Image flicker is a behavior commonly found on monitors where images shown on-screen will appear as a series of short duration impulses instead of staying on-screen constantly until they have to be replaced by the next image. Flicker has a large impact on the appearance of motion. Its appearance can either be an intentional method to improve motion clarity (usually referred to as black frame insertion or backlight strobing) or simply a side effect of the screen's brightness adjustment system (PWM Flicker). Learn more about our image flicker tests on monitors.
Using image flicker on LCD monitors is similar to how traditional CRT monitors would show the image, as it would periodically scan down the screen and send an impulse to every row as it was being fed the video signal.
To test the flickering on monitors, we measure, using a photodiode-equipped tool, the frequency at which the screen flickers, and if it has options for it, the different settings which alter the function. We measure the different frequencies at which the screen's optional flickering feature can be used, if it has one.
If you want to see our test for TVs, check our "Image Flicker of TVs" article.
For our black frame insertion test, monitors need to offer a way to enable a dedicated flickering feature to introduce black intervals between each frame. This emulates the behavior of impulse-driven CRT screens, where the image is shown on-screen as soon as it is received and only for a short span of time. Monitors that flicker by default need to offer a way to adjust the flickering frequency to match the refresh rate of the screen to pass this test. This behavior can be observed using the same methodology used in our PWM dimming frequency test.
The BFI Maximum Frequency is the highest frequency the BFI feature of the monitor is capable of operating at. It is common for monitors to have a BFI frequency slightly below their maximum refresh rate. This causes the refresh rate of the monitor to be reduced when that feature is enabled. The optimal result in this test is a maximum BFI frequency that matches the screen's maximum refresh rate.
The BFI Minimum Frequency is the lowest refresh rate at which the black frame insertion feature can be enabled. This is useful for users that want to use this feature with a console that only outputs at a standard 60Hz frequency. The theoretical best value is 1Hz, but most people tend to find frequencies under 100Hz distracting.
While image flicker has a very strong effect on motion quality, other aspects like pixel response times and refresh rates are similarly impactful. Because of this, it's important to consider these factors as well when looking for optimal motion performance.
Some older gaming monitors like the Asus VG248QE were originally meant as 3D monitors and to use their higher refresh rate for active 3D functions. In later revisions of their 3D Vision program, NVIDIA also added a feature called "lightboost" which was meant to improve the quality of 3D in a few ways, one of them being reducing motion blur.
Users caught on to this feature and with a few hacks made it work in the monitor's normal 2D Mode. NVIDIA, later on, introduced G-SYNC (learn about it here) which included Ultra Low Motion Blur as a basic feature. While it is not usable in conjunction with variable refresh rates, it means that every G-sync monitor comes packaged with a backlight flicker feature.
Monitors have two major ways of displaying images on their screen, sample and hold or impulse-driving. Most LCD monitors use a sample and hold system where images stay displayed until they are replaced by the next frame. Some LCD monitors offer a feature that emulates the impulse-driving mechanism of older CRT monitors by flickering the backlight of their screen. This can greatly improve the clarity of motion, but it introduces flicker which some users find fatiguing. Some monitors also flicker by default, as they alter their flickering to adjust the brightness output of the screen. This is generally seen as undesirable. We test for flicker by measuring the frequency of the monitor's backlight using a photodiode under a number of conditions, as well as evaluating the monitor's ability to adjust flicker-related features.