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Image Flicker of TVs
Black Frame Insertion

What it is: Luminosity pattern when displaying images
When it matters: Sports, video games, when TV is used as a PC monitor
Score components:

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.

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 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 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). Frequencies that are multiples of 60Hz are better.

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 screen black between frames
When it matters: Reduces eye tracking blur in sports or video games
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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Questions & Answers

9 ANSWERED QUESTIONS
4
Hello. Thanks for all of the reviews! I am looking into getting a TV between 40 and 45 inches, and with help from your site, I've narrowed it down to the Samsung KU6300 or the Sony X800D. Motion handling is a main concern for me, as I play a lot of fast-motion games and get headaches from the blurred motion on some of the tvs in my house. In your reviews you give the X800D higher marks for motion, but in the pictures you provide under image flicker the KU6300 looks much less blurred. Could you explain that? Also, if you could provide video footage of each TV displaying an image blur test, it would be greatly appreciated! Thanks and keep up the good work!
A few years ago we experimented with providing video for our motion blur test, but it was both very difficult to film properly and not very useful. The images are an accurate representation of what you see, so we consider them adequate.

The images don't factor into the scores, the scores are calculated from the numerical measurements. The X800D has a faster response time and a flicker free backlight, so it gets a higher score.

The image in the image flicker test is when Black Frame Inertion (BFI) is enabled. The X800D has a 100 Hz BFI, yet our test video is at 60 fps. 60 fps shown on 100 Hz doesn't look good because some frames are shown twice while others are shown only once. The KU6300 has a BFI of 60 Hz, so our 60 fps test video looks much better. Unfortunately the KU6300 cannot use BFI in game mode. Also while a BFI of 60 Hz can greatly reduce motion blur, it also makes the screen less bright and adds visible flicker to the screen, which can bother some people.

1
I was looking at the comparison chart between the X800E and MU8000 and you scored a 60hz television higher than the 120hz Samsung for sports? You may want to correct that. You have the Samsung higher in every category other than viewing angle and black frame insertion. Just saying that most opinions and even your own numbers would make it seem the Samsung is the clear winner.

The lower viewing angle of the Samsung MU8000 is the main issue that causes it to fall behind the Sony X800E for the Sports category. With a 12% weight for that score component for Sports, the relatively large difference in viewing angle between the two models is the main cause of the difference you're seeing in the final score. The Samsung MU8000 has a lower viewing angle which means the image loses accuracy when viewed at an angle. If you watch sports with a group of people or have a wide seating arrangement, the X800E is a better choice, but if you watch them from directly in front, then both TVs are very similar for Sports viewing. You can use the '+ Create your own' function to create a Rating that best reflects your viewing conditions.

Regarding the refresh rate differences, since all sports channels currently broadcast at a maximum of 60 Hz, a 120 Hz monitor will use Motion Interpolation to increase the 60 fps input to display it at 120 fps. Motion Interpolation makes movement appear smoother, but may add artifacts to the final image or make motion appear unnatural to some people. Choosing between 60 Hz video and 120 Hz Interpolated video is mostly a matter of personal preference, and is not included in the Sports rating.

0
I have the KS8000 and was wondering if I turn off motion interpolation altogether am I avoiding image flicker or is that solely dependent on the backlight? Also does this affect UHD movies running at 24fps? Regards
The KS8000 uses PWM at 120 Hz to dim the backlight, so the only way to avoid flicker is to have the backlight near maximum, which is very bright. "LED Clear Motion" changes the frequency of the backlight flicker to 60 Hz, which helps to clear up motion but also makes the flicker more visible. Motion interpolation has no effect on backlight flicker, except the "LED Clear Motion" setting, which is off by default. The KS8000 will not flicker the backlight at 24 Hz to match 24 fps content, the lowest it will flicker is 60 Hz when "LED Clear Motion" is enabled.
0
What are the best TVs for motion? I love video games and use my TV for that most of the time. I'm wondering what the best TVs are for motion in game mode. I would think OLED since their response time doesn't change but I'm not sure. Are there brands that are better at motion overall, and are there TVs will all produce good motion or does it vary from TV to TV? I saw the Q7's motion is good, surprisingly since Sony are the ones that usually excel in that area. Is the Q7F's motion any different in game mode?
The very best TV for motion with video games would be the Sony A1E OLED TV. As you mentioned, OLEDs have next to no response time, meaning there are no visible trails following fast moving objects. In addition to this, the A1E has a black frame insertion feature that further enhances motion clarity. If you do not mind the flicker, this is the best choice. If you don't plan to use the flickering feature, you should go with the LG C7 OLED TV, which will otherwise perform the same but lacks the feature and will usually be found at a lower price.

OLEDs, because of their virtually perfect response times can look stuttery at lower frame rates (below 30 FPS). If this is an issue for you, it might be worth getting an LED TV with good motion such as the Sony X930E and the Samsung Q7F. The motion shouldn't vary between picture modes, except if the motion interpolation feature (soap opera effect) is turned on. Generally, Sony as a brand is usually a good bet for motion, but we've found some of their larger TVs to have slower response times.

0
In your review of the Sony X700D, it says that the TV is capable of BFI at 60Hz. But in the screenshot, there is very noticeable image doubling, which is usually caused by the frame rate not matching the flicker rate. Can you confirm that it is indeed 60hz? If so, do you know what could be the cause of that doubling?
Thank you for your very interesting question! This visible doubling is likely caused by a frame transition timing issue: the LCD layer pixel transition is occurring while the LED backlight is On instead of when it is Off. This is an issue, as one of the goals of BFI is to hide the LCD transition by turning the backlight off momentarily and changing the displayed frame during the dark phase, which helps clear up motion blur. We do not know whether this is only an issue with our X700D test unit, or if it affects every X700D. We did attempt to take multiple pictures at different times to see if this issue was intermittent, but every picture showed the same duplications.
0
I'm curious about flicker effects when using a TV as a monitor and for sports. When using a TV as a monitor, I plan on doing the snap-to-corners to effectively have quad 24.5" 1080p displays. I am looking at two of your highest overall rated 49" options, the Sony X720E and the Samsung MU6500. Based on your reviews, it appears this is a major reason of the Sony's higher score for use as a TV monitor. First, the Samsung flickers at 120Hz which makes images blurry, while the Sony can enable 120Hz BFI to clarify motion. Can you explain this in more detail? What is the difference between 120Hz BFI and 120Hz flicker? Second, the Samsung review mentions changing settings to improve fast-moving content while also increasing flicker. How would enabling these settings change the flicker rating? How would this different flicker rating (if any) compare to Sony's ratings as default vs. with optimized settings? Third, for a screen this large as a monitor for a single person, how important is curve?
Very good questions. We recommend the Sony X720E, for a few reasons. Our flicker box score is designed around the philosophy that the TV should not have flicker by default, but should be able to add flicker via Black Frame Insertion (BFI) for those who do want it. Sony usually does this extraordinarily well by having no PWM flicker by default, but the ability to add 60 Hz or 120 Hz BFI flicker as desired; while Samsung TVs usually have 120 Hz PWM flicker nearly all the time, and their BFI setting makes the flicker more severe and changes it to 60 Hz or 120 Hz depending on input. There is no fundamental difference between PWM flicker and BFI, but BFI usually has stronger flicker.

Adding flicker always reduces persistence based motion blur, but if you play content that doesn't exactly match the flicker frequency there will be double image artifacts, which look like this, because multiple backlight flashes happen per frame. For PC monitor use we generally recommend no flicker, because people often watch 24 Hz and 30 Hz movies and TV shows, which will have double image artifacts with 60 Hz or 120 Hz flicker. The Sony X720E is much better for this than the MU6500. If you play a lot of video games however then 60 Hz flicker will be very beneficial. Both the X720E and MU6500 can do 60 Hz BFI equally well.

The curve is important for single person because it reduces the viewing angle of the edges of the screen. On a flat TV with a VA panel, like the Samsung MU6300, the edges of the screen will look washed out if you sit at a typical PC monitor distance. The curve of the MU6500 helps to alleviate this. The Sony X720E doesn't need a curve because it uses an IPS panel with wide viewing angles, so the edges of the screen don't look washed out.

0
Can you clarify the difference between 120 Hz CRC (CMI) SRS depending on the brand and just plain old 120 Hz. I know there is a difference. Also are there any 4k TVs that put out true 120 Hz in 4k on HDMI inputs?
Most Hz advertising numbers are meaningless marketing numbers, as detailed in our fake refresh rate article. Most high end TVs have 120 Hz real refresh rates, while budget TVs have 60 Hz real refresh rates, as discovered in our testing. Backlights can have a variety of different PWM flicker rates, however flicker is usually undesirable. Few TVs can accept 120 Hz input over HDMI, and no 2017 or older TV can accept 4k @ 120 Hz over HDMI because it uses more bandwidth than HDMI 2.0 can provide. However almost all 120 Hz TVs have motion interpolation that can interpolate lower fps content up to 120 fps, at the cost of some artifacts and more input lag.
0
What happens on a non-PWM BFI-enabled TV when you reduce the brightness? Is the luminosity reduced or is the 'on time' decreased (and, consequently, the 'off time' increased)? In the last case, this would mean that reducing the brightness on a non-PWM BFI-enabled TV will decrease motion blur even further, as is the case on PWM TVs. I am particularly interested in (non-PWM) Sony TVs, but also in the general case.
We just did a quick BFI test on the Sony X900E, X850E and Samsung MU8000, and it seems like Sony TVs always reduce the amplitude, while Samsung TVs reduce the duty cycle ("on time") which makes motion even clearer at low backlight settings. It may be that TVs that dim primarily through PWM will have BFI that changes duty cycle, while TVs that dim primarily through lowering amplitude will do the same to their BFI. Plots of BFI at 100%, 50% and 0% backlight are shown here for the Sony X900E, here for the X850E, and here for the Samsung MU8000.
0
Can I use BFI + 1080@120hz on x905e? Will it increase input lag?
You can use both the 1080p@120hz input and BFI simultaneously. Input lag remains essentially the same.
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