LED Backlight Flicker in LCD Displays
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.
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.
- There is good evidence that fluctuations in the light signal are detected by the nervous system up to perhaps 200 Hz.
- 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.
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?
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.