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LED Backlight Flicker in LCD Displays

What causes flicker on LCD monitors?

Computer monitors have come a long way from the bulky and heavy CRT to the sleek, modern LCDs of today. One of the most significant improvements in display technology lately is the LED backlight, which results in a brighter and sharper image while reducing energy consumption. Manufacturers trumpet the benefits of LEDs over the old CCFL (fluorescent) backlights. However, some people experience problems such as unusual eye strain, pain or headaches that seem to be caused by their new monitor. And the most likely culprit is the flicker of the LED backlight.


"But wait," he might say: "I thought LCD screens should be free of flickers!" In fact, while CRT monitors redraw the entire screen many times per second (determined by their refresh rate), LCD monitors provide a constant image by simply changing the colors of the pixels from one to the other. The speed of this process depends on the response time of the monitor, which is different from the refresh rate. Therefore, the source of flicker in modern liquid crystal displays is not the LCD matrix itself, but the backlight.


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


How does PWM work on LED LCD screens?


More information about pulse width modulation attenuation on LCD screens


Pulse width modulation, or PWM, is a technique used to attenuate backlighting of the LCD, among other things. It involves turning the backlight on and off faster than the human eye can see. The PWM attenuator goes into action as soon as it reduces the brightness setting on your monitor to its maximum value. It has many advantages over analog attenuation, which provides continuous illumination: PWM is more economical to implement, offers a wider range of adjustment and prevents color change in lower brightness settings.


So, if the PWM-dimmed LEDs are so cool, can they really cause problems like extra eye strain or headaches? After all, humans can not see the backlight of the LCD screen blinking with the naked eye. Certainly, whenever a sufficiently high PWM switching frequency, such as 500 Hz, is used, even the most sensitive people should not 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 not perceive the backlight on and off at that frequency, but that does not mean that this flickering LED PWM does not It affects your eyes or affects your nervous system. Consider the following information:


• According to studies, 1 in 4,000 people are highly susceptible to flashing lights that cycle in the range of 3 to 70 Hz [...] Less known is the fact that long-term exposure to a higher frequency flicker (in the 70 to 160 Hz) range) can also cause general 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 blink rates above the critical perceptual blink 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 velocity (CFF) sensitivities. Each cell type integrates signals differently. [...] some ganglion cells in the retina can maintain firing rates up to 250 Hz.

CCFL vs LED backlighting on LCD screens
Why PWM in CCFL backlights can be easier for the eyes



You may be wondering why people who never had a problem with CCFL backlit screens are now complaining that their new LED monitors or TVs have hurt their eyes. CCFLs on LCD panels also use PWM attenuation, so why do not they cause the same problems? There are two main reasons for this. First, an LED can be turned on and off almost instantly, but a fluorescent lamp will light up briefly even after cutting

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