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).
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.
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.
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.
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'.
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.
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.
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.
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.