LED Backlight Flicker in LCD Displays
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://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/veitch_flicker_philly2010.pdf
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.
CCFL vs LED Backlights in LCD Screens
Why PWM on CCFL backlights might be easier on the eyes.
LED Backlight LCD Discussion
Share your experience with LED backlights in modern monitors and TVs.
Do you experience extra eye strain when using LED backlit LCDs?
Yes, LED monitors cause eye strain or other problems for me.
Andrea says:
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.
Peter says:
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?
Vladimir B says:
Yes! All times!
James says:
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.
Jesse Thompson says:
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....
Chadrew says:
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.
SR says:
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.
np says:
yes
Mark. says:
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.
Greg says:
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.
Ted says:
Yes. I purchashed a HP laptop with LED, becasue 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.
DevotedDad says:
This is a great article but I do not understand what the solution is.
Should I turn up the backlight and brightness or lower them?
Will increasing the backlight reduce the flicker?
If i increase the backlight but lower the brightness, will this help stop the headaches?
Can you give me an idea of an optimal setting?
I have been using the same computer for 2 years without a problem. Now it is simply unbearable. I am using a mid 2010 Macbook pro 13".
Mike says:
To add to last post, I have not tried ones advertised as "flicker free". So maybe there is hope for me if the flicker is the cause of the LED dizziness and nausea.
Mike says:
Yes. It really screws up my vision and I get dizzy and nausea after a few seconds. I was hoping things have gotten better but went to local computer stores the other day and after looking at the LED monitors felt dizzy and nauseous. I do not have the problem with regular LCD displays or CRTs. It is really an awful feeling to be dizzy and nauseous like that. It feels like I was staring at the sun.
When they first came out I noticed the problem and switched back to LCD. I also stocked an extra LCD monitor but I am now using my last LCD monitor. I am very worried about this since I work in IT and have for 20 years. I can't believe something like this is going to end my career. I have been lucky to be in a management position that afforded me the ability to avoid looking at LED's. That position ended and I am worried my next position (whatever it is) of likely looking at many different monitors is just not going to work.
I see many people having the same problem and believe the industry and media is either ignorant to it or intentionally ignoring it to favor the LED green agenda.
Nick says:
YES, its bad enough to make me walk away from using computers all together which would make it hard to work in the IT industry. Headaches, nausea, blood shot eyes you name it and most all screens on laptops now have a gloss cover. CRAP
Xiaoyong Zhou says:
Yes
Jenny says:
Serious trouble!!! I cant use a led screen without getting a headache. I wonder how people sensitive to this technology will work in the future. I work 95% of my time in my laptop.
Panos says:
This article is amazing. I now understand why i get headaches after 30mins of work on my MBP while in my 2009 Mac i had no issues at all after 10 hours of work on the screen!
Diego says:
I just realized mine was flickering by pointing my phone camera at it today to show something to my Video caller.
I then understood why I did not feel it comfortable when working hours in front of it.
I'm using a brand new $2000 13" LED ultrabook from one of the leaders in this domain. It is flickering at any brightness unfortunately.
Rutabaga says:
My theories are that its not the flicker in itself that is the cause of some peoples problem. Its the base light intensity of the monitor. When dimming a very bright screen to acceptable level, the result will be a quite short duty cycle period. This will amplify the problems. Not only do you have more irritant flicker as a result, but the intensity of the peaks will be much higher and probably cause issues in itself.
Therefore you cannot really compare this to most research on 'flicker' since they dont emulate these conditions.
Just keep up the holler, folks. There are better technology to deal with LED PWM dimming out there that doesent cost anything extra for monitor manufacturers. They're just stuck in their ways (production lines).
Karemeos says:
Definitely. After a period of use I am simply unable to continue. My vision becomes so blurry and my eyes so fatigued that it is truly impossible to look at the monitor anymore.
flycatcher says:
Yes, actually... and now I have an idea why that might be. Thanks!
Zebedee32 says:
Yes, but that's because I have bad eyesight lol
Chadrew says:
I had to go back from a shiny new 23" widescreen LED LCD to a CCFL-backlit one that I had been using for over five years. The reason? My eyes would start hurting after using the new screen for just a few minutes. And it was so bright I had to set it to 0 Brightness when reading or typing text (which I do a lot).
To be fair, I have no proof that this was caused by backlight flicker as opposed to, say, harsher light tones of the WLED, or some sort of an eye problem. All I can say is that I can use my old display for hours without any issue.
Speaking of light tones, I've also read that WLEDs emit more blue light which can cause insomnia by making your body think it's daytime. Of course this is not related to the backlight flicker, but it shows that there might be more drawbacks to this technology being used in computer screens than most people think.
No, I can look at LCDs with LED backlight for hours without issues.
Tc says:
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.
Clement says:
The minimum distance between me and monitor is always more than 75cms and brightness and contrast levels are set to low values.
pristi says:
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?
Alexander says:
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.
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 dangers of fluorescent lighting in general.
If you know of any other interesting and useful articles on the topic, please mention them in the comments below.
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