today I decided against buying a monitor because I found out it's LED backlight system runs at 170Hz. I first experienced this with my first LED monitor. Using it for more than a few hours causes me terrible eye strain. I usually like my monitors on the dim side since too bright also causes me eye strain if used for too long. I've had to find a balance with my LED monitor, I can't make it too dim or the backlight flicker will cause problems and I can't make it too bright or it'll again cause problems. Needless to say it's been relegated to a secondary monitor that never gets more than a couple of hours of use a day. I'm sure there are many people this affects that don't even know it. Lots of people seem to get eye strain from staring at a computer monitor for too long. I wonder if it's just because they're staring at a bad/poorly setup computer monitor for too long.

    • anonymous 4 years ago

      Unbelievable Eye strain, they make me extremely sick and so do overhead LED lights, I have just lost my job because of this and do not have a proper diagnoses yet so I do not have disability insurance approved yet....

    • carny 4 years ago

      I have a problem with my CCFL monitor only when viewing something on a dark background (e.g. white text on black background that some websites use). After reading such text for a long time I also see bright lines when I close my eyes (afterimage). I honestly have no idea why this only happens with dark backgrounds and not light ones.

    • anonymous 4 years ago

      Yes. I experience it with both CCFL and LED. I can close my eyes and see white lines from PWM after using my A-MVA BenQ panels. I have difficulty concentrating/focusing and have had seizure issues with the HP CCFL panel I've used briefly. In fact, of all the panels I've used, that HP had the worst effect. I tried it on more than one occasion just to be sure.

    • anonymous 4 years ago

      yes

    • anonymous 4 years ago

      Especially if used for extended periods of time.

      Legislation should be passed requiring manufactures TO PUBLISH their PWM rate for EVERY monitor where it is used. Also limit the MINIMUM PWM frequency to 2-5 KHz for LED.

      It will be cheaper then dealing with increasing numbers of blind users/workers.

    • anonymous 4 years ago

      I own both LCD and LED monitors. The LED has 180hz PWM. I am very sensitive to flicker and used to purchase 100hz+ CRTs in the past. The LED monitor causes me pain and eye strain, the LCD monitor does not.

    • anonymous 4 years ago

      Yes. I purchashed an hp laptop with LED, because that is all that is offered now. I can look at it a max of 30 minuts. I bought a toshiba before that and it gave me a splitting headache. My old toshiba with CCFL no problems, or my monitor at work. LED has a huge problem that is not being addressed. I truly hope they do not get rid of cffl backlit LCD's or I will have to just keep buying used ones I guess. I am sure the problems with LED's will finally get out, but who knows.

    No, I can look at LCDs with LED backlight for hours without issues.

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      • KRJ 8 months ago

        I have recently purchased a DELL XPS15 9550 laptop to comply with certain memory / disk requirements in my current contract job.

        I obviously had to reduce the display brightness and it was then that I started noticing the intermittent flickering.

        DELL support could not do much apart from reinstalling display drivers. One of their reps got closer to the conclusion of this article by suggesting I should increase brightness to max.

        Now that I see this is not a new issue, I guess the only workaround is to maybe add a physical screen dimmer (as we used to with desktop monitors, while leaving the display brightness at 100%).

      • Flashlight Guy 15 months ago

        I believe the real issue is the low frequency of the flickering. This has all been discussed in the LED flashlight world and there seems to be a consensus that with a high enough PWM frequency (well over 1Khz), most people are fine.

        I am surprised to read that monitors use a frequency as low as 100hz to 200hz... When it comes to lighting that would definitely annoy me big time, but I seem to have no issues with LED monitors and TV's other than being on the computer too long puts me to sleep while it used to keep me up till sunrise with CRTs... coincidence or age?

      • Jason 2 years ago

        Has anybody ever figured out any supplements to increase you eye strength in these situations. I know Bilberry helps me a little and higher doses of vitamin B2 for the migraines that follow. I have tried Roobis Red and Passion flower with no success. One thing for me that has worked was the Ben Q flicker free monitor. I can use that thing for hours and hours with no pain. Lighting is still the worst. I'm so sick of this problem and having no cure for it. Those stupid fl41 tinted glasses do nothing. I find Prevencia glasses help a little. All of it is still not nearly enough to make a difference. I'm tired of being in constant pain and no doctor or anybody can help. It's frustrating that LED's are be coming common place as well. Not happy about it.

      • Jason 2 years ago

        Plasma T.V's give me no Issues. I have my computer hooked up to my TV. I have an LED flicker free monitor at work. That gives me no issues. Any regular CCFL or LED monitor will destroy me in minutes. My biggest problem is lights, you can't avoid them. Especially in the winter when it gets dark so fast. Most of the cars and street lights are LED lights. This problem has wasted way too much of my life. I have been dealing with it for 6 years. I'm sick of it and doctor's are useless.

      • anonymous 4 years ago

        Personally I think many are just believing this nonsense just because you hear someone else state it. More than likely your headaches are coming from eye strain sitting close to a bright monitor with tiny text. Even the hand waving video here proves nothing. In fact I see no difference between each of those monitors.

      • anonymous 4 years ago

        The minimum distance between me and monitor is always more than 75cms and brightness and contrast levels are set to low values.

      • anonymous 4 years ago

        I never personally experienced a problem with the LED's per se, but I would like to recommend that it could be the lighting that is too intense? I'm slightly photosensitive and always turn down the lighting as low as I can (without it being a strain).

        Alternatively, I've been reading that it's the blue light of LEDs which are bad for the eyes. I've recently downloaded a free software called stereopsis which 'tells' your computer to reduce the blue lights. Perhaps those who have problems could try this and let everybody know if it helps?

      • anonymous 4 years ago

        No, it does not issue extra eye strain. May be because usually I keep my eyes at at least 70cm distance from the screen and I got used to CRT monitors some time ago before LED LCD.

        But sometimes it's very noticeable.

      Flicker-Free, No-PWM LED LCD Monitor

      If PWM-controlled LEDs hurt your eyes, consider this monitor

      Dell UltraSharp U2414H is one of the new LED-backlit displays which do not use PWM dimming, as tested by TFTCentral using an oscilloscope. In other words, it uses a constant control backlight and deserves the title of a truly "flicker-free" LCD monitor. Other advantages include IPS matrix for great image quality, fast response time and low input lag, as well as a thin bezel. The minimum brightness was measured to be just over 32 cd/m2, which means you can dim this monitor to almost nothing, and use it in a dark room comfortably.

      Links & Resources

      Interesting articles for further reading

      • prad.de is probably the first website on the net to bring up the topic of PWM switching in liquid crystal displays, and the potential issues associated with this method. Their recommendation to manufacturers is increasing the PWM frequency to at least 250 Hz, or using other methods of dimming which do not produce any flicker at all.
      • TFTcentral.co.uk has a detailed article on pulse-width modulation in LCD monitors. They offer a lot of excellent illustrations, and include instructions on measuring the PWM frequency of a display backlight using a digital camera, something you can do yourself without any complex equipment.
      • Vasya from Ukraine is a computer enthusiast who modded his LCD monitor to use a regular incandescent light bulb as a backlight. His page has a lot of information on the topic of screen flicker, and also many testimonials from other people who experience eye strain, pain, stress, migraine, and other issues when using digital screens. He also talks about the potential issues with fluorescent lighting in general. (Note: the webpage is no longer available, so the link leads to a copy on Archive.org).

      If you know of any other interesting and useful articles on the topic, please mention them in the comments below.

      Comments

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          anonymous 4 years ago

          Prad discovered another monitor that is 100% flicker-free by having an always-on backlight: BenQ BL2411PT

          Also, have a look at Asus PB248Q. Prad tested it to be flickering, but with the very high rate of 9400 Hz!

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          anonymous 4 years ago

          @anonymous: If you read the article you would understand that ccfl never entirely turns off during the off part of the cycle because of the fluorescent glow from the phosphorus in the ccfl even when it's in the off phase, so the flicker is not coticed by the brain. I can used ccfl fine, but not led.

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          anonymous 4 years ago

          Funny those claiming the flicker on LEDs give you headaches yet ccfl doesn't. You can't have it both ways. Flicker is flicker.

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          anonymous 4 years ago

          Took me the better part of five years to figure out what my 'flicker' problem was. I spent a small fortune on replacement hardware thinking the problem lay either with my graphics card, cables, EMI, RFI, peripherals, manufacturers etc, the list was endless. It's a very difficult problem to resolve if, like me, you have limited technical knowledge about such things: the flicker effect (i.e with the monitor backlight) lies hidden behind the image. horrid.

          I'm very glad to see these articles appearing on the net. More of us need to respond, especially gamers, who are a significant demographic of the flat-screen market.

          I would not wish the frustration I went through on anyone...

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          anonymous 4 years ago

          I too faced same issue with LED-LCD. So, I switched backed to CCFL-LCD.

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          anonymous 4 years ago

          Great post, glad I found it.... explains the issues i have with the newer laptops out. If it aint broke don't fix it is my theory. CCFL displays were just fine, CRT monitors didn't even bother me that much, but LED gloss screens..... the worst!

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          anonymous 4 years ago

          Great Post! I wonder if this is the cause with the screen in the ipad. I get eye strain and headaches after 15 mins of use.

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          TopFiveBestDeals 5 years ago

          cool lens!

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          Matt_Lowe 5 years ago

          Very informative. Now I know why I've been experiencing such horrible eye strain lately.

          Thanks!

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