How to test my Monitor for PWM flicker?
There are several devices which you use the test your monitor for PWM flicker but I want to show you something simpler.
The test below is based on a simple moving line on your screen.
You need to test your monitor at MAX hardware brightness and at MIN hardware brightness.
By hardware brightness, I mean to change the brightness from your monitor or laptop buttons.
Below you can see a white line moving really fast.
You can open this in your own browser TestUFO Blur Trail test.
We will use this to determine if your monitor has PWM flicker or no at both max and min brightness.
I will tell you what to look for but first a little intro on
PWM flicker is a cheap way that manufacturers use to control the brightness of the screen by adding some breaks without light.
Basically, on a given interval your monitor is OFF then after some milliseconds ON again.
Our brain is slow and we don’t perceive this but we get huge eye strain and eye pain from this.
This is one of the main reasons why our eyes hurt in front of the computer.
Because of this constant ON and OFF of the backlight our eyes contract all the time like this
This is why PWM flicker causes so much eye strain and is undesirable.
Most monitors flicker only when the brightness is lowered so when the hardware brightness is at 100% there is no flicker.
This is why my recommendation when you download Iris is to set your hardware brightness to the MAX and control the brightness with Iris.
Iris uses another method to control the brightness which doesn’t cause eye strain and eye pain.
Depending on how you see the moving line you can determine if your monitor uses PWM to control the brightness.
If your monitor doesn’t use PWM you will see a smooth moving line like this
You can test this by setting your hardware brightness to 100%.
When your hardware brightness is set to 100% your monitor doesn’t use PWM in all cases and you should see this smooth line.
Now set your hardware brightness to the minimum possible value.
If your monitor uses DC dimming instead of PWM the test line should look smooth like this
If your monitor uses PWM to control the brightness you will see multiple lines on the screen separated by some space like this
1. Set your monitor Hardware brightness to 100% using your monitor buttons.
2. Use Iris to control the brightness without PWM
If you liked this article and you learned new things about why we feel eye pain in front of the monitors
If you want more and more people to learn about this
If you want your friends to protect themselves from the monitor harmful rays and emissions
Flicker, the display affliction
All screens flicker to some degree — be they TV screens, car navigation displays, monitors, tablets, and yes, even smartphone displays. In this article, we will talk a little about what flicker is, what can cause it (on smartphones in particular), and how we at DXOMARK test for it, quantify it, and measure its impact on the end-user experience.
Setup for testing a smartphone using a computer-controlled flickermeter (DXOMARK engineers conduct the tests in very dark conditions).
Photo credit: DXOMARK; for illustration only
What is flicker?
Flicker is a quick oscillation of light output between on and off; it is measured in hertz (Hz) to quantify the frequency at which the oscillation occurs. While we may not be consciously aware of the flicker phenomenon, it’s important to understand that our eyes still physically respond to it — that is, our irises expand and contract in response to these changes in brightness. This involuntary physiological response can certainly explain why we may have a headache and particularly why our eyes can feel tired after looking at a display for an extended period of time — they have been working hard! (This is especially true when looking at a display in dark ambient conditions, such as reading in bed with the lights turned off, for reasons we’ll touch on a bit more below.)
What causes flicker on smartphones?
Given the ubiquity of smartphones, it is unfortunate that the flicker on their displays (especially OLED displays) is still an issue for many people. But wait! Why do they flicker? Well, let’s remember that smartphone display hardware is based on either LCD (liquid crystal display) or OLED (organic light-emitting diode) technology. LCDs don’t emit their own light; rather, they are back-illuminated by a strip of LEDs whose light intensity is quite powerful so as to compensate for the brightness drop due to the low transmission rate of the LCD panel (caused mainly by the RGB color filter). By contrast, in an OLED display, every pixel is itself an OLED that produces its own light.
Since both LCDs and OLED smartphone displays are composed of light-emitting diodes, let’s describe how these diodes are driven. Because of a diode’s intrinsic physical properties, it cannot be dimmed by changing the intensity of the current (mA) without impacting the color of the light. So how do phone manufacturers dim displays? They make use of a technique called pulse-width modulation (PWM), which means that they turn the diodes off and on at varying rates. Because we normally should not be able to see this switching between off and on (in other words, the flicker!), our brains are fooled into perceiving the screen as simply dimmer overall (a phenomenon known as the “brain averaging effect”). How dim depends on how long the diodes are off versus how long they are on: the longer they’re off, the dimmer the screen will appear.
So both LCDs and OLED displays power their light sources differently, but both technologies are subject to flicker effect; however, it is usually more noticeable on OLED displays than on LCDs. For one thing, OLED displays and LCDs show PWM at different frequency ranges — the PWM of OLED displays range from ~50 to ~500 Hz, whereas the PWM of LCDs starts at around 1000 Hz or higher. Second, as the human eye may experience flicker sensitivity up to about 250 Hz (at least for most people), it should come as no surprise that OLED displays are more likely to cause eyestrain than LCDs.
An on/off modulation pair is called a period, and the amount of time that the diode is switched on in a period is called a duty cycle. The chart below illustrates how different PWMs affect the perceived brightness of a display:
Perceived brightness levels for 25%, 50%, 75%, and 100% PWM duty cycles.
A significant disadvantage to using PWM technology can be that when a display adjusts to its minimum brightness in very dim or completely dark ambient light conditions, the duty cycle is very short and the interval when the diode is off is proportionately much longer (for example, minimum brightness may translate to a 10% duty cycle, meaning that the diode is off for 90% of the period). At lower PWM frequencies, flicker can become much more noticeable, which helps explain why reading text or watching videos in bed at night is more likely to cause headaches and eyestrain than when viewing screens in brighter conditions.
The video below was shot with a Phantom VEO-E 340L camera at 1500 fps (as were the other videos further below), slowed down to 4 fps to show display pulse-width modulation (PWM) — the white areas separated by black lines that extend across the screen when brightness diminishes at regular intervals. You can see the difference between the Samsung Galaxy S20 Ultra 5G on the left, which has a medium duty cycle (around 60%), and the Huawei P40 Pro and the Oppo Find X2 Pro, which have long duty cycles (roughly 90%; the black lines show that the OLEDs are turned off, albeit briefly):
Differences in PWM among the Samsung Galaxy S20 Ultra 5G, the Huawei P40 Pro, and the Oppo Find X2 Pro, shot in indoor low-light conditions.
How we measure flicker
So how does DXOMARK measure flicker? One major way is with a device called, appropriately enough, a flickermeter (specifically, a TRD-200 from Westar Display Technologies), whose sole purpose is to measure quick oscillations in brightness. Our engineers follow a strict protocol for measuring flicker on each smartphone display: all devices are individually tested using their default settings under the exact same dark (< 0.1 lux) ambient lighting conditions, and are placed at the same distance from the flickermeter. We chart the output on this graph (which we use to compare up to four phones in our display reviews; note that you can click on the name of a phone in the legend on the bottom of the graph to remove or redraw its results):
Yes, it’s a cool-looking graph, but what does it mean? How should we read this? Well, first of all, keep in mind that these results correlate with each device’s PWM — the on/off power cycle that helps control screen brightness. The horizontal X axis show the frequency of the oscillations over time measured with the flickermeter in hertz (Hz). The vertical Y axis shows the SPD(dB)— spectral power density in decibels, which is the amount of power associated with one frequency of the signal that the display generates.
The first spike in our flicker graph appears at a phone’s listed refresh rate, but it is the highest spike — that is, the one that comes closest to or surpasses 0 dB — that is of interest to us in terms of flicker, as it indicates the PWM frequency; in this case, 241 Hz for the Samsung (S20), 362 Hz for the Huawei, 481 Hz for the OnePlus, and 240 Hz for the other Samsung (Note20). (Just in passing, you can nearly always ignore values below -40 (dB) on the graph, as they correspond to testing noise.)
The table below shows the numeric values from the flicker graph above:
|Device||Refresh rate (Hz)||PWM frequency (Hz)|
|Samsung Galaxy S20 Ultra 5G||60||241|
|Huawei P40 Pro||90||362|
|OnePlus 8 Pro||120||481|
|Oppo Find X2 Pro||120||483|
|Samsung Galaxy Note20 Ultra 5G||120||240|
The very slow-motion video below imitates the results of a flickermeter test. What is interesting to note is that from left to right, the devices scroll faster, which indicates different PWM frequencies.
PWM: the Samsung Galaxy S20 Ultra 5G, the Huawei P40 Pro, and the Oppo Find X2 Pro, shot in a dark room at < 0.1 lux.
In this second very slow-motion video, we included the Samsung Galaxy Note20 Ultra 5G that has a refresh rate of 120 Hz; interestingly enough, however, its PWM frequency is 240 Hz (as the flicker graph above also showed). In the video of the Note20 Ultra 5G, you can see that it has one frame on (bright) to five frames off (dark); the P40 Pro ends up with one frame on to three frames off; and the Find X2 Pro varies between one frame on to two or three frames off. All this is to say that where flicker is concerned, even a phone with a fast refresh rate like the Samsung Galaxy Note20 Ultra 5G can have a low PWM frequency and thus noticeable flicker under certain conditions. If you are sensitive to flicker, you will likely notice it on the Samsung devices at this brightness level and these PWM frequencies, but not on other devices with higher PWM frequencies.
PWM: Samsung Galaxy S20 Ultra 5G (240 Hz), Huawei P40 Pro (362 Hz), Oppo Find X2 Pro (483 Hz; 4th shot: Oppo with flicker reduction);
shot in a dark room at < 0.1 lux.
You will no doubt notice the striking difference between the two Oppo Find X2 Pro devices on the right in the video above. While the Oppo device already benefits from a fast PWM rate, the shot of it on the far right shows the very noticeable effect of the Find X2 Pro’s flicker reduction feature (something we do not test in our current protocol, but that our technical team felt was worth pointing out).
Keep in mind that our engineers base their evaluations and the scores they assign to smartphone displays not only on the objective tests they perform with flickermeters and other instruments, but also on perceptual tests that they conduct after being specially trained to see flicker.
To further illustrate flicker, our engineers used a DSLR mounted on a translation rail and moved it quickly while it took a slow (1/10 second) shot of the three mounted smartphone displays shown below to imitate the effects of PWM. In the image of the Samsung Galaxy Note20 Ultra 5G on the left, you can see each individual white dot; on the Huawei P40 Pro in the middle, the individual dots are much closer together, but are still largely discernible; in the image of the OnePlus 8 Pro, however, the dots look more like an almost continuous line. Unsurprisingly, flicker is stronger on the devices where the white dots are further from one another — that is, devices with a lower PWM frequency.
Samsung Galaxy Note20 Ultra 5G, PWM at 240 Hz
Huawei P40 Pro, PWM at 362 Hz
OnePlus 8 Pro, PWM at 481 Hz
Let’s wrap things up by first repeating that flicker on smartphones is caused by the use of pulse-width modulation that turns light-emitting diodes off and on to control screen brightness levels. As we normally perceive flicker via our peripheral vision rather than via our “attending vision” (that is, what we specifically focus our eyes on), the small size of a smartphone screen makes it less likely that we will see flicker on it (unless we hold the phone very close to our eyes) than we might when viewing content on a laptop screen or monitor. When we do see flicker, however, it’s the PWM that is the culprit; and while flicker can be reduced on a phone with a higher refresh rate, you may sometimes see flicker on it anyway if the phone’s PWM is slow (as we saw with the Samsung Galaxy Note20 Ultra 5G).
Finally, it’s also important to remember that some people are more sensitive to noticing flicker than others; in fact, even people who may not consciously perceive flicker may nonetheless be sensitive to it, winding up with headaches or eyestrain after overdoing their screen time. Such people could choose an OLED smartphone with an anti-flicker feature, or one with an LCD. As you can see in the table below, the last entry shows the data for the Xiaomi Mi 10T Pro; since it uses LCD technology, its PWM frequency is so high that it in essence eliminates the flicker issue.
|Device||Panel technology||Refresh rate (Hz)||PWM frequency (Hz)|
|Samsung Galaxy S20 Ultra 5G||OLED||60||241|
|Huawei P40 Pro||OLED||90||362|
|OnePlus 8 Pro||OLED||120||481|
|Oppo Find X2 Pro||OLED||120||483|
|Samsung Galaxy Note20 Ultra 5G||OLED||120||240|
|Xiaomi Mi 10T Pro||LCD||144||2360|
Table showing refresh rates and PWM frequencies for several smartphones, including the Xiaomi Mi 10 Pro, whose display uses LCD technology.
This all said, you can rest assured that if our testers do discover a smartphone that has noticeable problems with flicker at its default settings, we will let you know about it as part of its Display review. (And by the way, we’ll also mention if a smartphone comes with a “flicker-free” feature or setting.)
Flicker-Free Technology ensures a comfortable viewing experience by preventing screen flickering as a means of reducing the monitor’s brightness.
Flicker-Free Technology (also referred to as Flicker-Less, Anti-Flicker, and similar) indicates that a monitor doesn’t use PWM (Pulse-Width Modulation) to regulate the display’s brightness.
The PWM method includes turning the backlight on and off rapidly, and even though the flickering frequency is invisible to the human eye, it can still cause eye strain and headaches to those sensitive to it after prolonged use.
Luckily, most modern monitors are flicker-free, meaning that they use DC (Direct Current) modulation to regulate the brightness instead, which provides a constant stream of light at any brightness level.
Now, keep in mind that some monitors may be advertised as ‘Flicker-Free’ but actually are not, such as the Samsung U32J590. Displays that use PWM introduce flicker only at lower brightness settings (lower than 20%-30%, sometimes below ~50%).
This is still an issue for those who prefer lower brightness levels, whether because they work in dim-lit environments or because their monitor’s luminance is simply too strong, even below 50%.
On the other hand, some completely flicker-free displays might not include that information in the monitor’s datasheet.
In our monitor reviews, we always include information on whether a display uses PWM to regulate brightness. While most new monitors use DC modulation, there are exceptions.
Visit our best gaming monitors and best monitors for office work buying guides to check out the best flicker-free displays available.
To test whether your monitor uses PWM, go here. If your display does use PWM at low brightness, there is something you can do to prevent it.
You can download a third-party application that will decrease the brightness by altering your graphics card’s white point value while allowing the monitor’s brightness to be set at 100%, thus avoiding PWM.
Other things that can affect your viewing comfort are proper anti-glare screen treatment and Low Blue Light Technology.
Curved vs Flat Monitor – Which Should You Choose
Rob is a software engineer with a Bachelor’s degree from the University of Denver. He now works full-time managing DisplayNinja while coding his own projects on the side.
DC dimming = Low flickering!
Looking at a screen from a close distance can cause eye fatigue over time. A computer monitor can wear your eyes out faster than other devices, such as TV's. So how do we reduce eye fatigue? At EIZO, we have researched the causes of eye fatigue in order to create the ideal eye-friendly monitors.
- Subject 1 LED Backlight
- Subject 2 Bright Screen
- Subject 3 Blue Light
LED backlight causes eye fatigue?
LCD monitors use backlights to display images. The number of reported cases of eye fatigue caused by screen flickering has increased since the popularization of LED-backlit Monitors. LCD monitors can even affect the eyes of people who don't notice backlight flickering.
How does flicker work?
Flicker is the result of backlight dimming (brightness control). There are 2 dimming techniques applied to LCD monitors: PWM (Pulse-Width Modulation and DC (Direct Current)
|PWM Dimming||Controls brightness by cycling the backlight on and off.|
Pros Wide brightness adjustment range. Simple circuit design.
Cons High speed cycles may cause flicker on LED screens.
|DC Dimming||Controls brightness by adjusting power supply.|
Pros No flickering.
Cons Difficult color reproduction with dark images. Complicated circuitry.
Tested: PWM vs. DC
We conducted an experiment to find out how users experienced flickering.
Did you notice any flickering?
Did you feel eye fatigue?
Lastly, which was the easiest to view?
Source: EIZO Corporation; EyeCare Dimming Technology - October 2012
- Results vary from person to person.
Hybrid Dimming Method
The FlexScan frameless monitors utilizes a hybrid solution to regulate brightness and make flicker unperceivable without any drawbacks like compromising color stability - even on low brightness settings as follows.
- High Brightness: DC Dimming Method
Furthermore, the monitors implement the high PWM method (over 10,000Hz) and the phase-shifting PWM dimming control in some models.
White Paper: Seven Ergonomic Features of the FlexScan EV Series [PDF]
A bright screen causes eye fatigue?
At first glance, a bright screen seems clear and easy to see. However, a screen that is too bright is a primary cause of eye fatigue. On the other hand, if a display is too dark it is too difficult to see and also adds to eye fatigue. At EIZO, we tested the effects of our Auto EcoView, which automatically adjusts the screen's brightness to an appropriate level in response to ambient lighting.
Did you notice flickering?
- Brightness Sensor OFF: Brightness of 100%, Brightness Sensor ON: Automated Control (EcoView)
Source: EIZO Corporation, Seven Ergonomic Features of the FlexScan EV Series. December 2015.
Auto Brightness Control
The Auto EcoView brightness sensor detects changes in ambient brightness throughout the day and automatically adjusts the screen to the ideal brightness level to achieve maximum eye comfort.
Does Blue Light affect sleep quality?
Research indicates that exposure to blue light emitted by electronic devices after sundown impacts sleep.
How to reduce blue light on an LCD monitor
In order to prevent eye fatigue caused by Blue Light, EIZO tested how much we could reduce the amount of Blue Light emitted by making adjustments to the monitor.
Lowering the color temperature
Lowering the color temperature causes light distribution to shift towards longer wavelengths (more reddish colors). When we change the initial color temperature of our monitors (6500 - 7000K) to 5000K...
blue Light was cut by 20%
Lowering Color Temperature and Brightness
Overall energy usage is reduced by lowering the monitor brightness. A brightness reduction from highest settings to an adequate value (approx. 20 cd/m2 and a color temperature of 5000K)...
cut the blue Light by 80%
The Circadian Dimming function in Screen InStyle dedicated software for the FlexScan frameless monitors automatically changes the color temperature of your monitor as the day progresses. Maintain your body's natural circadian rhythm by setting the monitor to gradually reduce blue light during the evening, helping you sleep easier.
Sleep Tight with Less Blue Light
Auto Brightness Control
To prevent eye fatigue, a brightness sensor called Auto EcoView detects the changes in the ambient brightness that occurs throughout the day and automatically adjusts the screen to the ideal brightness level.
Minimum Brightness of Approximately 1 cd/m2
In a dimly-lit work environment, a monitor with low brightness settings is more comfortable to use. With its LED backlight, the monitor is adjustable to approximately 1 cd/m² which is less than 1% of the maximum.
FlexScan Frameless are treated with an anti-glare coating to reduce screen reflections caused by ambient lighting. The coating scatters incoming light, reducing the amount of light reflected into your eyes.
Reduce Blue Light by 80%
In the visible light spectrum, blue light has wavelengths adjacent to ultraviolet light. Compared to the factory preset setting of 6500 K of typical LCD monitors, Paper Mode is closer to the spectral distribution with long reddish wavelengths so it reduces the amount of blue light, a cause of eye fatigue, and helps prevent eyestrain when reading documents. When used in conjunction with Auto EcoView dimming function, blue light can be reduced by as much as 80%.
Due to the way brightness is controlled on LED backlights, a small number of people perceive flicker on their screen which causes eye fatigue. FlexScan Frameless monitors utilize a hybrid solution to regulate brightness and make flicker unperceivable without any drawbacks like compromising color stability – even on low brightness settings.
Clear View from All Angles
The monitor uses an LED-backlit IPS (in-plane switching) LCD panel with 178° viewing angle that minimizes color shift and contrast changes when viewing the screen at an angle. This means that two people sitting at the one computer can easily see the screen with high image quality.
FlexScan business monitors for eye comfortSours: https://www.eizoglobal.com/library/basics/eyestrain/
Flickering test pwm
Our Monitor Motion Tests
Response time and Refresh Rate
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.
ULMB and Lightboost
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.
- As seen in the Samsung monitor above, it is most common for flicker to appear at lower backlight settings of the monitor. To easily check the lowest point at which you can set your monitor without tripping the PWM switch, set it to minimum, then wave your hand in front of the screen. If your monitor uses flicker, you should see a stroboscopic effect (doubling). Raise the backlight setting until this effect is not visible anymore.
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.
PWM is short for pulse-width modulation and describes a flickering of the background illumination to reduce the luminance. Sensitive users may see this flickering below a certain frequency and get tired eyes or headaches. These problems might not even be obvious. More than 500 Hz are generally not an issue, but there seem to be many users with problems below 250 Hz. Beware, there are even reports from readers that suffer from frequencies above 10k Hz. These users should look at the individual reviews and look out for completly flat lines in our measurements as small brightness changes may lead to sore eyes, headache ... for them.
We use the following table to show the measured frequency of the flickering. Some devices do not use PWM at all, but still showed a measurable brightness flickering during our review (e. g. poor shielding of the power adapter). Those devices are listed here as well. Devices listed with a "0" did not show big brightness fluctuations in our measurements. A detailed analysis and a screenshot of the measurement is available in the linked review.
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