Image flicker is when each frame is only displayed for a short time, with black images inserted between. Flickering the image displayed on a TV influences how clear movement appears to be when tracked across the screen. A lack of image flicker can cause a trail to appear to follow moving objects, which can be an issue for sports or videogames. This is one of the causes of motion blur, along with response time.
When evaluating the performance of image flicker, we measure the presence of strobing under calibration settings and also other motion settings which enable a stronger flicker. We photograph the appearance of blur on the TV with this strong flicker enabled and test whether it is possible to activate this feature with the ideal gaming settings.
When it matters
Some TVs use flickering to decrease the brightness of images by rapidly turning the pixels or backlight on and off. This produces less smooth movement due to duplications following moving objects. Some people who are sensitive to this experience discomfort and prefer a TV with a continuous light output.
Most people aren't as sensitive to flicker and can use it to help improve the clarity of fast movement as shown below. This can be beneficial for sports and video games, because of the fast movement occurring in both. With movies and regular TV the low frame rate and slow shutter speed of the camera means the video often includes significant amounts of blur intentionally, which can't be improved through flicker.
The two photos above are from the same TV, the Sony X750D. You can see that the image with flicker is significantly darker due to the time that the backlight is off. The amount of flicker that a person will find acceptable is subjective. Some people are much more sensitive than others.
PWM Dimming Frequency
Consider this comparison of the backlight implementation on the Sony X850D (first below), which does not flicker, and the Samsung KS8500 (second below), which has a kind of flickering known as pulse-width modulation (PWM).
For the Sony TV, you can see that whatever the level of luminosity, the backlight is pretty much static – it just stays at the same level of brightness. This corresponds to a video sampling method called ‘sample and hold’ (present on static LED backlit & OLED TVs), which presents each frame of the video for an amount of time proportional to the frame rate of the video (ex: 1/60 of a second for 60 fps), and displays the next frame right after. That means there is no break in the outflow of images, and if you refer to the image above, you can see that this makes for smooth movement, but also makes details look blurry.
With the Samsung model, you can see there is a regular pattern of brightening and dimming. This is ‘pulse-width modulation’ flickering at work. With flickering TVs (LED w/ PWM, plasma, CRT), each image is displayed for a shortened amount of time, and a dark frame is flashed before the next frame appears, which breaks up the flow of images. As you can see from the image above, details are defined a bit better with a flickering light, but you can also see there’s slight duplication to the image. This is because with PWM and flickering, while there might not be as much perceivable blur as with a static light, the movement isn’t as smooth. Some viewers also report issues like eye strain and headaches caused by PWM, though this isn’t common.
With this test, we check for flickering by analyzing the pattern of each TV’s lighting at three different levels of luminosity: maximum luminance, medium luminance, and minimal luminance. To measure the TV lighting we use a Rigol DS1102E oscilloscope and a photodiode, with a white image displayed on the TV.
Black Frame Insertion (BFI)
To pass our black frame insertion test, the TV must have an option to introduce more black frames into a 60 fps source. A TV which doesn't flicker when using our calibration settings even when the backlight is dimmed requires an option to add flicker to pass this test. For a TV which does flicker it needs to be able to reduce this backlight frequency. This is visible in the backlight oscilloscope as a change in the pattern. The blur photo with black frame insertion enabled is taken with the same methodology described here.
Black Frame Insertion Frequency
The black frame insertion frequency is the lowest flicker frequency possible on the TV for a 60Hz input. Low black frame insertion frequencies will produce a stronger, more visible flicker which helps to reduce eye tracking motion blur. The best performing TVs can match the flicker frequency to the same rate as the source material.
BFI In Game Mode
On some TVs, the low input lag modes which are best for gaming disable the black frame insertion feature. This feature can be useful to reduce eye tracking blur in video games, and so we test if it's possible to activate the feature.
Elements of motion blur
The response and overshoot time of a TV has a significant impact on the visible motion blur. See our response time article here for more information about this element.
How to get the best results
There are some TVs which allow you to fine tune the amount of flicker with a slider. Others have preset values to select from, or a single option to increase the amount of flicker. For the least eye tracking motion blur, choose the maximum flicker setting. If you prefer a smoother image with more blur then disable the BFI feature.
For most TVs which support image flicker, adjust the following settings:
Samsung: Go to Menu > Picture > Picture Options, set 'Auto Motion Plus' to 'Custom' and adjust 'LED Clear Motion'.
Sony: Go to Menu > Picture adjustments > Advanced settings > Motion, enable 'Motionflow' and adjust the 'Clearness' slider. On some Sony TVs the option is located in a different place. Go to Menu > Picture > Advanced Settings and adjust the 'LED Motion Mode'.
Vizio: Go to Menu > Picture > More Picture and adjust the 'Clear Action' toggle.
Reducing the backlight on TVs which use PWM dimming will increase the amount of flicker, as the backlight spends more time off to reduce the average brightness. Increasing the 'Backlight' often decreases the amount of flicker.
- The frame rate of the source also has a significant impact in the amount of visible blur. When a frame is displayed for more time the amount of blur from an eye moving past is increased. On some TVs it is possible to interpolate frames to create higher frame rates, see our discussion here for more information.
- Adjusting the backlight level of some TVs will affect the image flicker due to PWM dimming. On these TVs reducing the backlight will produce the clearest images and increasing the backlight will produce more smooth motion.
Frames which are displayed on a TV for a longer time increases the smoothness of motion due to the continuous image, but also makes movement less clear due to eye tracking blur. This can be annoying for sports fans or gamers, so some TVs have an option to insert black frames which increases the clarity of motion. To get an idea of how well image flicker works on each TV we measure the illumination pattern under calibration settings and with any black frame insertion features enabled. We also take a photo to show the amount of eye tracking blur visible on different TVs. To get the best results from your TV, adjust the backlight flickering option depending on your preference and the type of content.