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Image Flicker of TVs
Black Frame Insertion

What it is: How noticeable flicker is on the screen, when all optional flicker has been disabled.
When it matters: All usages, but particularly when viewing fast motion (such as in sports and video games) or when using the TV as a PC monitor.
Score components:
Score distribution

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 video games. 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.

If you want to see our test for monitors, check our "Image Flicker of Monitors" article. 

Test results

When it matters

Some TVs use flickering to decrease the brightness of images by rapidly turning the pixels or backlight on and off. This can produce duplications following moving objects if the flicker frequency doesn't match the content frequency. 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 TVs, the low frame rate and the slow shutter speed of the camera mean the video often includes significant amounts of blur intentionally, which can't be improved through flicker.

Sony X750D motion blur without flickerSony X750D motion blur without flicker
Sony X750D motion blur with flickerSony X750D motion blur with 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.

Our tests

PWM Dimming Frequency

What it is: Flickering pattern at different luminosities.
When it matters: For people sensitive to flickering.
Good value: N/A or high frequencies (> 300 Hz)

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).

Sony X850D motion blurSony X850D motion blur
Sony X850D backlight patternSony X850D backlight pattern

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.

Samsung KS8500 motion blurSamsung KS8500 motion blur
Samsung KS8500 backlight patternSamsung KS8500 backlight pattern

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)

What it is: Option to turn the screen black between frames.
When it matters: When flicker is desired by the user. Flicker is especially useful to make motion look clearer when viewing 60 fps content (sports, video games) and when using motion interpolation.
Good value: Yes

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 blurred photo with black frame insertion enabled is taken with the same methodology described here.

Sony X850D backlight pattern without BFI
Sony X850D backlight pattern without BFI
Sony X850D backlight pattern with BFI
Sony X850D backlight pattern with BFI

 

Black Frame Insertion Frequency

What it is: Lowest possible frequency of flickering pattern
When it matters: Reduces eye tracking blur in sports or video games
Good value: 60 Hz

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. 

 

Sony X750D BFI at 60HzSony X750D BFI at 60Hz

 

Sony X850D BFI at 120HzSony X850D BFI at 120Hz

BFI In Game Mode

What it is: Option to insert black frames when in the best settings for gaming
When it matters: Reducing eye tracking blur for video games
Good value: Yes

On some TVs, the low input lag modes that 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.

Additional information

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.

Related settings

  • 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.

Conclusion

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 increase 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.

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