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Our Monitor Motion Tests
Image Flicker

Updated
What it is: If the brightness of the screen fluctuates rapidly.
When it matters: Flickering can cause eye fatigue or headaches over time.
Score components:
Score distribution

Nearly all monitors use LED lights behind their LCD panels to help display an image. Sometimes, those backlights may flicker to reduce the intensity or brightness, known as pulse width modulation (PWM). Having a monitor with a flickering backlight can help improve the appearance of motion if used properly, but if it's not, it can create image duplication. However, having a monitor whose backlight doesn't flicker can also be beneficial because it helps reduce eye strain.

We test for the flicker frequency on monitors using a specialized photodiode tool, and we measure the flicker frequency at different backlight levels to see when/if the flicker starts.

You can learn about our image flicker tests on TVs here, which focuses more on black frame insertion (BFI).

Test results

When It Matters

Monitors introduce flicker to reduce the brightness of the backlight, which is called pulse width modulation (PWM). They do this by reducing the amount of time the backlight is on, resulting in a dimmer image. However, if the flicker isn't properly implemented, it could also create image duplication, especially if there's a low flicker frequency.

You don't notice the flicker frequency only by looking at the screen, but some people may feel like flicker causes eye strain, so in that case, it's better to have a flicker-free monitor. Most monitors we've tested are completely flicker-free; nearly 90% of monitors we've tested to date on the Test Bench 1.1 are completely flicker-free at all backlight levels.

Monitors may also flicker the backlight in a different way to reduce the appearance of motion, which is popular amongst gamers and is known as black frame insertion, which is a separate test besides this one.

Our tests

We test for the flicker frequency at the same time that we do the response time and input lag tests because we use the same tool. We place the photodiode tool at the center of the screen and use software to record and measure the flicker frequency with the backlight at 100% of its brightness, at 50% and 0%. If we notice anything strange, we also check the flicker with an oscilloscope. It's a very straightforward test, and as mentioned, most monitors are flicker-free.

Backlight Picture

The dedicated software plots charts of the backlight intensity at different levels, and we post a photo of them. As you can see in the charts below, the x-axis is time, and the y-axis is luminosity. A flicker-free backlight will have a straight line across the entire chart, meaning it doesn't flicker and change its luminosity. When there's flicker, as we see with the LG, the luminosity fluctuates, and how often it goes up and down tells us the flicker frequency. In this case, it has a 240Hz flicker, so it flickers every 4.17 ms, which we see in the chart.

Flicker free backlight - ASUS TUF VG27AQ Flicker-free backlight - ASUS TUF VG27AQ
Backlight flicker LG 32UL950-W 240Hz flicker - LG 32UL950-W

Flicker-Free

What it is: Whether or not the monitor has noticeable flickering (<1000Hz), or none at all.
When it matters: Flickering can cause eye fatigue or headaches over time.

We consider a monitor to be flicker-free when there's no flicker at all across any brightness level. Any sort of flicker at any of the brightness levels results in this getting a 'No'.

PWM Dimming Frequency

What it is: The rate at which the brightness of the image fluctuates.
When it matters: For people sensitive to flickering.
Good value: N/A or high frequencies (> 650Hz).

The PWM Dimming Frequency is simply the frequency at which the backlight flickers. It's better for a monitor with a backlight flicker to have it flicker at a multiple of 60Hz. There are a few monitors we've tested, like the Dell U3219Q, that have a flicker frequency that's not a multiple of 60Hz, which results in image duplication.

This result affects the scoring for the Image Flicker test. Although we give a perfect score of 10 to monitors that don't flicker, a monitor that flickers doesn't necessarily mean it won't get a perfect score either. We've tested monitors that flicker at such a high frequency (greater than 1000Hz) that you won't notice, and it scores 10. Also, OLED monitors are a unique case because they don't have a backlight; instead, they have a slight dip in brightness that coincides with the refresh rate, so we don't consider them flicker-free, but their PWM frequency is also 0, so they get a perfect score.

Flicker Mobile Pixels DUEX Plus Flicker frequency of over 1000Hz - Mobile Pixels DUEX Plus
Backlight flicker Gigabyte AORUS FO48UOLED flicker chart - Gigabyte AORUS FO48U OLED

Additional Information

image flicker vs black frame insertion

Backlight strobing, commonly known as black frame insertion (BFI), is an effect where the backlight flickers itself to try and improve the appearance of motion. We check for this in a separate test, but the BFI feature is tied into the flicker frequency; the only difference is that the image flicker is during regular use, while the BFI feature is usually something you can turn on and off. Below you can see an example of how introducing flicker on the LG 29UM69G-B helps improve the appearance of motion. However, there are times that the BFI features isn't good and creates more image duplication, as you can see here.

LG 29UM69G-B motion blur without flicker Without flicker
LG 29UM69G-B motion blur with flicker With flicker introduced

Learn more about black frame insertion on monitors

Types of pulse width modulation

Manufacturers implement different techniques of pulse width modulation, but one of the more common techniques is shortening the duty cycle. The duty cycle refers to the amount of time the pulse is sent for, and shortening the duty cycle reduces the intensity. Below are two examples from TVs that use different types of PWM, but the same techniques are applied with monitors that use PWM. You can see with the LG that the backlight flickers at all brightness levels, and the difference between the 100%, 50%, and 0% luminosity is the duty cycle. The backlight stays on for less time as you decrease the brightness. The Vizio starts to flicker at lower brightness levels with a short duty cycle, and by the time it reaches 0%, the cycle is almost 0.

LG NANO75 2021 120Hz flicker - LG NANO75 2021
Vizio D3 Series flicker 480Hz flicker - Vizio D3 Series 2021

Other Notes

  • A monitor can introduce image flicker at lower backlight levels, even if it's flicker-free at its max brightness. If you're concerned that your monitor flickers at lower backlight levels, set the brightness setting to its lowest, and wave your hand (or any object) in front of the screen. If you notice your hand is moving like it's in front of a strobe light, then it has flicker. Increase the backlight until you don't see this. If you don't see this effect, then there's no flicker.
  • We test the flicker on TVs similar to monitors, but on TVs, we also check to see which backlight setting the flicker starts at. We don't do that for monitors. You can use the test above to see when the flicker starts exactly.
  • This test is meant for LED-backlit displays and not OLEDs because they don't have a backlight. Still, OLED monitors get a perfect 10 because they don't have any flicker.

Conclusion

LED-backlit monitors have a backlight to display an image on the screen. Sometimes, these monitors will use a technique called pulse width modulation in order to dim the backlight, where it sends short impulses, creating a flicker effect. We want to know which monitors do this and at which frequencies the backlight flickers. Most monitors we've tested are completely flicker-free, but there are a few that flicker. Introducing flicker can help with the appearance of motion but may also create eye strain, so having a monitor that flickers or not is entirely up to you.

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